Preparation And Li Conductivities Of LiBH₄-Based Solid State Electrolytes

LiBH₄-based solid-state electrolytes（SSEs）have great potential for integration into advanced all-solid-state batteries（ASSBs）due to their light weight,low grain boundary resistance,ion selectivity,excellent stability to Li anode and excellent mechanical properties.Its main disadvantage is the poor conductivity(<10^-7 S cm^-1)at room temperature（RT）although it shows a high conductivity of 10^-3 S cm^-1 at 120°C.This is because the highly conductive P63mc phase transforms to the less conductive Pnma phase when temperature reduces to<110°C.Here,there effective modification strategies involve second-phase incorporation,interface effect and anions-cation introduction were taken to synthesis new high-performance LiBH₄-based solid-state electrolytes.The composition,morphology,electrochemical performance and mechanism were systematically studied.Herein,we demonstrated a novel room-temperature ultrafast Li-ion conductor,lithium borohydride ammonia borane complexes（（LiBH₄）_x·AB）.The incorporation of AB into LiBH₄ structure intrinsically increased the cell volume and decreased the volume density of Li ion,which substantially facilitated the Li-ion conduction.The LiBH₄·AB complex delivered up to 4.04×10^-4 S cm^-1 of ion conductivity at 25°C with nearly negligible electron conductivity.The Li-ion transference number was higher than0.999 at 40°C.The obtained room-temperature Li-ion conductivity was largely superior to other LiBH₄-based solid-state electrolytes reported previously under identical conditions.Moreover,the ultrafast Li-ion conduction remained stable upon heating and cooling cycling（18–55oC）.Ab initio molecular dynamics simulations displayed a kind of 1D Li diffusion channel along the b direction in the LiBH₄·AB structure,which offered a much low activation energy barrier（0.12 eV）,consequently enabling superior Li-ion conductivity at room temperature.LiLa（BH₄）₃Cl@SiO₂ samples with fast Li-ion conduction and improved stability against Li anode were successfully synthetized by mechanochemistry effect.SiO₂ and LiLa（BH₄）₃Cl are fused uniformly and tightly by high energy ball milling.The LiLa（BH₄）₃Cl@25 wt%SiO₂ sample showed an impressive conductivity of 1.18×10^-4 S cm^-1 at 35oC.The sample also exhibited a wide apparent electrochemical stability window（0 to 7 V vs Li/Li⁺）and a high Li-ion transference number of 0.9999.The activation energies for Li-ion conduction was as low as 0.42 eV showing typical fast ion conductor characteristics.Interface engineering effectively extended the cycle life of Li/LiLa（BH₄）₃Cl/Li symmetrical battery from 125 h to 250 h and also increased its limit current density for Li anode from 1.6 mA cm^-2 to 2.5 mA cm^-2.xLiBH₄:ZrCl₄ samples with fast Li-ion conduction and ultra stability against Li anode were successfully prepared by high energy ball milling LiBH₄ and ZrCl₄.Among them,22LiBH₄:ZrCl₄ composite material showed the best performance.At 50oC,the ionic conductivity reaches 1.7×10^-4 S cm^-1,the activation energy at low temperature zone was 0.59 eV and at the high temperature zone was 0.39 eV.The sample also exhibited a wide apparent electrochemical stability window（0 to 5 V vs Li/Li⁺）and a high Li-ion transference number of 0.9999 which suggested its electronic conduction can be ignored and can be regarded as a pure ionic conductor.Meanwhile,22LiBH₄:ZrCl₄ composite exhibited ultra stability against Li anode.The Li/22LiBH4:ZrCl4/Li batteries showed very stable responded voltage during the galvanostatic cycling curves at 50oC with current density of 0.2 mA cm^-2.The step current test showed that the material had a limit current density exceeding 5.6 mA cm^-2 which enabled 22LiBH₄:ZrCl₄ strong large current charge/discharge stability.The synergy of the resulted LiZrB_xH_y during the mill process and the Cl^-ion accounted for the superior electrochemical performance.