A Novel Solid Electrolyte Li₃La（PO₄）₂:Synthesis And Application In Solid State Batteries

Nowdays,although many solid-state electrolyte materials have been reported,there is still a lack of solid-state electrolyte materials with excellent overall performance in all aspects.Among thousands of lithium-containing inorganic materials,only a few fast ion conductors can meet the requirements of solid-state lithium battery applications.Therefore,it is of great significance to develop a universal synthesis route of solid-state electrolytes and expand the candidate materials for solid-state electrolytes.To this end,in this thesis,Na₃La（PO₄）₂（NLPO）,which is similar to the structure of fast sodium ion conductor（NASICON）,is used as the precursor to synthesize a novel lithium-ion solid-state electrolyte by liquid-phase ion exchange,providing a new strategy for the development of ionic conductors.In order to obtain high purity NLPO powders,two different synthesis pathways are designed,namely the sol-gel method and the high-temperature solid-phase method.The results indicate that the both synthetic routes could produce high purity NLPO powders with similar particle size（10μm）at 1100°C.In contrast,the high-temperature solid-phase synthesis route is simpler than the sol-gel method.The electrochemical impedance spectroscopy（EIS）indicates that the NLPO synthesized by the high-temperature solid-phase method has a higher ionic conductivity,and its bulk conductivity is 9.1×10^-6 S·cm^-1.In addition,the low ion conductivity of NLPO is mainly attributed to the high coordination structure of Na⁺ions and the lack of sodium vacancies.Firstly,the ion exchange reaction is carried out at 180℃,and the evolution of the crystal structure during the exchange process is initially determined by X-ray diffraction（XRD）experiments,and the core-shell model of ion exchange within large particles is proposed.Subsequently,the controlled ion exchange process is achieved by controlling the reaction temperature and the duration of the reaction time.The results show that the low-coordination Na⁺（NaO₆,NaO₇）is replaced first when the exchange percentage is less than 82%,and the reaction rate is faster.However,when the exchange percentage is over 82%,the high-coordination Na⁺（NaO₈）exchange process takes place,the framework structure collapses,and the reaction rate slows down significantly.The electrochemical impedance spectroscopy also shows that the bulk ion conductivity of the exchanged samples increases with the increase of the exchange percentage during the framework retention stage,and Li_2.7Na_0.3La（PO₄）₂ with the exchange percentage of82%has the highest bulk ion conductivity(3.19×10^-4 S·cm^-1),and its activation energy is 0.28 e V.With the introduction of succinate,a denser composite ceramic pellet（NLLPO-PCSE）with a total conductivity of 1.74×10^-4 S·cm^-1 is prepared by heat-assisted low-temperature sintering method.The composite electrolyte pellet exhibits a high oxidation voltage of～5.02 V,which can be matched with most current cathode materials.In addition,NLLPO-PCSE exhibits excellent stability to lithium compared to pure NLLPO ceramic pellet,with a stable cycle of more than 600 h at 0.1m A·cm^-2.Meanwhile,the Li/NLLPO-PCSE/Li Fe PO₄ with an integrated pressed composite electrolyte-cathode assembly has a first-cycle discharge specific capacity of143.3 m Ah·g^-1 at 0.1 C,and maintains a discharge specific capacity of 112.1 m Ah·g^-1after 100 cycles,while also showing good magnification performance.