Preparation Of PEO-based Solid Polymer Electrolyte And Study Of Electrolyte/Cathode Interface

Li metal anode is considered as the ideal anode material for high-energy batteries due to its high theoretical capacity(3 860 m A h g^-1)and low redox potential（-3.04 V vs.standard hydrogen electrode）.However,matched with traditional liquid electrolytes,it faces serious safety problems.With high thermal stability,solid-state electrolytes,instead of organic liquid ones,are promising to fundamentally solve the safety problem and break the upper limit of energy density of traditional lithium-ion batteries.Poly（ethylene oxide）（PEO）-based polymer electrolytes are mainly composed of PEO and conductive lithium salt,which have good chemical stability,thermal stability,and flexibility,as well as low cost,easy processing,and environmental friendliness.Nevertheless,in addition to low room-temperature ionic conductivity,their practical applications in high-energy power batteries are still plagued by the interfacial problems between PEO-based electrolytes and cathodes from chemical,electrochemical,and poor physical contact.In this thesis,we firstly summarized the interface failure mechanisms with PEO-based electrolytes/cathodes and related modification strategies,and then focused on studying the interface modifications between PEO-based electrolytes and cathodes:A novel PEO-based composite polymer electrolyte is developed via introducing a functional filler into pure PEO electrolytes,which is applied in all-solid-state lithium batteries（ASSLBs）.In the PEO-based composite polymer electrolyte（CPE-FA）,flyash-treated（FA）submicro/micron particles were used as inorganic fillers and PEO as matrix,and lithium bistrifluoromethanesulfonimide（Li TFSI）as lithium salt.After the addition of FA filler,the Li⁺transference number increases from 0.21 to 0.37,and electrochemical stability window reaches 5.20 V.FA,as the filler,can efficiently immobilize TFSI^-anions,reduce the concentration of TFSI^-anions migrating toward cathode side during the charging process,and effectively mitigate interface side reactions from their irreversible oxidative decomposition due to the internal interaction between TFSI^-anions and FA particles.Li Fe PO₄||CPE-FA||Li ASSLBs exhibit excellent cycling performance.The capacity retention of Li Fe PO₄||CPE-FA||Li cell is as high as 94.5%after 350 cycles at 30^oC and 0.5 C.And the capacity retention of Li Fe PO₄||CPE-FA||Li cell is 94.6%after 100cycles at 60 ^oC and 1 C.The pouch cell with a sandwich configuration（Li Fe PO₄||CPE-FA||Li）keeps the capability of lighting the light-emitting diode after puncture,shearing and folding,indicating its high safety.NCM622||CPE-LLZTO||Li ASSLBs with LiNi_0.6Co_0.2Mn_0.2O₂（NCM622）cathode,PEO-based electrolytes(CPE-LLZTO,where the commercial garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）powder was used as inorganic filler,PEO as matrix,and Li Cl O₄ as lithium salt),and lithium metal anode were used as the research object.The influence of different lithium salts,the content of conductive agent,and test temperature on the electrochemical stability window of PEO-based electrolytes were systematically analyzed.Li Cl O₄ was selected as the lithium salt in PEO-based electrolytes to exclude the influence on the interface stability of cathode side from the irreversible oxidative decomposition of free anions from lithium salt.The NCM622||CPE-LLZTO||Li cell after charging（at the cut-off voltage of 4.30 V）generated a sharp drop of 0.62 V for 10-day storage.This phenomenon can be explained as follows:when NCM622||CPE-LLZTO||Li cell is charged at a high cut-off voltage,both high-valent nickel ions on the surface of the NCM622 active material and Super P in the cathode will accelerate the oxidative decomposition of PEO,and PEO with ether groups tends to lose electrons.In order to remain charge balance,the insertion of Li⁺into NCM622 resulted in a rapid drop in voltage.The failure of NCM622 active materials and the increase of the interfacial resistance on the cathode side are responsible for the performance degradation of NCM622||CPE-LLZTO||Li ASSLBs.NCM622||CPE-LLZTO||Li cell after 10-day storage exhibits serious performance degradation with a capacity retention of only 24.4%and an average coulombic efficiency of 96.3%after 30 cycles.To ameliorate the interfacial problems between CPE-LLZTO and NCM622 cathode material,the construction of an interfacial nanolayer with aromatic polyamide（APA）on the surface of NCM622 active particles as a result of its high thermal stability,good chemical resistance,and strong mechanical strength.The strong interaction between NCM622 and APA interfacial nanolayer is attributed to O atoms in APA being easy to combine with Ni atoms on the NCM622 particle surface.APA interfacial nanolayer can compensate the volume change of electrode during the cycling process,and form a close regional contact between two active particles rather than a point contact,which is beneficial to improve the interface stability and reduce the interfacial charge transfer resistance.The construction of an interfacial nanolayer with APA on the surface of NCM622 active particles can not only effectively mitigate interfacial side reactions between PEO-based electrolytes and NCM622 cathodes to protect the crystal structure of NCM622,but also provide a steady interfacial environment for Li⁺diffusion.The solid-state cell with modified NCM622 exhibit an acceptable voltage drop of 0.27 V after 10-day storage and a substantially improved electrochemical performance with an average coulombic efficiency of 99.1%and capacity retention of 76.7%after 80 cycles.