Design Of Electrolyte For High Stability Lithium/lithium-ion Batteries And Its Properties

Consumption and demand for energy have risen dramatically,paralleling the advancement of human society.Therefore,the development of new energy storage systems is currently the focus of research and attention in the face of current energy and environmental problems.Lithium/Li–ion batteries play an important role in secondary battery development and research.With the growing demand for secondary batteries in electric vehicles and portable devices,it is critical to improve their energy density and safety.Because of their high energy density,nickel-rich ternary materials are thought to be the most promising cathode materials.However,the safety concerns raised by the instability of ternary materials are an urgent issue that must be addressed.One of the primary reasons for this is the instability of the electrolyte system with ternary materials,which causes safety issues.As a result,the development of functional electrolytes that are compatible with ternary materials is critical for the development of high energy density and high safety secondary batteries.In this thesis,with high stability as the foundation,several functional electrolyte additives were developed to address the issues of nickel-rich ternary cathode materials being prone to electrolyte corrosion during charging and discharging,resulting in leaching of transition metal ions and exacerbating interfacial side reactions and lithium dendrite growth on the negative side,by employing functional additive strategies to improve the electrode-electrolyte interface and lithium dendrite growth.The following are the main points of the thesis:1.To address the instability of Ni-rich ternary materials susceptible to electrolyte corrosion,a 1-propyl cyclic phosphonic anhydride(C₉H₂₁P₃O₆,PACA)electrolyte functional additive matched to the positive electrode of Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂（NCM 811）was investigated for improving the electrochemical performance of NCM 811/graphite full cells over a temperature range of-20 ℃～45 ℃.When compared to conventional carbonate-based electrolytes,the results show that PACA,as an electrolyte functional additive,exhibits excellent electrochemical performance in the cell over a wide temperature range of-20 ℃～45℃.After 550 cycles at room temperature,the capacity retention of the full cell with PACA additive increased from 26%to 92%at 1 C current denisty when compared to the full cell without PACA.The capacity retention of the full cell at 1 C current denisty increased from 75%to 87%after 350 cycles at 45 ℃.The full cell with the PACA additive has a higher discharge capacity at 0.2 C,0.5 C,and 1 C current denisty at-20 ℃.Electrochemical analysis and interfacial characterisation show that PACA can be preferentially redoxed to form a stable,low–impedance,lithium ion diffusion–friendly interfacial film at the positive and negative electrode interfaces.As a result,the electrolyte corrosion phenomenon on the positive electrode material is slowed,as is the leaching of transition metal ions on the positive side and the growth of lithium dendrites on the negative side.2.Based on the designability of ionic liquid,the cyano-functionalized pyrrolidine ionic liquid 1-methyl-1-cyanopropylpyrrolidine bis（fluoromethanesulfonyl）imide salt was designed and prepared,and PYR_1（4CN）TFSI was investigated and optimized as a functional additive for NCM 811/Li lithium metal battery electrolyte.PYR_1（4CN）TFSI improves the electrochemical performance of NCM 811/Li lithium metal battery by preferentially participating in the formation of CEI/SEI films and increasing electrical conductivity and Li⁺transport capacity.The Li/Li symmetric cell exhibited excellent stability at a PYR_1（4CN）TFSI addition of 0.5 wt.%,with a stable lifetime of 750 h at a current density of 0.5 m A cm^-2 and 250 h at a current density of 1 m A cm^-2,and the NCM 811/Li lithium metal battery had a discharge capacity of107 mAh g^-1 after 200 cycles at a rate of 1 C at room temperature.3.The addition of cyanide increases the viscosity of the electrolyte because cyano has a high electron cloud density.By introducing a cyanide followed by an ether group in the cationic side chain of pyrrolidine,we created a bifunctionalized ionic liquid 1-cyanopropyl-1-ethyletherpyrrolidine bis（fluoromethanesulfonyl）imide salt(PYR_{1（4CN）（2O2）}TFSI),and we investigated PYR_{1（4CN）（2O2）}TFSI as a functional additive in NCM 811/Li lithium metal battery electrolyte.The results showed that adding the ether group reduced the viscosity of the ionic liquid while increasing the conductivity and Li⁺transformation of electrolyte.When 0.5 wt.%PYR_{1（4CN）（2O2）}TFSI was added,the electrolyte’s Li⁺transform number increased from 0.661 to0.770,and the NCM 811/Li Li metal battery had a discharge capacity of 131 mAh g^-1 after200 cycles at a rate of 1 C at room temperature.