| Based on the concepts of solid solution of LiAO2-Li2BO3(A=Co, Ni, Cr, Fe etc.;B=Mn, Ti) to design new lithium ion materials, we put forward the existing might ofNiO-Li2BO（3B=Mn, Ti）solid solution materials. NiO has unordered rock salt structurewithout electrochemical activity as cathode material. But stabilized by Li2BO3（B=Mn,Ti）structure, it can present electrochemical activity and deliver considerablecapacity. Moreover, there is great probability to be applied in certain particularsituations due to structural stability, non-toxic, low cost and sound high-heat thermalstability.NiO-Li2TiO3(x=0.0-1.0) and NiO-Li2MnO3(x=0.0-1.0) series materials have beenprepared, respectively. Relations between x value and their crystal structure, latticeparameters, electrochemical characteristics have been studied and discussed. XRDresults show that NiO-Li2TiO3(x=0.0-1.0) series materials can be identified as a purephase based on hexagonal lattice. With the increase of x value within the given rangeof0.2-1.0, series materials cause a gradual transformation from layered orderedstructure to rock-salt structure, especially for x=0.2material the structure has become acompletely disordered rock-salt one. Lattice parameters, a and c, increase with theincrease of x value,resulting in the increase of unit cell volume. Therefore, we believethe series materials can form solid solution materials when x is from0.2to1.0.However, as for NiO-Li2MnO3(x=0.0-1.0) series materials, a trace of NiO impurity isobserved in x=0.5material. Therefore, we cannot analysis the change of the latticeparameters.0.5NiO-0.5Li2MnO3(LiNi0.5Mn0.5O2) as a cathode material for Li-ion battery hasbeen prepared by the metal acetate decomposition method, sol-gel method andcarbonate co-precipitation method, respectively. The influences of synthesis methodson the physical and electrochemical behaviors of LiNi0.5Mn0.5O2have beencharacterized by XRD, SEM and electrochemical tests. XRD patterns show that boththe sol-gel and carbonate co-precipitation methods can form single phase of layeredstructure, while a trace of NiO impurity is observed via the metal acetatedecomposition method. SEM results show the as-prepared carbonate particle has aspherical morphology with an average diameter of10μm, consisted of primarynano-sized particles with particle diameter of200nm. The sample prepared by thecarbonate co-precipitation method exhibits highest discharge specific capacity and best cycling stability, which results from the steady homogeneity of precursor constant bythe fixation of CO32-anion group. It can deliver an initial discharge specific capacity of186.3mAh/g and retain170mAh/g after100cycles at a current rate of20mA/g in thevoltage range of2.5-4.7V at25oC. Moreover, even at the high temperature of55oC, itstill delivers a reversible specific capacity of222.6mAh/g with little capacity loss after30cycles.The effects of pH, different sintering ways, and lithium carbonate contents onelectrochemical performance of LiNi0.5Mn0.5O2prepared by carbonate co-precipitationmethod are investigated. When pH reaches8.15, the as-prepared LiNi0.5Mn0.5O2particles have a more uniformity of15.5μm and its average composition is closer todesired composition. The two different sintering routes reveals that there are nodifference for the LiNi0.5Mn0.5O2material in terms of crystal structure, latticeparameters, initial charge/discharging specific capacity and cyclic specific capacity,which is direct calcination of the precursor and lithium carbonate and calcination ofpre-sintering oxide and lithium carbonate. The effects of lithium carbonate contents onelectrochemical performance of LiNi0.5Mn0.5O2also are investigated. Lithium excessexperiments reveal that although rich-Li products slightly reduce its initialcharge/discharge specific capacity, it can greatly improve its cyclic and ratecharacteristics. But when too much, un-reacted Li2CO3might attach to the surface andprevents insertion and emigration of Li+, lead to poor electrochemical characteristic. |
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