Grid‐scale energy storage systems (ESSs) that can connect to sustainable energy resources have received great attention in an effort to satisfy ever‐growing energy demands. Although recent advances in Li‐ion battery (LIB) technology have increased the energy density to a level applicable to grid‐scale ESSs, the high cost of Li and transition metals have led to a search for lower‐cost battery system alternatives. Based on the abundance and accessibility of Na and its similar electrochemistry to the well‐established LIB technology, Na‐ion batteries (NIBs) have attracted significant attention as an ideal candidate for grid‐scale ESSs. Since research on NIB chemistry resurged in 2010, various positive and negative electrode materials have been synthesized and evaluated for NIBs. Nonetheless, studies on NIB chemistry are still in their infancy compared with LIB technology, and further improvements are required in terms of energy, power density, and electrochemical stability for commercialization. Most recent progress on electrode materials for NIBs, including the discovery of new electrode materials and their Na storage mechanisms, is briefly reviewed. In addition, efforts to enhance the electrochemical properties of NIB electrode materials as well as the challenges and perspectives involving these materials are discussed.

The insertion of guest species in graphite is the key feature utilized in applications ranging from energy storage and liquid purification to the synthesis of graphene. Recently, it was discovered that solvated-Naion intercalation can occur in graphite even though the insertion of Na ions alone is thermodynamically impossible; this phenomenon enables graphite to function as a promising anode for Na-ion batteries. In an effort to understand this unusual behavior, we investigate the solvated-Na-ion intercalation mechanism using in operando X-ray diffraction analysis, electrochemical titration, real-time optical observation, and density functional theory (DFT) calculations. The ultrafast intercalation is demonstrated in real time using millimeter-sized highly ordered pyrolytic graphite, in which instantaneous insertion of solvated-Na-ions occurs (in less than 2 s). The formation of various stagings with solvated-Na-ions in graphite is observed and precisely quantified for the first time. The atomistic configuration of the solvated-Na-ions in graphite is proposed based on the experimental results and DFT calculations. The correlation between the properties of various solvents and the Na ion co-intercalation further suggests a strategy to tune the electrochemical performance of graphite electrodes in Na rechargeable batteries. Broader contextThis study represents the most comprehensive work on the mechanism of guest ion-solvent co-intercalation in graphite forming ternary GICs, which has been poorly understood because of its complexity and difficulty in quantifying the intercalation of ion-solvent complexes compared with simple intercalation forming binary GICs, such as LiC 6 or KC 8 . Our results reveal unexplored co-intercalation mechanisms and the formation of ternary GICs in terms of the stoichiometry, staging structure, and solvated ion configuration. This work also advances our understanding of the correlation between the electrochemical properties and the players in the co-intercalation process; we demonstrate that the intercalation potential of solvated Na-ether complex ions into graphite is tunable by tailoring the length of the solvent species, which determines the thermodynamic stability of the intercalation products by screening the repulsion among charge-carrier ions. Our results will lead to the further advancement of graphite as a practically important potential anode for Na ion batteries and enrich the pool of electrode materials, which has been limited to the binary systems of guest ion-host materials to versatile ternary systems of guest ion-solvent-host materials for energy storage. † Electronic supplementary information (ESI) available: Experimental and computational details, graphite staging models, ex situ XRD analysis results, and supplementary discussions. See