Silicon has been developed as a promising anode in the next generation lithium-ion batteries due to its high gravimetric capacity density of 3580mAh g⁻¹ at room temperature. However, the lithiation and delithiation reactions are accompanied by large volume changes, resulting in the fracture and pulverization of Si particles and leading to the failure of electrical contacts and poor cycleability. To suppress the physical stress due to its large volume change, we have reported Si Leaf Powder® (Si-LP, Oike & Co., Ltd.), which is amorphous Si nano-flake powder exhibited a good cycleability.^1-3 Another serious problem for Si-LP anode such as a large irreversible capacity in the initial and during cycleability remain to be solved for practical use. In previous studies, we have reported Li pre-doping technique using direct contact method with Li foil and Li-naphthalene complex solutions, which completely compensated a initail large irreversible capacity. ^4-5Unfortunately, irreversible capacity was continually generated during cycleability and coulombic efficiency was hovered at 97.9 % even after 10 cycle, where part of charging Li was considered to be consumed for the decomposition of electrolyte solution as solid electrolyte interface (SEI).

 In this study, the effects of adsorbed water on nanolized amorphous silicon and its electrodes were investigated by meas of analysis of Karl-Fischer method and in-situgas generation measurement under different drying condition. In addition, their electrochemical properties including the charge and discharge characteristics were carried out to elucidate the effects of containing water.

 1,743 ppm of water in nano-flake amorphous silicon powder were detected at 250°C Karl-Fischer method even after 1 hour drying at 120°C under 3Pa. This result indicated that nano-flake amorphous silicon were strongly bonded with water and the water was not easy to remove in electrode drying process, which were usually conducted around 100°C in practical CMC/SBR binder electrode. In coparision with graphite electrode, more than twenty times of adsorbed water were observed in nano-flake amorphous silicon electrodes and considerred to track inside the cell, leading to low coulombic efficiency not only initial but during cycleability. Actually, H₂ gas generation was increased and initial coulombic efficiency was enhanced after dried at 180°C under 3Pa. In addition, remarkable coulombic efficiency improvement of 99.9 % (2140 mAh g⁻¹) was observed after 10 cycles. The results of this study show that nanoized silicon strongly adsorbed water , which continuously caused the decomposition of electrolyte solution.

References: [1] K. Nakai et al., Electrochemistry, 78 (2010) 438.; [2] M. Saito et al., J. Power Sorces, 196 (2011) 6637.; [3] M. Saito et al., Solid State Ionics, 225 (2012) 506.; [4] T. Okubo et al., Solid State Ionics, 262 (2014) 39.; [5] S. Yoshida et al., Electrochemistry, 83 (2015), 843.