Electrochemical Compatibility Of Organic Electrolytes For Sodium Ion Batteries

Due to rich resources,low price and environmental friendliness of sodium,sodium ion batteries have potential advantages in large-scale energy storage.However,the current research on sodium ion batteries is mainly focused on the development of electrode materials and performance improvement.There are few reports on the study of electrolyte solutions.At the same time,due to the high reducibility of sodium metal,it reacts with many organic solvents.Therefore,it is preferred that the chemically and electrochemically stable electrolyte component is important for the development of high energy density and long-life sodium ion batteries.In this paper,different non-aqueous organic solvents（esters and ethers）and sodium salts are selected,and the chemical and electrochemical stability of the electrolyte system composed of them to metal sodium and hard carbon anodes are systematically studied.And their electrochemical properties were compared and analyzed.The possible reaction mechanism and applicable stable electrolyte systems were proposed,which provided a reference system for the development of sodium ion battery application.The main research contents and results are as follows:1.The carbonate electrolyte was studied by using vinyl carbonate（EC）with high ionization constant,propylene carbonate（PC）and two kinds of 1:1 mixed solvents as the base solvent,respectively.The chemical and electrochemical stability of different linear carbonates(dimethyl carbonate（DMC）,diethyl carbonate（DEC）,methyl ethyl carbonate（EMC）and their binary or ternary electrolytes were studied,and the reaction mechanism was discussed.Furthermore,the electrochemical properties of hard carbon negative electrodes in different electrolytes were studied.It is found that the change of linear carbonate,solvent ratio,salt,temperature and the type of additives have great influence on the chemical and electrochemical stability of electrolyte,the impedance and polarization of metal sodium and the electrochemical performance of hard carbon negative electrode.Generally speaking,when the carbonate solvent can not form a stable SEI film on the surface of metal sodium,the electrolyte will react continuously with metal sodium,and the polarization and impedance of Na/Na symmetrical cell increase with the cycle.The capacity of hard carbon negative electrode decays rapidly.While the carbonate electrolyte can form SEI film on the surface of metal sodium,the SEI film formed is thick but not dense enough.The electrolyte will not react with metal sodium for a short time,but it will still react at long time or high temperature.The initial polarization and impedance of Na/Na symmetrical cells are very large.With the decrease of the cycle,the capacity of the hard carbon electrode will decline after a long cycle.When the SEI film is thin and dense,the electrolyte will not react with the metal sodium.Although the initial polarization and impedance of the Na/Na symmetrical battery increase,the capacity of the hard carbon electrode does not decay after a long cycle.In addition,the reasons for the instability of PC electrolyte on the surface of sodium metal were revealed by characterization,and the reaction mechanism of PC electrolyte was expounded.2.Based on DEGDME and TEGDME in ether electrolyte,by changing conditions such as salt,temperature and additive,the chemical and electrochemical stability of ether-based electrolytes,the impedance and polarization of sodium metal,and the electrochemical performance on hard carbon anode were systematically explored.In conclusion,we found that the compatibility of DEGDME with sodium metal was better than that of TEGDME electrolyte in two kinds of ether electrolytes,and the compatibility of NaPF₆ with ether electrolyte was the best one among several salts.The cycling performance of ether electrolyte in hard carbon electrode is not much different from that of carbonate electrolyte,but the rate performance is better than that of carbonate,among which DEGDME-NaPF₆ has the best rate performance.This is due to the fact that the ether electrolyte has higher stability to the metal sodium anode and the resulting SEI film has a lower impedance.3.Sodium salts（NaClO₄,NaSO₃CF₃,NaTFSI,NaPF₆ and NaFSI）were taken as the object of study.The properties of different salts in PC solvent are also different.The electrolytes containing NaClO₄ and NaSO₃CF₃ on the surface of the metal sodium are unstable due to its non-formed-film property,and the polarization and impedance of the Na/Na symmetric battery gradually increase during the cycle.The electrolytes containing NaTFSI,NaPF₆ and NaFSI salt electrolytes have good stability but large impedance and polarization.The salt with the best cycling performance in the hard carbon electrode is NaFSI,but it reacts with the aluminum foil at high potential.