Modification And Electrochemical Performance Of PVDF-based Composite Polymer Electrolyte

Solid polymer electrolytes have great potentials to overcome safety issues of organic liquid electrolytes for lithium-ion batteries?LIBs?.However,the low ionic conductivity of solid polymers(<10^-5 S cm^-1)impedes the broad applications.In this thesis,two special one-dimensional inorganic fillers were respectively developed to optimize the physical,chemical,mechanical and electrochemical properties of the polyvinylidene fluoride?PVDF?-based membrane.The first one-dimensional inorganic filler is an oxygen-ion conductor Sm_0.2Ce_0.8O_1.9?SDC?,and the second one is earth-abundant halloysite nanotubes?HNTs?.In the first work,composite polymer electrolytes?CPEs?were prepared by dispersing oxygen-ion conducting SDC nanowires into the PVDF matrix.The ionic conductivity and the lithium-ion transfer number were greatly enhanced after the introduction of the SDC nanowires to CPEs,thanks to the increase of amorphous regions resulted from the SDC filler and the Lewis acid-base interaction due to the positively charged oxygen vacancies and partial dehydrofluorination.Specifically,CPE-10 with 10 wt%of the SDC nanowires reached the ionic conductivity of 9.09×10^-5 S cm^-1 at 30°C with a lithium-ion transfer number of?0.40.Also,CPE-10 exhibited a relatively high oxidation potential of 4.89 V versus Li⁺/Li.Impressively,a quasi-solid-state CR2032 Li Fe PO₄/CPE-10-20?L/Li battery exhibited the initial capacity of as high as 155.1 m Ah g^-1 at 1 C,and it still delivered 155.3 m Ah g^-1 after 130 cycles,as well as the favorable rate performance(e.g.,135.1 m Ah g^-1 at 4 C).Besides,a lithium-oxygen battery?Super P/CPE-10-5 wt%Li I/Li?exhibited the initial discharge capacity of 5325m Ah g^-1_{carb on}with the coulombic efficiency of 85.7%.Our work demonstrated that CPEs with the oxygen-ion conducting SDC nanowires could be potentially applied in quasi-solid-state LIBs and lithium-oxygen batteries.In the second work,we employed natural HNTs and PVDF to fabricate composite polymer electrolytes.CPE-5?HNTs 5 wt.%?showed an ionic conductivity of?3.50×10^-4 S cm^-1,which was?10 times higher than the CPE-0?without the addition of HNTs?at30°C.The greatly increased ionic conductivity was attributed to the negatively-charged outer surface and a high specific surface area of HNTs,which facilitated the migration of Li⁺in PVDF.To make a further illustration,a solid-state lithium-ion battery with CPE-5 electrolyte,Li Mn₂O₄ cathode and Li metal anode was fabricated.An initial discharge capacity of?71.9 m Ah g^-1at 30°C in 1C was obtained,and after 250 cycles,the capacity of 73.5 m Ah g^-1 was still maintained.This study suggested that a composite polymer electrolyte with high conductivity can be realized by introducing natural HNTs,and can be potentially applied in solid-state lithium-ion batteries.