Study On Preparation And Electrochemical Properties Of Polymer/Ceramic Nanofiber Composite Solid Electrolytes With High Ionic Conductivity

Since the 21st century,the demand for energy has been increasing.Among the existing energy storage technologies,lithium-ion batteries（LIBs）have been used in applications such as portable electronics,electric vehicles,etc.However,LIBs currently have reached their theoretical capacity.There is an urgent need to develop more suitable clean energy sources.Lithium metal batteries have attracted the attention of researchers because of their high theoretical specific capacity(3860 m Ah g^-1)and the lowest electrode potential（-3.04 V with respect to hydrogen electrodes）.However,there are many problems that need to be solved before lithium metal batteries can be used in practical applications,such as the growth of lithium cathode dendrites,contamination of organic electrolyte and flammable and explosive explosions.The electrolyte is an important part of the battery,as a bridge between the positive and negative electrodes.It is not only a necessary path for lithium ion transport,but also can block the direct contact between positive and negative electrodes to avoid short circuit of the battery.The use of new organic-inorganic composite solid electrolytes instead of the traditional organic electrolyte is expected to solve the safety problems caused by electrolyte’s easy leakage and flammability.In addition,the composite solid-state electrolyte can provide higher ionic conductivity compared with polymer solid-state electrolyte,and is expected to solve the problems of poor interfacial contact of inorganic solid-state electrolyte.Based on this,polymer/inorganic composite solid-state electrolytes with ceramic nanofibers as the reinforcing phase are designed and their electrochemical performance in lithium-metal batteries are systematically investigated from the perspective of lithium-metal battery safety issues and the interface between electrolyte and electrode.（1）We prepared a polymer/ceramic nanofiber composite solid-state electrolyte（PHLL）with electrospun Li_0.33La_0.557Ti O₃（LLTO）and Li₇La₃Zr₂O₁₂（LLZO）ceramic nanofibers as a bilayer self-supporting backbone and polyvinylidene fluoride-hexafluoropropylene（PVDF-HFP）as a flexible filler.Among them,PVDF-HFP can effectively improve the interfacial contact between ceramic fibers and lithium cathode,while LLTO fibers could provide ultra-high ionic conductivity at room temperature as inorganic fast ionic conductor.Moreover,in order to prevent the side reactions between Ti⁴⁺in LLTO with lithium metal anode,a thinner LLZO nanofibrous protective layer is introduced which can further accelerate Li-ion transport.As a result,the PHLL composite solid-state electrolyte with a bilayer structure exhibits a high ionic conductivity of up to 8.89×10^-3 S cm^-1 at room temperature.Due to the efficient inter-fiber Li⁺conduction and the stable and compact electrode-electrolyte interface,the corresponding Li||Li symmetric cell achieves a stable cycle of600 h at 0.2 m A cm^-2.A wide electrochemical window of 4.8 V is also obtained when coupled with a Ni Co Mn high-voltage electrode.It was found that the cell is able to reach an initial discharge capacity of 158 m Ah g^-1 at 0.1 C,indicating that the electrolyte can be well matched to high-voltage cathodes.（2）In order to further improve the interfacial interactions between electrolyte and lithium anode,an in situ polymerization strategy is applied to prepare the composite solid electrolyte（PLL）by the homogeneous infusion of precursor solution（PEGMEA）in the bilayer self-supporting LLZO/LLTO fiber skeleton.Compared with the non-in situ polymerization method,the p（PEGMEA）obtained by in situ polymerization can further improve the interfacial compatibility between electrolyte and lithium metal,effectively reduce the interfacial resistance,and thus increase the lithium ion transport rate.The composite solid-state electrolyte has a lithium ion mobility of up to t⁺_Li=0.68 and can provide a wide electrochemical stability window over 5.4 V.The lithium iron phosphate/Li all-solid-state battery assembled using the in situ polymerization strategy is able to provide an initial discharge capacity of 103 m Ah g^-1 at 0.5 C and 60°C,and still maintains a discharge capacity of 83 m Ah g^-1 after 30 cycles with a capacity retention rate of 81%.