stability Li-ion Battery Anode

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Robust Micron-sized Silicon Secondary Particles Anchored by Polyimide as High-capacity High-stability Li-ion Battery Anode Pui-Kit Lee, Tian Tan, Shuo Wang, Wenpei Kang, Chun-Sing Lee, and Denis Yau Wai Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09566 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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ACS Applied Materials & Interfaces

Robust Micron-sized Silicon Secondary Particles Anchored by Polyimide as High-capacity Highstability Li-ion Battery Anode Pui-Kit Lee†‡, Tian Tan†, Shuo Wang†‡, Wenpei Kang‡, Chun-Sing Lee‡, and Denis Y. W. Yu†‡* †School of Energy and Environment, City University of Hong Kong, ‡Center of Super-Diamond and Advanced Films (COSDAF), Hong Kong SAR KEYWORDS: Lithium-ion battery, secondary silicon particle, high energy ball-mill, polyimide, mechanical stability

ACS Paragon Plus Environment

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Abstract: Silicon is an attractive high capacity anode material for lithium-ion battery. With the help of nanostructures, cycle performance of silicon anode has improved significantly in the past couple of years. However, three major shortcomings associated with nanostructures still need to be addressed, namely their high surface area, low tap density and poor scalability. Herein, we present a facile and practical method to produce micron-sized Si secondary particle cluster (SiSPC) with high tap density and low surface area from bulk Si by high energy ball-milling. By coupling SiSPC with a mechanically robust polyimide binder, more than 95% of the initial capacity is retained after 500 cycles at 3500 mA g-1 (1 C rate). Reversibility of electrode thickness change is confirmed by in-situ dilatometry. In addition, the polyimide binder suppresses surface reaction of the particles with electrolyte, resulting in a high coulombic efficiency of 99.7%. Excellent cycle performance is obtained even for thick electrodes with an areal capacity of 3.57 mAh cm-2, similar to those in commercial lithium-ion batteries. The presented Si electrode system has a high volumetric capacity of 598 mAh cm-3, which is higher than that of commercial graphite anode materials.

1. INTRODUCTION Lithium-ion batteries (LIBs) are widely used in small-scale portable devices such as mobile phone and notebook. Even though LIBs have now been implemented into large-size application such as electric vehicles, future applications still require batteries with higher capacity and energy density.1 One method to increase the energy density of LIBs is to replace the graphitic anode with a high-energy-density material such as silicon (Si), germanium (Ge) and tin (Sn).2-10 Among these potential candidates, Si is one of the most promising materials because of its high theoretical gravimetric capacity of 3579 mAh g-1 and theoretical volumetric capacity of 2200 mAh cm-3. However, Si particles undergo large volume expansion of more than 300% during


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lithiation, leading to mechanical issues in the electrodes such as delamination and loss of electrical contact.11-13 Si particles also pulverise under large strain, which creates fresh surfaces and consumes electrolyte with the formation of a solid–electrolyte interphase (SEI).13-14 So, fast capacity decay along cycles is observed when bulk Si is tested with conventional binders such as polyvinylidene fluoride (PVdF) and carboxymethyl cellulose (CMC).15-16 The use of polyimide binder helps to increase electrode stability and reduce electrode expansion, but cracking of particles during cycling and formation of new surfaces still leads to decrease in capacity.17 The common approaches to solve the mechanical issues in Si are to make nanostructured materials and silicon-carbon composites by using void spaces and carbon matrix to accommodate the large volume change.2-4,14,18-21 However, these materials typically have low tap density (~0.15 g cm-3) and high production cost.2,21 They also have large surface area, resulting in lower capacity due to the native surface oxide that is inactive and also lower coulombic efficiency (typically between 98% to 99.5%) with cycling due to excessive solid-electrolyte interphase formation and side reactions during charge and discharge.14,19-21 Moreover, nano-Si materials are difficult to handle and mass-produce. Our strategy here to stabilize Si for battery applications is to form micron-sized Si secondary particle clusters (SiSPC) by high energy ball milling (HEBM). The micron-sized secondary clusters are made up of nano-sized Si primary particles, which prevent particle pulverisation.22 Morever, the secondary clusters decrease the surface area of the particles, thus reduce the contact with electrolyte and suppress excessive formation of SEI along cycles. Furthermore, the clusters minimise the inter-particle space and increase the electrode density of the Si electrode. Another key technology in this work is to combine SiSPC with a high-modulus polyimide (PI) binder (SiSPC-PI) to sustain the large volume expansion. The SiSPC-PI electrode shows excellent cycle


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stability, with more than 95% of initial charge capacity after 500 cycles under a current of 3500 mA g-1 (1 C rate). Average coulombic efficiency is more than 99.7% from 4th cycle to 500th cycle. The SiSPC-PI electrode system enables highly reversible thickness changes during charge and discharge, as verified by in-situ dilatometry tests, leading to superior cycle stability. In addition, the combination of SiSPC-PI allows us to make thick electrodes up to 3.57 mAh cm-2 (equivalent to the mass loading of graphite electrode in commercial lithium-ion batteries)21 with stable cycle performance. Our results are superior than other works on micron-sized Si reported in the literature (see Table S1).

2. EXPERIMETAL SECTION 2.1. Synthesis and characterization of silicon secondary particle clusters (SiSPC). High energy ball-mill (HEBM) technique (Fritsch pulverisette 7 planetary ball-mill) is applied to synthesize the SiSPC. Commercial micron-sized Si particle (m-Si, Sigma-Aldrich) with particle size ranging from 5 to 10 µm was used as a precursor. Ball-milling was conducted with tungsten carbide balls and bowls in Ar atmosphere with a ball to material ratio of 40 to 1 by weight. The rotation speed of ball-milling machine was set to 500 rpm for 6 h to provide high mechanical energy to form SiSPC. A control sample was ball-milled with 200 rpm for 6 h (Si-200) to investigate the effect of particle size on electrode stability. For comparison, nano-Si particles with a size less than 100 nm (n-Si, Sigma Aldrich) were also tested. Brunauer–Emmett–Teller (BET) surface area of the samples was measured using N2 adsorption and desorption isotherms on Micromeritics ASAP 2020 system. The morphology of the particles and electrode were characterized by scanning electron microscopy (SEM) (ZEISS EVO MA10) and Energydispersive X-ray Spectroscopy (EDX) (Philips XL30 FEG). X-Ray diffraction patterns were


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measured by X-ray diffractometry (X’Pert3 Powder X- 90 ray Diffractometer, One PANalytical). The Raman spectra were collected on Raman Spectroscopy Instruments (Reishaw 2000). Fourier transform infrared spectra were collected on FTIR spectrophotometer (Shimadzu IRAffinity-1) over the range of 2000−500 cm-1 using KBr pellets. The tap density is determined by tapping a cylindrical container with 0.3 g of powder for 3 minutes with 90 repetitions. The structure of SiSPC was characterized by transmission electron microscope (TEM) (Philips CM 20). 2.2. Electrochemical evaluation of the samples. The four samples (m-Si, n-Si, Si-200 and SiSPC) were used as active material and were first ground and mixed uniformly with acetylene black (AB) in a mortar. PI binder from I.S.T. (Japan) was added together with N-Methyl-2pyrrolidone (NMP) to make slurry and coat with a doctor blade onto copper current collectors (8 µm) to form the electrode. The typical mass ratio of Si: AB: PI is 60:20:20, but electrodes with 30% PI (Si: AB: PI=52.5: 17.5: 30 wt%) were also tested. The electrodes were cut into 16 mm diameter discs and calendered by a roll pressed. Typical active mass loading is ~1.6 mg cm-2 with an electrode density of ~1.4 g cm-3 for the m-Si, Si-200 and SiSPC electrodes. Electrode with n-Si can only be packed to 0.9 g cm-3. The pressed electrodes were then loaded into a tube furnace for thermal polymerization at 400°C for 2 hrs at a ramping rate of 1°C min-1. After annealing, the electrodes were pressed again to 1.4 g cm-3 and dried at 110°C under vacuum. The dried electrodes were transferred into an Ar-filled glove box. These electrodes were assembled into 2032 type coin cell with Li foil as counter electrodes. 1 M LiPF6 in fluoroethylene carbonate (FEC)/ diethyl carbonate (DEC) with a volume ratio 1:1 was used as the electrolyte. Electrolyte with 2 vol% vinylene carbonate additive was also used for long-term cycle tests. Coin cells were tested between 0.01 and 1 V vs. Li/Li+. 1 C in the tests refers to a current rate of 3.5 A g-1.


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Specific capacity of Si electrode was calculated with respect to the weight of the Si active material. 2.3. Mechanical strength of the binder. Micro-indentation (Shimadzu DUH-211S) is used to show the difference in mechanical strength between carboxymethyl cellulose (CMC) and polyimide (PI). Comparison between PI and CMC is made since CMC is widely used as a binder in LIB. The polymers, which are about 100 µm in thickness, are first coated onto copper foil. During the indentation test, an increasing load force is applied onto the film until a pre-set maximum force of 60 mN at a rate of 40 mN min-1. The load is then gradually reduced. Microindentation enables us to investigate Young's modulus of polymer films by equation 1. 1 1 − 1 −

= 1

E E E where Er is Young’s modulus of systems, Ei is Young’s modulus of the indenter, v i is Poisson’s ratio of the indenter, Eit Young’s modulus on sample and v is Poisson’s ratio of samples. The mechanical work consumed during indentation (Wtotal) and that released from elastic portion (Welastic) can also be measured during loading and de-loading processes, and the efficiency is given by equation 2.

η % =

W 2

W

This value indicates the elasticity of the tested polymer film. 2.4. Thickness change of electrode during Li insertion and removal. The thickness change of Si electrode upon charge and discharge was monitored by an in-situ dilatometer (EL-Cell) using a 9 mm Si electrode. The thickness change is measured by a linear voltage displacement


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transducer (LVDT) with a height resolution of 50 nm. The validity of the in-situ dilatometer was verified in our previous study on bulk Si electrodes.17 3. RESULTS AND DISCUSSION 3.1. Physical characterization of the materials. Micron-sized Si secondary particle clusters (SiSPC) are formed by ball-milling 5 to 10 µm sized commercial Si material (m-Si) (Figure 1a) at 500 rpm for 6 h in Ar atmosphere. Particle fracturing, cold welding and re-welding of the Si particles by repeated collisions induced by high energy ball-milling is verified by SEM after different durations of ball-milling (Figure S1). The fracture of Si particles is observed after 0.5 hr ball-mill treatment in Figure S1b. The size of Si particles reduces further after 1 and 2 hrs ballmilling in Figure S1c and d. After that, cold welding and re-welding become predominant which lead to the formation of secondary particle as illustrated in Figure S1e. The secondary particle size grows with increasing ball-mill time as illustrated in Figure S1f.23-27 SEM images reveal that the secondary particles of the SiSPC have a size of about 3-10 µm, similar to that of the bulk material (Figure 1b). The primary particles are between 100 and 200 nm in size (Figure 1d), larger than that of typical commercial nano-Si (n-Si) materials of about 100 nm (Figure S2b). A schematic diagram of SiSPC formation is proposed in Figure 2d. To study the effect of particle size, another sample is made by ball-milling m-Si at 200 rpm for 6 h in Ar (Si-200). With lower rotation ball-milling speeds, the Si particles are broken down into larger particles of 0.5 to 8 µm (Figure S2a). Figure S3a shows the top-view SEM image of the SiSPC after it is made into an electrode. As seen in the inset, the SiSPC particles can be clearly distinguished. After the calender process, the electrode is smoothened (Figure S3b). Some of the SiSPC on the surface may be partially destroyed by the pressing process, though the Si cluster can still be observed


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within the electrode, based on the cross-sectional SEM image of the calendered electrode in Figure S4a. TEM observations (Figure 2a and b) confirm that SiSPC are made up of nano-sized primary particle (>200 nm) that are closely packed. The lattice fringes from Si (111) plane with an interplanar spacing of ~0.315 nm could still be seen in Figure 2c. The formation of closely packed primary particles within SiSPC results in a high tap density and good electrical conductivity between the nanoparticles. SiSPC shows a tap density of 0.81 g cm-3, which is similar to that of m-Si but significantly higher than that of 0.17 g cm-3 of n-Si (see Table 1 and Figure 1e for a comparison of the appearance of the Si samples after tapping). The effect of HEBM on the structure of the material is studied by XRD and Raman spectroscopy. HEBM leads to partial amorphization of the crystalline Si, as indicated by a reduction and broadening of the X-ray diffraction peaks of Si as compared with those of n-Si and m-Si (Figure S5a).28 The presence of amorphous Si in SiSPC is also confirmed by Raman spectroscopy, which shows an additional peak at 480 cm-1 (Figure S5b).29 Despite the small primary particles, SiSPC has a BET surface area of 12.6 m2 g-1, half of that of 26.5 m2 g-1 of n-Si. Nitrogen sorption curves of SiSPC and n-Si are both type II (Figure 1f), but the hysteresis loop corresponds to H3 type and H4 type, respectively. It indicates the presence of mesopores within SiSPC and macropores within n-Si.30-31 The smaller size of the pores suggests that the primary Si particles within SiSPC are tightly packed.


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Figure 1. (a) SEM image of commercial micro-Si particle. (b-d) SEM images of SiSPC under different magnifications. (e) Digital picture of appearance of 0.3 g Si samples. (f) N2 adsorption and desorption isotherms of different Si particles.

Table 1. BET surface area and tap density of Si particles. m-Si

SiSPC

n-Si

BET surface area (m2 g-1)

1.4

12.6

26.5

Tap density (g cm-3)

0.94

0.81

0.17


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Figure 2. (a-b) TEM images of SiSPC, clearly showing highly connected primary Si nanoparticles. (c) HRTEM image of SiSPC. (d) Schematic diagram showing the formation of SiSPC from m-Si. 3.2. Anchoring SiSPC with polyimide in electrode. Since Si expands with lithiation, the particles must be combined with a mechanically strong binder to hold them together. Here, we compare the mechanical properties of polyimide (PI) and carboxymethyl cellulose (CMC). Loadpenetration depth curves (Figure 3a) show that Young’s modulus of PI is 14.5 GPa, about 2.5 times higher than that of CMC film of 5.7 GPa. The tested Young’s modulus of CMC is similar to the reported value.32 Indentation efficiency, the ratio of elastic work over total work, of PI film is 72.5%, and that of CMC is only 23.3%. The significant difference of indentation efficiency between PI and CMC suggests that PI is about three times more elastic than CMC. The higher


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modulus and elasticity makes PI a more suitable binder than CMC for Si electrodes, which

b 1793 cm

80

PI

70

Relative Absorbance (a.u.)

a

Carboxymethyl cellulose

60 50 40 30 20 10 0 0

0.5

1

1.5

2

2.5

3

3.5

Indentation depth (µm)

4

4.5

-1

1780 cm -1 1720 cm-1

1800 1760 1720 1680

5

Wavenumber (cm-1)

Relative Absorbance (a.u.)

exhibits substantial volume changes with Li insertion and extraction.

Load (mN)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PI-SiSPC

PI

C-N

C=N

C-N-C

1800 1700 1600 1500 1400 1300 1200 1100 1000

Wavenumber (cm-1)

900

800

700

600

500

c Further thermal treatment

Thermal treatment

Silyl benzoate bond

Crosslink between PI

Figure 3. (a) 60 mN load-penetration depth curves of PI and CMC. (b) FTIR spectra of pure polyimide (PI) and PI with SiSPC (PI-SiSPC). (c) Schematic diagram showing the chemical interaction between PI and Si. In addition to the superiority in mechanical properties of PI, we also found that the mechanical stability is reinforced by a chemical bonding between PI and SiSPC. Results from Fourier Transform Infrared Spectroscopy (FTIR) measurements on pure PI and PI mixing with SiSPC are shown in Figure 3b. PI is characterized by absorption peaks at 1780 cm-1 and 1720 cm-1, corresponding to the asymmetric stretching vibration of C=O imide carbonyl and symmetric stretching vibration of C=O imide carbonyl, respectively.33-35 The peak located at 755 cm-1 can be ascribed to the asymmetric stretching vibration of the C-N-C bond.36-37 These indicate the ring-closing reaction of PI material as shown in Figure S7. When PI is polymerized in the presence of SiSPC, peaks at 1730 cm-1 and 1793 cm-1 appear, corresponding to the stretching vibrations of C=O of the intermolecular polymeric chain

34,36,38

and isoimide39-40 respectively.


11

400


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The emergence of peaks at 1379 and 1666 cm-1 representing the stretching of C-N and C=N bond replaces asymmetric stretching vibration of the C-N-C bond located at 755 cm-1.39 These behaviours suggest that the PI intermediate, namely poly(amic acids), undergoes condensation polymerisation with the silanol group (Si-OH) on the surface of SiSPC forming silyl benzoate covalent bond with PI intermediate, and a schematic diagram is suggested in Figure 3c. Upon further heat treatment, the PI molecules will form crosslinks between them as the crosslink reaction takes place at lower annealing temperature (180ºC) comparing with that of ring-closing condensation (350ºC).39,41. The interaction between PI and SiSPC ensures the physical connection between SiSPC and PI binder and the integrity of electrode even when electrode undergoes drastic volume expansion.32,42 3.3. Electrochemical evaluation of SiSPC. m-Si, Si-200, SiSPC and n-Si were made into electrodes with 20 wt% of polyimide (PI) binder and tested at a current of 250 mA g-1. The first charge-discharge curves are shown in Figure S6. First reversible capacities of the four samples are 3196, 3078, 3154 and 1998 mAh g-1, respectively. m-Si, Si-200 and SiSPC all give 1st charge capacities of more than 3000 mAh g-1, close to the theoretical capacity of Si. In comparison, n-Si gives a much lower capacity, similar to that reported in the literature.32,43-44 The lower capacity of n-Si is attributed to the large amount of inactive native oxides on the surface of n-Si, and the loosely packed nano-particles, which give rise to lower electrode density, larger inter-particle resistance and longer electron transfer pathway.2,21 Our results show that the ball-milling process does not significantly affect the capacity of the Si material. First cycle efficiency (FCE) of n-Si is only 71.4%, while that of SiSPC is about 81.4%. The higher FCE for SiSPC suggests that SiSPC with smaller surface area suppresses SEI formation during the first cycle.2,21,45-47


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Delithiated Capacity (mAh g-1)

a 4000

SiSPC with 30wt% PI binder

3500

SiSPC

3000 2500 2000 1500

n-Si

1000

Si-200

500

250 mA

0

0

m-Si

g-1

10

20

30

40

50

60

70

Cycle Number Coulombic Efficiency (%)

b 101.5

SiSPC Si with 30wt% PI binder

101 100.5 100 99.5 99 98.5 98 97.5 97 96.5 96 95.5

SiSPC

m-Si

n-Si

250 mA

g-1

10

20

0

Si-200

30

40

50

60

70

Cycle Number

c Delithiated Capacity (mAh cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 5

2.54 mg cm-2

4 3 2

1.79 mg cm-2

1 0

250 mA 0

10

g-1 20

30

40

50

60

70

Cycle Number

Figure 4. (a) Cycle performance of electrode with different Si particle size at 250 mA g-1. (b) Coulombic efficiency of electrodes with different Si particles size at 250 mA g-1. (c) Cycle performance of SiSPC with high areal mass loading (up to 2.54 mg cm-2 electrode mass) with 30wt% PI. The cycle performance and coulombic efficiency are shown in Figure 4a and b. Despite the use of PI binder, m-Si electrode shows poor cycle stability. After 70 cycles, only 23.57% of capacity is retained and the average coulombic efficiency after the 3rd cycle is 97.75%. The poor cycle performance and coulombic efficiency are attributed to cracking of the large particle and formation of SEI on the surface of the cracks. Si-200 exhibits an improvement in terms of both


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stability and columbic efficiency comparing with m-Si due to less particle cracking. The capacity retention after 70 cycles enhances to 68.59% and average coulombic effcicency after 3rd cycle improves to 98.86%. In comparison, SiSPC shows further improvement on cycle stability with a capacity retention of 78.3% after 70 cycles. The better cycle performance is attributed to the small primary size of SiSPC of 100-200 nm, smaller than the critical size for particle cracking.22 The average coulombic efficiency of SiSPC from 4th to 70th cycle is 99.25%, higher than the average CE of n-Si of 98.86%. The enhanced coulombic efficiency is attributed to the smaller BET surface area of SiSPC through the agglomeration process compared to n-Si, leading to less side reaction such as solid electrolyte interphase between Si particle surface and electrolyte. Further improvement in cycle stability can be achieved with SiSPC by increasing the PI binder content to 30%, resulting in a 89.8% capacity retention after 70 cycles. a

b

c

d

e

f

Figure 5. (a) SEM image of calendered electrode (commerical Si) before cycling. (b) Commercial micro-Si particle with 20 wt% polyimide (PI) after 10 cycles. (c) 200 rpm-Si particle with 20 wt% PI after 10 cycles. (d) SiSPC with 20 wt% PI after 10 cycles. (e) SiSPC


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with 30 wt% PI after 10 cycles (scale bar: 20 µm). (f) Dilatometry result of SiSPC electrode with 30 wt% PI with a current of 250 mA g-1. The correlation between cycle stability and electrode integrity is verified by SEM (Figure 5a-e). Electrode cracks are observed on m-Si and Si-200 electrode after 10 cycles in Figure 5b and c. The connectivity of the electrode is improved with SiSPC electrode with 20 wt% PI binder, in Figure 5d. The reversibility of the SiSPC electrode with PI binder is also demonstrated by in-situ dilatometry (Figure 5f). During lithiation, the thickness of the SiSPC electrode with 30% PI increases linearly once the voltage is below 0.2 V vs. Li/Li+, without the accelerated increase in electrode thickness often observed due to mechanical failure of the binder.45,48-49 The electrode thickness increases by 320% when the electrode is lithiated to 0.01 V vs. Li/Li+, but over 90% of the change is recoverable upon de-lithiation. At the end of de-lithiation, the overall electrode thickness is only 26% thicker than that of the initial electrode thickness. Cross-sectional SEM images and corresponding EDX images of the pristine SiSPC electrode and those after lithiation and de-lithiation (10th cycle) were taken to verify the electrode thickness change measured by dilatometry (Figure S4). Before testing, the thickness of the SiSPC electrode is about 12.6 µm (Figure S4a). The electrode thickness expands to about 52.9 µm at the lithiated state, corresponding to an expansion of 320% (Figure S4d). After de-lithiation, the electrode thickness is reduced back to about 15.5 µm (Figure S4g). This corresponds to an electrode swelling of about 23% compared to the pristine electrode, which is consistent with the dilatometer result. Despite the significant electrode expansion (>300%) during lithiation, our results show that the SiSPC-PI configuration is able to keep the mechanical integrity of the electrode. The highly reversible electrode thickness change with charge and discharge is a precursor to excellent cycle performance.


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3.4. Designing thick SiSPC electrode for practical applications. When testing cycle performance, most literature tested thin electrodes with electrode mass loading of less than 1 mg cm-2 or low packing density (~0.5-0.7 g cm-3).32,48,50-57 Though, the areal capacity is much lower than that in commercial lithium-ion batteries. Our electrode with SiSPC and PI binder can also give excellent cycle stability when the areal capacity is increased. Figure 4c shows a comparison of SiSPC electrode (30 wt% PI; packing density of 1.4 g cm-3) with the areal loading of 1.79 and 2.54 mg cm-2, corresponding to an electrode thickness of 14 and 19 µm respectively. Even when the areal loading is increased to 2.54 mg cm-2, good cycle stability can still be obtained. After 70 cycles, the capacity was still as high as 2780 mAh g-1 with only 10% capacity loss. The higher electrode mass loading corresponds to an averaged de-lithiated capacity of 3.57 mAh cm-2. Average coulombic efficiency from 4th to 70th cycle is about 99.57%. Post-mortem SEM analysis of the SiSPC electrode with 30 wt% PI after cycles (Figure 5e) shows that the electrode surface is still smooth. It proves that the electrode system with SiSPC and 30wt% PI binder is highly stable even with high mass loading. A comparison of our work with that of others is shown in Table S1. Our results are among the best in the literature regarding stability with high areal capacity and long-term stability.


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Figure 6. (a) Cycle performance of SiSPC with 30% PI for 500 cycles under 3500 mA g-1. (b) Cycle performance of SiSPC with 30% PI capacity with 1000 mAh g-1 capacity limitation for 1400 cycles under 3500 mA g-1. Our electrode system can also be optimized for high rate applications. As shown in Figure 6a, an electrode with a mass loading of 0.6 mg cm-2 gives an initial charge capacity of 3250 mAh g-1 with a current of 250 mA g-1. The reversible capacity is still about 1760 mAh g-1 when it is tested with a high current of 3500 mA g-1 (about 1 C) (54% of the capacity at 250 mA g-1). The capacity of the electrode can maintain over 1690 mAh g-1 after 500 cycles, corresponding to a capacity retention of more than 95%. The average coulombic efficiency from 4th to 500th cycle is 99.71%. If the capacity of the electrode is limited to 1000 mAh g-1, the electrode can be cycled reversibly at 3500 mA g-1 for more than 1280 cycles with a high coulombic efficiency of 99.83% from 4th to 1280th cycle, as illustrated in Figure 6b.

4. CONCLUSIONS SiSPC with extremely high tap density (~0.81 g cm-3) is successfully designed and fabricated using a scalable, facile and technologically mature high energy ball-mill technique. In combination with the high modulus PI binder, the presented Si electrode system exhibits excellent cycle performance. At 1 C current rate, 500 stable cycles with an average capacity of 1860 mAh g-1 and 95.1% capacity retention are achieved. Moreover, superior cycle performance is also achieved with high electrode mass loading (2.45 mg cm-2) and high electrode density ~1.4 g cm-3 (~90% capacity retention), corresponding to volumetric capacity of 598 mAh cm-3. This volumetric capacity is about 37-81% bigger than that of graphite (330-430 mAh cm-3).58 Average coulombic efficiencies are as high as 99.83% (1000mAh g-1), 99.71% (fully lithiated) and


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99.57% (fully lithiated high mass loading electrode). The excellent capacity and cycle stability are attributed to the small primary particle size in SiSPC, fused together into densely packed secondary particles and held together by a mechanically strong PI binder.

ASSOCIATED CONTENT Supporting Information. SEM images of Si particles after 0 hr, 0.5 hr, 1 hr, 2 hrs, 4 hrs and 6 hrs. SEM image of Si-200 and n-Si. SEM images of SiSPC electrode surface (a) before and (b) after calendering. Crosssectional SEM images of calendered SiSPC electrodes. X-ray diffraction patterns and Raman spectroscopy of m-Si and SiSPC. 1st charge-discharge curves of m-Si, Si-200, SiSPC and n-Si with 20 wt% PI binder. Schematic of ring closing reaction of polyimide. Table of studies using micro-sized Si particles as precursor. Summary of the result of this work and other works.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Tel: (+852) 3442-6885 Present Addresses Academic 1, School of Energy and Environment, City University of Hong Kong, Tat Chee Ave, Hong Kong SAR. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The work described in this paper was supported by a grant from the Research Grants Council (CityU 21202014) of the Hong Kong Special Administrative Region, China Reference (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Lin, D.; Lu, Z.; Hsu, P.-C.; Lee, H. R.; Liu, N.; Zhao, J.; Wang, H.; Liu, C.; Cui, Y. A High Tap Density Secondary Silicon Particle Anode Fabricated by Scalable Mechanical Pressing for Lithium-Ion Batteries. Energy Environ. Sci. 2015, 8, 2371-2376. (3) Li, Y.; Yan, K.; Lee, H.-W.; Lu, Z.; Liu, N.; Cui, Y. Growth of Conformal Graphene Cages on Micrometre-Sized Silicon Particles as Stable Battery Anodes. Nat. Energy. 2016, 1, 15029. (4) Zong, L.; Jin, Y.; Liu, C.; Zhu, B.; Hu, X.; Lu, Z.; Zhu, J. Precise Perforation and Scalable Production of Si Particles from Low-Grade Sources for High-Performance Lithium Ion Battery Anodes. Nano Lett. 2016, 16, 7210-7215. (5) Li, X.; Yang, Z.; Fu, Y.; Qiao, L.; Li, D.; Yue, H.; He, D. Germanium Anode with Excellent Lithium Storage Performance in a Germanium/Lithium–Cobalt Oxide Lithium-ion Battery. ACS Nano 2015, 9, 1858-1867. (6) Li, D.; Wang, H.; Liu, H. K.; Guo, Z. A New Strategy for Achieving a High Performance Anode for Lithium Ion Batteries-Encapsulating Germanium Nanoparticles in Carbon Nanoboxes. Adv. Energy Mater. 2016, 6, 1501666. (7) Mo, R.; Rooney, D.; Sun, K.; Yang, H. Y. 3d Nitrogen-Doped Graphene Foam with Encapsulated Germanium/Nitrogen-Doped Graphene Yolk-Shell Nanoarchitecture for HighPerformance Flexible Li-ion Battery. Nat. Commun. 2017, 8, 13949. (8) Qin, J.; He, C.; Zhao, N.; Wang, Z.; Shi, C.; Liu, E.-Z.; Li, J. Graphene Networks Anchored with Sn@Graphene as Lithium Ion Battery Anode. ACS Nano 2014, 8, 1728-1738. (9) Zhao, Y.; Li, X.; Yan, B.; Li, D.; Lawes, S.; Sun, X. Significant Impact of 2D Graphene Nanosheets on Large Volume Change Tin-Based Anodes in Lithium-Ion Batteries: A Review. J. Power Sources 2015, 274, 869-884. (10) Zhang, H.; Shi, T.; Wetzel, D. J.; Nuzzo, R. G.; Braun, P. V. 3D Scaffolded Nickel-Tin Liion Anodes with Enhanced Cyclability. Adv. Mater. 2016, 28, 742-747. (11) Bridel, J. S.; Azaïs, T.; Morcrette, M.; Tarascon, J. M.; Larcher, D. Key Parameters Governing the Reversibility of Si/Carbon/CMC Electrodes for Li-ion Batteries. Chem. Mater. 2010, 22, 1229-1241. (12) Bridel, J. S.; Azaïs, T.; Morcrette, M.; Tarascon, J. M.; Larcher, D. In Situ Observation and Long-Term Reactivity of Si/C/CMC Composites Electrodes for Li-ion Batteries. J. Electrochem. Soc. 2011, 158, A750- A759.


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Table of content

Highly reversible electrode thickness change

Copper foil Silicon secondary particle cluster

High modulus and elasticity polyimide binder


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stability Li-ion Battery Anode

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