Unveiling the correlation between structural alterations and enhanced high-voltage cyclability in Na-deficient P3-type layered cathode materials via Li incorporation

Abstract

With exceptional capacity during high-voltage cycling, P3-type Na-deficient layered oxide cathodes have captured substantial attention. Nevertheless, they are plagued by severe capacity degradation over cycling. In this study, tuning and optimizing the phase composition in layered oxides through Li incorporation are proposed to enhance the high-voltage stability. The structural dependence of layered Na_2/3Li_xNi_0.25Mn_0.75O_2+δ oxides on the lithium content (0.0 ≤ x ≤ 1.0) offered during synthesis is investigated systematically on an atomic scale. Surprisingly, increasing the Li content triggers the formation of mixed P2/O3-type or P3/P2/O3-type layered phases. As the voltage window is 1.5–4.5 V, P3-type Na_2/3Ni_0.25Mn_0.75O₂ (NL_0.0NMO, R$\bar{3}$m) material exhibits a sequence of phase transformations throughout the process of (de)sodiation, that is, O3⇌P3⇌O3′⇌O3″. Such complicated phase transitions can be effectively suppressed in the Na_2/3Li_0.7Ni_0.25Mn_0.75O_2.4 (NL_0.7NMO) oxide with P2/P3/O3-type mixed phases. Consequently, cathodes made of NL_0.7NMO exhibit a substantially enhanced cyclic performance at high voltages compared to that of the P3-type layered NL_0.0NMO cathode. Specifically, NL_0.7NMO demonstrates an outstanding capacity retention of 98% after 10 cycles at 1 C within 1.5–4.5 V, much higher than that of NL_0.0NMO (83%). This work delves into the intricate realm of bolstering the high-voltage durability of layered oxide cathodes, paving the way for advanced sodium-ion battery technologies.