Multiple-dimensional Synthesis And Local-structural Controlling Study Of Lithium-excess Layered Cathode Material

The lithium-excess manganese based layered solid solution system is becoming the most promising cathode material of lithium ion batteries because of its anomalously high discharge capacity of more than 280 mAh g-1 and shows important scientific research value and application prospect.However,the further development of this series of material has been districted by the voltage hysteresis and capacity decay phenonenon,which is caused by the structure evolvement in the process of charge-discharge cycles because of the local ion rearrangement due to the formation and migration of oxygen vacancies at 4.5 V of the first charging process.In this thesis,in order to realize the high energy density,high power density and good cycling stability,following works are carried out by taking the lithium-excess layered oxide Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ as object:Lithium-excess oxide Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ with different morphologies were synthesized by electrospinning and coprecipitation method respectively.For the first time,we reported a comprehensive research of morphology,specific area,crystal structure,electrochemical performance and redox mechanism between the nanofibers and nanoparticles lithium-excess layered oxide.The specific surface area of nanofibers is 6.53,m2 g-1 which is larger than the nanoparticles 4.388 m2 g-1 according to the Multipoint BET analyses.The nanofibers electrode delivers higher capacity than the nanoparticles electrode at the various rates because of its good kinetics condition for lithium ions migration,particularly,the capacity of the nanofibers electrode reaches to 126.6 mAh g-1 at 5 C while only 101.1 mAh g-1 is obtained for the nanoparticles electrode.The lithium ion diffusion coefficients at both 4.0 V and 4.5 V of nanofibers electrode are greater than that of the nanoparticles electrode and from which we know that the filamentous morphology is beneficial to both the conductivity and the diffusion of solid state lithium ion.In addition,step-by-step cyclic voltammetry verified the oxygen evolution from Li2MnO3 at 4.5 V plateau and the subsequent activity of Mn.The morphology and microstructure depend on the heating rate and sintering temperature and time of the electrospun precursor.The end-product exhibits a petal-like nanoplates microstructure at the heating rate of 5 ? min-1.The petal-like nanoplates sample heated at 800 ?.for 8 h shows the best rate capability,delivers 127 mAh g-1 at 5 C because of the good kinetics during the electrochemical process.The nanofibers and petal-like nanoplates microstructure facilitate the sufficient contact of electrode and electrolyte and help to reduce the polarization during the electrochemical reaction process,especially when the cathode is under large current.Lithium and oxygen ions are chemically extracted when the lithium-excess oxides are leached in acid solution,which decreases the irreversible capacity loss for the initial cycle.However,the acid with high concentration damages the crystal structure of the active material,which leads to the deterioration of the cycling performance.Here,a mild acid treatment method was firstly proposed with acid of pH=2 and the initial coulombic efficiency is improved from 82.4%to 89.7%.Meanwhile the damage to the crystal structure is controlled and the cycling performance is ensured.Innovative in-situ CV measurement proclaimed this was attributed to the improvement of the reductive catalytic activity of oxygen released during initial charge process.A low temperature surface ions incorporation method was adopted with NH4BF4 as the modification compound for the first time.A continuous modification layer is gradually formed and BF4-is incorporated into the surface crystal structure of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ during the entire electrochemical cycling process.Specific capacity of 308.7 mAh g-1 at 0.05 C and 140.0 mAh g-1 at 5 C are achieved.The voltage decay is relieved from 202 mV to 65 mV after 55 cycles analyzed via a simplified method based on the reduction peaks of differential capacity curves.And a retained discharge capacity of 286.9 mAh g-1 after 55 cycles was obtained.HRTEM of both pristine and modified materials after 50 cycles revealed that the modification could effectively prevent the formation of lattice imperfection at the particle surface region.The absence of the 530.5 eV peak at 2.0 V for the modified electrodes of XPS analyses verified better redox reversibility of oxygen.