Investigations Of The Thermal Runaway Mechanisms And The Corresponding Solutions Of Lithium-Metal Batteries

As the energy crisis and global warming become more and more serious,the traditional energy structure must be transformed and the renewable energy sources such as wind,tidal,and solar power should be developed vigorously.However,these energy sources are intermittent,random,and fluctuating,leading to the urgency of designing novel energy storage devices with high energy density and excellent thermal safety.Unfortunately,the commercial lithium-ion batteries with graphite anode have almost reached their theoretical energy density without meeting the market demand.In comparison,lithium-metal batteries（LMBs）are regarded as one of the most promising next-generation batteries due to the lithium（Li）metal anode with the high theoretical energy density(3860 m Ah g^-1)and the lowest potential（-3.040 V vs.the standard hydrogen electrode）.However,until now,only the LIBs have been widely used in the energy storage system though the first rise of LMBs is even twenty years earlier than LIBs.LMBs are facing serious safety problems such as fire and explosion etc,resulting in the stagnation of the development of LMBs for a long time.Therefore,it is essential to identify the influence factors on the thermal safety of LMBs and propose the suitable strategies to inhibit or alleviate the thermal runaway of LMBs.In this paper,the key factors affecting the thermal safety of cycled lithium–sulfur batteries（Li–S）are systematically investigated.In addition,the relationship between Li powder caused by the generation of Li dendrites during the cycling processes and the thermal safety of LMBs is quantitatively demonstrated.Finally,the electrode-electrolyte interface is successfully modified to improve the thermal safety of LMBs.The specific research contents and results are as follows:（1）Considering the huge application prospect of Li–S batteries in the field of energy storage in the future,the key factors related to the thermal safety of cycled Li–S batteries are identified.It is discovered that the cycled Li–S batteries（16 cycles and 45 cycles）are thermally stable because of the high viscosity of the cycled electrolyte though a large amount of Li dendrites and dead Li are involved in these batteries.In addition,the thermal runaway does not occur for batteries after 45 cycles despite the viscosity of electrolyte decreases a lot because of the addition of the fresh electrolyte,which means that the fresh electrolyte is not harmful to the thermal safety of Li–S batteries.In contrast,,the Li–S batteries after 16 cycles with additional fresh electrolyte serve serious thermal runaway at 147.9^oC,demonstrating that the thermal safety of Li–S batteries is closely related to the viscosity of electrolyte and the species contained in it.The higher-order polysulfides(Li₂S_x≥6)are founded in Li–S batteries after 16 cycles by inductively coupled plasma spectra while the lower-order polysulfides(Li₂S_x≤4)exist in batteries after 45 cycles.In addition,differential scanning calorimeter（DSC）results reveal that Li metal is thermally stable with Li₂S_x≤4 while it reacts violently with Li₂S_x≥6 at 153^oC.As a result,it can be carried out that the electrolyte viscosity has a positive effect on the thermal safety of LMBs and the strong exothermic reactions between Li metal and Li₂S_x≥6dissolving in cycled electrolyte drive the thermal runaway of cycled Li–S batteries（2）The negative effect of Li powder caused by the generation of Li dendrites during the cycling processes on the thermal safety of LMBs is demonstrated quantitatively.Considering the key function of the thermal stability between Li metal and electrolyte on the thermal safety of LMBs,the thermal behaviors between Li deposition with different specific surface areas and electrolyte were investigated by DSC.The results figure out the positive relationship between the specific surface areas of Li metal and the exothermic reactions at electrode-electrolyte interface.In detail,the initial exothermic peak of the reactions between Li foil and electrolyte locates at 194^oC while that temperature between Li deposits obtained at a current density of1.59 m A cm^-2and electrolyte is just 142^oC.In addition,it is discovered by accelerating rate calorimeter that the thermal runaway temperature（T₂）of pouch cell with Li deposits is 111^oC,which is 100^oC lower than the cell with Li foil.These results reveal the importance of promoting uniform plating/stripping of Li anode during the charge/discharge processes of LMBs from the view of thermal safety.Furthermore,the thermal behaviors between Li deposits and electrolytes with different ratios of solvents were further investigated.The composition of electrolyte is discovered to be closely related to the thermal safety of LMBs,which provides a novel idea for designing electrolyte from the thermal safety view.（3）The thermal safety modifier can increase the electrolyte viscosity at high temperature,thereby alleviating the exothermic reactions in LMBs and improving the thermal safety of LMBs effectively.Polyethylene glycol（PEG）with low melting point and high viscosity is miscible with electrolyte;therefore,it is chosen as the thermal safety modifier.With the increase of electrolyte viscosity at high temperature,the charge transfer and solvent diffusion at electrode-electrolyte interface are poorer,leading to the improved thermal compatibility between electrodes and electrolyte.As a result,the exothermic peaks of the reactions of battery components（cathode+anode+electrolyte）locate at 144^oC and 263^oC with a total heat release of 26.44 W g^-1 while these for battery components with the addition of PEG are 187^oC,298^oC and 10.15 W g^-1.（4）Apart from the thermal safety modifier with core-shell structure,thermoresponsive electrolyte including vinyl carbonate（VC）+azodiisobutyronitrile（AIBN）keeps liquid at room temperature and polymerizes into gel state at high temperature,thereby balancing the cycle performance and thermal safety of batteries.On the one hand,the solid-electrolyte interface and cathode-electrolyte interface obtained in thermoresponsive electrolyte include plenty of poly（VC）,which are more thermally stable with lithium hexafluorophosphate than lithium carbonate and lithium oxide.Therefore,the self-heating temperature of batteries with thermoresponsive electrolyte is 65.9^oC higher than that with routine electrolyte.On the other hand,VC is induced into poly（VC）at high temperature with the assistance of AIBN initiator.Then,poly（VC）can not only serve as a barrier to prevent the direct contact between anode and cathode,but limit the free movement of liquid solvents,thereby alleviating the occurrence of exothermic reactions at the electrode-electrolyte interface.As a result,the T₂and internal short-circuit temperature of batteries with thermoresponsive electrolyte are 203.6^oC and 176.5^oC respectively,which are much larger than these with routine electrolyte（100.3^oC and 126.3^oC）.