Sodium-ion batteries (SIBs) are now being actively developed as low cost and sustainable alternatives to lithium-ion batteries (LIBs) for large-scale electric energy storage applications. In recent years, various inorganic and organic Na compounds, mostly mimicked from their Li counterparts, have been synthesized and tested for SIBs, and some of them indeed demonstrate comparable specific capacity to the presently developed LIB electrodes. However, the lack of suitable cathode materials is still a major obstacle to the commercial development of SIBs. Here, we present a brief review on the recent developments of SIB cathodes, with a focus on low cost and high energy density materials (> 450 Wh kg⁻¹ vs Na) together with discussion of their Na-storage mechanisms. The considerable differences in the structural requirements for Li- and Na-storage reactions mean that it is not sufficient to design SIB cathode materials by simply mimicking LIB materials, and therefore great efforts are needed to discover new materials and reaction mechanisms to further develop variable cathodes for advanced SIB technology. Some directions for future research and possible strategies for building advanced cathode materials are also proposed here.

Nanocrystalline Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ was prepared by a layered-template method and was tested as a high-capacity and high-power cathode for Li-ion batteries. Structural characterization demonstrates that the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ nanoparticles have a high crystallinity with a monoclinic (C/2m) structure. This material exhibits an initial discharge capacity of 277.4 mAh g^–1 and a high coulombic efficiency of 87.3 %, with a very small capacity fade of 0.046 % per cycle over 100 cycles. Such excellent electrochemical performance is likely to result from its monoclinic structure that enables a stable solid solution structure and reversible structural changes during cycling. Therefore, monoclinic Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ may meet the high-capacity and high-rate requirements for an alternative cathode for a new generation of Li-ion batteries.

A simple ball-milling method is used to synthesize a tin oxide-silicon carbide/few-layer graphene core-shell structure in which nanometer-sized SnO₂ particles are uniformly dispersed on a supporting SiC core and encapsulated with few-layer graphene coatings by in situ mechanical peeling. The SnO₂-SiC/G nanocomposite material delivers a high reversible capacity of 810 mA h g⁻¹ and 83% capacity retention over 150 charge/discharge cycles between 1.5 and 0.01 V at a rate of 0.1 A g⁻¹. A high reversible capacity of 425 mA h g⁻¹ also can be obtained at a rate of 2 A g⁻¹. When discharged (Li extraction) to a higher potential at 3.0 V (vs. Li/Li⁺), the SnO₂-SiC/G nanocomposite material delivers a reversible capacity of 1451 mA h g⁻¹ (based on the SnO₂ mass), which corresponds to 97% of the expected theoretical capacity (1494 mA h g⁻¹, 8.4 equivalent of lithium per SnO₂), and exhibits good cyclability. This result suggests that the core-shell nanostructure can achieve a completely reversible transformation from Li_4.4Sn to SnO₂ during discharging (i.e., Li extraction by dealloying and a reversible conversion reaction, generating 8.4 electrons). This suggests that simple mechanical milling can be a powerful approach to improve the stability of high-performance electrode materials involving structural conversion and transformation.