Interface Regulation And Performance Optimization Of Polyoxyethylene High-voltage Solid-state Lithium Batterie

The lithium batteries have been deemed as promising electrochemical energy-storage devices due to their high energy density,low self-discharge rate and long cycle life.The traditional liquid lithium batteries are faced with critical issues including unsatisfactory safety and limited energy density.The solid-state lithium batteries（SSLBs）employing non-flammable solid electrolytes instead of traditional organic electrolytes avert the risks of electrolyte leakage,combustion,and other safety problems,while the application of solid electrolytes enables the inhibition of dendrite growth,and coupling with lithium anode with high theoretical specific capacity(～3860m Ah g^-1).Therefore,it is imperative to develop SSLBs with high safety and energy density.However,the solid-solid interfacial contact between electrodes and electrolytes greatly limits the improvement of battery performance.Compared with inorganic solid electrolytes,the flexible polymer electrolytes present better interfacial contact with the electrodes.The poly（ethylene oxide）（PEO）as a commercial polymer electrolyte plays a crucial part in SSLBs because of its low cost,low density,good film forming property,and easy large-scale production.However,PEO electrolytes cannot directly couple with high-voltage cathode materials due to the limited electrochemical window（<4 V）,leading to the insufficient energy density.To address the above problem,in this thesis,the in-situ solidified interlayer with high-oxidation resistance is constructed in between the high-voltage cathode and PEO electrolyte.Differed from the traditional interfacial layer,the in-situ interfacial layer avoids the increased impedance induced by introduction of the secondary interfacial phases,and improves the interfacial contact.The obtained research results are listed as follows:（1）In term of the LiNi_0.6Co_0.2Mn_0.2O₂（NCM）||PEO||Li SSLBs,the high oxidation-resistance succinonitrile（SN）with Li Cl O₄ interlayer was constructed between NCM cathode and PEO electrolyte via in-situ solidification at room temperature.The introduction of SN-Li Cl O₄ interlayer physically isolates the direct contact of PEO with high-voltage NCM as well as inhibits the oxidation decomposition of PEO.Simultaneously,such soft SN-Li Cl O₄ interlayer also enables the construction of ionic conducting networks inside the cathode and intimate contact at NCM/PEO interfaces.As a result,the promoted comprehensive performance of batteries can be achieved.The SN-Li Cl O₄ interlayer has an electrochemical window up to 5.2V and ionic conductivity of 2.45×10^-4 S cm^-1 at 30℃,which contributes to the increased cycling stability of PEO electrolyte under elevated voltage.Consequently,the corresponding NCM||Li batteries based on the proposed interface engineering strategy deliver a good cycling performance under various cutoff voltages of 4.2,4.25,and 4.3 V at 0.1C and 30℃.Furthermore,a high cathode loading of 10.3 mg cm^-2(i.e.,1.85 m Ah cm^-2)accompanied with a high capacity retention of 80.8%after 50 cycles is realized.（2）In order to broaden the operating temperature range of solid-state NCM||Li batteries,the in-situ solidification polymer electrolyte interlayer was introduced at NCM/PEO interface,endowing the high-voltage SSLBs with an elevated operating temperature up to 55℃.The in-situ solidification polymer electrolyte was fabricated through UV-curing of poly（ethylene glycol）diacrylate（PEGDA）monomer with Li salts.The ionic conductivity of such interlayer is 8.25×10^-4 S cm^-1 at 60℃.The electrochemical window is widened to higher than 5.5 V.The precursor solution with high mobility can fully penetrate the porous cathode and spread out on cathode surface,achieving the in-situ solidified ionic conducting network inside the cathode and soft contact at cathode-electrolyte interface.This leads to the decreased interfacial resistance and smooth ionic transfer on cathode side,avoiding oxidation decomposition of PEO under high cutoff voltage.The resultant NCM||PEO||Li batteries deliver the satisfactory reversible specific capacity with cutoff voltages of 4.2 and 4.3 V after 100cycles at 55℃,which put forward a feasible strategy for pushing the application of commercialized PEO-based electrolytes in high-voltage SSLBs.