Preparation And Study On Polymer-based Quasi-solid-state Electrolytes For Lithium Metal Batteries

The rapid development of electric vehicles and portable electronic devices has put forward higher requirements for the energy density and power density of energy storage devices.At present,the energy density of lithium-ion batteries with graphite as the anode was close to the upper limit and still cannot meet the demand,while lithium metal was considered the“Holy Grail”for anode material of lithium batteries due to its high theoretical specific capacity(3860 m Ah g^-1).When lithium metal was used in combination with commercial liquid electrolyte,the uncontrolled growth of lithium dendrites during charging and discharging will penetrate the separator and cause internal short circuit.Moreover,the extensive use of flammable and volatile liquid electrolytes causes serious safety hazards.The use of solid electrolyte instead of liquid electrolyte was considered to be an effective way to solve the above problems.Polymer electrolytes show their merits among the solid electrolytes studied in the past years.However,it is still challengingto prepare polymer electrolytes that combine satisfactory mechanical strength,high ionic conductivity,and stable electrode/electrolyte interfacial properties.Polyurethane（PU）consists of a hard segment and a soft segment,and the mechanical properties of PU can be regulated by selectively choosing the types of monomers for polymerization.Methacrylate monomers exhibit high reactivity and offer a wide range of options for adjusting the electrolyte performance.This thesis presents the synthesis of a novel polymer-based lithium metal battery electrolyte using cross-linked methacrylate polymers,and systematically investigates the relevant properties of the electrolyte,such as ion transport and electrode/electrolyte interface stability.（1）Repairable polyaddition type polymer/inorganic hybrid solid electrolytes:Firstly,methacrylate-terminated polyaddition type polymer precursor（m-PPS,m represent methacrylate,P for PEG and POSS,respectively,and S for disulfide）was obtained by polyaddition reaction among trimethylhexamethylene diisocyanate（TMDI）,bis（2-hydroxyethyl）disulfide（HEDS）,1,2-propanel diollsobutyl polyhedral oligomeric silsesquioxane（POSS）,polyethylene glycol（PEG）followed by methacrylate end group modification.Afterwards,UV assisted in-situ crosslinking was carried out on the polymer precursor solution containing m-PPS and NMAc/Li TFSI DES on the cathode surface before cell assembly.The electrolyte film product was named X-PPS-D_n（n stands for the molar ratio of NMAc:Li TFSI and D refers to DES）.X-PPS-D₄ has a highσ(2.03×10^-4S cm^-1 at 30°C)and an electrochemical oxidation potential of 5.0 V vs.Li⁺/Li.In addition,spectral characterization and DFT calculations confirmed that the movement of TFSI^-in X-PPS-D₄ was hindered by the polymer molecular chain,leading to an increase in t_Li⁺.Stable long-term plating/strippingtests proved that X-PPS-D₄ had benign interfacial compatibility with lithium electrodes.At 30°C,Li/LFP cells assembled with the X-PPS-D₄ electrolyte exhibited excellent performance with a specific capacity exceeding 100m A h g^-1 at a rate of 5 C and a low overpotential.The in situ generated X-PPS-D₄ enabled the Li/LFP cell to be stably cycled more than 1000 cycles at 1 C with a high specific capacity.Good performance was also achieved for Li/NCM811 cells using X-PPS-D₄.（2）Polymer-based solid electrolytes containing polyionic liquids:A polymer-based solid electrolyte（EFA-G）containing Li[G₄]TFSI as additive and PEGMA,PEGDMA,pyrrolonium-based ionic liquid（MAIL）and pentafluorostyrene（PFS）as matrix was prepared on the surface of lithium electrodes by in situ free radical copolymerization.Spectral analysis and molecular dynamics simulations were used to reveal the complex form of Li⁺in EFA-G and the interaction between TFSI^-and other components in the electrolyte.The EFA-G exhibited ionic conductivity close to 10^-3 S cm^-1(9.87?10^-4 S cm^-1)at 30°C,with an improved lithium ion transferencenumber(t_Li⁺=0.33).The PFS introduced in the polymer could participate in the formation of a stable SEI layer.The lithium symmetric cells assembled with EFA-G as electrolyte could be cycled stably for1050 h at 0.1 m A cm^-2 and 0.1 m A h cm^-2 without short circuit.The Li/EFA-G/LFP cell revealed a discharge capacity of 78.3 m A h g^-1 at 8 C and retained a discharge capacity of127.8 m A h g^-1 after 600 cycles at 1 C.The excellent rate performance and cycling stability of the Li/EFA-G/LFP cell can be attributed to the high ionic conductivity and stable solid electrolyte interphase.The Li/EFA-G/NCA cell also showed good cycling performance due to the improved electrochemical stability.