Preparation And Modification Of Layered Ni-Mn Solid Solution Ni-based Cathode Materials LiNi Mn _0.3 O ₂

Nickel rich layered oxide cathodes have been identified as a potential candidate with high reversible capability and low cost for lithium ion batteries. However, for their further practical applications, there are still lots of challenges to be resolved, such as the difficulty of preparation, low cyclability and rate capability. In this work, nickel rich layered Li Ni0.7Mn0.3O2 materials have been synthesized through co-precipitation and high temperature solid-state method, which can be optimized by changing synthesis conditions or doping Al3+/Co3+. The optimum synthesis condition for Li Ni0.7Mn0.3O2 material has been investigated in detail. The crystal structure, morphology, valence state of elements and electrochemical performance of the cathode material are characterized and tested by means of powder X-ray diffraction(XRD), scanning electron microscopy(SEM), X-ray photoelectron spectroscopy(XPS) and galvanostatic charge/discharge test. The main research contents are summarized as follows:(1) The results show that the best synthesis conditions for Li Ni0.7Mn0.3O2 can be obtained by calcining the Li/(Ni+Mn) mixtures with a molar ratio of 1.1: 1 at 820 °C for 15 hours during high purity oxygen flows. The obtained materials have a typical α-Na Fe O2 layered structure and homogeneous spherical morphology. XPS results suggest that Ni3+ and Mn4+ exist mainly in this compound. The initial discharge specific capacity of the material is 161.6 m Ah g-1 at 0.2 C in the voltage range of 2.75-4.2 V. After 100 cycles, this composition still maintains 97.4 % of its initial capacity. Although Li Ni0.7Mn0.3O2 can deliver about 190 m Ah g-1 in the voltage range of 2.75-4.35 V, the capacity fading can be clearly observed after 70 cycles.(2) The effects of doping Al3+ on the electrochemical performance of Li Ni0.7Mn0.3O2 by the liquid and solid state method are not significantly improved, but the degrees of lithium/nickel cation mixing are decreased and its cyclic performance can be enhanced with a small capacity loss during cycling.(3) The influence of doping Co3+ on the electrochemical performance of Li Ni0.7Mn0.3O2 is also studied and compared. The obtained materials doped with Co3+ still have the typical α-Na Fe O2 layered structure. With the increase of the amount for Co3+, the electrode performance becomes better. When x is 0.15, the initial discharge specific capacity is 189.8 m Ah g-1 at 0.2 C in the voltage range of 2.75-4.35 V and it can still maintain 97.2 % of the initial discharge capacity after 100 cycles. Even discharged at 6 C in the voltage range of 2.75-4.35 V, the specific capacity is still over 140 m Ah g-1. Furthermore, the environment temperature is at-20 °C,its discharge efficiency still maintains 77.9 %.