Synthesis And Characterizations Of Li[li_(1/3.x/3)Cp_xMn_(2/3.2x/3)]O₂Cathode Materials For Li-ion Batteries

Great efforts have been done on lithium-ion batteries since SONYintroduced the first commercial lithium-ion battery in1991. Nowadays,conventional cathode materials such as layered LiCoO₂, spinel LiMn2O4, andolivine LiFePO4have been successfully applied in lithium ion batteries. Thesecathode materials typically deliver capacities of100-160mAh g-1. To satisfythe requirements for electric vehicles and hybrid electric vehicles, it is desiredto develop new cathode materials with superior performance such as highercapacities, lower cost, and better thermal stability. Recently, a series ofLi-riched layered materials, which can be simplified as Li[Li_xM_1-x]O₂(M=Mn,Ni, Co, Cr), have been studied as promising cathode materials for nextgeneration lithium ion batteries because of their low cost, low toxicity, highdischarge capacities. However, there are lots of arguments on true crystalstructure of such materials, not enough lucubrating on kinetics character andmechanism during the charge-discharge processes. Moreover, Li-richedlayered materials have to be charged over4.5V for the high specific energy,but the commercial electrolyte could not worke for long at such high voltage.Therefore, this work is devoted to research on the charge mechanism, crystalstructure and the electrochemical properties of Li[Li_(1/3-x/3)Co_xMn_(2/3-2x/3)]O₂ cathode materials.Firstly, we successfully prepared Li[Li_0.2Co_0.4Mn_0.4]O₂at900°C for15hours using the sol-gel method. We studied the structure properties of thematerial using varies techniques including X-ray diffraction(XRD), Fouriertransform infrared spectroscopy (FTIR) and Raman scattering,based on whichwe determined Li[Li_0.2Co_0.4Mn_0.4]O₂material was a solid solution rather thana composite of nano Li2MnO3and LiCoO₂. The additional shoulder band at670cm-1should be attributed to the local distortion, because that the Li+andMn4+superlattice was substituted by Co³⁺. FTIR and X-ray photoelectronspectrum (XPS) showed there was a little Li2CO3in the surface layer of thematerials resulting from the oxidation reaction of the residual Li.Secondly, we studied the electrochemical kinetics of the material usingGalvanostatic intermittent titration technique (GITT) and Electrochemicalimpedance spectroscopy (EIS). It is observed that the material showed lowestLi⁺diffusion coefficients (10^-19cm²s^-1), increased resistance of solidelectrolyte interface (SEI) film and huge charge-transfer resistance at the firstcharge plateau because of the high kinetic barriers associated with theconcurrent Li+diffusion,oxygen loss and structural rearrangement. Thesecomplicated reactions slowed down the electrochemical kinetics process. Atthe end of first charge, the material changed to a LixMO2layered structure,and the diffusion coefficients increased in evidence.Then, we compared the electrochemical properties of the material at-20°C and room temperature by charge-discharge cycling, cyclic voltammetry(CV), XPS and EIS. The resultes showed that the Mn4+was actived at roomtemperature, and deoxidized to Mn3+during the discharge process. The Mn³⁺was prone to dissolve into the electrolyte, resulting in bad cycle performance. EIS study showed that the charge transfer resistance severely increased withthe temperature dropped to-20°C, which was due to insufficient oxygen loss,and the material showed a low capacity. XPS and CV analysis showed that theinactive Mn⁴⁺ions in the electrode suppressed the dissolution of manganeseand the Jahn-Teller distortion of the material lattice, both of which resulted instable structure and excellent cycle life at low temperature.Finally, we studied the electrochemical properties at different voltagewindows, our experiments showed more sufficient oxygen loss and higherdischarge capacities were obtained when the material charged to4.8V, but thebig charge transfer resistance at high voltage decreased the cycle performance.The material showed a high discharge capacity of213mAh g-1and excellentcycle life in the voltage window of2.0-4.6V, so we determined this voltagewindow was a proper choice for Li-riched layered material. Moreover, weprecharged the materials with high voltage and low rate before cycling. CVshowed that the Mn4+was actived during the precharge treatment, andparticipated in the flowing cycles in the narrow voltage window, so thedischarge capacity and cycle performance were enhanced. This prechargetreatment technique brought high discharge capacity in the voltage window,which was fit for the commercial electrolyte for Li-ion Batteries.On all accounts, this work gives us a comprehensive understanding on thepreparation of Li[Li_(1/3-x/3)Co_xMn_(2/3-2x/3)]O₂cathode materials, as well as itsstructural and electrochemical properties.