Studies On Li₃V₂?PO₄?₃ Cathode Material And High-safety Ionogel Electrolytes For Lithium Metal Batteries

Lithium-ion batteries are an important part of global clean energy,and have been widely used in electronics,satellite solar storage,electric vehicles and other fields.With the change of communication methods,the increase of energy demand and the enhancement of environmental protection awareness,people have put forward higher requirements on the energy density,power density,cycle life and safety of lithium-ion batteries.The key to improving battery energy density and power density is to increase the output voltage of the battery's positive electrode material,while maintaining high capacity,high rate performance,and long-term cycle stability.In terms of improving safety performance,an effective method is to replace the traditional flammable and volatile liquid electrolyte with a highly safe gel polymer electrolyte to eliminate potential safety hazards such as electrolyte leakage,combustion,toxic gas volatilization,and internal short circuits.In this paper,we focus on the key materials of solid-state lithium metal batteries with high energy density,long life and high safety--including the preparation and modification of high voltage cathode materials,and the synthesis of ionogel polymer electrolytes and their applications in high voltage lithium metal batteries.The monoclinic lithium vanadium phosphate?Li₃V₂?PO₄?₃,LVP?is a NASICON structural cathode material with a three-dimensional lithium ion transport channel capable of fast lithium ion transport.LVP is one of the candidates for high energy density cathode materials due to its high operating voltage and high theoretical specific capacity,excellent structural stability and high safety.However,the low intrinsic electronic conductivity severely limits the high rate performance of the LVP materials.It has been proved that carbon coating can effectively enhance the electronic conductivity of lithium vanadium phosphate,but the electrochemical performance of the material is still unsatisfactory.At high voltage?>4.5 V?,LVP faces the problem of unstable electrochemical properties at the electrode/electrolyte surface interface,which affects the rate performance and cycling stability of the material.In addition,under low temperature conditions,the de-solvation ability of the solvation lithium ion decreases,resulting in lower first coulomb efficiency and discharge specific capacity,which seriously restricts the practical application of LVP cathode materials in lithium ion batteries.In view of the above problems,the contents of this thesis are as follows:?1?In order to solve the problems of low-rate and low-temperature capacity of LVP,YPO₄ and carbon dispersion coating were used to optimize the cathode material.Firstly,the carbon coated LVP cathode material was synthesized by sol-gel method.Next,yttrium source was added after the initial pre-sintering of LVP,and then YPO₄dispersed in carbon layer was obtained after high temperature sintering,which was named YPO₄-LVP/C).On the one hand,the YPO₄ distributed in the carbon layer shows the adsorption ability of lithium ions at the surface interface,which not only enhances the kinetic properties,but also significantly improves the rate performance of the material.At the same time,it improves the desolvation ability of the solvation lithium ion and improves the electrochemical stability of the surface interface.For the above reasons,YPO₄-LVP/C sample exhibit excellent electrochemical performance.On the voltage of 3.0-4.8 V,the first discharge specific capacity increases from 140-160 mAh g^-11 to 160-180 mAh g^-11 at 0.5 C rate.At 50 C rate,the discharge specific capacity increases from 30-50 mAh g^-11 to 90-110 mAh g^-1.Moreover,the electrochemical performance at the low temperature has been obviously improved.At low temperature-40?,the first Coulomb efficiency of the material has been increased from 73.4% to 85.3%,and the discharge specific capacity has been increased from 60-80 mAh g^-1 to 110-130 mAh g^-1.?2?To solve the problem of poor cycling stability and rate performance of LVP,lithium polyacrylate?LiPAA?was used as binder to prepare LiPAA-LVP electrode.LiPAA can more closely connect the uniform wrapping electrode materials,enhance the ability of electron transport and ion transport between active substances.The LiPAA-LVP exhibited excellent electrochemical performance compared to the PVDF-LVP electrode prepared by conventional binder polytetrafluoroethylene?PVDF?.The charge-discharge test results show that the LiPAA-LVP discharge specific capacity can reach 107 mAh g^-1(1 C=197 mA g^-1,3-4.8 V vs.Li/Li)at a rate of 70 C.The long cycle stability shows that the discharge specific capacity is 120mAh g^-1,after 1400 cycles,the capacity retention is 91%at 10 C rate.DFT calculations show that LiPAA has better electrochemical stability than PVDF.Moreover,techniques such as infrared spectroscopy?FTIR?,x-ray photoelectron spectroscopy?XPS?,cyclic voltammetry?CV?,and electrochemical impedance spectroscopy?EIS?demonstrate the reversible H⁺/Li⁺ exchange of carboxyl functional groups that LiPAA binders during charge/discharge processes.This can not only increase the reactive active sites on the surface of the material and improve the rapid charge-discharge ability,but also neutralize the protons and HF which was released by the electrolyte to avoid the side reactions of the electrolyte at high voltage,thus significantly enhance the long cycle stability of the material.In view of the safety problems such as leakage,volatile and internal short circuit caused by lithium dendrite growth,ionogel polymer was used as electrolyte of lithium metal battery to construct LVP//Li battery system with high safety and excellent electrochemical stability.The specific findings are as follows:?3?A novel ionogel electrolyte was prepared by one-pot method,imidazole ionic liquid as plasticizer,poly?ionic liquid??PIL?copolymer as polymer skeleton,cross-linked PIL copolymer was obtained by ring-opening reaction between PIL amino group and epoxy group of poly?ethylene glycol?diglycidyl ether?PEGDE?,and ionic liquid?IL?was perfused to prepare ionogel electrolyte.The electrolyte membrane exhibits excellent mechanical properties,non-flammability,high ionic conductivity and electrochemical stability.The cross-linked structure of this ionogel ensures the uniform deposition of lithium ions on the surface of lithium metal and effectively inhibits the growth of lithium dendrites.The Li/ionogel/Li₃V₂?PO₄?₃ battery was assembled and the first discharge specific capacity is 124 mAh g^-1,the capacity retention was 95.4%after 350 cycles.?4?In order to expand the application of ionogel electrolytes in high voltage lithium metal systems,we synthesized ionogel interpenetrated by organic-inorganic networks.The specific use of imidazole ionic liquid?IL?perfusion in cross-linked PIL polymer and glass fiber network interpenetrating organic-inorganic frameworks significantly enhance electrolyte mechanical properties to inhibit dendrite growth,enhance lithium ion migration and broaden the electrochemical stability window.In addition,combined with the excellent structural stability of LVP,the assembly of LVP/ionogel/Li battery exhibits excellent long cycle stability and high voltage working stability.In operating voltage of 3.0-4.3 V(1 C=132 mAh g^-1)at a current density of 0.5 C,the first discharge capacity was 127 mAh g^-1,the capacity retention was 82%after 1000 cycles.Moreover,under the operation at 3.0-4.8 V(1C=197mAh g^-1),the first discharge specific capacity is 150 mAh g^-1,and the capacity retention of was 91%after 100 cycles.Compared with the traditional electrolyte system and the pure organic or inorganic frame system,the electrochemical performance was significantly improved.In summary,aiming at the problems of poor rate performance,side reaction between LVP and commercial electrolyte at high voltage,we respectively carried out surface functional coating and electrode structure optimization to enhance the specific capacity,cycle stability,fast charge/discharge ability and low temperature capacity of LVP.On the other hand,were designed and prepared high safety ionogel electrolytes,and applied in LVP//Li batteries to inhibit lithium dendrite growth and provide aluminum collector from corrosion to realize long life cycle and high safety operation at high voltage.