The Design Of Polycaprolactone Based Composite Solid Electrolytes With The Investigation On Performance Of All-Solidstate Lithium Metal Batteries

The liquid organic electrolytes in Li metal batteries cannot form a stable and homogeneous solid electrolyte interphase（SEI）,and incurring nonnegligible safety issues and limited life span.Solid state electrolytes have good mechanical properties,high safety,high ionic conductivity and Li-ion transference number,which is very promising to become the next generation of commercial electrolytes,and its application in all-solid-state lithium metal batteries（ASSLMBs）have been investigated deeply by researchers.Especially,composite solid electrolytes（CPEs）not only take into account the advantages of inorganic solid electrolytes（ISEs）and solid polymer electrolytes,but also exhibit good compatibility and stability with lithium metal anode.In this thesis,the polycaprolactone（PCL）is selected as the polymer matrix because of their lower binding energy of Li⁺-PCL.Besides,optimizes the design of CPEs such as the type of lithium salts and the content of ISEs are optimized and then systematically characterizes the performance of CPEs,and explores the Li-ion transport mechanism of CPEs.The designed CPEs are matched with different cathode materials such as Li Fe PO₄（LFP）,Li Ni_0.5Mn_0.3Co_0.2O₂（NMC）,Li Mn_0.5Fe_0.5PO₄（LMFP）to assemble ASSLMBs,and investigate the rate performance,cycling performance,etc of ASSLMBs.Additionally,the failure mechanism of the ASSLMBs is investigated,the specific research contents are as follows:（1）The PCL show unique properties such as good physical and chemical properties,low cost,environmental friendless,and a low binding energy with Li-ion,which is beneficial for Li-ion migration.And the Li_1.4Al_0.4Ti_1.6（PO₄）₃（LATP）has high ionic conductivity,high Li-ion transference number,good thermal stability and mechanical properties.Based on the advantages above,the PCL-Li Cl O₄-LATP CPEs are prepared by adding the LATP and Li Cl O₄ into the PCL matrix.The CPEs with 40 wt%LATP,hereafter abbreviated as CPEs-40%,exhibits the better performance than CPEs with other content of LATP.The CPEs-40%obtain a Li-ion transference number（t₊）of 0.5,and high ionic conductivity of 3.64×10^-5 S·cm^-1 at 55 ^℃,which offers a wide electrochemical potential window of 5 V vs.Li/Li⁺.Besides,the thermal stability and thermal shrinkage of the CPEs are improved obviously after adding LATP.The pathways for Li-ion migration will gradually transfer to the LATP while more LATP are added,and Li-ions prefer to transport in LATP.ASSLMBs based on LFP presents a high discharge capacity of 149 m Ah·g^-1at 0.2C,which exhibits outstanding cycling performance with high initial discharge specific capacity of 136.6 m Ah·g^-1,and when the active materials increase to 80 wt%,the ASSLMBs show a high initial discharge capacity of 127.7 m Ah·g^-1 at 0.3C,and good cycling performance with the capacity retention of 80.8%after ultralong 200 cycles at55 ^℃.The ASSLMBs based on NMC and LMFP with CPEs-40%also good performance.The work exemplifies the CPEs based on PCL with a wide prospect,and the introduction of rational LATP can improve the electrochemical performances of CPEs and the battery performances of ASSLMBs.In the ASSLMBs based on CPEs-20%,the CPEs decomposes in order to maintain the stability of the interface,which incurs the decreasing performance of CPEs and the internal resistance increasing of the ASSLMBs.Those results cause the ASSLMBs failure.（2）PCL-Li TFSI-LATP-40%（hereafter abbreviated as TF-CPEs-40%）is designed to solve the problem that CPEs-40%system have not rapid charging and discharging.It is found that TF-CPEs-40%have high Li-ion transference number（t₊=0.8）,and the ionic conductivity of TF-CPEs-40%is 0.87×10^-4 S·cm^-1 at 55 ^℃.The ASSLMBs based on LFP with TF-CPEs-40%present a high discharge specific capacity of 143.7 m Ah·g^-1 at 0.2C at 55 ^℃,which exhibits good cycling performance with the superior capacity retention of 81.2%after ultralong 500 cycles at 1 C and the specific capacity of initial discharge is130.2 m Ah·g^-1,55 ^℃.Additionally,the ASSLMBs exhibited high initial discharge capacity of 108.2 m Ah·g^-1and the excellent cycling performance with the capacity retention of 75%after ultralong 500 cycles at 2C（55 ^℃）.Moreover,even at 3C,it still delivers good cycling performance（500 cycles）and high capacity retention（71%）,and the ASSLMBs based on NMC and LMFP with TF-CPEs-40%also show good performance.In contrast,the ASSLMBs based on PEO-Li TFSI-LATP-40%（hereafter abbreviated as EO-CPEs-40%）have a higher initial discharge capacity of 118.5 m Ah·g^-1and the Coulombic efficiency shown a lot of volatility after 300 cycles with the capacity retention of 50.5%after 500 cycles at 2C（55 ^℃）.Then,the X-ray photoelectron spectroscopy（XPS）and atomic force microscopy（AFM）are used for analyzing the failure mechanism of ASSLMBs.The results exhibit that the Li F,Li₂CO₃ and organic lithium salt play an important role in SEI interface for maintaining the stability and cycling,and the decrease of the relative content of any component will lead to the decline of the stability and compactness of SEI interface,which will cause the deterioration of ASSLMBs performance.Besides,the results of AFM have shown the lower Young’s modulus of EO-CPEs-40%（0.93 GPa）than that of TF-CPEs-40%（2.8 GPa）,which cannot effectively alleviate the material deformation that causes by stressing in the process of charging and discharging in the long cycling.The uniform deposition of Li-ion cannot be guaranteed because the deformation incurs the change of the transmission interface.The ASSLMBs will eventually fail as the cycling goes deeper.（3）To reduce the operating temperature of ASSLMBs,the Poly（vinylidene fluoride）（PVDF）is introduced to PCL to form a blending system.Li⁺-PVDF has lower binding energy than Li⁺-PCL by Density Functional Theory（DFT）,which is beneficial for Li-ion migration and improves the electrochemical performance of CPEs,and the blending system helps obtain the performance of ASSLMBs at low temperature.When the PCL/PVDF molar ratio is 5/1（hereafter abbreviated as P5-CPES-40%）,the conductivity of P5-CPES-40%system is 1.17×10^-4 S·cm^-1（55 ^℃）,and the Li-ion transference number is 0.89,which is close to single ion solid polymer electrolytes.Moreover,P5-CPEs-40%based on LFP have excellent ASSLMBs performance with the discharging capacity of155 m Ah·g^-1 at 0.2C.Additionally,the initial discharging capacity of the ASSLMBs is122.1 m Ah·g^-1with capacity retention of 71%after 500 cycles at 2C（55 ^℃）,and the capacity retention is 84.9%after 500 cycles at 3C,55 ^℃.More importantly,the ASSLMBs based on P5-CPEs-40%show good performance at 40 ^℃,even at 30 ^℃,the ASSLMBs can complete the charging and discharging with stable Coulombic efficiency values of 99¹00%.The improvement of performances might come from the competition between PCL and PVDF in the process of Li-ion migration and transmission,resulting in an effect like the“eel effect”.This synergistic effect not only promoted the electrochemical performance of CPEs,but also meliorated the stability of the interface with lithium anode.After analyzing failure mechanism of the cycled ASSLMBs by XPS and AFM.It is found that when the molar ratio of PCL/PVDF is 2/1 that exhibit excessive surface roughness causing poor contact between the electrolyte and the electrode by the“point to face”rather than“face to face”.Therefore,Li-ion cannot be uniformly diffused and incur the accumulation of electrons because of poor infiltration,which aggravates the polarization effect and causes damping of performance.Besides,with the deeper of cycling,the damping of performance will be further aggravated.In addition,the differences in the various components of the SEI interface in ASSLMBs may also cause large changes for ASSLMBs performances.Poor contact between electrolytes and electrodes and the amount of active ingredient decreasing in the SEI interface are the main causes of ASSLMBs failure.In summary,PCL-based CPEs has high-speed charging/discharging ability,which is beneficial to design and develop the CPEs with high power density and high energy density by the optimization of the design,and exhibits good performance at 30°C.It is significance for the development of environment-friendly,low-cost,suitable for large-scale production and high-performance CPEs,and provides a certain theoretical basis for in-depth study of the failure mechanism of ASSLMBs.