In Situ Preparation Of Composite Solid Electrolyte And Its Application In Lithium Ion Batteries

Due to the continuous development of social civilization,people’s demand for mobile terminals and electric new energy vehicles is also increasing day by day.All–solid state batteries have attracted attention for their safety.However,existing all-solid-state lithium-ion batteries still face severe challenges in practical applications,such as the low ionic conductivity of solid-state electrolytes at room temperature and how to improve the interfacial issue between solid-state electrolyte and electrodes.Solid electrolytes could be segmented into inorganic materials and organic polymer materials.Inorganic electrolytes has good properties of excellent ionic conductivity,wide electrochemical window,good stability to lithium,and excellent mechanical properties.However,inorganic materials are relatively fragile,and have a large interfacial impedance in solid-solid contact with electrodes,which makes it difficult to obtain widely scale applications in the short term.The polymer electrolyte has low interface impedance with the electrode,but the ion conductivity at room temperature is too low which limits its use.In addition,the problems such as poor stability to Li and low Li⁺transfer number are proposed to be solved.The composite solid electrolytes as very promising solid electrolyte materials have the characteristics of inorganic electrolytes and polymer electrolytes.However,the problem of poor interface contact between solid electrolyte and electrode is still difficult to solve.The in-situ method to generate solid electrolyte on the electrode surface could enhance the interfacial contact,enhance the interface compatibility,and reduce the solid-solid interface impedance.In the past,research work mainly focused on coating solid electrolyte and electrode materials and introducing buffer layers.Although these studies have reduced the interface impedance to a certain extent,they still have not fundamentally solved the interface problem.In this paper,a highly performance LLZTO/PVCA composite solid electrolyte was prepared by using in-situ polymerzation of a solid electrolyte,which successfully optimized the performance of the composite solid electrolyte and improved the interface between the solid electrolyte and the cathode material;based on the above result,the development an all-solid-state lithium-ion battery with good room temperature performance.The major research contents are described as follows:（1）LiDFOB（lithium difluorooxalate borate）and LLZTO(Li_6.4La₃Zr_1.4Ta_0.6O₁₂)are added into vinylene carbonate（VC）,and the solution are stirred and disperse well,then AIBN（azobisisobutyronitrile）is added into the mixture as initiator to obtain the precursor solution.The precursor solution is dropped into the cellulose membrane and sandwiched between the electrodes to assemble the battery,and then are heated at 60℃.to complete the polymerzation and obtain a composite solid electrolyte battery with cellulose frame.As the ratio of LLZTO is 3 wt%,LLZTO/PVCA has the greatest ionic conductivity at room temperature(7.06×10^-5 S cm^-1)and 1.4×10^-4 S cm^-1 at 60℃.Using the scanning electron microscope,it is confirmed that the solid electrolyte membrane formed in-situ is in close contact with the electrode material.In comparison to the solid electrolyte cell prepared by ex-situ way,the interfacial resistance is decreased by 80%in the way of in-situ polymerzation.The all solid-state Li Fe PO₄|LLZTO/PVCA|Li cells presents high initial capacity 151.8 m Ah g^-1 with 83.71%remained after 120 cycles at0.1 C,indicating improved capacity and boosted cycling life.（2）We design a PTFE mold.The prepared precursor solvent is injected into the mold,packaged and thermally polymerized to obtain composite solid electrolyte membrane without any support framework.After taken out,the membrane is cut intodisc,the precursor solution is dripped between the solid electrolyte membrane and the electrodes,and then assembled into a button cell,heated at 60 ^oC for secondary in-situ polymerization,to obtain the in-situ polymerized frameless composite solid electrolyte battery.The study find that by adjusting the ratio of LLZTO to 3wt%,LLZTO/PVCA has the greatest ionic conductivity.The ionic conductivity of the prepared solid-state battery can reach 7×10^-4 S cm^-1 at room temperature,and the electrochemical window is（4.5 V vs Li⁺/Li）,the measured Young’s modulus is18.35MPa,and the Tg is 10.8℃.The assembled Li Fe PO₄|LLZTO/PVCA|Li cells can be cycled for 300 cycles at 1C at room temperature,and the capacity retention rate is as high as 92.8%,own the specific capacity of 143 m Ah g^-1.（3）LiDFOB and AIBN are dissolved in VC and AN（acrylonitrile）and heated at60℃ to obtain PVN（polyvinylene carbonate-acrylonitrile）.Take PVN into the mixture of ethylene glycol and DMSO,stir at 70℃ to dissolve it completely,and then coat it on the Li Ni_0.5Mn_1.5O₄ positive electrode sheet,and dry the coated positive electrode sheet in vacuum.The modified PVN-Li Ni_0.5Mn_1.5O₄ positive electrode sheet was assembled with the composite solid electrolyte membrane prepared in the second work.This work has shown that by adjusting the ratio of VC and AN for optimization,the highest electrochemical window（5.8 V vs Li⁺/Li）can be obtained after optimizing the molecular ratio,and the ion conductivity is 4.4×10^-4S cm^-1 at the room temperature.After 40 cycles at 0.2 C,the capacity retention rate is as high as 95%.