Recent advances in P2-type Ni–Mn-based layered oxide cathodes for sodium-ion batteries

As concerns over environmental contamination and rapid consumption of fossil fuels continue to grow, it is important for energy storage technology to reduce the intermittency of clean and renewable energy sources. So far, lithium-ion batteries (LIBs), commercialized by SONY corporation in 1991, have been the most widely used rechargeable batteries for various energy storage devices. Due to the ever-increasing demand for lithium employment in mobile electronic devices and electric vehicles (EVs), the price of Li resources is rising year by year. It is well known that worthwhile lithium resources are only found in a few countries (mainly in South America). Recently, sodium-ion batteries (SIBs) have been regarded as promising alternatives to LIBs for future large-scale energy storage systems (ESSs) owing to their low cost, abundant reservoirs of Na resources and similar characteristics to LIBs. Developing high-performance cathode materials is crucial to realize the commercialization of the SIB technology. Sodium transition metal oxides (Na_xTMO₂), especially for Ni–Mn-based compounds, have received significant attention thanks to their high specific capacity and operating voltage. Normally, layered Na_xTMO₂ materials have two types of crystal structures: P2 and O3, according to the surrounding Na environment and the number of unique oxygen layers occupied within the lattice. Compared with the O3 phase, the P2-type structure has open diffusion channels for the transport of Na⁺ and relatively rare phase transitions, which make P2-type Na_0.67Ni, MnO₂ (NNMO) one of the most promising cathodes for SIBs. However, NNMO materials generally suffer from irreversible P2–O2 phase transformations, Na⁺/vacancy ordering transitions and Jahn–Teller distortion of Mn^(III)O₆ octahedra, leading to structural deterioration and performance degradation during the charge and discharge processes. In detail, the P2–O2 phase transition inevitably causes significant lattice volume change (~20%) and even the formation of cracks, resulting in the stripping of active substances from the collector and serious capacity decay during cycling. The Na⁺/vacancy ordering in NNMO causes the multi-step two-phase reactions, which may increase the activation energy barrier for Na⁺ hops between adjacent prismatic sites, consequently hindering Na⁺ diffusion. Additionally, the lattice distortion and P2-P’2 phase transition induced by the Jahn–Teller effect also impede Na⁺ migration, leading to the sluggish kinetics of Na⁺ (de)intercalation. In this review, the recent progress on NNMO cathodes is summarized, including ion-doping, surface modification and composite structure. The comprehensive and integrated explanation of the structure–function–performance relationship of these optimized cathodes is further presented. Moreover, the existing challenges of NNMO and possible remedies are also discussed. It is expected that this review can provide new insights into the commercialization of NNMO for SIBs.