Research On The Surface And Interface Properties Of Key Materials For Solid-State Lithium Batteries

All-solid-state lithium batteries(ASSLBs),which is supposed to achieve higher energy density and safety simultaneously is considered as a potential alternative to traditional lithium-ion batteries(LIBs).Although a variety of solid electrolytes have been developed,and the Li⁺conductivity of many solid electrolytes has reached or even exceeded that of liquid electrolytes,there is still a huge gap between the electrochemical performance of ASSLBs and the current advanced LIBs.The main reason for this gap is the poor surface and interface stability in ASSLBs,which severely limits the overall performance.Therefore,in this thesis,we mainly focus on the key surface/interface problems of two important solid electrolytes:(1).The poor chemical/electrochemical stability of PEO(polyethylene oxide)and high-voltage cathodes interface severely limits the application of PEO in high-voltage batteries.In this paper,combining differential electrochemical mass spectrometry(DEMS)testing and theoretical calculations,the reaction mechanism between PEO and the high-voltage layered cathode interface is studied,and surface coating of ionic conductor LATP(Li_1.4Al_0.4Ti_1.6(PO₄)₃)and mixed ion conductor AZO(Al-doped Zn O)were conducted to modify Li Co O₂(LCO)to improve the interface stability.(2)The solid garnet elelctrolyte has poor chemical stability to air,it reacts with air to produce an alkaline passivation layer resulting in high cost in production and storage.The kinetics of the passivation layer growth and the reaction mechanism and law of the solid garnet elelctrolyte and the organic solvent are studied combining experiments and theoretical calculations.Firstly,DEMS was used to study the gassing behavior of LCO|PEO-Li TFSI|Li solid battery.The experiments,together with theoretical calculations,revealed that the surface catalytic effect of Li_1-xCo O₂ is the root cause for the unexpected H₂ gas release of PEO-based SPBs at 4.2 V.The surface coating of LCO with a stable solid-electrolyte LATP can mitigate such surface catalytic effect and therefore extend the stable working voltage to over 4.5 V.The crossover effect of HTFSI,which is generated at cathode side due to oxidation/dehydration of PEO and reacts with lithium at anode side,is proposed to explain the H₂ generation behavior.After uncovering the importance of the chemical oxidation of Li_1-xCo O₂ to PEO electrolyte decomposition,in the second part of this thesis,a comparative study between high nickel cathodes NCM 622,NCM 811 and LCO was conducted.Gassing kinetics and interface reaction mechanism in PEO ASSLBs were comparatively studied.It is found that although NCM 622 and NCM 811 cathodes have a higher capacity than LCO in the same potential range(3.0-4.6 V),the gas generation and interface impedance are far lower than that of LCO,indicating that Li_1-xCo O₂ can more violently catalyze the oxidative decomposition of PEO electrolyte.NCM 622 cathode is the best among the three cathodes in terms of gassing,interface dynamics and cycle stability.In addition,the gassing behavior of solid polymer batteries is totally different from that of traditional LIBs:One is that PEO reacts with the surface oxygen of cathodes to generate small molecules such as aldehydes and carbon dioxide,without oxygen release,which is favorable for safety;The second is that gassing in solid polymer batteries will continue to occur upon cycling,and a stable interface layer cannot be formed,which further emphasizes the importance of artificially constructing a stable interface.In the third part of this thesis,a mixed ion conductor AZO was used to coat LCO to improve the stability of the cathode/electrolyte interface.Compared with other coating materials,AZO has a high electronic conductivity(10⁴ S cm^-1).And it is found that AZO has a one-dimensional Li⁺transport channel with low Li⁺diffusion activation energy(1.8 e V)through bond valence calculations.AZO coating can effectively promote the electron and ion transport at the interface thus make it possible to carry on a fully-covered coating.The AZO coated LCO cathode has obtained long-term cycle stability at 4.5 V(650 cycles,capacity retention>80%)and the rate performance was significantly improved.In addition,the optimized cycling stability of AZO coated LCO was verified in ASSLBs with PEO as electrolyte,which proves that proper cathode surface modification has an essential impact on both the performance of LIBs and ASSLBs.Finally,the surface chemical stability of Garnet-type Li_6.34La₃Zr_1.4Ta_0.6O₁₂(LLZTO)solid electrolyte was studied.First,ex-situ XPS and in-situ Raman were conducted to explore the growth kinetics of the passivation layer on the surface of LLZTO,and a functional model of the growth behavior of Li₂CO₃ was obtained by fitting.Subsequently,in the last part of the thesis,the reaction mechanism and law of LLZTO and different organic solvents with typical functional groups are studied.The results show that the types of atoms and functional groups near the H atom will affect the charge arrangement around H,thus affecting the difficulty of H⁺detachment.Solvents with higher acidity are more prone to react with LLZTO through Li-H exchange,and consequently,result in more severe lattice swelling.Besides,LLZTO was found to catalyze the polymerization of nitriles and carbonyl compounds.These reactions will have a serious impact on the practical application of LLZTO.The results in this work may provide guiding significance for researchers to select suitable solvents in different scenarios of LLZTO applications.In addition,the reaction mechanism of LLZTO with different functional groups is of reference significance for understanding the stability of garnet and polymer matrix with different functional groups in the composite solid electrolyte.