Study On The Preparation And Surface Modification Of Lithium Ion Battery Cathode Material 5V Spinel LiMn_1.5Ni_0.5O₄

Lithium ion batteries as the most efficient commercialized energy storage device, has been widely used in mobile electronic devices. It is used in portable electronic devices such as mobile phones, laptops, digital cameras, and is expected to be further used in electric vehicles, hybrid vehicles, military and aerospace. But at the expense of traditional cathode material, safety and environmental aspects, the Lithium ion batteries no longer able to meet the high efficiency and high energy and other aspects of more pressing needs. In order to further improve the energy density of lithium-ion batteries, the need to develop a high-capacity and high-voltage lithium-ion battery cathode material.Compared with the traditional 4 V LiMn2O4 spinel and layered LiCoO2, or the 3.5 V olivine LiFe PO4 cathodes, LiMn1.5Ni0.5O4 provides access to the Ni2+ to Ni4+ at about 4.7 V(versus Li/Li+), which has intrigued great interest from scientists and engineers. Except its attractive high operating voltage, Li Mn1.5Ni0.5O4 has been regarded as one of the most promising candidate for its good cycling performances(without Jahn-Teller effect related to the presence of Mn3+), low cost, environmentally benign property and good thermal stability. However, the material still suffers from unsatisfied cycle life due to severe electrolyte decomposition at high voltage and metal dissolution in HF-contained LiPF6-based electrolytes.In order to amend these disadvantages, the surface modification used to improve the electrochemical performance of cathode material. This method is coating a layer of barrier on the surface of the material to avoid the direct contact with electrolyte and thus improves the electrochemical performance in cycling. The biggest advantage of surface modification is that this method can avoid the structural distortion induced by the substituted ions, and also can optimize the physical and chemical property and the electrochemical performance. Lots of the studies have validated that the electrochemical performance get notable improvements after surface modification.In our works, first of all, we optimize the synthesis condition in order to obtain LiMn1.5Ni0.5O4 with the best electrochemical performance. Then, our orientation is to improve the capacity of LiMn1.5Ni0.5O4 through a surface modification method. The chemistry-stable AlF3 and MgF2 are chosen as a coating layer for its chemical inactivity and structural stability.1. The spinel LMNO material is synthesized by sol-gel method. X-ray diffraction spectroscopy(XRD), Scanning electron microscopy(SEM) are used to characterize the physical properties of samples. The investigation on their cycling performance demonstrates that the sample synthesized at 870 °C displays much better crystallinity and distinct spinel morphology.2. Spinel LiMn1.5Ni0.5O4 cathode material is modified by different contents of AlF3 through simple chemical deposition. The effects of AlF3 coating on the structural and electrochemical properties of LMNO cathodes are investigated using X-ray diffraction(XRD), Raman spectroscopy, field emission scanning electron microscopy(FESEM), electrochemical impedance spectroscopy(EIS), Fourier transform infrared spectroscopy(FTIR) and differential scanning calorimetry(DSC). It is found that the Al F3 surface modification layers do not change the bulk structure of LMNO. Compared with the pristine sample, the AlF3-coated LMNO materials display enhanced cycling stabilities. Especially, the 1 wt% AlF3-coated LMNO demonstrates the best reversibility, with capacity retention of 93.6% after 50 cycles, much higher than that of the pristine material, 77.6%. EIS and FTIR data illustrate that the improvement of electrochemical performance can be attributed to the suppression of solid electrolyte interface(SEI) growth by AlF3 coating layer. DSC tests show that the AlF3 coating layer also helps in enhancing the thermal stability of LMNO cathode.3. Spinel LiMn1.5Ni0.5O4 cathode material is modified by different contents of MgF2 via wet coating strategy. The results of X-ray diffraction(XRD), Raman spectroscopy, field emission scanning electron microscopy(FESEM) and high resolution transmission electron microscopy(HRTEM) showed that the Mg F2 surface coating layers do not physically change the bulk structure of pristine material. Compared with the pristine compound, the MgF2-coated LMNO electrodes display enhanced cycling stabilities. Especially, the 5 wt% MgF2-coated LMNO demonstrates the best reversibility, with capacity retention of 89.2% after 100 cycles, much higher than that of the pristine material, 70.0%. Electrochemical impedance spectroscopy(EIS) and flourier transform infrared spectroscopy(FTIR) data illustrate that the improvement of electrochemical performance can be attributed to the suppression of solid electrolyte interface(SEI) growth by MgF2 coating layer. Differential scanning calorimetry(DSC) tests show that the MgF2 coating layer also helps in enhancing the thermal stability of LMNO cathode.