Improving The Electrochemical Performance Of Li_1.2Mn_0.6Ni_0.2O₂ Cathode

Recently, cobalt-free layered Li-rich cathodes are considered to be one of the most promising alternatives for next generation lithium ion batteries due to the high capacity, low cost and environmentally-friendly nature. However, these materials always suffer from lower columbic efficiency, poor capability, fast capacity loss and voltage decay, which hinder their commercial applications. To tackle those problems, in this thesis, surface modification and dual doping were chosen to improve the electrochemical performance of Li-rich cathode Li_1.2Mn_0.6Ni_0.2O₂?LMNO?.Firstly, electrochemical performances of LMNO modified with different amounts of conductive acetylene black ?CAB? were studied. The obtained results show that surface modification with CAB could improve the cycling stability and rate capability of LMNO. Comparing to 33.7% capacity retention of pristine after 80 cycles at 5C ?1C= 200mA/g?, the modified LMNO at 3% CAB delivered a capacity as high as 203.6 mAh/g with a capacity retention of 83.2% after 100 cycles at the same rate. The improved performance is supposed to be related to the main roles of CAB, i.e. an improved electronic conductivity and suppression for side reactions between cathode and electrolyte.Secondly, Na⁺ and Al³⁺ ions were chosen as the dopants for dual doping. Serial samples NaxLi_1.2-xMn_0.6-xAlxNi_0.2O₂ were synthesized by a simple sol-gel method. The effect of dual doping on the crystal structure, covalency of chemical bonds between transitional metal ion and oxygen, electrochemical performance and interface behavior between cathode and electrolyte were studied systematically. It is shown that dual doping with Na⁺ and Al³⁺, to some degree, could increase the lattice parameters ?a and c? without altering the layered structure. Although the feature of Ni-O bond remains unchanged after dual doping, the covalency of Mn-O was strengthened, which accounts for the enhanced cycling stability of dual doped sample. Furthermore, among all the doped samples, sample with x= 0.03 exhibited the highest columbic efficiency and the best cycling stability.When cycled at 30 ?, its discharge capacity retention at different rates all remains beyond 85% after 200 cycles. Moreover, when testing at high temperature of 55?, the discharge capacity retention at 2C after 100 cycles is higher than 90%, far better than those for the mono-doped or un-doped samples.