Interface Engineering And Performance Improvement Of Solid-state Batteries Based On Garnet Electrolytes

Solid-state batteries?SSBs?are considered as the next-generation lithium-ion batteries due to the high energy density and excellent safety.As one of the key components in SSBs,garnet-type solid-state electrolytes?SSEs?have attracted much attention because of their high ionic conductivity,large electrochemical window,and the excellent stability with lithium metal.Currently,the researches based on garnet electrolytes can be divided into two categories.One is polymer/garnet hybrid electrolytes?PGEs?,where garnet powders are well distributed into various polymer matrices to fabricate flexible membranes.The other is the garnet ceramic electrolytes?GCEs?,which are fabricated by calcinating the garnet powders at a high temperature.The solid-solid interfacial contact is a critical issue during the studies of SSBs,which greatly affects the electrochemical performance of SSBs,such as the Coulombic efficiency,and cycle life.For flexible PGEs,the interfacial issue mainly comes from the interactions between the inorganic fillers and the polymer matrix.The slow Li+ transport through the polymer/garnet interface can lead to the low ionic conductivity of PGEs.While for GCEs,the ceramic bulk with a relative density over 99% shows no obvious grain boundary,leading to the ionic conductivity over 10-3 S cm-1 at room temperature.Under this circumstance,the interfacial issue could be attributed to the poor interfacial contact between the GCEs and Li metal anodes.It can induce the large interfacial resistance as well as the lithium dendrite growth.This dissertation focuses on the aforementioned interfacial issues,various targeted strategies are proposed to construct the excellent interfacial structure,thus significantly improving the performance of SSBs.The main achievements are shown as follows:?1?The Li+ transport at the polymer/garnet interface is adjusted to enhance the ionic conductivity of PGEs.A small amount of ionic liquid is added in the system of polyethylene oxides?PEO?and Li6.4La3Zr1.4Ta0.6O12?LLZTO?,thus forming the highly conductive interface for Li+ migration.The interfacially wetted PEO/LLZTO electrolytes show the conductivity of 2.2×10-4 S cm-1 at 20 oC,which is one order of magnitude greater than that of the PGEs without the ionic liquids.In addition,the LLZTO with the surface basicity can have an interaction with some certain polymers by the Lewis acidbase effect,which can lead to the improvement of ionic conductivity.Therefore,the highpolarization polyvinylidene fluoride?PVDF?is chosen as the polymer to combine with LLZTO powders.The partial dehydrofluorination of PVDF is promoted in an alkalinelike environment,turning the PLLZ slurry from transparent to brown.The increased dissociation of lithium salt and efficiently immobilized anions can greatly improve the ionic conductivity of PVDF/LLZTO electrolyte to 1.4×10-4 S/cm at room temperature.?2?The flexible interface between the PGE and the silicon anode is constructed to alleviate the volume change of Si anode,thus achieving excellent performance.Silicon anode is very promising due to the high specific capacity?4200 m Ah g-1?and low cost.However,the large volume change of 300% causes the fast decay of the charge/discharge capacity.Beneficial from the flexibility of PGEs,the interfacial stress by large volume variation of silicon anode can be greatly alleviated during the charge/discharge process.As a result,the Si/Li cells exhibit 2520 m Ah g-1,2260 m Ah g-1,1902 m Ah g-1,and 1342 m Ah g-1 at 0.1 C,0.2 C,0.5 C,and 1 C,respectively.?3?A sandwich-type PGE with hierarchical garnet particles is constructed to simultaneously achieve the dendrite suppression and conductivity improvement.While the ceramic-in-polymer electrolyte with 20 vol% 200-nm LLZTO particles?CIP-200 nm?exhibits the highest ionic conductivity of 1.6×10-4 S cm-1 at 30 oC and excellent flexibility,the polymer-in-ceramic electrolyte with 80 vol% 5-?m LLZTO?PIC-5 ?m?shows the highest tensile strength of 12.7 MPa.A sandwich-type PGE?a PIC-5 ?m interlayer is sandwiched between two CIP-200 nm thin layers?enables highly stable lithium plating/stripping cycling for over 400 h at 0.2 m A cm^-2 at 30 oC.?4?A universal and simple method of rapid acid treatment is proposed to perfectly remove the surface Li2CO3 and retrieve a lithiophilic SSE surface.The LLZTO/Li interfacial resistance dramatically decreases from 940 ? cm2 to 26 ? cm2 at 30 oC.The acid-treated LLZTO exhibits the interfacial resistance comparable to the pellets with various surface coatings.In addition,the Li2CO3-free LLZTO/Li interface remains stable during cycling,which enables the lithium symmetric cells to continuously cycle over 700 h under 0.2 m A cm^-2 at 30 oC.?5?A mixed conductive layer?MCL?consisting of electron-conductive nanoparticles embedded in an ion-conductive network is introduced at the interface between the LLZTO and the lithium anode.Such MCL not only leads to the transition from lithiophobicity to lithiophilicity,but also homogenizes electric-field distribution inside the MCL and relieves the electronic attacks to the LLZTO.As a result,the Li/MCL/LLZTO/MCL/Li cells show a critical current density?CCD?as high as 1.2 m A cm^-2 and stable cycling for over 1000 h at 0.1 m A cm^-2.?6?Three-dimensional?3D?LLZTO/Li interface is constructed to improve the electrochemical performance of SSBs.Three different interfaces are fabricated based on LLZTO electrolytes and the lithium penetration behaviors across the interfaces are investigated.The lithiophilic LLZTO/Li interface with initially low resistance can be gradually broken and the larger and larger voltage hysteresis drives lithium nucleation and proliferation through the grain boundary of the garnet ceramic bulk.The results indicate that a 3D LLZTO/Li interface can remain stable due to the decreased local current density and relived lithium volume change,thus suppressing the dendrite.The 3D stable interface enables a CCD as high as 1.4 m A cm^-2 and over 600 h long-term stable cycling at 0.5 m A cm^-2.