Research On Separator Wettability In Li Metal Batteries

Lithium(Li)metal battery has higher energy density than lithium ion battery.However,the Li dendrite growth on Li metal anode limits the commercialization of Li metal battery.Separator wettability toward electrolyte is vital to electrochemical performance of Li metal batteries,especially the Li dendrite growth.On the one hand,separator with good wettability is favorable to Li ion transportation,which contributes to achieve low battery internal resistance.On the other hand,good separator wettability is beneficial to the well-distributed of Li ions flux over Li metal anode and suppresses the dendrite growth.This thesis focuses on the research of separator wettability toward electrolyte in Li metal battery.Firstly,the commonly used polyolefin separator and nonaqueous electrolyte were used to study the factors that affect separator wettability.Secondly,various electrolyte wetting additives were examined to enhance separator wettability toward electrolyte,including F-EAE,F-EPE,trace of water,and P123.Except for this,the impacts of these additives on battery electrochemical performance were also studied.Lastly,we studied the effect of additives mentioned above on Li dendrite suppression in Li metal battery.In the first chapter,a general introduction is given on following aspects: 1)Li metal battery and recent research progresses.2)Separator wettability toward electrolyte and the commonly used methods for its enhancement have been reviewed.In the second chapter,a brief introduction to the chemicals,methods and equipment used in the thesis were given.In the third chapter,1)carbonates organic solvents were used to investigate the solvent effect on separator wettability.The results show that solvent with high viscosity and high dielectric constant are more inclined to poor separator wettability.2)Li salts with different anion molecular structure were used to investigate the Li salts effect on separator wettability.The results show that LiTFSI prefer to achieve good separator wettability toward electrolyte(especially in high viscosity electrolyte)due to the non-polar group(-CF3)which contribute to low electrolyte surface tension.3)The research on separator wettability along with varies of Li salts concentration indicates electrolyte with high concentration Li salts lead to poor separator wettability.The main reason could attribute to the increase of electrolyte viscosity.In the fourth chapter,we developed three kinds of electrolyte additives to enhance separator wettability including(F-EAE and F-EPE),trace of water and P123.Except for this,the impacts of these additives on battery ratability and cycle ability were also studied.The results show that: 1)the effect of F-EAE and F-EPE get optimum when the additive content is 2% and 5% respectively.The separator electrolyte uptake has increased from 30% to 110% and 105% respectively.The discharge capacity in LiFePO4||Li at 10 C has increased from 40 mAh g-1 to 110 mAh g-1 when 0.2% F-EAE is added in the basic electrolyte.2)After adding trace of water(200ppm)in the electrolyte for 48 hours,the electrolyte uptake of separator has increased from 30% to 100% while the discharge capacity of LiFePO4||Li at 10 C has increased from 96 mAh g-1 to 115 mAh g-1.3)The electrolyte uptake of separator has increased from 30% to 85% when 0.2% P123 has been added in basic electrolyte,and the conductivity of separator increased from 0.045 mS cm-1 to 0.50 mS cm-1?In chapter five,the positive impact of the enhanced separator wettability on Li dendrite suppression was studied by using Cu||Li sells based on LiPF6-LiTFSI electrolyte.The results show that the Li deposition micro-morphology evolved from needle-like to island type along with the increase of separator wettability.Except for this,the effects of P123 on dendrite suppression were also studied in this chapter.The results indicate 0.2% P123 can suppress the Li dendrite most effectively.CV and XPS tests show that P123 can absorb on the electrode surface strongly and served as “artificial SEI” to protect Li metal from dendrite and interface reaction.In the last chapter,we give an overview on the achievement and the deficiency in this dissertation,and some suggestions and possible directions of future work are also pointed out.