Unique Structural Design and Strategies for Germanium-Based Anode Materials Toward Enhanced

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1 PROGRESS REPORT Lithium Storage Unique Structural Design and Strategies for Germanium-Based Anode Materials Toward Enhanced Lithium Storage Dan Li, Hongqiang Wang, Tengfei Zhou, Wenchao Zhang, Hua Kun Liu, and Zaiping Guo* Germanium-based materials are arousing increasing interest as anodes for lithium-ion batteries, stemming from the intrinsic physical and chemical advantages of germanium. This progress report provides a brief review on the current development of germanium-based materials in lithium storage. The state-of-the-art strategies to achieve enhanced electrochemical properties are highlighted, with their main aim being to resolve the trickiest issue: vast volume changes in germanium during cycling. These strategies include structural modification, modification by surface coating, forming germaniumbased alloys, and forming binary or ternary germanium-based composites. The recent work on a novel composite of germanium and tin particles encapsulated in double-concentric carbon hollow spheres is also presented here, with an emphasis on the relationship between structural design and improved performance. 1. Introduction Triggered by increasing and urgent demands for electrical portable devices and hybrid electric vehicles, tremendous efforts had been devoted to research on energy storage systems with high energy and power density. [1] Compared with the previous commercial batteries, such as lead-acid, metal hydride, and alkaline batteries, lithium-ion batteries have been considered the most suitable energy storage system for production and suitability for daily life. As in the use of any technology, lithium-ion batteries have some disadvantages, including relatively high cost, Dr. D. Li College of Chemistry and Molecular Engineering Zhengzhou University Zhengzhou, Henan Province , P. R. China Dr. D. Li, Dr. H. Wang, Dr. T. Zhou, W. Zhang, Prof. H. K. Liu, Prof. Z. Guo Institute for Superconducting and Electronic Materials University of Wollongong NSW 2500, Australia zguo@uow.edu.au Prof. Z. Guo School of Mechanical Materials and Mechatronics Engineering University of Wollongong NSW 2500, Australia DOI: /aenm suffering from ageing, needing protection from being over charged/discharged, and needing their current to be kept within safe limits. Nevertheless, graphite, the conventional anode material, cannot satisfy the continuously surging demands on the lithium-ion battery industry for high power and energy densities due to its low theoretical specific capacity of 372 ma h g 1 (LiC 6 lithiated state). [2,3] The elements in Group IV, silicon, [4] tin, [5] and germanium, [6 8] are very promising candidates as high-performance anode materials for lithium-ion batteries due to their high theoretical capacity compared with carbon. [9] Among the elements in Group IV, silicon has the highest theoretical specific capacity of 4200 ma h g 1 (for the Li 4.4 Si state) and is low cost and environmentally friendly, which has attracted great attention in the anode material field for lithium-ion batteries. [4,10] The obstacles to its application stem from its low electrical conductivity and unstable solid electrolyte interphase (SEI) layer, as well as its devastating volume changes. Germanium has been demonstrated to possess high electrical conductivity (100 times higher than silicon), [6,11] superior lithium-ion diffusion (400 times faster than in silicon), [12] and a relatively high theoretical specific capacity of 1600 ma h g 1, although the huge volume expansion (about 300% for the Li 4.4 Ge lithiated state) after lithiation is also a thorny issue for its use as an anode material. The root causes for the unsatisfactory electrochemical performance of germanium-based anode materials can be summarized as (i) devastating huge volume changes during lithiation/delithiation processes, leading to cracking and pulverization of the electrode materials, and (ii) aggregation of the germanium particles in cycling progresses, leading to increased resistance and longer diffusion paths for lithium-ions. The massive volume change could have a serious impact on the cycling performance. The repeated volume changes generate mechanical strain in the lithiation/delithiation processes, which results in gradual deterioration of the electrode and thus capacity decay during cycling. In addition, the large volume changes could destroy the particle contact in the active materials. The weak contact leads to the peeling off of active particles and thus rapid loss of capacity. The aggregation/agglomeration of germanium particles will extend the diffusion paths for lithium ions and degrade their mobility, (1 of 16)

2 so that the cores of the aggregated germanium particles are insufficiently involved in electrochemical reactions. This interior sluggish kinetics and increased transmission distance will ultimately result in poor cyclability. To alleviate or eliminate the adverse influence of these problems, some strategies have been applied to optimize the electrochemical properties, including structural modification, [11,13 20] modification by surface coating, [21 24] forming germaniumbased alloys, [25 27] and forming binary [28 33] or ternary [34 36] germanium-based composites. Based on our previous studies and experience, we found that the complete and effectively surface coating on nanostructures with high structural robustness is vital to achieving good electrochemical performance in terms of high specific capacity, superior long-term cyclability, and good rate capability. Therefore, in this progress report, we provide a brief review on these modification strategies to obtain good electrochemical properties, with an emphasis on the surface coating aspect. Importantly, we include our recent progress on the coatings to produce germanium nanostructures with improved properties. 2. Structural Design and Strategies 2.1. Structural Modification Nanosized Structures Due to the vast volume changes characteristic of germanium, bulk germanium-based materials usually present inferior electrochemical performance, especially cycling performance, resulting from the cracking and pulverization of the electrode materials. Furthermore, the bulk-form germanium materials have limited active area, which leads to sluggish lithium-ion diffusion. In contrast, nanosized structures have some intrinsic advantages, including shortened diffusion distances for lithium ions, more active sites due to high specific surface area, and good contact between electrode materials and electrolytes. Therefore, tremendous efforts have been devoted into fabricating nanostructured materials, such as nanoparticles, [17,37 39] nanowires, [13 15,40] nanotubes, [16,41 43] nanofibers, [44,45] and nanorods. [18,46] Notably, nanostructured modifications are always accompanied by surface coating or embedding/encapsulating in a matrix to avoid the agglomeration of particles due to the high active energy for nanostructures. Cho et al. reported a simple gas phase laser photolysis of tetramethyl germanium followed by an annealing process for preparing germanium nanoparticles and germaniumbased composites nanoparticles (GeS and GeO 2 ), as shown in Figure 1a. [39] The obtained germanium nanoparticles exhibited excellent cyclability with a stable capacity of 1100 ma h g 1 after 100 cycles at a current density of 160 ma g 1. Their X-ray diffraction (XRD) patterns and high-resolution transmission electron microscope (HRTEM) images confirmed that there was a phase transformation from cubic to tetragonal in the obtained samples because the tetragonal phase was more thermodynamically stable than the cubic phase, based on first-principles calculations. In another study, a facile synthetic route was applied by Ngo et al. to prepare germanium nanoparticles interconnected Dan Li received her Ph.D. degree in 2015 from the Institute for Superconducting and Electronic Materials (ISEM) at the University of Wollongong (Australia). Since 2015, she has been a lecturer in the College of Chemistry and Molecular Engineering at Zhengzhou University (China). Her research interests focus on materials design and synthesis for high-electrochemical-performance lithium- and sodium-ion batteries. Hongqiang Wang received his Ph.D. degree in 2016 from the ISEM at the University of Wollongong. Since 2017, he has been a lecturer in the College of Chemistry and Environmental Science at Hebei University. His research topics focus on smart design and synthesis of nanostructured sulfur cathodes for high-performance lithium sulfur batteries. Zaiping Guo obtained her B.S. degree in 1993 from the Department of Chemistry at Xinjiang University, China, and her Ph.D. in 2003 from the University of Wollongong, Australia. After her postdoctoral research with Prof. Huakun Liu at the Institute for Superconducting and Electronic Materials, University of Wollongong, she joined the Faculty of Engineering in She is now a full professor in the Faculty of Engineering and Information Sciences. Her research interests include synthesis and applications of nanomaterials for energy storage and conversion. by carbon via thermal decomposition followed by calcination treatments. A schematic illustration is shown in Figure 1b. [38] The germanium nanoparticles were buffered by the carbon layers against aggregation by high open porosity, delivering a high specific capacity of 1232 ma h g 1 after long-term cycling for 1000 cycles at a rate of 0.5 C (800 ma g 1 ). In terms of 1D structural fabrication, Liu et al. proposed carbon-coated (2 of 16)

3 Figure 1. Nanoparticle and nanowire structures designs: a) Cycling performance and high-resolution transmission electron microscope (HRTEM) images of the obtained germanium nanoparticles before (cubic phase) and after cycling (tetragonal). Reproduced with permission. [39] Copyright 2013, American Chemical Society. b) Schematic illustration of the two-step synthetic route and TEM images of germanium nanoparticles interconnected by carbon that were produced via thermal decomposition and calcination treatments. Reproduced with permission. [38] Copyright 2014, John Wiley and Sons. c) Schematic illustration of Ge/C nanowires produced from organic-inorganic hybrid GeO x /EDA nanowires via a self-assembly process, and transmission electron microscope (TEM) images of the final sample. Reproduced with permission. [13] Copyright 2014, American Chemical Society. d) Schematic illustration of carbon germanium nanowires carbon nanofibers synthesized via an in situ vapor liquid solid process, and scanning electron microscope (SEM) and TEM images of the final sample. Reproduced with permission. [8] Copyright 2015, John Wiley and Sons. germanium nanowires prepared from organic inorganic hybrid GeO x /ethylenediamine (EDA) nanowires via a self-assembly process (Figure 1c). [13] The as-synthesized Ge/C presented superior electrochemical performance, including high capacity retention under cycling and good rate capability ( 770 ma h g 1 after 500 cycles at the rate of 10 C), which stemmed from the synergistic effects of the carbon coating and well-defined 0D in 1D structure. Li et al. also synthesized uniform carbon-coated germanium nanowires grown on carbon nanofibers (CNFs) via an in situ vapor liquid solid process (Figure 1d). [8] Compared with the reference sample (germanium on CNFs), the obtained sample had increased electroactive interface, high electronic conductivity, and robust mechanical flexibility, and therefore exhibited a remarkable cycling performance with an approximate capacity of 1520 ma h g 1 after 100 cycles at the rate of 0.1 C with only 0.05% capacity fading per cycle. In addition to the fabrication of the nanoparticle and nanowire structures discussed above, nanotube, nanofiber, and nanorod structured germanium anode materials are also interest to researchers for study and investigation. Liu et al. prepared a germanium nanotube array anode through templateassisted (Au-sputtered polycarbonate membrane) electrodeposition from an air and water stable ionic liquid (Figure 2a). [42] The scanning electron microscope (SEM) and transmission electron microscope (TEM) images showed that the obtained germanium nanotubes were about 1.7 µm in length and 250 nm in diameter. The anode material featured remarkable capacity retention, with of 98% retained (with respect to the capacity in the 50th cycle) after 250 cycles. Moreover, Park et al. applied a unique synthetic method based on the Kirkendall effect, coating antimony precursor on germanium nanowires, which was followed by an annealing treatment, to obtain germanium (3 of 16)

4 Figure 2. Nanotube, nanofiber, and nanorod structural designs: a) Schematic illustration of germanium nanotube array synthesis via template-assisted deposition, and SEM and TEM images of the sample. Reproduced with permission. [42] Copyright 2015, Royal Society of Chemistry. b) Schematic illustration of the synthetic route for germanium nanotubes utilizing the Kirkendall effect and the TEM images corresponding to each synthetic step (scale bar: 100 nm). Reproduced with permission. [16] Copyright 2011, John Wiley and Sons. c) Schematic illustration of synthesis of carbon-coated via chemical vapor deposition and electrospinning techniques and TEM images of the sample. Reproduced with permission. [44] Copyright 2014, Elsevier. d) Schematic illustration of synthesis of Si/Ge nanorod array using nanosphere lithography, inductively coupled plasma dry etching, and ultrahigh-vacuum chemical vapor deposition techniques, and TEM images of the sample. Reproduced with permission. [46] Copyright 2013, Royal Society of Chemistry. nanotubes (Figure 2b). [16] The as-prepared material exhibited a high charge capacity of 1002 ma h g 1 at the 50th cycle at a current rate of 0.2 C with a coulombic efficiency of over 99%. Carbon-coated germanium@cnfs material was synthesized via chemical vapor deposition (CVD), electrospinning, and a following thermal carbonization, as reported by Li et al. (shown in Figure 2c). [44] In order to illustrate its structural merits, germanium nanoparticles@cnfs and CNFs-coated germanium@c composites were also prepared as the reference samples. The electrochemical test results showed that the carbon-coated germanium@cnfs presented the highest cyclability of 89% after 50 cycles, which was attributed to the additional carbon confinement and robust thorn-like structure. Yue et al. synthesized 3D hexagonal bottle-like Si/Ge nanorod arrays on silicon substrates by nanosphere lithography, inductively coupled plasma dry etching, and ultrahigh-vacuum chemical vapor deposition (Figure 3d). [46] The unique structure endowed the material with improved lithium-ion kinetics and space to accommodate the volume expansion after lithiation. In summary, compared with the 0D nanostructure (germanium nanoparticles), 1D germanium nanostructures, including nanowires, nanotubes, nanofibers, and nanorods, have attracted more attention due to their good strain relaxation ability, short diffusion paths for lithium ions, large interfacial contact, and facile electron transport along the longitudinal direction. Various synthesis techniques, such as vapor liquid solid, [8] pyrolysis, [13] chemical vapor deposition, [41] and solution liquid solid, [14,47] have been focused on producing 1D germanium nanostructures. As discussed above, however, the fabrication of pristine germanium nanostructures is insufficient to improve their electrochemical performance. Carbon matrices are always introduced into composites with germanium nanostructures as volume buffers to modify their cyclability and rate capability Porous/Mesoporous Structures Fabricating porous/mesoporous structures is considered to be common strategy in structural design to improve the electrochemical properties of germanium-based materials because the pores in the active materials and/or matrix can provide sufficient space or voids to accommodate the volume expansion after lithiation and more diffusion paths for lithium ion and electrons, as well as increased contact area and thorough soaking of the electrode materials by the electrolyte. [11,19,20,48 51] Recently, hollow mesoporous germanium nanoparticles were successfully synthesized via a unique redox-transmetalation reaction from hollow GeO 2 particles, which were reacted with zinc vapor. After the reaction, the by-product ZnO was removed by hydrochloric acid (HCl), and the final mesoporous germanium nanoparticles were obtained (Figure 3a). [48] The as-synthesized materials exhibited good long-term cyclability, with 99.5% (4 of 16)

5 Figure 3. Porous/mesoporous structural designs: a) Schematic illustration of mesoporous germanium synthesized through a redox-transmetalation treatment, and TEM image of a single germanium particle with mesopores. The inset is a magnified image showing the crystalline structure. Reproduced with permission. [48] Copyright 2015, American Chemical Society. b) Schematic illustration of porous germanium obtained via magnesiothermic reduction from GeO 2, and SEM images of the sample. Reproduced with permission. [49] Copyright 2014, American Chemical Society. c) Schematic illustration and TEM images of germanium inverse opals with porous walls. Reproduced with permission. [19] Copyright 2012, Royal Society of Chemistry. d) Schematic synthesis route for mesoporous Ge@C spheres via a chelation reaction and the extended Stöber method. Reproduced with permission. [50] Copyright 2014, Royal Society of Chemistry. capacity retention after 300 cycles at 0.5 C. Jia et al. employed a simple magnesiothermic reduction treatment to synthesize 3D porous germanium particles with a hexagonal-like morphology (Figure 3b). [49] The SEM images showed uniformly distributed nanosized pores after the removal of MgO/MgGe 2 by HCl. The obtained materials presented a high rate capability of 717 ma h g 1 at 5 C and a good cycling stability, with a capacity of 1131 ma h g 1 at the 200th cycle at 1 C. In another study, a germanium opal with porous walls was synthesized by Song et al. using a CVD technique to deposit germanium on silica opal (Figure 3c). [19] Compared with germanium inverse opal with dense walls, the germanium inverse opal with porous wall showed 1.5 times higher surface area, meaning a larger flux of lithium ion between the active material and the electrolyte, and facile internal stress release. The porous walled germanium inverse opal exhibited good rate capability and cycling performance, showing a capacity retention of 88.9% after 100 cycles. Liu et al. synthesized novel mesoporous Ge@C spheres based on the chelation reaction of resorcinol-formaldehyde and an extended Stöber method followed by thermal carbonization treatment (Figure 3d). [50] The obtained Ge@C spheres were uniform in size ( 500 nm) with 14 nm sized mesopores, and they had a high specific surface area of 348 m 2 g 1. When tested as an anode material, the mesoporous Ge@C spheres delivered a specific capacity of 1099 ma h g 1 at the 100 th cycle at 0.1 C with a coulombic efficiency of 99% Forming Alloys It has been demonstrated that alloying germanium with other elements in Group IV, especially silicon and tin, can combine the advantages inherent in both components of the alloys to achieve high electrochemical performance due to their similar physical and chemistry properties. Moreover, the alloy components are all electrochemically active, which can reduce the irreversible capacity and avoid overall capacity loss. [52,53] Alloying is also believed to lead to materials with higher working potential and smaller volume change. [53,54] Therefore, many efforts have been devoted to fabricating Ge/Si and Ge/Sn alloys and investigating their electrochemical performance. [6,18,25 27,53,55 57] Kim et al. proposed a facile thermal annealing treatment in a hydrogen environment to prepare Ge/Si nanowire alloy by manipulating the atomic arrangement, which enabled fine tuning of the lithium-ion diffusion and thereby fine control of overpotential (Figure 4a). The remaining unlithiated alloy served as a support matrix to maintain the structural integrity, leading to a high capacity retention of 89% after 400 cycles at a current (5 of 16)

6 Figure 4. Strategies for forming alloys: a) Schematic illustration of the transformation from uniform SiGe nanowire alloy to germanium-rich SiGe nanowire alloy, and their corresponding TEM images. Reproduced with permission. [27] Copyright 2015, American Chemical Society. b) Schematic illustration of 3D nanoporous SiGe alloy synthesis and SEM images of the obtained materials. Reproduced with permission. [56] Copyright 2013, Elsevier. c) Schematic illustration of SiGe nanotube synthesis via a one-pot electrolytic treatment utilizing the Kirkendall effect, and SEM and TEM images of the obtained material. Reproduced with permission. [26] Copyright 2016, John Wiley and Sons. d) Comparison of rate capability of the various Si 1 x Ge x alloys. Reproduced with permission. [55] Copyright 2013, American Chemical Society. e) Schematic illustration of Sn Ge alloy nanorods synthesized via a colloidal process, and TEM images of the obtained alloy. Reproduced with permission. [18] Copyright 2014, American Chemical Society. f) Schematic representation of the formation of Sn 78 Ge alloy nanowires from Sn 78 Ge 22 clusters, and TEM images of the obtained alloy. Reproduced with permission. [6] Copyright 2007, American Chemical Society. rate of 0.2 C. In another study, 3D porous SiGe alloy nanostructures were synthesized via a template-assisted (opal-like silica) method, and the synthetic route is shown in Figure 4b. The 3D porous structure endows the alloy with good electrical contacts and open channels for lithium-ion diffusion, as well as good internal stress accommodation. Therefore, the resulting material delivered a high capacity of 1047 ma h g 1 at a current rate of 16 A g 1, showing a relatively good rate capability. Recently, Xiao et al. reported a novel Si/Ge alloy nanotube anode material obtained through electroreduction of an SiO 2 and GeO 2 mixture in molten chloride, utilizing the Kirkendall effect (Figure 4c). [26] With the advantage of the resultant hollow structure to absorb the volume expansion after lithiation, the obtained alloy showed enhanced electrochemical performance compared with the reference samples of bare germanium and silicon. The glancing angle deposition (GLAD) technique was applied by Abel et al. to synthesize a series of Si 1 x Ge x alloys, and their electrochemical properties were investigated (Figure 4d). [55] A trade-off between the capacity and rate performance was demonstrated by adjusting the ratio of silicon to germanium in the alloy. In addition to Si Ge alloys, Sn Ge alloy nanorod material was studied by Bodnarchuk et al., who prepared it via a facile colloidal (6 of 16)

7 Figure 5. Structural designs for binary and ternary germanium-based compounds: a) Schematic illustration of the synthesis of GeO 2 /C and GeO 2 / Ge/C from GeO 2, and TEM images of the obtained GeO 2 /Ge/C. Reproduced with permission. [31] Copyright 2013, American Chemical Society. b) TEM images, cycling performance, and rate capability of GeS and GeS 2 samples. Reproduced with permission. [32] Copyright 2013, Royal Society of Chemistry. c) Schematic illustration of crystalline Zn 2 GeO 4 /GO nanocomposite synthesis via the ion-exchange reaction, and SEM and TEM images of the obtained material. Reproduced with permission. [58] Copyright 2013, Royal Society of Chemistry. d) Schematic illustration of the structural evolution of urchin-like Ca 2 Ge 7 O 16 hollow spheres, and SEM and TEM images of the obtained material. Reproduced with permission. [36] Copyright 2015, Nature Publishing Group. synthesis based on the solution liquid solid growth mechanism (Figure 4e). [18] TEM images confirmed that the alloy nanorods were no more than 50 nm in length with an aspect ratio of Under the synergistic effects from combining tin and germanium, enhanced electrochemical performances were achieved with a high specific capacity of over 1000 ma h g 1 at a current rate of 1 A g 1. Lee and Cho also fabricated Sn 78 Ge alloy nanowires from butyl-capped Sn 78 Ge 22 clusters through a thermal annealing treatment (Figure 4f). [6] The obtained alloy showed high capacity retention of 94%, which can be attributed to the reduced activation energy barrier and maximum hoop strain. In summary, compared with Sn Ge alloys, Si Ge alloys are far more practical attractive because of the combination advantages of high capacity from silicon and good electrical conductivity from germanium. Importantly, forming Si Ge alloys could dilute the price of germanium without sacrifice its electrochemical properties. For the further development of Si Ge alloys, more efforts should be put into the optimal composition of silicon and germanium to realize making the most of their merits Forming Binary and Ternary Germanium Compounds In order to improve the cyclability of germanium, the fabrication of binary or ternary germanium compounds is an alternative method to restrict the volume changes of germanium during lithium-ion insertion/extraction processes. The binary germanium compounds form lithiated compounds in situ in the initial lithiation process, which can serve as a volume buffer for germanium particles. [29 31,59,60] In terms of ternary germanium compounds, similarly, the lithiated compounds formed in situ and metal oxides act as volume buffers as well as separators to prevent the aggregation of germanium particles. A brief summary of binary and ternary germanium compounds is given below from the perspective of structural design improve the electrochemical properties. GeO 2 /Ge/C was prepared from GeO 2 precursor via a simple thermal annealing in acetylene gas, in which carbon coating and partial GeO 2 reduction to germanium can take place (Figure 5a). [31] The obtained germanium nanoparticles interconnected with GeO 2 particles under the uniform covering of the carbon layer form clusters over 30 µm in size. The composite exhibited a high capacity of over 1680 ma h g 1 at a current rate of 10 C, which was attributed to the catalytic effect of the germanium in the composite. In addition, nanostructured GeO 2 (synthesized by a facile precursor pyrolyzation) demonstrated a high specific capacity of 1340 ma h g 1 at a current density of 0.1 A g 1 after 50 cycles. [29] Another interesting core shell type of GeO 2 dispersed in nitrogen-doped porous carbon was constructed by the sol gel method followed by an annealing treatment. [30] This GeO 2 /N-C presented superior capacity retention of 91.8% after 200 repeated cycles at a current rate of 0.5 C (7 of 16)

8 In addition to GeO 2, germanium sulfides also drew the attention of researchers for investigation of their electrochemical properties. Cho et al. synthesized a series of germanium sulfides (amorphous GeS and GeS 2, crystalline GeS and GeS 2 ) via gas-phase laser photolysis followed by thermal calcination (Figure 5b). [32] The crystalline GeS and GeS 2 showed better cycling performance (1010 ma h g 1 for GeS and 740 ma h g 1 for GeS 2 after 100 repeated cycles) than amorphous counterparts. In another study, GeS 2 glass was chosen for study due to its air stability. [33] Compared with germanium metal and GeO 2, GeS 2 glass exhibited better cycling performance, which was attributed to the greater amount Li Ge alloy phases formed in the lithiation process compared to GeO 2 glass. Moreover, substoichiometric germanium sulfide thin films were prepared by the GLAD technique. [61] The obtained amorphous and homogeneously deposited germanium sulfide thin films exhibited enhanced rate capability and capacity retention. Ternary germanium-based compounds, such as CuGeO 3, [62,63] Co 2 GeO 4, [64] SrGe 4 O 9, [65] BaGe 4 O 9, [65] Zn 2 GeO 4, [35,58] and Ca 2 Ge 7 O 16, [34,36] have been studied for application as anode materials for lithium-ion batteries. Zn 2 GeO 4 has drawn increasing attentions due to its low cost (only 27% mass percentage of germanium in the compound). [35] A sandwich type Zn 2 GeO 4 graphene oxide (GO) composite was fabricated via an ion-exchange reaction, and an illustration of the synthesis is shown in Figure 5c. [58] Due to the buffer effect of GO, the obtained composite delivered a high specific capacity of 1150 ma h g 1 at a relatively high current density of 200 ma g 1. It is believed that the formation in situ of the metal oxide in the initial lithiation process with negligible volume change favors the structural stability of the overall composite. Ca 2 Ge 7 O 16 is expected to afford stable cycling performance due to the nearly zero strain on the CaO during lithium insertion and extraction processes. A unique urchin-like Ca 2 Ge 7 O 16 hierarchical hollow microsphere material was developed through a solvothermal reaction using hexadecyl trimethyl ammonium bromide (CTAB) as surfactant. With the benefits of the urchin-like hollow structure, including sufficient space for accommodation the volume changes, shortened lithium-ion diffusion distances, and increased contact area between the active material and the electrolyte, the obtained Ca 2 Ge 7 O 16 showed superior rate capability (341.3 ma h g 1 at a current density of 4000 ma g 1 ) and cyclability (over ma h g 1 at a current density of 100 ma g 1 over 100 cycles). In summary, although lithiated compounds formed in situ can play the role of a volume buffer, binary germanium compounds still suffer from low electrical conductivity and large volume change during the charging/discharging processes. Therefore, modification by introducing a conductive matrix is always involved in the synthesis to improve the electrochemical performance. In contrast, pristine ternary germanium materials present good cyclability without other modifications. Furthermore, large-scale synthesis can be realized for germanates due to their low cost, abundance, and simple synthetic routes. These appealing features enable ternary germanium materials to be promising active materials in anodes for lithium-ion batteries Surface Coating Modification Based on our research (Figure 6), we believe that the fabrication of nanostructures alone is unable to guarantee the structural stability of germanium anode materials due to their strong tendency toward aggregation of nanoparticles, while effective and complete modification of nanostructures by surface coating could not only prevent the electrochemical aggregation of germanium nanoparticles in lithiation/delithiation processes, but also buffer the large volume changes of germanium and thus maintain the structural integrity of the anode material. We found an effective synthetic route for the fabrication of carbon-coated germanium materials with various morphologies through carbon coating GeO 2 precursor via chemical vapor deposition under acetylene gas and then thermal reduction to obtain Ge@C composites as the final products. Importantly, the thermal reduction introduces voids and space into the carbon shells, which can accommodate the volume expansion of germanium during lithiation processes. Self-assembled nanostructures of carbon-coated germanium clusters was prepared by hydrolysis of GeCl 4 (Figure 6a). [22] Compared with the reference sample of nonclustered carboncoated germanium, the target sample exhibited exceptionally high rate capability with capacity of 480 ma h g 1 at the high rate of 40 C. The carbon-coated cluster nanostructure was proposed to mitigate the internal stress of the composite by accommodating the volume changes by the voids and space that are formed in carbon layers. In another study (Figure 6b), carboncoated mesoporous Ge@C hollow structures were obtained from hollow ellipsoidal GeO 2 precursor (Figure 6b). [66] The hollow structure and pores (providing space for volume expansion) and uniform carbon coating (serving as a buffer matrix) ensured that the structure remained robust under repeated lithiation/delithiation. Therefore, the hollow Ge@C structures exhibited better high rate capability (over 800 ma h g 1 at 20 C) and superior capacity retention (really 100% after 200 cycles at 0.2 C) when they were compared with the solid ellipsoidal sample. Graphene is an optimal surface modification choice for germanium-based materials [28,45,58,60,63,64] due to its remarkable mechanical flexibility, electrical conductivity, and open channels for lithium-ion diffusion. It is difficult, however, to thoroughly and uniformly encapsulate germanium particles in graphene sheets during the synthesis. The germanium particles will form a loose aggregation after several charge/discharge processes, resulting in pulverization of the electrode material. Therefore, an additional carbon coating was applied on the GeO 2 /graphene precursor to provide double protection against cracking of the material in electrochemical reactions (Figure 6c). The obtained C/Ge/graphene nanocomposite delivered a high specific capacity of over ma h g 1 after 160 cycles and 746 ma h g 1 at 20 C. Similar nanostructures of germanium@ graphene@tio 2 core shell were fabricated by Wang et al. using the electrospinning technique followed by annealing. [67] The structural design was chosen from the consideration of robust capacity retention of TiO 2 under cycling. In the study shown in Figure 6d, germanium ethoxide was introduced into hollow carbon spheres, which was subsequently subjected to an annealing treatment to form (8 of 16)

9 Figure 6. Structural design with surface coating modification: a) Schematic illustration of self-assembled germanium/carbon nanostructures and TEM images of the nanostructures corresponding to each synthetic step. Reproduced with permission. [22] Copyright 2012, John Wiley and Sons. b) Schematic illustration of the formation of GeO 2 hollow ellipsoidal structures and TEM images of the sample synthesized with a 1:1 ratio of germanium to tin. Reproduced with permission. [66] Copyright 2013, Royal Society of Chemistry. c) Schematic illustration of the synthesis of sandwich-structured C/Ge/ graphene nanocomposite, and SEM and TEM images of the final sample. Reproduced with permission. [24] Copyright 2013, Royal Society of Chemistry. d) Schematic illustration of the synthesis of germanium encapsulated in hollow carbon spheres, and TEM images at each synthesis step. Reproduced with permission. [23] Copyright 2014, Royal Society of Chemistry. e) Schematic representation of the synthesis of germanium encapsulated in carbon nanoboxes, and SEM images at each synthesis step. Reproduced with permission. [21] Copyright 2015, John Wiley and Sons. germanium encapsulated in hollow carbon spheres. [23] The hollow carbon spheres were initially modified by CTAB, which formed pores after the annealing, enabling the successful introduction of germanium precursor. The as-prepared samples exhibited an excellent high rate capability of ma h g 1 at 20 C. Another interesting work involved encapsulating germanium nanoparticles in carbon nanoboxes through carbon coating a precursor of GeO 2 nanocubes and a subsequent reduction reaction. The synthetic route is presented in Figure 6e. [21] The robust carbon nanoboxes can increase the electrical conductivity of the composite and mitigate the internal mechanical stress induced by lithium-ion intercalation/deintercalation. Notably, the carbon nanobox encapsulated germanium offered a high tap density due to the densely stacked arrangement. Compared with the Ge@C bulk, the as-synthesized sample showed a stable cycling performance and better rate capability, with a high capacity of 497 ma h g 1 at 30 C and long-term cyclability with a specific capacity of ma h g 1 at the 500th cycle at 0.5 C. In brief, the enhanced electrochemical performance shown in the works discussed above can be attributed to the free space and voids formed after the release of oxygen (in the form of steam) from the GeO 2 precursor (reduced to germanium) during the annealing treatment, because these voids can accommodate the volume expansion during lithiation and thereby guarantee the structural integrity of the anode. Inspired by the merits of nanostructures and surface coating modification, we recently designed a novel double-carbonshell strategy for synthesizing a tin/germanium carbonaceous (9 of 16)

10 Figure 7. Schematic illustration of the synthesis procedure for a) b) and c) samples. For the the hollow SnO 2 spheres are covered successively by carbon (from acetylene), GeO 2, and carbon again to form SnO which is transformed to the final product Sn@C@Ge@C by hydrogen via a reduction reaction. composite to buffer the volume changes of tin and germanium. Specifically, as shown in the schematic illustration in Figure 7a, tin oxide (SnO 2 ) hollow nanospheres were first synthesized as the template for the following procedures. Acetylene/argon gas was chosen as the carbon source for the carbon shell coating on the surfaces of hollow SnO 2 spheres. After the carbon coating, the asobtained black powder, SnO was immersed in germanium ethoxide to coat the SnO with a germanium dioxide (GeO 2 ) layer (SnO 2 ). Finally, the outer carbon shell (generated by the acetylene/argon gas) was coated on the SnO GeO 2 via the CVD technique and then was reduced by hydrogen/ argon gas to obtain the final product, Sn@C@Ge@C. Powder XRD confirmed the phase changes in each synthesis step, as shown in Figure 8a. For the initial SnO nanospheres, all the peaks were well indexed to the tetragonal rutile phase of SnO 2 (Joint Committee on Powder Diffraction Standards (JCPDS) card no ). After hydrolysis of the germanium ethoxide coated on the surface of SnO the additional peaks that appeared were indexed to hexagonal phase GeO 2 (JCPDS card no ). Then, SnO 2 and GeO 2 were completely converted to metallic tin and germanium after the reduction treatment, corresponding to the tetragonal phase (JCPDS card no ) and diamond cubic phase (JCPDS card no ), respectively. In all the XRD patterns, no diffraction peak from carbon can be detected, which can be ascribed to the disordered nature of the carbon and overlapping with the high intensity (111) peak of germanium. The Raman spectra (shown in Figure 8b) further reveal the disordered nature of the carbon shells due to the relatively stronger intensity of the peak centred at 1333 cm 1 band compared to that of the peak at 1603 cm 1, corresponding to the disorder-induced D band and the graphitic G band, respectively. The peaks appearing at 174 cm 1 and 300 cm 1 can be related to the optical modes of metallic tin and germanium, respectively, which match well with previous reports in the literature. [68,69] Figure 8c shows a field-emission scanning electron microscope (FESEM) image of Sn@C@Ge@C with uniform sphere-shaped particles about 500 nm in size. The broken Sn@C@Ge@C sphere shown in the Figure 8c inset confirms its double-shell structure. To gain further insight into the double-shell structure of the obtained Sn@C@Ge@C composite, TEM was conducted, as shown in the images in Figure 8d,e. The TEM images revealed that the Sn@C yolks covered by a thin layer of germanium nanoparticles are homogeneously encapsulated by the outer carbon shells with a thickness of 35 nm. The elemental mapping images (Figure 8f h) give more specific details on the double-shell structure and the distribution of tin and germanium within the ball-in-ball carbon spheres. It can be observed that the tin yolks are about 300 nm in size, which matches the SEM images of SnO 2 precursor shown in Figure S1 in the Supporting Information. The shape of the tin particles is not typically round, meaning that there is volume shrinkage of the SnO 2 nanoparticles during the reduction process in hydrogen gas. Notably, the germanium nanoparticles are randomly distributed between the double carbon shells, demonstrating that the GeO 2 nanoparticles hydrolyzed from the germanium ethoxide coating the surface of the inner carbon shell are in a very loose or porous form. Therefore, after the reduction treatment, there are abundant void spaces generated between the double carbon shells, which can provide ample room to accommodate the volume changes during the lithiation/delithiation processes. It also worth noting that a small amount of tin nanoparticles is present in the germanium shell, which could be attributed to the penetration of tin through the inner carbon shell at the relatively high temperature of 500 C in the reduction process due to its low melting point of 232 C. In the case where the inner carbon shell originated from the carbonization of glucose (in the sample denoted as Sn@C(glucose)@Ge@C), the tin would diffuse out of the pores [70] that exist on the carbon shell, which are created from pyrolyzed glucose, to alloy with germanium. The Sn Ge alloy grows and aggregates at the relatively high temperature under the catalytic effect of tin, [18] resulting in bursting of the outer carbon shell and the escape of the Sn Ge alloy. Therefore, (10 of 16)

11 Figure 8. a) Powder X-ray diffraction patterns of SnO 2 hollow spheres (top), SnO 2 (middle), and Sn@C@Ge@C (bottom). b) Raman spectra of Sn@C@Ge@C in two wavenumber ranges: (I) from 140 to 1900 cm 1, and (II) from 140 to 350 cm 1. c) FESEM image of Sn@C@Ge@C, with the inset showing an enlarged broken sphere, d) bright field and e) dark field images of Sn@C@Ge@C collected by scanning transmission electron microscopy (STEM), and corresponding element mapping of f) carbon, g) germanium, and h) tin. robust and complete carbon shells are essential for effectively avoiding the penetration of tin and keeping the tin and germanium nanoparticles in their own spaces. To better understand the effects of this unique double-shell structure on the electrochemical performance, Sn/Ge@C composite was synthesized as a reference sample by grinding SnO 2 and GeO 2 nanoparticles together, based on the mass ratio of Sn: Ge of Sn@C@Ge@C, and then carbon coating and reducing the mixture. Sn@C(glucose)@Ge@C was also chosen as a reference to investigate the scaffolding role of stiff carbon in maintaining the structure. The lithium-ion storage performances of the as-prepared multishelled Sn@C@Ge@C, Sn/Ge@C, and Sn@C(glucose)@Ge@C composites are evaluated in Figure 9. On the basis of our calculations, the mass content ratio of tin, germanium, and carbon in Sn@C@Ge@C is 63.0%:18.6%:18.4%, respectively. Therefore, the theoretical specific capacity of the Sn@C@Ge@C composite was determined by CSn@C@Ge@C = CSn 63.0% + CGe 18.6% + C 18.4% C where C Sn@C@Ge@C, C Sn, C Ge, and C c correspond to the theoretical capacities of Sn@C@Ge@C composite, tin (993 ma h g 1 ), germanium (1600 ma h g 1 ), and carbon (372 ma h g 1 ), respectively. The calculated theoretical capacity (1 C) of Sn@C@Ge@C composite is 992 ma h g 1. Figure 9a shows the rate performances of the three samples from 0.1 to 30 C for five cycles each. The Sn@C@Ge@C electrode exhibited obvious capacity decrease in the first few cycles (at 0.1 C), which can be attributed to the large areas of electrochemically active sites in/between the double shells and in the grain (1) (11 of 16)

12 Figure 9. a) Comparison of rate capability of and at various current rates from 0.1 to 30 C, b) galvanostatic charge/discharge profiles for selected cycles of composite at different current rates from 0.1 to 30 C (corresponding to (a)), c) cyclic voltammograms of Sn@C@Ge@C at the scan rate of 0.1 mv s 1 for the first five cycles, d) comparison of cycling performances of Sn/ Ge@C, Sn@C(glucose)@Ge@C, and Sn@C@Ge@C at the 2 C current rate, e) long-term cycling performance of Sn@C@Ge@C at a current density of 1 C. The Coulombic efficiency is plotted on the right axis of (e). boundary areas of the tin and germanium nanoparticles. [69,71] The average capacity is 860.1, 788.2, 731.3, 677.0, 547.4, and ma h g 1, at 1, 2, 5, 10, 20, and 30 C, respectively. The capacity recovers to ma h g 1 when the current rate returns to 0.1 C. On the contrary, the Sn/Ge@carbon and Sn@C(glucose)@Ge@C composites exhibited a much faster decrease in capacity as the current rate increased to 30 C, and the capacity declined to 70.8 ma h g 1 and ma h g 1 at the 35th cycle, respectively. Representative discharge charge voltage profiles of Sn@C@Ge@C composite under different current rates are shown in Figure 9b. The lithiation and delithiation processes are demonstrated in the cyclic voltammograms in a voltage window of V to give closer insight (Figure 9c). The peaks at about 1.1 V in the first reduction cycle correspond to the formation of the SEI layer and the irreversible reactions of lithium and functional groups on the carbon surfaces. [2,72] The cascade of peaks in the reduction and oxidation scans below 1 V can be associated with the alloying and dealloying reactions, respectively, corresponding to various types of Li Sn alloys and Li Ge alloys. [2,14,73] In terms of cycling performance (Figure 9d), the Sn@C@Ge@C composite exhibited good cycling stability at a current rate of 2 C and delivered a high capacity of ma h g 1 at the 400th cycle, which is almost 70% of its theoretical specific capacity. In contrast, the Sn/Ge@carbon and Sn@C(glucose)@Ge@C composites exhibited much inferior cycling stability, showing continuously decreasing capacities during cycling. Only a low capacity of 51.3 ma h g 1 is delivered for Sn@C(glucose)@Ge@C at the 400th cycle. To assess the durability of the cycling performance of Sn@C@Ge@C composite, we conducted a long-term cycling test. The Sn@C@Ge@C composite demonstrated good longterm cycling performance at 1 C (Figure 3e), and it maintained 82.2% of the theoretical specific capacity (816.3 ma h g 1 ) after 1000 cycles with almost 100% Coulombic efficiency (12 of 16)

13 Figure 10. Schematic illustration of changes in the morphology of and during lithiation/delithiation processes. The rapid decay in the capacity of and can be ascribed to the pulverization of the Sn Ge alloy, which normally has a large particle size when there is no restraint or only ineffective restraint from the carbon shell, as the lithiation/delithiation processes continue, leading to cracking and peeling off of the electrode material from the current collector, as shown in the schematic illustration of the morphology changes in Figure 10. In particular, for the Sn@C(glucose)@Ge@C composite, the carbon shells arising from the carbonization of glucose are too porous to prevent the diffusion of tin as the temperature is increased during the reduction process (above 232 C). The diffused tin from the glucose-sourced carbon shell acts as a catalyst for the growth and aggregation of germanium. The uncontrollable growth of Sn Ge alloy bursts the outer imperfectly developed carbon shell, even in the acetylene carbonization process. The thusformed Sn Ge alloy has no or extremely thin carbon shells to restrain it due to the simultaneous processes of alloying and carbon coating, which cannot tolerate the huge volume changes stemming from lithiation/delithiation of both tin and germanium, resulting in steep capacity decay. As expected, the introduced germanium enhances the specific capacity and rate performance of the tin/germanium/ carbon composite compared with the tin/carbon composites reported in the literatures. [74] The as-obtained Sn@C@Ge@C composite also shows better electrochemical properties than those of published Sn/Ge alloy composites. [18,53,75] The remarkable rate capability and cycling performance can be attributed to the virtues of the unique double-shell structure in the following aspects. First, the double carbon layer grants highly robust structural stability to the Sn@C@Ge@C composite by buffering the volume changes in both the tin and the germanium during lithiation/delithiation processes, and thus, good electrical contact between the active materials and the current collector is maintained by preventing pulverisation. Second, tin and germanium can be separated effectively by the stiff inner carbon layer, so that they do not form widely distributed Sn/Ge alloy, which maintains the integrity of the double-shell structure in the synthesis process and protects the large electrochemically active surfaces and abundant active sites in the Sn@C@ Ge@C composite. Third, the reduction reactions from SnO 2 and GeO 2 to metallic tin and germanium provide extra space for the volume expansion after lithiation, guaranteeing that the carbon shells remain intact during the electrochemical cycling. 3. Conclusions and Outlook Herein, we have presented recent progress on the development of germanium-based materials for lithium-ion batteries. Table 1 summarized the electrochemical performance of germaniumbased materials. The most serious challenge for germanium is the huge volume change, resulting in electrode collapse, high irreversible capacity, and poor cycling stability. The widely applied strategies to address the issue can be classified into the following categories: i. Nanostructure fabrication: Nanostructures can offer shortened diffusion distances for lithium ions and large interfacial contact areas between the electrode materials and electrolyte. The proposed nanostructures include nanoparticles, nanowires, nanotubes, nanofibers, nanorods, etc. ii. Designing porous/mesoporous structures: Pores or mesopores can provide sufficient space or voids to accommodate the volume expansion after lithiation and provide more diffusion paths for lithium ions and electrons, as well as allowing thorough soaking of the electrode materials in the electrolyte (13 of 16)