Improvement of the cyclic deterioration and structural evolution of Li[Li_0.2Ni_0.2Mn_0.6]O₂ cathode material by controlling initial charging voltages

One of the Li-rich manganese metal layered materials, Li[Li_0.2Ni_0.2Mn_0.6]O₂ (LNMO), is a very promising cathode material with high energy density for lithium-ion batteries, but its cyclic stability faces great challenges. In this paper, we focus on the effect of the initial charging cut-off voltages on cyclic and structural changes upon cycling. The activation method of the Li₂MnO₃ phase in rich-lithium materials is very important, because the degree of activation from this phase strongly affects the reversible discharge capacity and cycle of Li-rich materials. It is found that stepwise charging can nearly fully activate the Li₂MnO₃ phase so that it obviously reduces the first-cycle irreversible capacity, enhancing discharge capacity and cyclic stability. The biggest discharge capacity reached 282 mA h g^-1 at a 0.1C rate and 239 mA h g^-1 was maintained after 100 cycles. Combining the CV tests and TEM observation, it was revealed that the way of controlling voltages could lessen the oxygen release, which is a crucial element to stabilize the layered structure, and delay the transformation from the layered to spinel. Moreover, EIS measurement results showed that the alleviated transformation resulted in smaller R_SEI and R_ct values, and this is the reason that stepwise charging can strengthen the cyclic performance of LNMO cathode material.