It is essential to replace lithium-ion batteries (LIBs) from the perspective of the Earth's resources and the sustainable development of mankind. Sodium-ion batteries (SIBs) are important candidates due to their low price and abundant storage capacity. Hard carbon (HC) and graphite have important applications in anode materials of SIBs. In this review, the research progress in electrolyte and interface between HC and graphite anode for SIBs is summarized. The properties and performance of three types of widely used electrolytes (carbonate ester, ether, and ionic liquid) with additives, as well as the formation of solid electrolyte interface (SEI), which are crucial to the reversible capacity and rate capability of HC anodes, are also discussed. In this review, the cointercalation performance and mechanism of solvation Na + into graphite are summarized. Besides, the faced challenges and existing problems in this field are also succinctly highlighted.

The development of large‐scale energy storage systems (EESs) is pivotal for applying intermittent renewable energy sources such as solar energy and wind energy. Lithium‐ion batteries with LiFePO4 cathode have been explored in the integrated wind and solar power EESs, due to their long cycle life, safety, and low cost of Fe. Considering the penurious reserve and regional distribution of lithium resources, the Fe‐based sodium‐ion battery cathodes with earth‐abundant elements, environmental friendliness, and safety appear to be the better substitutes in impending grid‐scale energy storage. Compared to the transition metal oxide and Prussian blue analogs, the Fe‐based polyanionic oxide cathodes possess high thermal stability, ultra‐long cycle life, and adjustable voltage, which is more commercially viable in the future. This review summarizes the research progress of single Fe‐based polyanionic and mixed polyanionic oxide cathodes for the potential sodium‐ion batteries EESs candidates. In detail, the synthesized method, crystal structure, electrochemical properties, bottlenecks, and optimization method of Fe‐based polyanionic oxide cathodes are discussed systematically. The insights presented in this review may serve as a guideline for designing and optimizing Fe‐based polyanionic oxide cathodes for coming commercial sodium‐ion batteries EESs.

Na super ionic conductor (NASICON)-type Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) has been regarded as a prospective candidate of cathode materials for sodium-ion batteries due to its excellent structural stability, relatively high capacity and working voltage. However, the poor cyclability and rate capability, resulting from its low intrinsic electronic conductivity, have become a serious obstacle to their practical large-scale application. In this work, N-doped carbon coated NVPF composites (NVPF@NC) were successfully synthesized via a simple sol−gel method, in which low-cost polyvinylpyrrolidone was introduced as a nitrogen source. After high-temperature pyrolysis, a highly conductive N-doped carbon layer was in-situ constructed on the particle surface to enhance the sodium storage performance of NVPF. The optimized NVPF@NC cathode delivered high reversible capacity, excellent rate capability and long-term cycle life compared to pristine NVPF@C. The remarkable electrochemical performance of NVPF@NC cathode benefits from the modification strategy of introducing a heteroatom-doped carbon layer, triggering the formation of extrinsic defects and active sites in the N-doped amorphous carbon layer, which greatly enhances the electrical conductivity and the diffusion rate of sodium ions. This work provides a facile and effective approach for the preparation of N-doped carbon coated NVPF with remarkable sodium storage properties, which could be extended to other electrode materials electrochemical for energy storage. KEYWORDS: sodium-ion batteries, Na 3 V 2 (PO 4 ) 2 F 3 , nitrogen-doped carbon, cathode material, electrochemical energy storage

and electric vehicle broadly, [1] while the lower lithium resource reserve in the earth limits the application for the large scale energy storage in the future. Owing to the earth abundant element reserve of Na in the earth's crust and excellent electrochemical performance of sodium-ion battery, which is an alternative choice to meet the large-scale energy storage system (ESS) for the construction of smart grid and energy internet.Solid sodium-ion battery enjoys high security, high energy density, and shape variability, which is very promising for the application in large-scale ESS. [2,3] Solid electrolytes are key component to provide the transfer of charges by ion movement between two electrodes. However, the low ionic conductivity and poor interface between solid electrolyte and electrode limit the industrial application of solid sodium-ion battery. It is very important for the comprehensive understanding of solid electrolyte/electrode interface mechanism and science and technology problem, as well as the development tendency of solid sodium-ion battery.The overall sodium-ion battery technologies have been reviewed broadly. However, solid electrolyte and interface is rarely concerned yet. Review on sodium-ion battery solid electrolyte and interface is meaningful to design solid sodium-ion battery and improve the safety. The research focus on Na-ion batteries has drastically increased in recent years after 2010, our review mainly provides detailed and comprehensive research progress for sodium-ion battery solid polymer electrolyte and inorganic solid-state electrolyte systems (Figure 1).

P2-type Na 0.67 Ni 0.33 Mn 0.67 O 2 has been considered as the potential cathode for sodium-ion batteries. However, its practical application is plagued by Na + /vacancy ordering, harmful phase transition, and lattice oxygen loss. Herein, we develop a dual site-selective substitution strategy to fabricate a P2-type Na 0.63 Ca 0.05 (Ni 0.26 Li 0.07 Mn 0.67 )O 2 cathode. The substitution of Li + for Ni 2+ introduces lone pair oxygen via forming a Li−O−Li configuration and make O 2p close to its Fermi level due to the weakened TM−O (TM: transition metal) bond, which triggers the anionic redox for charge compensation, while the introduction of Ca 2+ in a Na layer enhances the electrostatic cohesion of neighboring TM layers by forming a strengthened O−Ca−O configuration, which suppresses the glide of adjacent TM layers and reduces the excessive lattice oxygen loss. Therefore, with a dual site-selective substitution strategy, the P2-type Na 0.63 Ca 0.05 (Ni 0.26 Li 0.07 Mn 0.67 )O 2 cathode can suppress the Na + /vacancy ordering, P2−O2 phase transition, and lattice oxygen loss even at a potential of 4.35 V, achieving a reversible anionic redox and solid-solution reaction. The P2-type Na 0.63 Ca 0.05 (Ni 0.26 Li 0.07 Mn 0.67 )O 2 cathode exhibits high discharge capacity (142.7 mA h g −1 at 20 mA g −1 ), excellent rate capability (57.1 mA h g −1 at 2 A g −1 ), and cyclic stability (a capacity retention of 83.2% after 700 cycles).