Multi-scale Structure Of High-capacity Electrode Materials For Lithium-ion Batteries

With the rapid development of consumer electronics,electric vehicles,and largescale energy storage,the demand of high-energy-density lithium-ion batteries（LIBs）becomes more and more urgent.Pursuing higher capacity electrode materials is of great significance for further improving the energy density of LIBs.In-depth understanding of the multi-scale structure in the electrode materials and its evolution mechanism during the electrochemical reaction processes,is crucial for the design and optimization of high-capacity electrode materials,which is also conducive to the overall battery performance.In this thesis,the high-capacity electrode materials（such as cationdisorder cathode materials,silicon oxide anode materials,and high-voltage layered cathode materials,etc.）are taken as research models.Combined with advanced characterization techniques such as pair distribution function（PDF）and high resolution transmission electron microscopy（HR-TEM）,the local short-range ordering structure,nano-cluster structure,and particle structure of the high-capacity electrode materials are investigated at multiple length scales from the atomic,nano to micro scale.At the atomic scale,the short-range ordering structure in the novel cation-disorder cathode materials and its effect on the lithium-ion conduction networks are studied by employing Li_1.2Ti_0.35Ni_0.35Nb_0.1O_1.8F_0.2（LTNNOF）as a model material.The Li-ion diffusion channels and three-dimensional Li-ion conduction networks in LTNNOF is quantitatively analyzed by neutron pair distribution function（PDF）combined with reverse Monte Carlo method（RMC）modelling.Quantitative statistical analysis of the different types of Li-ion diffusion channels is conducted to explore the effect of TM’s types on the proportion distribution of Li-ion diffusion channels.By comparing the Liion conduction network models with the actual capacity of LTNNOF cathode materials,it is found that the Li-ion conduction network in LTNNOF is mainly composed of 0-TM diffusion channels and some extended 1-TM diffusion channels.These results demonstrate the effectiveness of the supercell-scale structural analysis method for the precise study of short-range ordering structure in cation-disordered cathode materials,and the important role of short-range ordering structure regulation in the design of high performance cation-disordered cathode materials.At the nano scale,the structural evolution of nano-Si clusters in Si O anode materials during electrochemical cycling is explored by X-ray pair distribution function（PDF）analysis based on high-energy synchrotron.The size growth and fracture behavior of nano-Si clusters in Si O upon long-term cycling are detected by ex-situ PDF combined with high-resolution transmission electron microscopy（HR-TEM）.The Si O anode materials of nano-Si clusters with different initial sizes are applied as model materials.The size evolution of nano-Si clusters in Si O is closely related to its initial size and affects the reversible capacity and cycling stability of Si O anodes.The results show that by adjusting the initial size of nano-Si clusters,the cluster agglomeration along with size growth and the subsequent fracture process of nano-Si clusters in Si O anode materials can be retarded during the long-term cycling,which is beneficial to improve the electrochemical cycling performance of Si O anode materials.At the micro scale,we take the conventional layered oxide cathode material as Li Co O₂ as the model material and investigate the effect of solid oxide electrolyte Li_6.5La₃Zr_1.5Ta_0.5O₁₂（LLZTO）on the surface structure stability of charged Li Co O₂ particles.It’s found that LLZTO can provide Li-ions for the delithiated oxide cathode at high temperatures on the Li Co O₂ particle surface.This re-lithiation process increases the Li content in the surface region of Li Co O₂ at highly charged state,which significantly delays the structural decomposition and oxygen release,improving the thermal stability of charged Li Co O₂.Using this advantage of LLZTO,adding a small amount of LLZTO（1 wt.%）into Li Co O₂ electrodes can significantly enhance the safety performance of Li Co O₂/graphite full cells.These results demonstrate that the oxide solid electrolyte LLZTO can not only replace the combustible liquid electrolyte,but also enhance the intrinsic structural stability of layered oxide cathode by surface relithiation reaction.