Research On The Modification Of Low Temperature Electrolyte For Primary Lithium/Fluorinated Carbon Batteries

Lithium/fluorinated carbon（Li/CF_x）primary battery possesses the advantage of high theoretical specific capacity（865 mAh/g）,high energy density（2180 Wh/Kg）,low self-discharge rate,stable voltage platform.It is widely used in the civil and military fields with excellent development prospects.However,the battery performance of discharge capacity and voltage decrease sharply at low temperature.As the important medium for electrons and ions transport,electrolyte is one of the key factors for determining the low temperature performance of battery.On the one hand,the viscosity of the electrolyte increase when temperature drops below 0℃,resulting in the decrease of conductivity obviously;On the other hand,the kinetic of chemical and electrochemical reactions become slower owing to the increased charge transfer resistance and the decreased Li⁺diffusion coefficient,which affects the low-temperature performance of battery.At present,the LiBF₄ PC/DME electrolyte is commonly used in Li/CF_x primary battery.However,the poor performance of Li/CF_xis due to the high viscosity of PC solvent and the low dielectric constant of DME solvent.In this work,based on the solvent and additives of the electrolyte,the low temperature performance of the LiBF₄ PC/DME is studied.Satisfiying performance can be obtained in the low temperature range by reducing the viscosity of the electrolyte,broadening the electrochemical window,improving the conductivity and reducing the electrochemical impedance.The main research contents of this paper are as follows:（1）Appropriate addition of SN（Succinonitrile）to LiBF₄ PC/DME electrolyte can improve the battery electrochemical at low temperature.The wettability between electrolyte and diaphragm has been improved,which is attributed to the high conductivity,low viscosity,high solubility of SN.The battery with 10%SN addition amount shows the discharge capacity of 571.5 mAh/g,486 mAh/g and 260 mAh/g under 0.5C current density at 0℃,-10℃and-20℃respectively,which is higher than the battery without the electrolyte additive.However,the improvement for the wettability and viscosity of electrolyte with the addition of SN is limited,increasing the low temperature performance of Li/CF_x primary battery still exsist a wide space.（2）A variety of solvents are selected to synthesis electrolyte with different components.Among these electrolytes,the battery with LiBF₄ DME/GBL electrolyte reveal a discharge capacity of 717.6 mAh/g and 383.1 mAh/g under 0.5 C current density at 0℃and-20℃,respectively.The corresponding discharge plateau are about2.3 V and 2.0 V.Additionally,according to the results of GITT,Li/CF_x primary battery based on LiBF₄ DME/GBL exhibits higher Li⁺diffusion,ensuring the fast reaction kinetics at low temperature and confirming the good low-temperature performance.（3）The influence of different additives on the LiBF₄ DME/GBL electrolyte is explored at low temperature.The introduction of 3%ADN reduces the viscosity of electrolyte,increases the conductivity,broadens the electrochemical window,which effectively improves the rate performance of Li/CF_x primary battery at low temperature.At the rate of 0.5 C and 1 C,the discharge capacity increased from 383.1 mAh/g,282.4mAh/g to 489.5 mAh/g,345.2 mAh/g at-20℃.At the same time,the fast Li⁺diffusion coefficient across the interphase and the electrode facilitates the operation of batteries at ultra-low temperature.However,the discharge capacity and voltage decreased with less electrolyte quantity,which attributed to the lack of infiltration between the electrolyte,the diaphragm and electrodes.