Study On The Doping And Electrochemical Performance Of Li-rich Layered Li-Ni-Mn-O Materials

How to further improve the energy density is one of the most challenging problem in the science and technology of lithium（Li）ion batteray.Since cathodes have been an important limit for energy density improvment,the Li-rich layered cathode materials（LLO）that possess extra high capacities(200³00 mAh g^-1)and energy density have attarcted much interest in recent years.However,thus far the LLO materials have been suffering from drawbacks such as poor first-cycle Coulombic efficiency,degradation of voltage during cycling,and low rate performance,which prevents their practical industry applications.Hereinthisthesis,with0.5Li₂MnO₃·0.5Li[Mn_0.5Ni_0.5]O₂(Li_1.2[Ni_0.2Mn_0.6]O₂)as a representative model system of LLO materials,we studied the elemental doping of LLO by barium（Ba）ion.It is found that Ba doping can surpraisingly improve the superior electrochemical performance LLO via suppression of the oxidation of O^2-ions in the first charging.Remarkbaly,an extremely low concentration of the Ba(x=0.005 in Li_1.2[Ba_xNi_0.2-x.2-x Mn_0.6]O₂)yielded an improved first-cycle efficiency.The oxygen generation in the first charge was effectively suppressed by the Ba doping.This effect can be attributed to the stabilizing effect of the Ba cations on the oxygen radical intermediates generated during the oxidation of O^2-.Meanwhile,because the stabilized oxygen radicals likely facilitate the charge transportation in the layered-layered structure,the barium-doped Li_1.2[Ni_0.2Mn_0.6]O₂exhibits significant improvement in rate performance.Those results suggests that stabilizing the oxygen radicals could be a promising strategy to improve the electrochemical performance of LLO materials.Apart from studies of the alkaline earth metal elements doping,this thesis also studied the aluminum（Al）ions doping of Li_1.2[Ni_0.2Mn_0.6]O₂.It is found that a low concentin of Al³⁺doping can improve the first-cycle efficiency of LLO materials but a higher extend of Al³⁺doping results no further improvment.Though most of the Al dopants can be inserted into the TM（transition metal）layer,there will also be a portion of Al³⁺stayed in the Li layer at the same time,because of the small radius of Al³⁺.This will hinder Li⁺transport and thereby reduce the electronic conductivity of LLO.In order to clarify phase transition mechanism of LLO,we further systematically studied the influence of synthetic temperature,calcining duration,atmosphere and cooling rate on the variation of phase structures of LLO.When increasing the synthesis temperature from 900 to 1100℃and duration up to 24h,it was found that a new layered phase of LiNiO₂ would be formed and the parent phase remained to be of LLO structure with the coexistence of trace of spinel LiMn₂O₄ phase.This phase segregation phenomena causes the deterioration of electrochemical performance of LLO,i.e.,the decrease of capacity,the aggravation of voltage fading and the shortening of cycle life.Importantly,this voltage fading behavior is similar to the case of common LLO material at late cycling,indicating that the voltage fading problems of LLO have strong correlation to the gradual loss of Ni.In addition,we also investigated the influences of pure oxygen atmosphere and cooling rate on the phase segregation processes.By quenching in air after high temperature roasting,we obtained a pure-phase sample with the ordered microstructure and little Ni segregation,which can evidently increase the capacity,improve the cycling performance and voltage fading problems.These new experimental findings may open a new way to for the future research and development of pure-phase LLO material.