Reviewing Li-Rich Disordered Rocksalts as Next-Generation High-Energy Cathode Material

To keep up with the ever increasing demand for improved energy storage technologies, huge improvements in the capacity and energy density of intercalated cathode materials used in lithium batteries are required. Due to their crystal structure and chemistry, current layered and spinel-type cathodes can reversibly cycle only a maximum of one Li+ per formula unit (pfu). Radical improvements are needed, and this will require the adoption of new chemistries. Li-rich disordered rocksalt oxide (LDRS) cathodes can store more than one Li+ pfu. They have a crystalline rocksalt structure with a disordered cation lattice. LDRS has demonstrated the potential to provide capacities over a wide range of Li compositions without structural changes. Cation disorder results in unique Li transport properties, electrochemical profile, local structure, and very little to no structural change during charge–discharge cycling. A cationically disordered lattice also makes the material compositionally flexible. This reduces the dependence on scarce and expensive raw materials, such as nickel and cobalt. A higher operating voltage, which leads to the higher energy density of LDRS materials, is generally achieved by the introduction of anion redox in addition to cation redox and/or by fluorination. While cation-disordered rock salts (DRS) have been studied for over a decade, high-energy DRS materials are a relatively new class of compounds. This Perspective reviews the DRS synthesis and Li transport properties of DRS materials and, more specifically, the design strategies that have been employed to achieve high-energy-density DRS cathodes. Despite their structural stability against cation migration, DRS cathodes face challenges. These include poor capacity retention. We review the possible mechanisms for capacity and voltage degradation and suggest possible strategies to improve the performance of high-voltage/high-power DRS to make it a viable high-energy-density alternative to the present Li–Ni–Mn-Co oxide (NMC) cathodes.