Investigation On The Electrochemical Performances Of Materials With Surface Modification For Lithium-Ion Batteries

In recent years, tremendous efforts have been paid to the development of power Li-ion batteries for electric vehicles (EVs), hybrid electrical vehicles (HEVs) and plug-in hybrid vehicles (PHEV). Although much progress has been made, power Li-ion batteries suffer from low energy density, poor cyclic stability and poor high-temperature safety as well, which greatly hinders its realistic applications. In an attempt to overcome the significant drawbacks, several strategies have been exploited including doping, surface modification and nanosized structure as well. It is considered that nanosized structure of active materials is an efficient way to obtain high-performances batteries due to its shorter li-ion diffusion length and larger contact area between active materials and electrolyte. In present work, we focus on the synthesis of nanosized structure of active materials including Li4Ti5O12, LiNi1/3Co1/3Mn1/3O2 and Li1.2Ni0.13Co0.13Mn0.54O2. The structural properties and electrochemical performances are investigated by XRD, SEM, N2 adsorption-desorption analysis, galvanostatic charge/discharge tests, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV), respectively. Main contents are as follows,Firstly, hole-rich Li4Ti5O12 composites are synthesized by spray drying using carbon nanotubes as additives in precursor solution, subsequently followed calcinated at high temperature in air. The structure, morphology, and texture of the as-prepared composites are characterized with XRD, Raman, BET and SEM techniques. The electrochemical properties of the as-prepared composites are investigated systematically by charge/discharge testing. In comparison with the pristine Li4Ti5O12, the hole-rich Li4Ti5O12 induced by carbon nanotubes exhibits superior electrochemical performance. Cyclic voltammograms and AC impedance spectroscopy show that the obtained excellent electrochemical performances of should be attributed to the hole-rich structure of the materials, which offers more connection-area with the electrolyte, shorter diffusion-path length as well faster migration rate for both Li ions and electrons during the charge/discharge process.Secondly, layered LiNi1/3Co1/3Mn1/3O2 cathode material is synthesized via a sol-gel method and subsequently surface modified with Eu2O3 layer by a wet chemical process. As prepared composites are characterized with XRD, Raman, SEM. EDS. In comparison, the Eu2O3-coated sample demonstrates better electrochemical performances and thermal stability than that of the pristine one. Analysis from the electrochemical measurements reveals that the surface-modified composites are mainly ascribed to the presence of Eu2O3-coating layer, which could efficiently suppress the undesirable side reaction and increasing impedance, and enhance the structural stability of active material.Finally, Li1.2Ni0.13Co0.13Mn0.54O2 nanoparticles are prepared by ultrasonic atomization and then surface modified with gapless PbPdO2 layer by a wet chemical process. XRD, SEM, EDS are carried out to characterize the structure, morphology and element distribution of the as-prepared composites. Analysis from the electrochemical measurements reveals that Li1.2Ni0.13Co0.13Mn0.54O2@PbPdO2 exhibits better cycle stability, rate capability, low temperature performance and thermal stability than the pristine one. AC impedance test indicated that the PbPdO2-coating would suppress the increase of charge transfer resistance, relieve the side reactions between the electrolyte and active materials and faciliate lithium ion diffusion, resulting in the enhanced performances.