Low-Temperature Aqueous Na-Ion Batteries: Strategies and Challenges of Electrolyte Design

Qiubo Guo(郭秋卜)$^{\dag}$, Shuai Han(韩帅)$^{\dag}$, Yaxiang Lu(陆雅翔)$^{\hyperlink{s}{*}}$,\\ Liquan Chen(陈立泉), and Yong-Sheng Hu(胡勇胜)$^{\hyperlink{s}{*}}$

doi:10.1088/0256-307X/40/2/028801

⇦Chinese Physics Letters, 2023, Vol. 40, No. 2, Article code 028801Review⇨ Low-Temperature Aqueous Na-Ion Batteries: Strategies and Challenges of Electrolyte Design Qiubo Guo (郭秋卜)†, Shuai Han (韩帅)†, Yaxiang Lu (陆雅翔)*, Liquan Chen (陈立泉), and Yong-Sheng Hu (胡勇胜)* Affiliations Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Received 22 December 2022; accepted manuscript online 10 January 2023; published online 4 February 2023 ^†Qiubo Guo and Shuai Han contributed equally to this work.
^*Corresponding authors. Email: yxlu@iphy.ac.cn; yshu@iphy.ac.cn Citation Text: Guo Q B, Han S, Lu Y X et al. 2023 Chin. Phys. Lett. 40 028801 Abstract Aqueous Na-ion batteries (ANIBs) are considered to be promising secondary battery systems for grid-scale energy storage applications and have attracted widespread attention due to their unique merits of rich resources of Na, as well as the inherent safety and low cost of aqueous electrolytes. However, the narrow electrochemical stability widow and high freezing point of traditional dilute aqueous electrolytes restrict their multi-scenario applications. Considering the charge-storage mechanism of ANIBs, the optimization and design of aqueous Na-based electrolytes dominate their low-temperature performance, which is also hot off the press in this field. In this review, we first systematically comb the research progress of the novel electrolytes and point out their remaining challenges in ANIBs. Then our perspectives on how to further improve the low-temperature performance of ANIBs will also be discussed. Finally, this review briefly sheds light on the potential direction of low-temperature ANIBs, which would guide the future design of high-performance aqueous rechargeable batteries.

DOI:10.1088/0256-307X/40/2/028801 © 2023 Chinese Physics Society Article Text 1. Introduction. With the constant consumption of fossil energy and the increasing demand of human society for energy, the development of renewable energies has become the universal consensus and concerted action of all countries in the world. However, the volatility, randomness, and intermittent characteristics of renewable energies restrict their practical applications. Therefore, it is urgent to develop new electrochemical energy storage technologies with high security and high energy density.[1-4] In recent years, although Li-ion batteries have been widely used, the lack of lithium resources and inherent safety problems also restrict their further development.[5-8] Compared with traditional organic battery systems, aqueous alkali metal ion batteries have the advantages of non-flammability, low cost, and high ionic conductivity.[9-12] Among them, aqueous Na-ion batteries (ANIBs) have great advantages in large-scale energy storage system applications due to rich resources of Na and inherent safety of their aqueous electrolytes.[13-15] However, so far, the development of ANIBs still faces many challenges, especially the high freezing point of traditional dilute aqueous electrolytes restricts their electrochemical performance in many extreme operating environments.[16] Thus, improving the low-temperature tolerance of the aqueous Na-based electrolytes is of great significance for broadening practical applications of ANIBs. Since the thermodynamic freezing point of water is 0 ℃, dilute aqueous electrolytes will suffer severe degradation on the ion diffusion and electrode–electrolyte contact at subzero conditions, given their abundant free water molecules.[17,18] Recently, “water-in-salt” electrolytes (WiSE) have been explored to widen their stable electrochemical windows. The high-concentration salts in the electrolytes could lower the activity of water molecules and counteract the freezing problem of aqueous electrolytes.[19-21] However, the large amount of salts will increase the cost and ion-diffusion barriers, and the problem of salt precipitation at low temperature will also be a big challenge. Apart from this, electrolyte additives can also play a critical role in improving the low-temperature performance of aqueous electrolytes. Among them, ionic additives could act as inter-support salts to lower the freezing point of aqueous electrolytes, which arises from their strong polarization effects.[22] Non-ionic additives mostly could form strong hydrogen bonds with water molecules, thus damaging the hydrogen bond network within water molecules and preventing hydrogen bond grids from forming in ice crystals.[23-26] However, the effects of the ionic additives and non-ionic additives on the system need to be further explored. Accompanied by the optimization of electrolyte design, composite strategies, for instance, the combination of ionic and non-ionic additives or the co-introduction of electrolyte additive and hydrogel has also been investigated to improve the low-temperature electrochemical performance of ANIBs.[27-30] In this review, we focus on the development of aqueous Na-based electrolytes against low-temperature tolerance, summarize recent strategies (shown in Fig. 1) to improve the low-temperature performance of ANIBs and point out corresponding deficiencies. We also intend to provide insights for future investigation and development of the low-temperature ANIBs at the end.

Fig. 1. Summary of strategies to improve the low-temperature performance of ANIBs.

Fig. 2. (a) The snapshot of the molecular dynamics (MD) simulations of 1 m NaClO$_{4}$ and 17 m NaClO$_{4}$ electrolytes. Atom colors: Na, purple; O, red; H, white; Cl, yellow; (b) the schematic of Na$_{3}$V$_{2}$(PO$_{4})_{3}$(NVP)//17 m NaClO$_{4}$// Na$_{3}$V$_{2}$(PO$_{4})_{3}$(NVP) aqueous sodium ion micro-batteries (ANIMS); (c) cycling performances at 0.3 mA$\cdot$cm$^{-2}$ of Na$_{3}$V$_{2}$(PO$_{4})_{3}$(NVP)//17 m NaClO$_{4}$//Na$_{3}$V$_{2}$(PO$_{4})_{3}$(NVP) aqueous sodium ion micro-batteries (ANIMS) at various temperatures.[19] (d) DSC test of 17 m NaClO$_{4}$ electrolyte; [(e), (f)] GCD curves of the NiHCF//PT full cells under the current density of 5 A$\cdot$g$^{-1}_{\scriptscriptstyle{\rm PT}}$ from 25 to 100 ℃ or 0.5 A$\cdot$g$^{-1}_{\scriptscriptstyle{\rm PT}}$ from $-10$ to $-40\,^{\circ}\!$C.[20] (g) GCD curves of the Na$_{0.44}$MnO$_{2}$(NMO)//PNZ full cells at various temperatures.[21]

2. “Water-in-Salt” Electrolytes. Since Suo et al. reported water-in-salt electrolytes (WiSE) as a new family of aqueous electrolytes for Li-ion batteries (LIBs) in 2015, plenty of research has adopted the WiSE strategy to expand the stable electrochemical windows of the aqueous electrolytes, which arises from the decreased free water molecules.[31] Considering that the low freezing point of water begets the poor low-temperature tolerance of aqueous electrolytes, the WiSE strategy may also lower the operating temperature of the ANIBs by reducing the number of free water molecules in the aqueous electrolytes. Wang et al.[19] reported an aqueous planar symmetric Na-ion micro-batteries system using 17 m (mol$\cdot$kg$^{-1}$) NaClO$_{4}$ as the electrolyte, which can operate at a low temperature of $-40\,^{\circ}\!$C. As shown in Fig. 2(a), the molecular dynamics (MD) simulations of the 1 m and 17 m NaClO$_{4}$ electrolytes demonstrate the formation of hydrogen bonding networks among abundant free water molecules in the low-concentration electrolyte, whereas most water molecules connected with Na$^{+}$ ions in the high concentration electrolyte to damage the hydrogen bonds, thus lowering the freezing point of electrolyte. Figures 2(b) and 2(c) show the schematic illustration and cycling performance of the Na$_{3}$V$_{2}$(PO$_{4})_{3}$(NVP)//NaClO$_{4}$//NVP full cell, which can still display a high capacity of 16 mAh$\cdot$cm$^{-3}$ at $-40\,^{\circ}\!$C, revealing the good low-temperature electrochemical performance in the highly concentrated NaClO$_{4}$ electrolyte. Zhang et al.[20] also reported an all-climate ANIB system with 17 m NaClO$_{4}$ as the electrolyte. As shown in Fig. 2(d), the 17 m NaClO$_{4}$ electrolyte shows a low freezing point of $-31.3\,^{\circ}\!$C. Figures 2(e) and 2(f) show the galvanostatic charge–discharge (GCD) curves of the NiHCF//17 m NaClO$_{4}$//PT full cell at various temperatures. Although the achieved capacity decreases with the reduction of operation temperatures, it can still display a discharge capacity of 85.6 mAh$\cdot$g$^{-1}_{\scriptscriptstyle{\rm PT}}$, maintaining 68.7% of the room-temperature capacity. It is well known that alkaline or acid electrolytes usually exhibit lower freezing points compared with neutral electrolytes.[32-35] However, most traditional cathode and anode materials cannot work stably in non-neutral electrolytes. Sun et al.[21] reported phenazine (PNZ) as a stable anode material in 10 m NaOH, and Na$_{0.44}$MnO$_{2}$ (NMO) can be synthesized in the aqueous NaOH solution, which indicates that it can be employed as the cathode in an alkaline system. Though the authors did not show the freezing point of the 10 m NaOH electrolyte, they tested the NMO//PNZ full cells at subzero temperatures. As shown by the GCD curves in Fig. 2(g), the NMO//PNZ full cell delivers a high capacity of $\sim$ $120$ mAh$\cdot$g$^{-1}$ at $-10\,^{\circ}\!$C with a current rate of 2 C. The full cell can even operate at $-20\,^{\circ}\!$C and achieve a capacity of 67 mAh$\cdot$g$^{-1}$ at 10 C, indicating the good low-temperature tolerance of the 10 m NaOH electrolyte.

Fig. 3. (a) Polarized-light microscope images of 3.86 m CaCl$_{2}$ + 1 m NaClO$_{4}$ electrolyte and 1 m NaClO$_{4}$ electrolyte during cooling process, respectively; (b) the GCD curves of the Na$_{2}$CoFe(CN)$_{6}$//AC full cells at room temperature and under $-30\,^{\circ}\!$C for different cycles at a current rate of 1 C.[36] (c) Polarized-light microscopes images of the 3.5 m Mg(ClO$_{4})_{2}$ + 0.5 m NaClO$_{4}$ electrolyte during cooling process; (d) DSC curves of various electrolytes recorded during scans from $-150$ to 30 ℃ with a heating rate of 5 ℃/min; (e) the GCD curves of AC//NaTi$_{2}$(PO$_{4})_{3}$(NTP)@C full cells at $-60\,^{\circ}\!$C.[37] (f) DSC curves of different electrolytes between 60 and $-120\,^{\circ}\!$C, and the optical photograph of 25 m NaFSI + 10 m NaFTFSI after storage at ambient temperature for six months; (g) cycling performance of Na$_{3}$(VOPO$_{4})_{2}$F//NaTi$_{2}$(PO$_{4})_{3}$ full cells at $-10\,^{\circ}\!$C. The inset shows GCD curves for different cycles.[38]

3. Ionic Additives in Aqueous Na-Based Electrolytes. Although the WiSE could improve the low-temperature tolerance of the electrolyte significantly, the high-concentration salts will decrease the ion conductivity and increase cost of the WiSE.[39,40] As the temperature decreases, high-concentration electrolytes will experience a severe salting-out phenomenon, which will cause cell failure. Some ionic additives with multivalent cations that have strong interaction with water molecules may also lower the freezing point of aqueous electrolytes. Zhu et al.[36] reported a low-temperature ANIB system with CaCl$_{2}$ as the inert ionic additive to improve the low-temperature performance of batteries. As shown in Fig. 3(a), the solidification phenomenon appeared between $-20\,^{\circ}\!$C and $-30\,^{\circ}\!$C in the 1 m NaClO$_{4}$ electrolyte, by contrast, no obvious ice crystals appeared in the optimal 3.86 m CaCl$_{2}$ + 1 m NaClO$_{4}$ electrolyte even when the temperature decreased to $-100\,^{\circ}\!$C. This phenomenon indicates that the anti-freezing additive CaCl$_{2}$ can regulate the proportion of hydrogen bonds of electrolytes by strong interaction with water molecules due to Ca$^{2+}$ being a hydrogen-bonding acceptor, thus lowering the freezing point of electrolytes and improving the low-temperature electrochemical performance of ANIBs. As shown in Fig. 3(b), the constructed Na$_{2}$CoFe(CN)$_{6}$//active carbon full cell delivers a high capacity of 74.5 mAh$\cdot$g$^{-1}$ at $-30\,^{\circ}\!$C at the current rate of 1 C, corresponding to 64.8% capacity retention at room temperature. Later, this group reported a new electrolyte system with the same strategy, where 3.5 m Mg(ClO$_{4})_{2}$ was introduced into the 0.5 m NaClO$_{4}$ electrolyte.[37] Due to the high ionic potential of Mg$^{2+}$ with a strong polarization effect, the chemical environment of water molecules and the ice formation temperature of electrolytes were greatly affected. As shown in Figs. 3(c) and 3(d), the 3.5 m Mg(ClO$_{4})_{2}$ + 0.5 m NaClO$_{4}$ electrolyte shows no obvious salting-out phenomenon or ice crystallization during the cooling process and demonstrates an ultra-low phase transition temperature at around $-122\,^{\circ}\!$C, indicating that low freezing point was achieved. The GCD curves of the active carbon//NaTi$_{2}$(PO$_{4})_{3}$@C full cell delivers a high capacity of 83.2 mAh$\cdot$g$^{-1}$ under $-60\,^{\circ}\!$C at the current rate of 0.2 C [Fig. 3(e)]. As mentioned above, the multivalent cations could lower the freezing point of aqueous electrolytes, whereas anions can also improve the low-temperature electrochemical performance of ANIBs significantly. Reber et al.[38] found that the asymmetric anions of the electrolyte additive could suppress the crystallization of high-concentration electrolytes at low temperature. As shown in Fig. 3(f), the 25 m NaFSI + 10 m NaFTFSI electrolyte shows no crystallization during the cooling scan and can still remain liquid at room temperature after six months, where the asymmetric anions could suppress the formation of long range order, thereby effectively suppressing crystallization. The NaFTFSI additive with asymmetric anion not only suppresses the crystallization of high-concentration electrolytes but also decreases its freezing point. The NTP/NVPOF full cell with the mixed-anion electrolyte delivers an initial capacity of 48 mAh$\cdot$g$^{-1}$ at $-10\,^{\circ}\!$C and the capacity retention can achieve 99% relative to the 1$^{\rm st}$ cycle.

Fig. 4. (a) The schematic illustration of the charge-storage mechanism in the ethanol-water system (light-colored H$_{2}$O molecules represent that water whose activity is suppressed) and water system; (b) cycling performance of Na$_{0.44}$MnO$_{2}$//Zn full cells at 0 ℃.[23] (c) The snapshot and its conformation analysis from the MD simulations of the system with $\chi_{_{\scriptstyle \rm DMSO}}=0.3$; (d) solvation structure of the $\chi_{_{\scriptstyle \rm DMSO}}=0.3$ system from MD simulations; (e) GCD curves of AC// NaTi$_{2}$(PO$_{4})_{3}$ full cells at $-50\,^{\circ}\!$C and 25 ℃.[24]

4. Non-ionic Additives in Aqueous Na-Based Electrolytes. In addition to ionic additives, organic solvents or organic solutes can also be employed as electrolyte additives to improve the low-temperature tolerance of aqueous electrolytes. The non-ionic electrolyte additives should have the following characteristics: (1) The organic compound should be soluble in or mutually soluble with water. (2) They should simultaneously improve the low-temperature electrochemical performance and should be compatible with the fabrication process of ANIBs. (3) They should not affect the dissolution and operation of the Na salts. (4) The non-flammability of ANIB system should be maintained. Based on the above considerations, ethanol, which could exert a strong hydrogen-bonding effect on the water with its hydroxyl group, should be a good choice to lower the activity of free water molecules in aqueous electrolytes. Chua et al.[23] reported that the hybridization of ethanol with water could rearrange the solvation structure by breaking the hydrogen-bonding network between water molecules. The ethanol-water co-solvent could not only lower the freezing point of the aqueous electrolyte but also improve the electrode/electrolyte contact. As shown in Fig. 4(a), most of the free water molecules form strong hydrogen bonding with oxygen atoms of the hydroxyl group in ethanol for the ethanol-water system. Furthermore, the strongly bonded proton atoms cannot co-insert into the NMO electrode with Na$^{+}$, thus resulting in the much better structural stability of the electrode and higher cycling stability of the battery system. Finally, the Na$_{0.44}$MnO$_{2}$//1 m NaAc-ethanol/water//Zn full cell delivers a capacity of 44.5 mAh$\cdot$g$^{-1}$ at the current rate of 50 mA$\cdot$g$^{-1}$ at 0 ℃, and can achieve capacity retention of 94% after 50 cycles [Fig. 4(b)]. Except for the hydroxyl groups, the sulfoxide (S=O) groups could also form hydrogen bonds with H of the O–H in water molecules. Nian et al.[24] reported a 2 m NaClO$_{4}$ aqueous electrolyte with the dimethyl sulfoxide (DMSO) as an additive with a molar fraction of 0.3 (2 m-0.3 electrolyte). The freezing point of the electrolyte is lower than $-130\,^{\circ}\!$C, which is the lowest freezing point that the hybrid aqueous Na-based electrolyte can achieve so far. As shown in Figs. 4(c) and 4(d), DMSO and water molecules form various solvent molecule configurations, the mixture of various configurations would not generate the ordered crystal-like structure at low-temperature, thus contributing to improving the low-temperature tolerance of the electrolyte. The GCD curves of the AC//2 m-0.3//NTP full cell, as shown in Fig. 4(e), deliver a capacity of 68 mAh$\cdot$g$^{-1}$ at a rate of 0.5 C at $-50\,^{\circ}\!$C, which achieves 61% capacity retention at 25 ℃. Although the introduction of ethanol and DMSO additives could improve the low-temperature tolerance of the aqueous electrolytes significantly, excessive ethanol and DMSO will beget inevitable safety risks, and a green and safe electrolyte additive, which could widen the stable electrochemical window and lower the freezing point of aqueous electrolyte, is urgently demanded. Sun et al.[25] constructed a dilute hybrid electrolyte with a low freezing point under $-80\,^{\circ}\!$C [Fig. 5(a)], that is, 1 m NaNO$_{3}$ in a mixture of glycerol (Gly) and water (Di), the optimal mass fraction of Gly in the co-solvent is 66.7% ($2\!:\!1$ Gly/Di). The Gly could not only damage the hydrogen bond network between water molecules but also expand the stable electrochemical window of the electrolyte. As shown in Fig. 5(b), the Ni$_{2}$ZnHCF//PTCDI full cell provides a capacity of 40 mAh$\cdot$g$^{-1}$ at the current rate of 100 mA$\cdot$g$^{-1}$ at $-10\,^{\circ}\!$C.

Fig. 5. (a) DSC curves of the hybrid electrolytes with different Gly-to-water mass ratios; (b) GCD curves of Ni$_{2}$ZnHCF//PTCDI full cells at 25 ℃ and $-10\,^{\circ}\!$C.[25] (c) Local structures of the AWE electrolyte from MD simulation; (d) the freezing point data obtained from the differential scanning calorimetry (DSC) curves of AWE and 9 m NaTFSI electrolytes; (e) cycling performances of Na$_{3}$V$_{2}$(PO$_{4})_{3}$(NVP)//NaTi$_{2}$(PO$_{4})_{3}$(NTP) full cells in AWE and 9 m NaTFSI electrolytes at low temperature.[26]

Requirements of the aqueous electrolytes for non-ionic additives are complex, which means that the electrolyte additive should not only improve the low-temperature electrochemical performance of ANIBs but also suppress the dissolution of electrodes. Wang et al.[26] found that the constant dissolution of vanadium-based electrodes and solid-electrolyte interphase (SEI) in aqueous electrolytes severely limits the cycling stability of ANIBs. They introduced a high-voltage additive of adiponitrile (ADN), which is commonly used for LIBs, into an aqueous NaTFSI electrolyte, and a clear hybrid electrolyte was obtained with a NaTFSI:H$_{2}$O:ADN molar ratio of $1\!:\!1\!:\!2.5$ (named AWE). As shown in Fig. 5(c), the solvation structure of the AWE electrolyte demonstrates that one TFSI$^{-}$ anion, one water molecule, and two ADNs appear in the primary solvation shell of Na$^{+}$. Since the molar ratio of Na$^{+}$ to H$_{2}$O is $1\!:\!1$, this suggests that almost all of the water molecules are confined in the solvation shell of Na$^{+}$. This phenomenon can be attributed to the less donor number of ADNs than that of water, which means Na$^{+}$ will preferentially coordinate with water instead of ADNs. Meanwhile, the cyano groups from ADNs could serve as hydrogen bond acceptors and damage the hydrogen bond network among water molecules. All these features make the designed AWE electrolyte achieve a low freezing point at around $-76\,^{\circ}\!$C [Fig. 5(d)], as well minimize the dissolution of the vanadium-based electrodes, and the NaF-rich SEI generates from the decomposition of F-containing anions. As shown in Fig. 5(e), the NVP//NTP full cell in the AWE electrolyte delivers a high capacity of $\sim$ $91$ mAh$\cdot$g$^{-1}$ at $-20\,^{\circ}\!$C at the current rate of 1 C, which is much better than the traditional 9 m NaTFSI electrolyte. 5. Composite Strategies. We have summarized the single strategies to improve the low-temperature tolerance of the aqueous electrolyte. However, the introduction of composite strategies may improve the electrochemical performance of the ANIBs more comprehensively. Cheng et al.[27] combined the hydrogel concept and the electrolyte additives, and reported a Na$_{2}$SO$_{4}$-SiO$_{2}$ hydrogel-type electrolyte with fumed silica as the gel matrix and methyl alcohol as the anti-freezing additive. The composite electrolyte can support the operation of battery at the low temperature of $-30\,^{\circ}\!$C, which is attributed to the following features: (1) The inter-molecular bonding between SiO$_{2}$ and Na$_{2}$SO$_{4}$ could suppress the precipitation of Na$_{2}$SO$_{4}$ and the growth of the Na$_{2}$SO$_{4}$ grain at low temperature. (2) The anti-freezing function of methyl alcohol could lower the freezing point of water. (3) The three-dimensional structure of hydrogel composed of SiO$_{2}$ and mixed co-solvent of water and methyl alcohol make the Na$_{2}$SO$_{4}$ stay at a metastable state. The schematic illustration of the freezing process for the designed electrolyte is shown in Figs. 6(a)–6(d), the precipitation of Na$_{2}$SO$_{4}$ and the crystallization of water were suppressed even at $-30\,^{\circ}\!$C. The CV curves and cycling performance of the AC//NaTi$_{2}$(PO$_{4})_{3}$ full cell in the designed electrolyte at the temperature of 25 ℃ and $-30\,^{\circ}\!$C are shown in Figs. 6(e)–6(h). Even though the lowered temperature increased the polarization of the full cell, it can still achieve a capacity of 61.8 mAh$\cdot$g$^{-1}$ at $-30\,^{\circ}\!$C at a current rate of 0.13 A$\cdot$g$^{-1}$ with the improved cycling performance.

Fig. 6. (a)–(d) The schematic illustration of freezing process for aqueous Na$_{2}$SO$_{4}$ electrolytes and Na$_{2}$SO$_{4}$-SiO$_{2}$ hydrogel electrolytes at 25 ℃ and $-30\,^{\circ}\!$C; [(e), (f)] CV curves of AC//Na$_{2}$SO$_{4}$-SiO$_{2}$//NaTi$_{2}$(PO$_{4})_{3}$ full cells at a rate of 0.05 mV$\cdot$s$^{-1}$ at 25 ℃ and $-30\,^{\circ}\!$C; [(g), (h)] cycling performances of AC//Na$_{2}$SO$_{4}$-SiO$_{2}$//NaTi$_{2}$(PO$_{4})_{3}$ full cells at a current rate of 1 C (1 C = 0.13 A$\cdot$g$^{-1}$) at 25 ℃ and $-30\,^{\circ}\!$C.[27] (i) The schematic illustration of Na$^{+}$/Zn$^{2+}$ charge-storage mechanism in the Na$_{3}$V$_{2}$(PO$_{4})_{3}$//Zn aqueous hybrid batteries; (j) optical photographs of different electrolytes at $-20\,^{\circ}\!$C; (k) GCD curves of Na$_{3}$V$_{2}$(PO$_{4})_{3}$//Zn full cells at different rates.[28]

The cooperation of ionic and non-ionic additives could also improve the cycling stability and low-temperature performance of ANIBs significantly. Liu et al.[28] constructed an aqueous Na–Zn hybrid battery system, with the carbon-coated single crystalline Na$_{3}$V$_{2}$(PO$_{4})_{3}$ nanofiber as the cathode, Zn as the anode, and 10 m NaClO$_{4}$-0.17 m Zn(CH$_{3}$COO)$_{2}$-2 wt% VC as the electrolyte [Fig. 6(i)]. Notably, the ClO$_{4}^{-}$ anion can take part in the solvation shell of Na$^{+}$ and break the hydrogen bond network of the water molecules in the electrolyte, which could expand the conversion energy gap between water and ice, thus lowering the freezing point of the electrolyte [Fig. 6(j)]. During the charge and discharge process, the SEI layer derived from the composition of VC could also improve the ionic conductivity of the battery system. As shown in Fig. 6(k), the Na$_{3}$V$_{2}$(PO$_{4})_{3}$//Zn full cell delivers a high capacity of 94.4 mAh$\cdot$g$^{-1}$ at 0.2 A$\cdot$g$^{-1}$ at $-10\,^{\circ}\!$C, and the over-potential is less than 0.1 V. 6. Conclusions and Outlooks. To understand the above strategies more intuitively, we summarized the electrochemical performance of the reported ANIBs at low temperatures, as shown in Table 1. Despite the current achievements in electrolyte design for low-temperature ANIBs, there are still considerable challenges and opportunities promising in this field. (1) WiSE possesses low freezing points due to the reduced free water molecules that arise from the high-concentration salts in the electrolytes. However, the concentrated salts will experience a severe salting-out phenomenon and decrease the ionic conductivity of electrolytes at low temperatures, which will cause cell failure. Among various salts, NaClO$_{4}$ is one of the most commonly used salts, where it indeed demonstrates good electrochemical performance in different ANIB systems. However, NaClO$_{4}$ is a strong oxidizer, and too much NaClO$_{4}$ contained in the electrolytes is potentially unacceptable for industrial applications. (2) The ionic additives with the strong interaction between multivalent cations and water molecules can lower the freezing point of diluted aqueous electrolytes significantly. However, the narrow electrochemical window of low-concentration electrolytes are not suitable for the cathode and anode materials with extreme operation potentials at room temperature.

Table 1. The performance for the previous reported ANIBs at low temperatures. RT: room temperature.
Systems	Electrolyte	Temperature (electrolytes) (℃)	Temperature (batteries) (℃)	Average voltage (V)	Specific capacity (mAh$\cdot$g$^{-1}$)	Energy density (Wh$\cdot$kg$^{-1}$)	References
Na$_{0.44}$MnO$_{2}$ //PNZ	10 m NaOH		$-20$	$\sim$ $0.8$ (RT)	67$_{\rm PNZ}$ (10 C)	58.9$_{\rm total}$ (RT)	[21]
Na$_{3}$V$_{2}$(PO$_{4})_{3}$// Na$_{3}$V$_{2}$(PO$_{4})_{3}$ (micro-batteries)	17 m NaClO$_{4}$	$-50$	$-40$	1.5 (RT)	16$_{\rm total}$ (mWh$\cdot$cm$^{-3}$)	77$_{\rm total}$ (mWh$\cdot$cm$^{-3}$) (RT)	[19]
NiHCF//PT	17 m NaClO$_{4}$	$-31.3$	$-40$	$\sim$ $1$ (RT)	85.6$_{\rm PT}$ (0.5 A$\cdot$g$^{-1}_{\rm PT}$)	45.3$_{\rm total}$ (RT)	[20]
Na$_{3}$(VOPO$_{4})_{2}$ F// NaTi$_{2}$(PO$_{4})_{3}$	25 m NaFSI $+$ 10 m NaFTFSI	$-32$	$-10$	1.44 (30 ℃)	65$_{\rm cathode}$ (C/5)	64$_{\rm total}$ (30 ℃)	[38]
Na$_{2}$CoFe(CN)$_{6}$// AC	3.86 m CaCl$_{2}$ + 1 m NaClO$_{4}$	$< -100$	$-30$	$\sim 1.05$	74.5$_{\rm cathode}$ (1 C)		[36]
AC// NaTi$_{2}$(PO$_{4})_{3}$	3.5 m Mg(ClO$_{4})_{2}$ + 0.5 m NaClO$_{4}$	$-122$	$-60$	$\sim 1.1$	83.2$_{\rm NTP}$ (0.2 C)		[37]
AC// NaTi$_{2}$(PO$_{4})_{3}$	10 m NaClO$_{4}$- 0.3DMSO	$-130$	$-50$	$\sim 1.2$	68$_{\rm NTP}$ (0.5 C)		[24]
PB-Na//AC	2 m NaNO$_{3}+$ Super- Concentrated Sugar	$< -50$					[41]
Na$_{0.44}$MnO$_{2}$/ Zn	1 m NaAc-Et/Di		0	$\sim 1.1$ (25 ℃)	44.5$_{\rm cathode}$ (50 mA$\cdot$g$^{-1}$)	102$_{\rm cathode}$ (25 ℃)	[23]
Na$_{3}$V$_{2}$(PO$_{4})_{3}$ /NiHCF// NaTi$_{2}$(PO$_{4})_{3}$	NaClO$_{4}$: H$_{2}$O: urea: DMF = 1:2:2:1	$< -50$					[42]
LiMn$_{2}$O$_{4}$// NaTi$_{2}$(PO$_{4})_{3}$	LiClO$_{4}$: NaClO$_{4}$: urea = 1:1:8		0	1.67 (RT)	6$_{\rm total}$ (Ah) (10 A) (70% capacity retention relative to 25 ℃)	100$_{\rm total}$ (RT)	[43]
Na$_{3}$V$_{2}$(PO$_{4})_{3}$// NaTi$_{2}$(PO$_{4})_{3}$	Molar ratio NaTFSI: H$_{2}$O: ADN = 1:1:2.5	$-76$	$-20$	$\sim 1.2$ (RT)	$\sim 91_{\rm NVP}$ (1 C)		[26]
Ni$_{2}$ZnHCF// PTCDI	1 m NaNO$_{3}$-2:1 Gly–Di	$ < $ $-80$	$-10$	$\sim 0.65$	$\sim$ $13.3_{\rm total}$ (100 mA$\cdot$g$^{-1}$)		[25]
AC// NaTi$_{2}$(PO$_{4})_{3}$	Na$_{2}$SO$_{4}$-SiO$_{2}$ hydrogel electrolyte		$-30$	$\sim 1.18$	61.8$_{\rm NTP}$ (1 C)		[27]
Na$_{3}$V$_{2}$(PO$_{4})_{3}$ nanofiber//Zn	10 m NaClO$_{4}$-0.17 m Zn(CH$_{3}$COO)$_{2}$ -2wt% VC		$-20$	1.48 (RT)	90$_{\rm total}$ (2 C)		[28]
Na$_{2/3}$Mn$_{2/3}$Co$_{1/3}$ O$_{1.98}$ F$_{0.02}$// hard-carbon	2 m NaTFSI- PEGMA-BEMA water-in-ionogel		$-25$	0.73	32$_{\rm total}$ (1 C)	23.4	[29]
AC// PNTCDA	(NaClO$_{4})_{1.7}$- (H$_{2}$O)$_{5.5}$-(FA)$_{5.81}$	$< -50$	$-50$	$\sim 0.9$	78.58$_{\rm PNTCDA}$ (RT)		[30]
Ni(OH)$_{2}$// NaTi$_{2}$(PO$_{4})_{3}$	2 m NaClO$_{4}$		$-20$	1.25 (RT)	$\sim$ $70_{\rm anode}$ (10 C)	40.1$_{\rm total}$ (RT)	[44]

(3) Non-ionic additives, which contain oxygen functional groups, could form strong hydrogen bonds with free water molecules and damage the hydrogen bond network between water molecules, thus lowering the freezing point of aqueous electrolytes and improving the low-temperature electrochemical performance of ANIBs obviously. However, the flammability of the introduced additives should be carefully concerned. Moreover, although the freezing point of the designed electrolytes with organic compounds is extremely low, the operation temperature of the ANIBs can only work at a much higher temperature, which means that the freezing point of electrolytes is not the only factor affecting the low-temperature performance of batteries. (4) Composite strategies, such as the combination of the hydrogel concept and the electrolyte additives, not only improve the low-temperature tolerance but also widen the electrochemical window of aqueous electrolytes. However, the complex preparation processes need to be evaluated for the construction of ANIBs. (5) Most of the current reports focus on the electrolyte design to improve the low-temperature electrochemical performance of ANIBs, in addition to this, other factors such as electrode/electrolyte interface and electrode features will also greatly affect the low-temperature performance of ANIBs, which needs to pay more attention to study. This review systematically summarizes the recent progress of electrolyte design to achieve low-temperature ANIBs and provides general insights into the merits and challenges of reported strategies. The water-in-salt electrolytes, non-/ionic additives, and composite strategies offer trains of avenues for the development of low-temperature ANIBs. Since the non-/ionic additives can significantly lower the freezing point of the electrolyte, which may be the preferred strategy for designs of low-temperature ANIBs, and should further combine with research of SEI and electrode designs. Considering the significance of low-temperature ANIBs for energy storage applications, we believe that novel electrolyte designs are indispensable and will promote continuous development of this field. Acknowledgments. This work was supported by the Beijing Municipal Natural Science Foundation (Grant No. 2212022), the National Natural Science Foundation of China (Grant Nos. 51725206, 52122214, and 52072403), Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2020006), and Jiangsu Province Carbon Peak and Neutrality Innovation Program (Industry tackling on prospect and key technology BE2022002-5). References Assessment of utility energy storage options for increased renewable energy penetrationRecent Progress on Integrated Energy Conversion and Storage SystemsSustainability and in situ monitoring in battery developmentElectrical Energy Storage for the Grid: A Battery of ChoicesEvolution of Strategies for Modern Rechargeable BatteriesElectrochemical Energy Storage for Green GridThe future cost of electrical energy storage based on experience ratesCobalt‐Phthalocyanine‐Derived Molecular Isolation Layer for Highly Stable Lithium AnodeAqueous Rechargeable Li and Na Ion BatteriesVoltage issue of aqueous rechargeable metal-ion batteriesRecent Advances in Aqueous Zinc-Ion BatteriesAqueous rechargeable lithium batteries as an energy storage system of superfast chargingProgress in Aqueous Rechargeable Sodium-Ion Batteries“Water‐in‐Salt” Electrolyte Makes Aqueous Sodium‐Ion Battery Safe, Green, and Long‐LastingRecent Progress in Presodiation Technique for High-Performance Na-Ion BatteriesThe prospect and challenges of sodium‐ion batteries for low‐temperature conditionsNavigating at Will on the Water Phase DiagramThe equilibrium low‐temperature structure of iceHigh-voltage aqueous planar symmetric sodium ion micro-batteries with superior performance at low-temperature of −40 ºCAll-climate aqueous Na-ion batteries using “water-in-salt” electrolyteA phenazine anode for high-performance aqueous rechargeable batteries in a wide temperature rangeHow Ions Affect the Structure of WaterHydrogen-Bonding Interactions in Hybrid Aqueous/Nonaqueous Electrolytes Enable Low-Cost and Long-Lifespan Sodium-Ion StorageAqueous Batteries Operated at −50 °CDilute Hybrid Electrolyte for Low‐Temperature Aqueous Sodium‐Ion BatteriesHybrid Electrolytes Enabling in‐situ Interphase Protection and Suppressed Electrode Dissolution for Aqueous Sodium‐Ion BatteriesCost attractive hydrogel electrolyte for low temperature aqueous sodium ion batteriesHigh Energy, Long Cycle, and Superior Low Temperature Performance Aqueous Na–Zn Hybrid Batteries Enabled by a Low-Cost and Protective Interphase Film-Forming ElectrolyteA free-sealed high-voltage aqueous polymeric sodium battery enabling operation at −25°CAqueous sodium ion hybrid batteries with ultra-long cycle life at -50 ℃“Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistriesUniversal quinone electrodes for long cycle life aqueous rechargeable batteriesHigh Power and Energy Density Aqueous Proton Battery Operated at − 90 ° CAn Ultralow Temperature Aqueous Battery with Proton ChemistryAqueous rechargeable zinc air batteries operated at −110°CInorganic Electrolyte for Low‐Temperature Aqueous Sodium Ion BatteriesTailoring Pure Inorganic Electrolyte for Aqueous Sodium‐Ion Batteries Operating at −60 °CSuppressing Crystallization of Water-in-Salt Electrolytes by Asymmetric Anions Enables Low-Temperature Operation of High-Voltage Aqueous BatteriesAll-climate aqueous supercapacitor enabled by a deep eutectic solvent electrolyte based on salt hydrateModulating electrolyte structure for ultralow temperature aqueous zinc batteriesA Universal Approach to Aqueous Energy Storage via Ultralow‐Cost Electrolyte with Super‐Concentrated Sugar as Hydrogen‐Bond‐Regulated SoluteElectrolyte solvation structure manipulation enables safe and stable aqueous sodium ion batteriesAqueous Rechargeable Li⁺ /Na⁺ Hybrid Ion Battery with High Energy Density and Long Cycle LifeAll-Climate Aqueous Dual-Ion Hybrid Battery with Ultrahigh Rate and Ultralong Life Performance

[1]	Evans A, Strezov V, and Evans T J 2012 Renewable Sustainable Energy Rev. 16 4141
[2]	Luo B, Ye D, and Wang L 2017 Adv. Sci. 4 1700104
[3]	Grey C P and Tarascon J M 2017 Nat. Mater. 16 45
[4]	Dunn B, Kamath H, and Tarascon J M 2011 Science 334 928
[5]	Goodenough J B 2013 Acc. Chem. Res. 46 1053
[6]	Yang Z G, Zhang J L, Kintner-Meyer M C W, Lu X C, Choi D, Lemmon J P, and Liu J 2011 Chem. Rev. 111 3577
[7]	Schmidt O, Hawkes A, Gambhir A, and Staffell I 2017 Nat. Energy 2 17110
[8]	Dai H L, Dong J, Wu M J, Hu Q M, Wang D N, Zuin L, Chen N, Lai C, Zhang G X, and Sun S H 2021 Angew. Chem. Int. Ed. 60 19852
[9]	Kim H, Hong J, Park K Y, Kim H, Kim S W, and Kang K 2014 Chem. Rev. 114 11788
[10]	Liu Z X, Huang Y, Huang Y, Yang Q, Li X L, Huang Z D, and Zhi C Y 2020 Chem. Soc. Rev. 49 180
[11]	Fang G Z, Zhou J, Pan A Q, and Liang S Q 2018 ACS Energy Lett. 3 2480
[12]	Tang W, Zhu Y, Hou Y, Liu L, Wu Y, Loh K P, Zhang H, and Zhu K 2013 Energy & Environ. Sci. 6 2093
[13]	Bin D, Wang F, Tamirat A G, Suo L, Wang Y, Wang C, and Xia Y 2018 Adv. Energy Mater. 8 1703008
[14]	Suo L M, Borodin O, Wang Y S, Rong X H, Sun W, Fan X L, Xu S Y, Schroeder M A, Cresce A V, Wang F, Yang C, Hu Y S, Xu K, and Wang C S 2017 Adv. Energy Mater. 7 1701189
[15]	Xie F, Lu Y X, Chen L Q, and Hu Y S 2021 Chin. Phys. Lett. 38 118401
[16]	Wang M, Wang Q, Ding X, Wang Y, Xin Y, Singh P, Wu F, and Gao H 2022 Interdiscip. Mater. 1 373
[17]	Pipolo S, Salanne M, Ferlat G, Klotz S, Saitta A M, and Pietrucci F 2017 Phys. Rev. Lett. 119 245701
[18]	Leadbetter A J, Ward R C, Clark J W, Tucker P A, Matsuo T, and Suga H 1985 J. Chem. Phys. 82 424
[19]	Wang X, Huang H, Zhou F, Das P, Wen P, Zheng S, Lu P, Yu Y, and Wu Z S 2021 Nano Energy 82 105688
[20]	Zhang Y, Xu J, Li Z, Wang Y, Wang S, Dong X, and Wang Y 2022 Sci. Bull. 67 161
[21]	Sun T J, Liu C, Wang J Y, Nian Q S, Feng Y Z, Zhang Y, Tao Z L, and Chen J 2020 Nano Res. 13 676
[22]	Hribar B, Southall N T, Vlachy V, and Dill K A 2002 J. Am. Chem. Soc. 124 12302
[23]	Chua R, Cai Y, Lim P Q, Kumar S, Satish R, Jr W M, Ren H, Verma V, Meng S, Morris S A, Kidkhunthod P, Bai J, and Srinivasan M 2020 ACS Appl. Mater. & Interfaces 12 22862
[24]	Nian Q, Wang J, Liu S, Sun T, Zheng S, Zhang Y, Tao Z, and Chen J 2019 Angew. Chem. Int. Ed. 58 16994
[25]	Sun Y, Zhang Y, Xu Z, Gou W, Han X, Liu M, and Li C M 2022 ChemSusChem 15 e202201362
[26]	Wang H, Liu T, Du X, Wang J, Yang Y, Qiu H, Lu G, Li H, Chen Z, Zhao J, and Cui G 2022 Batteries Supercaps 5 e202200246
[27]	Cheng Y G, Chi X W, Yang J H, and Liu Y 2021 J. Energy Storage 40 102701
[28]	Liu S, Lei T, Song Q, Zhu J, and Zhu C 2022 ACS Appl. Mater. & Interfaces 14 11425
[29]	Rong J Z, Cai T X, Bai Y Z, Zhao X, Wu T, Wu Y K, Zhao W, Dong W J, Xu S M, Chen J, and Huang F Q 2022 Cell Rep. Phys. Sci. 3 100805
[30]	Zhu K, Sun Z, Li Z, Liu P, Chen X, and Jiao L 2022 Energy Storage Mater. 53 523
[31]	Suo L M, Borodin O, Gao T, Olguin M, Ho J, Fan X L, Luo C, Wang C S, and Xu K 2015 Science 350 938
[32]	Liang Y L, Jing Y, Gheytani S, Lee K Y, Liu P, Facchetti A, and Yao Y 2017 Nat. Mater. 16 841
[33]	Sun T J, Du H H, Zheng S B, Shi J Q, and Tao Z L 2021 Adv. Funct. Mater. 31 2010127
[34]	Yue F, Tie Z W, Deng S Z, Wang S, Yang M, and Niu Z Q 2021 Angew. Chem. Int. Ed. 60 13882
[35]	Chen S, Wang T, Ma L, Zhou B, Wu J, Zhu D, Li Y Y, Fan J, and Zhi C 2022 Chem (in press)
[36]	Zhu K J, Li Z P, Sun Z Q, Liu P, Jin T, Chen X C, Li H X, Lu W B, and Jiao L F 2022 Small 18 2107662
[37]	Zhu K J, Sun Z Q, Jin T, Chen X C, Si Y C, Li H X, and Jiao L F 2022 Batteries Supercaps 5 e202200308
[38]	Reber D, Kühnel R S, and Battaglia C 2019 ACS Mater. Lett. 1 44
[39]	Bu X D, Zhang Y R, Sun Y G, Su L J, Meng J N, Lu X G, and Yan X B 2020 J. Energy Chem. 49 198
[40]	Zhang Q, Ma Y, Lu Y, Li L, Wan F, Zhang K, and Chen J 2020 Nat. Commun. 11 4463
[41]	Bi H B, Wang X S, Liu H L, He Y L, Wang W J, Deng W J, Ma X L, Wang Y S, Rao W, Chai Y Q, Ma H, Li R, Chen J T, Wang Y, and Xue M Q 2020 Adv. Mater. 32 2000074
[42]	Ao H S, Chen C Y, Hou Z G, Cai W L, Liu M K, Jin Y A, Zhang X, Zhu Y C, and Qian Y T 2020 J. Mater. Chem. A 8 14190
[43]	Zhang X Q, Dong M F, Xiong Y L, Hou Z G, Ao H S, Liu M K, Zhu Y C, and Qian Y T 2020 Small 16 2003585
[44]	Nian Q S, Liu S, Liu J, Zhang Q, Shi J Q, Liu C, Wang R, Tao Z L, and Chen J 2019 ACS Appl. Energy Mater. 2 4370