Study On The Synthesis And Electrochemical Performance Of Li-Rich Layered Li-Ni-Fe-Mn-O Materials

With the ever-developing of electronic devices and electric vesicles（EVs）,the demand for energy is growing.Developing lithium-ion batteries（LIBs）with higher energy density and lower cost has become an important issue and a long-standing goal for researchers.Li-rich layered material Li₂Mn O₃-Li MO₂（M=Ni,Co...）is one of the most promising cathode candidates for next-generation LIBs owing to their high reversible capacity.However,the current layered transition metal oxide materials are overly dependent on cobalt,resulting in constant rising of the cobalt resources’price.Therefore,it is imperative to develop new cobalt-free cathode material.Based on that,we studied a low-cost,cobalt-free Fe-and Ni-substituted Li-rich Mn-based layered material Li-Ni-Fe-Mn-O.The synthesis conditions,electrochemical properties,structural stability and thermal stability of this material are involved.The results provide strong support for the evaluation of Li-Ni-Fe-Mn-O.（1）Hydroxide precursor containing Ni,Fe and Mn was synthesized via a co-precipitation method.A lithium-rich material Li_1.16Ni_0.19Fe_0.18Mn_0.46O₂（NFM）with well layered structures and high average discharge voltage was obtained by adjusting the calcination temperature.It delivered a reversible capacity of 214.3 m Ah g^-1 at the initial cycle at a rate of 0.1 C and its average voltage for first discharge was 3.56 V（vs.Li/Li⁺）.After 50 cycles,the capacity retention was 81.2%and the average discharge voltage decreased only 0.13 V.Results of X-ray photoelectron spectroscopy（XPS）and density functional theory（DFT）calculation revealed that the capacity of NFM is derived from the reversible redox couples of Ni²⁺/Ni⁴⁺,Fe³⁺/Fe⁴⁺ and 2O^2-/O₂^n-.And the anionic oxidation prefers occur on oxygen with Li-O-Li configuration and with oxidized Fe and Ni coordination.（2）It was found that the low coulombic efficiency of NFM during cycling in the conventional electrolyte is related to the dissolution of Fe and the phase transition of the material.By replacing ethylene carbonate（EC）with fluoroethylene carbonate（FEC）,a cathode electrolyte interfacial（CEI）film with 10-20 nm thickness formed on NFM.The film effectively suppresses the phase transition during cycling,which improved the coulombic efficiency of NFM from 93%to 99%,and it exhibited better rate performance.（3）Cells with traditional NCM cathode material triggered thermal runaway during the accelerated rate calorimeter（ARC）tests.Transition metal segragation was observed on NCM after the thermal runaway.Ni and Co were reduced to Ni（0）and Co（0）while Mn exited in form of Mn O.NFM had higher onset temperature and lower exothermic heat than high nickel NCM material for their decomposition reactions in differential scanning calorimeter（DSC）tests,which confirmed well thermal stability of NFM.In addition,the CEI film grown on NFM in the FEC electrolyte also improved the thermal stability of NFM.