Atomic-scale Investigation On The Structural Evolution Of Layered LiCoO₂ Cathode For Lithium Ion Batteries

There is no doubt that lithium-ion batteries(LIBs)are indispensable in the field of energy storage due to their high energy density,high operating voltage,excellent cycling performance and rate capability.However,problems remain,especially in the persuit of high energy density.Hence,more attention is paid to the layered cathodes to solve the capacity bottleneck,which is the fundamental to develop more powerful LIBs.Among all the candidates,the layered O3-LiCoO2 cathode has been commercialized with a large market share,especially in 3C(Computer,Communication&Consumer Electronics)consumer electronics products due to its long cycling life,reasonable rate performance and high energy density.The present consecutive investigations on layered cathode LiCoO2 implies the potential improvement on its practical application,which is actually just the beginning on making full use of this high energy density material.To date,the LiCoO2 still has some unresolved problems,such as the low practical capacity that is about half of its theoretical capacity,electrochemical degradation at high voltage,oxygen releasing and so on.All these issues are rooted from the layered structure of LiCoO2,because the LIBs are substantially the structure-determinng energy storage and conversion devices.In this context,this paper focuses on the atomic scale structural evolution during charge and discharge using in/ex-situ techniques,and the innovative progress is summarized as follows:(1)In-situ X-ray diffraction(XRD)and ex-situ electrochemical impedance spectroscopy(EIS)techniques are employed to characterize the changes in surface and bulk of LiCoO2 cathode upon delithiation.The structural changes and the charge transfer with the involvement of lattic oxygen are clearly elucidated in the formation cycle.A readable physical picture is draw to describe the dellithiation process of LiCoO2 electrode;(2)The ex-situ scanning transmission electron microscopy(STEM)technique images the atomic-scale surficial changes that are induced by the LiCoO2-electrolyte interfacial side reactions upon Li extraction.Of special interest is that different chemical valences of Co ions are revealed in a gradient distribution from the surface to bulk as attested from the electron energy loss spectroscopy(EELS),namely the Co2+/Co3+/Co4+ions;Moreover,it is found that lattice oxygen can participate in charge compensation reversibility,while the Li-Co antisites contribute to stabilize the surfacial structure,thus stabilize the electrochemically active lattice oxygen and afford the internal stress introduced by the Li removal;(3)The LiCoO2 cathode is coated with Al2O3 by a facile solution method to improve its electrochemical performance.This coating behavior achieves a dual function:(1)physically isolating the LiCoO2 material and the electrolyte to suppress the obvious side reactions and(2)unintentionally introducing a small amount of aluminum into the lattice of LiCoO2 material to stabilize the lattice,thereby optimizing the electrochemical performance.