Reducing the Rate of Mn Dissolution in LiMn 0.8 Fe 0.2 PO 4 /Graphite Cells with Mixed Salt and Low Salt Molarity Electrolytes

Abstract

Li-ion cells using LiMn x Fe 1-x PO 4 (LMFP) as the positive electrode can fail via Mn dissolution from the positive electrode and subsequent deposition on the negative electrode. Developing methods to reduce the amount of Mn on the negative electrode is critical for the commercialization of this material. Blends of LiPF 6 and LiFSI salts are used in varying ratios, 5%, 30%, or 40% LiPF 6 , and at varying concentrations, 1.5, 1.0, or 0.5 M total salt to determine the impact of reducing salt concentration on the lifetime of LMFP/artificial graphite cells. Cycle life was improved for mixed salt cells compared to cells containing pure LiPF 6 , and cells containing low salt concentration electrolytes showed the best performance. Mn deposition on the negative electrode was quantified by X-ray fluorescence spectroscopy, which showed that lowering the salt concentration reduced the rate of Mn deposition per cycle on the negative electrode. Nuclear magnetic resonance spectroscopy showed that LiPF 6 was preferentially consumed from the electrolyte during cycling.