Impact of Mixed Salt Electrolytes for LiMnxFe1-XPO4/Graphite Li-Ion Cells

Lithium Manganese Iron Phosphate, LiMn x Fe 1-x PO 4 , (LMFP), is a promising positive electrode material for Lithium-ion batteries. Introduced by Pahdi et al. in 1997 [1], it is an alternative to the high-cost Nickel Manganese Cobalt (NMC) layered oxides and the low energy density Lithium Iron Phosphate (LFP) positive electrode materials. The LMFP material has a higher operating voltage than LFP, shows good cyclability, is low cost, and has a stable structure. However, the LMFP material suffers from poor rate capability due to its poor electronic conductivity and poor ionic diffusion. This hampers its use for high C-rate applications. Tailoring electrolytes to ensure better ionic diffusion at the solid/electrolyte interface is one solution to improve rate capability of these LMFP cells. Herein, we study the effect of different salts and mixed salts on LMFP/graphite Li-ion pouch cells and come up with a suitable electrolyte combination for LMFP cells. The salts used in this study are readily available salts and the mixed salts were made with a combination of these readily available salts at different ratios. The tests were conducted in a 70 °C temperature box to accelerate failure in these cells, as a method to screen these salts/mixed salts. Our observations show that mixed salts play a significant role in improving the lifetime of these cells, with the best cell reaching the end of life (EOL) after ~280 cycles at 70 °C. As LMFP is prone to manganese dissolution, the amount of manganese deposited on the negative electrode was quantified using X-ray Fluorescence spectroscopy (XRF). The cells using mixed salts display a lower amount of manganese deposited on the graphite compared to the individual salts. The use of readily available mixed salt electrolytes seems to improve cycle life, which is promising for the future commercialization of the LMFP positive electrode Li-ion cells. References: K. Padhi et al. 1997 J. Electrochem. Soc. 144 1188 Figure 1. a) Discharge capacity, b) normalized discharge capacity, and c) normalized voltage hysteresis of LMFP cells containing different salts/mixed salts, cycled at 70 °C. Figure 1