Scholarly article on topic 'High capacity group-IV elements (Si, Ge, Sn) based anodes for lithium-ion batteries'

High capacity group-IV elements (Si, Ge, Sn) based anodes for lithium-ion batteries Academic research paper on "Nano-technology"

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Abstract of research paper on Nano-technology, author of scientific article — Huajun Tian, Fengxia Xin, Xiaoliang Wang, Wei He, Weiqiang Han

Abstract Tremendous efforts have been devoted to replace commercial graphite anode (372 mAh g−1) by group IV elements (Si, Ge, Sn) based-materials with high capacities in lithium-ion batteries (LIBs). The use of these materials is hampered by the pulverization of these particles due to the high volumetric change during lithiation and delithiation cycles, which leads to particles pulverization and destabilization of solid electrolyte interphase (SEI) films. These problems result in fast capacity fading and low Coulombic efficiency. Nanostructured materials show significant improvements in rate capability and cyclability due to their high surface-to-volume ratio, reduced Li+ diffusion length, and increased freedom associated with the volume change during cycling. However, the nanostructured active materials with high ratio of surface-to-volume increase the irreversible capacity due to the formation of more SEI films. Although the nanostructured materials active materials keep relatively stable during repeated cycles of lithiation/delithiation process, the SEI film continually breaks/reforms, lowing the Coulombic efficiency. Meanwhile, the high-cost, low Coulombic efficiency and low tapping density limit the commercialization of the nanostructured electrode materials. Therefore, it is urgent to find solutions which could take advantage of both long cycle life of nanomaterials within the group IV elements (Si, Ge, Sn) and high volumetric/gravimetric capacity of micro-materials in the group IV as well as elements (Si, Ge, Sn). This report presents an overview of the recently developed strategies for improving the group IV elements (Si, Ge, Sn)-based anodes performances in LIBs to provide a further insight understanding in designing novel anodes.

Academic research paper on topic "High capacity group-IV elements (Si, Ge, Sn) based anodes for lithium-ion batteries"

Accepted Manuscript

High capacity group-IV elements (Si, Ge, Sn) based anodes for Lithium-ion Batteries Huajun Tian, Fengxia Xin, Xiaoliang Wang, Wei He, Weiqiang Han

PII: S2352-8478(15)00047-7

DOI: 10.1016/j.jmat.2015.06.002

Reference: JMAT 21

Materiomics

To appear in: Journal of Materiomics

Received Date: 15 April 2015 Revised Date: 20 May 2015 Accepted Date: 9 June 2015

Please cite this article as: Tian H, Xin F, Wang X, He W, Han W, High capacity group-IV elements (Si, Ge, Sn) based anodes for Lithium-ion Batteries, Journal of Materiomics (2015), doi: 10.1016/ j.jmat.2015.06.002.

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Graphical abstract

We present an overview of the recently developed strategies for improving the high capacity group IV elements (Si, Ge, Sn)-based anodes performance in Lithium-ion Batteries. And we hope to give a further understanding in designing novel high-performance anodes for practical application.

Keywords:

High capacity; group-IV elements; (Si, Ge, Sn) based; anodes; Lithium-ion Batteries

High capacity group-IV elements (Si, Ge, Sn) based anodes for Lithium-ion Batteries

Huajun Tiana, Fengxia Xina, Xiaoliang Wangat, Wei Hea and Weiqiang Hanab*

aNingbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China

bSchool of Physical Science and Technology, Shanghai Tech University, Shanghai200031, P. R.China

^Present address: Seeo Inc., 3906 Trust Way, Hayward, CA 94545 *Corresponding Author:

E-mail address: hanweiqiang@nimte.ac.cn(W. Han)

Abstract

Tremendous efforts have been devoted to replace commercial graphite anode (372 mAh g-1) by group IV elements (Si, Ge, Sn) based-materials with high capacities in lithium-ion batteries (LIBs). The use of these materials is hampered by the pulverization of these particles due to the high volumetric change during lithiation and delithiation cycles, which leads to particles pulverization and destabilization of solid electrolyte interphase(SEI) films. These problems result in fast capacity fading and low Coulombic efficiency. Nanostructured materials show significant improvements in rate capability and cyclability due to their high surface-to-volume ratio, reduced Li+ diffusion length, and increased freedom associated with the volume change during cycling. However, the nanostructured active materials with high ratio of surface-to-volume increase the irreversible capacity due to the formation of more SEI films. Although the nanostructured materials active materials keep relatively stable during repeated cycles of lithiation/delithiation process, the SEI film continually breaks/reforms, lowing the Coulombic efficiency. Meanwhile, the high-cost, low Coulombic efficiency and low tapping density limit the commercialization of the nanostructured electrode materials. Therefore, it is urgent to find solutions which could take advantage of both long cycle life of nanomaterials within the group IV elements (Si, Ge, Sn) and high volumetric/gravimetric capacity of micro-materials in the group IV as well as elements (Si, Ge, Sn). This report presents an overview of the recently developed strategies for improving the group IV elements (Si, Ge, Sn)-based anodes performances in LIBs to provide a further insight understanding in designing novel anodes.

1. Introduction

The group IV elements (silicon-Si, germanium-Ge, tin-Sn) have a much higher specific capacities (are 3579 mAh g-1,1600 mAh g-1, 994 mAh g-1, respectively) than that of commercial carbon-based anodes (372 mAh g-1). [1-20] They have been considered the most promising anode candidates for the next-generation LIBs. However, the use of bulk Si, Ge and Sn is hampered by the pulverization of the particles due to the high volumetric change of ~300% (are 297%, 270%, 257%, respectively) during lithiation and delithiation cycles, which leads to particle pulverization and destabilization of a solid electrolyte interphase (SEI) film. These problems result in fast capacity fading and low Coulombic efficiency. In the past 20 years, extensive efforts have been made to improve the electrochemical behavior of the group IV elements (Si, Ge, Sn)-based anodes. The most effective approaches mainly include: (1) reducing particle size to nanoscale for alleviating mechanical strain; (2) forming the hierarchical porous structure in order to provide a stable SEI layer and the inside pore providing adequate space for the group IV elements (Si, Ge, Sn) expansion; (3) dispersing nano-sized the group IV elements (Si, Ge, Sn) in a conductive matrix (such as carbon-based materials) to accommodate volume change and maintain mechanical integrity of the composite electrode; (4) forming amorphous MOx(M= Si, Ge, Sn) with small particle size. (5) narrowing down the voltage window and fixing the lithiation level. (6) using intermetallic alloys with a composite structure that contains an active or inactive host matrix.

In this work, we present an overview of recently developed strategies combined with our research group progress in the group IV elements (Si, Ge, Sn)-based anodes for improving their performances in LIBs, We hope to give a further understand of designing novel high-performance anodes.

2. High-Capacity Si-based anodes

Among these (Si, Ge, Sn) anodes, Si has the highest specific capacity (3579 mAh g-1) at a low charge-discharge potentials of < 0.5 V (vs. Li/Li+), corresponding to form the Li15Si4 phase, which delivers as 10 times higher theoretical specific capacity than that of conventional graphite anode.[10, 21-27] The strategies to overcome the main problems, including low Coulombic efficiency, the unstable cycling life, low conductivity and tapping density, are summarized and discussed in this section.

Firstly, one successful method to alleviate mechanical strain induced by volume change of Si is to minimize the Si particle size to nano-scale, which has been proved to be an effective method. Nano-Si with a higher surface-to-volume ratio increases the irreversible capacity due to the formation of more SEI film. Although the nano-Si does not pulverize during repeated cycles of lithiation/delithiation process, the SEI film still continually breaks/reforms, lowing the Coulombic efficiency. Meanwhile, usually the nano-Si is very expensive of costly synthesis process and has low tapping density, which constrain the nano-Si commercialization.

In 1998, Wang et al. prepared nano-Si by ball-milling using micro-sized silicon mixed with graphite.[28] The nano-sized silicon particles could reversibly insert lithium, increase the capacity of the high lithium alloying capacity of silicon and retained the high reversibility of carbon. In 1999, Li et al. reported nanometer-scale Si

powder as anode in LIBs prepared by laser-induced silane gas reaction method, which exhibited an high reversible capacity(1700 mAhg-1) with better cycling performance than normal micro-sized Si[29]. The slurry weight ratio of Si, carbon black, and PVDF was 4:4:2. The electrochemistry testing indicated the nano-Si worked well even at a high current density. The authors suggested that enhanced performance was attributed to larger surface area, smaller diffusion length, and faster diffusion rate along extensive grain boundaries existing in nanoscale materials.

Secondly, nanostructuring of micro-sized electrode materials is considered as an effective solution to increase volumetric energy density and reduce the irreversible capacity. Hierarchical porous structure with carbon coating was designed in order to provide stable SEI layer and the inside pore provides adequate space for Si expansion.

Recently, Yi et al. successfully prepared porous micro-sized Si-C composite using SiO as the Si source, which demonstrated a high capacity of 1459 mAh g-1 and retained 97.8% of initial capacity after 200 cycles.[30] Micro-sized SiO was heated to form a Si/SiO2 composite of interconnected Si nanoparticles embedded in a SiO2 matrix due to the disproportionate of SiO. After etching, the SiO2 was removed forming a large portion of the original pores. After that, the micro-sized porous Si was coated carbon by thermal decomposition of acetylene. Si and carbon were three-dimensionally interconnected in nano level, which could maintain internal electrical contact and sustain cycling stability.

Li et al. prepared a large (>20 ^m) mesoporous silicon sponge by electrochemical etching of single crystal Si wafers. It delivered a capacity of ~1.5 mAh cm- with 92% capacity retention over 300 cycles.[31] The mesoporous Si sponge had a highly porous structure with thin crystalline Si walls surrounding large pores that were up to ~50 nm in diameter. Authors indicated that the suppression of pulverization and the low volume expansion of mesoporous Si sponge particles could summarized with three factors: (1) pores accommodate the Si volume expansion during lithiation; (2) ~10 nm Si walls are sufficiently thin that they reversibly expand/shrink during lithiation/de-lithiation without breaking; (3)solid surface oxide layer formed at the pore wall surface serves to confine and reinforce the nanostructures.

Kim et al. reported the 3D, porous Si particles, which consisted of bulk sizes greater than 20 |im. It was prepared by simple method using thermal annealing of SiO2 particles and butyl capped Si gel at 900 °C under an Ar stream.[32] The capacity retention of this Si at a rate of 0.2 C was 99% (~2800 mAh g-1) after 100 cycles, while at a rate of 1 C it was 90% . The capacities at a rate of 1, 2 and 3 C were 2668, 2471, 2158 mAh g-1, respectively. This work indicated that the superior rate capability was attributed to the interconnected 3D porous structure which provided fast lithium-ion mobility.

Tian et al. in our group also designed a hierarchical porous structure Si-C anode.[33] In this work, micro-sized (2~10^m) Si/C composites consisting of 20 nm carbon coated secondary Si were synthesized from the abundant and low cost Al-Si alloy ingot by acid etching, ball-milling and carbonization procedures. The nano-porous Si/C composites provided capacity of 1182 mAh g-1 at a current density of 50 mAg-1, 952 mAh g-1 at 200 mAg-1, 815 mAh g-1 at 500 mAg-1, and maintained

86.8% of initial capacity after 300 cycles. The superior rate capability and cycling stability of micro-sized nano-porous Si/C anodes are because it takes advantage of both long cycle life of nano-Si and high volumetric/gravimetric capacity of micro-Si. Al-Si ingot is quite cheap (~$2500/ton). The exceptional electrochemical performance and low cost scalable synthesis provide new diversion for high energy Li-ion batteries development. The synthesis of Si/C composite is schematically summarized in Figure 1. Figures 2a-b presented the electron microscope images of micro-sized porous Si after A1 was etched out. The size was 2~10|im and consisted of -20 nm secondary Si and -15 nm pores. XRD in Figure 2f confirmed that A1 was basically removed from the micro-sized eutectic Al-Si powders (JCPDS #27-1402), which was also confirmed by the energy dispersive X-ray spectra (EDS), Figure 2c). The porous Si formed by etching Al-Si alloys normally showed poor cycling stability, due to the weak connection between Si dendrite in micro-sized porous Si.[34] To increase the connection between the Si particles in porous Si, porous Si was ball milled for 24h. As shown in Figures 2d-f, the -2 |im porous Si (Figure 2d and 2e) aggregated into 5 [j,m primary Si particles that consisting of 200 nm nano-Si cluster formed by -20 nm Si particles. The nano-Si and nano-Si cluster were well connected each other, which would improve the cycling stability. Carbon coating on the micro-sized porous Si could greatly increase the rate performance and cycling stability. The SEM and TEM images of Si/C composite were illustrated in Figures 2g-l. As shown in Figures 2j-k, a layer of carbon with 15 nm thickness was uniformly coated on the nano-Si surface.

In this work, the low cost hierarchical structured porous Si/C anode retained 86.8% of initial capacity after 300 cycles at 500 mAg"1, demonstrating one of the best performances for micro-sized low-cost Si (Table 1).

Table 1. Comparison of electrochemical performance of micro-size Si based anodes in literature

Si-based anodes Si source particle size (|im) Highest capacitances obtained (mAh g"1) Capacity retention Ref.

Si-C composit e Al-Si alloy ingot 2-10 952 mAh/g@0.2Ag_1 86.8% capacity retention over 300 cycles [33]

-92.0%

silicon sponge Si wafer >20 790 mAh/g@0.1Ag_1 capacity retention over 300 cycles [31]

Si-C composit -97.8%

SiO -20 1630 mAh/g@0.4Ag_1 capacity retention over [30]

e 200 cycles

-99.0%

porous c-Si SiCl4 >20 2800 mAh/g@0.4Ag_1 capacity retention over 100 cycles [32]

multi-di Si 5-8 -2400m Ah/g@~0.4 Ag"1 -95.0% [35]

mension al Si-C powder (30|m, 99.9%) capacity retention over 70 cycles

3D micropor ous Si-C Si powder (10|m, 99.9%) ~7 ~2500 mAh/g@0.2Ag-1 87.0% capacity retention over 50 cycles [36]

C-Si nano-co mposite SiH4 15-35 ~1950 mAh/g@ ~0.2Ag-1 — [1]

(Si-SiO-SiO2)-C composit e SiO (325 mesh) Micro size ~1280 mAh/g@ 0.2C 99.5% capacity retention over 200 cycles [37]

graphene /Si-C composit e SiO (2|m) Micro size 1100 mAh/g After 100cycles, ramains capacity of 3.2 mAh/cm2 [38]

prickle-li ke Si@C Si powder ~5 1980 mAh/g@0.1Ag-1 65.0% capacity retention over 100 cycles [39]

Si-C composit e Triethox ysilane Micro size ~1600 mAh/g@0.4Ag-1 ~90.0% capacity retention over 150 cycles [40]

Si/C composit es Si powder (1-2|m, 99.99%) Micro size 1860 mAh/g@0.1Ag-1 ~68.9% capacity retention over 100 cycles [41]

Graphen e sheet-wr apped Si Si powder 1-5 1525 mAh/g@0.2Ag-1 After 30 cycles, ramains capacity of 1500 mAh g-1 [42]

Si-C composit e silicon powder( <78|m) Micro size 620 mAh/g@ 0.5C After 50 cycles, ramains capacity of 525 mAh g-1 [43]

Si@ self-healing polymer (SHP) silicon micropar ticles(3-8 lm) Micro size 2617 mAh/g@0.4Ag-1 80.0% capacity retention over 90 cycles [44]

The doping approach has been proven useful for silicon anode materials, for example Al, Fe and B.[33, 45] The mechanism of conductivity enhancement for Al and Fe doping in Si was investigated using the first-principle calculations. As shown in Figure 3a, there are two sites for Al, Fe doping: silicon site (Si) and vacancy site (V). Al, Fe tended to co-doped silicon and form Al-Fe pairs (Al substituting the Si site and Fe occupying the vacancy site)[46]. Thus, Fe would mainly exhibit the Fe-Al co-doping pairs in the silicon sample. Figure 3b and 3 c showed the corresponding

PDOS (atom-projected density of states) for spin-up and spin-down state of Al-Fe co-doped silicon, respectively. It could be seen that the spin-up state exhibited obvious impurity states near the Fermi level and the activation energy was about 0.2 eV, while the spin-down state affected less the edges of the Fermi level for doped silicon. The emerging impurity excitation was more favorable to increase the carrier concentration than intrinsic excitation, leading to higher electrical conductivity. On the other hand, the Al content was much higher than that of Fe, and the excessive Al atoms would form other point defects in the doped silicon sample. The PDOS of Al substituting the Si site and Al occupying the vacancy site were plotted in Figure 3d and 3e. Al substituting the Si site and Al occupying the vacancy site would move the Fermi level toward the conduction bands and valence bands, respectively, which could increase the electron/hole concentration in systems, although no impurity states were introduced. Another benefit for partial Al-doping was that it could effectively prevent expanding of Si lattice (C (~10%), Al (~94%) and Si (~280% for amorphous Si), respectively, due to their different Li uptake capacities).[47] Therefore, the low concentration Fe and Al impurities could increase the electrical conductivity, reduce the lattice expansion of silicon sample, and improve the performance of Si as anode materials in LIBs.

Third, one-dimensional(lD) Si nanomaterials possess efficient electron transport and allow lateral relaxation and reduced mechanical, leading to improvements in cycling stability, thus making them promising as anodes for high performance Li-ion batteries. Chan et al. synthesized silicon nanowires by vapour-liquid-solid process on stainless steel substrates using Au catalysts.[2] The Si NWs also displayed high capacities at higher currents. Even at the 1C rate, the capacities remained >2,100 mAh g-1. Silicon nanowire can accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances.

Ge et al. reported silicon nanowires synthesized by direct etching of boron-doped silicon wafers.[48] The reported porous silicon nanowires exhibited superior electrochemical performance and long cycle life. Even after 250 cycles, the capacity remained stable above 2000, 1600, and 1100 mAh g-1 at current rates of 2, 4, and 18 A/g, respectively. Porous silicon having a large pore size and high porosity could maintain its structure during long-term lithiation/delithiation process. Meanwhile, this work indicated that boron doping could increase electron conductivity in silicon, which would help to reach a high capacity at high current rates. On the other hand, the alginate as a binder, due to its high viscosity, could further improve the structural stability during long-term cycling.

Liu et al. reported a novel scaffold of hierarchical silicon nanowires-carbon textiles anodes fabricated via a CVD method.[49] The hierarchical structure Si NWs-carbon electrode exhibited high capacity (2950 mAh g-1 at 0.2 C) and long cycle life (200 cycles). The excellent performance could be ascribed to: (1) 1D Si nanowires are beneficial to the insertion/extraction of Li+; (2) the Si nanowires coated by carbon facilitates lithium-ion/electron transport; (3) the current silicon nanowires/carbon textiles matrix can obtain outstanding electronic conductivity.

Xue et al. prepared a new electrode composed of Si/C composite nanofibers using

electrospinning method.[50] The mass loading of the active material was about 2 mg cm- . The cycling stability of the Si@C/CNF mat was improved and the capacity retention was increased to 92% in the first 15 cycles after by carbon coating. It was ascribed that the Si nanoparticles with a carbon layer enhanced the electric connection and bonding between Si particles and the fiber matrix.

Xu et al. synthesized a novel flexible 3D Si/C fiber paper anode.[51] The flexible 3D Si/C fiber paper electrode demonstrated a very high overall capacity of -1600 mAh g-1 with capacity loss less than 0.079% per cycle for 600 cycles and excellent rate capability. The overall capacity still retained -500 mAhg-1 at a high current density of 8 A g-1. The excellent performance was ascribed to the unique architecture of the flexible 3D Si/C fiber paper including the resilient and conductive carbon fiber network matrix, carbon-coated Si nanoparticle clusters, strong adhesion between carbon fibers and Si nanoparticle clusters, and uniform distribution of Si/C clusters in the carbon fiber frame.

Wang et al. prepared porous single-crystal silicon nanowires via an electroless HF/AgNO3 etching process. The single-crystal silicon wafers [p-type, 0-100 Qcm, (100)] were used as the silicon source.[52] In this work, the formed Ag nanoparticles were etched, and porous nanowires were formed (Figure 4). Si nanowires with carbon black (NW + CB) as the conductive additive showed an initial capacity of 1,066 mAh g-1, as shown in Figure 5a. The initial Coulombic efficiency was 49%. The capacity on the second cycle was 1,256 mAhg-1; after 20 cycles, it was 815 mAhg-1, i.e., 76% of the initial value. Capacity retention was much better than that of solid nanowires. In the latter case, the capacity dropped by ~50% after 10 cycles. Furthermore, graphene nanosheets, massless charge carriers with high mobility were employed as effective 2D conductive additive to assure the high performance of Si nanowires. The NW+G (Si nanowire/graphene) anode had an initial capacity of 2,347mAhg-1 (Figure. 5a). The Coulombic efficiency was 64% on the first cycle and quickly increased upon cycling, reaching close to 100%. Graphene could enhance the rate performance of Si nanowires. In this research, the electrochemical activity of Si nanowires with graphene was better than that of those with carbon black.

Overall, Si-based materials have been considered one of the most promising alternatives as anodes in LIBs. The main challenges for the practical implementation of Si-based anodes are the high volumetric change of ~300% during long-term lithiation and delithiation process and the unstable SEI films. However, breakthroughs have been achieved by the advanced nanotechnologies. Substantial process is needed to improve the first Coulombic efficiency, capacity retention and rate performance. It is believed that nanostructuring of micro-scale electrode materials is considered as an effective solution to increase volumetric energy density and reduce their reversible capacity. But it remains a challenge to develop a facile approach simultaneously solve the practical problems of Si-based anode materials including low first Coulombic efficiency, capacity retention, unsatisfied rate performance and fabrication costs.

3. High-Capacity Ge-based anodes

Compared with Si, Ge has attracted less attention than Si because of its relatively

higher cost. Nonetheless, Ge exhibits higher electronic conductivity (104 times) and lithium ion diffusivity (400 times) at room temperature than Si, which could exhibit higher rate performance as anode in LIBs.[53-66] However, similar to other Li alloy anodes, Ge-based anodes are hampered by the pulverization of the particles due to the high volumetric change of -300%. To overcome those problems, many strategies have been utilized, mainly including: (1) using carbon as a buffer layer to mitigate the considerably large volume expansion; (2) decreasing the size of Ge-based particles to nanoscale; (3) adopting 3D porous structure in order to minimize and accommodate the volume changes of the Ge during lithiation/delithiation. (4) forming amorphous GeOx with small size.

Table 2. Comparison of electrochemical performance of Ge-based alloy anodes in literature

Electrodes Particle Initial Reversible Capacity Ref.

size Coulombic efficiency capacity (mAhg1) retention

Ge02/Ge/C 30 |j,m 82% 1860 mAhg"1 90% over [62]

Nano composite @ 1C 50 cycles

Three-Dimensional 700 nm 80.3% 1131 mAhg"1 96.6% [67]

Microporous Ge @ 1C after 200 cycles over 200 cycles

Ge/C Nanowires 50-100 nm 82% 1200 mAh g"1 @ 0.2C after 50 cycles 84% over 50 cycles [54]

Graphene/Ge Nanowire -46 nm 69% 1059 mAhg"1 @ 4C after 200 cycles 90% over 200 cycles [68]

Amorphous Hierarchical Porous 3.7 nm 70% 1268 mAh g"1 @ C/2 after 96.7% over 600 [69]

GeOx 600 cycles cycles

mesoporous Ge@C sphere 500 nm 87% 1099 mAhg"1 @ 0.1 Cafter 100 cycles [70]

Ge-carbon hybrid 200 nm 76% 895 mAh g"1 88% over [71]

nanoparticles @ 2C after 2000 cycles 2000 cycles

Ge@C/RGO 10-15 52% 940 mAh g"1 @ SOmAg"1 after 50 cycles — [72]

nanocomposite nm

Ge-RGO-CNT 20-30 79.7% 863.8 mAhg"1 — [73]

composites nm @ 100 mAg"1 after 100 cycles

nano-GeCh 5-20 — 452 mAh g"1 93% over [74]

/mesoporous carbon nm @ 1C after 380

composite 380 cycles cycles

Ge02/RG0 30 nm 69% 1150 mAhg"1 — [75]

composite @ 1C after 500 cycles

Ge@graphene/VAGN hybrids 42 nm 1014 mAh g-1 @ 260 mA g-1 after 90 cycles 97% over 90 cycles [76]

Electrochemical performances of reported Ge-based alloy anodes are summaried in Table 2. Xue et al. reported a facile method preparing Ge@C/RGO nanocomposite, which was used as an anode in LIBs.[72] After 50 cycles under a current density of 50 mAg-1, the Ge@C/RGO nanocomposites still retained a reversible capacity of ~940 mAh g . Even under the very high current density of 3600 mAg , the Ge@C/RGO nanocomposites still exhibited a high specific capacity of 380 mAh g-1. The authors indicated that this excellent performance could be ascribed to: (1) carbon shell acts as a buffer that plays a role to minimize volume changes; (2) RGO networks serve as the elastic and electronically conductive substrate. It affords good dispersion of the NPs and guarantees a high electrical conductivity of the overall electrode.

Park et al. reported an amorphous GeO2/C composite without carbon black as an electrode for Li-ion batteries.[77] The amorphous GeO2/C electrode exhibited excellent electrochemical stability with a 95.3% charge capacity retention after 400 cycles. The authors also checked the practical applicability in full cell. The capacity of the full cell was reduced by only 7.6% after 50 cycles.

Cho et al. synthesized Ge nanoparticle from 0D Hollow to 3D Porous structure prepared by etching a thermally annealed physical mixture of SiO2 and ethyl-capped Ge gels at 800 °C.[78] The first discharge capacities of the 0D and 3D could reach 1428 and 1380 mAh g-1, respectively. The 3D porous Ge retained almost 99% of its capacity after 100 cycles at 1C rate.

Li et al. reported a novel approach via reduction and carbonization of germanium chelate synchronously to in situ forming uniform Ge with a size of about 30 nm dispersed uniformly in a carbon buffering layer.[57] This anode exhibited cycling capacity (ca. 895 mAh g-1) over 2000 cycles at a high rate of 2 C. The exceptional performance was attributed to the interconnected carbon buffering network and mesoporous structure.

Wang et al. reported 3D hierarchical porous structure amorphous GeOx with very long cycle life (600 cycles) and high capacity (1250 mAh g-1).[69] The high-capacity GeOx with its very long cycle life was ascribed to four beneficial characteristics: small primary particles, amorphous state, porous structure, and the incorporation of oxygen. Firstly, the primary small size could tolerate the change in volume, because the size of the particles was only ~3.7 nm (Figure 6). Second, the electrical contact could be maintained because of the absolute volume change of each primary particle was small. Third, smallness may enhance electrochemical activity by shortening charge-transportation distances and offering more surfaces for charge transfer. In summary, the small primary particles were the basis for the hierarchical porous structure's stability and integrity upon high-capacity cycling. It also indicated that the amorphous state was another important factor. Ultrafine crystallites (<10 nm) still showed fading capacities that might be related to the harmful anisotropic

expansion/extraction, and the stresses from phase transitions occurring in crystalline anode. Meanwhile, the hierarchical porous structure accommodated such alterations and preserved the intactness of the micron-sized GeOx agglomerates, thus preventing changes in the microscopic charge-transport pathway and the particles' locations whilst maintaining the electrical contact. In Figure 7, the GeOx has a very stable capacity of ~1250 mAh g and retained its capacity very well for 600 full charge/discharge cycles.

The initial GeOx was evaluated in full-cell configuration, a very challenging key step toward real-world applications. As shown in Figure 8, it obtained an initial Coulombic efficiency of 99.5% with an open-circuit voltage (OCV) of 0.74V. After that, Li(TNriCoMnji/302(NCM) was used as the active cathode in full cells. As shown in Figure.8, a full cell discharged 164 mAh(g of NCM)-1 at C/20 (based on LifNiCoMnji/30^). The full cell exhibited stable charge/discharge cycling; and the average capacity loss over 200 cycles at C/2 was only 0.028% per cycle. Therefore, the initial GeOx anode in a full cell had excellent reversibility and stability.

Compared with the silicon, Germanium has good lithium diffusivity and high electrical conductivity. However, the practical usage of Ge-based material as an anode is also hindered by dramatic volume changes caused by insertion/extraction of lithium ions, which results in crack and pulverization, and loss of electrode contact. Furtherly, because of its relatively higher cost, the performance of the Ge-based anodes still do not meet the commercial demands. The steadily improving performance of Ge-based anodes was attained through nanostructuring, carbon coating, porous and amorphous structuring et.al. To gain commercial success, continued fundamental advances about the preparation, engineering and fabrication need to execute for overcoming the technology barriers in high energy density LIBs.

4. High-Capacity Sn-based anodes

Since Idota et al. showed that a tin-based amorphous oxide had a reversible Li-ion storage capacity > 600 mAh g"1, [79] offering specific volume capacity > 2200 mAh cm" corresponding to more than twice that of existing state-of-the art carbon materials, much attention has been focused on high capacity Sn-based anode materials.[80-95] In 2005, Sony's NP-FP71 lithium-ion batteries using the anode consisting of a Sn-Co based alloy (Sn:Co = -1.1:1 mol, with possible titanium of -5%) and graphite were commercialized. The particle sizes of the primary particles and the aggregated secondary particles for anode composite were -5 nm and 1 [j,m, respectively. [96] The novel battery system, which alleviated the large volume change (257%) upon lithium insertion/extraction causing the pulverization of the particles and loosing electrical disconnection within the anode, was representative of the understanding tin alloys as anodes with high performance and exploring ideal second metal element.

Wang et al. successfully synthesized a series of M-Sn (M=Fe, Cu, Co, Ni) nanospheres[97] with size of 30-50 nm by a conversion chemistry,[98-100] which could rigorously control both the shape and the size of these nanoparticles for comparing their different performances. The theoretical capacities are CoSn3 (852

mAh g l) > FeSn2 (804 mAh g l) > Ni3Sn4 (725 mAh g l) > Cu6Sn5 (605 mAh g 1). However, the practical value of the alloy compounds could be listed in the following order: FeSn2 > CueSns ^ CoSn3 > Ni3Sn4. The higher capacity of FeSn2 among these intermetallic nanospheres could be ascribed to open channels located within the FeSn2 crystal lattice which promotes the penetration and alloying with Li in the Sn host.

During the synthesis of the Fe-Sn system, a reborn FeSns phase, that is not in the Fe-Sn phase diagram representing a new binary structure type, was discovered as shown in Figure 9. [101] The crystal structure including lattice parameters, thermal factors, atomic coordinates, and occupancies was solved using the charge-flipping method. Differing from room-temperature FeSn and FeSn2 phases, and high-temperature FesSn3 and Fe3Sn2 phase, new FeSns phase belongs to tetragonal in the P4/mcc space group. Fe vacancies always exist in the pure intermetallic FeSm, implying strongly that the stable FeSm structure could maintain in the condition of existing large numbers of Fe vacancies. Structurally, the antiprisms form only weakly interconnected a one-dimensional (ID) network along the c-axis in FeSm, leading to strong quasi-lD characteristics, drastically different from the 3D network of FeSn2. The Feo.74Sn5 nanospheres (-45 nm) possesses the highest theoretical capacity of 929 mAh g 1 among the reported M (electrochemically inactive metal)-Sn intermetallic anodes by then.

During the preparation of the Co-Sn system, an almost identical crystal structure as FeSns binary intermetallic phase was achieved by CoSns due to similarity between Fe and Co.[102] The Coo.83Sn5 (Co-deficient CoSns) anode demonstrated one of best-in-class theoretical capacity of 917 mAh g 1 among the existing Sn-M alloys. The cell exhibited very good capacity retention (ca. 450 mAh g-1) over 100 cycles at a rate of 0.05 C. The increase in cycle capacity early in the cycling of the Coo.83Sn5 alloy could be ascribed to (i) an activation process of anode (especially for high capacity electrodes with large volume expansion/shrinkage) in initial few lithiation/delithiation cycles (ii) the formation and stabilization of the SEI (iii) the improvement of Li uptake/removal kinetics (iv) an electrode structural readjustment upon lithium insertion/extraction (v) the creation of new electrical contacts between nanospheres/ carbon black (vi) the reaction of a thin oxide shell with lithium.

Fe and Co atoms could co-exist in the MSm structure, forming Feo.5Coo.5Sn5 ternary structure, which had the highest theoretical capacity of 931 mAh g 1 to date among the reported Sn-based ternary intermetallic anodes.[103] Monodisperse 30-50 nm MSn5 (M=Fe, Co and FeCo) nanospheres were obtained by using Sn nanospheres as templates by conversion chemistry strategy. The formation mechanism of FeSm, Feo.5Coo.5Sn5, CoSn5 nanospheres was schematically summarized in Figure 10 a. In this work, 30-50 nm tin nanospheres were synthesized in a three necked flash by adding of SnCl2 into a hot tetraethylene glycol (TEG) solution containing surface stabilizers (PVP and PEtOx) at 170°C, followed by reduction with sodium borohydride (NaBFD.With the increasing of the temperature of Sn mixture to 205°C, 200°C, 195°C, followed by the adding of FeCl3/TEG, FeCl3+ CoC12/TEG, CoC12/TEG solution, precipitates metal Sn nanocrystals had been converted to

intermetallic compounds FeSns, Feo.5Coo.5Sn5, CoSns NCs, typically via diffusion-based processes where Fe/Co diffused into Sn. The similar particle distribution of these three compounds was demonstrated using electron microscope images in Figure 10 b, e, h. HRTEM images revealed that all nanospheres had core-shell structure consisting of a -30 nm single-crystalline intermetallic core and a ~4 nm amorphous oxide shell (Figure 10c, f, i). The STEM EDS elemental mapping images clearly demonstrated homogeneous distribution of transition metals (Fe or Co) and Sn in the nanospheres with the ratios 7:1, 7:1, and 6:1 for the Sn/Fe, Sn/Fe+Co respectively as evidenced by TEM-EDS.

The reversible reactions mechanism during the first lithiation/de-lithiation cycle for these compounds could be described as:

xLi+Mj.Sns^U.MySnstM =Fe, Co or FeCo, 0.7- 0.3 V); (1)

zLi + UvM,Sn5 <->Li.v.zSn5 + yM (x + z < 22, 0.3-0.01 V); (2)

At the first discharge process(Figure 11), the formation of Li-Sn alloy could be responsible for the reversible capacity up to the theoretical limit of LizuSn. Further charge to 1.5 V, the reformation of MSns phase was attributed to high reversibility, differing from known Fe-Sn or Co-Sn phases. FeSns anode exhibited a high specific capacity of 750 mAh g 1 along with dramatic derogation after 15 cycles due to gradually separation of Sn from Fe aggregated into large particle in FeSm phase. After 100 cycles under a current density of 0.05 C, the CoSns intermetallic nanospheres still retained a reversible capacity of -460 mAh g 1 which was attributed to alloying of Sn and Co although the ratio of Sn to Co decreases from 6:1 to 4:1. Feo.5Coo.5Sn5 phase can take advantages of high capacity and cycling life, providing 736 mAh g"1 and maintaining 92.7% of initial capacity after 100 cycles with an average capacity loss of only 0.07% per cycle. The performance of reported Sn-based alloy anodes are summarized in Table 3.These works provide insight towards exploring and designing new Sn-based alloy anode materials for Li-ion batteries.

Table 3. Comparison of electrochemical performance of Sn-based alloy anodes in literature

Sn-based alloy anodes method particle size (11111) Highest capacitances obtained (mAh g"1) Capacity retention Ref.

M-Sn (M = Fe, Cu, Co, Ni) conversion chemistry -40 -510,350,280, 250 mAh/g @0.05C -94.1,71.4, 94.6, 76% capacity retention 100 cycles [97]

FeSn5 conversion chemistry 30-50 768 mAh/g @0.05C 78.9% capacity retention ¿520 cycles [101]

C0S115 conversion chemistry 30-50 548 mAh/g @0.05C -81.8% capacity retention over Ïï 100 cycles [102]

Feo.5Coo.5Sn5 conversion chemistry 30-50 736 mAh/g@0.05C -71.0% capacity retention ^100 cycles [103]

Cu.Siis chemical reduction -40 815 mAh/g i/0.1 mA cm"" -33.0% capacity retention 100 cycles [104]

Cu6Sn5-Sn electrodeposit ion Micro size 1020 mAh/g(5>,0.08 mA cm " -34.3% capacity retention 5 5 cycles [105]

Cu.Siis electron beam deposition -100 720 mAh/g@50 mAg"1 -76.4.0% capacity retention ^15 cycles [106]

Sn2Fe:SnFe3C ball-milling -10-20 -650 mAh/g@37 mAg"1 48.3% capacity retention ¿540 cycles [107]

FeSn2 chemical reduction 30-70 -635 mAh/g@80 mAg"1 78.7% capacity retention ^20 cycles [108]

Feo.5Coo.5Sn2 reduction -20 -750 mAh/g@80 mAg"1 73.3% capacity retention ¿530 cycles [109]

[Sll0 55COo 45]l-v Cv sputtering — 700 mAh/g@C/12 — [110]

S1131C028C41 ball milling Nano Size -500 mAh/g c/0.25 C 100.0% capacity retention 5 100 cycles [111]

Sn-Co-C mechano-chemical -50 478 mAh/g@100 mAg"1 -91% capacity retention 5 100 cycles [112]

Ni3Sn2 electron beam deposition 10 800 mAli/cm3@30 uA cm"2 -100% capacity retention 5500 cycles [113]

Sn-Ni electrodeposit ion Nano Size -800 mAh/g@250 mAg"1 -82.5% capacity retention 5=* 70 cycles [114]

Ni3Sn2 solvothennal route 2-5 -771 mAh/g@0.2 C -90.3% capacity retention 5^400 cycles [115]

By introducing carbon matrices with high electrical conductivity, good mechanical stability and the ability to store lithium for Sn electrode is one of successful methods to effectively accommodate the strain caused by the volume expansion/shrinking and provide good electronic conductivity for overall electrode. Graphite, carbon nanotubes (CNT), graphene, ordered mesoporous carbon and amorphous carbon are usually exploited as conductive network and an inert for Sn-based materials, offering many different Sn/C composite anode including Sn@carbon nanoparticles in bamboolike hollow carbon nanofibers,[116] Sn@C@CNT nanostructures,[117] graphene-confined Sn nanosheets,[118] 3D hollow Sn@carbon-graphene hybrid material[119] et.al.

In 2013, Wang's group reported nano-Sn/C composite with uniformly dispersed 10 nm nano-Sn within a spherical carbon matrix using facile and scalable aerosol spray pyrolysis technique.[120] The nano-Sn/C composite sphere exhibited excellent electrochemical stability with charge capacity of 710 mAh g 1 after 130 cycles at 0.25

C. Even at a high rate of 20 C, discharge capacity could maintain high rate performance (~600 mAh g-1). The exceptional performance of nano-Sn/C anodes was attributed to the carbon matrix to accommodate the stress, prevented Sn nanoparticle agglomeration and provided continuous path for charge transfer. In 2013, Zhu et al. reported ultrasmall Sn nanoparticles (~5 nm) embedded in nitrogen-doped porous carbon network by carbonizing of Sn at 650 °C under Ar atmosphere.[121] The initial discharge capacity of Sn/C anode could reach 1014 mAh/g with 71.2% capacity retention over 200 cycles at the current density of 0.2 A/g. At higher current density of 5 A/g, the reversible capacity could still retain ~480 mAh/g. The remarkable electrochemical performance can be summarized as follows: ultrasmall Sn nanoparticles, homogenous distribution, and porous carbon network structure. 2014, Qin et al. fabricated 5-30 nm Sn nanoparticles anchoring 3D porous graphene networks encapsulated in 1 nm graphene shells by using NaCl particles as a template with a three-dimensional (3D) self-assembly and metal precursors as a catalyst.[122] The 3D hybrid anode showed excellent rate performance that 1022 mAh g-1 at 0.2 C, 865 mAh g-1 at 0.5 C, 780 mAh g-1 at 1 C, 652 mAh g-1 at 2 C, 459 mAh g-1 at 5 C, and 270 mAh g-1at 10 C could be obtained. At high rate (2 A/g), the anode had extremely long cycling stability that high capacity could reach 682 mAh g-1 and maintain 96.3% charge capacity retention after 1000 cycles

Xin et al. synthesized Fe074Sn5@RGO nanocomposite which could achieve capacity retention 3 times than that of the nanospheres alone after 100 charge/discharge cycles by a one-pot wet chemistry synthesis.[123] The excellent electrochemical performance of Fe074Sn5@ RGO nanocomposite could be related to the following characteristics: (1) the flexible and conductive RGO sheets offers mechanical support to accommodate the stress from large volume change during charge/discharge process; (2) RGO sheets prevent Fe074Sn5 nanoparticle agglomeration can relieve irreversible aggregation and/or stacking of individual RGO nanosheets. (3) RGO sheets provide good electrical conductivity (4) large voids between the nanoparticles and RGO sheets promote easy penetration of the electrolyte. Similar with Si and Ge anode, the small size of primary particles and porous structure can effectively alleviate mechanical strain resulted from volume fluctuation.[124, 125]

We summarized the general method to address the problems of Sn-based anodes, including: (1)using Sn-M or Sn-M-C alloys(M= Fe, Cu, Co, Ni); (2)reducing the particle size of Sn to nanoscale; (3)intergrating nano-Sn with a conductive matrix such as carbon. Especially, among M-Sn intermetallics, sustained effort has been devoted to develop high capacity Fe-Sn, Co-Sn, Fe-Co-Sn systems in the academic and industry. It is quite promising that continual development of these and other strategies will promote the practical applications of Sn-based anodes material in LIBs. 5. Conclusions

The group IV elements (Si, Ge, Sn)-based anodes have been considered very promising anodes in the next-generation LIBs due to their high capacities. In the past 20 years, great efforts have been made in exploring group IV elements (Si, Ge, Sn) based-anodes with high energy/power density, good cycling stability, environmental

friendliness, and low cost for practical applications. However, the use of bulk Si, Ge and Sn is always hampered by the pulverization of the particles due to the high volumetric change during the lithium insertion/extraction process, which causes electrode agglomeration, pulverization, and thus fast capacity fading. In this article, we not only review the main problems of group IV elements (Si, Ge, Sn)-based materials as anodes in LIBs, but also introduce the main solutions, especially using our group works as examples. The main solutions are summarized as: (1) reducing particle size to nanoscale for alleviating mechanical strain; (2) forming the hierarchical porous structure in order to provide stable SEI layer and the inside pore to provide adequate space for (Si, Ge, Sn) expansion; (3) using carbon as a buffer layer to accommodate volume change and maintain the mechanical integrity of the composite electrode; (4) narrowing the voltage window and fixing the lithiation level. (5) forming amorphous MOx(M= Si, Ge, Sn) with small size. (6) using intermetallic alloy with a composite structure that contains an active or inactive hostmatrix.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51371186), the "Strategic Priority Research Program" of the Chinese Project Academy of Science (Grant No. XDA09010201), Zhejiang Province Key Science and Technology Innovation Team (Grant No.2013TD16), Ningbo 3315 International Team of Advanced Energy Storage Materials, and Ningbo Natural Science Foundation (Grant No. 2014A610046).

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Huajun Tian is currently working as a research fellow in Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences. He received his B,S. degree in China University of Geosciences(Wuhan) and earned his Ph.D. degree in Materials Physics and Chemistry from Institute of Plasma Physics, Chinese Academy of Sciences in 2013. From 2012 to 2014, he conducted postdoctoral research in Professor Weiqiang Han's research group at NIMTE, Chinese Academy of Sciences. His research interests are energy storage devices, including Lithium-ion Batteries and Sodium ion Batteries.

Fengxia Xin is currently a PhD candidate at Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences. She obtained her BS degree in China Jiliang University in China in 2012. Her research interests are mainly on nanomaterials for energy storage in Li-ion and Na-ion batteries.

Xiao-Liang Wang is currently a R&D Manager at Seeo Inc. Before coming to Seeo, he was a research associate at Brookhaven National Laboratory (BNL). At BNL he was using nanostructuring as a tool to explore the synthesis of high-performance electrode nanomaterials for lithium batteries and to study fundamental mechanisms. He has 12 papers as the first author and more than 600 citations. He is an inventor on 4 US patent applications and 2 Chinese patents.

Wei He is currently a M.S. candidate under supervision of Prof. Wei-Qiang Han at Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences. He received his B.S. degree from Department of Materials Science and Engineering at Zhejiang University in 2013. His research mainly focuses on Si-based anode materials for energy storage devices in Li-ion batteries.

Wei-Qiang Han is currently a professor and the director of Institute of New Energy Technology at the Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences. He has published more than 100 paper in peer-reviewed journals. He has been developing novel nanomaterials for the applications of renewable energies, especially for advanced lithium batteries and catalysts.

Figure caption

Al-Si alloy Porous Si Ball milled porous Si Si/C

Figure 1. Schematic illustration of the preparation process from Al-Si alloy to the Si/C composite. (Reprinted with permission from Tian et al. [33]. Copyright 2015, Elsevier)

Figure 2. Typical (a), (b) SEM images and (c) EDS of the prepared etched preliminary Si. (d), (e) SEM images and (f) EDS of ball milled etched Si. (g), (h), (j) SEM images and (i) EDS of Si/C composite. (k) HRTEM image of Si/C composite, (l) the selected area electron diffraction (SAED) patterns of the core in Si/C. (Reprinted with permission from Tian et al. [33]. Copyright 2015, Elsevier)

tt 20 r\

1 1 1 1 1 i

•0.5 0.0

Energy (eV)

tt 20 /■s

1 ■ 1-- T T I T I 1 -Fe 1 -Al 1 1 -Si

-r-Ni wA. .Ju/Sfc

-1.0 -0.5 0.0 0.5 1.0

II 1 1 1 L (c> 1 I 1 1 1 1 l -Fc 1 -Al i _c;

1 N k: . .JJM

-1.0 -0.5 0.0 0.5 1.0

X s II v

•0.5

0.0 0.5 Energy (eV)

Figure 3. (Color online) The whole atomic is a cubic 3 x 3 x 3 supercell in first-principle calculations, (a) is part of the atomic model for doped silicon, and (b), (c), (d), (e) are the atom-projected density of states (PDOS) for Al, Fe codoping spin-up state, spin-down state, Al substituting the Si site (Si), and Al occupying the vacancy site (V), respectively. (Reprinted with permission from Tian et al [33]. Copyright 2015, Elsevier)

Figure. 4 Porous single-crystal Si nanowires. a SEM image of the wafer; b XRD patterns; c TEM image; and d SAED pattern along the [011] direction [15] (Reprinted with permission from Wang et al. [52]. Copyright 2010, The American Chemical Society)

% 4000 ■

< 3000E

^ 2000< I 1000-

Graphite

—I-'

10 Cycles

~~r~ 15

100 Ï

o c o>

Ö jb-Hi

300 I 250

Graphene Carbon Black

^H * M M W

0,5 Rate / C

Figure 5. (a) Charge capacities and Coulomb efficiency of cells for 20 cycles. NW: Si nanowires; G: graphene; CB: carbon black; NP: Si nanoparticles. We removed the contribution to capacity from graphene and carbon black (b). (c) Rate capacities from 0.1C to 2C, with the rate for discharge fixed to 0.1C. (Reprinted with permission from Wang et al. [52]. Copyright 2010, The American Chemical Society)

c -J 20k-

o 15k-

U) 10 k-

<D 5k-

(b) r-

* * 200 nm '

10 20 30 2e / degree

9 25 < 2.0

(©) / -GeO

■ I 1 \ — - Ge ref. (\----Ge02 ref.

1 I1 A

—>—r

2 3 4 5 6 R/A

Figure 6 Nano- and atom-structure of the initial GeOx. a-c Hierarchical nanoporous structure, with a Low-magnification SEM image, b Enlarged SEM image corresponding to the area enclosed by a square in (a) and c TEM image corresponding to the area in the square in b d Synchrotron XRD profile (A=0.72958 A). e Synchrotron EXAFS profile. Powder Diffraction File (PDF) peaks of Ge (#00-004-0545) and GeO2 (#01-085-0473), and synchrotron EXAFSs of Ge and GeO2 references, respectively, also are shown in (d) and (e) (Reprinted with permission from Wang et al. [69]. Copyright 2011, The American Chemical Society)

T 2,500 ™ 2,000-1

Initial

300 °C 700 °C

9 1.0-1

200 400 Cycle number

2000 Capacity / mAh g 1

Figure.7 Anode performance in half cells. a Delithiation capacities.b Charge-discharge curves.(i) The first cycle of the initial GeOx (C/20). (ii) The second cycle of the initial GeOx (C/5). (iii)The first cycle of the 700 °C sample (C/20). (iv) The second cycle of the 700 °C sample (C/5) (Reprinted with permission from Wang et al. [69]. Copyright 2011, The American Chemical Society)

>4.0 - 3.6

£3.2 "Ö 2.8 >2.4H 2.0

Full cell Li/Li(NiCoMn)

0 40 80 120 160 200° Capacity / mAh g

40 80 120 160 200 Cycle number

Figure. 8 Cell performance of the initial GeOx anode. a Initial profiles of the Li-compensated GeOx/NCM full cell in comparison with those of the Li metal/NCM half-cell. b Reversible battery discharge capacity of NCM in the full cell (Reprinted with permission from Wang et al[69]. Copyright 2011, The American Chemical Society)

26 / deg. (c) FeSns [001]

(d) FeSn2 [001]

5.91 6.54

- 5.90

■' T ' I ' I T ■' I ' T ' T

0 60 100 150 200 250 300 350

6.53 -

- 5.33

- 5.32

- 5.31

1—i—|—i—|—i—p-i—|—i—|—i—r

0 50 100 150 200 250 300

equilibrium position

potential / a.u.

bond length / a.u.

bond length / a.u.

Figure 9 (a) Synchrotron XRD pattern and Rietveld refinement of Fe0.74Sn5. Black dots, observed profile; red line, calculated profile; blue line, difference profile; and olive line, background, with the inset illustrating the crystal structure. (b) The crystal structure of FeSn2, in which the color designation is the same as (a). (c) Fe0.74Sn5 and (d) FeSn2 crystal structures from [001] view direction. (e and f) The variation of lattice constants with temperature, (e) Fe074Sn5 and (f) FeSn2. (g and h) The illustration of bond energy diagrams showing the influence of temperature on the thermodynamic equilibrium lattice parameters a and c, (e) Fe074Sn5 and (f) FeSn2. (Reprinted with permission from Wang et al [101]. Copyright 2011, The American Chemical Society)

(a) SnCl2 NaBH4 Fei+/CoI+

PEtOx 7 ' ^

pyp Sn MSn5 (M=Fe or/and Co)

Figure 10 (a) Synthesis process for FeSn5, Fe0.5Co0.5Sn5 and CoSn5 nanospheres; (b, e, h) SEM; (c, f, i) HRTEM; and (d, g, j) STEM-EDS elemental mappings images of FeSn5, Fe0.5Co0.5Sn5 and CoSn5 nanospheres (Reprinted with permission from Xin et al [103]. Copyright 2015, Royal Society of Chemistry)

Figure 11 (a, d, g) Charge and discharge curves of the FeSn5, Feo.5Coo.5Sn5 and CoSn5 nanospheres electrode for the first cycle at a current density of 0.05 C; (b, e, h) Synchrotron ex situ XRD patterns of at different potentials during discharge and charge processes of FeSn5, Fe05Co05Sn5 and CoSn5 nanospheres electrode and (c, f, i) the set of FTs of the Sn K-edge XAFS spectra taken during the first cycle(Reprinted with permission from Xin et al [103]. Copyright 2015, Royal Society of Chemistry).

Table caption

Table 1. Comparison of electrochemical performance of micro-size Si based anodes in literature

Si-based anodes Si source particle size (lm) Highest capacitances obtained (mAh g-1) Capacity retention Ref.

Si-C composite Al-Si alloy ingot 2-10 952 mAh/g@0.2Ag-1 86.8% capacity retention over 300 cycles 33

~92.0%

silicon sponge Si wafer >20 790 mAh/g@0.1Ag-1 capacity retention over 300 cycles 31

~97.8%

Si-C composite SiO ~20 1630 mAh/g@0.4Ag-1 capacity retention over 200 cycles 30

~99.0%

porous c-Si SiCl4 >20 2800 mAh/g@0.4Ag-1 capacity retention over 100 cycles 32

multi-dim ensional Si-C Si ~95.0%

powder (30|m, 99.9%) 5-8 ~2400mAh/g@~0. 4Ag-1 capacity retention over 70 cycles 35

3D microporo us Si-C Si powder (10|m, 99.9%) ~7 ~2500 mAh/g@0.2Ag-1 87.0% capacity retention over 50 cycles 36

C-Si nano-com posite SiH4 15-35 ~1950 mAh/g@ ~0.2Ag-1 — 1

(Si-SiO-S iO2)-C composite SiO (325 mesh) Micro size ~1280 mAh/g@ 0.2C 99.5% capacity retention over 200 cycles 37

graphene/ Si-C composite SiO (2|m) Micro size 1100 mAh/g 100cycles, ramains capacity of 3.2 mAh/cm2 38

prickle-lik e Si@C Si powder ~5 1980 mAh/g@0.1Ag-1 65.0% capacity retention over 100 cycles 39

~90.0%

Si-C composite Triethox ysilane Micro size ~1600 mAh/g@0.4Ag-1 capacity retention over 150 cycles 40

Si/C composite Si powder Micro size 1860 mAh/g@0.1Ag-1 ~68.9% capacity 41

s (l-2|im, 99.99%) retention over 100 cycles

Graphene sheet-wrap ped Si Si powder 1-5 1525 mAh/g@0.2Ag_1 After 30 cycles, ramains capacity of 1500 m Ah g"1 42

Si-C composite silicon powder( <78|im) Micro size 620 mAh/g@ 0.5C After 50 cycles, ramains capacity of 525 m Ah g"1 43

Si@ self-healing polymer (SHP) silicon micropar ticles(3-8 |im) Micro size 2617mAh/g@0.4Ag"1 80.0% capacity retention over 90 cycles 44

Table 2 Comparison of electrochemical performance of Ge-based alloy anodes in literature

Electrodes Particle Initial Reversible Capacity Ref.

size Coulombic efficiency capacity (mAhg1) retention

GeCVGe/C 30 |j,m 82% 1860 mAhg"1 90% over 62

Nano composite @ 1C 50 cycles

Three-Dimensional 700 nm 80.3% 1131 mAhg"1 96.6% 67

Microporous @ 1C after over 200

Ge 200 cycles cycles

Ge/C Nanowires 50-100 nm 82% 1200 mAh g"1 @ 0.2C after 50 cycles 84% over 50 cycles M

Graphene/Ge -46 nm 69% 1059 mAhg"1 90% over 68

Nanowire @ 4C after 200 cycles 200 cycles

Amorphous 3.7 nm 70% 1268 mAh g"1 96.7% 69

Hierarchical Porous @ C/2 after over 600

GeOx 600 cycles cycles

mesoporous Ge@C sphere 500 nm 87% 1099 mAhg"1 @ 0.1 Cafter 100 cycles '/0

Ge-carbon hybrid 200 nm 76% 895 mAh g"1 88% over 71

nanoparticles @ 2C after 2000 cycles 2000 cycles

Ge@C/RGO 10-15 52% 940 mAh g"1 @ SOmAg"1 after 50 cycles — '/2

nanocomposite nm

Ge-RGO-CNT 20-30 79.7% 863.8 mAhg"1 — Ti

composites nm @ 100 mAg"1 after 100 cycles

nano-GeCh 5-20 — 452 mAh g"1 93% over 74

/mesoporous carbon nm (ca 1C after 380

composite 380 cycles cycles

GeO2/RGO composite 30 nm 69% 1150 mAh g-1 @ 1C after 500 cycles 75

Ge@graphene/VAGN hybrids 42 nm 1014 mAh g-1 @ 260 mA g-1 after 90 cycles 97% over 90 cycles 76

Table 3 Comparison of electrochemical performance of Sn-based alloy anodes in literature

Sn-based alloy anodes method particle size (nm) Highest capacitances obtained (mAh g-1) Capacity retention Ref.

M-Sn (M = Fe, Cu, Co, Ni) conversion chemistry ~40 ~510, 350, 280, 250 mAh/g@0.05C ~94.1, 71.4, 94.6, 76% capacity retention over 100 cycles 97

FeSn5 conversion chemistry 30-50 768 mAh/g@0.05C 78.9% capacity retention over 20 cycles 101

CoSn5 conversion chemistry 30-50 548 mAh/g@0.05C ~81.8% capacity retention over 100 cycles 102

Fe0.5Co0.5Sn 5 conversion chemistry 30-50 736 mAh/g@0.05C ~71.0% capacity retention over 100 cycles 103

Cu6Sn5 chemical reduction ~ 40 815 mAh/g@0.1 mA cm-2 ~33.0% capacity retention over 100 cycles 104

Cu6Sn5-Sn electrodeposition Micro size 1020 mAh/g@0.08 mA cm-2 ~34.3% capacity retention over 55 cycles 105

Cu6Sn5 electron beam deposition ~100 720 mAh/g@50 mAg-1 ~76.4.0% capacity retention over 15 cycles 106

Sn2Fe:SnFe3 C ball-milling ~10-20 ~650 mAh/g@37 mAg-1 48.3% capacity retention over 40 cycles 107

FeSn2 chemical reduction 30-70 ~635 mAh/g@80 mAg-1 78.7% capacity retention over 20 cycles 108

Fe0.5Co0.5Sn 2 reduction ~20 ~750 mAh/g@80 mAg-1 73.3% capacity retention over 30 cycles 109

[Sn0.55Co0.45 ]1-VCV sputtering — 700 mAh/g@C/12 — 110

Sn3jCo28C41 ball milling Nano Size ~500 mAh/g@0.25 C 100.0% capacity retention over 100 cycles 111

Sn-Co-C mechano-chemical ~50 478 mAh/g@100 mAg-1 ~91% capacity retention over 100 cycles 112

Ni3Sn2 electron beam 10 800 mAh/cm3@30 ~100% capacity 113

deposition uA cm-2 retention over 500 cycles

Sn-Ni electrodeposition Nano Size ~800 mAh/g@250 mAg-1 ~82.5% capacity retention over 70 cycles 114

Ni3Sn2 solvothermal route 2-5 ~771 mAh/g@0.2 C ~90.3% capacity retention over 400 cycles 115