Study On The Effect Of Electrolyte Additives On The Electrochemical Properties Of Carbon Materials For Sodium Storage

With the increasing demand for new energy batteries in electric vehicles and large-scale energy storage systems,limited lithium resources have limited the sustainable development of lithium-ion batteries.Because sodium is abundant in the Earth’s crust and has similar properties to lithium,sodium-ion batteries are considered an ideal substitute for lithium-ion batteries,with broad research prospects.At present,researchers’research on sodium-ion batteries mainly focuses on the development of positive and negative electrode materials.Further research is needed on the electrolytes and electrolyte additives used in sodium ion batteries.The configuration of electrolytes suitable for electrode materials has a crucial impact on the performance of sodium-ion batteries.Hard carbon and graphite are the most widely used negative electrode materials.Unfortunately,hard carbon has disadvantages such as low initial Coulombic efficiency and poor rate performance,while graphite exhibits excellent initial Coulombic efficiency and rate performance in ether-based electrolytes.Therefore,this thesis selects commercial spherical graphite as the negative electrode material for sodium-ion batteries,and selects different organic solvents,electrolyte additives,and sodium salts to study the electrochemical performance of graphite in electrolytes containing different electrolyte additives,different sodium salt concentrations,and different sodium salts.The main research content and results are as follows:（1）Based on ether-based electrolytes,the effects of different electrolyte additives on the electrochemical performance of graphite for sodium storage in ether-based electrolytes were studied.The addition of typical film forming additives（FEC and VC）inhibits the co intercalation reaction of Na⁺in the graphite layer;The addition of HN makes the stability of ether-based electrolytes worse;SUL as an additive will reduce the reversible sodium storage capacity of graphite electrodes during cycling.The addition of DOL,THF,and TTE to ether-based electrolytes can improve the stability of the electrolyte,with graphite having the best sodium storage and long cycle performance in 1 M Na PF₆ in DEGDME with 1 vol%TTE.It is worth noting that DMSO as an additive can improve the sodium storage performance of graphite.Graphite has a first discharge capacity of up to 488.6 m Ah g^-1 in an ether-based electrolyte containing 5 vol%DMSO.However,SEM testing of the graphite electrodes after cycling found that the addition of DMSO can cause the structure of spherical graphite to be destroyed during the sodium removal cycle,making it impossible to achieve sustained and stable charging and discharging cycles.（2）In order to suppress the detachment of graphite by DMSO,high concentration sodium salts and electrolytes containing different dual additives were configured,and the electrochemical performance of graphite in these electrolytes was studied.When the concentration of sodium salt is increased to 2 M,the stripping of graphite by DMSO can be inhibited to a certain extent,and selecting TTE as an auxiliary additive can stabilize the flake graphite layer structure of spherical graphite stripped by DMSO.The simultaneous use of high concentration sodium salts and dual additive strategies not only inhibits the detachment of DMSO from graphite,but also greatly improves the sodium storage capacity of graphite in ether-based electrolytes.The reversible capacity of the graphite electrode reached 591.9 m Ah g^-1 after 100 cycles in 2 M Na PF₆ in DEGDME with 3vol%DMSO+1vol%TTE.XPS and FTIR tests confirmed that the additives（DMSO and TTE）in the electrolyte participated in the electrochemical reaction during the co-embedding of sodium ions and ether solvents into the graphite layer.（3）Select sodium salts containing different anions and configure them as ether-based electrolytes without additives and with dual additives respectively to study the effect of sodium salts on the sodium storage performance of graphite.Electrochemical tests have found that the differences in anions in sodium salts have a significant impact on the performance of the electrolyte.Among them,Na FSI and Na Cl O₄ as sodium salts are not conducive to the sodium storage behavior of graphite in ether-based electrolytes.After adding 3vol%DMSO and 1vol%TTE to an electrolyte with a sodium salt concentration of 1 M,Na BF₄ and Na OTf as sodium salt electrolytes can achieve stable charge discharge cycling of the graphite electrode.The addition of dual additives improves the sodium storage performance of the graphite electrode.The graphite electrode has a reversible specific capacity of 336.3 m Ah g^-1 after 70 cycles in Na BF₄as sodium salt electrolyte.After increasing the concentration of sodium salts to 2 M,the selection of Na OTf and Na PF₆ as electrolytes for sodium salts also greatly improved the sodium storage capacity of graphite.Among them,graphite has a more stable charge discharge capacity in electrolytes with Na OTf as the sodium salt,but after long-term cycling in electrolytes with Na PF₆ as the sodium salt,higher reversible specific capacity can be obtained.