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Abstract：Rechargeable aqueous Zn-ion batteries are attractive cheap，safe and green energy storage technologies but are bottlenecked by limitation in high-capacity cathode and compatible electrolyte to achieve satisfactory cyclability. In this work，we report on a ternary mixture of urea，LiTFSI，and Zn（Tfo）2 that forms a molten electrolyte. The molten electrolyte is prepared easily，as a new type of eco-friendly and low-cost electrolyte with high performance of ionic conductivity. By virtue of a molten electrolyte，two reversible reactions of Li+ extraction/insertion（cathode）and Zn dissolution/deposition（anode）can be realized in Zn batteries. That full Zn-ion cell has been assembled using the molten electrolytes，with LiCoO2 the positive electrode and Zn as the negative electrode.
Keywords：Zn batteries；LiCoO2；molten electrolyte；Eco-friendly
1. Introduction
In recent years，with the environment be destroyed and the decreasing of non-renewable energy，the energy storage devices play more obvious roles [1]. Presently，lithium-ion batteries（LIBs）play an important role in meeting this demand. However，it is well known that LIBs have safety issues as their components are toxic and flammable [2].
Zn batteries（ZBs）receive increasing attention due to their intrinsic non-flammable nature [3]. But so far there are still many problems concerning Zn anode in the presence of aqueous electrolytes，like dendrite formation，passivation and H2 evolution，which significantly limit cycle ability and further commercialization [4].
In the work，we report urea-based molten salts（UMS）composed of urea，LiTFSI，and Zn（Tfo）2，and study their application in ZBs. Designed Zn/UMS/ LiCoO2 battery shows a high energy density of 140Wh/kg.
2. Experimental
The mechanical mixing of LiTFSI and urea with molar ratios between 1：3 and 1：3.8 and the addition of a small amount of Zn（Tfo）2（molar ratio：n = Li+/Zn2+ = 20）lead to the formation of homogeneous liquid gradually at room temperature after stirring and storing overnight. The coin cells were assembled in air for Zn/LiCoO2 batteries with UMS electrolyte and glass fiber seperator.
3. Results and discussion
Fig. 1a shows the SEM image of LiCoO2. Fig. 1b shows the XRD pattern of LiCoO2. Fig. 1c exhibits a series of DSC curves of the UMS system. A endothermic peak for the sample of LiTFSI-urea（molar ratio of 1：3.8）are observed at ?60 °C，corresponding to the eutectic temperature. When Zn（Tfo）2（molar ratio：n = Li+/Zn2+ = 20）is added，the eutectic temperature keeps constant. The variation of ionic conductivity as a function of temperature is exhibited in Fig. 1d. It is found that the UMS with LiTFSI-urea ratio of 1：3.8 yields the highest ionic conductivity 0.3 mS cm?1 at room temperature. Fig. 2.（a）CV curves of Zn.（b）XRD pattern of LiCoO2 after the cycle.（c）Galvanostatic charge/discharge curves of Zn/LCO.
The cyclic voltammogram（CV）curves of the UMS electrolyte recorded on stainless steel foil are presented in Fig. 2a. The CV indicating a reversible Zn deposition/stripping process. It is noted that the Coulombic efficiency for the UMS electrolyte gradually increases to ～99%. Fig. 2b shows XRD pattern of LiCoO2 keeps constant after the cycle. The charge-discharge profiles of Zn/UMS/LiCoO2 batteries obviously exhibit typical voltage plateaus（Fig. 2c）. By the virtue of the high electrochemical stability of the UMS，the Zn/UMS/LiCoO2 cell possesses a high limited charge voltage of 2.2 V. Note that，for the first cycle，a high capacity of 140 mAh/g based on the active material of cathode.
4. Conclusions
In this work，we report on a ternary mixture of Urea，LiTFSI，and Zn（Tfo）2 that forms a molten electrolyte. By virtue of a molten salt electrolyte，two reversible reactions of Li+ extraction/insertion（cathode）and Zn dissolution/deposition（anode）can be realized in ZBs. Base on the above results，UMS electrolyte can be further used for high-performance secondary ZBs.
Reference：
[1]Tarascon，J.M.；Armand，M.，Issues and challenges facing rechargeable lithium batteries. Nature 2001，414（6861），359-367.
[2]Xu，K.，Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chemical reviews 2004，104（10），4303-4418.
[3]劉鹏，童叶翔，杨绮琴，熔盐体系及有关应用的新进展.电化学2007，13（4），351-359.
[4]Hu，Y.；Li，H.；Huang，X.，Novel room temperature molten salt electrolyte based on LiTFSI and acetamide for lithium batteries. Electrochemistry communications 2004，6（1），28-32.
Keywords：Zn batteries；LiCoO2；molten electrolyte；Eco-friendly
1. Introduction
In recent years，with the environment be destroyed and the decreasing of non-renewable energy，the energy storage devices play more obvious roles [1]. Presently，lithium-ion batteries（LIBs）play an important role in meeting this demand. However，it is well known that LIBs have safety issues as their components are toxic and flammable [2].
Zn batteries（ZBs）receive increasing attention due to their intrinsic non-flammable nature [3]. But so far there are still many problems concerning Zn anode in the presence of aqueous electrolytes，like dendrite formation，passivation and H2 evolution，which significantly limit cycle ability and further commercialization [4].
In the work，we report urea-based molten salts（UMS）composed of urea，LiTFSI，and Zn（Tfo）2，and study their application in ZBs. Designed Zn/UMS/ LiCoO2 battery shows a high energy density of 140Wh/kg.
2. Experimental
The mechanical mixing of LiTFSI and urea with molar ratios between 1：3 and 1：3.8 and the addition of a small amount of Zn（Tfo）2（molar ratio：n = Li+/Zn2+ = 20）lead to the formation of homogeneous liquid gradually at room temperature after stirring and storing overnight. The coin cells were assembled in air for Zn/LiCoO2 batteries with UMS electrolyte and glass fiber seperator.
3. Results and discussion
Fig. 1a shows the SEM image of LiCoO2. Fig. 1b shows the XRD pattern of LiCoO2. Fig. 1c exhibits a series of DSC curves of the UMS system. A endothermic peak for the sample of LiTFSI-urea（molar ratio of 1：3.8）are observed at ?60 °C，corresponding to the eutectic temperature. When Zn（Tfo）2（molar ratio：n = Li+/Zn2+ = 20）is added，the eutectic temperature keeps constant. The variation of ionic conductivity as a function of temperature is exhibited in Fig. 1d. It is found that the UMS with LiTFSI-urea ratio of 1：3.8 yields the highest ionic conductivity 0.3 mS cm?1 at room temperature. Fig. 2.（a）CV curves of Zn.（b）XRD pattern of LiCoO2 after the cycle.（c）Galvanostatic charge/discharge curves of Zn/LCO.
The cyclic voltammogram（CV）curves of the UMS electrolyte recorded on stainless steel foil are presented in Fig. 2a. The CV indicating a reversible Zn deposition/stripping process. It is noted that the Coulombic efficiency for the UMS electrolyte gradually increases to ～99%. Fig. 2b shows XRD pattern of LiCoO2 keeps constant after the cycle. The charge-discharge profiles of Zn/UMS/LiCoO2 batteries obviously exhibit typical voltage plateaus（Fig. 2c）. By the virtue of the high electrochemical stability of the UMS，the Zn/UMS/LiCoO2 cell possesses a high limited charge voltage of 2.2 V. Note that，for the first cycle，a high capacity of 140 mAh/g based on the active material of cathode.
4. Conclusions
In this work，we report on a ternary mixture of Urea，LiTFSI，and Zn（Tfo）2 that forms a molten electrolyte. By virtue of a molten salt electrolyte，two reversible reactions of Li+ extraction/insertion（cathode）and Zn dissolution/deposition（anode）can be realized in ZBs. Base on the above results，UMS electrolyte can be further used for high-performance secondary ZBs.
Reference：
[1]Tarascon，J.M.；Armand，M.，Issues and challenges facing rechargeable lithium batteries. Nature 2001，414（6861），359-367.
[2]Xu，K.，Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chemical reviews 2004，104（10），4303-4418.
[3]劉鹏，童叶翔，杨绮琴，熔盐体系及有关应用的新进展.电化学2007，13（4），351-359.
[4]Hu，Y.；Li，H.；Huang，X.，Novel room temperature molten salt electrolyte based on LiTFSI and acetamide for lithium batteries. Electrochemistry communications 2004，6（1），28-32.