Study Of Charge And Discharge Properties For Sodium/Lithium Storage Materials Via The Synchrotron-based Techniques

In-depth understanding of the electrochemical reaction mechanism of the battery materials is necessary for developing high-performance lithium/sodium-ion batteries.Revealing the reaction mechanism of the battery materials can help us to improve the electrochemical performance and design new battery materials.In this thesis,combining the in-operando synchrotron-based techniques,especially synchrotron X-ray imaging techniques,with electrochemical experiments,the lithium/sodium storage mechanisms especially the initial irreversibility of the materials during battery operation of serval battery materials are investigated.Based on these understandings,the new material with excellent performance has been developed for lithium ion batteries through introduction of nanoporosity and ion doping.Layered transition metal compounds have attracted much attention due to their high theoretical capacity and energy density for sodium-ion batteries.However,this kind of material suffers from serious irreversible capacity decay when charged to a high cut-off voltage.Here,combining synchrotron-based in operando transmission X-ray microscopy and high-energy X-ray diffraction with the electrochemical experiments,we visualize the structural evolution of the NaNiO₂ cathode material in sodium-ion battery and elucidate the irreversible phase transformation mechanism during the initial electrochemical cycling.At single particle level,the electrochemically active phase transformation is observed by in operando transmission X-ray microscopy.The origin of initial irreversibility associated with the charge process at the voltage range of below 3.0V and above 4.0 V is confirmed through the quantification analysis of the chemical phase mapping results.The irreversibility at the high voltage zone is caused by the irreversible structural distortion and electrolyte decomposition during the first charge process.The correlation of the capacity evolution with reaction kinetics during the first charge and discharge process is also confirmed by galvanostatic intermittent titration technique measurement.These findings reveal the origin of the irreversibility of NaNiO₂ and offer valuable insight into the phase transformation mechanism,which will provide important guidance for further development of high-performance sodium-ion batteries.Transition metal sulfides are promising high capacity anode materials for sodium-ion batteries in terms of the conversion reaction with multiple electron transfer.Nonetheless,some challenges such as sluggish sodium ion diffusion kinetics,large volume change and poor cycle stability limit their implementation.Addressing these issues necessitates a comprehensive understanding about the complex sodium ion storage mechanism especially at the initial cycle.Here,taking nickel subsulfide as a model material,we reveal the complicated conversion reaction mechanism upon the first cycle by combining in operando 2D transmission X-ray microscopy with X-ray absorption spectroscopy,ex-situ 3D nano-tomography,high-energy X-ray diffraction and electrochemical impedance spectroscopy.This study demonstrates that the microstructure evolution,inherent slow sodium ion diffusion kinetics,and slow ion mobility at the two-phase interface contribute to the high irreversible capacity upon the first cycle.Such understandings are critical for developing the conversion reaction materials with the desired electrochemical activity and stability.Based on the proposed reaction mechanisms of the various battery materials,we developed a 3D nanoporous channels structure with an enhanced electrochemical performance.A hollow-structured nanoporous LiMn_0.8Fe_0.2PO₄ material was developed by a facile ion exchange solvothermal method from a hollow microspherical Li₃PO₄precursor.This microsphere architecture provides interconnected open mesopores that facilitate homogenous carbon coating and electrolyte infiltration,enhancing the electronic conductivity and reducing the diffusion path of the lithium ions.Furthermore,based on the porous architecture,the electrochemical performance of the LiMnPO₄material is enhanced by magnesium-ion doping.Trace-magnesium doping can stabilize the crystal structure of the LiMnPO₄ material during battery operation.After magnesium ion doping via a facile solvothermal method,the porous structure was further architected by the spray drying method.Finally,the lithium storage mechanism of the developed materials is revealed through in operando synchrotron-based high-energy X-ray diffraction.In summary,the understandings of the lithium/sodium storage mechanisms of the serval battery materials can provides improtant guidance for development of high-performance battery materials.