Mechanical and Electrochemical Properties of Highly-Dense Li-Si Alloy Anodes Fabricated by Arc Plasma Deposition

Abstract

Silicon is one of the most promising anode materials because it has a very high theoretical capacity (4200 mAh g− 1). This capacity is about 10 times higher than that of carbon anode (372 mAh g− 1).[1] However, enormous volumetric change and mechanical stress occur in a silicon anode during electrochemical lithiation-delithiation. Consequently, the specific capacity is degraded during repetition of charge-discharge because of cutting off conductive paths caused by mechanical fracture. It is reported that pre-lithiation by sputtering and electrochemical methods suppresses fracture of silicon anodes by decrease in elastic modulus and yielding stress.[2] However, the reported Li-Si alloy anodes possesses the mechanical properties derived from both the deposited silicon and the voids generated during the synthetic processes. Therefore, a highly-dense alloy is necessary to investigate relationships between mechanical and electrochemical properties of Li-Si itself. We have already fabricated highly-dense amorphous silicon films by an arc plasma deposition (APD) method.[3] Thus, we attempted to fabricate highly-dense Li-Si alloy films by an APD method. The obtained Li-Si alloy films were investigated with focusing on relationships between mechanical properties and electrochemical properties.Li x Si (x = 0-3.56) alloy films were prepared on Al2O3(0001) single-crystal and Cu substrates by an APD method. Metallic lithium and B-doped silicon coated with carbon were used as target materials in the APD apparatus. The physical structures and chemical states of the obtained Li x Si alloy films were investigated with Raman spectroscopy, hard X-ray photoelectron spectroscopy (HAXPES), and neutron reflectometry (NR). Elastic modulus and hardness of the Li x Si alloy films were calculated from load-displacement curves gained by nanoindentation measurement. Constant-current charge-discharge tests were carried out with 2032-type coin cells assembled using the Li x Si films as the working electrodes, Li foils as counter electrodes, and 1 M LiPF6 in EC: DEC 3:7 vol% as electrolytes. The charge-discharge tests were conducted with constant current of 0.01-0.16 mA (1C rate), lower and upper voltages of 20 mV and 1.5 V vs. Li/Li+.In HAXPES spectra, Si 2p peaks derived from Si0 shifted to lower binding energy compared to the Si (x = 0) film as x increased. Therefore, Li x Si alloy films with different compositions were successfully fabricated by the APD method. The peaks because of Si-Si bonds in Raman spectra also shifted to lower wavenumber with the increase in x. The shifts suggest that the length of the Si-Si bonds extended with the increase in x. The NR analyses revealed that Li x Si alloy films possessed the smooth surface with the roughness less than 3 nm. The elastic modulus and hardness of Li x Si alloy films decreased as x increased, meaning that Li x Si alloy films were softer and more easily deformed compared to Si films. In addition, the elastic modulus and hardness of Li x Si alloy films were higher than those of Li x Si films previously reported,[2] suggesting that the APD method more densely made the Li x Si alloy films than the reported sputtering and electrochemical methods. Thus, the objective highly-dense films of Li x Si alloys were successfully obtained. The plateaus corresponding to the reactions of Li x Si ↔︎ Li x ’Si ↔︎ Li y Si (x < x’ < y) were observed in charge-discharge curves of Li x Si alloy films. The first discharge capacities of Li x Si alloy films were lower than the theoretical capacity of silicon. This is due to the existence of lithium in the structure by pre-lithiation. Cycle stabilities of Li x Si alloy films were superior to the Si film. To reveal factors of the cycle stabilizing, the charge-discharge reactions were analyzed with dQ/dV plots. The peaks around 0.5 V in the delithation process decreased about Si film. This decrease in the reaction peak is attributed to the decrease in the active species during the reaction process because of the fracture. In contrast, the peaks of Li x Si alloy films around 0.5 V hardly decreased. This result suggests that delamination and fracture by volumetric change during charge-discharge processes were suppressed in Li x Si alloy films because of their softness. Thus, the analyses of the highly-dense Li x Si alloy films fabricated by the APD method clarified that cycle stabilities of active materials were improved by controlling mechanical properties.[1] Ashuri, M.et al., Nanoscale 2016, 8, (1), 74-103.[2] Sitinamaluwa, H. et al., RSC Adv. 2017, 7, 13487-13497.[3] Asano, S. et al., Highly-dense and smooth amorphous silicon films fabricated by arc plasma deposition and electrochemical properties., ACSSI-17, 3A-06, 9.15.2022.