Study On Doping Modification Of LiTa₂PO₈ Electrolyte And Electrochemical Performance Of Its Composite Electrolyte

As a new type of ceramic solid electrolyte LiTa₂PO₈（LTPO）has high theoretical ionic conductivity with simple preparation conditions,and has the potential to replace liquid electrolytes as the next generation of electrolyte materials for batteries.However,due to the insufficient relative density of the sample prepared by conventional solid-phase sintering method,the actual electrolyte properties are significantly different from the theoretical values,which affects the performance of LTPO in solid-state batteries.For the development of all-solid-state batteries and the preparation of high-energy-density batteries,LTPO needs to be modified to optimize its performance in lithium new energy solid-state batteries.The specific experimental contents are as follows:（1）Li_1+xTa_2-xTe_xPO₈（LTTPO）electrolyte sheets with different Te⁴⁺doping contents were prepared by conventional solid phase sintering.The results show that the doped Te⁴⁺replaces the position of part of Ta⁵⁺,and exists in the original TaO6octahedral position in the form of TeO6 octahedron,which changes the surrounding environment of Ta⁵⁺,increases the binding capacity of Ta-O,broadens the a axis and b axis of LTPO crystal,and reduces the attraction of oxygen to lithium ions in the system.The excess liquid phase generated at high temperatures promotes the growth of LTPO crystals and increases the density of the sample.The prepared high-density Li_1.04Ta_1.96Te_0.04PO₈ ceramic electrolyte has a low activation energy of 0.193 e V and an ionic conductivity of 4.5×10^-4 S/cm,which is four times higher than that of undoped samples.In addition,with PEO polymer electrolyte as the interface modifier between LTPO and electrode,Li_1.04Ta_1.96Te_0.04PO₈ showed stable cycling performance in symmetrical Li|Li cells and Li|PEO|Li_1.04Ta_1.96Te_0.04PO₈|LiFePO₄ batteries.（2）Li_1+xTa_2-xCe_xPO₈（LTCPO）with polyvalent rare earth element Ce⁴⁺doped by conventional solid-phase sintering method were prepared.The doping of appropriate amount of the large radius element Ce⁴⁺promoted the growth of LTPO,increased the connection between grains,and improved the compactness.Meanwhile,Ce⁴⁺doping replaces part of Ta⁵⁺positions,broadens the lithium ion transport channel,and increases the LTPO room temperature ionic conductivity.The reaction of the polyvalent element Ce with lithium metal reduces the reduction of lithium metal to Ta⁵⁺,reduces the electronic conductivity of LTPO,and improves the interface compatibility of LTPO to lithium metal.The relative density of the prepared Li_1.04Ta_1.96Ce_0.04PO₈ sample reaches94.13%and the room temperature ionic conductivity is 7.26×10^-4S/cm nearly 4.5 times higher than that undoped(1.65×10^-4S/cm).The lithium-symmetrical battery can be stably cycled for 400 hours at a current density of 0.1 m A/cm².Moreover,using anhydrous acetonitrile containing Lithium bis（trifluoromethanesulfonylimide）（LiTFSI）as the interface adjuster for LTPO and electrode,the prepared solid-state battery can be stably cycled for 50 cycles at room temperature at a current density of 0.1 m A/cm².（3）The prepared LiTa₂PO₈ was added to the polyethylene oxide（PEO）-based polymer electrolyte as an inorganic filler to prepare PEO-LiTFSI-x wt%LiTa₂PO₈organic-inorganic composite electrolyte.The prepared composite electrolyte does not burn and has good safety.The introduction of LTPO reduces the crystallinity of PEO-based polymers in the composite electrolyte,and at the same time,the micropores caused by the addition of LTPO can offset the stress caused by strain and increase the toughness of the composite electrolyte.The decrease of crystallinity increases the kinematic ability of the chain segment,which is beneficial for the Li⁺to move by complexing-uncomplexing.At the same time,LTPO has Lwies acid-base interaction with TFSI^-in it,which increases the content of free mobile lithium ions in the matrix,thereby increasing its lithium ion conductivity.Prepared Li|PEO-LiTFSI-10 wt%LTPO|LiFePO₄ solid-state battery has good cycle performance and rate performance at0.3 C,and the discharge specific capacity is increased from 94 m Ah/g to 106.5 m Ah/g at 60℃.