Mechanism Study On The Inhibition Of Mn(Ⅱ) Deposition On Graphite Anode By Modulating Solvation Structure

The solvation structure of ions in electrolyte is an important factor affecting the electrochemical stability of lithium-ion batteries（LIBs）,and it has a significant impact on suppressing the dissolution and deposition of transition metal ions（TMIs）in lithium-ion batteries.Therefore,optimizing the solvation structures of electrolyte through electrolyte engineering is an important means to improve the electrochemical performance of batteries.This thesis studied the difference in Mn²⁺tolerance between high-concentration electrolyte and conventional-concentration electrolyte,revealed the mechanism of high-concentration electrolyte in inhibiting Mn²⁺from damaging the cycle performance of the cell,and optimized the solvation structures of the electrolyte by introducing weakly-solvating solvent into the conventional-concentration electrolyte system through the above mechanism,so as to achieve the effect of suppressing Mn²⁺deposition on the anode surface in a more economical and efficient way.The main research content of this thesis is as follows:（1）By comparing and analyzing the electrochemical stability of electrolytes containing1000 mg kg^-1 Mn²⁺in 1 mol L^-1 lithium bis（trifluoromethanesulfonyl）imide（Li TFSI）/dimethyl carbonate（DMC）and 3 mol L^-1 Li TFSI/DMC in the graphite||Li half cell system,to illustrate that by increasing the electrolyte concentration,Mn²⁺can be effectively suppressed from damaging the cycle performance of lithium-ion batteries.Combined with spectroscopic analysis and density functional theory（DFT）calculation,the solvation structures of different concentration electrolyte systems were analyzed,and the mechanism of high-concentration electrolyte in inhibiting the destructive effect of Mn²⁺was revealed.The results show that when the lithium salt concentration increases from 1 mol L^-1 to 3 mol L^-1,the solvation structures of the electrolyte will change from separated ion pairs(SSIPs,Li⁺-4DMC and Mn²⁺-4DMC)to contact ion pairs(CIPs,Li⁺-2DMC-TFSI^-and Mn²⁺-2DMC-TFSI^-)and aggregates(AGGs,Li⁺-2TFSI^-and Mn²⁺-2TFSI^-),which will raise the lowest unoccupied molecular orbital（LUMO）values of Li⁺and Mn²⁺solvation structures(LUMO values of Li⁺-4DMC,Li⁺-2DMC-TFSI^-,Li⁺-2TFSI^-Mn²⁺-4DMC,Mn²⁺-2DMC-TFSI^-and Mn²⁺-2TFSI^-are-2.619,-0.059,3.071,-6.707,-4.115 and-1.404 e V,respectively),thereby inhibiting their continuous reduction and decomposition on the graphite surface and Mn²⁺deposition,and avoiding excessive growth of solid electrolyte interphase（SEI）.（2）Considering that high-concentration electrolyte has the disadvantages of high cost and high viscosity,the solvation effect theory obtained from the above research is used to regulate the ion solvation structure in the electrolyte by introducing weakly-solvating solvent tetrahydrofuran（THF）,thereby inhibiting the destructive effect of Mn²⁺on the cycle performance of lithium-ion batteries.Through electrochemical performance testing,spectroscopic characterization,and DFT calculation,the influence of 1 mol L^-1 Li TFSI/THF,1mol L^-1 Li TFSI/DMC,and 3 mol L^-1 Li TFSI/DMC on electrochemical performance were summarized from the perspective of solvation structures.The results show that after 100 charge-discharge cycles,the capacity of the weakly-solvating electrolyte system containing Mn²⁺does not significantly decay,while the capacity retention rate of the conventional-concentration electrolyte system is only 36.26%.The solvation structures with a larger number of CIPs and AGGs constructed by the weakly-solvating electrolyte are not only superior to the conventional-concentration electrolyte in inhibiting the destructive effect of Mn²⁺to the cycle performance of lithium-ion batteries,but also superior to the high-concentration electrolyte.It can be seen that introducing weakly-solvating solvent is an efficient way to construct a good solvation structure and optimize the electrochemical performance of lithium-ion batteries.