Performance Optimization Of Garnet Electrolyte And Interfacial Modification Of Garnet Based Solid-state Batteries

There have been widespread concerns over the safety of conventional lithium-ion batteries as employing liquid/polymer organic electrolyte,which may leak or fire in some certain conditions.Moreover,the energy density is restricted by the complex structure since using some extra components.Fortunately,the properties of safety and energy density could be improved greatly if solid-state electrolyte?SSEs?replace liquid organic ones.Among these SSEs,garnet-structured Li₇La₃Zr₂O₁₂?LLZO?has attracted considerable attention,owning to its high ionic conductivity and high electrochemical stability.However,there are still some fundamental problems need to be solved in the application of solid-state batteries?SSBs?,such as low room-temperature ionic conductivity,high interfacial resistance.In this work,we have worked on these problems and the details are as follows:?1?Garnet-structured Li_7-xLa₃Zr_2-xNb_xO₁₂?LLZNO?was synthesized by a conventional solid-state reaction method and sintered to dense ceramic pellets by hot isostatic pressing technology.We evaluated the influence of Nb doping amount and synthesized temperature on the performance of LLZNO pellet.Pure cubic phase was obtained as x?0.375 and the optimal synthesized temperature was 950?as impurity phases could be found at the higher sintering temperature.Hot isostatic pressed LLZNO pellets existed high relative densities with grain boundaries and pores free.The optimal relative density was 99.5%while x=0.625.?2?Hot isostatic pressed LZNO electrolytes existed favorable Li⁺conductivities as the value reached 10^-4 S cm^-1 while x?0.125.The optima value was 1.37×10^-3 S cm^-1 corresponting to x=0.625.LSV revealed the pellet was pure ionic conductor since the electronic conductivity was 5 orders of magnitude lower than that of ionic conductivity.CV suggested the pellet was stable aginst Li metal as the electrochemical window was higher than 5 V.?3?LLZNO pellet was sandwitched with Li-metal anode,Li FePO₄?LFP?composite cathode to fabricated SSB.CV showed Li⁺could insert/extract reversibly as there was no side reaction between the voltage of 2.7-4.0 V.The SSB could ran steady as the interfacial resistance decreased at the elevated temperature of 60?.It delivered an initial discharge capacity of 135 mAh g^-1 with a 98.9%platform of Coulombic efficiency until the 100^th cycle,in which the discharge capacity remained 98.7%of the initial value.Further measurement of elevated current densities revealed that the SSB existed favorable cycling and rate performance.?4?The poor contact performance inhibit the application in SSBs since the large interfacial resistance induced Li dendrite at high current density.We introduced a Sn nanolayer that improved the contact performance by forming a self-limited Li-Sn interlayer after heating or cycling.The Sn layer of 10 nm thickness delivered the optimal modification effect since it decreased the interfacial resistance from 758?cm²to 46.6?cm².Li⁺could insert/extract steady over 500 cycles at the current density of0.5 m A cm^-2 under RT.Compared with liquid battery,the battery of Li/Sn-LLZNO/LFP enhanced the intial discharge capacity from 84.18%of theoretical to 89.24%,and the initial Coulombic efficiency rised from 88.03%to 90.87%.The Coulombic efficiency pleataued at 99.6%in the following cycles that showd a good cycling performance.?5?In order to improve the interfacial contact performance,we fabricated a flexible composite cathode by introducing LiTFSI-SCN crystalline electrolyte,electronical additive of KB and binder of PVDF in LiFePO₄.LiTFSI-SCN crystalline electrolyte could improve the interface performance as it formed a three dimension ion-conductive frameworks and was flexible enough to accommodate the volume change of LiFePO₄particles during lithiation and delithiation process.Electrochemical characterizations showed LiTFSI-SCN crystalline electrolyte decreased half of the SSB's inner resistance,and Li⁺inserted/extracted steady between the voltage of 2.6-4.1 V.At the current density of 0.05 C under RT,the SSB delivered an initial discharge capacity of 149.8mAh g^-1 which remained 96.7%even after 100 cycles with the Coulombic efficiency plateaued at 99%.Further measurement of higher current density revealed that the battery adopted the flexible composite electrolyte showed an excellent rate performance.