Chemical and structural instability of the chemically delithiated (1 – z) Li[Li1/3Mn2/3]O2·(z) Li[Co1–yNiy]O2 (0 ≤ y ≤ 1 and 0 ≤ z ≤ 1) solid solution cathodes

Abstract

The delithiated samples obtained by chemically extracting lithium from a series of O3 type layered (1 – z) Li[Li_1/3Mn_2/3]O₂·(z) Li[Co_1–yNi_y]O₂ solid solution samples by reacting with the oxidizer NO₂BF₄ in acetonitrile medium have been chemically and structurally characterized. The oxygen loss in the delithiated samples has been found to be determined by the amount of lithium in the transition metal layer of the O3 type layered structure. However, high Mn⁴⁺ content in the samples causes a decrease in oxygen loss and the lithium content in the transition metal layer is sensitively influenced by the tendency of Ni³⁺ to get reduced to Ni²⁺ and the consequent volatilization of lithium during synthesis. The chemically delithiated samples were found to adopt either the parent O3 type structure or the new P3 or O1 type structures depending upon the composition and synthesis temperature of the parent samples and the proton content inserted into the delithiated sample. For example, a high proton insertion resulting from a good cation ordering or a larger surface area seems to favor the transformation of the initial O3 structure to the P3 or O1 structure. However, such phase transformations were not evident in the electrochemically delithiated solid solution samples.