Study On The Preparation And Properties Of Polyester Solid Electrolyte At Room Temperature

Human beings have been lived in the era of the great mobile internet in the 21st century.Power storage has been one of the most critical needs of the society.Lithium ion batteries（LIBs）have been extensively used in portable electronic devices and electric automobiles due to their high energy density,long cycle life cycle,and no memory effect.However,the traditional liquid lithium ion secondary battery contains a large number of volatile,flammable,and explosive organic electrolytes,which can cause serious security risks.Therefore,replacing liquid electrolytes with solid electrolytes is one of the effective ways to address the above problems.From the current research of solid polymer electrolyte（SE）,it is critical to increase the ionic conductivity of SE and improve the interface compatibility between electrodes and electrolytes.To solve the above problems,the purpose of this paper is to investigate and design a new polymer electrolyte system.Taking into account the mechanical properties and electrochemistry properties of SE membrane,the key point is to improve the ionic conductivity and interface stability.We chose the poly（propylene carbonate）（PPC）and poly（ε-caprolactone）（PCL）as the polymer matrix,respectively.We designed and prepared PPC-based SE membranes,sandwich-like PPC-based SE membranes and PCL-based SE membranes.The properties of ionic conductivity,thermal stability,mechanical properties and electrochemical stability of SE membranes were studied systematically.Its application in LiFePO₄/Li battery achieved good cycling performance and the specific results are as follows:1.PPC was used as the polymer matrix and polypropylene（PP）separator was used as supporting membrane.The effects of the amount of lithium salt and plasticizer on the performance of the solid state electrolyte membrane were investigated.First,different amounts of bis（trifluoromethane）sulfonimide lithium salt（LiTFSI）were added.By comparing the ionic conductivity and electrochemistry properties,it was found that the content of LiTFSI was 30%of the total PPC system（PPCE）,the PPCE showed the higher ion transference number（0.5）and the lower glass transition temperature.In order to further improve the ionic conductivity of the SE membrane,we then added the plasticizer succinonitrile（SN）into the optimized PPC lithium salt system and obained a PPC-based SE membrane with high ionic conductivity.When the content of SN was 60%of the total PPC system（SPE）,the battery showed the best performance.The results of XRD indicated that PPC,LiTFSI and SN were well compatible and no extraneous peaks appeared.Moreover,the Li/SPE/LiFePO₄ battery exhibited a good initial discharge specific capacity(131mA h g^-1)at room temperature.It showed that the suitable lithium salt content and addition of SN not only improved the lithium ion transmission capacity,but also improved the cycling performance of the LIBs.2.A sandwich-like structured PPC-based SE membrane was designed,which made the SE have both high ionic conductivity and good stability to lithium anode.Although the addition of SN can greatly increase the ionic conductivity and cycle specific capacity of SE membranes,it has been found that the compatibility was poor between SN and metallic lithium,which is detrimental to the long term cycling stability of the LIBs.So we proposed a concept of sandwich-like solid polymer electrolyte membrane（CSE）.The sandwich-like structure of the composite SE consisted of three parts:pure PPC-based solid polymer electrolyte layer A,PP support separator and PPC/SN solid electrolyte layer B.Pure PPC as a buffer layer was closed to the lithium anode,which could improve the interface compatibility,Coulombic efficiency（CE）,and cycle life of batteries.A PP separator was used as a support membrane to enhance the strength of the SE membrane.SN in the PPC-based SE was used to increase the ionic conductivity.The sandwich-like composite SE exhibited good comprehensive properties such as a high ion transference number（0.65）,sufficient ionic conductivity(2.18×10^-4 S cm^-1)for ambient temperature.Furthermore,a Li/Li FePO₄ cell with the sandwich-like composite SE delivered a discharge capacity close to 140 mA h g^-1（0.1 C）at 25℃.After 150 cycles,the CE was 99.8%.The stress-strain curve showed that the CSE membrane can provide reliable mechanical properties and effectively improve the safety and cycling performance of LIBs.The pouch type battery was cut into pieces;the red LED lamp was still lighted up when the cell was cut off.This test fully proved the reliability of CSE in the application of solid-state LIBs.3.A PCL/PPC composite solid state polymer electrolyte with good mechanical properties and high ionic conductivity at room temperature was developed.Firstly,we used PCL as a polymer matrix,and added different amounts of PPC to prepare a self-supporting membrane by a simple solution casting method.Taking into account mechanical properties and ionic conductivity,the optimum content of PPC was 40%in the total PCL system.Then we introduced the porous polypropylene separator（HP）as supporting membrane,and the HP-PCLC0.4 electrolyte membrane was successfully prepared.The results of FESEM showed that the surface of the PCL-based self-supporting membrane was smooth and non-porous and the HP-PCLC0.4 membrane was observed to be of good flexibility by electrophotography.The stress-strain curves showed that the mechanical properties were greatly improved after the introduction of HP as a support membrane.LiFePO₄ batteries assembled with HP-PCLC0.4 membrane exhibited a first discharge specific capacity of 142.5 mA h g^-1 and 99%first-time Coulombic efficiency.The LiFePO₄/HP-PCLC0.4/Li battery exhibited good cycle performance and low voltage polarization,indicating that PCL-based polymer electrolytes have valuable application prospects in LIBs.