Interface Optimization And Design Of Garnet Solid-State Lithium Metal Battery

Solid-state lithium metal batteries（SSLMBs）possessing high energy and power density as well as enhanced safety are regarded as one of the powerful candidates for next-generation battery technology.They use non-flammable solid electrolytes to replace the flammable and volatile traditional electrolytes,targeting to fundamentally solve the safety hazards of conventional lithium-ion batteries.Meanwhile,matched with high-specific-capacity(3860 m Ah g^-1)lithium（Li）metal and high-voltage cathode materials,they can break the upper limit of energy density of the existing lithium-ion battery system(350 Wh kg^-1).In recent years,the garnet-type solid electrolyte Li₇La₃Zr₂O₁₂（LLZO）has received extensive attention due to its high room-temperature ionic conductivity and good stability with Li.However,LLZO-based SSLMBs also have some problems.For the anodic interface,the non-wetting of Li and LLZO results in poor interfacial contact and large interfacial impedance,which severely restricts its application.The non-wetting behavior is mainly due to the large surface energy difference of Li/LLZO and the presence of surface impurities.Furthermore,the growth of lithium dendrites or the nucleation of lithium inside the LLZO will cause the battery short circuit.For the cathodic interface,insufficient interfacial contact between complex electrode particles and LLZO is the primary factor that makes the battery difficult to operate.The lack of electron/ion conductive paths inside the electrode and the interfacial volume effect during the cycle also hinder the application of all-solid-state batteries.In this regard,semi-and quasi-solid-state batteries have been developed,and the control of the amount of electrolyte added and the guarantee of battery safety are the focus of the research.In this dissertation,Ta-doped LLZO（LLZT）was chosen as a solid electrolyte.For anodic interface,we control the interfacial modification layer by tuning the components and their structure,which improves the interfacial contact and inhibits Li dendrite growth.For cathodic interface,from the perspectives of electrolyte additives and gel-like modified layers,the progress of semi-solid-state batteries to quasi-solid-state batteries is achieved.The major researches are summarized as follows:（1）Phosphoric acid-induced stable Li/garnet solid electrolyte interfacial layerPhosphoric acid is used to react with the pre-passivated LLZT surface to form an ion-conducting Li₃PO₄ modified layer,which ingeniously turns the unfavorable passivation layer into a favorable interfacial layer.Such a layer will further react with molten lithium during the lithium-melting process to in situ generate Li₃P and Li₂O,which effectively reduces the surface energy gap between Li and LLZT,promotes interfacial wetting and builds a stable anodic interface.Furthermore,the evolution of highly conductive Li₃P can promote Li⁺transport and induce uniform deposition on the interface,thus suppressing the dendrite growth.The modified interface shows a low interfacial impedance of 7.0Ωcm²,and an increased critical current density of 0.8 m A cm^-2.During galvanostatic cycling,the modified cell can be cycled stably for 1600 h and 450 h at 0.1 and 0.5 m A cm^-2,respectively,which proves the excellent electrochemical stability and dendrite-resisting ability of the interface.（2）Construction of a 3D lithiophilic and electronically insulating modified layerThe surface of LLZT is treated with acid-salt solution to form a three-dimensional（3D）functional modified layer under specific conditions.The submicron pore structure of the modified layer can cause capillary absorption,promoting the wetting of Li and LLZO and building a uniform stable interface.Furthermore,by controlling the layer components,Li F and Li Cl with low electronic conductivity are introduced to effectively block electrons through the interface and inhibit the nucleation and growth of lithium inside the electrolyte.The modified interface shows a largely-reduced impedance of11.6Ωcm² and can be cycled stably for 1000 h at 0.5 m A cm^-2.More importantly,the time-constant and capacity-constant modes of the critical current density tests are also explored,and the concept of critical areal capacity is defined for the first time,which provides a reference standard for the subsequent determination and comparison of critical current density under a specific capacity.（3）Iodine additives applied in garnet-type semi-solid-state lithium-sulfur batteriesIodine（I₂）is first added to the ether electrolyte and used in the LLZT-based semi-solid-state lithium-sulfur battery.LLZT as an intermediate layer can directly inhibit the shuttle effect of lithium polysulfides,while the addition of iodine can induce the ring-opening reaction and gradually polymerization of the solvent 1,3-dioxolane in the electrolyte,forming an organic cathodic solid electrolyte interphase（CEI）.The CEI inhibits the dissolution of polysulfides,thereby stabilizing the capacity of the battery.Moreover,iodine delivers a discharge capacity and the battery with iodine exhibits the first-cycle discharge capacity of 1323 m Ah g^-1.In addition,the concentration of I₂additive is optimized and the best concentration is determined as 3 g L^-1.（4）3D polydimethylsiloxane composite with plastic crystal electrolyte to realize a quasi-SSLMBsThe succinonitrile（SN）-based plastic crystal electrolyte is combined with a 3D crosslinked polydimethylsiloxane（PDMS）prepared by the salt template method,forming a PS modified layer.In this gel-like system,PDMS has good chemical/electrochemical stability and thermal stability,maintaining its safety.Meanwhile,PDMS can fix some anionic groups and effectively increase lithium-ion transference number of PS,while SN with high ionic conductivity can provide the interfacial and internal ionic transport pathway.With thermogravimetric analysis,ignition experiment and related electrochemical tests,PS modified layer is proven to have excellent thermal stability and fire resistance,a room-temperature ionic conductivity of 1.25×10^-3 S cm^-1,a wide electrochemical window（>4.95 V vs Li⁺/Li）,and high lithium-ion transference number（0.67）.The modified quasi-solid-state Li|Li Fe PO₄ battery can still operate stably at a high current density of 0.6 m A cm^-2,while the Li|Li Co O₂ battery can stably cycle for 200 cycles at 0.1～0.3 m A cm^-2,and the coulombic efficiency is maintained over 99.5%.