Preparation And Modification Of Garnet-type Solid Electrolyte LLZTO

The garnet-type oxide solid electrolyte Li₇La₃Zr₂O₁₂（LLZO）and its derivatives have great application prospects due to their high ionic conductivity and good chemical stability.In particular,Ta-doped LLZO solid electrolyte LLZTO has an ionic conductivity of 10^-4-10^-3 S cm^-1 at room temperature,which is much higher than LLZO and comparable to the conventional liquid electrolytes.However,LLZTO solid electrolytes also face problems such as high interfacial resistance with cathode materials and poor stability in the air.There are two main sources of interfacial resistance between LLZTO and cathode materials:（1）LLZTO reacts with the cathode material to form an interfacial phase;（2）LLZTO reacts with CO₂ and H₂O in the air to form an insulating layer containing Li OH and Li₂CO₃.In view of the above problems,this paper mainly carries out the following aspects of work:（1）Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）solid electrolyte was prepared by mechanical ball milling and solid phase reaction,and the effect of different sintering temperatures were discussed.When the sintering temperature is lower than 900°C,the particle morphology is cubic,there is not only LLZTO phase in the system,but also a small amount of lanthanum zirconate phase（La₂Zr₂O₇）;when the sintering temperature rises to 900°C and above,the particle morphology is spherical,the lanthanum zirconate phase in the system disappears,the XRD patterns shows that only the LLZTO cubic phase exists in the system.As the sintering temperature increases,the LLZTO solid electrolyte particles are growing,indicating that elevated sintering temperature promotes the growth of solid electrolyte particles.The conductivity of LLZTO at different temperatures was tested,with the increase of temperature,the impedance of LLZTO solid electrolyte decreased and the conductivity increased.（2）Non-sintering method was used to improve the ion conductivity of LLZTO solid electrolyte.Use the HBO₃,Li NO₃ to modify LLZTO by ball milling,Li NO₃ supplemented the loss of lithium,H₃BO₃ reacted with the Li₂CO₃ and Li OH on the solid state electrolyte surface to form Li₃BO₃ and Li₂B₄O₇,increasing the lithium-ion conductivity of LLZTO solid electrolyte ceramic grain boundary.In this way,the ceramic pellets obtained by cold pressing also have a high ionic conductivity.The conductivity of the modified electrolyte increases by two orders of magnitude from 10^-6 S cm^-1 to 10^-4 S cm^-1.When we add the modified LLZTO into the cathode LR114,the reaction products Li₃BO₃ and Li₂B₄O₇ are also the intermediate phases,promoting a close contact interface between LLZTO and LR114 cathode,thereby ensuring that the LLZTO-LR114 interface has a high lithium ion mobility,inhibiting the element diffusion between LLZTO and the cathode material,and inhibiting the formation of low conductive interlayer phases such as Li OH and Li₂CO₃.When adding 2 wt.%of 5 wt.%LLZTO/HBO₃/Li NO₃ powder to the LR114 cathode material,the capacity increased by 5-11m Ah/g.（3）High safety electrolyte is particularly important for the construction of lithium-ion battery system,in addition to the oxide-type solid-state electrolyte LLZTO has high safety,lithium-ion battery system also has a high stability electrolyte,that is,the water electrolyte system,the choice of anode materials for high-voltage aqueous batteries beyond 1.5 V is still far from satisfactory,due to the narrow electrochemical stability window of aqueous electrolytes.With the inherent advantages of low working voltage and fast ionic diffusion,niobium phosphates with polyanionic framework groups built of NbO₆ octahedra and PO₄tetrahedra are very promising anode materials for high-voltage aqueous batteries.Herein,carbon-coated NbOPO₄（NPO/C）materials with monoclinic structure has been prepared by the solid-state reaction&chemical vapor deposition method.Between-1.34 V and 0.6 V vs.standard hydrogen electrode,it could deliver a specific capacity of 116 m Ah/g with a high coulombic efficiency of 92.0%at a low current rate of 0.33C in aqueous electrolytes based on methylsulfonylmethane,Li Cl O₄ and H₂O（DES）.Upon the reversible insertion/extraction of lithium ion into/from NPO/C,it maintains the monoclinic structure with a small lattice expansion,which is a characteristic of the zero-strain effect.Finally,the full cell based on NPO/C anode,Li Mn₂O₄ cathode and DES electrolytes with a high specific energy 101.6 Wh/kg and a high voltage output beyond 2 V has been validated.It also shows excellent cycling performance with the capacity retention of 94.3%after 1000 times at 2C rate,which suggests that monoclinicβ-NbOPO₄can be used as a promising electrode material for aqueous lithium-ion batteries with high voltage output for energy storage in the future.