Elucidating the Role of Excess Li in the Electrochemical Performance of Li1+x[Ni0.5Mn0.5]1-xO2 Layered Oxides

Layered cathode materials comprising of Ni and Mn can possess comparable theoretical capacities to Ni-rich cathode materials. However, to draw upon this capacity, they need to overcome rate capability issues and operate to higher voltages. Incorporating excess Li during synthesis can allow much of this capacity to be accessed. This work compares the effects of excess Li on electrochemical properties of Li1+x[Ni0.5Mn0.5](1-x)O-2 layered oxides in the conventional voltage window as well as with higher upper cut-off voltages. Materials with different amounts of excess Li were systematically compared based on specific capacity, first-cycle irreversible loss, cycling stability, and rate capability in the voltage ranges of 3.0 V-4.3 V, 3.0 V-4.5 V, and 3.0 V-4.8 V. In all samples, excess Li improves the rate capability and cycling stability in all these voltage ranges while significant gains in specific capacity can only be attained when operating these materials at higher voltage cut-offs. The improved rate capability performance in presence of excess Li can be attributed to enhanced electronic conductivity and Li+ ion diffusion arising from reduced amounts of Ni in the Li layer.