Study On Ionic Conductivity And Interface Of Fluorinated Polyester Based Electrolyte In Solid State Lithium Batteries

The all-solid-state（ASS）lithium batteries not only eliminated the liquid electrolyte and improve the safety performance,but also enhance the energy density owing to the application of higher capacity Li-metal or alloy anodes.In ASS cells,the development of high-performance solid-state electrolytes is the most critical challenge.Among them,solid polymer electrolytes（SPEs）are considered as more promising candidates for future ASS batteries,owing to their higher flexibility,better interfacial contact,and easier scale production.To fulfill the advantageous energy density of ASS batteries,the high voltage cathodes are necessarily applied in ASS batteries.However,the currently reported high-voltage stable SPEs are limited to low Li⁺-conductivity and poor interfacial stability with Li metal anode or high-voltage cathodes.In order to obtain high-performance SPEs with both high Li⁺-conductivity and wide electrochemical windows,it is necessary to understand the factors influencing Li⁺-conductivity,electrochemical windows,and interfacial stability of SPEs,respectively.In this thesis,with the goal of obtaining high performance SPEs that can stabilize both lithium metal anode and high voltage cathode,a series of fluorinated modified polyester SPEs was designed by modulating the polymer structure and the factors influencing Li⁺-conductivity and interfacial stability were systematically investigated.The details of the study are as follows:（1）Three fluorinated poly（propylene glycol）oxalate SPEs with straight chain and methyl side chains were prepared.The strong coordination between the oxalate group and Li⁺on the chain segments of these polymers facilitates the dissociation of lithium bis（trifluoromethane）sulfonimide（Li TFSI）,while the terminal trifluoroacetyl group can generate a solid electrolyte interface layer（SEI）containing Li F in situ to improve the interfacial stability of the electrolyte and Li-metal anode.Due to the coordination interaction between the polymer matrix and Li⁺,the straight-chain structure of SPEs has lower glass transition temperature and higher Li⁺-conductivity.In addition,this class of fluorinated polyoxalate SPEs has good interfacial stability with Li-metal anode and high-voltage Li Ni_0.6Co_0.2Mn_0.2O₂（NCM622）cathode,and both assembled Li/Li and Li/NCM622 cells achieve long-term stable cycling.（2）To investigate the factors influencing the ionic conductivity of SPEs,straight-chain polyoxalate electrolytes（POE）,polycarbonate electrolytes（PCE）,and polymalonate electrolytes（PME）with different diol chain segments were prepared.It was found that the SPEs composed of pentanediol（C5-diol）showed a higher Li⁺-conductivity than the corresponding SPEs composed of butanediol（C4-diol）due to the higher flexibility and stronger asymmetry of the pentanediol-based polymer.In addition,with the same C5-diol segments,the Li⁺-conductivities of PME and POE were around 10 and 5 times of PCEs,respectively,owing to a stronger coordinating capability of malonate/oxalate with Li⁺and so the easier dissociation of Li TFSI.When 5mol%of 2,2,3,3-tetrafluoro-1,4-butanediol chain segment was introduced into the PCE/POE/PME composed of pentanediol the Li⁺-conductivity of SPEs were both significantly reduced due to the strong electron-absorbing nature of the-CF₂-CF₂-unit leading to the delocalization of electrons on the carbonyl oxygen to the F-atom thus leading to the reduced solubility of Li TFSI.Therefore,the coordination capability with Li⁺,the molecular asymmetry and existing modes of elemental F are key factors that affect the Li⁺-conductivity of the SPEs.（3）Considering the application of fluorinated polymerester-based electrolyte in high-voltage lithium batteries,the stability of SPEs with lithium metal interface and NCM622 cathode was investigated using1,5-pentanediol-based PCE/POE/PME electrolytes.The-OH group at the end of the polymer is an important factor affecting the stability of SPEs at the interface with lithium metal.The use of terminal trifluoroacetyl not only facilitates the reduction of the HOMO energy level of the polymer and improves the oxidation resistance,but also allows the transfer of HOMO electrons from the terminated unit to the middle part of the polymer chain segment,improving thermal stability of the polymer.Compared with the-O-CH₂-CF₂-CF₂-CH₂-O-（TF-diol）units in all PCE/POE/PME mainchains the terminal trifluoroacetyl-unit（CF₃-COO-）is demonstrated to show better interfacial stability with Li-metal.With C5-PCE-F-Li（fluorinated polycarbonate pentylene glycol electrolyte）and C5-POE-F-Li（fluorinated polyoxalate pentylene glycol electrolyte）as SPEs,Li/NCM622 ASS cells deliver stable cycling at 60^oC and 45^oC,respectively,since C5-POE-F-Li possesses higher conductivity than C5-PCE-F-Li.However,when C5-PME-F-Li is employed,the ASS Li/NCM622 cell gives poor cycling stability,owing to the enol isomerization and the following transition metal dissolution.To suppress this enol isomerization,the dimethyl group introduced into the malonate of PME giving DM-PME,with which,the cycling stability of ASS Li/NCM622 is recovered at a slightly lower temperature of 40^oC,with the help of slightly higher conductivity of DM-PME than C5-POE-F-Li.（4）In order to further improve the ionic conductivity of fluorinated polyethylene glycol ester electrolyte and obtain high performance SPEs.In this thesis,a class of fluorinated polybutanedioic acid/glutaric acid/glutaryl adipate SPEs（C4-C5-F-Li,C5-C5-F-Li,and C6-C5-F-Li）was prepared by further modulating the flexibility of the polymer segments and controlling the molecular weight of the polymer.By utilizing the high flexibility and coordination of fluoropolymer segments,the ionic conductivity of C6-C5-F-Li and C6-C5-F-M-Li reached 2.13×10^-5S cm^-1and 3.11×10^-5S cm^-1at 25^oC,respectively,with a wide electrochemical window,showing good compatibility with lithium metal anode and high voltage cathodes.The assembled Li/Li symmetrical cell and Li/Li Fe_0.5Mn_0.5PO₄cell can achieve long-term stable cycling at 30^oC.In addition,fluorinated polyester-based SPEs were used for the recovery of polymers and Li TFSI considering environmental protection and sustainable use of resources.Taking advantage of the different solubility of fluorinated polyester-based polymers and Li TFSI in organic solvents,they were efficiently recycled to obtain Li TFSI with high purity and excellent electrochemical properties.The process effectively reduces the production cost of SPEs and realizes the recycling of resources.