Study On The Synthesis And Electrochemical Properties Of LiNi_0.5Mn_1.5O₄ Spinel For Lithium Ion Batteries

As the cathode material of lithium ion batteries, LiMn₂O₄ spinel has many advantages, such as low toxicity, low cost, and abundance of raw materials etc., and it has high potential in application. However, the electrochemical performance of pure LiMn₂O₄ is very unsatisfying, because its capacity fades fast, especially at elevated temperature. In order to overcome these drawbacks, two methods are mainly employed: substitution and surface modification. In all the substituted spinels, LiNi_0.5Mn_1.5O₄ shows excellent electrochemical performance. Compared with LiMn₂O₄, LiNi_0.5Mn_1.5O₄ has larger practical capacity, higher discharge plateau at 4.7V, larger specific power, more stable crystal structure, better capacity retention rate, and it is emerging as an active research topic. This paper focuses on LiNi_0.5Mn_1.5O₄ spinel and explores new methods of preparation of it and betterment of its performance.Firstly, based on and improving the already reported preparation methods, the copricipitation method was employed to synthesize LiNi_0.5Mn_1.5O₄ spinel, and the effect of calcination temperature on the physical and chemical character of the samples was studied. XRD and SEM analysis reveals that particle size and crystallinity are closely related with the calcination temperature. The sample obtained at 700℃has undeveloped crystallinity and somewhat congregates, and the everage particle size is about 200 nm. On the other hand, the sample obtained at 900℃has high crystallinity, small specific surface area and excellent electrochemical performance, its initial capacity is 127 mAh/g. After 100 cycles, its capacity retention rate remains over 95%.Secondly, the LiNi_0.5Mn_1.5O₄ samples prepared by sol-gel method was coated with fumed silica and the effect of coating amount of fumed silica on the electrochemical performance of LiNi_0.5Mn_1.5O₄ and the mechanism of electrochemical performance improvement was explored. 1.0wt% fumed silica forms a loose and porous film on the surface of LiNi_0.5Mn_1.5O₄. In this case, electrolyte can penetrate through the film and contact with the active mass, so this sample shows no obvious reduction of initial capacity but improvement of capacity retention rate. When the coating amount comes to 3.0 wt%, the coating film becomes compacted, negatively affecting the contact of electrolyte with active mass, so the initial capacity of this sample is reduced, but it shows good cycling performance. XPS analysis reveals that the coating silica has changed the content of SEI film formed on the surface of the cathode, and suppressed the generation of LiF, remains good kinetics of active mass. Because silica can neutralize HF in electrolyte and reduce the dissolution of SEI film and active mass caused by HF. EIS analysis exhibits that LiNi_0.5Mn_1.5O₄ can remain good electrochemical performance if only the interface between the active mass and electrolyte is stabilized.