Preparation Of Lithium Lanthanum Titanate Based Fiberous Membranes And Applications On Modificating Li-metal Anodes

With the booming of small electronic products,electric vehicles and smart grids,people have put forward higher requirements for the endurance and fast charging ability of electrochemical energy storage devices.However,due to the limitations of intercalation chemistry,the actual energy density of lithium-ion batteries is close to their theoretical limits.Therefore,it is urgent to develop high-capacity electrode materials to meet the needs of energy.Lithium（Li）metal is considered to be the most promising anode material due to its ultrahigh capacity(3860 m A h g^-1)and the lowest redox potential（-3.040 V vs standard hydrogen electrode）that significantly increases the energy density of batteries.However,the practical application of Li metal anode still faces the following serious obstacles:first of all,the uncontrollable growth of Li dendrites leads to serious safety problems;second,the huge volume expansion of Li metal during repeated charge and discharge processes;and third,the high reactivity between Li metal and liquid organic electrolytes leads to the continuous consumption of active Li and electrolytes.Therefore,in view of the above problems,it is of great significance to conduct reasonable modification studies on the Li metal anode in order to achieve long cycling life and high safety of Li metal batteries.In recent years,it has been found that one-dimensional micro-nanofiber materials exhibit unique functional characteristics and advantages in the application of Li metal anode modification:1)Micro-nanofibers provide a continuous and rapid transmission channel for charged particles,thereby facilitating charge transfer and ion diffusion;2)Micro-nanofibers provide a larger reaction contact area and a wealth of active sites to reduce local current density and promote uniform deposition of Li;3)Micro-nanofibers can accommodate volume expansion,thereby inhibiting structural degradation of Li metal anodes in repeated cycles.Among the various methods for preparing micro-nanofiber materials,electrospinning has become one of the most commonly used methods due to its advantages of simple equipment,wide source of raw materials,good fiber continuity and good structural tunability.Lithium lanthanum titanium Li_3xLa_（2/3）-x□_（1/3）-2xTiO₃（LLTO）with perovskite structure is a typical oxide fast ion conductor material,which has the advantanges of high room temperature ion conductivity,good electrochemical stability and good thermal stability etc.,is an excellent material for optimizing the Li metal anode.In the study of using it to modify the Li metal anode,the stability and safety of the Li metal anode have been significantly improved.Therefore,this study aims to combine the advantages of nanofiber materials and LLTO itself.Self-supporting LLTO-based nanofiber film materials with different characteristics were designed and prepared by electrospinning and high-temperature calcination processes.Then,these materials were used as buffer layers or current collectors to combine with Li metal,which could effectively alleviate problems such as Li dendrite growth,and realizing the improvement of safety performance and electrochemical properties of Li metal batteries.The main contents are summarized below:（1）A mixed ion-electron conductive fiber interface was constructed to solve the problem of Li dendrite growth.First,taking polyvinylpyrrolidone as the polymer template,by regulating the content of the polymer template,and selecting the appropriate solvents and complexing agents,a uniform and stable spinning solution was obtained.The self-supporting LLTO nanofiber film materials were prepared by using sol-gel electrospinning and high temperature calcination process.The crystallization of precursor fibers at different calcination temperatures was studied,and the calcination conditions required for the formation of tetragonal phase LLTO nanofibers were determined in combination with thermogravimetric analysis.The deformation mechanism of oxide ceramic nanofiber membranes was preliminarily analyzed.Secondly,a room temperature Li metal reduction strategy was developed,that is,the rapid redox reaction between Li metal with high reactivity and LLTO nanofiber film was used to introduce oxygen vacancy defects into the LLTO lattice,Ti⁴⁺was reduced to Ti³⁺or even lower titanium ions.LLTO nanofiber membrane changed from white to black in a few minutes,and the electron conductivity was increased to 0.32 S m^-1,thereby becoming a mixed ion-electron conductor.Finally,in order to solve the problem of uncontrollable growth of Li dendrites,an artificial protective film of mixed ion-electron conductive nanofibers was constructed on the surface of Li metal using the above reaction mechanism.The protective film could balance the potential of the Li anode surface and induce uniform Li-ion deposition behavior,thereby realizing dendrite-free Li deposition,effectively improving the interface stability and the electrochemical performances of the Li-metal anode.（2）The three-dimensional mixed ion-electron conductive fiber current collectors were constructed to solve the problem of volume expansions of the Li metal anodes.Firstly,by combining electrospinning and one-step carbonization,a carbon nanofiber membrane material attached with LLTO nanoparticles（LLTO@C）was successfully prepared,in which carbon nanofibers provide electronic transmission in the whole,and LLTO nanoparticles with a mass ratio of up to 93%were uniformly dispersed in carbon nanofibers to provide rapid ion transport.At the same time,porous carbon nanofiber films（MCNF）and carbon nanofiber films（CNF）were also prepared.In order to solve the problem of large volume fluctuations of Li metal anodes,the above three kinds of nanofiber films and commercial copper foils were used as current collectors of Li metal anodes,respectively.Nucleated overpotential tests shown that the mixed ion-electron conductive LLTO@C films could effectively reduce the nucleation barrier of Li,and the synergy of LLTO and C provided a dual continuous ion/electron channel that could guide the uniform deposition of Li metal on the surface of the nanofibers,thereby inhibiting the growth of Li dendrites and effectively alleviating the volume change of the Li anode.However,the MCNF needed a large thickness to provide sufficient pores to accommodate Li metal,and the CNF and copper foil could not achieve the adjustment of Li deposition behavior.The composite anodes with LLTO@C films as the current collectors could operate stably for more than 400 hours with a low over-potential of 40 m V at a high current density of 3 m A cm^-2,significantly improving the interfacial stability of the battery and prolonging battery life.（3）An integrated solid-state battery containing a mixed conductive three-dimensional Li anode was constructed to solve the problem of continuous side reactions due to thermodynamic instability between Li metal and liquild electrolytes.Through a simple roll-pressing and hot-pressing process,the Li foil was physically embedded in the LLTO@C nanofiber frame,and the ion conductor molten interphase plasticized by Li-salt was introduced to constructed a composite three-dimensional Li anode,which solved the problem of unstable interface and large impedance of solid-state lithium batteries.Among them,the ion-electron coupling effect and lithiophilicity of the mixed conductor nanofiber frame could promote uniform Li deposition and reduce the volume fluctuation of the anode,while the deformable molten interphase act as a strong binder could ensure continuous ion contact between the Li anode and the electrolyte during the cycle.Thanks to the above structure,the symmetrical batteries could operate stably for more than 1200 hours at a current density of 0.6m A cm^-2,and the battery interface impedance remained stable.In addition,the electrolyte-infiltrated cathodes were designed to improve the interfacial contact between the cathode and the electrolyte,and the integrated solid-state full batteries were constructed by pairing with the above composite3D Li anodes.The full batteries delived high capacity retention of 98.7%after 100 cycles at 0.2 C,and even at high rates of 0.5 C and 1 C,stable cycle performance and high coulombic efficiency could still be maintained.