Phosphonium-Based Ionic Liquid Electrolyte in Combination with Silicon Anodes and LiNi0.5-XMn1.5+XO4 Cathodes for Lithium Ion Batteries

Abstract

Ionic liquids, like bis(fluorosulfonyl)imide (FSI) ionic liquids, are promising solvents for lithium ion batteries due to their high thermal stability and wide electrochemical window. The performance of ionic liquid electrolytes is however hampered by the low conductivity at room temperature, but the high thermal stability allows for operation at elevated temperatures, at which the conductivity is significantly improved.Electrolytes based on LiFSI and ionic liquids of the cations; pyrrolidinium (PYR13 +), and phosphonium (P111i4 +), with FSI anions, have been shown to exhibit stable cycling at room temperature, and good rate performance for cells composed of silicon anodes and LiFePO4 cathodes at 60 °C [1,2]. The lithium ion mobilities, relative to standard carbonate electrolytes, were determined from the limiting current densities at room temperature and 60 °C. Significant differences in the temperature dependence of the transport properties were revealed, with a considerable increase in the lithium ion mobility from 20 to 60 °C for electrolytes based on P111i4FSI. The best rate performance at 60 °C was demonstrated for the P111i4FSI based electrolyte, with discharge capacities around 1000 mAh/gSi at a 2C rate [2]. The improved performance was attributed to a combination of the improved lithium ion mobility and formation of a more conductive solid electrolyte interphase (SEI) for this electrolyte.In view of the excellent high temperature performance of P111i4FSI, including high thermal stability and a large electrochemical window [1,2], this electrolyte has been selected for further studies as electrolyte for high voltage LiNi0.5-xMn1.5+xO4 (LNMO) cathodes. LiNi0.5-xMn1.5+xO4 (LNMO) is an attractive candidate as a next generation Li-ion battery (LIB) cathode material because it has a high energy density, due to the high operating voltage, and a relatively low cost (Co-free) compared to alternative cathode chemistries. However, the stability of LNMO is not yet satisfactory for commercial applications, especially in full-cell configuration with graphite as the anode material, and at elevated temperatures. One challenge is the transition metal (TM) dissolution, which poisons the graphite anode by degrading the solid electrolyte interface (SEI) [4]. The TM dissolution is affected by the formation of hydrofluoric (HF) acid in carbonate-based electrolytes utilizing the LiPF6 salt [3]. Additionally, the decomposition processes occur with an increased rate at elevated temperatures [4,5]. Another challenge is the side reactions occurring at the high operating voltage, which limits the coulombic efficiency. Previous works have indicated that the carbon conductive additive is more prone to side reactions than the LNMO active material [6].Here, we present a study on the phosphonium-based ionic liquid P111i4FSI, with two different salt concentrations: 0.79 m LiFSI and 3.0 m LiFSI, as an alternative electrolyte for LIBs with an LNMO cathode.The focus of the research is to study the side reactions separately on the carbon conductive additive, and on the LNMO active material. Carbon conductive additives differing in structure and particle size are studied. The electrolyte is tested both in half-cell and full-cell configuration with a graphite anode, as well as at room temperature and elevated temperatures. Using 0.79 m LiFSI P111i4FSI results in similar capacity retention, but higher coulombic efficiencies after 200 cycles in half-cells at room temperature compared to 1 M LiPF6 in 1:1 EC:DEC (v/v). Post-mortem characterization is carried out with SEM, TEM, Raman spectroscopy, and FT-IR.