In recent years, considerable attention has been focused on the development of sodium-ion batteries (SIBs) because of the natural abundance of raw materials and the possibility of low cost, which can alleviate the concerns of the limited lithium resources and the increasing cost of lithium-ion batteries. With the growing demand for reliable electric energy storage devices, requirements have been proposed to further increase the comprehensive performance of SIBs. Especially, the low-temperature tolerance has become an urgent technical obstacle in the practical application of SIBs, because the low operating temperature will lead to sluggish electrochemical reaction kinetics and unstable interfacial reactions, which will deteriorate the performance and even cause safety issues. On the basis of the charge-storage mechanism of SIBs, optimization of the composition and structure of electrolyte and electrode materials is crucial to building SIBs with high performance at low temperatures. In this review, the recent research progress and challenges were systematically summarized in terms of electrolytes and cathode and anode materials for SIBs operating at low temperatures. The typical full-cell configurations of SIBs at low temperatures were introduced to shed light on the fundamental research and the exploitation of SIBs with high performance for practical applications.

As lithium (Li) metal has the highest specific capacity (3860 mAh g −1 ) and lowest anode potential (−3.04 V vs SHE), it is considered as the optimal choice of anode materials for new energy storage devices. However, the unstable Li plating/stripping behavior of the solid electrolyte interphase (SEI) on the Li anode surface triggers the common Li dendrite growth and side reactions between Li metal and electrolytes. The construction of an artificial SEI on the surface of lithium anodes is one of the effective approaches to improve the ionic conductivity, suppress the growth of lithium dendrite, and address the problem of low reversibility of lithium anodes. However, it remains a difficult problem to construct a uniform artificial SEI due to the lack of stability. Herein, an artificial Li 3 BO 3 SEI film is synthesized through a facile, environmentally friendly, and inexpensive in situ reaction between boric acid and anodes to address the difficult issues. The artificial Li 3 BO 3 protective layer exhibits a stable dendrite-free cycling behavior after 900 h at 1.0 mA cm −2 in the Li symmetrical cell. The Coulombic efficiency of the Li|LiFePO 4 batteries is close to 100% after 500 cycles, which is better than those of the unmodified bare samples. In addition, Li-Li 3 BO 3 also shows excellent electrochemical performance in lithium−sulfur (Li−S) batteries. These innovative findings provide new insights into the interfacial issues of Li metal anode protection and are expected to be a promising strategy for stable Li metal batteries.

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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

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