Degradation of LiNi0.5Mn1.5O4Cathodes in the P111i4FSI Ionic Liquid Electrolyte and Carbonate Electrolytes

Abstract

LiNi0.5Mn1.5O4(LNMO) is a promising material for the cathode of lithium-ion batteries (LiBs); however, its high operating voltage causes stability issues when used with carbonate battery electrolytes. Ionic liquids are a viable alternative to conventional carbonate solvents due to their thermal stability and electrochemical window. This work reports the performance of LNMO/Li half cells with an ionic liquid electrolyte (ILE) composed of 0.79 molal LiFSI in trimethyl isobutyl phosphonium bis-fluorosulfonyl imide (P111i4FSI). The long-term stability of the cells cycled at 25 °C in ILE is superior compared to all the other cycling conditions, as shown by the Coulombic efficiency (>99.5%) and capacity retention after 210 cycles (>87.9%). Spectroscopy measurements showed that the LNMO in the LP40 cycled cells had severe structural damage, with visible holes in the surface region of the particle, extending 15–20 nm away from the surface. On the other hand, the structure of the LNMO used in the cells with ILE was similar to that of the pristine spinel after 210 cycles, the only difference being a rock-salt layer on the surface. The surface chemistry of the LNMO particles was analyzed by electron energy-loss spectroscopy and revealed that the surface region of the LNMO cycled in LP40 adopted a (MnxNiy)3O4-type structure in the previously described holes, while the surface chemistry was nearly unaffected by cycling in ILE. XPS highlighted the influence of the electrolyte on the nature of the cathode electrolyte interface (CEI), which showed the presence of a predominantly organic CEI after cycling in LP40. The CEI formed after cycling in ILE was thinner and dominated by species like Li2CO3and salt decomposition products. Overall, the cycling stability of LNMO with LiFSI in P111i4FSI was improved, and the structural integrity was maintained with this electrolyte, as opposed to the conventional LP40.