Preparations,Structures And Conductivities Of LiBH₄-based Solid State Electrolytes

All solid-state lithium-ion batteries employ solid electrolytes instead of traditional organic liquid electrolytes,which are expected to fundamentally solve the issue of batteries safety,while further improving the energy density and cycle life of lithium batteries.It is in line with the demands of safety and high energy density of lithium-ion batteries.Compared with other solid electrolytes,the lithium borohydride solid electrolyte described in this paper owns the advantage of high ionic conductivity at high temperature,wide electrochemical window,perfect compatibility with electrode materials and high thermal stability.However,there are still some gaps in the system compared with the application standard of all-solid-state electrolyte.The reason is that the ionic conductivity of the system at room temperature still not meets the requirement of application.Therefore,it does not play a role in all-solid-state batteries.In view of above problems,this paper intends to study lithium borohydride from the following aspects:?1?The LiBH₄-NaBH₄ composite was prepared by means of simply mechanical milling.Through tuning the molar ratio of LiBH₄ over NaBH₄,the most favoarble ratio of LiBH₄-NaBH₄ compound was identified as 4LiBH₄-NaBH₄.The phase transition temperature of 4LiBH₄-NaBH₄ is the lowest?373 K?,of which the conductivity of lithium ion is the highest(2.0?10^-6 S cm^-1).This conductivity is two orders of magnitude higher than that of pure LiBH₄ phase at room temperature(10^-8 S cm^-1).Its activation energy is the lowest?0.8029 eV?.The LiBH₄-NaBH₄ composite can be stabilized at around 4.5 V after five CV cycles,indicating that the potential window is wide.?2?Various amount of MgH₂ was doped into 4LiBH₄-NaBH₄ after ball milling to form ternary phase composites.The ionic conductivity of the sample at room temperature can reach 2.01?10^-6 S cm^-1 when the additive amout is 30 wt.%.The ionic conductivity of the sample at room temperature is two orders of magnitude higher than that of pure LiBH₄phase(10^-8 S cm^-1),as well as binary phase 4NaBH₄-LiBH₄.This may be resulted from the disordered boundaries between phases during ball milling,which is beneficial to the transportation of lithium ions.?3?LiBH₄ is combined with NaCl/NaI.When the ratio of the two is LiBH₄:NaI=2:1,the room temperature ionic conductivity is up to 1.28?10^-6 S cm^-1,which is 100 times higher than that of pure LiBH₄.At the same time,according to the phase transition temperatures of all samples in the system,the phase transition temperature of 2LiBH₄-NaI is365 K,17 K lower than those of pure LiBH₄.This is mainly due to the ion exchange reaction in the parent phases of LiBH₄-NaX?X=Cl,I?after ball milling,yielding LiBH₄-NaBH₄ eutectic composite and Li?BH₄?_1-yX_y solid solution.Due to synergistic effects from both species,the ionic conductivity of the system is enhanced.