Thermal Runaway Mechanism Of Advanced Battery Systems And High Safety Battery Design

The rapid development of lithium-ion battery technologies pushed the increases of the markets of electric vehicles and energy storages.However,the constantly occured fire and combustion accidents,which caused by battery thermal runaway,severely hinders the further development of these aeras.Moreover,with the battery energy density increases,the safety issue could be more serious.All those issues make the battery safety chanllenge become imminent.It was found that safety accidents tend to occur in those batteries which introduced new matearials or new technologies,especially at early stage of the new technology applications,such as the fires accidents related to high-nickel ternary battery in recent years and lithium metal battery fires in the 1990s.For those novel battery systems,investigations and tests of the safety features of those batteries in their whole life-cycle is needed to obtained the comprehensive understandings about their safety features.Then the battery safety precaution,prediction,and early warning could be realized.However,currently,the major role of battery safety researches is the battery which has been commercialized already.The safety investigations about the novel battery materials and systems,such as silicon anode,lithium metal anode,and solid-state batteries,are inogred to some extent.While only the safety characteristics of those batteries are intensively investigated before mass production,the safety control and prevention of battery products can be achieved.This is why the safety investigations of the next generation batteries should be initiated as soon as possible.Here,this thesis studies the key reactions in the thermal runaway process of silicon oxide anode system,lithium metal anode system and all solid state battery system,and gives the potential thermal runaway reaction chain of those novel battery systems.Specifically,first,we studied the thermal runaway characteristics of high-nickel/Si O pouch cells during their whole life cycle.It was found that,on the contrary of graphite anode-based battery system,the thermal runaway of this battery system was significantly accelerated after cycling to the end of the life,while the safety of the graphite battery could be much improved after the cycle.Combined with electrochemical analysis,thermal behavior analysis,3D X-ray tomography,and inductive coupled plasma emission spectrometer（ICP）testing,we found for the first time that the electrochemical-thermal runaway coupling failure mechanism existed in the NMC/Si O battery system:the repeated expansion and contraction of the Si O anode lead to the particle aggregation and the battery polarization,which raises the intercalation potential of Si O anode and results to excessive de-lithiation in the positive electrode under the condition of unchanged charging strategy.The over delithiatied cathode delivers inferior thermal stability and finally leads to the accelerated thermal runaway of the battery after aging.Secondly,we studied the thermal stability of lithium metal deposited in different electrolytes and electrolytes.The main factors affecting the lithium/electrolyte thermal stability were found:thermodynamically,the solvent of different electrolytes has different reactivity to lithium metal;Kinetics,on the one hand,the composition and coverage of SEI will affect the initial temperature of reaction between lithium metal and electrolyte.On the other hand,the specific surface area of deposition morphology of lithium metal will directly affect the heat release of reaction between lithium metal and electrolyte.Thirdly,for all-solid-state batteries,the thermal stability of four mainstream oxide solid electrolytes（SE）and lithium metal was quantitatively studied by adiabatic accelerated calorimeter for the first time.It was found that Li_1.4Al_0.4Ti_1.6（PO₄）₃（LATP）and Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）solid electrolytes,which is reactive with lithium metal,all experienced violent thermal runaway during the test.No thermal runaway happened for Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZO）garnet solid electrolyte,which is stable to lithium metal chemically,when contacted with metallic lithium.Combining First principles calculation and XRD refinement analysis,the multi-step thermal runaway mechanism between metallic lithium and solid electrolytes are reported:as the temperature rises,Li/SE interfacical reactions happened,and then inducing solid electrolyte interface reaction local oxygen activation,the acviated oxygen species reacted with metallic lithium,and finaly lead to Li/SE interfacial thermal runaway.In addition,we synthesized SE ceramic pellet toinvestiagte the thermal stability of solid electrolyte at the quasi-practical situation.Combined with X-ray computerized tomography（CT）and in situ electrochemical impedance spectra（EIS）test,we found that lithium metal can penetrate to the inside solid electrolyte under high temperature,and the higher activity of metallic lithium on defects sites inside the ceramic pellet（which displays higher reaction activity）will exacerbate interface thermal runaway.Based on the above mechanism,we introduced the low melting point Li PO₂F₂ as sintering additive into the ceramic during sintering,and effectively improve the thermal stability of the reaction between ceramic pellet and lithium metal.Finally,based on the safety investigations on different battery systems,the evolution diagram of the key reaction evolution of battery thermal runaway with the change of the battery materials and systems.The many-to-many relationship between the exothermic reactions and the improved methods in different battery systems are given.It is found that the oxygen generation is the common safety issue in all the high energy density batteries.Therefore,a facial and effective cathode thermal stability improve method is proposed and vertified on practical sized pouch cells.The lithium transferring behaviors between SE and cathode materials under high temperature are reported.It is found that the LLZO solid electrolyte can supply lithium ion to charged state cathode and improve its thermal stability.The full pouch cells with layered cathode and graphite anode by 1 wt.%LLZO addition on cathode side dilivers significantly enhanced thermal safety.These research results in this thesis indicates that the thermal runaway characteristics of these new battery system are much different with the traditional batteries,the underlined mechanism on material level are investigated,and the safety improvement method are proposed.These results will effectively promote the theoretical understanding of the safety of the new battery system,and provides guidance for the large-scale application of advanced battery system with high energy density.