Study of the lithium-rich integrated compound xLi₂MnO ₃ · (1-x)Li[Mn_yNi_zCo_w]O ₂ (x around 0.5; M = Mn, Ni, Co; 2:2:1) and its electrochemical activity as positive electrode in lithium cells

We investigated the structural characteristics of Li-rich xLi ₂MnO₃ · (1-x)Li[Mn_yNi_zCo _w]O₂ cathode material (x around 0.5, y:z:w around 2:2:1) and its electrochemical performance in lithium cells at 30 and 60°C. It was established that nanoparticles of the xLi₂MnO₃ · (1-x)Li[Mn_yNi_zCo_w]O₂ compound are intergrown on the nano-scale and are built of thin plates of 40-50 Å. We demonstrated that xLi₂MnO₃ · (1-x)Li[Mn _yNi_zCo_w]O₂ electrodes exhibited at 60°C high capacities of ̃270 and ̃220 mAh/g at 1C and 2C rates, respectively. They can be cycled effectively at 30 and 60°C providing capacity ̃250 mAh/g in the initial cycles, but it fades upon prolonged cycling due, to some extent, to increasing the electrode impedance (charge-transfer resistance) especially at the elevated temperature. The effective chemical diffusion coefficient of Li+ in these electrodes measured during charge to 4.7 V by potentiostatic intermittent titration technique (PITT) was found to be ̃10-10 cm²/s. From convergent beam electron diffraction and Raman spectroscopy studies we established, for the first time, that partial structural transition from layered-type to spinel-type ordering in xLi₂MnO₃ · (1-x)Li[Mn_yNi _zCo_w]O₂ electrodes occurred in the initial charge to 4.7 V and even at the early stages of charging at 4.1 V-4.4 V. The thermal behavior of the xLi₂MnO₃ · (1-x)Li[Mn _yNi_zCo_w]O₂ material and electrodes are also discussed.