Recent Progress and Perspective: Na Ion Batteries Used at Low Temperatures

Tuning the Electrolyte and Interphasial Chemistry for All-Climate Sodium-ion Batteries

Sodium-ion batteries (SIBs) present a promising avenue for next-generation grid-scale energy storage. However, realizing all-climate SIBs operating across a wide temperature range remains a challenge due to the poor electrolyte conductivity and instable electrode interphases at extreme temperatures. Here, we propose a comprehensively balanced electrolyte by pairing carbonates with a low-freezing-point and low-polarity ethyl propionate solvent which enhances ion diffusion and Na+-desolvation kinetics at sub-zero temperatures. Furthermore, the electrolyte leverages a combinatorial borate- and nitrile-based additive strategy to facilitate uniform and inorganic-rich electrode interphases, ensuring excellent rate performance and cycle stability over a wide temperature range from -45 °C to 60 °C. Notably, the Na||sodium vanadyl phosphate cell delivers a remarkable capacity of 105 mAh g-1 with a high rate of 2 C at -25 °C. In addition, the cells exhibit excellent cycling stability over a wide temperature range, maintaining a high capacity retention of 84.7 % over 3,000 cycles at 60 °C and of 95.1 % at -25 °C over 500 cycles. The full cell also exhibits impressive cycling performance over a wide temperature range. This study highlights the critical role of electrolyte and interphase engineering for enabling SIBs that function optimally under diverse and extreme climatic environments.

Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries

The shortage of resources such as lithium and cobalt has promoted the development of novel battery systems with low cost, abundance, high performance, and efficient environmental adaptability. Due to the abundance and low cost of sodium, sodium-ion battery chemistry has drawn worldwide attention in energy storage systems. It is widely considered that wide-temperature tolerance sodium-ion batteries (WT-SIBs) can be rapidly developed due to their unique electrochemical and chemical properties. However, WT-SIBs, especially for their electrode materials and electrolyte systems, still face various challenges in harsh-temperature conditions. In this review, we focus on the achievements, failure mechanisms, fundamental chemistry, and scientific challenges of WT-SIBs. The insights of their design principles, current research, and safety issues are presented. Moreover, the possible future research directions on the battery materials for WT-SIBs are deeply discussed. Progress toward a comprehensive understanding of the emerging chemistry for WT-SIBs comprehensively discussed in this review will accelerate the practical applications of wide-temperature tolerance rechargeable batteries.

A Carbonyl and Azo-Based Polymer Cathode for Low-Temperature Na-Ion Batteries

Due to flexible structure tunability and abundant structure diversity, redox-active polymers are promising cathode materials for developing affordable and sustainable Na-ion batteries (NIBs). However, polymer cathodes still suffer from low capacity, poor cycle life, and sluggish reaction kinetics. Herein, we designed and synthesized a polymer cathode material bearing carbonyl and azo groups as well as extended conjugation structures in the repeating units. The polymer cathode exhibited exceptional electrochemical performance in NIBs in terms of high capacity, long lifetime, and fast kinetics. When coupled with a low-concentration electrolyte, it shows superior performance at low temperatures down to -50 °C, demonstrating great promise for low-temperature battery applications. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were employed to study the reaction mechanism, interphase structure, and morphological evolution, confirming reversible redox reactions between azo/carbonyl groups in the polymer and Na+/electrons, a NaF-rich interphase, and high structure stability upon cycling. This work provides an effective approach to developing high-performance polymer cathodes for affordable, sustainable, and low-temperature NIBs.

Advantages of a Solid Solution over Biphasic Intercalation for Vanadium-Based Polyanion Cathodes in Na-Ion Batteries

The efficient operation of metal-ion batteries in harsh environments, such as at temperatures below -20 °C or at high charge/discharge rates required for EV applications, calls for a careful selection of electrode materials. In this study, we report advantages associated with the solid solution (de)intercalation over the two-phase (de)intercalation pathway and identify the main sources of performance limitations originating from the two mechanisms. To isolate the (de)intercalation pathway as the main variable, we focused on two cathode materials for Na-ion batteries: a recently developed KTiOPO4-type NaVPO4F and a well-studied Na3V2(PO4)2F3. These materials have the same elemental composition, operate within the same potential range, and demonstrate very close ionic diffusivities, yet follow different (de)intercalation routes. To avoid any interpretation uncertainties, we obtained these materials in the form of particles with merely identical morphology and size. A detailed electrochemical study revealed a much lower capacity and energy density retention for phase-transforming Na3V2(PO4)2F3 compared to NaVPO4F, which exhibits a single-phase behavior over a wide range of Na concentrations. The reasons for the inferior rate capability and temperature tolerance for the phase-separating Na3V2(PO4)2F3 material should be affiliated with slow phase boundary propagation. We hope that the comprehensive information on limiting factors provided for both mechanisms is useful for the further optimization of electrode materials toward a new generation of high-power and low-temperature metal-ion batteries.

Vanadium Ferrocyanides as a Highly Stable Cathode for Lithium-Ion Batteries

Owing to their high redox potential and availability of numerous diffusion channels in metal-organic frameworks, Prussian blue analogs (PBAs) are attractive for metal ion storage applications. Recently, vanadium ferrocyanides (VFCN) have received a great deal of attention for application in sodium-ion batteries, as they demonstrate a stable capacity with high redox potential of ~3.3 V vs. Na/Na⁺. Nevertheless, there have been no reports on the application of VFCN in lithium-ion batteries (LIBs). In this work, a facile synthesis of VFCN was performed using a simple solvothermal method under ambient air conditions through the redox reaction of VCl₃ with K₃[Fe(CN)₆]. VFCN exhibited a high redox potential of ~3.7 V vs. Li/Li⁺ and a reversible capacity of ~50 mAh g^-1. The differential capacity plots revealed changes in the electrochemical properties of VFCN after 50 cycles, in which the low spin of Fe ions was partially converted to high spin. Ex situ X-ray diffraction measurements confirmed the unchanged VFCN structure during cycling. This demonstrated the high structural stability of VFCN. The low cost of precursors, simplicity of the process, high stability, and reversibility of VFCN suggest that it can be a candidate for large-scale production of cathode materials for LIBs.