Regulation Of Electrode-electrolyte Interfaces In Lithium Metal Batteries With Eutectic Electrolytes

Lithium metal batteries display significant application prospects due to their much higher energy density than conventional lithium-ion batteries.However,its application still suffers from uncontrollable lithium dendrite growth and unstable solid electrolyte interface（SEI）.In addition,the commercial electrolytes suffer from safety hazards such as poor thermal stability and flammability.In this thesis,we design new eutectic electrolytes to suppress lithium dendrite growth,improve SEI films,and enhance the stability of electrolytes,which effectively solve the problems in lithium metal battery applications.The main research contents and results are as follows:In this study,a eutectic electrolyte composed of lithium bis（trifluorosulfonyl）imide（Li TFSI）and butyrolactam（BL）is designed.By introducing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether（TTE）as a co-solvent,the ionic conductivity of the electrolyte is improved,the viscosity is reduced,and the compatibility with lithium metal is improved.In addition,the co-solvent eutectic electrolyte forms a SEI film rich in inorganic components such as Li F,Li₃N and Li-N-C,which promotes Li⁺migration in SEI and induces homogeneous lithium deposition.The LFP||Li cell exhibits capacity retention of 89.0%after 650 cycles at 1 C of room temperature and 87.0%after 150 cycles at 1 C of 60°C,demonstrating excellent cycling stability at room temperature and high temperature.In this study,the ternary eutectic electrolyte is prepared by introducing succinonitrile（SN）with Li TFSI and BL,which possesses high thermal stability,non-flammability,high ionic conductivity(2.83 m S cm^–1),low lithium salt concentration(1.15 mol kg^–1),and low viscosity.It is found that there is a competitive solvation mechanism between BL and SN,where BL would preferentially participate in the solvated structure of Li⁺and SN would not participate in the first solvated structure shell.This structure facilitates the formation of stable SEI film rich in Li F and Li_xN,induces a uniform and dense deposition of Li⁺,reduces the side reactions between SN and lithium metal,and improves the stability of the electrolyte.The LFP||Li cell exhibites 90.0%capacity retention after 500cycles at 2 C.Lithium difluoroxalate borate（Li DFOB）is found to significantly improve the electrochemical performance of ternary eutectic electrolytes in NCM811||Li and NCA||Li cells as an additive,with a capacity retention of 80.0%after 200 cycles at 1 C in NCA||Li cells.In addition,Li DFOB as an additive forms a thinner and more homogeneous CEI film,as well as a flatter and denser SEI film,which reduces the side reactions between electrolyte and electrode materials.In this study,a sulfone eutectic electrolyte is synthesized by replacing BL with dimethyl sulfone（MSN）,an organic ligand without active hydrogen.The sulfone eutectic electrolyte significantly improves the interfacial stability with lithium metal and enables the stable operation of Li||Li symmetric cells for more than 1000 h.In addition,the sulfone eutectic electrolyte possesses a high Li⁺transfer number of 0.78.It is found that the sulfone eutectic electrolyte forms a stable SEI film rich in inorganic components Li F-Li₃N-Li₂S_x-Li₂SO_x,which promotes the rapid migration of Li⁺in the SEI film,inhibits the side reactions between the electrolyte and electrode materials,and improves the long cycling stability.The capacity retention of the LFP||Li cell is 93.1%and 96.1%after 3000and 1200 cycles at 1 C and 5 C at room temperature,respectively,and 90.0%after 2000cycles at 1 C and 50°C.In this study,a non-flammable dual salt eutectic electrolyte consisting of Li TFSI,Li DFOB and SN is designed to form a eutectogel electrolyte by combining with polyvinylidene fluoride-hexafluoropropylene（PVDF-HFP）,polyethylene oxide（PEO）and 1-ethyl-3-methylimidazolium bis（trifluoromethanesulfonyl）imide salt（EMITFSI）,which promotes the formation of a stable SEI film,induces the uniform and dense deposition of Li⁺,and reduces the side reactions between the lithium metal and the electrolyte.The LCO||Li cell has a capacity retention of 72.5%after 1500 cycles at 0.5 C under 3.0-4.45 V.In addition,the eutectogel electrolyte reduces the decomposition of lithium salts,forms a CEI film with high passivation ability,which effectively inhibits the structural deterioration of LCO cathodes at high voltage.It is demonstrated by in situ and ex situ XRD that the LCO cathode undergoes a stable and reversible O3-H1-3 phase transition between 4.5-4.6 V in eutectogel electrolyte.