Compositionally Graded Cathode Material with Long-Term Cycling Stability for Electric Vehicles Application

Abstract

Al is introduced into a compositionally graded cathode with average composition of Li[Ni0.61Co0.12Mn0.27]O2 (FCG61) whose Ni and Mn concentrations are designed to vary continuously within the cathode particle. The Al-substituted full concentration gradient (Al-FCG61) cathode is tested for 3000 cycles in a full-cell, mainly to gauge its viability for daily charge/discharge cycles during the service life of electric vehicles (≈10 years). The Al-substitution enables the Al-FCG61 cathode to maintain 84% of its initial capacity even after 3000 cycles. It is demonstrated that the Al-substitution strengthens the grain boundaries, substantiated by the mechanical strength data, thereby delaying the nucleation of microcracks at the phase boundaries which is shown to be the main reason for the cathode failure during long-term cycling. It also shows that the Al-substitution decreases the cation mixing and suppresses the deleterious formation of the secondary phase that likely initiates the microcracks. Unlike an NCA cathode, whose depth of discharge (DOD) must be limited to 60% for long-term cycling, the proposed Al-FCG61 cathode is cycled at 100% DOD for 3000 cycles to fully utilize its available capacity for maximum energy density and subsequent reduction in cost of the battery.