In Situ Transmission Electron Microscopy Study On Failure And Reaction Mechanisms Of Electrode Materials For Lithium/Sodium Metal Batteries

Owing to the high specific capacity and the low electrode potential,lithium/sodium（Li/Na）metal anode can significantly improve the energy density of present Li/Na-ion battery system,so Li/Na metal batteries are promising candidates for a new generation of high energy density energy storage devices.However,there are still many problems in the development of Li/Na metal batteries.Although lithium is inert to oxygen at room temperature,it may react with oxygen released from the cathode of lithium metal batteries（LMBs）at elevated temperatures to cause thermal runaway of LMBs,the microscopic mechanism of the reaction between lithium and oxygen at high temperatures remains unknown.During electrochemical cycling in sodium metal battery,structural degradation of layered cathode materials induces fast capacity fading and thus severely limits the lifetime of batteries.A major root cause of the structural degradation is the accumulation of electrochemically-induced extended defects such as dislocations and cracks,which tend to cause much more severe damage than the commonly studied point defects.However,the origins of formation and evolution of these extended defects with cycles remain largely unexplored to date.High temperature Na-S batteries have been commercially used for large-scale energy storage and conversion applications.The reported high temperature Na-S batteries failed to achieve one-third of the theoretical capacity.Formation of polysulfides is speculated to be the culprit for the low energy density of high temperature Na-S batteries.Direct imaging of the polysulfide evolution of high temperature Na-S batteries has not been possible due to technical difficulties,and the electrochemical behavior of sodium polysulfides in high temperature Na-S batteries remains unclear.Transmission electron microscope（TEM）is one of the most powerful techniques for nano or atomic scale structural characterization.In this thesis,in situ TEM was used to study the high temperature oxidation reaction of lithium dendrites generated from lithium anodes in LMBs,and also to study the degradation mechanism of layered oxide cathodes in sodium metal batteries and the reaction mechanism of sulfur cathode at different temperatures.The specific research is as follows:We investigated the oxidation mechanism of lithium dendrites at high temperature in real time using in situ TEM.We show that lithium exhibits interesting oxidation chemistry at high temperatures.Namely,it features the formation of Li₂O thin film around the surface of lithium dendrite at temperatures lower than 160℃governed by the Wagner oxidation mechanism,which is essentially controlled by the diffusion of lithium.However,the oxidation product is nanosheet Li OH when the temperature is between 160 and 200℃,which turns into Li₂O nanocubes at temperatures higher than 300?C.Density functional theory（DFT）calculations reveal that these oxidation mechanisms are governed by the thermodynamics and kinetics of the interactions between O₂ or H₂O and Li₂O at high temperatures.These results provide important understanding to the microscopic oxidation mechanism of Li at elevated temperatures,which sheds lights on the thermal runaway of LMBs.We investigated the degradation mechanism of P2-type Na_0.7Ni_0.3Mn_0.6Co_0.1O₂（Na-NMC）layered cathode materials.Kinks form in the delaminated layers due to severe local bending,and each kink consists of a vertical array of dislocations,resulting from easy slip between transition metal oxide layers.In situ mechanical compression experiments directly reveal the kink formation due to strong mechanical anisotropy parallel and perpendicular to the intercalation layers in single-crystal Na-NMC.In situ electrochemical experiments indicate that kinks form during the desodiation process.Our results unveil a new mechanism of electrochemically-induced mechanical degradation stemming from weak interlayer bonding in layered cathode materials.This work has broad implications for mitigation of degradation associated with irreversible interlayer slip in layered cathode materials.We built microscopic nanobatteries with sulfur and metallic sodium as cathode and anode electrodes using a microelectromechanical system（MEMS）heating device in TEM and performed electrochemical reaction mechanism studies.The formation and evolution of transient polysulfides during cycling are revealed in real-time.Upon discharge,sulfur transforms to long-chain polysulfides,short-chain polysulfides,and finally Na₂S or its mixture with polysulfides,and the process is reversible during charge at high temperatures.Surprisingly,by introducing nanovoids into the sulfur cathode to buffer the large volume change thus preserving the integrity of the electronic/ionic pathways and reducing the diffusion distance of Na⁺ions,the sulfur cathode is fully discharged to Na₂S rather than the conventionally observed Na₂S₂ at 300℃.Moreover,the electrochemical reaction is swift and highly reversible.The in situ studies provide not only new understanding to the polysulfide electrochemistry but also critical strategies to boost the capacity and cyclability of high temperature Na-S batteries for large-scale energy storage applications.