Electrodeposition of Si on Cu Substrate As Anode for Li-Ion Battery Application

Abstract

Lithium ions battery (LIBs) has been considered as one of the furthermost capable power sources in the portable electronic devices and electric vehicles.1 Silicon (Si) is a promising negative electrode material for LIBs, owing to its cost-effectiveness, non-toxicity, and high specific capacity of 4200 mAhg-1.2 The main challenges of applying Si-based LIBs are low electronic conductivity (≈10−4 S m−1) 3 and volume expansion (~ 300%) during the charge-discharge process.4,5. For a sustainable and practical approach, room temperature (RT) methods are an essential requirement. Recent studies suggest that the aqueous phase synthesis of Si-nanoparticles affects its phase purity, therefore the use of ionic liquids (ILs) as media for the electrodeposition of Si-nanostructures can resolve sustainability issues. In this work, electrodeposition of Si-nanoparticles on Cu substrate was successfully fabricated in the ILs media (1-butyl-3-methylimidazolium-bis (trifluoromethyl sulfonyl) imide (BMImTf2N)) at room temperature.Cyclic voltammetry (CV) revealed the sequential reduction of Si4+ to Si2+ followed by Si2+ to Si-nanoparticles (SiNPs). The morphology of the electrodeposited SiNPs from 1 M SiCl4 in BMImTf 2 N at room temperature on Cu substrate is shown in Fig. 1 (a). The electrodeposited SiNPs were interconnected with an average particle size of ⁓50-70 nm. The X-ray photoelectron spectroscopy (XPS) data of surface compositions of the electrodeposited SiNPs showed peak at 99.1 eV for elemental Si (Fig 1 (b)). Further, X-ray diffraction (XRD) analysis of SiNPs revealed sharp peak at 28.7° for the crystalline Si (111) phase (JCPDS NO: 800005; Fig 1(c)). The first discharge and charge capacity of Si anode material have been found to be 2386 and 2300 mAhg-1, respectively, at a high current density of 4 Ag-1 (Fig 1(d)). The initial irreversibility of discharge-charge process can be attributed to the solid electrolyte interface (SEI) formation via electrolyte decomposition and trapped Li+ into inner pores of SiNPs. After 10 cycles, charge-discharge profiles have been stabilized, with a reversible capacity of 2286 mAhg-1 at 4 Ag-1 current density. The stabilization of charge-discharge capacity may be due to stable SEI formation. The coulomb efficiency (CE) of first cycle was 96 %, which improved to 98 after 10 cycles. The high CE is attributed to the reversible kinetics of the Li + ion on the SiNPs in the charging and discharging process. The high surface area of SiNPs accommodates the volume expansion and stabilizes SEI layer. The reversible capacity 2286 mAhg-1 is only ~ 5 % less than that of first discharge capacity. The retention of very high reversible capacity with respect to the initial discharge capacity shows that SiNPs of crystalline (111) phase orientation is a promising material for LiBs. The effect of SEI formation and battery testing on the electrical conductivity of SiNPs was investigated by electrochemical impedance spectroscopy (EIS). The resistance due to SEI formation (RSEI) was increase from 27 Ω to 30 Ω after 100 charge-discharge cycle. The large volume expansion during charge-discharge leads to pulverization of SiNPs, and hence electrical conductivity of SiNPs was decreased. However, charge-discharge data suggests that there is only ~ 5 % decrease in discharge capacity from the first cycle, demonstrating the improvement of the interfacial charge transfer kinetics. The pure electrical conductivity of Si can be improved by the incorporation of CNTs for stable SEI formation and accommodate large volume expansion during charge-discharge cycle. In summary, we have demonstrated the electrodeposition of SiNPs with controllable size in the presence of water contaminated RTILs. The electrodeposited SiNPs shows high reversible charge capacity of ~ 2286 mAhg-1 and stability up to 100 cycles at high current density of 4 Ag-1. The specific charge capacity is further improved by incorporation of carbon nanomaterials and tuning of current density. Figure 1