With the rapid development of energy storage technologies, all-solid-state sodium-ion batteries (ASSBs) have emerged as promising candidates for next-generation batteries, due to their potential for high energy density,safety and cost-effectiveness. However, one of the most significant challenges hindering the widespread adoption of ASSBs is the interface between the ceramic electrolyte and the metallic sodium electrode. Specifically, the interface between the ceramic electrolyte Na_3.4Zr₂Si_2.4P_0.6O₁₂ (NZSP) and the metallic sodium electrode has long been identified as a critical factor limiting the overall performance of these batteries. This interface is particularly problematic because it is prone to have defects that lead to high polarization during battery operation. High polarization, in turn, results in low capacity and poor cycling stability, which are detrimental to the practical application of ASSBs. To tackle these challenges, various strategies have been explored to optimize the interface between NZSP and the metallic sodium electrode.

One particularly innovative approach involves the introduction of a polymer film onto the surface of the NZSP electrolyte. Among the polymer films, polyethylene oxide (PEO) and polyethylene glycol diacrylate (PEGDA) have shown great promise. These polymers are chosen not only for their chemical compatibility with the ceramic electrolyte but also for their ability to enhance the interfacial properties between the electrolyte and the electrode. The introduction of the polymer film has multiple purposes. Firstly, it significantly enhances the interfacial wettability and contact between the NZSP electrolyte and the metallic sodium electrode. This improved contact is crucial because it allows ions to smoothly transport across the interface, thereby reducing the resistance that contributes to high polarization. Secondly, the polymer film acts as a protective barrier that effectively prevents unwanted side reactions at the interface. These side reactions, often involving the reduction of the electrolyte or the oxidation of the electrode, can lead to the formation of insulating layers or the consumption of active materials, both of which degrade battery performance.

When a polymer film is applied to the surface of the NZSP electrolyte, the resulting Na/NZSP/Na symmetric battery demonstrates remarkable improvements in electrochemical performance and cycling stability. For instance, at current density of 0.01 m A·cm^-2, the polymer-coated symmetric battery can be stably cycled for up to 1,000 h, which is a significant enhancement as compared with the uncoated battery, which experiences a short circuit after operation for only 80 h. The ability to achieve such long-term stability is a major step forward in the development of ASSBs, as it addresses one of the primary concerns regarding their practical application.

The experimental results show that the Na/NZSP/Na symmetric cell coated with polymer film can be stably cycled for 1000 h at a current density of 0.01 m A·cm^-2, exhibiting excellent electrochemical performance and cycling stability, in contrast to the symmetric cell without polymer film, which was shorted after only 80 h. This result indicates that the introduction of polymer film provides an effective strategy for the interfacial optimization of all-solid-state sodium-ion batteries, which not only improves the performance of the batteries, but also lays the foundation for the commercial application of all-solid-state sodium-ion batteries.

Sodium-ion batteries (SIBs), as an emerging sustainable energy storage technology, hold strategic significance in multiple aspects. Their abundant, widely distributed raw materials and low cost reduce reliance on scarce lithium resources, enhancing energy security. SIBs also offer unique advantages in safety, cycle life and operating temperature range, with a higher thermal runaway temperature than lithium batteries and stable charge/discharge in low temperatures. Additionally, their lower manufacturing costs and broad application prospects in large-scale energy storage and electric vehicles make them a promising technology. The development of SIBs is not only a technological advancement but also a response to global energy transition and sustainable development. The traditional energy system, heavily dependent on fossil fuels, faces challenges such as resource scarcity, environmental pollution and climate change. SIBs, with their unique advantages, can play a significant role in constructing a clean, low-carbon, safe and efficient energy system. They can be used in renewable energy storage, smart grids and electric vehicles, promoting the transformation of the energy system towards sustainability. Moreover, the rise of SIBs reflects the innovation and exploration in the field of energy storage. They have broad application prospects in various fields. In large-scale energy storage, SIBs can be used for grid-connected energy storage, peak shaving and frequency modulation and renewable energy storage. In the field of electric vehicles, SIBs can be applied in pure electric vehicles, hybrid electric vehicles and commercial vehicles. In addition, they can also be used in consumer electronics, aerospace and other fields.

In recent years, significant progress has been made in SIB. Aiming at the difficulties of the electrolyte/sodium metal interface in all-solid-state SIB, the research progress in reducing the interfacial impedance and improving the interfacial contact through material optimization, interfacial chemical regulation, interfacial structural regulation, interfacial layer construction and heat treatment, is summarized, by taking the Na/Na₃Zr₂Si₂PO₁₂ interface as an example. Interfacial material optimization focuses on the development, design and optimization of electrolyte and electrode materials to improve the performance and stability of the interface. By regulating the structure, composition and surface properties of the materials, the ion transport rate can be increased, the interfacial resistance can be reduced and the interfacial side reactions can be suppressed. The reaction behavior of the interface between inorganic solid electrolyte and electrode can be optimized by regulating the chemical environment of the interface, i.e., the stability of the interface can be improved, the interfacial resistance can be reduced, and the electrolyte loss can be suppressed by adding interfacial modifier, controlling the interfacial redox state and regulating the interfacial charge density. By regulating the structural characteristics of the interface to improve the interfacial properties, i.e., by controlling the morphology, thickness, porosity, etc. of the interfacial layer, one can optimize the ion and electron transport paths, reduce the interfacial impedance and improve the energy storage and release rate. By making full use of the surface modification of various materials to construct interfacial interlayers, the buffer interlayer usually plays two key roles in the Na/NZSP interface, both as a filler for the voids and defects on the NZSP surface, which can connect the SSEs to the Na metal anode to regulate the Na⁺ ion transport and reduce the interfacial resistance, while serving as a protective layer to prevent the occurrence of the Na and SSEs in the long term cyclic process of the side reactions. Heat treatment is a common method for interface improvement, which can promote the increase in interfacial bonding strength, crystallization and mutual diffusion of interfacial particles, thus improving the electrochemical properties and stability of the interface, through high-temperature sintering, thermal annealing and other heat treatment processes.

In summary, SIBs show great potential in terms of resource utilization, safety and cost control, but still face challenges in terms of energy density and interfacial stability. Future research should focus on optimizing electrolyte/electrode interfacial compatibility, as well as further improving battery energy density and cycle life, so as to accelerate its commercialization in all-solid-state sodium-ion batteries and contribute more to the development of sustainable energy storage field. With the advancement of technology and the maturity of the industrial chain, SIBs are expected to be widely used in large-scale energy storage and electric vehicles, and make greater contributions to the development of sustainable energy storage.