In Situ Tem Studying Of The Li Storage Mechanism And Mechanical Behaviors Of Anode Materials For Lithium Ion Batteries

The high-energy anode component in Li ion batteries provide an excellent example to study the intimate coupling between mechanics and electrochemistry.The electrochemical process of high concentrated Li⁺insertion and extraction induces such rich phenomena of elemental mechanics as large elasto-plastic deformation,cavitation,reactive plasticity,and corrosive fracture.Meanwhile,mechanical stresses modulate the thermodynamics and kinetics of ionic transport,surface charge transfer,interfacial reaction,and phase transition of materials.These mechanics and electrochemistry changes could substantially influence ion and electron transport and affect the performance of the entire battery system.And thus,it is urgent to in situ,real-time,and dynamically understand the structure–property relationships for high capacity anode materials during electrochemical reaction processes.In this dissertation,equipped with in situ transmission electron microscopy-scanning tunnel microscopy（STM-TEM）technique,we have real-time systematically investigated the dynamic structural evolution and mechanical behavior of distinguished anodes,like metal oxide（α-MoO₃,Co₃O₄）,layered transition metal chalcogenides（MoS₂）,and element Se electrode during the lithiation–delithiation cycles.And combined with the simulation,the corresponding（de）lithiation and mechanical characters were uncovered and proposed.The main achievements are summarized as following:1.Metal oxides hold the promise of high-capacity anodes for Li-ion batteries.1)We find that these metal oxides likeα-MoO₃ and Co₃O₄ show the general characters of structural irreversablilty in the lithiation-deliathiation process,which result in the low columbic efficiency.2)Furthermore,we also find some individual（de）lithiation characters.We characterize the two-step lithiation behavior ofα-MoO₃:namely,intercalation accompanied with kinetically fast/a minor volumetric change,and the conversion reaction with kinetically slow/large deformation.Furthermore,instead of showing significant Li-embrittlement as seen in typical oxides,the lithiated MoO₃products still show large plascity,even approaching 11%bending strain.Moreover,as for Co₃O₄ anode,the delithiation induced incomplete reaction and structural collapse under higher bias can be attributed to the capacity loss at higher current density.These results give deep understanding of the Li storage mechanism in metal oxide anodes,and provide a guideline for achieving metal oxide anode based high-performance LIBs.2.MoS₂ has received considerable interest for electrochemical energy storage and conversion.Aim at achieving Li storage mechanism,we have investigated the deep lithiation process of MoS₂ anode that occurs along[001]direction by in situ TEM.Impressively,we find a novel conversion mechanism of MoS₂ anode that Li ions induce structural destruction following a dynamic layer-by-layer dissociation with Mo/Li₂S composites left,rather than a multistep phase transformation in bulk.The first-principles computations verify that the surfacial relaxation of Li₂S to form an anti-fluorite structure on higher electric conductive Li_x MoS₂ surface is the primarily thermodynamic driving force for activating layer-by-layer conversion reaction.The interface between the Li₂S and Mo surface become metallic due to the widely dispersive free electrons unveiled by electron local function（ELF）,which would overcome the above kinetic issues to facilitate next layer S conversion.Moreover,based on the layer-by-layer conversion mechanism derived from the in situ TEM observation,we design the assembled MoS₂ sheets with large surfaces supported on highly conductive tubular graphene as a composite anode,which preserving excellent performance with rate and cyclic capability.These results are envisaged to be helpful for designing durable conversion-type MoS₂ anodes by surface engineering,and further points out a new protocol for graphene based hybrid anode for enhanced lithium-ion batteries.3.Li-Se battery,analogous to Li-S battery presever the merits of high energy density and better rate capability.Herein,We have in situ investigated the type of lithium transportation,structural evolution,and coupling mechanical behavior of carbon conformably coated Se nanowire（NW）cathode reacted with Li.Intriguingly,We find a unique lithiation mechanism that the“leapfrog phase transformation”occurs at interface between carbon coating and Se NW cathode.More importantly,interface between coating and Se is the fastest and primary Li iond transport channel in the lithiation process,which uniquely differs from the surface-coating directed Li transportation engineered wherein Li ions initially diffuse into coatings and then react with core materials of electrodes;The lithiaition-delithiation cycles demonstrate the structural reversibility;Furthermore,we note a threshold diameter region of Se NWs with¹15-120 nm.These observations unveiled a new interfacial lithiation behavior and enriched the electrochemical reaction mechanism for core-shell electrode materials.