Understanding the Role of Ni in Stabilizing the Lithium-Rich High-Capacity Cathode Material Li[Ni_xLi_(1–2x)/3Mn_(2–x)/3]O₂ (0 ≤ x ≤ 0.5)

The lithium-rich high-capacity cathode material Li[Ni x Li (1–2 x )/3 Mn (2– x )/3 ]O 2 was investigated by X-ray absorption spectroscopy (XAS) to understand the role of Ni in the lithium-rich layered line in the pseudoternary system. The electronic structural changes that occur at different charged states were examined with soft XAS to understand the role of the secondary transition metal in controlling the stability. The Mn L II,III -edges reveal a relationship between Ni and the degree of structural transformation after the first cycle. Close examination of Li 2 MnO 3 shows that Mn changes its oxidation state slightly during charging; however, there is a more dramatic change upon reduction from 4+ to a value close to 3+ at discharge, where it participates in the subsequent charge-compensation mechanism. The Ni L II,III -edges reveal that nickel acts as a stabilizer preventing the complete transformation of Mn by substitution reduction that is thought to couple with the anionic redox couple during the extended oxygen-activation plateau. The O K-edge reveals that loss of stability might be related to diminished d–sp hybridization occurring after oxygen activation, which is improved by the incorporation of nickel as a stabilizing agent. These findings clarify the complex relationships among nickel, manganese, and oxygen within lithium-rich high-capacity cathode materials in controlling stability while simultaneously obtaining high capacities exceeding 250 mA·h/g.