Pure Silicon Nano-Flake Powder As a High Capacity Anode in Lithium-Ion Batteries

Li-Si alloy is a promising high capacity anode; however, the poor cycleability should be improved for commercial applications. The poor cycleability has been attributed to a large volume during the charge/discharge cycling, resulting fracture and pulverization of Si particles. Therefore, relaxation of the physical stress is important to attain a good cycleability. It has been well recognized that Si nano-particles effectively reduces the stress by the expansion and shrinkage, which gives a good cycleability. However, the use of nano-particles results in a low packing density of the electrode. We designed Si nano-flakes called Si Leaf-Powder^®(LP), which enables us to reduce the stress by the expansion and shrinkage and to make highly dense electrode composites.  We prepared Si nano-flakes of different thickness (50-400 nm, Fig. 1) by a vapor phase deposition, and have investigated their charge and discharge characteristics [1-3].  Here we discuss the cycleability, the irreversible capacity, and the surface film (SEI) growth during cycling of the Si nano-flakes.

The cycleability of Si-LPs of different thicknesses at C/6 in 1 M LiPF₆/EC+DEC(1:1) +10 wt% VC is shown in Fig. 2 [1].  The cycleability of Si-LPs greatly depended on the thickness.  Thinner samples (50-200 nm) showed relatively good cycleability, while thicker samples (300 and 400 nm) showed poor cycleability.  For the thinner samples, the diffusion of metallic Li in the Si flakes is facile.  The resulting uniform distribution of Li within the nano-flake during charging and discharging suppressed the physical stress and resulted in good cycleability.

The large irreversible capacity (Q_irr) of Si anodes is a serious problem for use in practical lithium-ion cells.  The addition of VC does not reduce Q_irr.  The origin of the large Q_irr has been attributed to the reduction of surface SiO_x and solvent decomposition (and the following SEI formation), but such an extremely large Q_irr of > 1000 mAh g^-1is most probably owing to the latter phenomenon.

We found that coating of Si-LPs with thin carbon layer is effective for reducing Q_irr [2]. However, crack formation of the carbon layer decreased the Coulombic efficiency the initial cycles, and we concluded that further remarkable reduction of Q_irr cannot be expected.  To reduce Q_irr, we also found that a pre-doping technique is effective.  We can reduce Q_irr  practically to zero by the pre-doping technique, and we obtained good cycleability similar to that without pre-doping [3]. 

Figure 3 shows the variations of the discharge capacity and Coulombic efficiency of Si-LP (100 nm) upon a long-term cycle test.  The Si-LP sample showed good cycleability up to the 500^th cycle.  The initial capacity was ca. 2000 mAh g^-1, and gradually decreased to ca. 1500 mAh g^-1at the 500 cycle.  Unfortunately, the capacity suddenly dropped after the 560 cycle, the reason of which is not clear at present. 

Though Si-LP (100 nm) showed good cycleability in the long-term cycle test, we found another serious problem.  The Coulombic efficiency was initially low and gradually increased to 98% at the 9^th cycle and to 99% at the 80^th cycle.  It did not reach 100% even at the 500^th cycle (99.8%).  This fact indicates that solvent decomposition (and SEI growth) slowly proceeds and does not stop even at the 500^thcycle.  It is therefore important to completely suppress the solvent decomposition, e.g. by a good additive, for use in practical lithium-ion cells.

Acknowledgment:This work was supported by JST (ALCA), Japan.

References: [1] M. Saito et al., Solid State Ionics, 225, 506 (2012).; [2] T. Okubo et al., Electrochemistry, 80(10), 720 (2012).; [3] M. Saito et al., Extended Abstracts of The 38^thConference on Solid State Ionics in Japan, 1A02, Kyoto (2012).