Preparation And Surface Modification Of High-Capacity Li-Rich Layered Cathode Materials For Lithium-Ion Batteries

Owing to the advantages of high working voltage,high energy density and power density,stable discharge and good safety,lithium ion batteries occupy an important position in the field of energy conversion and storage.At present,lithium ion batteries have made great achievements in the market of small mobile electronic devices.However,they cannot satisfy the increasing market demand.Therefore,it is particularly urgent to research and develop lithium ion batteries with high energy densities.Electrode materials are the key to determine the performance of lithium ion batteries.In particular,the specific capacity of the cathode materials severely restricts the energy density of lithium ion batteries.Lithium-rich layered cathode materials of xLi₂MnO₃·（1–x）LiMO₂ have the advantages of high specific capacity(>250 mA h g^-1),high working voltage（～4.5 V）and low cost,which have attracted the extensive attention from basic research and industry.Whereas,LLOs still face many problems to be solved,such as difficult issues like low initial Coulombic efficiency,poor cyclability,severe voltage decay and poor rate performance.These problems are often related to the surface/interface of electrodes.In this dissertation,nano-structured high-capacity lithium-rich layered cathode materials are prepared,and their surface modification is also explored to enhance the electrochemical performance.The main content and results of this dissertation are listed as follows:（1）A uniform Co₃O₄ layer was coated on the surface of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂（LLOs）nanoplates by a hydrothermal method and subsequent heat treatment successfully.Through tuning the coating amount and control the heating temperature,the relationship between the structure and the performance of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂（LLOs）materials is explored.It is found that the electrochemical performance of the sample is optimal when the coating content of Co₃O₄ is 2 wt%and the heat treatment temperature is 450 ℃.The Co₃O₄-coated material delivers a high discharge capacity of 296 mA h g^-1 at 0.1 C with an initial Coulombic efficiency of 84.3%.After 160 cycles at 0.2 C,the capacity retention can reach 83.2%with a discharge capacity of 221.6 mA h g^-1.The electrochemical performance is improved obviously after Co₃O₄ coating.The main reasons are as follows.The Co₃O₄ layer can serve as a protective layer,and restrain the erosion of the electrolyte on the electrode materials.The Co₃O₄ layer can also absorb extracted Li₂O,thus forming the new product of Li_xCoO_y that is electrochemically active.This will greatly reduce the irreversible capacity loss of the cathode material and improve the initial Coulombic efficiency.Meanwhile,the induced new spinel phase on the surface improves the cycle stability of the LLOs.（2）A uniform film of TiO₂ with continuous controllable thickness on the surface of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂（LLOs）electrodes by the atomic layer deposition（ALD）method successfully.The dependence of the deposition cycles（TiO₂ deposition thickness）on the electrochemical properties of the material was systematically studied.After the deposition of40 cycles,the performance of the sample is the best.The deposition thickness of TiO₂ is about2.2 nm.The capacity retention is 82.8%with a discharge capacity of 180.5 mA h g^-11 after 100cycles at 1 C.The discharge capacity is still 143.4 mA h g^-1 at 4 C.It can be seen from the SEM analysis,that the electrode of the TiO₂-coated LLOs electrode can still retain the original structure and morphology after 100 cycles.The TiO₂ coating layer,therefore,can protect the structure and morphology of the electrode,inhibit the side reactions and the erosion of the electrolyte on the electrode,and improve the cyclability of the cathode,thus enhancing the electrochemical properties of the LLOs electrode material.