Design Strategies for Sodium Electrode Materials: Solid-State Ionics Perspective

Sodium-ion battery is one of the most promising and feasible energy storage candidates. However, compared to the lithium ion, the larger ionic radius and higher molecular mass of the sodium ion lead to inferior electrochemical performance of sodium-ion batteries. Therefore, achieving the rational design and construction of high-performance electrode materials is a key point and remains a great challenge for sodium-ion batteries. In this work, we focus on the transport properties of sodium ions and electrons and discuss design strategies of sodium electrodes from the perspective of solid-state ionics. First, for the bulk sodium electrode materials, investigating their transport properties, such as ionic conductivity, electronic conductivity, and diffusion coefficient, is a prerequisite for electrode design. Although there are various methods of measuring the diffusion coefficient, separately achieving the intrinsic ionic and electronic conductivity of the pure materials is highly important. Doping and carbon-coating are the most useful approaches to improve the specific transport properties of the investigated materials. Building defect chemistry models based on measured transport properties and relevant defect chemistry theory is crucial but remains a great challenge for the design of sodium electrodes. Second, for the nano sodium electrodes, size effects can be applied to design and construct electrodes from a nanoionics perspective. Thermodynamically, the equilibrium shape and equilibrium voltage change with a reduction in the particle size and facilitate the discovery of new electroactive electrode materials. Kinetically, according to tau similar to L-2/D (where tau is diffusion time, L is particle radius, and D is diffusion coefficient), a smaller particle size leads to better kinetic behavior (higher rate performance) and also improves the diffusion coefficient in some cases. In terms of sodium transport and storage mechanisms, size effects result in the transition from a two-phase to a single-phase mechanism, an increase in the interfacial storage and surface reaction, as well as a variation of the sodium storage mechanism in pores, further leading to variation of the discharge voltage plateau. Finally, whether for bulk or nano-electrode materials, constructing efficient electrochemical circuits by the optimization of the phases and dimensionalities based on the transport properties of electrode materials is significant in achieving the rational design of sodium electrode materials and optimizing the electrochemical performance of sodium-ion batteries. We believe that this study will serve as a useful guide for the development of sodium electrode materials and will certainly contribute to the commercialization of sodium-ion batteries.