Recent Advancements in Polymer Based Composite Electrolytes for Rechargeable Lithium Batteries

Transcription

1 Electrochemical Energy Reviews (2018) 1: REVIEW ARTICLE Recent Advancements in Polymer Based Composite Electrolytes for Rechargeable Lithium Batteries Shuang Jie Tan 1,3 Xian Xiang Zeng 2 Qiang Ma 2 Xiong Wei Wu 2 Yu Guo Guo 1,3 Received: 7 February 2018 / Revised: 11 April 2018 / Accepted: 7 May 2018 / Published online: 18 May 2018 Shanghai University and Periodicals Agency of Shanghai University 2018 Abstract In recent years, lithium batteries using conventional organic liquid electrolytes have been found to possess a series of safety concerns. Because of this, solid polymer electrolytes, benefiting from shape versatility, flexibility, low-weight and low processing costs, are being investigated as promising candidates to replace currently available organic liquid electrolytes in lithium batteries. However, the inferior ion diffusion and poor mechanical performance of these promising solid polymer electrolytes remain a challenge. To resolve these challenges and improve overall comprehensive performance, polymers are being coordinated with other components, including liquid electrolytes, polymers and inorganic fillers, to form polymer-based composite electrolytes. In this review, recent advancements in polymer-based composite electrolytes including polymer/ liquid hybrid electrolytes, polymer/polymer coordinating electrolytes and polymer/inorganic composite electrolytes are reviewed; exploring the benefits, synergistic mechanisms, design methods, and developments and outlooks for each individual composite strategy. This review will also provide discussions aimed toward presenting perspectives for the strategic design of polymer-based composite electrolytes as well as building a foundation for the future research and development of high-performance solid polymer electrolytes. Keywords Solid batteries Solid electrolytes Polymer electrolytes Lithium anode Interface PACS Wx- Polymers and organic materials in electrochemistry F- Energy storage technologies 1 Introduction After the commercialization of lithium-ion batteries (LIBs) by the Sony Corporation in the 1990s, Li-based secondary batteries have received significant attention because of their high energy density, long cycle life and minimal memory effects [1 3]. Currently, commercially available lithiumion batteries utilize liquid electrolytes as the lithium-ion (Li + ) carrier, offering benefits such as high conductivity * Yu Guo Guo ygguo@iccas.ac.cn CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing , China College of Science, Hunan Agricultural University, Changsha , Hunan, China University of Chinese Academy of Sciences, Beijing , China and excellent wetting ability for the electrodes [4 6]. However, these liquid electrolytes also suffer from poor electrochemical and thermal stability, low ion selectivity and poor safety, limiting their application in next-generation high-energy density battery systems [7 9]. Furthermore, the rise of portable electronic devices, electric vehicles and gridenergy storage systems has increased lithium-ion battery requirements [10 16], including the need for higher energy densities, longer cycle lives and higher levels of safety. Because of these increased requirements, solid-state electrolytes (SSEs) have been attracting increasing attention around the world and are believed by researchers to be promising in lithium-ion battery applications, with the potential to endow batteries with longer cycle lives, higher energy densities, fewer packaging and state-of-charge monitoring circuit requirements [17 21]. In general, SSEs can be classified into two categories: inorganic ceramic electrolytes and polymer electrolytes. Inorganic ceramic electrolytes, including Li phosphorus nitride [22], Li 3 N [23 25], Li 7 La 3 Zr 2 O 12 [26, Vol.:( )

2 114 Electrochemical Energy Reviews (2018) 1: ], Li 3x La (2/3) x TiO 3 [28], Li 1+x Al x Ge 2 x (PO 4 ) 3 [29], Li 14 Zn(GeO 4 ) 4 [30], Li 10 GeP 2 S 12 [31, 32] and xli 2 S (1 x) P 2 S 5 [33, 34], have been extensively investigated and developed in recent years. And among these, pioneering studies such as the one proposed by Kanno et al. [31, 32] have produced inorganic ceramic electrolytes that can provide superior conductions rivaling that of liquid electrolytes in ionic conductivity at room temperature. However, drawbacks do exist with these electrolytes which limit their applications, including poor compatibility between the solid inorganic electrolyte and the electrode (e.g., stability against Li [35, 36] and high-voltage stability [37]), poor air stability [38], high interface resistance [39] and complex synthesis processes. Several excellent reviews on inorganic ceramic electrolytes are available in literature [40 44], and in this review the main focus will be on polymer-based composite electrolytes. Polymer electrolytes, possessing shape versatility and flexibility, are light weight and inexpensive to produce. They have the ability to form good electrode/electrolyte contact and are believed to be promising candidates for high-energy density battery systems [45 47]. From a practical application standpoint, polymer electrolytes for Li polymer batteries should inherently possess the following properties: (1) high ionic conductivities(> 10 4 S cm 1 at ambient temperature) [46] (2) appreciable Li + transference numbers (close to unity if possible) [48], (3) good mechanical strength(> 6 GPa) [49 51], (4) wide electrochemical stability windows (up to 4 5 V vs. Li/Li + ) [20], (5) excellent chemical and thermal stability and beneficial compatibility with electrode materials [37, 52] and (6) low-cost and facile synthesis processes [53]. However, it is difficult for single-polymer electrolytes to meet all the requirements, and most possess low ionic conductivity ( S cm 1 at room temperature) and poor long-term stability as a result of the structural reorganization of polymer chains, severely limiting practical applications [54, 55]. As proposed, an effective method to improve the comprehensive performance of polymer electrolytes is to combine polymer electrolytes with other proper components to take advantage of synergistic effects in the construction of polymer-based composite electrolytes [56 58]. Based on this, the combination of polymer electrolytes with liquid components, other polymers and inorganic fillers to synthesize polymer/liquid hybrid electrolytes [59 61], polymer/polymer coordinating electrolytes [62 64] and polymer/inorganic composite electrolytes [56, 65] are all being widely studied. In this article, a comprehensive review is provided of the recent advancements in polymer-based composite electrolytes. Here, polymer-based composite electrolytes refer to composite electrolytes which contain polymers that can contribute to the improvement of comprehensive performance. Three different composite polymer-based electrolyte synthesis methods are discussed, including polymer/liquid hybrid electrolytes, polymer/polymer coordinating electrolytes and polymer/inorganic composite electrolytes. Various strategies are also discussed to further develop and conquer key issues such as low ion conductivity as well as thermal, mechanical and (electro)chemical stability. Finally, outlooks will be provided toward the future development of polymer SSEs and polymer SSEs-based Li batteries. 2 Polymer/Liquid Hybrid Electrolytes Solid polymer electrolytes (SPEs), possessing high flexibility and security assurance, have become increasingly attractive over the years and are considered to be favorable candidates to replace conventional organic liquid electrolytes [66, 67]. However, the inferior room-temperature ionic conductivity of SPEs is a detrimental characteristic that restricts its further development in the field of solid-state lithium batteries. To resolve this, hybrid systems consisting of polymer/liquid hybrid electrolytes have been explored in recent years and have demonstrated the ability to provide a balance between the characteristics of SPEs and conventional organic liquid electrolytes. These polymer/liquid hybrid electrolytes containing certain amounts of liquid ingredients in a polymer matrix have especially demonstrated increasing promise in the application of electrochemical energy harvest devices because of its ability to provide superior interfacial contact between the cathode and anode, as well as possessing high ionic conductivity and flexibility [68, 69]. In addition, a stable protection layer can be constructed by polymer/liquid hybrid electrolytes in solid-state lithium batteries, guaranteeing better safety performances as compared with conventional organic liquid electrolytes. SPEs without additives such as the ceramic filler, the liquid component or the functional polymer, also suffer from poor ionic conductivity at ambient temperatures [70 72]. This poor ionic conductivity can be attributed to large ion diffusion restrictions caused by the high crystallinity of the polymer which severely affects the transport of carriers. Up until now, numerous polymer/ liquid hybrid electrolytes along with the individual components have been studied and reported with equal intensity. These include polymer matrixes such as polyethylene oxide (PEO) and its derivate [73], poly(vinylidene fluoride) (PVDF) [74], poly(ethylene glycol) diacrylate (PEGDMA) [75], polymeric ionic liquid (PIL) and so on [76], as well as conventional liquid electrolytes and ionic liquids [77 79]. Here, the study of the liquid ingredients in polymer matrixes has been mainly devoted to the resolution of the diffusion issue as well as the realization of better interfacial adhesion between the two electrodes. 2.1 Polymer/Liquid Hybrid Electrolytes with Organic Liquids LIBs using conventional organic liquid electrolytes have rapidly and widely been applied in the consumer electronic

3 Electrochemical Energy Reviews (2018) 1: market in recent years. Despite this, because of the growing safety concerns arising from the solvent volatilization, leakage and explosion potential of organic liquid electrolytes, SSEs with excellent mechanical properties and favorable safety features are in high demand and are urgently needed [46]. However, insufficient ionic conductivity is a major obstacle for the wide spread application of SPEs at ambient temperatures. To overcome this, polymer/liquid hybrid electrolytes, which can combine the advantages of the higher room-temperature ionic conductivity of organic liquid electrolytes and the superior mechanical properties of SPEs, are being extensively explored. Furthermore, the volatility and leakage characteristics of organic liquid electrolytes can be efficiently inhibited in polymer/liquid hybrid electrolytes and thermal stability and safety performance can be increased. For example, Zhang et al. [80] designed an honeycomb-like polymer matrix based on poly(vinylidene difluoride-cohexafluoropropylene) (PVDF-HFP) that produced a high electrolyte uptake of 86.2% and exhibited an excellent thermal stability of up to 350 C (Fig. 1a b). In another example, Li et al. [81] synthesized a pentaerythritol tetraacrylate (PETEA)-based polymer/liquid hybrid electrolyte using a radical polymerization synthetic route and performed nail penetration tests to investigate the safety performance in which they built a pouch battery using the synthesized PETEA-based electrolyte as well as a pouch battery using a commercially available liquid electrolyte for comparison (Fig. 1c d). In their study, the liquid electrolyte battery experienced violent combustion during the nail penetration test, whereas much lower gas productions were observed for the polymer/liquid hybrid electrolyte pouch battery. In addition, Li et al. [81] reported that the anode and cathode of the polymer/liquid hybrid electrolyte-based battery in their study could hold higher capacity retentions (Fig. 1e f) and that as compared with the liquid electrolyte, the prepared polymer/liquid hybrid electrolyte possessed improved safety performances, higher ionic conductivities and better cycling stabilities, indicating the construction of a stable interface layer using the liquid gradient within the polymer/liquid hybrid electrolyte. In another study, drawing lessons from Soggy sand electrolytes [82], Zeng et al. [83] reported an infiltrated quasi-solid electrolyte (i-qse) with nano-sized AlPO 4 interspersed throughout the interconnected polymer network (Fig. 2a). This concentrated electrolyte confined in an infiltrated structure was found to be capable of facilitating the wetting of the interface and enhancing the adhesion between the electrodes and the polymer electrolyte. The infiltrated structure was also found to be effective in the inhibition of lithium salt exhaustion and the significant elimination of the uneven electrodeposition of Li, efficiently improving Li + transport and achieving a lower activation energy of 0.12 ev (Fig. 2b, c). And because of these beneficial effects, a Li symmetric battery constructed using the 115 i-qse demonstrated superior cycling stability as compared with commercial liquid electrolytes at elevated temperatures (55 C) (Fig. 2d). These novel interface-improved methods using the in situ generation of safeguards for Li anodes shed light on promising novel synthesis methods from an interface-engineering level. 2.2 Polymer/Liquid Hybrid Electrolytes with Ionic Liquids Ionic liquids (ILs), possessing characteristics such as low volatility, high ionic conductivity and excellent thermal stability, have been extensively researched in the field of electrochemical energy storage devices such as supercapacitors, fuel cells and LIBs [84 86]. The use of polymer/liquid hybrid electrolytes doped with ILs has been found to be an effective method to lower the crystallinity of polymers and promote the segmental motion of polymeric chains. The role of ionic liquids within the polymer matrix is similar to that of liquid electrolytes, and polymer/liquid hybrid electrolytes based on ILs exhibit much more reliable security assurances. Up to now, solvent casting, electrospinning and phase inversion are the most commonly used preparation methods to construct polymer/liquid hybrid electrolytes with ILs. However, in recent years, other ingenious methods have been developed by researchers such as in situ thermal polymerization and UV-light polymerization, both of which are beneficial to reducing interface resistances and allow for easy assembly. For example, Wang et al. [87] designed a poly(ionic liquids) electrolyte comprised of phosphonium ionic liquids which presented stable electrochemical performances in a Li symmetric battery. A hierarchical poly(ionic liquid)-based gel polymer electrolyte synthesized using an in situ synthesis route was also reported by Zhou et al. [88]. In addition, polymer/liquid hybrid electrolytes containing ionic liquids can also contribute to the uniform electrodeposition of Li in which parts of the anion can be immobilized by the bulky cation, reducing ion concentration gradients within the electrolyte system. Poly(ionic liquids) possess high chemical stability and superior compatibility with ILs as compared with other polymer matrixes and can effectively restrain phase separation and leakage. In this regard, the design of various types of structures is a potential method to further improve ionic conductivity. For example, interconnected ion transfer pathways were created by Wang et al. [76], using 2D silica nano-fillers and ILs confined in a polymeric ILs network and their results showed higher ionic conductivities and better transference numbers. And although polymer/liquid hybrid electrolytes with organic liquids and ionic liquids are both capable of optimizing interfacial contacts and achieving high ionic conductivities, the safety of polymer/liquid hybrid electrolytes with ionic liquids is superior to those with organic liquids. This

4 116 Electrochemical Energy Reviews (2018) 1: Fig. 1 a 3D architecture of the PVDF-HFP polymer membrane and b the porous gel polymer electrolyte with a combination of two membranes as one integrated separator (two back sides are exposed to the outside). Reprinted with permission from Ref [80], Nature Publishing Group c and d An examination of the safety performance of NLGS and NPGS batteries by nail penetration testing. e Specific capacity of the graphite anode and f NCA cathode before and after 280 cycles at 0.5C charge/1c discharge and 45 C. Reprinted with permission from Ref [81], The Royal Society of Chemistry 2017 can be attributed to the lower volatility and better thermal stability of ionic liquids and the fact that organic liquids suffer from gas production. Despite this, the development of polymer/liquid hybrid electrolytes with ionic liquids for large-scale SSE applications is difficult because of the associated high costs of the materials. Therefore, the search for comprehensive liquid materials for polymer/liquid systems is necessary. And although numerous achievements have been made, meeting the demands of high voltage and high energy density for polymer/liquid hybrid electrolytes is a difficult task. In addition, with the upsurge of flexible electronics, safety has also become an increasingly important aspect. To achieve all of this and to allow for large-scale applications, further research into polymer/liquid hybrid electrolytes is required, especially for the Li + transport mechanisms of the electrolytes, along with the optimization of the interface between the electrolyte and the corresponding two electrodes, the preparation of functional polymer matrixes and the optimization of preparation methods.

5 Electrochemical Energy Reviews (2018) 1: Fig. 2 Schematic illustration of the preparation and characterization of i-qse. a Schematic illustration of the preparation of i-qse; b DC polarization with a 10 mv amplitude in the Li symmetric cell and impedance curves before and after polarization (insert in Figure b); c Impedance spectra of the i-qse versus temperature; d Voltage profiles versus time of Li symmetric cells in a liquid electrolyte (up panel) and i-qse (down panel) at a current density of 4 ma cm 2 for 2 ma h cm 2. Reprinted with permission from Ref [83], Elsevier Inc. 2018

6 118 Electrochemical Energy Reviews (2018) 1: Polymer/Polymer Coordinating Electrolytes Polymer coordinating composites are widely used to manufacture specific functional polymers and in cases in which a polymer with a single monomer cannot meet requirements, polymer/polymer coordination is considered. Emblematical examples include ethylene vinyl acetate, vulcanized rubber and NORYL, all of which have created large and profitable markets. In relation to SPEs, pioneer studies by Wright et al. [89] who in 1973 discovered that alkali metal salts, if complexed with PEO, can produce ion conductivity, and Armand et al. [90] who in 1994 proposed the use of polymer electrolytes for batteries, have resulted in numerous polymers being developed as the host for polymer electrolytes, including PAN, PMMA, PVDF and PVP along with PEO being the most studied example. These polymer electrolytes are still far from being optimal, however, with low roomtemperature ionic conductivity and poor mechanical property hindering practical application. Still, polymer/polymer coordination provides opportunities to combine the merits of different hosts to fabricate preeminent polymer matrixes with many studies so far having focused on polymer/polymer synergism. Among these studies, the most widely researched methods include copolymerization, crosslinking, interpenetration and blending. 3.1 Copolymerization Copolymerization is the process in which two or more different monomers are polymerized together to form a product called a copolymer [91]. These copolymers can be classified according to how the polymer units are arranged along a chain, including random copolymers, block copolymers and graft copolymers. Block copolymer electrolytes (BCEs) have been the most studied in the literature with early studies on BCEs for Li batteries being reported by Giles et al. [92] in BCEs are attractive because they have the ability to self-assemble into periodic structures with domain spacing in the order of 10 nm [62] and can potentially provide opportunities for the design of materials with attractive transport and mechanical properties. The most common architectures of BCEs are the AB di-block and the BAB tri-block, in which A is the ionic conductor block and B is the block providing other functionalities such as mechanical strength [93]. The A block in these cases generally consists of either a linear PEO block [94, 95] or a low molecular weight poly(ethylene glycol) block grafted onto a macromolecular backbone such as poly(ethylene glycol methacrylate) [96]. For the mechanical reinforcement B block, a wide variety of polymers have been tested including polystyrene [94, 97] and poly(alkyl meth acrylates) [96]. In past decade, the mechanical and morphological properties of salt-doped BCEs have attracted increasing interest, and several studies have examined the effects of salt-doping on ionic conductivity, mechanical strength and morphology [98]. Recently, numerous efforts have been devoted to the coupling of high transfer numbers, large shear modulus, acceptable conductivities and mechanical strength. A representative work of block copolymers for polymer electrolytes was conducted by Bouchet et al. [99] who fabricated a single-ion BAB tri-block copolymer (Fig. 3a) consisting of TFSI anions, and whose structure enabled the important delocalization of the negative charge. This fabricated copolymer electrolyte exhibited an anionic conductivity of S cm 1 (60 C) with a Li + transport number close to unity ( t Li + > 0.85 ), as well as excellent mechanical properties (10 MPa at 40 C) and an electrochemical stability window spanning 5 V versus Li + / Li. Furthermore, Devaux et al. [93] developed an approach to rationalize the physiochemical properties of block copolymer electrolytes to find optimal polymer architectures using compositions of Li batteries based on the PS PEO PS copolymer as a model system. Here, it was demonstrated that linear PS PEO PS BCEs presented the best compromise between high conductivity and good mechanical property in which an intermediate molecular weight ( M n ) of the PEO block with a composition of 70 wt% PEO is the optimal direction for improving performances at low temperatures. Research on the specific interactions of different components is also key to the fabrication of better composite electrolytes. For example, the impact of grain sizes [100] and side-chain branches [101] on ionic conductivity was recently studied and it was found that dangling side-branches can impart higher chain mobility at the electrode/electrolyte interface, preventing the deposition of reaction by-products at the interface which in turn improves capacity retention. Gradient copolymers [102] and graft copolymers [62] have also been researched for application in polymer electrolytes. Considerable research efforts have also been devoted to improving the fundamental understanding of copolymerbased electrolytes to gain a deeper understanding of the detailed structure activity relationship between copolymers and to provide better strategies to couple high transfer numbers, large shear modulus, acceptable conductivities and mechanical strengths. Overall, copolymer room-temperature conductivity still needs to be improved and different combinations of polymer monomers; not only AB di-block and ABA tri-block copolymer systems should be tested to obtain optimal performances. 3.2 Crosslinking Crosslinking is a method in which one polymer chain is linked to another through a bond. Research has demonstrated that crosslinking can induce improvements in dimensional

7 Electrochemical Energy Reviews (2018) 1: Fig. 3 a The chemical structure of the single-ion conductor tri-block copolymer P(STFSILi)-b-PEO-b-P(STFSILi). Reprinted with permission from Ref [99], Nature Publishing Group b The chemical structure of the polyethylene/poly (ethylene oxide) solid polymer electrolyte. Reprinted with permission from Ref [109], American Chemical Society c A schematic of the synthesis of the 3D cross-linked polymer electrolyte. Reprinted with permission from Ref [73], WILEY VCH 2017 stability and increase dynamic storage modulus of polymer electrolytes [103]. Moreover, increases in the amorphous phase can be obtained in polymers through crosslinking, providing polymers with rubber-like characteristics [63, 104, 105]. Because of these benefits, numerous crosslinked polymers have been investigated for Li batteries, including polyether polymers [64, 106], acrylate polymers [107] and polyurethane polymers [108]. These obtained network polymers are mechanically stable, but low room-temperature ionic conductivities ( S cm 1 ) limit practical applications. Therefore, improving ionic conductivity and obtaining a balance between high room-temperature ionic conductivity and stable mechanical property is needed. In one example, Khurana et al. [109] synthesized a novel electrolyte composed of stiff semi-crystalline polyethylene (PE) chains covalently crosslinked by PEO segments for Li metal batteries (Fig. 3b) in which the length of the PE backbone between the crosslinks as well as the length of the PEO segments can be adjusted as needed. In their results, their obtained PE-PEO cross-linked SPE exhibited a high conductivity (> 10 4 S cm 1 at 25 C) and an exceptional dendrite growth resistance. In a continuous work, Zheng et al. [110] conducted a systematic structure property study of cross-linked hydrocarbon/poly(ethylene oxide) electrolytes and found that the crystallinity of the hydrocarbon backbone plays a key role in the regulation of the size and morphology of Li dendrites. An initiator-free one-pot synthesis strategy based on a ring-opening polymerization reaction to prepare a tough and compact 3D network gel polymer electrolyte (Fig. 3c) was proposed by Lu et al. [73]. The researchers proposed this synthesis method based on the fact that radical initiation processes have inherent disadvantages in which by-products such as free radicals and residual monomers are highly reactive with Li metal, increasing electrode resistances and severely degrading battery performances [111]. Moreover, Cui et al. [67] developed an in situ synthetic chemistry strategy to fabricate a crosslinked polymer electrolyte using Li salts (LiBF 4 ) as the initiator. This self-catalyzed strategy toward the preparation of crosslinked polymer electrolytes can provide excellent contacts between the electrolyte and the electrode, decreasing interface resistances and ensuring excellent electrochemical properties. In general, crosslinking provides SPEs with numerous fabrication possibilities, and a series of outstanding studies have been reported recently. Initiator-free and in situ polymerization are promising preparation strategies that are being actively investigated. However, these strategies still require more development to allow the crosslinking polymerization process, especially initiator-free and in situ polymerization, to be more controllable so as to provide excellent batch stability for battery production.

8 120 Electrochemical Energy Reviews (2018) 1: Interpenetration The interpenetrating polymer network (IPN), a concept devised by Klempner [112] and Sperling [113] independently in 1969, is a novel type of polymer blend composing of crosslinked polymers and is used to improve the compatibility between the immiscible phases in traditional polymer/ polymer composite processing [114]. IPNs are intimate mixtures of two or more distinct crosslinked polymer networks with no covalent bonds between the polymers (i.e., polymer A crosslinks only with other molecules of polymer A, and polymer B crosslinks only with other molecules of polymer B) which can be synthesized sequentially (from polymer A and monomer B) or simultaneously (from monomer A and monomer B). And because of the inherent crosslinking with each other, these networks can satisfy the primary requisite of dimensional stability, along with the ability to solvate ions. The IPN morphology can also reduce the existence of crystalline domains to almost none. As for the majority of the IPN composition, the T g is seen to shift inwards, leading to improved mechanical properties at ambient temperatures. The presence of 3D-crosslinked networks also reduces the effects of contact ion-pair formations and ion clusters and therefore can also minimize the trapping of mobile charge carriers within the matrix. Moreover, IPNs are versatile composite materials that can be custom designed to suit specific uses. Considering the superiority of IPNs, researchers have developed various polymer electrolytes with interpenetrating polymer networks [ ]. Of these, sequentially synthesized IPNs are preferred because of its controllable process and wide material choices of polymer matrix. In one example, Zheng et al. [120] fabricated a novel solid-state polymer electrolyte with an interpenetrating poly(ether-acrylate) (ipn-pea) network using a photo-polymerizing branched acrylate in the presence of ion-conductive PEO (Fig. 4a, b). Their obtained composite electrolyte was found to possess decreased crystallinity (Fig. 4c) and produced both high ion conductivity (0.22 ms cm 1 ) at room temperature and high mechanical strength [ca. 12 GPa (Fig. 4d)] owing to the desired combination of plasticizing PEO and flinty PEA. This electrolyte was subsequently paired with a cathode at a working potential within 4.5 V (vs. Li + /Li) and a Li metal anode, and the resulting battery delivered admirable specific capacity and cycling stability. Here, the in situ formed electrolyte possessing high mechanical strength demonstrated a notable ability to block Li dendrite growth as evidenced by the dendrite-free surface after Li deposition for 2 ma h cm 2 at 0.5 ma cm 2 (Fig. 4e, f). Although sequentially synthesized IPNs provide a more controllable process, simultaneously synthesized IPNs are more effective in the fabrication of a fully interpenetrating network. As an example, Ma et al. [121] synthesized a cross-linking poly(acrylic anhydride-2-methyl-acrylic Fig. 4 a Illustration of the preparation of the ipn-pea electrolyte. b Proposed electrochemical deposition behavior of Li metal with the ipn-pea electrolyte. c XRD spectra of PEO, polyacrylate and the ipn-pea electrolyte. d Top view and e side view SEM images of Li stripping. f Young s modulus mapping of the ipn-pea electrolyte. Reprinted with permission from Ref [120], American Chemical Society 2016

9 Electrochemical Energy Reviews (2018) 1: acid-2-oxirane-ethyl ester-methyl methacrylate) (PAMM) IPN through the in situ polymerization of the monomers and this obtained composite gel electrolyte demonstrated greatly improved high-voltage resistances. A solvent-free method to prepare all-solid IPN composites was also developed by Duan et al. [122], in which an in situ plasticized SPE with two conducting polymer networks was fabricated through the facile polymerization of two types of liquid polymer monomers with appropriate chain lengths. This prepared IPN exhibited enhanced ion conductivity, high mechanically flexibility, wide electrochemical window (4.7 V vs. Li + /Li), high thermal stability (stable up to 200 C) and good dendrite suppression ability. Interpenetrating polymer networks can provide composite polymer electrolytes with enhanced ion conductivities and mechanical properties. However, more efforts will be needed to develop appropriate systems with ionic conductivities at ~ 10 3 S cm 1. Comparing with ex situ manufacturing, in situ curing demonstrates superior performance and the exploration of optimized in situ polymerization strategies along with improvements in controllability are essential for practical applications. 3.4 Blending Polymer blending is another effective strategy to improve the thermal, mechanical and electrical properties of polymer electrolytes and has been investigated for a number of years [123, 124]. Compared with copolymerization and cross-linking, polymer blending is more easily processed and economically friendly [125]. In the bending of two polymers, the polymers need to be able to dissolve in a common solvent and should have complementary properties (i.e., one polymer possesses the ability to solvate a wide variety of salts and the other polymer possesses excellent thermal or mechanical properties). And by combining the beneficial properties of two or more polymers, blend polymer electrolytes can produce improved ionic conductivities, mechanical properties, electrochemical performances and cycling stabilities. In general, polymers that are selected for blending are usually ones that have already been studied and have been proven to have advantageous characteristics, including PEO, PAN, PVDF, P(VDF-HFP), PMMA, PE and PVC. Research on various polymer combinations has been ongoing over the past two decades and some typical examples include P(VDF- HFP)/PAN, PVDF/PMMA and P(VDF-HFP)/PMMA [57, 58]. Mendez et al. [126] comprehensively summarized existing polymer blend electrolytes in 2015, and therefore, in this review, only progresses after 2015 are discussed and a brief outlook will be provided. Currently, many novel combinations of polymers have been explored to fulfill the different requirements of different battery systems. Here, single-ion conducting polymer 121 electrolytes of proven conductivity are preferred because of a variety of advantages such as the absence of the detrimental effects of anion polarization and the low rates of Li dendrite growth [127]. Polymer blending provides an effective method to fabricate polymer electrolytes with high transference numbers and outstanding comprehensive performances by simply blending an anion-immobilized polymer with other proper matrixes [128, 129]. Ma et al. [130] synthesized a new type of lithium polymer salt LiPSsTFSI to blend with PEO. The complex electrolyte exhibited a high Li-ion transference number ( t Li + = 0.91) and a Li-ion conductivity as high as S cm 1 at 90 C. Highvoltage tolerance is another direction people are pursuing. In one example, Zhao et al. [131] developed a self-supporting PVDF/P(VC-VAC) blended polymer electrolyte possessing high oxidation-resistance potentials of up to 5.4 V (vs. Li + /Li). This self-supporting blend polymer electrolyte also possessed superior compatibility with a high-voltage LiNi 0.5 Mn 1.5 O 4 cathode and exhibited high discharge capacities and cycle stabilities. Polymer blending requires blending components to possess complementary properties, and the exploration of components beyond conventional polymer blends is worthwhile to improve performances. In addition, for practical applications, blending polymers with specific structures (e.g., hierarchical nano- to micropore structures) and specific functions (e.g., high-voltage tolerance and electrode compatibility) are needed. 4 Polymer/Inorganic Composite Electrolytes Inorganic ceramic electrolytes exhibit satisfactory ion conductivity and mechanical property but suffer from large interfacial resistances and poor machinability. In contrast, polymer electrolytes offer shape versatility, flexibility and low processing costs [46, 53]. Therefore, the synergistic integration of inorganic and polymer electrolytes to construct composite electrolytes to balance ion conductivity, /mechanical strength and interfacial stability is a promising strategy to comprehensively improve electrolyte performances [56]. In general, polymer/inorganic composite electrolytes contain a polymer matrix (e.g., PEO, PVDF- HFP) and an inorganic micro- or nanoscale filler (e.g., SiO 2, Al 2 O 3, TiO 2 ). Here, in theory, the polymer matrix serves to improve mechanical flexibility and processing ability and simultaneously contributes to good physical contact between the electrolyte and the electrodes, negating interface resistances. The inorganic filler on the other hand, if ideal, acts as a solid plasticizer that can reduce crystallinity and enhance transportation properties because of increased dielectric constants.

10 122 Electrochemical Energy Reviews (2018) 1: Recent developments in polymer/inorganic composite electrolytes have attracted much attention, and there have been many excellent reports in the literature dealing with fabrication methods, fillers, matrixes and combination modes. Categorically, polymer/inorganic composite electrolytes can be divided into two types, amalgamated and layer stacked, depending on the degree of dispersion of the inorganic fillers. For amalgamated electrolytes, inorganic fillers are uniformly dispersed in a polymer matrix where as for layer stacked electrolytes, inorganic fillers and polymers remain as two separate layers with a clear but tight coupling boundary in between. 4.1 Polymer/Inorganic Amalgamated Electrolytes The homogeneous mixing of inorganic fillers with polymer matrixes at the micro- or nanoscale has been widely researched. This is because the dispersion of inorganic fillers in polymer matrixes creates surface interactions that can reduce the crystallization tendencies of the polymer and promote the dissociation of lithium salts [132]. In addition, the resulting abundant interfaces of the composite electrolyte can provide numerous transport pathways for Li +, improving ionic conductivity [133]. In general, the type, particle size and shape of inorganic fillers as well as the interactions between the inorganic filler and the other component will all have impacts on the performance of composite electrolytes and these will be discussed in detail Types of Inorganic Fillers Many different types of inorganic fillers have been tested to fabric composite electrolytes and have been shown to be able to increase ionic conductivities and mechanical properties because of their better thermal and mechanical properties. Based on the capability for ionic conductivity, inorganic fillers can be divided into active fillers which participate in the conduction of ions or passive fillers which do not. Typically, passive fillers include Si [134], SiO 2 [135], Al 2 O 3 [136], TiO 2 [137, 138], ZrO 2 [139] and BaTiO 3 [140] and active fillers include Li 3 N [58], Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 [ ], Li 7 La 3 Zr 2 O 12 [ ], Li 0.33 La TiO 3 [147], Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 [148], Li 10 GeP 2 S 12 [149] and Li 2 S P 2 S 5 [150]. And all these fillers have attracted attention from researchers in recent years because of their ability to enhance ion conductivity Shapes of Inorganic Fillers Inorganic fillers with different dimensional shapes have attracted much attention from researchers recently. These shapes include zero-dimensional (0D) particles, 1D wire-like, 2D sheet-like and 3D network-like, and all of which have been tested as fillers to fabricate composite electrolytes. (1) Zero-Dimensional Particles Of all the shapes, zero-dimensional (0D) particles are the most frequently studied and used inorganic fillers for hybrid electrolytes. This is because 0D particles can be easily obtained and if uniformly dispersed, can hinder the reconstitution of polymer chains and reduce crystallization, proving to be an effective method to accelerate Li + transportation [136]. For zero-dimensional (0D) particles, synthesizing model and particle size are the most researched. The mechanical mixing method, being convenient and inexpensive, has been generally adopted as the go-to method to obtain composite electrolytes. However, composite electrolytes obtained using this method can have poor filler distribution because of the high surface energies of the inorganic fillers, leading to nanoparticle aggregations, which subsequently leads to the reduction in efficacy of the inorganic fillers [137]. In addition, nanoparticle aggregations can create numerous crystallized polymer regions in the electrolyte, leading to weak polymer ceramic interactions and adversely affect the comprehensive performance of the composite electrolyte. To resolve this, Lin et al. [151] proposed an in situ method to synthesis ceramic particles inside a polymer electrolyte to obtain a composite electrolyte. In their study, a PEO-monodispersed ultrafine SiO 2 (MUSiO 2 ) composite electrolyte was obtained through the well-controlled in situ hydrolysis of tetraethyl orthosilicate in a PEO solution (Fig. 5a) and possessed better particle distribution and mono-dispersity with PEO crystallinity being successfully suppressed. In subsequent testing, the ionic conductivity of this in situ synthesized composite electrolyte was one order of magnitude higher than samples prepared using a simple mechanical mixing method (Fig. 5b). The particle size of inorganic fillers also has vital impacts on ionic conductivity properties [58]. For pure solid inorganic electrolytes, a high ionic conductivity can be achieved by reducing the particle size to enrich Li-ion diffusion routes [152, 153]. In hybrid electrolytes, inorganic fillers are dispersed in a polymer matrix in which the matrix helps to bind particles together to produce high mechanical flexibility. Ideally, compared with micro-sized fillers, nano-sized inorganic fillers have been reported to be more effective in reducing crystallinity [154] and interfacial resistances [155], along with better compatibility with Li metal [65]. The nature of the interactions, which is believed to be a dipole dipole interaction, increases with decreased particle sizes. Recently, Zhang et al. [156] reported that a nano-sized filler (40 nm LLZTO) can exhibit an apparent conductivity

11 Electrochemical Energy Reviews (2018) 1: Fig. 5 a Schematic of the in situ hydrolysis and interaction mechanisms between PEO chains and MUSiO 2 and b Arrhenius plots of the ionic conductivity of ceramic-free SPE, PEO-fumed SiO 2 CPE, ex situ CPE and in situ CPE. Reprinted with permission from Ref [151], American Chemical Society c Schematic of flexible pouch cells and d the conductivity as a function of the LLZTO volume fraction for LLZTO particles with different sizes. Reprinted with permission from Ref [156], Elsevier 2016 of more than 10 4 S cm 1 at 30 C, which was nearly two orders of magnitude higher than microscale inorganic fillers (Fig. 5c, d). Yamada et al. [157] proposed that percolation effects were the reason for the conductivity enhancements because percolation thresholds decreased with decreasing particle sizes. In addition, smaller inorganic particles can provide composite electrolytes with abundant and highly conductive interfaces which further contribute to fast Li + transportation [158] and large specific surface areas can also lead to increased coherent conductivity paths [159]. Despite these promising performance improvements however, the agglomeration of nano-fillers can occur in electrolyte systems, severely affecting the effectiveness of the nano-fills as reported by Andreev et al. [160]. All these results imply that nanoscale particles are more effective in the improvement of conductivity as compared with microscale particles if agglomeration can be avoided. This is consistent with the results of studies employing in situ methods to synthesize nano-ceramic particles inside polymer electrolytes [137, 151, 161]. (2) One-Dimensional Wire-Like Fillers Compared with 0D particles, 1D wire-like fillers can create a larger contact area between the inorganic filler and the polymer electrolyte, drastically enhancing ionic conductivity. A nanowire filler possessing high aspect ratios can also create continuous ionic transport pathways to bridge much longer distances, reducing the large junction crossing between particles and particles. For example, Liu et al. [147] fabricated a novel solid composite polymer electrolyte using the electrospinning method to disperse 15 wt% Li 0.33 La TiO 3 nanowires into PAN-LiClO 4 (Fig. 6a). And because of the resulting fast surface ionic transport pathways, an unprecedented ionic conductivity of S cm 1 at room temperature was obtained (Fig. 6b). The mechanisms of conductivity enhancement were studied recently by Chan et al. [133] using solid-state NMR measurements, and they proposed that Li + preferentially travel through the LLZOmodified PAN regions rather than the unmodified PAN regions. Because randomly distributed nanowires, possessing a significant portion of ceramic materials, are unable to contribute to ion transportation if they are aligned in parallel

12 124 Electrochemical Energy Reviews (2018) 1: Fig. 6 a Schematic illustration of the synthesis of ceramic nanowirefilled polymer-based composite electrolytes, and the comparison of possible lithium-ion conduction pathways in the nanowire-filled and nanoparticle-filled composite electrolytes. b Arrhenius plots of the composite electrolyte with various LLTO nanowire concentrations, together with the data for the LLTO nanoparticle-filled PAN-LiClO 4 electrolyte. Reprinted with permission from Ref [147], American Chemical Society c Schematic of vertically aligned and connected ceramic channels for enhancing ionic conduction. d SEM image of the ice-templated LATP channels. e Ionic conductivities of the three structures at different temperatures. Reprinted with permission from Ref [162], American Chemical Society 2017 to the electrolyte surface, Zhai et al. [162] fabricated a vertically aligned and connected LATP ion-conductive ceramic filler using an ice-templating-based method (Fig. 6c). In this composite electrolyte, fast Li + transport pathways were supported by vertically aligned ion-conductive ceramic fillers and the polymer matrix provided flexibility and mechanical support (Fig. 6d). In testing, this aligned composite electrolyte exhibited a high ion conductivity of S cm 1, reaching theoretical values based on the conductivity and volume portion of LATP (Fig. 6e). Similarly, a meticulous study comparing well-aligned and randomly aligned nanowires was conducted by Lui et al. [163] (Fig. 7a), revealing that nanowires can provide better long-run surface pathways for Li + conduction than nanoparticles. In this study, randomly aligned and well-aligned nanowires with different orientation angles of 0 ± 5, 45 ± 9, 90 ± 8 were also fabricated and tested. In the case of 90, a low conductivity ( S cm 1, 30 C) similar to the polymer without filler was obtained. This may be a result of the fact that the long axis of the nanowires is parallel to the electrode direction and that the surface area of the nanowires does not cross between the electrodes. As for the case of 45, because of the longer continuous length (1.44 times of 0 ) of the nanowires across the electrodes, the obtained ion conductivity was S cm 1 and was also lower than those obtained at 0 ( S cm 1 ) (Fig. 7b). In the examination of the Li + transport mechanism as calculated from the experiment data, the surface conductivity of the nanowires ( S cm 1 ) in the polymer amalgamated electrolyte was found to be comparable to the ionic conductivity of liquid electrolytes. Moreover, Comsol Multiphysics numerical analysis of the current distribution in the composite polymer electrolyte using nanowires of various orientation angles (0, 30, 45, 60 and 90 ) was carried out and was in good agreement with experimental data (Fig. 7c, d). From this, it can be concluded that the highly conductive surfaces of amalgamated inorganic fillers in polymers play a key role in augmenting ionic conductivity. (3) Two-Dimensional Sheet-Like Fillers 2D nanomaterials, such as graphene [164], boron nitride [165] and transition-metal di-chalcogenides [166], have also been extensively studied for a wide range of applications. This is because of the attractive properties originating from their ultrathin structure and the high degree of anisotropy and chemical functionality [167, 168]. In particular, large enhancements in the physical and chemical properties of diverse composites have been reported even with the slight addition of 2D nanomaterials as functional nano-fillers, being ascribed to the exceptionally large interactive surface areas of 2D nanomaterials which can provide efficient interfacial interactions with polymer matrixes [169, 170].

13 Electrochemical Energy Reviews (2018) 1: Fig. 7 a Schematic illustration of the surface region of inorganic nanoparticles (NPs) and nanowires (NWs) acting as a pathway for Liion conduction. b Arrhenius plots of composite polymer electrolytes with aligned nanowire arrays at various orientations, together with the data for composite electrolytes with randomly dispersed nanowires and filler-free electrolytes. c Modelling of current densities for the composite polymer electrolyte with aligned nanowires (orientation angle is 0 ). d Conductivity versus angle for composite polymer electrolytes, presenting data for both simulated and experimental results. Reprinted with permission from Ref [163], Nature Publishing Group 2017 In one example, Yuan et al. [171] took advantage of the desirable properties of 2D single-atomic-thickness graphene oxide (GO), with its ultra-large surface area and excellent mechanical and electrical insulating properties, and used it as a filler to fabricate a solid PEO-LiClO 4 -GO polymer nanocomposite electrolyte (Fig. 8a). The resultant composite electrolyte with a 1 wt% GO resulted in a roughly two orders of magnitude enhancement in ion conductivity (~ 10 5 S cm 1, Fig. 8b) as compared with the pure polymer electrolyte. The obtained electrolyte also exhibited excellent mechanical properties with an over 260% increase in tensile strength (Fig. 8c). A modified GO was also prepared to increase compatibility with polymer matrixes by Shim et al. [170]. Recently, Shim et al. [172] used a 3 4 nm-thick functionalized BN as an additive along with P(VDF-co-HFP) as a matrix to fabric an composite electrolyte (Fig. 8d) and found that even with small amounts of BN (0.5 wt%), the resulting composite electrolyte is still capable of providing unexpectedly high ionic conductivities (Fig. 8e), t Li +, and mechanical modulus (Fig. 8f), making the resulting electrolyte strongly resistant to the formation and growth of Li dendrites by providing both an alleviated Li + concentration gradient and a mechanically robust blocking layer. A Li/Li cell using a thinner G-CFBN (8 mm) also exhibited much smaller voltage hysteresis and longer cycling stability (1940 h) without any short circuiting (Fig. 8g). (4) Three-Dimensional Network-Like Fillers 3D ceramic networks formed by interconnected nanofibers are also desirable fillers that have been extensively studied because they can create a continuous lithium-ion-conducting network. 3D ceramic networks are also continuous and self-supporting in which Li + can be transported through not only the polymer matrix but also the ceramic network. The rich polymer ceramic interface can also contribute to Li + transport. In addition, the self-supporting structure does not require the mechanical mixing of fillers with polymers to fabric composite membranes, allowing for the simplification of fabrication processes and the avoidance of filler agglomeration. In one example, Fu et al. [173] fabricated a flexible solid-state fiber-reinforced polymer amalgamated composite using a 3D porous garnet nanofiber network to reinforce a Li salt-polymer solution (Fig. 9a). The resulting interconnected 3D structure was found to provide long-range Li + transfer pathways and further provided structural reinforcements to enhance the polymer matrix. This composite membrane also exhibited excellent flexibility (Fig. 9b) and a high roomtemperature ion conductivity of S cm 1 (Fig. 9c), along with improved mechanical properties imparting the ability to block dendrite grow thin a symmetric Li cell during repeated Li stripping/plating processes. More recently, Bruce et al. [174] created a 3D bi-continuous structured hybrid electrolyte with improved mechanical properties

14 126 Electrochemical Energy Reviews (2018) 1: Fig. 8 a Schematic of the PEO/GO/Li salt membrane; b ionic conductivity at room temperature; and c stress strain curves of polymer electrolyte films. Reprinted with permission from Ref [170], The Royal Society of Chemistry d Schematic illustration of the overall procedure for the preparation of G-CFBNs. e Ionic conductivity and Li + transference numbers (25 C), f Stress strain curves, and g Galvanostatic cycling profiles of symmetric Li/Li cells (1.0 ma cm 2 ) using LE-Celgard, G-PVH, and G-CFBN. Reprinted with permission from Ref [172], The Royal Society of Chemistry 2017 using a template method (Fig. 9d). In this 3D bi-continuous structure, the polymer channels allowed for mechanical property improvements and the ceramic channels provided continuous, uninterrupted pathways and maintained high ionic conductivity between the electrodes. This fabricated 3D bi-continuous electrolyte also exhibited an equivalent ionic conductivity of S cm 1 at room temperature as well as superior mechanical properties because of the dense structure Interactions in the Composite Electrolyte The amalgamation of inorganic fillers into polymer-based electrolytes can increase interaction effects. In purely polymer electrolyte systems, interaction effects, including the effects between polymer and solvent, polymer and salt, and salt and solvent, have been widely studied in which interaction effects among each component can greatly affect overall electrochemical performances. In the construction polymer/inorganic amalgamated composite electrolytes, interaction effects in the composite will increase to include the effects between inorganic fillers and polymer matrixes, inorganic fillers and salts, and inorganic fillers and solvents. Of these interactions, inorganic filler and polymer matrix Lewis acid base interaction effects [175, 176], entropy effects [176, 177], ion aggregate formation change effects [178, 179] and weakening polymer cation association effects [180] have been the most investigated and a further understanding is essential for the fabrication of optimal composite electrolytes.

15 Electrochemical Energy Reviews (2018) 1: Fig. 9 a Schematic procedure to fabricate FRPC lithium-ion conducting membranes. b Photographs to demonstrate the flexibility and bendability of FRPC lithium-ion conducting membranes. c Arrhenius plot of the FRPC electrolyte membrane at elevated temperatures. Reprinted with permission from Ref [173], National Academy of Sciences of the United States of America d Schematic of the templating procedure used in the synthesis of structured hybrid electrolytes using the cube microarchitecture as an example. Corresponding SEM images of each synthesis stage of the cube LAGP-epoxy electrolyte are included below each schematic. Reprinted with permission from Ref [174], The Royal Society of Chemistry 2017 In general, inorganic fillers in polymer matrixes can act as a physical plasticizer to contribute to the decrease of crystallization and the increase of ionic conductivity [56, 181]. Here, the interaction between the inorganic filler and the polymer matrix as well as the solvent plays a significant role. Recently, Zhang et al. [146] prepared a flexible PVDF/LLZTO composite electrolyte using an conventional solution-casting method in which Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO) powders were dispersed into a PVDF matrix. This complex process led to partial dehydrofluorination in the composite polymer electrolyte and thereby enhanced the interactions between the PVDF matrix, lithium salt and LLZTO particles (Fig. 10a), significantly improving the performance of the composite electrolyte (e.g., a high ionic conductivity of ~ S cm 1 at 25 C, high mechanical strength

16 128 Electrochemical Energy Reviews (2018) 1: Fig. 10 a Schematic of possible complex structures in PVDF/ LLZTO-CPEs. b Raman spectra of PVDF-SPE and PVDF/LLZTO- CPE membranes. FTIR spectra of DMF and DMF mixing with different concentrations of LLZTO at wavelengths of c ~ cm 1 and d ~ cm 1. e 1H NMR spectra of pure DMF and DMF coupling LLZTO. f The computed charge density difference of DMFabsorbed LLZO-001. Reprinted with permission from Ref [146], American Chemical Society 2017 and good thermal stability). In this study, Raman spectra (Fig. 10b) revealed that the dehydrofluorination of PVDF chains can result from the alkaline-like conditions created by LLZTO for PVDF. A solvent/llzto interaction was also found to exist in this composite electrolyte in which FTIR (Fig. 10c, d) and 1H NMR (Fig. 10e) spectroscopy suggested that LLZTO can complex with N atoms and C=O groups of DMF in a high-electron-density state. First-principle calculations were also conducted (Fig. 10f), and the exchange and transfer of charges between the DMF molecule and the (001) surface of LLZO were observed, which were consistent with the results of the 1H NMR test. Ideal inorganic fillers should be able to interact with Li salts to contribute to the dissociation of Li salts, increasing free Li + concentrations and immobilizing anions to improve Li + transference numbers. Based on this, Liu et al. [182] synthesized a Y 2 O 3 -doped ZrO 2 (YSZ) nanowire with abundant positive-charged oxygen vacancies (Fig. 11a). The researchers subsequently composited this nanowire with PAN to fabric a hybrid polymer electrolyte, in which the rich oxygen vacancies can interact with anions and release Li +. The obtained FTIR spectra for the composite polymer electrolyte filled with YSZ nanowires of various dopants revealed that the percentage of peaks corresponding to bonded ClO 4 with Li + was significantly reduced with increasing Y 2 O 3 doping levels (Fig. 11b). Figure 11c showed that at a doping level of 7 mol%, both the Li + conductivity of the composite electrolyte and the oxygen-ion conductivity of the YSZ approached maximum values, proving that free oxygen vacancies with positive charges played a crucial role in the Li + ion conduction of composite polymer electrolytes. The composite polymer electrolyte tested in this study therefore predictably provided a high ionic conductivity of S cm 1 at 30 C and an improved electrochemical stability. In another study, Zhang et al. [183] proposed a flexible anion-immobilized ceramic polymer composite electrolyte in which garnet-type Al-doped Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 ceramic particles were well dispersed into a polymer Li salt matrix to synthesize a polyethylene oxide (PEO)-lithium bis(trifluoro methyl sulphonyl)imide (LiTFSI)-LLZTO (PLL) solid electrolyte membrane. Here, anions as large as TFSI can be firmly trapped by PEO chains and LLZTO particles so that they are immobilized at the interface between the ceramic filler and the polymer matrix, where as Li + can freely emerge and move rapidly along the extended interface (Fig. 11d). A high transfer number (Fig. 11e) was obtained, and a dendrite-free deposition was achieved for the solid electrolyte because of the interactions between the ceramic particles and the

17 Electrochemical Energy Reviews (2018) 1: Fig. 11 a Schematic illustration of the Li-ion transport in composite polymer electrolytes with positive-charged oxygen vacancy nanowire fillers. b Plot of the percentage of the peak according to bonded ClO 4 with Li + versus Y 2 O 3 doping levels. c Plot of the relationship between Y doping levels and conductivity, together with the conductivity of the YSZ bulk. Reprinted with permission from Ref [182], American Chemical Society d Schematic of immobilized anions tethered to polymer chains and LLZTO ceramic particles. e t Li + of (i) the PLL composite electrolyte, (ii) the PEO-LiTFSI solid electrolyte, (iii) the 1 M LiPF 6 -EC/DEC and (iv) the 1 M LiTFSI-DOL/ DME liquid electrolyte. Reprinted with permission from Ref [183], National Academy of Sciences of the United States of America 2017 polymer matrix in which the fabricated electrolyte exhibited a high Li + conductivity of S cm 1 at room temperature and a wide electrochemical window of 5.5 V. Polymer/inorganic composite electrolytes with an amalgamation structure have been widely studied and numerous inorganic compounds including active and passive types have been adopted as fillers for hybrid electrolytes. Fillers with different physical shapes and functions have also been researched, and significant progress has been made. Compared with simple polymer electrolytes, hybrid electrolytes exhibit a full spectrum of improvements, including ion conductivity and mechanical strength. However, challenges remain that needs to be resolved as follows: The mechanisms of ionic conductivity The identification of ionically conductive channels in hybrid electrolytes is meaningful and helpful in the design of optimal compositing strategies for electrolytes. To date, two separate theories have been postulated to explain ionic transportation in hybrid electrolytes in which one speculates that ion conduction within composite electrolytes mainly occurs at the ceramic-polymer interface [133, 163, 184], whereas the other disputes this [144]. Overall, the mechanisms of ion transport in hybrid electrolytes and the various factors influencing this transport remain unclear. However, the amount, shape and conducting capability of inorganic fillers may be contributing factors to ion transmission pathways. Therefore, effective characterization methods to intuitively catch Li + movements are needed to elucidate the mechanisms of ionic conductivity. The interactions of composite systems Because of the complexities of amalgamation systems, in-depth characterizations of the interactions between different components are hard to achieve, in which qualitative and even quantitative descriptions of the interactions between separate components are clearly lacking. Therefore, the understanding of the different interactions in each system can provide powerful theoretical guidance to novel and better hybrid electrolyte designs. 4.2 Polymer/Inorganic Layered Electrolytes Polymer/inorganic amalgamated electrolytes possess several key advantages; however, drawbacks remain such as Li dendrite growth which can grow and pierce along the soft part of the polymer matrix. To resolve this, heterogeneous structures such as layer-stacking structures have been investigated in the fabrication of hybrid electrolytes. Here, the layer-stacking structures may have two or multi components, with each component being in an independent continuous phase or two different components being tightly bounded and even interpenetrated with each other. In these layer-stacking structures, different components will have different functions and the synergism of each component can contribute to a high-performance composite electrolyte.

18 130 Electrochemical Energy Reviews (2018) 1: Inorganic ceramic electrolytes [e.g., garnet-type Li 7 LaZrO 12, perovskite-type Li 3x La 2/3-x TiO 3 and NASI- CON type Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 ] possess superior mechanical strength and demonstrate acceptable ionic conductivities at room temperature. However, large interfacial resistances, poor stability versus Li metal anodes and uncontrolled dendrite growth along grain boundaries restrict their further application. Because of this, the combination of a polymer layered electrolyte with a heterogeneous structure, being soft, stable and wets well with anodes, may be an effective method to overcome these drawbacks. For example, Li et al. [38] fabricated an all-solid-state Li/polymer/LLZT- 2LiF/LiFePO 4 battery using a Li + conducting electrolyte (PEO LiTFSI) as a buffer layer (Fig. 12a). This heterogeneous structure possessed low interfacial resistances and contributed to great improvements in electrochemical performances. Fu et al. [185] proposed a 3D-bilayer garnet SSE that included a dense layer (20 μm, Fig. 12b) that can act as a rigid barrier to prevent the penetration of Li dendrites as well as a porous layer that can support the thin dense layer and encapsulate cathode materials (Fig. 12c). A polymer coating layer was also adopted to compensate for interfacial roughness and to enable homogeneous Li + flux through the interface (Fig. 12d, e). This resulting symmetric layer by layer structure can solve issues simultaneously, whereas bi-layered electrolytes can only handle problems on one side. Tu et al. [186] developed a sandwich-type composite structure electrolyte (contains liquid electrolyte in working conditions, Fig. 12f) by laminating a high pore density nano-porous γ-al 2 O 3 sheet between macro-porous PVDF- HFP polymer layers for Li metal batteries and obtained satisfactory results. Similarly, Zhou et al. [187] proposed the concept of a polymer/ceramic/polymer all-solid composite electrolyte in which the cross-linked Li + polymer conductor was on the outside and the ceramic layer was sandwiched in between (Fig. 12g). In this proposed concept, the Fig. 12 a Schematic of a Li/Polymer/LLZT/LiFePO 4 battery. Reprinted with permission from Ref [38], Wiley VCH 2017.b Magnified SEM image of the dense grain microstructure, which can block soluble active materials and suppress Li dendrite penetration. c Cross section of the bilayer garnet structure in which two distinct layers with porous and dense garnet structures can be clearly observed. d Schematic of the bare dense garnet layer surface and polymer coated dense garnet layer surface. e Cross-sectional SEM image of the polymer coated dense garnet. Reprinted with permission from Ref [185], The Royal Society of Chemistry f Schematic of the structure of the PVDF-HFP/Al 2 O 3 separator. Reprinted with permission from Ref [186], Wiley VCH g Illustration of an all-solid-state battery design using a PCPSE electrolyte. h Illustration of the electric potential profile across the sandwich electrolyte. Reprinted with permission from Ref [187], American Chemical Society 2016

19 Electrochemical Energy Reviews (2018) 1: multifunctional polymer layer can suppress dendrite nucleation because of the uniform Li + flux and improve the wetting ability toward the electrodes, whereas the ceramic layer can block anion transport, reducing the double-layer electric field (Fig. 12h) at the Li/polymer interface and lowering the chemical/electrochemical decomposition of the polymer electrolyte with improved Coulombic efficiency. In practical testing, the proposed all-solid-state LiFePO 4 /Li cell was constructed and delivered superior long-term electrochemical stability and notably high Coulombic efficiencies up to %. More recently, Bucur et al. [188] proposed multilayer inorganic organic hybrid membranes to encapsulate sulfur particles for Li metal anode and achieved desirable outcomes. In battery systems using solid electrolytes, different sides of the solid electrolyte have different requirements. For example, on the Li anode side, a high modulus is required to block Li dendrite penetration, whereas on the cathode side, a low interface resistance is required. In relation to this, SPEs possess shape versatility and flexibility and provide low cathode/electrolyte interfacial resistances but suffer from dendrite penetration (Fig. 13a, b), whereas inorganic ceramic electrolytes (ICEs) possess a highly resistive interface (Fig. 13c). Inspired by this, a thin asymmetric hybrid 131 solid electrolyte (below 36 μm, Fig. 13d f) possessing two functionalized layers was proposed by Duan et al. [189]. On the cathode side, the electrolyte consists of a soft layer of the polymer electrolyte (5.4 μm, Fig. 13g) that provides sufficient contacts with the active materials, allowing for good interfacial connections and the facilitation of continuous ion transport. On the Li anode side, the electrolyte consists of a modified dense Li 7 La 3 Zr 2 O 12 polymer electrolyte layer (5.7 μm, Fig. 13h) that establishes a rigid barrier to prevent the penetration of Li dendrites. This ingenious arrangement of the electrolyte endows the solid battery with excellent electrochemical performances, with no apparent dendrites or defects being observed for the Li metal on the polymer side after Li plating/stripping for 3200 h, as well as an extremely high Coulombic efficiency (over 99.8% per cycle) being achieved in a LiFePO 4 /ASE/Li battery. Despite these promising results however, asymmetric electrolytes are often not compatible with batteries if both cathode and anode requirements are to be simultaneously met. Therefore, electrolyteanode contact and electrolyte-cathode contact are as required uniquely designed electrolytes. Based on the evidence presented in research, layer stacking is a viable new strategy to synergistically combine inorganic and polymer electrolytes and is equivalent to a series Fig. 13 Schematic diagrams of solid Li metal batteries with (a and b) SPEs, c ICEs and d ASEs as the electrolyte. Cathode/electrolyte is designated as C/E. e Cross-sectional SEM and f EPMA-EDS images of an assembled battery with ASE. g Cross-sectional SEM images of the polymer layer (5.4 μm) and h the LLZO-coating layer (5.7 μm). Reprinted with permission from Ref [189], American Chemical Society 2018

20 132 Electrochemical Energy Reviews (2018) 1: of electrolytes in space. Here, ionic conductivities depend largely on the development of simple inorganic and polymer electrolytes. And because series connections produce more interfaces than simple electrolytes, better interface construction is required to reduce interface resistances. In addition, the thickness of composite electrolytes greatly affects the energy density of battery systems and therefore, the control of hybrid electrolyte thickness to acceptable levels is essential to the practical application of layer-stacking structured electrolytes. Furthermore, the asymmetric electrolyte strategy provides additional design avenues for layer-stacking systems in which the fabrication of compatible asymmetrically structured hybrid electrolytes with suitable materials will lead to better performing batteries. Various types of amalgamated and layered polymer/inorganic composite electrolytes have been designed and studied, and the majority has proven to be effective in creating better electrolytes. In the case of polymer/inorganic composite electrolytes with amalgamated structures, inorganic fillers are uniformly dispersed in a polymer matrix and every inorganic nano- or macro-sized particle has contact with the polymer, leading to strong interactions between the inorganic component and the polymer, influencing properties such as ionic conductivity and mechanical strength. Currently, research on composite electrolytes with amalgamated structures mainly focuses on the regulation and control of the distribution, type and shape of the inorganic component as well as the inner interactions involving them. However, from a larger perspective, amalgamated structured electrolytes are homogeneous and symmetric, and may be difficult to meet the specific and different requirements of both the cathode and anode simultaneously. As for polymer/inorganic layered electrolytes, the interactions between the polymer and the inorganic component are limited (only on the boundary), and the synergistic effects of the layered structure can only be achieved spatially. In addition, the ionic conductivity of layered electrolytes tends to be smaller than those of single component electrolytes and increased interfaces may cause larger interface resistances. Nevertheless, layered electrolytes possess asymmetric structures which can enrich design strategies and have more potential in practical applications of full batteries. The development of desirable electrolytes is established based on the balance between battery requirements and component properties. The properties of different types of electrolytes are summarized in Fig. 14 in which single electrolyte systems are shown to always present noticeable weaknesses that limit application in high-energy density batteries. For example, liquid organic electrolytes suffer from poor safety and endless side effects, whereas solid polymer electrolytes have low room-temperature ionic conductivity and solid inorganic electrolytes are hard to process and possess large interface resistances. However, significant performance improvements can be seen in fabricated composite Fig. 14 The average performance profiles of different electrolyte materials. Radar plots of the performance properties of a liquid organic electrolytes, b solid polymer electrolytes, c solid inorganic electrolytes; d polymer/liquid composites, e polymer/polymer composites and f polymer/inorganic composite. Here, the more extended the plot is, the better the performance is

21 Electrochemical Energy Reviews (2018) 1: electrolytes, which synergistically improve the weaknesses observed in single electrolyte systems. From this, it can be seen that the fabrication of composite electrolytes is a viable method to drastically improve battery performance and achieve better electrolytes. 5 Summary and Outlook In this review, recent progresses of different types of polymer-based composite electrolytes including liquid/polymer hybrid electrolytes, polymer/polymer coordinating electrolytes and polymer/inorganic composite electrolytes have been comprehensively reviewed in which the composition of various components is an effective method to improve the comprehensive performance of polymer-based electrolytes. The reasonable design of composite electrolytes can also make up for the various shortfalls of its individual components, and the concept of composite electrolytes can extend the boundaries of electrolyte design and enrich the contents of solid-state chemistry, making it a promising candidate for all-solid batteries. Although many breakthroughs have been attained in solid-state electrolytes, there are still challenges that require attention in the research of composite electrolytes for high-energy density Li batteries: Finding better combinations of components Although numerous component combinations have been explored in recent years and great progress has been made, there is still much room for improvement with many materials still untested. An ideal electrolyte may include more than just two or three components, or even a mailing combination of more components. Developing advanced characterization methods Compared with liquid electrolyte systems, the characterization for solid batteries is still underdeveloped with characterization methods being usually borrowed from liquid systems. However, these characterization methods are not perfect because there are wide variations between liquid and solid systems. Therefore, it is important to explore in situ or ex situ characterization methods which are specialized for solid battery systems. Developing advanced processing technologies Although in situ polymerization using liquid monomers or precursor solutions with low viscosity has been developed, controllable polymerization with guaranteed product consistency is still challenging. Moreover, a battery system with high energy density requires thin solid electrolytes, so the balance between thickness and function requires advanced processing technologies. Understanding ionic conductivity mechanisms The identification and understanding of ionic conductivity mechanisms for composite electrolytes is of scientific significance and can better guide the fabrication of composite electrolytes. 133 Unfortunately, these mechanisms are still under debate in the scientific community. Therefore, continuous efforts should be made to discern the ionically conductive channels in composite electrolytes with convincing evidence and support. Understanding the interactions in composite electrolytes The interactions between different electrolyte components remain indefinable because of the complexity of amalgamation systems. The elucidation of the various interactions in each system can provide powerful theoretical directions for the design of novel hybrid electrolytes as well as the development of productive experimental and advanced characterization methods. Overall, great progresses have been achieved, but great challenges remain in the practical application of composite electrolytes in commercial rechargeable Li batteries. And with further research efforts and the information provided in this review, solid electrolytes and solid-state batteries will eventually become optimized to meet growing energy demands. Acknowledgements This work was supported by the Basic Science Center Project of National Natural Science Foundation of China under grant No , the National Key R&D Program of China (Gant 2016YFA ), the National Natural Science Foundation of China ( , ), Beijing Natural Science Foundation (L172023), and the Transformational Technologies for Clean Energy and Demonstration, Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA ). References 1. Tarascon, J.M., Armand, M.: Issues and challenges facing rechargeable lithium batteries. Nature 414, (2001) 2. 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