English

• 1. Introduction

  With the global population growing and the level of economic development improving, fossil energy has not only brought serious environmental problems, but also now faces resource depletion. It is urgent to develop new green, clean and sustainable energy technology [1–4]. The emergence of rechargeable battery technology provides a way to solve these problems. Meanwhile, the intermittency of clean energy such as wind, tide, and solar energy also requires the further development of energy storage grid technology, among which smart grid and large-scale energy storage devices are the key points [5–8]. Lithium-ion batteries, currently the preferred choice for portable electronic devices and electric cars, have been commercially developed for decades [9]. However, the lack of lithium resources and the uneven global distribution cannot meet the growing market demand, urging people to find a new generation of reliable alternatives [10–16]. Because sodium ions have similar structural and chemical properties to lithium ions, sodium-ion batteries have similar electrochemical storage mechanisms and are also “rocking chair” batteries. Compared with lithium-ion batteries, sodium-ion batteries are resource-rich and low-cost. It has a broad application prospect in low-speed electric vehicle and large-scale energy storage device, and is a strong competitor of lithium-ion batteries in the future [17–21].

  Large-scale energy storage and electric vehicles will be used on a large scale in low-temperature areas [22,23]. For ordinary secondary batteries, the energy density rapidly attenuates below 0 ℃, resulting in serious voltage drop, accompanied by extremely poor cycle stability, which has a great impact on electrical applications and causes potential safety hazards [24–26]. At present, military equipment, aerospace, diving equipment, polar scientific investigation, electric communication, electric vehicles and large-scale energy storage have put forward new requirements for low-temperature performance of batteries. The development of low-temperature and high-energy-density sodium-ion batteries is very important for the applications in these key fields [27–31]. Due to the slow mass transfer in electrolytes and sluggish interfacial and solid diffusion processes at low temperatures, conventional lithium-ion batteries only retain about 20% of their room-temperature capacity at −20 ℃, and some of them even stop working [32,33]. Sodium-ion batteries will also face the challenges such as slow kinetics at low temperature. Although the radius of sodium ion is larger than that of lithium ion, the Stokes radius of solvated structure [34,35] and the migration barrier of Na⁺ are lower than that of Li⁺, suggesting that sodium-ion batteries may have better performance at low temperature [36]. But at present, the research on low-temperature sodium-ion batteries is still in progress, not mature, many problems have not been solved, which limits the commercial application of sodium-ion batteries.

  Sodium-ion batteries have an advantage over lithium-ion batteries in large-scale energy storage and extreme environments, based on their greater resources and superior electrochemical performance at low temperature. Although some studies on improving the performance of low-temperature sodium-ion batteries from different perspectives have been reported recently, there is a lack of reviews on the low-temperature performance of sodium-ion batteries [37–42]. So, in this paper, the studies in recent years of low-temperature sodium-ion batteries are reviewed. First, we analyze the reasons for the performance degradation of sodium-ion batteries at low temperature and the storage mechanisms at low temperature (Fig. 1a). Then, modification strategies and specific applications are introduced from three perspectives of electrode materials, electrolyte and interphase. Coating, compositing, doping and structural design are used to improve the diffusion kinetics of sodium ions in electrode materials. The “cocktail strategy” is used to improve the overall performance of low-temperature organic electrolytes, while for aqueous electrolytes the strategy is to disrupt the ordered structure of the electrolyte to lower the freezing point. Design of the solventised structure to reduce the desolventisation energy or adopt a co-embedding mechanism to directly avoid the desolventisation process (Fig. 1b). Finally, we summarize all the factors, prospect low-temperature sodium-ion battery field, and point out the feasible direction.

  Figure 1

  

  Figure 1.  (a) Sodium storage mechanisms in sodium-ion batteries. (b) Challenges of low temperature sodium-ion batteries.

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  2. Energy storage mechanism of sodium-ion batteries at low temperature

  The slow mass transfer and struggling charge transfer at low temperature limit the performance of sodium-ion batteries (Fig. 1a). The capacity, energy/power density, rate performance and cycle stability of sodium-ion batteries have deteriorated significantly, greatly limiting their application and deployment at low temperature.

  At low temperature, the electrolyte freezes, the conductivity drops sharply, and the battery’s performance deteriorates dramatically. According to the freezing point reduction formula of dilute solution,

  $ \Delta T=k_{\mathrm{f}} m $

  (1)

  In the formula, ΔT is freezing point changes temperature, k_f is freezing point depression coefficient, and m is molality. To some extent, increasing the solute concentration could lower the freezing point. However, the problem is that when the concentration is too high, the formula fails and the high viscosity caused by high concentration will deteriorate the low-temperature performance.

  Ethylene carbonate (EC) is commonly used as one of the solvents in electrolytes because of its high dielectric constant and ability to form a stable solid electrolyte interphase (SEI) on the anode surface. But its melting point is 36.4 ℃, so its application is very limited at low temperatures. In order to extend the liquid-range, one common method is to introduce low freezing point solvent components to reduce the freezing point of electrolyte, such as propylene carbonate (PC, −48.8 ℃), diethyl carbonate (DEC, −43 ℃), 1,2-dimethoxyethane (DME, −58 ℃), ethyl acetate (EA, −84 ℃) and tetrahydrofuran (THF, −108 ℃).

  Factors affecting electrical conductivity include:

  $ \sigma=\sum\limits_i c_i z_i \mu_i F $

  (2)

  In the formula, σ is electrical conductivity, i is different ions, c_i is free ion concentration, z_i is ion charge number, μ_i is ion mobility, F is Faraday constant. Therefore, the key to improving the electrical conductivity of electrolytes lies in c_i the free ion concentration and μ_i ion mobility. c_i is related to the dielectric constant of solvent and the lattice energy of sodium salt. The concentration of free sodium ion in solution can be increased by increasing the dielectric constant and selecting sodium salt with low lattice energy. μ_i is related to the viscosity of electrolyte and solvation radius of solute ions.

  $ \mu=\frac{z e}{6 \pi \eta r} $

  (3)

  In the formula, e is unit charge, η is viscosity, and r is solvation radius. The radius of sodium ion (0.97 Å) is larger than that of the lithium ion (0.68 Å), but the solvation radius (Stokes radius) of sodium ion in electrolyte is 4.6 Å, which is smaller than that of the lithium ion (4.8 Å), which means the conductivity of the sodium electrolyte is higher than that of the lithium electrolyte for the same electrolyte concentration.

  At low temperatures the solvent solubility decreases and the solute precipitates, leading to a decrease in conductivity. Increasing the anion radius, increasing the charge delocalization and decreasing the force between cations and anions can promote the dissociation of solute. At the same time, to select the high dielectric constant solvent with strong binding force with Na⁺ can form a strong solvation structure, which is conducive to the dissolution of solute.

  Low temperature will also lead to a sharp rise in the viscosity η of the electrolyte and a serious decline in conductivity.

  $ \eta=\eta_0 \exp (E / k T) $

  (4)

  In the formula, η₀ is constant, E is activation energy, k is Boltzmann constant, T is absolute temperature. Viscosity η is the embodiment of the fluidity of the solution, and is related to the kinetic energy of the molecules in the solution and the intermolecular interaction forces. At low temperatures the kinetic energy of molecules decreases, the fluidity of electrolyte is weak, and the wetting of electrode and diaphragm becomes poor. High dielectric constant of solvents like EC, which has strong polarity and dipole-dipole interactions between solvents, improves the viscosity at low temperatures. Adding low viscosity (inert) cosolvents, such as dimethyl carbonate (DMC), dichloromethane (DCM), can alleviate this situation to some extent. At the same time, the proportion of high dielectric constant solvent in electrolyte is a quantity that needs to be considered comprehensively.

  From the perspective of electrode materials, the first is the slow diffusion process of Na⁺ in solid materials.

  $ D=D_0 \exp \left(-E_{\mathrm{A}} / R T\right) $

  (5)

  In the formula, D₀ is the maximum value of the diffusion coefficient at infinite temperature, E_A is the activation energy, R is the universal gas constant, and T is the absolute temperature. The long distance and unfavorable solid diffusion path contribute to the poor rate performance and capacity at low temperatures which results in intensified electrode polarization and reduced charge and discharge capacity. The large radius of Na⁺ severely hinders the reaction kinetics and causes huge volume changes during repeated discharging and charging. Good electrical contact between active electrode particles and current collectors is also a problem due to the huge volume changes.

  In addition, the charge transfer process associated with the electrode/electrolyte interphase also greatly influences the kinetics of chemical reaction in the battery, apart from the slow mass transfer process in electrolyte and the slow diffusion process in electrode material. The charge transfer process at low temperatures greatly limits the kinetics of chemical reaction and is considered as the decisive step. The charge transfer process includes: (1) Solvated sodium ions are desolventizing on the electrode surface; (2) Sodium ions migration in SEI; (3) Sodium ions receive electron and are embedded/deposited in anode to complete the reaction. This process can be described by the Arrhenius formula:

  $ \frac{1}{R_{\mathrm{ct}}}=A_0 \mathrm{e}^{-E_{\mathrm{a}} / R T} $

  (6)

  In the formula, R_ct is the charge transfer impedance, A₀ is the pre-exponential factor, related to the charge of ion or ion cluster, E_a is the activation energy, R is the gas constant, T is the absolute temperature. Thus, the charge transfer impedance increases exponentially as the temperature decreases. E_a, activation energy, includes ion desolvation energy and migration barrier in SEI. Ion desolvation energy is a major factor in the charge transfer process (usually 200–500 meV). The selection of solvents, solutes, and additives in the electrolyte determines the solvated structure of Na⁺, which further determines the composition as well as the morphology of the SEI. Different solvation structures correspond to different desolvation energy. The influencing factors of migration barrier include the compositions (inorganic and organic) and elements (F, C, O, etc.) of SEI.

  Furthermore, due to the decline in the conductivity of electrolyte and the slowed charge exchange at low temperatures, sodium metal precipitation and deposition will occur in the anode side, especially when the battery is subjected to high-rate charging and discharging. The irreversible reaction between the deposited sodium metal and electrolyte consumes a large amount of electrolyte, further increases the SEI thickness, which increases the impedance and polarization of anode/electrolyte interphase. As a result, the low-temperature performance, cycle life and safety of the battery is greatly damaged.

  3. Application and strategy for low temperature battery

  At low temperatures, the slow mass transfer in the electrolyte as well as the electrode and the struggling charge transfer at interphase are the primary causes of performance degradation in sodium-ion batteries. For electrode materials, enhancing the diffusion coefficient D of Na⁺ within the electrode through structural design and other approaches is an effective strategy to mitigate polarization. For electrolytes, selecting appropriate components to lower the freezing point ensures that the electrolyte remains unfrozen at low temperatures, which is essential for normal battery operation. Meanwhile, the conductivity σ is a critical parameter governing mass transfer in the electrolyte. By optimizing the electrolyte formulation to achieve high conductivity σ, which can reduce concentration polarization and improves battery performance. The charge transfer process at the interphase is intimately linked to the desolvation of Na⁺ and its migration through the SEI. Reducing the activation energy E_a associated with desolvation and migration in the SEI is an effective strategy to accelerate the charge transfer kinetics. Starting from these critical low-temperature parameters, the electrode materials, electrolytes, and interphase are systematically optimized to enhance kinetics as well as maintain stability at low temperatures, which is beneficial for promoting the large-scale application of sodium-ion batteries [43–47].

  3.1 Low temperature battery application and strategy of cathode

  Using highly conductive materials to composite or surface coat the cathode can reduce the impedance, and at the same time, the coating layer can reduce the side reaction between cathode and the electrolyte, increasing the stability of cathode structure [48,49]. Liu et al. adopted phosphate material Na₃V₂(PO₄)₃ with fast three-dimensional (3D) Na⁺ migration NASICON structure to improve Na⁺ diffusion coefficient (Fig. 2a), and improve its poor conductivity by carbon coating [50]. It delivers a capacity of 91.3 mAh/g under 10 C at −20 ℃. The retention rate was 85.2% (100% at room temperature). Among the known materials for sodium storage electrodes, polyanion materials have good prospects for development due to their stable structure, high safety and open sodium ion transport channels. What is more, Li et al. synthesized mesoporous titanium-niobium oxide and carried out carbon coating (Ti_0.88Nb_0.88O_4-x@C) through limited acid-base pair self-assembly strategy [51]. The oxygen vacancies in TNO not only provide active sites for Na⁺ storage, but also induce the excess electrons around the metal atoms to form negative charge centers, thus attracting Na⁺. In this way, the diffusion of Na⁺ can be accelerated and the graphitized carbon coating allows for fast electron migration, while also perfectly stabilizing the ordered mesoporous structure and inhibiting irreversible side reactions. Due to the large radius of sodium ion, the volume of electrode material changes greatly during charge/discharge progress, and the maintenance of electrical contact between electrode particles and collector becomes a problem. Due to the small volume change of Prussian blue during cycling, You et al. anchored monodisperse Prussian blue nanotubes to a network of conductive carbon nanotubes to form a robust and flexible PB/CNT (Fig. 2b), which keeps a good electrical contact between PB particles and the collector while reducing the temperature [26]. At −25 ℃, it delivers the discharge capacity of 142 mAh/g, the power density of 408 Wh/kg, and the Coulombic efficiency is 99.4%. Moreover, the low-temperature performance is better than that of LiFePO₄.

  Figure 2

  

  Figure 2.  Cathode material for low temperature sodium-ion battery. (a) Schematic diagram of Na₃V₂(PO₄)₃. Copied with permission [50]. Copyright 2016, Elsevier. (b) PB/CNT structure representation. Copied with permission [26]. Copyright 2016, Wiley. (c) Schematic diagram of 1D NaCrO₂ nanowires. Copied with permission [52]. Copyright 2019, American Chemical Society. (d) Structural design of radially aligned hierarchical columnar structure (RAHC) cathode. Copied with permission [53]. Copyright 2015, Springer Nature.

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  The slow diffusion of Na⁺ in electrode materials is one of the reasons for sodium-ion batteries’ poor performance at low temperatures. Although it is not a rate-determining step, it can improve the overall performance of sodium-ion batteries such as energy/power density by increasing the Na⁺ diffusion kinetics. Liang et al. synthesized ultra-long layered NaCrO₂ nanowires nanocrystals (Fig. 2c) [52]. Its synthetic one-dimensional (1D) NaCrO₂ nanostructure has directional and shortened electron/ion transport and is structurally remarkably resistant to stress changes during Na⁺ insertion/extraction. As a result, Na⁺ storage behavior is significantly improved in the wide temperature range from −15 ℃ to 55 ℃. With 1.0 mol/L NaPF₆ in EC/DEC as the electrolyte, the NaCrO₂/Na cell delivers the capacity of ~60.1 (−15 ℃) mAh/g at 10 C, and the capacity retention is ~80.6%. Hwang et al. designed cathode spherical particles with a radially aligned hierarchical columnar structure with different chemical compositions from the inner end Na[Ni_0.75Co_0.02Mn_0.23]O₂ to the outer end Na[Ni_0.58Co_0.06Mn_0.36]O₂ (Fig. 2d) [53]. Based on the electrochemical reaction of Ni^2+/3+/4+, the cathode material can easily provide a high discharge capacity, while the columnar structure provides a directional migration path, ensuring fast diffusion kinetics of sodium ions. With 0.5 mol/L NaPF₆ in ethyl methyl carbonate (EMC)/fluoroethylene carbonate (FEC) as electrolyte, the battery can maintain 92% capacity after 100 cycles under 75 mA/g at −20 ℃.

  The bulk phase doping of Mn, Al, Cr, Mg, F and other elements increases the layer spacing of the material to reduce the diffusion impedance of Na⁺ and improve the low-temperature performance. Guo et al. prepared Na₃V₂(PO₄)₂O₂F nanofour-prism (NVPF-NTP), showing two high working platforms, low strain (2.56% volume change) and superior Na transport kinetics, which enable it to have a long cycle life, superior low-temperature performance and excellent high rate capacity [54]. It introduced F with greater electronegativity into the NVP lattice, and V-F bonds replaced part of V-O bonds. In the process of Na⁺ insertion/extraction, the change in V valence results in a higher redox potential and thus an increase in energy density. The substitution of F for O helps to narrow the band gap and increase the concentration of electrons and holes, which is conducive to Na⁺ insertion/extraction. However, due to the induction effect, too much F element will enhance polarization but not be conducive to Na ion diffusion. Yang et al. prepared Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂ cathode material (1/12-NNMF), which inhibited P2-O2 high-voltage phase transition and improved structural stability through Fe substitution [55]. The introduction of Fe³⁺ with larger radius expanded transition metal layer spacing, promoted electron delocalization, and enhanced the diffusion of Na⁺. It has excellent rate capacity (65 mAh/g at 25 C) and excellent low-temperature performance (capacity is 84 mAh/g at −25 ℃, capacity retention is 63%). Shi et al. designed Nb doping of P2-type Na_0.78Ni_0.31Mn_0.67Nb_0.02O₂ cathode material, and found that Nb doping could enlarge the distance between the transition metal (TM) layer, from 0.376 nm to 0.389 nm, and Na-O band from 0.251 nm to 0.256 nm, which endows Na⁺ with enhanced de/intercalation capabilities [56]. Based on density functional theory (DFT), authors found that Nb doping reduces the electron band gap and ion diffusion barrier, that the mobility of Na⁺ is easier when Nb is involved. Yang’s group synthesized high entropy Na₄Fe_2.95(MgCaAlCrMn)_0.01(PO₄)₂P₂O₇ (HE-NFPP) by doping transition metals (TMs) ion (Mg, Ca, Al, Cr, Mn) into Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) through HE strategy, which could reduce the band gap between the conduction and valence bands and promote electron transfer. Meanwhile, the 3D network of Na⁺ diffusion channels formed in HE-NFPP can effectively improve the diffusion efficiency of Na⁺, thereby enhancing the electrode reaction kinetics at low temperatures. HE-NFPP batteries exhibit a remarkable capacity retention of 96.7% after 100 cycles under 1 C at −10 ℃, versus only 65.6% for NFPP [57].

  Reducing the particle size could shorten the Na⁺ migration path. However, it should be pointed out that this method will increase the specific surface area of the material and thus increase the side reaction with the electrolyte. Liu et al. transformed electrochemically passivated NaFePO₄ (NFP) into a highly active amorphous phase through the unique ultrafine NFP nanoclusters (NFPNCs) nanostructure [58]. The porous phase is formed due to the ultrafine nano effect. The ultrafine NFP@C subunit shortens the Na⁺/electron diffusion path and provides high conductivity, leading to high-rate performance and outstanding cryogenic applications. Ma et al. prepared the cathode electrode of Na₂Ni[Fe(CN)₆] by electrostatic spinning method, with slow crystallization, high sodium content, good crystallinity and Ni²⁺ inactivity. Na₂Ni[Fe(CN)₆] cathode has the best overall electrochemical performance in terms of stability, cost and temperature adaptability apart from its low discharge specific capacity [59]. It is suitable for SIBs in large-scale storage systems. At 0 ℃, 87% capacity remains after 440 cycles. A stable 54 mAh/g capacity can also be delivered after 440 cycles at −25 ℃. The special structural design of cathode materials to promote the transport of Na⁺ at the interface is also a new approach. Chen and Gao’s group connected the opening of carbon nanotubes (CNTs) to the surface of carbon-coated Na₃V₂O₂(PO₄)₂F cathode nanoparticles. By using CNTs to capture Na⁺ released from the cathode particles during charging, the movement of Na⁺ is restricted to the inside of the neuron-like cathode, eliminating ion transport between the electrolyte and cathode in traditional batteries. The design significantly reduced the interface charge transfer resistance and enables reversible operation of the battery at low temperatures down to −60 ℃ [60].

  3.2 Application and modification of anode materials

  In addition to the similar coating and doping of cathode materials, there are many delicate structural designs of anode materials [61]. Based on the electrochemical milling process (EMP), Wang et al. prepared crystallized electrode materials with ultra-small nanostructures and excellent electrochemical properties at room temperature (Fig. 3a) [28]. The ultrafine Bi nano particles prepared by EMP reduce the ion/electron diffusion path and realize the ultrafine kinetic process. With 1.0 mol/L NaPF₆ in DME as the electrolyte, the EMP-Bi@3DCF/Na cell delivers the capacity of 190 mAh/g (−20 ℃) and 237 mAh/g (25 ℃), with a current density of 5 A/g. Later, Li et al. proved that solvated Na⁺ in diglyme-based electrolyte could be directly stored by Bi anode through alloying reaction without the need for a desolvation process through in-situ FTIR/ATR analysis and in-situ XRD characterization [62]. This is important for low-temperature kinetics and also reminds us that behind the excellent low-temperature performance of many electrode materials, there may exist new energy storage mechanisms which need to be explored. Zhao’s group reported a hard carbon material doped with atomic Zn (Zn-HC). The Zn doping can regulate the bulk and interfacial structure of hard carbon. The optimized Zn-HC has a larger carbon interlayer spacing (d₀₀₂ = 0.408 nm), suitable nanopores (diameter ≈ 0.8 nm), and lower defect content, which facilitates fast Na⁺ storage. The Zn doping architecture can trigger a local electric field in the hard carbon bulk, reducing the Na⁺ diffusion energy barrier (0.60 eV vs. 1.10 eV) and enhancing the bulk Na⁺ storage kinetics. The obtained Zn-HC material exhibits high reversible capacity (546 mAh/g), remarkable rate capability (140 mAh/g at 50 A/g), decent initial coulombic efficiency (ICE, 84%), and noticeable low-temperature capacity (443 mAh/g at −40 ℃) [63].

  Figure 3

  

  Figure 3.  Low temperature sodium-ion battery anode material. (a) Bi@3DCF particle size was reduced to EMP-Bi@3DCF via electrochemical milling process. Copied with permission [28]. Copyright 2021, Elsevier. (b) Micropores and ultrapores of porous carbon. Copied with permission [64]. Copyright 2021, Springer. (c) Due to the induction of Sn, the activation energy of sodium ion diffusion in HTO-Sn decreases. Copied with permission [65]. Copyright 2020, American Chemical Society. (d) Na migration barrier decreases in FeS₂@G@CNFs. Copied with permission [66]. Copyright 2019, Wiley.

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  Yang et al. prepared microporous carbon anode by diffusion melting carbonization method (Fig. 3b). Microporous carbon anode plays an ion screening role, and the pores remove solvents to ensure more Na⁺ storage locations, which is conducive to capacity improvement [64]. The pore size is smaller than Na solvated structure, and the dissolution occurs around the pore size. As the pore diameter decreases, electrons tend to diffuse to all adjacent Na⁺ rather than a single Na⁺, thus making the tendency of Na⁺ to aggregate in the pore possible. The semi-cell was evaluated with 1.0 mol/L sodium triflate (NaOTf) in diethylene glycol dimethyl ether (DEGDME) as electrolyte at 0.2 mA/cm² (−20 ℃), and the capacity was maintained at 87% compared with room temperature.

  It is also a method to enhance Na⁺ diffusion kinetics by designing heterointerfaces. Que et al. use titanate to restrict Sn ions (Fig. 3c), Sn triggers TiO₆ reconstruction, and the synergy of electronic modulation and electrochemically induced layer-spacing expansion enhances ion diffusion dynamics and reduces interfacial charge transfer energy barrier [65]. Na⁺ diffusion coefficient becomes three times as the original, and this anode material delivers the capacity of 120 mAh/g at 1 A/g under −20 ℃, 91% capacity retention is achieved over 1200 cycles. TiO₂@rGO anode also plays a role in lowering the energy barrier. Deng et al. compounded titanium dioxide with carbon, taking advantage of the fact that pseudo-capacitance decreases more slowly than chemical capacitance at low temperatures [32]. There were a large number of heterojunctions and effective active sites in the composite material, and the capacitance contribution was high. The mesoporous and nanostructured TiO₂ films provide rapid ion insertion and removal and short diffusion distance, while graphene sheets provide continuous electronic conductive network. First-principles calculations show that the tight bonding between graphene and TiO₂ reduces the diffusion barrier, thus enhancing the Na⁺ intercalated pseudo-capacitance process. Enhanced pseudo-capacitance can increase the kinetic reaction of sodium-ion batteries, especially at high current density. Chen et al. prepared FeS₂@graphene@Carbon nanofibers anode material and used graphene coating to improve the conductivity [66]. DFT calculation showed that the graphene/FeS₂ heterointerface could improve the storage stability and effectively reduce the diffusion barrier of Na⁺(Fig. 3d). Its high storage performance can be attributed to graphene's high electrical conductivity and rapid reaction-diffusion kinetics. Meng et al. uniformly distributed ultrafine SnO₂ nanoparticles (~4 nm) on the surface of graphene sheets [67]. Graphene sheets provide an electronically conductive framework for anchoring highly dispersed tin dioxide nanoparticles, as well as adapting to changes in electrode volume during ion embedding/disembedding. Both the conversion of SnO₂ and the permissive reaction contribute to sodium storage. The main contribution of the conversion reaction is that the ultrafine nanoparticles trigger the conversion reaction activity of SnO₂ and sodium. At −20 ℃, it delivers the capacity of 97 mAh/g over 100 cycles.

  The diffusion rate of Na⁺ can be improved effectively by designing a fast Na⁺ migration path in electrode materials. The capacity of the anode material prepared by Li et al. decreases by 8.7% at −20 ℃ compared with room temperature, and the average voltage change is negligible [68]. The Na_0.8Ni_0.4Ti_0.6O₂/NaV_1.25T_i0.75O₄ cell, using 1.0 mol/L NaClO₄ in PC with 2 vol% FEC as electrolyte, delivers a capacity decrease of 4.1% at −20 ℃ compared with room temperature. The excellent results are due to the long durability and stability of the 1D channel and ultra-high speed ion diffusion in the temperature dependent range. Short Na⁺ diffusion path and stable SEI protection benefit low-temperature performance. Hou et al. designed self-interwoven carbon microstrip and self-supported large-size hard carbon paper (HCP), and its unique micro/nano structure was combined with ether electrolyte to prepare HCP with high ionic conductivity and ultra-fast ion transport path, which greatly improved Na storage performance [69]. The capacity retention rate at −25 ℃ is as high as 89% compared to the capacity delivered at 25 ℃ (50 mA/g). Even at 500 mA/g, it delivers the capacity of 217.1 mAh/g at −15 ℃, and 81% capacity retention is achieved over 1000 cycles.

  3D structural design of electrode materials can also enhance low-temperature performance. Wang et al. combined carbon-coated NaTi₂(PO₄)₃ anode material with CNTs [70]. The 3D CNTs network can absorb the electrolyte and reduce the internal diffusion resistance. The interconnected pores can also serve as an effective buffer to adapt to volume changes, providing a fast transmission path for Na⁺ and electrons, and improving the poor electronic conductivity of NTP/C anode. Fan et al. formed the backbone of MNCNTs into an interconnected network, with ZnS being uniformly and tightly anchored to the network [71]. Meanwhile, the introduction of pseudo-capacitance into the interconnected conductive microstructure plays a key role in Na⁺ storage. Similarly, nitrogen-doped carbon-coated Ni_1.8Co_1.2Se₄ nanoparticles [72], N_i1.5CoSe₅@NC@rGO [73], 3DSG anode materials [74], and foamy carbon coated NVP and NTP materials [75] are prepared.

  3.3 Principle and modification strategy of low temperature electrolyte

  The low-temperature performance of the electrolyte is closely related to the conductivity of the electrolyte itself. The higher conductivity of the electrolyte quickens the ion transfer, so more capacity can be delivered at low temperatures. The more sodium ions dissociate from the electrolyte, the higher the conductivity. The higher the conductivity, the faster the ionic conduction and the smaller the polarisation, the better the battery will perform at low temperatures. Therefore, high conductivity is a necessary condition for achieving good low-temperature performance of sodium-ion batteries.

  At the same time, the film-forming impedance between electrolyte/electrode interphase is also a key factor affecting the performance of sodium-ion batteries at low temperatures. The composition and formation conditions of SEI can be optimized by adjusting the composition of electrolyte, and the diffusion rate of sodium ions in SEI can be improved to increase the ionic conductivity of SEI.

  The influence of low temperature on organic electrolyte lies in: (1) The solvent will freeze at a certain low temperature, and different solvents have different freezing points. The common electrolyte composition of sodium ion batteries is EC:PC = 1:1, EC:PC:DMC = 0.45:0.45:0.1, etc. In this electrolyte, EC freezes at 35–38 ℃, PC freezes at −48 ℃, and DMC freezes at 2 ℃. The total freezing point can be reduced by introducing low-melting points solvent components, such as ethylene glycol (freezes at −12.9 ℃), ethyl acetate (freezes at −84 ℃), ether (freezes at −116.3 ℃) and ethanol (freezes at −114 ℃). (2) At low temperatures the viscosity of electrolyte increases, while the conductivity of high-viscosity electrolyte decreases. By introducing additives or low-viscosity solvents (DMC, EMC), the viscosity of electrolyte is reduced, so as to improve its low-temperature performance. The melting and boiling point and viscosity-dielectric constant of common organic solvents are presented in Figs. 4a and b [76–78]. As can be seen from the figures, carboxylic ester and ether components are worth expecting in order to obtain electrolyte with lower freezing point. Diffusion coefficient of Na⁺ in electrolyte is affected by dielectric constant and viscosity. However, the low freezing point, high dielectric constant and low viscosity are often in conflict, and most of the electrolyte components cannot be balanced, which requires researchers to employ a cocktail of strategies to modulate electrolyte components for optimal performance. PC and DEGDME are the most used electrolyte components, followed by EC, DEC and DMC. FEC is used as an additive in many low temperature electrolytes. (3) The conductivity of electrolyte decreases seriously at low temperature and is highly dependent on temperature (Fig. 4c) [77]. This is because the solubility and dissociation of sodium salt in the solvent at low temperature are reduced, and the conductivity of the electrolyte is reduced. The solubility of sodium salt in the electrolyte is affected by temperature. When the temperature is reduced, sodium salt may be precipitated in the electrolyte, and the conductivity of the electrolyte is decreased. Increasing the radius of anion can elevate the charge delocalization, reduce the interaction force between cations and anions, promote the dissociation of solutes, and improve the conductivity of electrolyte at low temperature. This is also the reason why the sodium salts in the electrolyte are basically NaPF₆, NaClO₄ and NaCF₃SO₃. (4) The desolvation of solvation structure formed by solvent and sodium salt is the rate-limiting step at low temperature. Therefore, to select the solvation structure with low desolvation energy is beneficial to facilitate Na⁺ transfer from electrolyte to electrode materials at low temperature [79,80]. Low desolvation energy corresponds to weak solvation structure and also corresponds to low dielectric constant. This is in contradiction with the high dielectric constant promoting the dissolution of sodium salt, which is also the key point for researchers to reconcile. Reducing the concentration of sodium salt may be one of the solutions, because compared with lithium salt, sodium salt only needs about 1/3 of the concentration of lithium salt to achieve the same conductivity. Deng et al. used a low-concentration electrolyte 0.3 mol/L NaClO₄ in EC/PC (1:1) + 5% FEC to spontaneously form a weak solvation structure and reduce the charge transfer energy barrier by synergistic effect (Fig. 4d) [29]. The half-battery assembled with NVPF as cathode material at −25 ℃ is 90.8% of the capacity of 25 ℃, and its capacity remains 93.4% after 1000 cycles at −25 ℃.

  Figure 4

  

  Figure 4.  (a) Statistical diagram of melting and boiling points of common solvents. (b) Diagram of common solvents’ dielectric constant and viscosity. (c) Conductivity solvent diagram and dielectric effect viscosity effect formula. Copied with permission [77]. Copyright 2020, Wiley. (d) Solvation structure of Na⁺ in electrolyte. Copied with permission [29]. Copyright 2022, Elsevier.

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  For solid and gel polymer electrolytes the conductivity is not high at room temperature. For polymer electrolytes, higher conductivity and lower the vitrification temperature of the system can be achieved by introducing plasticizers (such as polyethylene glycol (PEG) oligomers) or by increasing the salt concentration. For oxide electrolytes its grain boundary impedance is large, so high-temperature treatment is needed to improve material density and reduce grain boundary impedance. In terms of improving the interface contact between electrode materials and electrolyte, the commonly used strategies include filming, liquid wetting or polymer addition, in situ deposition, porous structure design, etc. Du et al. [81] explored a novel polymer-based solid-state electrolyte (SSE named PFSA-Na membrane) for solid-state sodium-ion battery (SSIB). It can be obtained by a simple large-scale ion exchange strategy and exhibited good mechanical flexibility before and after freezing test. The half batteries assembled by using HQ-NaFe (a Prussian blue cathode) as the cathode material and PFSA-Na membranes as the electrolyte, are still able to obtain the high ionic conductivity of 4.88 × 10⁻⁵ S/cm at the extremely low temperature of −15 ℃. The SSIB still has stable cycling performance at −35 ℃ and the Coulombic efficiency is almost 100%. Capacity can be restored to the original value when the temperature rises, indicating that it has extraordinary temperature tolerance. Li et al. also used PFSA-Na film, using NVOPF@rGO as cathode materials and HC as anode material to assemble full batteries [82]. Full batteries delivered 40.6 mAh/g capacity over 30 cycles at −25 ℃, and the slower ion transport rate of the HC anode at low temperatures may be the reason for its limited capacity.

  For aqueous electrolytes, the main factors affecting water freezing point are the forces between water molecules, including hydrogen bond, Coulombic effect, van der Waals’ force. During solidification, water molecules need to form neat and continuous hydrogen bonds to transform into a solid with regular crystal arrangement. Therefore, breaking or reducing the continuous interaction between water molecules can effectively enhance the performance of low-temperature electrolytes. (1) For example, alcohols (ethylene glycol) are introduced. Due to the strong interaction between ethylene glycol and water, the continuous hydrogen bonds between water molecules are destroyed, thus reducing the freezing point. (2) Another example, the method of reducing the content of free water can form high solubility electrolyte by enhancing the interaction between electrolyte and water and reducing the interaction between water molecules, or introducing ether groups into the aqueous electrolyte to form strong hydrogen bonds with free water and so on.

  Cheng et al. prepared a Na₂SO₄-SiO₂ hydrogel-type electrolyte, using methanol as an antifreeze agent, which worked at −30 ℃ (Fig. 5a) [83]. Due to the intermolecular bonding between SiO₂ and Na₂SO₄, the precipitation of Na₂SO₄ in the supersaturated electrolyte and the continued growth of Na₂SO₄ grains are highly inhibited at a low temperature. At −30 ℃, the ionic conductivity is 0.070 mS/cm. It provides a new way for the development of aqueous sodium-ion batteries at low temperatures.

  Figure 5

  

  Figure 5.  (a) Low-temperature aqueous sodium ion battery with Na₂SO₄-SiO₂ electrolyte. Copied with permission [83]. Copyright 2021, Elsevier. (b) Asymmetric anions reduce freezing point in 25.0 mol/L NaFSI+10.0 mol/L NaFTSI electrolyte. Copied with permission [84]. Copyright 2016, American Chemical Society. (c) Ratio of water molecules with strong, weak and no hydrogen bond in 1.0 mol/L NaClO₄+3.86 mol/L CaCl₂ electrolyte. Copied with permission [85]. Copyright 2022, Wiley.

  DownLoad: Full-Size Img PowerPoint

  Ionic liquid (IL) electrolytes are a viable solution for cryogenic battery operation due to their unique low freezing point and high electrochemical stability. Wang et al. developed an ionic liquid composed of 25.0 mol/L (fluorosulfonyl)imide (NaFSI) and 10.0 mol/L sodium (fluorosulfonyl)(trifluoromethylsulfonyl)imide (NaFTFSI), which inhibited the crystallization of high-concentration electrolyte by using asymmetric anions as solutes (Fig. 5b) [84]. The asymmetric distribution of anions can disrupt the arrangement of dense ions in the ionic liquid, improve the disorder degree of the system and reduce the freezing point. The batteries can operate at a temperature of at least −10 ℃ and maintain a capacity of 74% after 500 cycles at C/5.

  Due to the strong interaction of calcium chloride with water molecules, Zhu et al. used low-cost calcium chloride as an antifreeze additive in 1.0 mol/L NaClO₄ aqueous electrolyte (Fig. 5c) [85]. Calcium chloride can significantly improve the proportion of free water in electrolyte, and it is an inorganic inert additive, superior to the flammable and toxic organic antifreeze agents. The freezing point of the optimized electrolyte is significantly lower than −50 ℃, and it has ultra-high ionic conductivity (7.13 mS/cm) at −50 ℃. At 10 C of −30 ℃, the cell can achieve ultra-long cycle stability of 6000 times, and there is no significant capacity decay, indicating rapid Na⁺ transfer at low temperatures.

  3.4 Low temperature electrolyte/electrode interphase

  The interphase is an important part of connecting electrolytes and electrode materials. At low temperature, the impedance of electrolyte/electrode interface increases and the electrochemical dynamics slows down, which is the main reason for limiting the low-temperature performance of sodium-ion batteries.

  Sodium ions in the electrolyte do not exist alone, but form solvated structures with solvents and anions (Fig. 6a) [29]. Different electrolyte components will form different solvated structures. During the discharge process, solvated sodium ions first diffuse to the electrolyte/electrode interphase, and then desolvate into a single sodium ion, afterwards migrate through the SEI and are embedded into electrode materials. The charging process is reversed. The whole process dynamics can be described by the Arrhenius formula, which has an exponential relationship with temperature. Therefore, the interfacial process dynamics is slow at low temperature, which limits the low-temperature performance of sodium-ion batteries. Some studies have shown that the process of sodium ion desolvation at low temperatures may be the most important rate-determining step [86–88].

  Figure 6

  

  Figure 6.  (a) The charge transfer process on the interphase between NVPF cathode and 1.0 mol/L NaClO₄ in EC/PC (1/1) + 5 vol% FEC. Copied with permission [29]. Copyright 2022, Elsevier. (b) By introducing co-embedding mechanism, the dissolubilization step is skipped and the diffusion barrier is lowered. Copied with permission [89]. Copyright 2016, Elsevier. (c) Common inorganic substances and ionic conductivity in SEI. Copied with permission [5]. Copyright 2021, Springer Nature. (d) Effect of low temperature on metal deposition and thickness and composition of SEI. Copied with permission [90]. Copyright 2019, American Chemical Society.

  DownLoad: Full-Size Img PowerPoint

  Chen et al. assembled the full sodium ion batteries with artificial graphite (AG) as the cathode material, polytriphenylamine (PTPAn) as the anode material and 1.0 mol/L NaPF₆ in DEGDME as the electrolyte [33]. It combines cationic solvent co-intercalation with anionic storage chemistry and essentially eliminates the slow process of desolvation in the presence of ether-based electrolyte. Na-ether co-embedded on the anode side and anions doped in the cathode side, so as to bypass the slow desolvation process of traditional rocking chair batteries and achieve excellent kinetic performance. This insoluble mechanism enables the batteries to reach 61% of its room temperature capacity at ultra-low temperature of −70 ℃. It can deliver 70% capacity retention and 99.8% Coulombic efficiency under −40 ℃ over 400 cycles at 0.1 A/g. In addition, Sun et al. [89] designed HT-NW anode materials, and introduced defects by non-SEI and expanding the distance between electrode material’s layer to ensure the smooth process of Na⁺-solvent co-intercalation, thus fundamentally avoiding the slow desolvation process. Its unique Na⁺-solvent co-intercalation makes the defect HT-NW have faster diffusion kinetics, and a lower energy barrier (66.0 meV) at −25 ℃ (Fig. 6b). In addition, the defective HT-NW electrode maintained a high degree of structural stability during charge and discharge.

  In addition, the composition and thickness of SEI films also affect the diffusion rate of sodium ions to a certain extent (Figs. 6c and d) [5,90]. Generally speaking, the formation of a thin and stable SEI is beneficial to improve the stability of the batteries, reduce the occurrence of electrolyte side reactions, and shorten the diffusion path of Na⁺. SEI contains inorganic and organic components, which are composed of the products of side reactions between the electrode and the electrolyte after battery assembly. Inorganic components such as Na₂CO₃ and NaF provide a greater diffusion coefficient of Na⁺ than organic components. Therefore, it is necessary to adjust solvents, sodium salts and additives reasonably to form a stable and thin SEI which is rich in inorganic components.

  4. Low temperature sodium-ion batteries outlook

  Compared with lithium-ion batteries, sodium-ion batteries have a better prospect of application at low temperatures due to the weaker viscosity effect of sodium ions in the electrolyte and the lower desolvation energy brought by larger cationic radius. How to further play the advantages of sodium ion batteries at low temperatures mainly lies in (Fig. 7):

  Figure 7

  

  Figure 7.  Development and prospective of low-temperature sodium-ion batteries.

  DownLoad: Full-Size Img PowerPoint

  (1) Based on more advanced characterization technologies, we can explore more detailed electrochemical reaction mechanisms of electrode materials with low-temperature sensitivity, further investigate low-temperature energy storage mechanism, and develop new low-temperature sodium ion batteries electrode materials based on this.

  (2) We can introduce new energy storage mechanisms such as Na⁺-solvent co-embedding, anionic cathode energy storage mechanism, pseudo-capacitance energy storage, to avoid slow desolvation process to accelerate the kinetics, or to improve the baseline energy density to compensate for the loss of capacity at low temperatures.

  (3) The development of high-entropy electrolyte can increase the disorder degree of the system by mixing multi-component solute and solvent, which can delay the freezing of low-temperature electrolyte and may bring advantages of each component (cocktail strategy).

  (4) Based on first-principle calculations, we can optimize the electrolyte formula, develop new additives and new sodium salts, reduce the freezing point and viscosity of the electrolyte, and form a weak solvation structure with low desolvation energy at the same time, accelerate the slow interface desolvation process at low temperatures; Based on high-throughput technology and machine learning technology, we can design the electrode materials and electrolyte the most suitable for low temperature scenarios.

  (5) Based on the further exploration of Hofmeister sequence with strong/weak ability to break hydrogen bond and the successful application of water in ionogel electrolyte at low temperatures the development of low-temperature aqueous sodium ion batteries may have a place in the future.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Qiong Su: Writing – review & editing, Writing – original draft, Funding acquisition. Chao Hu: Writing – original draft. Sichan Li: Writing – original draft. Wenjun Huang: Writing – original draft. Jianyu Dong: Writing – original draft, Funding acquisition. Ren Song: Writing – original draft. Lan Xu: Writing – review & editing, Writing – original draft, Supervision, Funding acquisition. Guozhao Fang: Writing – review & editing, Writing – original draft, Supervision.

  Acknowledgments

  We thank the Scientific Research Project of Hunan Provincial Department of Education (No. 24A0675); The Natural Science Foundation of Hunan Province (No. 2024JJ5102); the Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education (No. QSQC2405) and the National Natural Science Foundation of China (No. 22378106) for supporting our work.

  
  
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• 

  Figure 1  (a) Sodium storage mechanisms in sodium-ion batteries. (b) Challenges of low temperature sodium-ion batteries.

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  Figure 2  Cathode material for low temperature sodium-ion battery. (a) Schematic diagram of Na₃V₂(PO₄)₃. Copied with permission [50]. Copyright 2016, Elsevier. (b) PB/CNT structure representation. Copied with permission [26]. Copyright 2016, Wiley. (c) Schematic diagram of 1D NaCrO₂ nanowires. Copied with permission [52]. Copyright 2019, American Chemical Society. (d) Structural design of radially aligned hierarchical columnar structure (RAHC) cathode. Copied with permission [53]. Copyright 2015, Springer Nature.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Low temperature sodium-ion battery anode material. (a) Bi@3DCF particle size was reduced to EMP-Bi@3DCF via electrochemical milling process. Copied with permission [28]. Copyright 2021, Elsevier. (b) Micropores and ultrapores of porous carbon. Copied with permission [64]. Copyright 2021, Springer. (c) Due to the induction of Sn, the activation energy of sodium ion diffusion in HTO-Sn decreases. Copied with permission [65]. Copyright 2020, American Chemical Society. (d) Na migration barrier decreases in FeS₂@G@CNFs. Copied with permission [66]. Copyright 2019, Wiley.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Statistical diagram of melting and boiling points of common solvents. (b) Diagram of common solvents’ dielectric constant and viscosity. (c) Conductivity solvent diagram and dielectric effect viscosity effect formula. Copied with permission [77]. Copyright 2020, Wiley. (d) Solvation structure of Na⁺ in electrolyte. Copied with permission [29]. Copyright 2022, Elsevier.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) Low-temperature aqueous sodium ion battery with Na₂SO₄-SiO₂ electrolyte. Copied with permission [83]. Copyright 2021, Elsevier. (b) Asymmetric anions reduce freezing point in 25.0 mol/L NaFSI+10.0 mol/L NaFTSI electrolyte. Copied with permission [84]. Copyright 2016, American Chemical Society. (c) Ratio of water molecules with strong, weak and no hydrogen bond in 1.0 mol/L NaClO₄+3.86 mol/L CaCl₂ electrolyte. Copied with permission [85]. Copyright 2022, Wiley.

  下载: 全尺寸图片 幻灯片
  

  Figure 6  (a) The charge transfer process on the interphase between NVPF cathode and 1.0 mol/L NaClO₄ in EC/PC (1/1) + 5 vol% FEC. Copied with permission [29]. Copyright 2022, Elsevier. (b) By introducing co-embedding mechanism, the dissolubilization step is skipped and the diffusion barrier is lowered. Copied with permission [89]. Copyright 2016, Elsevier. (c) Common inorganic substances and ionic conductivity in SEI. Copied with permission [5]. Copyright 2021, Springer Nature. (d) Effect of low temperature on metal deposition and thickness and composition of SEI. Copied with permission [90]. Copyright 2019, American Chemical Society.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Development and prospective of low-temperature sodium-ion batteries.

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•