Electrolyte Effect On The Electrochemical Performances Of Graphite As The Potassium Ion Battery Anode

People's increasing demand for a better life have support of convenient equipment,and lithium-ion batteries play an important place in electrochemical energy storage applications as a portable energy source.However,the lithium resources in lithium-ion batteries are unevenly distributed,limited and expensive,which greatly restricts the further development of lithium-ion batteries.The potassium element of the same main group of lithium is widely concerned by researchers because its chemical properties are similar to those of lithium,and its resources are more abundant and the cost is lower.The disadvantage is that the potassium ion radius is larger than lithium ion,and the volume change caused by the repeated intercalation/deintercalation of K⁺during the charge and discharge cycle eventually deteriorates the material properties.Studies have shown that K⁺and solvent may form solvated ions with smaller radius in the electrolyte,thereby reducing material structure changes and improving electrochemical performance.Therefore,in this thesis,we use to commericial graphite as a potassium ion battery anode and systematically its studies the electrochemical performance of potassium ion batteries in different kinds of solvents and different concentrations of potassium salts.Firstly,the phase structure and morphology of the graphite powder are characterized.It shows that the graphite sample has good crystallinity,the layer spacing is 3.36?,and the nanosheets with different sizes are stacked into 5-20?m ellipsoidal particles.It has a specific capacity of 330 mAh g^-1 at a current density of 0.2C in lithium ion battery,and the capacity retention is 97%after 100 cycles.However,the high rate performance is poor,and the capacity is only 25 mAh g^-1 at 10C.Secondly,the performance of graphite ion batteries in carbonate and ether solvent electrolytes is investigated.The capacity is severe decay in the KPF₆-EC/DMC electrolyte,and the specific capacity is only 17 mAh g^-1 after 700 cycles at 1C.In KPF₆-DME electrolyte,the rate performance is excellent.The specific capacity changes between 82 and 94 mAh g^-1 tested at rate of 0.5-10C,and the specific capacity is 94mAh g^-1 to 74 mAh g^-11 after 3700 cycles at 10C,which is superior to all reported graphite storage properties at present.The mechanism of potassium intercalation in the two electrolytes is different.There are stage structures changes in EC/DMC-based electrolyte.KC₈ and KC₂₄ are formed at the same time,and K⁺-DME co-insert process in DME-based electrolyte with small volume change in graphite.Moreover,a loose porous solid electrolyte interphase?SEI?film is formed on the graphite surface in EC/DMC-typed electrolyte,while almost no SEI film is formed in DME-typed electrolyte.Finally,the electrochemical performance of graphite as potassium ion batteries anode at different concentrations of KFSI-DME electrolyte was investigated.The performance of graphite is poor at electrolyte concentration of 3 M kg^-1 when the current rate is smaller than 560 mA g^-1,however,the specific capacity and the cycle stability are excellent at electrolyte concentration of 7 M kg^-1.The specific capacity of the battery at 1C is about 230 mAh g^-1,The specific capacity is 220 mAh g^-1 after 500cycles at 2C,and the capacity retention rate is 98.2%compared to 50 cycles(224 mAh g^-1).It is confirmed by GITT and CV tests that there is a different potassium intercalation mechanism between the low electrolyte concentration and the high electrolyte concentration.At electrolyte concentration 3 M kg^-1,there is a phenomenon in which potassium ions and solvent are co-embedded into graphite.However,potassium ions are embedded in graphite to form a stage structure at high electrolyte concentrations of 5 M kg^-1 and 7 M kg^-1.This indicates that a reasonable choice of solvent and potassium salt concentration in the electrolyte plays an extremely important role in improving battery performance.