Understanding Solid Electrolyte Interfaces Film Formation on SnSb Anode Electrodes for Li-ion Batteries

Abstract

The demand for higher energy and power density for Li-ion batteries (LIBs), which are made of transition metal oxide cathodes and graphite anode, motivates the research toward alternative electrode materials with larger specific charge capacities1. Among the several materials proposed to replace graphite (372 mAh.g-1) as an anode material, tin (Sn) and Sn-based anodes, such as SnSb, TiSnSb, CoSn, are very promising because of the superior theoretical capacity, e.g SnSb = 825 mAh.g-1 2. However, these materials suffer from the large volume change during the Li+ alloying/dealloying mechanism and formation of solid electrolyte interface (SEI) at the electrode surface3. The SEI film characteristics, such as film thickness, composition, and Li+ conductivity, dictate the electrochemical reaction occurring at the electrode surface, hence, the battery performance like first irreversible capacity loss, energy density, cycle stability, and cycle life performance4-5. Previous work shows in great detail the SEI formation mechanism for carbonaceous anodes5-6, while there are fewer studies for intermetallics materials. In this work, the SnSb anode electrode was fabricated using slurry cast method by mixing a SnSb powder, binding agent (PVDF), and conducting agent (Carbon Sp) with a ratio of 70:15:15. The electrochemical tests (GCPL, and PEIS) were performed using a standard three-electrode Swagelok cells assembled in a glove box filled with high purity argon. The half-cells consist SnSb as a working electrode, Li foil as the reference and counter electrode, and Whatman glass microfiber separator soaked in 1 M LiPF6 in EC:PC:3EMC with 1% VC,5 % FEC electrolyte solution. The PEIS were recorded by applying a dc bias and amplitude of 7 mV over the frequency range of 200 KHz to 10 mHz at various potential. The cell voltage at which PEIS measurement performed was achieved by galvanostatically cycling the cell at C/20. Fig1. shows the equivalent circuit used for analyzing the impedance spectra of SnSb/Li cell, where Re represents the total resistance of the electrode, electrolyte and separator; Rsei and Csei are the resistance and capacitance of the SEI film, respectively; Rct and Cdl are the charge-transfer resistance and double layer capacitance, respectively; and W is a Warburg impedance due to the semi-infinite diffusion of Li+ into SnSb electrode. Fig1. Equivalent circuit used for impedance analysis of SnSb/Li cell. Fig 2a & b, shows the impedance spectra for the first charge and discharge. At OCV before the first discharge, the impedance spectra has a semi-circle at high frequency (HF) due to the SEI film formation and a line at an angle close to 90 0 in lower frequency (LF) which is characteristics of capacitance, Fig 2a. This indicates that there is a formation of SEI film due to electrolyte decomposition by simple contact of the SnSb electrode surface with the electrolyte. The impedance spectra for the first alloying/de-alloying of Li+ into SnSb consists of two overlapped semi-circles at HF and medium frequency (MF) domain, which corresponds to the resistance of SEI and charge-transfer, followed by a spike at LF which is attributed to the diffusion of Li+. Fig 2. Typical Nyquist plot of SnSb electrode at various potentials for the (a) first discharge and (b) charge. During this work, the SEI formation on the surface of SnSb anode electrode for the first cycle will be discussed.