Nanoscale. Integrating Si nanoscale building blocks into micro-sized materials to enable practical applications in lithium-ion batteries

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1 FEATURE ARTICLE View Article Online View Journal View Issue Cite this:, 2016, 8, 1834 Received 31st October 2015, Accepted 17th December 2015 DOI: /c5nr07625k 1. Introduction Integrating Si nanoscale building blocks into micro-sized materials to enable practical applications in lithium-ion batteries Ran Yi, Mikhail L. Gordin and Donghai Wang* Fossil sources of energy have been exploited for several centuries to support industrial activities. Significant environmental, climate, and health risks are associated with the use of fossil fuels, and the large-scale implementation of green energy sources such as wind and solar is thus highly desired. 1 However, these renewable power sources require energy storage to be viable at a large scale, and even other power sources such as nuclear and hydroelectric are more efficient when combined with energy storage. 2 With their high energy density and long cycle life, lithium-ion batteries (LIBs) are an Department of Mechanical & Nuclear Engineering, The Pennsylvania State University, University Park, PA 16802, USA. dwang@psu.edu; Fax: ; Tel: This article highlights recent advances in micro-sized silicon anode materials composed of silicon nanoscale building blocks for lithium-ion batteries. These materials show great potential in practical applications since they combine good cycling stability, high rate performance, and high volumetric capacity. Different preparation methods are introduced and the features and performance of the resulting materials are discussed. Key take-away points are interspersed through the discussion, including comments on the roles of the nanoscale building blocks. Finally, we discuss current challenges and provide an outlook for future development of micro-sized silicon-based anode materials. attractive energy storage system and have been widely used in various portable electronic devices. 3 LIBs are also promising candidates for electric vehicle (EV) and hybrid electric vehicle (HEV) applications. However, major challenges in LIB design need to be addressed before LIBs can meet the technical requirement for their application in stationary energy storage systems, EVs, and HEVs. Of particular interest is increasing energy and power densities, lowering cost, and improving safety of LIBs. Conventional anode materials in commercial LIBs are primarily synthetic graphite-based materials with a theoretical capacity of 370 ma h g 1. 4 Considering cells with current cathode materials having capacities of 200 ma h g 1 and other conventional cell components (electrolyte, separator, current collector etc.), development of anode materials having a specific capacity on the order of ma h g 1 can Ran Yi Ran Yi received his B.S. (2007) and M.S. (2010) degrees in Materials Science from Central South University, China. Then he worked with Dr Donghai Wang in the Department of Mechanical Engineering, Pennsylvania State University and received his Ph.D. degree in Mechanical Engineering in His research focuses on development of functional materials for energy conversion and storage. Mikhail L. Gordin Mikhail L. Gordin received his Ph.D. degree in Mechanical Engineering from The Pennsylvania State University in 2015, based on his work in Dr Donghai Wang s Energy Nanostructure Lab. Before that he received his BSE degree in Mechanical Engineering from Duke University in His research interests have included development and surface analysis of materials for energy storage. 1834,2016,8, This journal is The Royal Society of Chemistry 2016

2 significantly improve the specific energy density of Li-ion battery full cells and thus enable high energy density for electric vehicle and plug-in hybrid electric vehicle (PHEV) applications. 5 Because of its high specific capacity (>3500 ma h g 1 ) and abundance, silicon is the most promising anode material for high-energy-density LIBs and has been intensively studied. 6 However, the severe pulverization of Si triggered by its large volume change during lithiation and delithiation, along with other serious issues stemming from this volume change such as an unstable solid-electrolyte interphase (SEI) and disintegration of the electrode structure, leads to poor cycling performance. 7,8 Tremendous efforts have been made to tackle this problem. For example, decreasing the dimensions of Si-based anode materials to the nanoscale (nanotubes, nanowires, nanoparticles, etc.) can mitigate fracture and thereby provide a large capacity with minimal fading However, Si nanomaterials suffer from three major disadvantages that prevent their practical application. First, although high gravimetric capacity can generally be obtained by Si nanomaterials, they show low volumetric capacity due to their intrinsically low tap density. 13 High volumetric capacity is a key parameter for LIBs, especially for space-conscious applications such as EVs and PHEVs. Second, the preparation of Si nanomaterials involves high-cost, laborious processes and is thus difficult to scale up. 14,15 Third, the fabrication of electrodes using nanoparticles may pose serious health and safety risks related to their inhalation or explosion. 16,17 The obvious way to increase tap density is to increase the Si particle size to the micron level, in line with commerciallyused micro-sized electrode materials. However, most microsized Si-based materials have very poor cycling stability. This article summarizes recent efforts into developing micro-sized Si anode materials with excellent cycling stability, high rate capability, and high volumetric capacity. The common strategy Dr Donghai Wang is currently an Associate Professor at the Department of Mechanical Engineering at The Pennsylvania State University. Before joining Penn State in 2009, he was a postdoc and subsequently became a staff scientist at Pacific Northwest National Laboratories. He received his B.S. and Ph.D. degree in Chemical Engineering from Tsinghua University and Tulane University in Donghai Wang 1997 and 2006, respectively. Dr Donghai Wang s research interests have been related to the design and synthesis of nanostructured materials for a variety of applications. His recent research is focused on materials development for energy storage technologies such as Li-ion batteries and beyond Li-ion batteries. shared by these advances is to combine the advantages of both micro-sized and nano-sized particles, using micro-sized particles containing nanoscale Si building blocks to achieve both high volumetric capacity and stable cycling. We have categorized these anode materials based on their preparation methods, and present the method highlights, electrochemical performance, and any salient insights and remaining challenges which can clearly be seen from that group of work. Finally, we present an outlook on future developments of micro-sized Si-based anode materials. 2. Preparation methods of micro-sized Si-based anodes 2.1. SiO x -derived (0 < x < 2) materials SiO x -based anode materials, which are typically micro-sized, have been extensively studied. The focus is commonly on size reduction and surface modification such as carbon coating to improve cycling life. In addition, they have already been used in some commercial LIBs. Other researchers have reviewed progress in SiO-based anode materials, 18 so a summary of such materials is not included in this article. Instead, SiO x - derived anode materials are discussed, most of which take advantage of the disproportionation of SiO x into nano-sized crystalline Si and amorphous SiO 2 at high temperatures. Park et al. demonstrated a Si SiO SiO 2 multicomponent anode via annealing of bulk SiO in the presence of NaOH. 19 The introduction of NaOH leads to the disproportionation of SiO at a lower temperature and over a shorter period of time than would otherwise be the case, and prompts the formation of crystalline rather than amorphous SiO 2. As a result, the Si SiO SiO 2 multicomponent anode is composed of a Si/SiO core and a cristobalite shell. The ratio of crystalline Si to cristobalite can be simply tuned by adjusting the ratio of NaOH to SiO and the annealing duration. After carbon coating, this multicomponent anode showed very stable cycling performance for 200 cycles and much less volume expansion at the electrode level than is seen in carbon-coated SiO disproportionated without NaOH. Park et al. also reported a porous Si SiO SiO 2 multicomponent anode. 20 A micro-sized porous SiO precursor was first prepared by an electroless etching process, the general method of which is discussed in more detail in section 2.2 below. Ag nanoparticles were deposited on the surface of SiO by a galvanic reaction of AgNO 3 and HF, and a mixture of HF and H 2 O 2 was then used to etch the SiO layer under the Ag particles. The porous SiO precursor was then subjected to a similar NaOH-assisted annealing process to obtain the porous Si SiO SiO 2 multicomponent anode. Benefiting from its large active material/electrolyte interface due to its porous structure, the anode exhibited excellent rate performance. The SiO 2 network in the Si/SiO 2 structure after disproportionation of SiO serves as a strong framework to maintain the integrity of micro-sized particles and as a buffer layer for a volume change of Si to ensure good cycling stability. However, the drawback of SiO 2 is clear it has very low electrical conduc- This journal is The Royal Society of Chemistry 2016,2016,8,

3 tivity which hinders charge transfer, an especially-important issue for micro-sized particles which already have a long Li ion diffusion length. To this end, we reported a micro-sized Si C composite with interconnected Si and C nanoscale building blocks. 21 As shown in Fig. 1a, SiO 2 was removed by HF etching to form porous Si. The pores were then filled with carbon by thermal decomposition of acetylene. The resultant Si C composite retained the micro-sized morphology of its SiO precursors (Fig. 1b). The size of Si building blocks was around 10 nm (Fig. 1c) and carbon was found to be uniformly distributed throughout the micro-sized particles (Fig. 1d), forming an interconnected conductive network for Si. The composite shows a reversible capacity of 1459 ma h g 1 after 200 cycles at 1Ag 1 (97.8% capacity retention) and an impressive high rate performance of 700 ma h g 1 at 12.8 A g 1, and also has a high tap density of 0.78 g cm 1. 3 The uniform carbon filling not only enables rapid charge and discharge by connecting each silicon building block and thus confining the electron transfer path within the less-conductive silicon to the nanoscale, but also ensures a high utilization degree of silicon even after pulverization of the micro-sized particles during cycling as shown in Fig. 1g. Following a similar strategy, Lu et al. prepared nonfilling carbon-coated porous silicon microparticles (nc-psimps) in which only the outer surface of the psimps was coated with carbon while the interior pore structures were left unfilled. 22 The void space between the outer surface and the Si core provided extra accommodation for volume expansion of Si. A long cycling life of 1000 cycles was demonstrated with a reversible capacity of around 1500 ma h g 1 at a mass loading of around 0.5 mg cm 2. Increasing mass loading generally led to an increase in areal capacity. An areal capacity of 2.84 ma h cm 2 was obtained after 100 cycles at 2.1 mg cm 2. However, no rate performance was reported. The results demonstrate the importance of maintaining an inner void space by using nonfilling carbon coating, different from our strategy of pore-filling carbon coating discussed above, which allows the Si to contract/expand freely and thus contributes to stable cycling. To understand the effects of key parameters in designing the micro-sized Si C composites on their electrochemical performance and explore how to optimize them, we investigated the influence of Si nanoscale building block size and carbon coating on the electrochemical performance of the micro-sized Si C composites. 23 The critical Si building block size was found to be 15 nm, which enables both a high capacity and good cycling stability. It is worth noting that in this study, 15 nm is the critical size that allows not just a lack of fracture but also a stable solid electrolyte interface (SEI) layer, evidenced by the high cycling efficiency and stable cycling. While a previous in situ TEM study by Liu et al. revealed that 150 nm is the critical size below which individual Si particles do not break upon charging/discharging, 24 to the best of our knowledge this size does not guarantee a stable SEI layer. In other words, avoiding primary particle fracture is not sufficient to achieve micro-sized nanocomposites with stable cycling and high efficiency. Instead, a much smaller Si primary particle size is required. It is important to remember this difference when designing high-performance Si-based anode materials. In addition to this, it was found that carbon coating at higher temperature can reduce SiO x (0 < x < 2) on the surface of porous Si and form higher-quality carbon with a higher degree of graphitization, both contributing to the improvement of the 1 st cycle coulombic efficiency (CE) and the rate capability. A key step towards the development of Si-based highenergy-density LIBs is to fabricate electrodes with high areal capacity (ma h cm 2 ) and thus increase the active/inactive material ratio (e.g., more capacity per mass of the current collector used). This requires a high mass loading (mg cm 2 )of active materials. In commercial Li-ion batteries, the areal capacity of anodes is about 4 ma h cm 2. The areal capacity of Si electrodes should be increased to at least that level in order to be used in practical applications. However, simply increasing mass loading does not necessarily lead to increase in areal capacity: tests have shown that increasing mass loading has little effect on the areal capacity of the Si C composite, with material utilization dropping as mass loading increases (Fig. 2c). To address this, we developed a novel micro-sized graphene/si C composite (G/Si C) to translate the performance of the micro-sized Si C composite from the material level to the electrode level. 25 The 2D graphene sheets create a conductive network connecting different micro-sized Si C particles, which together with the carbon coating within each micro-size particle provides dual conductive networks for the Si building blocks (Fig. 2a). Thanks to this structure, the G/Si C composite has high material utilization and exhibits increasing areal capacity with increasing mass loading (Fig. 2b), achieving a high areal capacity of 3.2 ma h cm 2 after 100 cycles while maintaining high coulombic efficiency (average 99.51% from 2 nd to 100 th cycles). Post-cycling TEM analysis showed that Si C particles were still connected by graphene sheets, similar to the structure before cycling. Besides various conductive coatings that help improve the electrical conductivity of the resultant Si-based composite, another approach is to boost the conductivity of Si itself. We developed a simple and effective method of boron doping to further improve the rate capability of Si-based anode materials. 26 A boron-doped Si C composite can deliver a high capacity of 575 ma h g 1 at 6.4 A g 1 without the addition of any conductive additives, 80% higher than that of an undoped composite. Compared to the obvious capacity fading of the undoped Si C composite, the boron-doped Si C composite also maintains its capacity well during long cycling at a high current density. Electrochemical impedance spectroscopy (EIS) measurements showed that the improved rate capability was attributed to a lower charge transfer resistance of the borondoped Si C composite. Other than SiO, different SiO x precursors were also employed to prepare micro-sized Si-based materials, such as SiO The synthesis process involves facile thermal disproportionation of silsesquioxane (SiO 1.5 ), etching of the resultant SiO 2 using HF to leave behind porous Si, and subsequent carbon coating. The Si C composites are composed of particles around 10 μm in size with sub-10 nm primary building 1836,2016,8, This journal is The Royal Society of Chemistry 2016

4 View Article Online Fig. 1 (a) The preparation process from the SiO precursor to the Si C composite. (b) SEM image, (c) TEM image, (d) cross-section SEM and EDS mapping of Si and C in the area marked by the white square, (e) cycling stability and (f ) rate performance of the Si C composite. (g) The breaking process of the micro-sized Si C composite showing the interconnected structure and the electrical contact is maintained between and within the broken parts. Reproduced with permission from ref. 21. Copyright 2013 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim. This journal is The Royal Society of Chemistry 2016, 2016, 8,

5 View Article Online Fig. 2 (a) Preparation process of G/Si C. Cycling performance of (b) graphene/si C composite and (c) Si C composite. Reproduced with permission from ref 25. Copyright 2014 Elsevier. blocks. In addition, carbon was uniformly coated throughout silicon particles. Si C composites exhibited a high reversible capacity of 1660 ma h g 1 and excellent cycling stability within 150 cycles. It also has a relatively high initial coulombic efficiency of 76%, rising to 99.5% during cycling. A high reversible capacity of 800 ma h g 1 can also be achieved at a high current density of 8 A g 1. Due to the nature of microsized secondary aggregation this Si C composite also featured a high tap density of 0.68 g cm 3, resulting in a high materiallevel volumetric capacity of 1088 ma h cm 3. Highly-toxic HF was used in the preparation of most of these SiOx-derived Si materials. To try and find an HF-free approach, we have demonstrated a micro-sized Si-based material (B Si/SiO2/C) with high rate performance prepared without the use of HF.28 The cycling stability and rate performance of B Si/SiO2/C were improved by size reduction through simple ball milling and boron doping. B Si/SiO2/C has an average particle size of 2 μm, much smaller than 20 μm of the pristine counterpart. B Si/SiO2/C delivered a capacity of 1279 ma h g 1 after 100 cycles, corresponding to a capacity retention of 92.7%. In comparison, a capacity of 990 ma h g 1 was obtained by a control sample prepared without ballmilling after 100 cycles, corresponding to only 75% of its initial capacity. At a high current density of 6.4 A g 1, B Si/ 1838, 2016, 8, SiO2/C delivers a capacity of 685 ma h g 1, 2.4 times that of the undoped Si/SiO2/C, showing significantly enhanced rate performance. The tap density of B Si/SiO2/C is measured to be around 0.8 g cm 3, in line with most micro-sized Si-based materials. The combination of boron doping and ball-milling thus enables the good performance of this etching-free material Si and Si-alloy-derived materials Many of the methods discussed in the previous section are based on selective etching of interpenetrating SiO2 networks formed by thermal disproportionation of SiOx species. However, micro-scale porous particles may also be produced by direct etching of silicon materials, most commonly Si wafers or alloys. The process of generating porous Si films by electrochemical etching of Si wafers has been known since the 1950s. When a positive current is passed through a silicon electrode in the HF electrolyte, either a four-electron oxidation of Si forming soluble SiF62, or two-electron oxidation of Si forming SiF2 which then chemically reacts with HF to form soluble SiF62, can be expected to occur. The latter reaction, which occurs at comparatively-low applied voltages, leads to pore formation, and the actual porous structure (e.g., pore size) This journal is The Royal Society of Chemistry 2016

6 depends on factors like the electrolyte composition, dopant type, concentration, applied voltage, and temperature. 29 One recent example of this technique for making porous microsized Si anode materials for Li-ion batteries is the mesoporous Si sponge (MSS) material generated by Li et al. through electrochemical etching of B-doped Si wafers in HF/ethanol solution. 30 After etching, the wafers were broken into particles >20 µm in size by ultrasonication. The MSS was found to have pores up to 50 nm in size, with 10 nm pore walls composed of crystalline Si wrapped in 1 3 nm of silicon oxide. Amorphous carbon was also coated onto the MSS to improve conductivity and stabilize the interface (Fig. 3a). Electrochemical tests of the electrodes with 46 wt% Si and 0.5 mg-si per cm 2 showed promising performance, with 81% capacity retention in 1000 cycles and a top capacity of around 640 ma h per g- electrode ( 1390 ma h per g-si), although the initial coulombic efficiency (ICE) was quite low at 56% (Fig. 3b). It was proposed that the high porosity ( 80%) allowed the volume change to be buffered well, while the thin pore walls were able to accommodate the stress of expansion, accounting for the material s cycling stability. In further tests, prelithiation with stabilized lithium metal powder was seen to dramatically improve the ICE, raising it to 94.5%. These cells showed somewhat higher maximum capacity ( 740 ma h per g-electrode, or 1610 ma h per g-si), but slightly worse capacity retention, with 80% retention after 800 cycles. In situ TEM charge/ discharge tests showed that particles of the material displayed only 30% volume expansion at full lithiation despite the nominal volume expansion of Si at this state being 300%, which helps to corroborate the buffering effect of the porous structure (Fig. 3c). A very similar etching process was used by Thakur et al. to generate macroporous Si particulates (MPSPs) which were µm in size. 31 A composite of these with pyrolized polyacrylonitrile (PAN) was then prepared for use as an anode material. These showed dramatically better performance than non-porous micron-sized Si, with stable cycling for 600 cycles when the charge capacity was controlled to 1000 ma h per g-si and the Si : PAN ratio was optimized at 7 : 3. This study showed significant differences in performance depending on the Si : PAN ratio, with both too-high and toolow contents of pyrolized PAN leading to poor stability, emphasizing the impact of the carbon coating procedure on the performance of Si C composite materials. Chemical etching procedures have also been used to generate porous micron-sized Si for battery applications. Electroless etching using noble metal catalysts is one common way of etching silicon. The silicon is first coated with noble metal nanoparticles from a precursor such as AgNO 3, and then etched with an oxidant and etchant combination such as HF and H 2 O 2. During this process, the oxidant is preferentially reduced at the noble metal surface, leading to hole (h + ) injection into the adjacent Si, oxidation of that Si, and etching of the oxide, creating pores near the noble metal nanoparticles. Depending on the rate of oxidation and etching, additional Fig. 3 (a) SEM [a, b] and TEM [c] images of a mesoporous silicon sponge material, with scale bars of 20 µm, 100 nm, and 100 nm, respectively. (b) Cycling performance of prelithiated mesoporous Si sponge. (c) In situ TEM of a mesoporous Si sponge particle during lithiation, from unlithiated (far left) to fully lithiated (far right), demonstrating the low particle-level volume expansion enabled by the porous structure. Adapted with permission from ref. 30. Copyright 2014 Nature Publishing Group. This journal is The Royal Society of Chemistry 2016,2016,8,

7 porosity can also be created in nearby areas. 32 One example is the synthesis of mesoporous Si powder by Zhao et al., produced by milling a B-doped Si wafer, soaking the powder in AgNO 3 /HF solution to generate Ag nanoparticles, and then etching with HF/H 2 O 2 solution. 33 The resulting material was composed of particles 3 10 µm in size and had mesopores ranging from 3 to 20 nm in diameter. This material was coated with phenolic resin and sintered at 850 C, resulting in carbon coating of the particle exteriors and partial filling of the pores. The carbon-coated porous Si showed excellent cyclability for 50 cycles at a set discharge capacity of 1500 ma h g 1, while a set capacity of 2000 ma h g 1 led to 84% capacity retention in 50 cycles. In a different approach, Zhang et al. used a Rochow reaction with a Cu-based catalyst to generate tens-of-micron-sized Si particles with 1 5 µm pores. 34 In this process, Si and Cu react to form copper silicide at high temperature, which then reacts with CH 3 Cl to form gaseous organosilicides, etching the material. After an additional carbon coating, the resultant composite showed good capacity retention after the first few cycles and had a capacity of 732 ma h per g-composite after 100 cycles. This approach is claimed to be very cost-efficient compared to other etching approaches due to factors like its use of metallurgicalgrade Si powder rather than Si wafers. Si alloys have also recently been used to generate porous micron-sized Si materials. This is quite different from the electrochemical and electroless etching processes described above; porous materials are instead prepared through dealloying, in which the non-si component of the alloy is selectively dissolved. During dissolution, the Si atoms lose their lateral coordination and diffuse across the material surface, spontaneously aggregating into Si islands and allowing access to the as-yet-alloyed material deeper within. This dissolution/ islanding process then leads to pore formation. 35 Several researchers used Al Si alloys for this method; conveniently, dealloying can be accomplished simply by immersion in hydrochloric acid (HCl) One example comes from Zhou et al., who used Al Si alloy powder (88% Al) with 1 10 µm spherical particles. 36 Etching led to the formation of a sunflower-like Si dendrite structure, with pores having a broad distribution between a few and 55 nm with a peak at 15 nm. Around 5 wt% Al remained after Al dissolution. Stirring of the material overnight in HCl led to fracture of the particles, but there was no further change in the composite. The fractured material had a capacity of above 1150 ma h g 1 after 60 cycles, with visible but slow capacity decay after the first few cycles. Jiang et al. used a similar Al Si alloy powder (80% Al) and additionally etched with HF after dealloying to remove oxides. 37 The resulting material had some resemblance in its dendritic structure to that in the previous example, but lacked the same radiating-from-the-center patterning. Pore size showed a wide distribution between a few and 175 nm with two peaks at around 15.5 nm and 66 nm. Performance was quite promising, with 1368 ma h g 1 retained after the 258 th cycle. They were able to further improve the performance by adding a coating of metal nanoparticles to the material, depositing 10 wt% Cu and 5 wt% Ag. 38 With a decreased amount of binder and conductive additive in their test electrodes compared to the previous test, they found that the coated porous Si achieved 1651 ma h g 1 after 150 cycles, compared to 600 ma h g 1 for the uncoated Si. Post-cycling SEM showed that the coated sample appeared to have better structure preservation than the uncoated sample, possibly due to the metal nanoparticles inhibiting aggregation of Si during volume expansion. As a general note, it is known that dealloying processes can be tuned to produce different length scales and structures by controlling the alloy composition, etchant composition, etch time and temperature, and other factors. 35,40 This offers opportunities to researchers seeking high-performance and low-cost materials, and to date this has not been fully explored. It is also worth noting that Al Si is not the only alloy which can be used. A ternary alloy Al Si Ag was used by Hao et al. as a precursor for a bimodal porous Si Ag composite via dealloying of Al in HCl solution. 41 The incorporation of Ag created an efficient electron-conducting pathway in the material and thus boosted the rate performance. A recent work by Wada et al. also demonstrated the formation of porous micron-sized Si from Mg Si alloys, using a Bi melt for the dealloying process and then etching away the solidified MgBi with HNO Changes in the melt temperature and immersion time were found to change the length scale of the nanostructures from tens to hundreds of nanometers. Under the conditions selected, this material had a much larger pore size than the materials synthesized from Al Si alloys, with pores 400 nm in diameter. However, the performance was still very promising: when the charge capacity was held at 1000 ma h g 1, the material retained a discharge capacity of 1000 ma h g 1 for 1500 cycles SiO 2 -derived materials Pioneered by Bao et al., 43 Mg reduction has been widely used as a general approach to prepare porous Si from preformed porous SiO 2 templates According to eqn (1), SiO 2 templates are converted into a MgO/Si mixture by molten Mg at high temperature. 2MgðgÞþSiO 2 ðsþ!2mgoðsþþsiðsþ MgO can easily be removed with dilute HCl solution, which can potentially eliminate the use of HF, as is needed for many other approaches. However, it is not uncommon for dilute HF to still be used to etch out the unreacted SiO 2. A key feature of Mg reduction is that if engineered properly it can retain the structure and morphology of the SiO 2 precursors, leveraging different porous SiO 2 materials that are already well established. For example, Jia et al. reported the synthesis of a threedimensional mesoporous Si-based anode material (mp-si) using SBA-15 silica templates via a Mg reduction process (Fig. 4a). 44 The mp-si inherited the porous structure of the SBA-15, and the particles were composed of 30 nm sized Si crystallites (Fig. 4b and c). After carbon coating, the mp-si@c ð1þ 1840,2016,8, This journal is The Royal Society of Chemistry 2016

8 Fig. 4 (a) Preparation process of mesoporous Si particles (mp-si) with SBA-15 as templates via a Mg reduction route. (b) SEM and (c) TEM images of mp-si. (d) Cycling stability of mesoporous Si-based anodes in comparison with commercial Si nanoparticles ( nm). (e) Rate performance of mp-si@c. Reproduced with permission from ref. 44. Copyright 2011 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim. composite showed good cycling stability (1500 ma h g 1 after 100 cycles) and excellent rate performance (Fig. 4d and e). Li et al. also synthesized micro-sized 3D interconnected porous silicon/carbon (Si/C) hybrid architectures from silica aerogels. The hybrid architectures had particle sizes above 10 µm and well-distributed pores. The interconnected porous structure was assembled from Si nanosheets that formed during Mg reduction. A reversible capacity of 1552 ma h g 1 was obtained after 200 cycles. 49 There are two major challenges for Mg reduction. First, the conversion yield is often low (<50%). To address this, a facile reduction method based on the control of reaction pressure was developed, which improved the conversion yield to >95%. 50 Second, the highly exothermic nature of Mg reduction may lead to an undesired formation of Mg 2 Si, collapse of the nanoporous structure, and aggregation of Si building blocks into large crystals, which adversely affect the electrochemical performance of the resultant Si. 51,52 To help maintain the porous structure and small building block size, NaCl was used as a heat scavenger: the fusion of NaCl significantly reduced the heat conduction to Si. 51 Another solution was to use Al instead of Mg as the reductant, since Al exhibits a much lower exothermic heat than Mg. 53,54 Besides metals, Mg 2 Si has also been used as a reductant following a similar mechanism; such reduction can provide twice the amount of Si compared with the Mg reduction of SiO Bottom-up chemical synthesis from SiCl 4 A major downside of many of the aforementioned approaches is inherent in their use of etching; since etching produces porosity by removing the material, the cost of the final product is increased by the cost of the material removed. This is especially exacerbated by using relatively costly starting materials like Si wafers. In addition, the etchants used (e.g., HF) are generally hazardous to work with and further add to the cost of the procedure. To avoid these downsides, some pro- This journal is The Royal Society of Chemistry 2016,2016,8,

9 cedures were developed to instead use a bottom-up approach, synthesizing Si-based materials from an inexpensive Si precursor like silicon tetrachloride (SiCl 4 ). One example of this comes from Kim et al., who synthesized porous micron-sized carbon-coated Si particles from SiCl 4 using SiO 2 spheres as pore templates. 56 The synthesis involved the reduction of SiCl 4 by sodium naphthalide, reaction with butyllithium, and removal of the NaCl and LiCl byproducts to form a butyl-capped Si gel, which was then mixed with 200 nm SiO 2 spheres and annealed to form a carbon-coated Si/SiO 2 composite. The SiO 2 spheres were etched with HF to form a porous carbon-coated Si (c-si). The material had 200 nm pores, matching the templates, and 40 nm pore walls, which could accommodate expansion stresses well. The amorphous carbon layer on the material s surface was found to be <10 nm in thickness. The electrochemical tests showed good stability at 400 ma g 1 rate for 100 cycles even without the use of fluoroethylene carbonate in the electrolyte, although tests at 2000 ma g 1 showed much worse stability. Although this method still requires a significant use of HF etching to remove the pore templates, it does avoid direct etching of silicon or use of noble metals (as in electroless etching). In order to avoid the use of etching entirely and also take advantage of the bottom-synthesis method, we demonstrated an amorphous Si/SiO x /SiO 2 composite material. 57 SiCl 4 was reduced using a sodium potassium alloy (NaK) while being heated under reflux, forming Cl-terminated Si particles; these were then mixed with water, first forming OH-terminated particles and then Si oxides. After heat treatment at 500 C, a mixture of amorphous Si, SiO x, and SiO 2 was formed which had an overall stoichiometric ratio of SiO 1.3. The material took the form of aggregated nanoparticles 10 nm in diameter, with interparticle pores also being 10 nm. Thanks to the buffering effect of the oxides, the material was able to achieve excellent stability, with a top capacity of 650 ma h per g-composite after 30 cycles and over 99% and 95% capacity retention after 100 and 350 cycles, respectively. However, the particle aggregates of this material were still of sub-micron-scale, leading to the aforementioned issues of Si nanomaterials. We recently also demonstrated a related method for generating porous micron-scale Si. 58 This procedure used NaK to reduce SiCl 4 at room temperature, a process which in situ forms pore templates composed of NaCl and KCl (Fig. 5a). After heat treatment to form a Si/salt matrix, these salt templates were removed simply by washing with water, leaving the highlyporous micron-sized Si particles (Fig. 5b and c). In addition, the particle and pore size could be controlled by varying the heat treatment temperature. Relatively mild etching with HF was also conducted in order to increase the surface area and decrease the oxide content. Although this type of synthesis requires a special system design for highly reactive NaK, we believe that it is quite promising for battery applications due to its high Si yield and use of less expensive SiCl 4 precursors. Fig. 5 (a) Scheme of the synthesis route, (b) TEM and (c) HRTEM of mpsi. Scale bars in (b) and (c) are 100 and 5 nm, respectively. Reproduced with permission from ref. 58. Copyright 2014 Nature Publishing Group. 1842,2016,8, This journal is The Royal Society of Chemistry 2016

10 The resultant material s Li-ion battery performance is presently being investigated Bottom-up assembly from Si nanoparticles Besides the chemical synthesis methods mentioned above, another effective bottom-up approach is to assemble Si nanoparticles, which are commercially available or can be prepared by various methods, into micro-sized agglomerates. This approach generally requires modification of the nanoparticles, since most of these nanoparticles have sizes around 50 nm and thus do not exhibit stable cycling. One modification that is widely applied is to construct core/ hollow shell structures with Si cores and conductive shells, carbon in most cases. The void between the core and the shell can accommodate the volume change of Si while the shell may prevent the penetration of the electrolyte and thus its direct contact with Si, enabling a stable SEI layer. Several groups have demonstrated encouraging results using this strategy and the optimal void/si volume ratio has been studied Liu et al. took this a step further to produce micro-sized secondary pomegranate-like particles composed of nano-sized hollow core/shell units. 62 Not only can the micro-sized particles provide a high tap density compared to the Si nanoparticles, but they can also maintain the original structure after extended cycling by taking advantage of the build-in void space. A similar structure, Si/porous-C composite with buffering voids, has also been reported. The composite both has a void space between the Si and the carbon coating and has pores within the carbon coating itself, providing an extra space to accommodate the volume change of Si. 63 Si nanoparticles have also been deposited on or wrapped by micro-sized conductive backbones to form composites. Magasinski et al. reported a hierarchical porous Si C composite by annealing carbon black particles to generate branching carbon structures, and then successively coating Si and carbon using CVD (Fig. 6a). 64 The particle size, pore size, and composition of the composite could be controlled by adjusting carbon black branch size and Si and carbon CVD deposition parameters. The composite had nm pores, which could accommodate the volume change of the deposited Si nano- Fig. 6 (a) Scheme of hierarchical bottom-up assembly of a C Si nanocomposite granule. (b) TEM image showing the Si nanoparticles. SEM images of a C Si nanocomposite granule (c) before and (d) after cycling. (e) Cycling performance of the C Si granule electrode. Reproduced with permission from ref. 64. Copyright 2010 Nature Publishing Group. This journal is The Royal Society of Chemistry 2016,2016,8,

11 particles with sizes around nm (Fig. 6b) and preserve the overall structure after cycling (Fig. 6c and d). The size of the composite particles was around 10 μm, giving a tap density of around 0.49 g cm 3. The composite showed stable cycling over 100 cycles and good rate performance (Fig. 6e). It is worth noting that the 1 st efficiency was about 85%, which is much higher than those of other Si C composites with a similar carbon content (50 wt%). This is because the carbon black underwent high-temperature annealing (over 2000 C), leading to graphitization and high purification (>99.9%). This significantly reduced the number defects which could trap Li ions and cause irreversible capacity loss. Two-dimensional structures such as graphene and graphite flakes were also used as conductive networks. Various Si nanoparticles graphene sandwich structures have been demonstrated, with the graphene sheets forming a continuous highly-conductive network and the interlayer space providing a buffer for Si expansion. Good cycling stability and rate performance could generally be achieved. However, the major issue of such structures is the usually-low 1 st cycle and/or cycling efficiency due to their large surface and high content of surface functional groups, since the graphene was processed below 1000 C in most cases. The use of graphite flakes with a lower surface area and higher purity has helped address this issue to some extent. An average cycling efficiency >99.9% was demonstrated by Yang et al. with a Si oxide-coated graphite flake composite. 72 This composite also showed 94% capacity retention after 500 cycles yet with a relatively low capacity below 500 ma h g Ball milling Ball milling is a well-established approach for preparing various nanomaterials, and can easily be scaled up. Ball milling has been used to reduce the primary and secondary particle sizes of Si anode materials, as well as to form composites. Zhou et al. prepared Al Si graphite powders by ball milling commercial eutectic Al Si alloy and graphite in a planetary machine. 73 The Al Si graphite had a spherical morphology and ranged from 1 to 8 μm in diameter. The material delivered an initial capacity of 650 ma h g 1 and retained 98% of it after 10 cycles. It should be noted that both Al and Si are electrochemically reactive to Li and the Si content in the composite is only 7.9 wt%, making Si a capacity booster rather than the primary active material. Other than Si composites, pure Si has also been produced by ball milling. Gauthier et al. reported a high-energy ballmilling process to prepare micro-sized Si using two coarsegrained Si precursors (1 5 μm and 20 mesh) with yield above 90%. 74 It was found that the particle size of the milled Si was Fig. 7 (a) Particle size distributions of the milled Si samples and their precursors. (b) TEM image of the milled 20 mesh Si powder. Cycling performance of milled Si and nanosized Si without (c) and with (d) capacity limitation at 1200 ma h g 1. (e) Scheme of the nanocrystalline structure of the milled Si showing grain boundaries which allow faster and more homogeneous Li diffusion. Reproduced with permission from ref. 74. Copyright 2013 The Royal Society of Chemistry. 1844,2016,8, This journal is The Royal Society of Chemistry 2016

12 not determined by the initial particle size of Si precursors, but rather by a balance between cold welding and fracturing of Si during ball milling, which depends on the mechanical properties of Si itself. As a result, similar final particle sizes were produced with the two precursors (centered at 5.2 to 10 μm for 1 5 µm and 20 mesh, respectively) (Fig. 7a). These micro-sized particles were composed of small cold-welded particles with a size of nm. Despite still being micro-sized, the milled Si had much smaller crystallites: ball milling reduced the average crystallite size from 44 nm to 10 nm, and also introduced many defects and dislocations (Fig. 7b). When paired with an acid-treated carboxymethyl cellulose (CMC) binder, the milled Si underwent nearly 900 cycles with a capacity limit of 1200 ma h g 1 and delivered over 1600 ma h g 1 without capacity limitation (Fig. 7c and d). It is important to note that such performance was achieved by a Si material without any porous structure or major buffer layers (only a very thin SiO 2 layer exists), which is contrary to conventional wisdom that such features are indispensable for high-performance Si-based anodes. The improved performance of the milled Si is believed to stem from its nanocrystalline structure. The increase in grain boundaries due to size reduction of crystallites promotes more homogeneous lithium diffusion into the Si, as they are efficient Li pathways, 75,76 generating a more gradual volume variation of Si (Fig. 7e). This limits cracking and disconnection of the Si particles and breaking of the SEI layer. As a result, cycling performance can be much enhanced. 3. Outlook As discussed above, great progress has been made in the research on micro-size Si anode materials, enabling good cycling stability, high rate performance, and high tap density. Table 1 summarizes the performance and properties of typical Si materials prepared by various methods. However, many challenges still remain. First, the silicon mass loading of anodes in the literature is usually low, generally leading to an areal capacity less than 2 ma h cm 2. Although superior cycling stability can be achieved at such low mass loading, this is not realistic; in order to reasonably explore the performance of Si anodes, electrodes with an areal capacity >3 ma h cm 2 should be fabricated and evaluated. However, increasing mass loading may cause poor cycling stability because of lower conductivity and more severe fracture with increasing electrode thickness. Other than constructing a conductive network at the electrode level as discussed above, possible approaches to improve areal capacity include developing (1) new Si materials with a built-in void space to minimize the volume change of the electrode, (2) advanced binders that could maintain the electrode integrity at high mass loading, and (3) rational cell design to allow for appropriate electrode porosity so that the volume change at the electrode level could be accommodated. In addition, the performance of micro-sized Si-based anodes has to date mainly been investigated in half-cell tests, while how they work in full cells has rarely been reported. The major difference between a half cell and a full cell is the amount of available lithium. In a half cell, a thick Li foil is generally used as the counter electrode, and can be seen as an unlimited source of Li (around 52 ma h cm 2 in the case of 0.25 mm thick foil, typically used for coin cells) considering the relatively low mass loading of the Si electrode (<4 ma h cm 2 ). On the other hand, a full cell s Li supply is limited by its cathode (around 4 ma h cm 2 ). Our recent study shows that the performance of Si anodes in half cells cannot necessarily be translated to full cells. Fig. 8 shows the cycling performance of two Si-based materials with different surface areas in both half and full cells. It is clear that both materials have very similar cycling stability and coulombic efficiency in half cells. Little capacity degradation is observed after 80 cycles and the average coulombic efficiency is around 99.7% (from 2 nd to 80 th cycles) for both. However, a pronounced difference is found in full cells with commercial NCM cathodes. While neither cell exhibits stable cycling, the one with the high surface area Si anode suffers from much faster capacity fading. This degradation of Si anodes in full cells is from parasitic reactions that are highly correlated to the specific surface area, involving continuous formation of new SEI layers and thus irreversible consumption of available Li. The results above suggest that, to Table 1 Summary of different preparation methods of micro-sized Si anode materials and performance of typical resulting products Method SiO x -derived (0 < x < 2) materials (section 2.1) Si and Si-alloy-derived materials (section 2.2) SiO 2 -derived materials (section 2.3) Bottom-up chemical synthesis from SiCl 4 (section 2.4) Bottom-up assembly from Si nanoparticles (section 2.5) Ball milling (section 2.6) Structure and morphology Carbon coated micro-sized particles (Fig. 1 and ref. 21) Carbon coated micro-sized porous particles (Fig. 3 and ref. 30) Carbon coated micro-sized porous particles (Fig. 4 and ref. 44) Carbon coated micro-sized porous particles (ref. 56) Porous carbon silicon spheres (Fig. 6 and ref. 64) Micro-sized particles with nanoscale crystallites (Fig. 7 and ref. 74) Tap density (g cm 3 ) Performance ma h g 1 after 200 cycles N/A N/A N/A 750 ma h g 1 based on the total electrode weight with >80% capacity retention over 1000 cycles 1500 ma h g 1 after 100 cycles 2780 ma h g 1 after 100 cycles 0.49 Around 1500 ma h g 1 after 100 cycles ma h g 1 after 600 cycles This journal is The Royal Society of Chemistry 2016,2016,8,

13 Fig. 8 Cycling performance of two Si-based materials with different surface areas in (a, b) half and (c, d) full cells. examine the performance of Si anodes in practical applications, full cell tests with cathodes having areal capacities close to those of the anodes should be conducted. Another key issue for Si anodes is the need for a better understanding of the properties and growth process of the SEI, especially for porous Si materials with large surface areas. For example, it is vital to find out where the SEI layer grows: on the outer surface of the secondary particle or the inner surface of the primary building blocks. If the latter is the case, continuous SEI formation could fill up the pores and even lead to fracture of primary particles, subverting the intended role of the pores, and the details of such behavior are poorly understood. An increased understanding of these and other factors of SEI formation would allow for better selection of Si-based anode material coatings and structures. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract no. DE-EE References 1 S. Chu and A. Majumdar, Nature, 2012, 488, H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li and Y. Ding, Prog. Nat. Sci., 2009, 19, J. M. Tarascon and M. Armand, Nature, 2001, 414, M. Winter, J. O. Besenhard, M. E. Spahr and P. Novák, Adv. Mater., 1998, 10, U. Kasavajjula, C. Wang and A. J. Appleby, J. Power Sources, 2007, 163, B. A. Boukamp, G. C. Lesh and R. A. Huggins, J. Electrochem. Soc., 1981, 128, H. Wu, G. Chan, J. W. Choi, I. Ryu, Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu and Y. Cui, Nat. Nanotechnol., 2012, 7, L. Y. Beaulieu, K. W. Eberman, R. L. Turner, L. J. Krause and J. R. Dahn, Electrochem. Solid-State Lett., 2001, 4, A137 A H. Wu, G. Zheng, N. Liu, T. J. Carney, Y. Yang and Y. Cui, Nano Lett., 2012, 12, J. K. Yoo, J. Kim, Y. S. Jung and K. Kang, Adv. Mater., 2012, 24, W. Wang and P. N. Kumta, ACS Nano, 2010, 4, ,2016,8, This journal is The Royal Society of Chemistry 2016