Studies On The Synthesis And Electrochemical Properties Of Lithium-rich Transition Metal Oxides

Lithium-rich transition metal layered oxide,Li1+xNiyCozMn1-y-zO2,is one of the most promising cathode materials owing to its high theoretical capacity and good electrochemical performance,which has been successfully and widely used in commercial lithium ion batteries at present.The nickel-cobalt-manganese ternary layered structure combines the advantages of LiCoO2,LiNiO2 and LiMnO2 layered cathode materials,meaning that the ternary is endowed with a comprehensive performance better than each of the three single-component oxides because of an obvious synergistic effect at the optimal Ni:Co:Mn molar ratio of 1:1:1.In fact,the high energy density of lithium-rich ternary transition metal layered oxides(LLOs;Li1+xNiyCozMn1-yzO2 or xLi2MnO3·(1-x)LiMO2,M=Ni1/3Co1/3Mn1/3)is due to the replacement of transition metal sites by "extra" lithium ions in lattice structure.Although LLOs cathode material in practice possesses a high specific capacity of ca.200-300 mAh g-1,its drawbacks of low initial coulombic efficiency and voltage/capacity fade emerging during charge-discharge cycling processes,as well as its presently ambiguous high-capacity mechanism,still need to be further prohibited for practical application purposes.As an ideal compound model of serial lattice structure-integrated solid solutions xLi2MnO3·(1-x)LiMO2(M=Ni1/3Co1/3Mn1/3),crystallographically phase-pure monoclinic Li2MnO3 is actually electrochemical inert at a low voltage of<4.5 V(vs.Li+/Li and ibid)but can be activated when operation voltage is higher than 4.5 V.Even at a high voltage of>4.5 V,the literature-reported practical specific capacity of pristine Li2MnO3 is no more than 30 mAh g-1.Anyway,considering a high theoretical specific capacity of Li2MnO3(i.e.,459 mAh g-1)calculated according a hypothetical two-electron transfer mechanism,the electrochemical activation of Li2MnO3 may play an important role in developing lithium-rich layered transition metal oxides for lithium-ion battery cathodes.As another component of LLOs,rhombohedral LiNi1/3Co1/3Mn1/3O2 belongs to a layered structure,which possesses a relatively high specific capacity,good cycling performance and safety by comparison with those of layered LiNiO2.LiCoO2 and LiMnO2.According to literature reports,one of the most successful synthesis procedure of LiNi1/3Co1/3Mn1/3O2 deals with three steps in order:co-precipitation of nickel-cobalt-manganese ternary precursor,mixing the ternary precursor with lithium salt,calcination at a high temperature.This dissertation mainly focuses on the optimal preparation and electrochemical properties of Li2MnO3,LiNi1/3Co1/3Mn1/3O2,their two-component mechanical mixture(Li2MnO3+LiNi1/3Co1/3Mn1/3O2)and solid solution(Li1.2Ni0.13Co0.13Mn0.54O2 or 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2),and the main content of which can be divided into the following two chapters/sections:(1)Pristine Li2MnO3,without any lattice defects,mainly plays a role in stabilizing the lattice structure of xLi2MnO3·(1-x)LiNi1/3Co1/3Mn1/3O2(0<x<1)solid solutions when serving as lithium-ion battery(LIB)cathodes.In this section,surface chemical modification of hydrophilic binder sodium alginate(Na/Alg)to well-crystallized nanoparticles of Li2MnO3 has been systematically investigated,aiming to prove a high-valence Mnn+-participating(n?5)excess-activation mechanism of deficiency-free Li2MnO3 and then to demonstrate its high-capacity contribution to serial admixtures xLi2MnO3+(1-x)LiNi1/3Co1/3Mn1/3O2(0<x<1).Owing to the participation of elemental O in Na/Alg carboxyl groups,within 2.5-4.7 V vs.Li+/Li the gradually activated phase-pure Li2MnO3 delivers a never-reported high reversible capacity of 195.6 mAh g-1 at 20 mA g-1 in the 147th cycle,and this gradual activation process can even improve both the coulombic efficiency and discharge capacity of a ball-milled mixture 0.45Li2MnO3+0.55LiNi1/3Co1/3Mn1/3O2 in each cycle.Therefore,these suggest that the gradual excess-activation of component Li2MnO3 may be an effective approach to solve the capacity-fading drawback of corresponding solid solutions for high-performance LIB cathodes.(2)Optimal preparation of the high-performance solid solution cathode material of 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn/3O2(LLO)has been one of the hottest research topics in recent decades.In this chapter,the above-mentioned three-step preparation of LiNii/3Co1/3Mn1/3O2(NCM)or LLO is modified by the addition of anionic surfactant or co-precipitant sodium dodecyl sulfate(SDS)into the co-precipitation system of nickel,cobalt and manganese ternary hydroxide precursor.Compared to the NCM obtained in the absence of SDS,nanocrystalline LiNi1/3Co1/3Mn1/3O2 prepared in the presence of SDS(SDS-NCM)has the smaller value of average particle size:NCM,450 ±100 nm;SDS-NCM,250 ± 40 nm.Cyclic voltammetry(CV)behaviors of NCM and SDS-NCM cathodes comparatively show that the cathodic/anodic peak potential difference of SDS-NCM is smaller than that of NCM(NCM?0.28 V;SDS-NCM?0.18 V),indicating the less electrode polarization or electrochemical reversibility of SDS-NCM.After the continuous 120 charge-discharge cycles within 2.0-4.7 V vs.Li+/Li at 20 mA g-1,the reversible specific capacity of SDS-NCM remains at 140.8 mAh g-1 with a retention rate of 76.4%,while those of NCM are only 104.3 mAh g-1 and 55.7%,respectively.As for the optimal preparation of solid solutions LLOs,the presence of SDS into the co-precipitation step makes the final product of SDS-LLO nanocrystallites to acquire a uniform shape of nanocube,whereas the counterpart of crystalline LLO nanoparticles is irregular in shape.During the galvanostatic chargeing-discharging cycle,SDS-LLO has higher electrochemical activity.Especially,the maximum activation of Mn3+Mn4+ redox pairs make a high specific capacity of 235.3 mAh g-1 within 2.0-4.7 V vs.Li+/Li at 20 mA g-1 in the 120th cycle,far higher than LLO without SDS(188.5 mAh g-1).