钠离子电池电极材料的设计策略——固态离子学视角

钠离子电池是目前最有前景及可行性的新兴储能候选体系。对于钠离子电池而言，如何实现其电极材料的理性设计及构筑，是重要的科学问题。本文立足于钠离子/电子输运这一核心问题，从固态离子学视角探讨钠离子电池电极材料的设计策略。首先，对于体相电极材料，输运特性的明晰、调控以及缺陷化学模型的建立，是传统电极材料开发的关键。其次，对于纳米电极材料，随着尺寸的减小，电极材料的热力学性质、动力学特性以及钠离子微观储输机制都会发生相应变化，因此从纳米离子学视角，以尺寸效应调控电极材料具有重要的科学价值及现实意义。最后，无论对于体相材料还是纳米材料，从材料的本征输运特性出发，通过电化学电路的设计和构筑来优化电极动力学，可以为钠电电极材料的理性设计及可控制备提供理论指导。我们相信，通过本文系统地对钠离子电池电极材料设计策略的梳理，必将对钠离子电池的开发，提供有意义的指导，并为最终的产业化打下良好的基础。

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 τ~L²/D (where τ 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.