Research On The External Characteristic And Mechanism Of The Lithium-sulfur Batteries

Lithium-sulfur battery is currently reported to be the highest specific energy in the secondary battery systems which use solid materials as the active substance.However,the problems of fast capacity fade,low coulombic efficiency and high self-discharge rate caused by the polysulfide shuttle effect limit the practical applications of lithium-sulfur battery.In this paper,the external characteristics such as self-discharge characteristics,thermal characteristics and resistance characteristics and its rules of lithium-sulfur pouch cell are tested and analyzed by controlling various conditions such as depth of discharge(DOD),discharge rate,temperature and idling time.Further more,the change of the conductivity of the electrolyte in the cell cycle was analyzed by the in-situ four electrode method and the related mechanism of lithium-sulfur battery are analyzed.The experimental results show that the self-discharge characteristics of lithium-sulfur battery are closely related to the electrochemical reaction mechanism and the process products of lithium-sulfur battery.There is no self discharge behavior of a lithium ion battery which is not recycled because it has no process product,lithium sulfur battery has different self-discharge characteristics under different depth of discharge.In the depth of discharge(DOD)is 0%,battery has the maximum self discharge.With the increase of depth of discharge,the self discharge rate decreases until DOD 30% where have the highest concentration in the electrolyte polysulfides,then in the low voltage platform self discharge is relatively stable,the corresponding transformation in sulfide to lithium sulfide,and at the end of the discharge has the minimum self discharge.In addition,the self-discharge has a positive correlation with temperature and idling time.Therefore,it is necessary to store the lithium sulfur batteries that have been cycled at the end of the discharge and low temperature environment.The lithium sulfur battery charge and discharge heat production research results show follow: In the high current charge discharge,at different positions of the surface will produce temperature change in different degree,in the upper part of the battery close to the pole ear has the maximum temperature rise.It is found that the maximum temperature rise of the battery increases with the increase of the number of cycles,the decrease of the ambient temperature and the increase of the charge and discharge current.It was found in one cycle,in early stage cycle,the discharge temperature rise is larger,DOD 30% stage is the most prone to the maximum temperature rise,with the number of cycles increased,the charging temperature is higher than the discharge temperature,at the end of charging have a maximum temperature rise.In a certain temperature range,the higher temperature has a positive effect on the thermal stability of the battery.With the increase of temperature,the polarization of the battery is small,the maximum temperature rise is more stable in charging.The resistance characteristics of lithium sulfur battery were tested by step method and impedance method.It is found that the variation law of the internal resistance is in good agreement with the variation law of the surface temperature.The increase or decrease of the internal resistance directly affects the temperature of the battery.It is found by in-situ four electrode method that with the depth of discharge increases,the battery internal impedance of the electrolyte increase untill 30% DOD reached the highest value,which corresponds to the generation of high valence polysulfides and then transformed into low valence polysulfide,the polysulfide concentration increases and reaches the highest value.The impedance decreases as the discharge continues,and the concentration of the soluble polysulfide decreases with the transformation of the low valent polysulfide to Li2S2 and Li2 S.With the increasing number of the cycles the impedance increases at the same DOD.The electrolyte resistance is related to the amount of solvent and solute concentration.Therefore,the author believes that the electrolyte solvent is consumed in the process of cycling,and the irreversible dissolution of polysulfide leads to the increase of the electrolyte impedance.