Abstract
P2-structured Na0.67Ni0.33Mn0.67O2 (PNNMO) is a promising Na-ion battery cathode material, but its capacity fading during cycling remains a hurdle. Li doping in layered transition-metal oxide (TMO) cathode materials is known to enhance their electrochemical properties. Nevertheless, the influence of Li at different locations in the structure has not been investigated. Here, the crystallographic role and electrochemical impact of lithium on different sites in PNNMO is investigated in Li x Na0.67–y Ni0.33Mn0.67O2+δ (0.00 ≤ x ≤ 0.2, y = 0, 0.1). Lithium occupancy on the Na layer is promoted in Na-deficient (Na < 0.67) PNNMO, evidenced by ex situ and operando synchrotron X-ray diffraction, X-ray absorption spectroscopy, and 7Li solid-state nuclear magnetic resonance. Partial substitution of Na with Li enhances cycling stability and specific capacity compared to PNNMO. In contrast, when lithium is located primarily on octahedral TM sites, capacity is increased but at the cost of stability.
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