Synergistic Effect of Cation and Anion for Low-Temperature Aqueous Zinc-Ion Battery

The ratio of hydrogen bonds in water molecules is significantly decreased by introducing oxygen-ligand Mg2+ and hydrogen-ligand ClO4−, resulting in an ultralow solidifying point of − 121 °C. The excellent low-temperature physicochemical properties and good compatibility with Zn metal of 3.5 M (mol L−1) Mg(ClO4)2 + 1 M Zn(ClO4)2 electrolyte gives fabricated Zn||pyrene-4,5,9,10-tetraone (PTO) battery and Zn||Phenazine (PNZ) battery a satisfactory low temperature performance. The ratio of hydrogen bonds in water molecules is significantly decreased by introducing oxygen-ligand Mg2+ and hydrogen-ligand ClO4−, resulting in an ultralow solidifying point of − 121 °C. The excellent low-temperature physicochemical properties and good compatibility with Zn metal of 3.5 M (mol L−1) Mg(ClO4)2 + 1 M Zn(ClO4)2 electrolyte gives fabricated Zn||pyrene-4,5,9,10-tetraone (PTO) battery and Zn||Phenazine (PNZ) battery a satisfactory low temperature performance. Although aqueous zinc-ion batteries have gained great development due to their many merits, the frozen aqueous electrolyte hinders their practical application at low temperature conditions. Here, the synergistic effect of cation and anion to break the hydrogen-bonds network of original water molecules is demonstrated by multi-perspective characterization. Then, an aqueous-salt hydrates deep eutectic solvent of 3.5 M Mg(ClO4)2 + 1 M Zn(ClO4)2 is proposed and displays an ultralow freezing point of − 121 °C. A high ionic conductivity of 1.41 mS cm−1 and low viscosity of 22.9 mPa s at − 70 °C imply a fast ions transport behavior of this electrolyte. With the benefits of the low-temperature electrolyte, the fabricated Zn||Pyrene-4,5,9,10-tetraone (PTO) and Zn||Phenazine (PNZ) batteries exhibit satisfactory low-temperature performance. For example, Zn||PTO battery shows a high discharge capacity of 101.5 mAh g−1 at 0.5 C (200 mA g−1) and 71 mAh g−1 at 3 C (1.2 A g−1) when the temperature drops to − 70 °C. This work provides an unique view to design anti-freezing aqueous electrolyte.


Introduction
The temperature in surface of earth is unevenly, which results in a great challenge for energy storage devices. For example, there are abundant wind and solar in severe cold regions, but how to store these energies becomes a problem. Aqueous zinc-ion batteries (AZIBs), with merits of high theoretical specific capacity and low redox potential of Zn anode, low cost and high ionic conductivity of aqueous electrolyte, and various cathode materials, have attached tremendous attention from researchers and have shown great potential for large-scale energy storage devices [1][2][3][4][5][6]. Unfortunately, AZIBs show terrible electrochemical performance at low-temperature condition, which hinders their application in harsh environments. For AZIBs, the ultralow and ultrahigh activation energies of the anodic and cathodic reactions result in insensitive electrode kinetics to varied temperatures based on the Arrhenius Equation [7]. While the aqueous electrolytes are extremely sensitive to temperature. As we know, the thermodynamic freezing point of solvent water is 0 °C. When temperature further drops, the aqueous electrolyte will freeze, and the ionic conductivity and interface wettability will rapidly deteriorate, which causes AZIBs cannot work normally [8,9]. Therefore, inbibition of aqueous electrolyte freezing is an effective strategy to improve the low-temperature performance of AZIBs.
The formation of ice crystal is driven by hydrogen bonds (HBs) between water molecules [9,10]. Modulating the chemical environment of O and H atoms in water to break the HBs network is possible to induce the freezing point depression of water. To date, several strategies have been reported including hybrid electrolyte with cosolvents or antisolvent additives, high-concentration electrolyte, and hydrogel electrolyte, etc. [11][12][13][14][15][16]. Although these methods improve the low-temperature performance of AZIBs to some extent, some inhere problems still hinder their practical application, such as low ionic conductivity and environmental unfriendliness of organic additives, high viscosity, and cost of high-concentration salt, and complex synthesis and assembly processes of hydrogel electrolyte. Essentially, these strategies alter coordination environment of H atoms by introducing HBs acceptors and further lower its freezing point. The researches about adjust chemical environment of O atoms to suppress ice up are ignored, which are worthy of further study.
Deep eutectic solvents (DES), as a kind of "green" solvent, have been studied to protect the Zn metal anode for AZIBs [17][18][19]. Meanwhile, it also exhibits a certain antifreezing property due to the interaction with water molecules. However, conventional DES systems (for example, aqueous-organic DES mixture) commonly show low ionic conductivity and high viscosity at low temperature due to the low water content. In contrast, the aqueous-salt hydrates DES, without organic compound, is worth to be considered for application in low-temperature AZIBs. Aqueous-salt hydrates possess rich hydrogen-bond (HB) acceptor and hydrogen-bond donor, which effectively destroy the HBs network of original water molecules [20,21]. By simultaneously regulating the coordination environment of O and H atoms in water, this DES system can obtain a low freezing point.
Here, an anti-freezing dual-cations EDS electrolyte of 3.5 M (mol L −1 ) Mg(ClO 4 ) 2 + 1 M Zn(ClO 4 ) 2 is reported for low-temperature AZIBs. It is discovered that the ratio of HBs in water molecules is significantly decreased by introducing oxygen-ligand Mg 2+ and hydrogen-ligand ClO 4 − , resulting in an ultralow solidifying point of − 121 °C. The novel aqueous-salt hydrate shows high ionic conductivity, low viscosity, and activation energy at − 70 °C due to the absence of organic additive. The excellent low-temperature physicochemical properties and good compatibility with Zn metal of this electrolyte give fabricated Zn||pyrene-4,5,9,10tetraone (PTO) battery and Zn||Phenazine (PNZ) battery a satisfactory low temperature performance. For example, when at − 70 °C, the Zn||PTO battery exhibits a high discharge capacity of 101.5 mAh g −1 at 0.5 C (200 mA g −1 ) and excellent rate performance (71 mAh g −1 at 3 C (1.2 A g −1 )).

Electrochemical Measurement
The PTO and PNZ electrodes are prepared by mixing PTO or PNZ, Ketjen black (KB), and polytetrafluoroethylene (PTFE) at an appropriate weight ratio of 5 The current density and specific capacity of full battery are based on the active mass of cathode in each electrode. The ionic conductivity is tested by fabricated coin cell, which includes filled electrolyte, cathode, and anode stainless-steel case (Φ 20 mm).

Calculation Details
The ionic conductivity is calculated follow Eq. 1: σ: Ionic conductivity; L: thicknesses of the electrolyte (0.3 cm); R: resistance of the electrolyte; S: area of the electrolyte (3.14 cm −2 ). The activation energy is calculated follow Eqs. 2 and 3: σ: Ionic conductivity; T: temperature; E a : activation energy; k: Boltzmann constant (1.3807 × 10 -23 J K −1 ). The − E a /k was fitted by different temperature ln (σT) and 1/T. The Ea was obtained.

DFT Calculation
All of the calculations are carried out using the Gaussian 16 program. Geometry optimization and frequency analysis are performed in water solvent with the SMD solvation model. C, H, O, N using B3LYP functional and 6-31 + G (d, p) basis set. Zn 2+ and Mg 2+ using B3LYP functional and def2tzvp basis set.

Synergistic Effect of Mg 2+ and ClO 4 −
As mentioned above, it is a key that finding suitable salt to construct anti-freezing aqueous-salt hydrates EDS  (Fig. S5). In addition, the dissolution Mg(ClO 4 ) 2 salt in water is a violent exothermic process,    (Fig. S6).
The interaction between anion and water molecules is systematically investigated by spectroscopic methods. The thus resulting in the peaks of strong and medium HBs have blue shift (Fig. S8). Meanwhile, the ratio of weak HBs is increased (Fig. 1f). In addition, the Cl-O stretching vibration at about 1093 cm −1 has a red shift (Fig. S10), which is consistent with DFT calculations (Fig. S9,

Low-Temperature Properties of 3.5 M Electrolyte
The freezing points of different concentration solution are tested by differential scanning calorimetry (DSC) (Fig.  S14). Figure 2a shows the V-shape relationship between the liquid-solid transition temperature and concentration of Mg(ClO 4 ) 2 . When the concentration of Mg(ClO 4 ) 2 increases to 3.5 M, an ultralow glass transition temperature of − 121 °C is obtained (Fig. 2b) [32]. The freezing temperature below 3.5 M is mainly dominated by HBs ratio among of water molecules. However, above 3.5 M, the freezing temperature is raised because of increased ions interaction [9]. Therefore, 3.5 M Mg(ClO 4 ) 2 + 1 M Zn(ClO 4 ) 2 (3.5 M) solution can be used as electrolyte for low-temperature AZIBs.
In situ polarizing light (PL) or non-PL microscope is applied to intuitively observe solidification state of different electrolytes. As shown in Fig. 2c, (Fig. S15). By contrast, the 3.5 M electrolyte still maintains liquid state at − 70 °C, and an uneven boundary appears when temperature reduces to − 130 °C, indicating the solution transforms into brittle glass (Fig. 2c). Thus, 3.5 M solution shows a good freezing resistance and its physicochemical properties are further investigated. The ionic conductivity of 3.5 M electrolyte at different temperature from + 25 ~ − 70 °C is calculated by impedance of electrolyte. As shown in Fig. 2d, it shows a high ionic conductivity of 1.41 mS cm −1 at − 70 °C. A low viscosity of 22.9 mPa s is achieved at − 70 °C, which enables fast ion transport (Fig. 2e). The conductive activation energy (E a ) of 3.5 M electrolyte is fitted by the relationship between ionic conductivity and temperature (Fig. 2f). The E a is calculated to be 0.23 eV, implying fast ions diffusion ability. The excellent physicochemical properties enable AZIBs to achieve a favorable low-temperature performance.

Compatibility Between 3.5 M Electrolyte and Zn Anode
The compatibility between 3.5 M electrolyte and Zn metal anode is further understood. It is well known that introduced metal cations in AZIBs electrolyte can promote uniform deposition Zn and alleviate its dendrite problem by electrostatic shield effect [33][34][35]. The Mg 2+ has concentrated surface charges on account of small ionic radius (0.72 nm) and high positive charge. Thus, the Mg 2+ has more distinct electrostatic shield effect than univalent cation in theory. The cyclic voltammetry (CV) curves Fig. S16a show excellent reversibility of Zn plating/stripping on stainless-steel mesh (SS) in 3.5 M electrolyte. Compared with 0 M electrolyte (1 M Zn(ClO 4 ) 2 electrolyte), the Zn||SS half cell shows smaller voltage polarization in 3.5 M electrolyte (Fig. S16b). An obvious Zn plating peak (wide peak, not sharp peak) in 3.5 M electrolyte is detected, implying a depression of side reaction. In addition, the symmetric Zn||Zn battery in 3.5 M electrolyte exhibits a long-term cycling life of 500 h with a low and stable voltage polarization of 50 mV at 0.5 mA cm −2 . While in 0 M electrolyte (1 M Zn(ClO 4 ) 2 ), the symmetric battery shows increased voltage polarization and finally shorted circuit at about 450 h (Fig. S17). The symmetric Zn||Zn battery also displays excellent cycling stability at 1 mA cm −2 (Fig. S18), Scanning electron microscopy (SEM) image shows compact and smooth Zn surface after cycling 10 times at 3.5 M electrolyte (Fig. S19). These results suggest Mg 2+ has significant protect effect for Zn plating process, showing the potential feasibility of applying 3.5 M electrolyte for AZIBs.

Reaction Mechanism of PTO and PNZ Electrodes
To fabricate a high-performance low-temperature AZIBs, suitable electrode materials are selected. Organic electrode materials, with many advantages such as low cost, environmentally friendly, fast reaction kinetics, and high capacity independence of temperature, have been seen as a feasible choose [36][37][38]. Thus, pyrene-4,5,9,10-tetraone 1 3 (PTO) with electroactive carbonyl groups and phenazine (PNZ) with electroactive conjugated amino groups are selected to construct low-temperature AZIBs. It is worth noting that a great number of Mg 2+ exist in this EDS electrolyte (3.5 M electrolyte), which may be involved in organic electrode reaction. To distinguish it, DFT calculations are firstly carried out. The negative electrostatic potential (ESP) of the carbonyl groups in PTO molecule in Fig. 3a reveals its reaction sites. As shown in Fig. 3b, the effective electron delocalization occurs in the conjugated structure when PTO is reduced PTO 4− with accepting four electrons, indicating it can occur to four-electron reduction [39]. Thus, the binding energy of two cations (Mg 2+ or Zn 2+ ) one PTO molecule are calculated. As shown in Fig. 3c, the bind energy of PTO with two Mg 2+ (− 10.46 eV) is lower than with two Zn 2+ (− 9.41 eV), and it also is lower than ZnMgPTO (− 9.47 eV). However, it cannot be ignored that the metal cation binding to PTO requires a de-solvation process. As mentioned above, the hydrated energy of Zn 2+ (− 1.72 eV) is higher than Mg 2+ (− 4.14 eV). The Mg 2+ needs more energy to break the interaction with H 2 O molecules and then combine with PTO. Considering the de-solvation energy of metal cations, the bind energy of Zn 2 PTO is re-calculated to be − 5.97 eV, which is smaller than ZnMgPTO (− 3.61 eV) and Mg 2 PTO (− 2.18 eV) (Fig. 3d), suggesting that the PTO tends to bind to Zn 2+ not Mg 2+ . In addition, the CV curves of Zn||PTO battery in 3.5 M and 0 M electrolyte (1 M Zn(ClO 4 ) 2 ) show similar shape and potential, while it is different from in 3.5 M Mg(ClO 4 ) 2 electrolyte (Fig. S20). The result implies that the redox reaction of PTO is independent of Mg 2+ . The PNZ also exhibits similar reaction mechanism by DFT calculations (Fig. S21). The reaction mechanism of PTO is further confirmed by ex situ FTIR and XRD patterns. As shown in Fig. 3f S22 and Table S1).

Low-Temperature Performance of Zn||PTO and Zn||PNZ Battery
The low-temperature AZIBs are constructed by 3.5 M Mg(ClO 4 ) 2 + 1 M Zn(ClO 4 ) 2 electrolyte, PTO cathode, and Zn metal anode (Fig. 4a). The CV curves of Zn||PTO battery at + 25 ~ − 70 °C show a good reversibility (Fig. S23). The voltage polarization of Zn||PTO battery has gradually increased when temperature dropped, which may be caused by increased activation process of PTO material and concentration polarization of the electrolyte. Figure 4b shows the charge-discharge curves of Zn||PTO battery range from + 25 ~ − 70 °C. It can work well at − 70 °C and exhibit a high discharge capacity of 101.5 mAh g −1 at 200 mA g −1 .
Even at ultrahigh current density of 3 C (1.2 A g −1 ), this system still maintains 71 mAh g −1 discharge capacity, which is 67% of the capacity at 100 mA g −1 (Fig. 4c). The discharge capacity recovers to initial level when current density increases to 0.25 C (100 mA g −1 ). The excellent rate performance of Zn||PTO battery benefits from 3.5 M electrolyte with high ionic conductivity, low viscosity and activation energy at low temperature. As shown in Fig. 4e, the Zn||PTO battery also can cycle 100 times with no obvious capacity fading at − 70 °C and achieve near 100% coulombic efficiency. In addition, the Zn||PNZ battery is fabricated and tested at + 25 ~ − 70 °C. The CV curves of Zn||PNZ battery at + 25 and − 70 °C are displayed in Fig. S24. This system obtained discharge capacity of 218.7 and 115.6 mAh g −1 at + 25 and − 70 °C, respectively (Fig. S25). The battery also shows a good rate capacity at − 70 °C. A high discharge capacity of 68.3 mAh g −1 is achieved at a current density of 1.5 C (435 mA g −1 ) (Fig. S26). Moreover, the Zn||PNZ battery exhibits an impressive cycling stability and maintains 100 mAh g −1 discharge capacity after 100 times at − 70 °C (Fig. S27).  Especially, the Zn||PTO battery delivers a high discharge capacity (101.5 mAh g −1 at 0.5 C), excellent rate performance (71 mAh g −1 at 3 C), and cycling stability (cycles 100 times with no obvious fading) at − 70 °C. This work highlights the design of low-temperature aqueous electrolyte and promotes the development of low-temperature AZIBs.