Study Of Lithium Excess Cathode Materials XLi₃NbO₄·（1-x）LiMO₂ （M=Mn,Ni,Co;0<x<1） For Li-Ion Batteries

Lithium ion batteries have great potential in electrical vehicles and energy storage.A critical criterial for lithium ion batteries is the energy density.There are two ways to achieve a higher energy density:to elevate the average discharge voltage or to improve the specific capacity of the battery.The specific capacity of the anode is much larger than that of cathode,so cathode is now the key point of the battery research,and it is also very important to the energy density of the whole battery.Lithium rich materials are possible to release more lithium ions than traditional layered materials,therefore they attracted much attention of the researchers all over the world.In this paper,we optimized the synthesis conditions and successfully synthesized a series of Li₃Nb_O4 based lithium excess materials xLi₃Nb_O4·（1-x）LiMnO₂（x = 0.11,0.25,0.43,0.67）.XRD results shows that the samples of 0.25≤x≤0.67 share the same structure.They are very well crystallized,have a cubic structure with space group of Fm-3m.Electrochemical study reveals that the samples with x = 0.25 or x = 0.43 has better electrochemical performance,they both deliver about 216 mAh/g in the initial cycle.Lithium rich materials always suffer from voltage decay,but the sample of x =0.25 decays more severely.We speculate that Li₃Nb_O4 is critical in stabilizing the structure,because the sample with x = 0.43 has more Li₃Nb_O4.We employed the sample 0.43Li₃Nb_O4·0.57 LiMnO₂ to research the mechanism of the electrochemical process.Ex-situ XRD results shows that the structure of the material is very stable,the extraction of lithium ions leads to the decrease of lattice parameter,but no new phase is observed in the whole process.XPS tests show that in the charging process,Mn³⁺ is oxidized to Mn⁴⁺,and the peaks of Niobium also shifts to higher binding energy,it proves that Niobium transfers extra electron to Oxygen.EX-situ XAFS of Mn proves that Mn³⁺ is oxidized to Mn⁴⁺before 4.3V.So the charging process can be divided into two stages,in the first stage,lithium ions are extracted from the structure and Mn³⁺ is oxidized to Mn⁴⁺,in the second stage,the oxidization of oxygen accounts for the charge compensation.We also successfully synthesized Co³⁺ doped material 0.43 Li₃Nb_O4·0.57 LiMn_1-xCoxO₂（x=0.25,0.5）,XRD results indicates that the Co³⁺ doped materials also has a rock-salt structure with Fm-3m space group.The lattice parameter decreases as the amount of Co³⁺ increases.The electrochemical tests shows that a moderate amount of Co³⁺ doped material has a decreased charge transfer resistance,the rate capability is improved,and the cycle performance is also better than the undoped material.