Rational Design Of High-Performance Separators And The Application In Lithium-Ion Batteries

Lithium ion battery has become one of the most widely used energy storage devices due to its high specific energy and high voltage.However,the high power density,energy density and high safety peroformnce of power batteries are urgently required for the rapid development of electric vehicles.As an important part of the battery,the separator is crucial to electrochemical and safety performance of the powe battery.Commercial microporous separators are used widely for their outstanding electrochemical stability and high mechanical strength.However,the poor affinity to electrolyte,poor heat resistance,low porosity and uneven pore size distribution of separator limit their application in high-power and high-safty lithium ion battery for vehicles.Aiming at the above problems,this thesis focused on the design of new lithium ion battery separator and investigated the influence of the composition and structure of the separator on the electrochemical performance of lithium ion battery.The main work is as follows:First,the CP-PVDF composite membrane was prepared via vacuum filtration technology by polyvinylidene fluoride(PVDF)solution filling micron channel in cellulose.The structure characterization showed that the CP-PVDF composite membrane had a uniform nanometer pore structure and abundant oxygen-containing groups,which enhanced the wettability and conductivity of the separator.Electrochemical analysis showed that LiFePO4/Li cells assembled with CP-PVDF separator exhibited dramatic rate performance with high discharge capacity of～113.8 mAh g-1 at 5C and maintained 100.3 mAh g-1 after 500 long cycles.Secondly,a polydopamine(PDA)modified cellulose paper(CP)separator has been prepared by a simple and efficient soaking process.The PDA-CP separator exhibited a nano-sized pore structure,excellent electrolyte storage capacity(320 wt.%),electrical conductivity(1.29 mS cm-1),and excellent thermal stability(300℃).The assembled Li/PDA-CP/Li battery demonstrated a stable voltage within 500 cycles(3000 h)at a current density of 0.5 mA cm-2,which shows the stability of the repeated electrodeposition of Li and the LiFePO4/PDA-CP/Li battery exhibited a high reversible discharge capacity of 110 mAh g-1 at a high rate of 5C with a negligible capacity loss in 600 cycles,which was much higher than that of the commercial separator(80 mAh g-1).Thirdly,a in-situ modifying PVDF-MOF composite separator was prepared by a phase conversion method.The microscopic morphology shows that the porous MOFs particles grow uniformly and orderly on both sides of PVDF membrane.Such uniform nanopores were favorable for electrolyte infiltration(300 wt.%)and ionic conductance(1.2 mS cm-1)of the separator.As a result,the assembled LiFePO4/PVDF-MOF/Li battery exhibits a high capacity of 110 mAh g-1 under 5C and maintains 102.5 mAh g-after 400 cycles.In addition,low interface resistances were obtained during the repeated cycles,indicating good interface compatibility,which ensures the stable cycle stability and high power performance of the battery.In summary,this work aimed at the rational design of high-performance separators with high electrolyte affinity and thermal stability,and it is an impotant direction to develop the cheap and high-security cellulose separator for the high-performance lithium-ion battery in the future.Therefore,this work have provided some new ideas and theoretical support for the development of high-performance separators.