All-solid-state Li Batteries Based On The Fourth Subgroup Metal Halides

To fundamentally solve a series of safety problems such as leakage,combustion,and explosion of combustible organic electrolytes in conventional commercial lithium-ion batteries,researchers have proposed to design all-solid-state lithium batteries using non-combustible inorganic solid electrolytes instead of organic electrolytes.Moreover,all-solid-state lithium batteries are expected to break the energy density bottleneck of commercial lithium-ion batteries.However,the practical industrial application of allsolid-state lithium batteries has still been hindered by many challenges regarding solid electrolyte and electrode materials.In terms of solid electrolyte materials,there is no solid electrolyte that can simultaneously satisfy all the demands of industrialization,i.e.,high ionic conductivity,wide electrochemical window,excellent deformability,and low material and manufacturing costs.In terms of cathode materials,the new cathode materials for all-solid-state lithium batteries cannot simultaneously exhibit high ionic conductivity,easy deformability,and a good capacity retention ratio.The halide material system which can display high ionic conductivity,wide electrochemical stability windows,and easy deformation,seems promising to solve the above issues related to solid electrolytes and cathodes.However,the high raw material costs of the halide solid electrolytes with excellent performances significantly impede the industrial application of the halides.Additionally,the halides that can be used as cathodes also face the problem of low ionic conductivity.The sluggish lithium-ion migration will lead to poor electrochemical performances.To solve the challenges in solid electrolytes and cathodes mentioned above,we conducted several innovative work which is as follows:(1)Although the recently developed solid halide electrolytes(Li3MCl6)have the advantages of high ionic conductivity and wide electrochemical stability window,their commercialization is still hindered considerably by the expensive raw materials,such as the chlorides containing trivalent rare-earth Sc-Lu elements and In element.Because of this,a new zirconium-based chloride Li-M-Cl system was successfully synthesized using the cheap tetravalent Zr as the M element.The material not only retains the desirable properties of the Li3MCl6 systems,but also exhibits its unique humidity tolerance against 5%relative humidity.More importantly,if Li2ZrCl6 is used as the solid electrolyte layer with a thickness of 50 μm,the raw material cost is only $1.38/kg.The value is not only much lower than the cheapest cost previously reported for the Li3MCl6 systems($23.05/kg for Li3YCl6)but also below the recognized threshold to ensure cost-competitiveness of all-solid-state batteries($10/kg).In addition,the allsolid-state lithium cells assembled with zirconium-based chloride and an uncoated 4 Vclass cathode also exhibit excellent electrochemical performances at room temperature.(2)Based on the work in the previous section,we futher improve the room temperature ionic conductivity,thermal stability,and moisture reversibility of the zirconium-based chlorides by doping the heterovalent soft acid element indium.When the partial Zr is replaced by an indium element with a larger atomic radius,the induced lattice expansion will reduce the migration energy barrier of lithium ions and effectively improve the room temperature ionic conductivity of the zirconium-based chlorides.Furthermore,in this work,we assembled all-solid-state lithium cells by using indiumdoped zirconium-based chloride as the solid-state electrolyte,single-crystal NMC811 as the cathode,and Li-In alloy as the anode.When being cycled at 25℃ and 0.2 C,the cells exhibit an initial Coulomb efficiency of 87.1%and a discharge specific capacity of 175.7 mAh g-1.Even at a rate of 1 C,the cell can still retain 80%capacity after stably operating for 325 cycles.The results provide a new strategy to simultaneously improve the ionic conductivity,thermal stability,and moisture reversibility of solid halide electrolytes.(3)Furthermore,by introducing variable-valence transition metal elements,the halide solid electrolyte material with a single function was developed into a multifunctional material that can be used as the cathode,electrolyte,and anode simultaneously.In this part,titanium-based chloride Li3TiCl6 was successfully synthesized by introducing trivalent titanium into Li3MCl6 solid electrolyte systems.It not only possesses all the excellent properties of Li3MCl6 solid electrolytes,such as high ionic conductivity(1.04 mS cm-1),wide electrochemical window,and easy deformability but also exhibits new functions as electrode materials.It is worth noting that when Li3TiCl6 is added to the composite cathode as the solid electrolyte,it can not only assist lithium ion transport but participate in charge and discharge to provide capacity.This provides a new idea for improving the energy density of all-solid-state lithium batteries.When Li3TiCl6 is used as the electrode material,its theoretical capacity is 95.2 mA g-1,corresponding to one Li per formula unit of Li3TiCl6;its theoretical capacity is 190.4 mA g-1,corresponding to two Li per formula unit of Li3TiCl6.The content of the active material(Li3TiCl6)in the composite electrodes can reach 95 wt%(the other 5 wt%is carbon black).The all-solid-state lithium cells assembled with the composite electrode can be stably operated for more than 1000 cycles at room temperature.In addition,we assembled a type of single-material allsolid-state lithium cells with multifunctional Li3TiCl6 as cathode,solid-state electrolyte,and anode simultaneously.The cell can stably operate for more than 2500 cycles at room temperature.The discovery of the multifunctional chloride material is beneficial to expanding the design ideas of halide materials and designing new all-solid-state lithium batteries.The above research results show that our work has achieved admirable results in solving the problem of solid electrolytes and cathode materials in all-solid-state lithium batteries.We successfully reduced the high raw material cost of chlorides as solid electrolytes and increased the ionic conductivity of chlorides as cathodes.Additionally,we proved the feasibility of chlorides containing variable valence metal elements in extending the performances of halide material systems.The conclusions of this paper are expected to be used to solve various challenges of all-solid-state lithium batteries in terms of materials and to accelerate the actual industrialization of all-solid-state lithium batteries.