Single-Crystal Nickel-Rich Layered Oxide Cathodes Under High Voltage

Abstract

Nickel-rich layered oxide materials LiNixCoyM1-x-yO2 (x>0.5, M=Mn or Al, NMC or NCA) have been considered as a mainstream choice for electric vehicle (EV) batteries in recent years, which have higher energy density than polyanion-type materials and can remarkably extend the mile range of EVs. In NMC/NCA, the energy density can be improved by increasing the nickel content or the cut-off voltage. Increasing the nickel content can effectively improve the energy density but lead to low thermal stability. Increasing the cut-off voltage to improve the energy density may result in poor structure stability, such as cation mixing on surface and intergranular cracks in bulk.Here we synthesized the Zirconium coating single-crystalline LiNi0.6Co0.2Mn0.2O2 (ZNMC622) by molten salts method to achieving both high energy density and high thermal stability. ZNMC622 can deliver over 190 mAh g-1 at 4.5 V, which is close to the specific discharging capacity (~195 mAh g-1) of LiNi0.8Co0.1Mn0.1O2 at 4.3 V. The cycling tests show that ZNMC (4.5 V) has much higher capacity retention and retained 80% of initial capacity after 600 cycles than the capacity retention of NMC811 (4.3 V). It can be concluded that Zirconium coating can improve the particle surface stability at 4.5 V and single-crystalline structure can eliminate the intergranular cracks in bulk. Meanwhile, lower nickel content of ZNMC has better thermal stability, even at 4.5 V, the exothermic peak temperature and generated heat of ZNMC is obviously smaller than the counterpart of NMC811 (4.3 V). In conclusion, ZNMC is a promising cathode material with both high energy density and safety, especially suitable for EV batteries.