Study On The Preparation And Surface Modification Of5V Spinel LiMn_1.5Ni_0.5O₄

Considering the intrinsic excellent rate performance arising from the three-dimensional path of lithiumions in the spinel lattice, as a derivative of spinel LiMn₂O₄, the spinel LiMn_1.5Ni_0.5O₄（LMNO） has recentlyattracted great attentions for its nearly flat operating voltage. It shows a high capacity of130mAh g^-1(theoretical capacity:147mAh g^-1) and much higher operating voltage at around4.7V. However, thespinel LMNO still has some non-negligible drawbacks, such as the formation of Li_xNi_1-xO impurity phaseduring synthesis process, as well as cathode and electrolyte interface instability at high operating voltageabove4.8V. when the voltage reaches4.8V, the formation of SE（Isolid electrolyte interface）layer causedby the decomposition of electrolyte will deteriorate the electrochemical performance of LiMn_1.5Ni_0.5O₄inevitably.How to amend these disadvantages? Many researchers have practiced various methods in their worksto find an appropriate answer. Among their studies, two methods have been taken commonly in improvingthe electrochemical performance. They are structual doping and surface modification.Structural doping with other metal elements is one of the effective way in improving electrochemicalperformace. Many researchers have reported that Mn and Ni elements substituted by other transfer metalelements in LMNO structure can reduce the amouts of impurity phase and increase the electronconductivity. All of the evidences proved that this method is an effective way in improving electrochemicalperformance. Surface modification have the same function in improving electrochemcal performance. Thebiggest advantage of surface modification is that this method can avoid the structural distortion induced bythe substituted ions, and also can optimize the physical and chemical property and the electrochemicalperfprmance. Lots of the studies have validated that the electrochemical performance get notableimprovements after surface modification.In our works, first of all, we optimizes the synthesis condition in order to obtain LMNO with the bestelectrochemical performance. Then, our orientation is to improve the capacity of LiMn_1.5Ni_0.5O₄through asurface modification method. The chemistry-stable FePO₄is chosen as a coating layer for its chemicalinactivity and structural stability. 1. The spinel LMNO material is synthesized by sol-gel method with different annealing temperatures.The sample synthesized at800°C displays much better crystallinity and distinct spinel morphology. Itsgalvanostatic results show a capacity of121mAh/g in the fist cycle with coulombic efficiency of96.2%and exhibit the best capacity retention. The results of CV （Cyclic Voltammetry）measurement are alsoconsistent with previous reports very well. So we can conclude that800°C is the optimal temperature inthe preparation of LMNO by sol-gel method.2. Cycling stability of5V spinel LiMn_1.5Ni_0.5O₄（LMNO） is improved by surface modification withFePO₄through a chemical deposition method. The pristine,1wt.%and3wt.%FePO₄-coated LMNO arecharacterized by X-ray diffraction, fourier transform infrared spectroscopy, transmission electronmicroscopy and field emission scanning electron microscopy. It is found that the coating process is in favorof the disorder-order phase transition. The investigation on their cycling performance demonstrates that1wt.%FePO₄modified LMNO cathode exhibits the best cycling performance, with the capacity retentionratio of99.3%after50cycles, much better than that of the pristine LMNO （only79%）. Electrochemicalimpedance spectroscopy is applied to explain the galvanostatic results. The enhanced cycling performanceof the surface-modified samples can be attributed to the decreasing contact area between the electrode andelectrolyte and the suppression of undesirable thick SEI （solid electrolyte interfacial） layer.