Phase evolutions of sodium layered oxide cathodes during thermal fluctuations

Layered transition metal oxide (Na_xTMO₂) cathodes are considered highly appropriate for the practical applications of sodium-ion batteries (SIBs) owing to their facile synthesis and high theoretical capacity. Generally, the phase evolution behaviors of Na_xTMO₂ during solid-state reactions at high temperature closely related to their carbon footprint, prime cost, and the eventual electrochemical properties, while the thermal stability in various desodiated states associated with wide temperature fluctuations are extremely prominent to the electrochemical properties and safety of SIB devices. Therefore, in this review, the influences of sintering conditions such as pyrolysis temperature, soaking time, and cooling rates on the phase formation patterns of Na_xTMO₂ are summarized. The competition between thermodynamics and kinetics during phase growth is extensively discussed. An overview on thermal stability enhancement strategies, such as elemental composition modulations, surface reconstructions, and polyphase symbiosis reported in previous works is provided. Furthermore, phase transition behaviors together with alterations in battery system temperatures under various electro-/chemical environments are discussed. Finally, we prospect the highly efficient preparations and high-heat-resistance designs of Na_xTMO₂ layered oxides.