This review summarizes the development history, basic characteristics, special co-intercalation mechanism, origin of the superior performance of ether-based electrolytes in sodium-ion batteries and their advancements in other batteries.

Clarifying the microstructure of hard carbon is essential to reveal its sodium storage mechanism and to develop hard carbon negative electrodes for high-performance sodium ion batteries. Currently, although various sodium storage mechanisms for hard carbon models are proposed, they are still controversial. Besides, the puzzling and abnormal variation of a Na⁺ diffusion coefficient during the discharge process cannot be well explained. Inspired by amorphous alloys, we propose and confirm the dispersion region at the junction between amorphous structures and graphite microcrystals, which is closely related to the structure of graphite microcrystals. The special dispersion region plays a buffer role in the sodium ion diffusion process and provides satisfactory storage capacity. Therefore, the effect of synthesis conditions on the local structure in the dispersion region should be considered when designing hard carbon. In this work, a specific graphite microcrystalline structure of hard carbon is precisely synthesized by screening organic molecules, and the constraint relationship between the parameters of the graphite microcrystalline structure is revealed. Importantly, this work is of great significance for resolving the current controversy about the sodium storage mechanism and making clear the anomalies of sodium ion diffusion in the low-voltage interval (<0.1 V) in hard carbon.

It is of great significance to design and innovate electrode materials with unique structures to effectively optimize the electrochemical properties of the secondary battery. Herein, inspired by neuron networks, an ingenious synthesis is proposed to fabricate NiSe with multidimensional micro-nano structures, followed by in situ construction of NiSe/NiO heterostructures via a temporary calcination. The major structure of bulk NiSe synthesized by the solvothermal method is 3-dimensional micron cluster spherical particles interwoven by uniform one-dimensional nanofibers. Such structures possess the synergistic advantages of nano and micro materials. After a temporary calcination in air, NiSe/NiO heterostructures should be formed in the bulk NiSe, which provides a built-in electric field to enhance diffusion kinetics of sodium ions. This special neural-like network and heterojunction structures ensure the excellent structural stability combined with rapid kinetics of the electrode, releasing 310.9 mAh g ⁻¹ reversible capacity after 2,000 cycles at 10 A g ⁻¹ . Furthermore, the electrochemical storage and ion transport mechanisms are elaborated by electrochemical analysis and theoretical calculation in more detail.