Amide-Based Eutectic Electrolyte Solvation Structure Regulation And Application In Lithium Metal Battery

With the increasing application of lithium-ion batteries in various aspects,the current commercial graphite cathode can hardly meet the increasing demand for high energy density,and lithium metal is beginning to attract attention as an electrode material with extremely high theoretical capacity.The traditional commercial carbonate electrolyte is not suitable for lithium metal anode because the surface is prone to dendrites and passivation layers,which affect the electrochemical performance of the battery and cause safety hazards.In addition,the carbonate electrolyte itself is flammable and explosive,which further aggravates the safety risk.Therefore,to balance the energy density and safety of lithium batteries,there is an urgent need to develop electrolytes that are non-flammable and can inhibit the growth of lithium dendrites.Eutectic electrolytes based on deep eutectic solvents have shown potential as electrolytes for lithium metal batteries because of their non-flammability,high thermal stability,wide electrochemical stability window,and designability.However,the research on eutectic electrolytes is still in its initial stage,and there are few studies on their solvation structure.In this thesis,we start from the regulation of the solvation structure of eutectic electrolytes and introduce co-solvents and additives to optimize the structural composition of electrolytes,which effectively improve the lithium-ion transference number,reduce the viscosity,and improve the stability of lithium metal anode.The main research components include:A eutectic electrolyte with non-flammable,high thermal stability and wide electrochemical stability window was prepared based on lithium bis（trifluoromethanesulphonyl）imide（Li TFSI）and N-methylacetamide（NMAC）.The four-coordination tendency of Li⁺in the first solvated shell of the electrolyte was determined by simulations analysis,and the coordination relationship between Li⁺and different contents of NMAC was investigated,as well as the optimal ratio of 1:4（Li TFSI:NMAC）was confirmed.The film-forming additive fluoroethylene carbonate（FEC）was introduced into LN4（optimal ratio electrolyte）,which does not participate in the coordination of Li⁺so that the modified LN4-5F（LN4+5%FEC）electrolyte can maintain the anion-rich solvation structure,thus inducing the formation of spherical lithium deposition morphology and inhibit lithium dendrite growth.The LFP||Li cell based on LN4-5F electrolyte has a capacity retention rate of96%after 1000 cycles at 1 C at room temperature.Compared with carbonate electrolytes,LN4-5F electrolyte has significantly improved in lithium metal battery performance.However,its high viscosity led to lower lithium-ion transference number and presents lower capacity problems when applied to LFP||Li cells.In this thesis,by introducing 1,2-dimethoxyethane（DME）as a non-solvation co-solvent for the eutectic electrolyte,the solvation structure of the electrolyte was adjusted to a localized highly concentrated cluster structure,which reduces the overall salt concentration while retaining the coordination structure of individual Li⁺unchanged.The introduction of the co-solvent can effectively reduce the electrolyte viscosity and increase the lithium-ion transference number from 0.18to 0.43 while retaining the non-flammability and high thermal stability of the eutectic electrolyte and achieving a flatter and more uniform lithium deposition morphology.The LFP||Li cell based on the electrolyte with DEM has a capacity retention rate of95%after 1200 cycles at a high current rate of 5 C at room temperature and 81%after750 cycles at 2 C and 50℃.Lithium nitrate（LiNO₃）can effectively improve the deposition behavior of lithium metal,but its solubility in carbonate electrolytes is extremely low and difficult to apply.Based on the particular solvation structure of the LN4 electrolyte,the solubility of LiNO₃ can reach 4 wt.%.According to the simulation and calculation results based on the density functional theory,it was probed that there is an interaction force between LiNO₃ and NMAC in the electrolyte,and the Li⁺in LiNO₃ tends to coordinate with NMAC and dissociate with NO₃^–,which in turn promote its dissolution,and this mechanism is similar to the eutectic process of Li TFSI-NMAC.In addition,the presence of LiNO₃ helps to adjust the solvation structure of Li⁺in the electrolyte to de-solvation,and Li⁺tended to coordinate with the anion more,which was favorable to the formation of anion-derived interphases.The SEI layer based on LN4-2N（LN4+2 wt.%LiNO₃）electrolyte is enriched with Li-F,Li_xN,and LiN_xO_y,effectively accelerating the Li⁺transport rate and induces the formation of uniform spherical lithium deposits.The electrolyte can also promote the formation of thin and uniform CEI film without the metal oxide bond,which has an improved protective effect on the cathode interphase.The capacity retention of LN4-2N-based NCM622||Li cell was 84%after 600 cycles at 0.5 C and room temperature,which is significantly better than that of carbonate electrolyte（～56%）,and this pattern is also verified in high theoretical capacity NCM811 and NCA cathode materials.In addition,employing eutectic electrolyte to LMO||Li cells can effectively alleviate the Mn₂⁺/Mn₃⁺dissolution problem during LMO cathode cycling,and the solubility of Mn element in the eutectic electrolyte is only one-ninth of that in carbonate electrolyte.The capacity retention of LMO||Li cell based on LN4-2N5F（LN4+5 wt.%FEC+2 wt.%LiNO₃）electrolyte is 80%after 1000 cycles at 0.5 C and room temperature,and 47%for the LMO||EC-DEC||Li cell under the same conditions.In addition,the LMO||LN4-2N5F||Li battery still has a high-capacity retention of 73%after 3000 cycles at 1 C.