Study On Electrode Materials For Lithium Secondary Batteries

In view of increasing the energy density of Li-ion batteries, besides a highcapacity, a low average charging voltage of the anode material is another importantfactor. Among various transition metal oxides, manganese monooxide （MnO） is apromising anode material for LIBs because of its high theoretical lithiation capacity(756mAh g^-1), relatively lower electromotive force value （1.032V vs. Li⁺/Li）, highdensity (5.43g cm^-3), low cost, as well as environmental benignity. However, MnOsuffers from some drawbacks, such as limited electronic conductivity, large volumeexpansion （170%） and rapid capacity decay upon cycling. In this paper, composites ofN-MnO mixed by high electron conductive TiN is prepared through two simplemethods, which is expected to obtain improved lithium storage performance becauseof the synergetic effect between N-MnO （serving as a superior lithium storagematerial） and TiN （offering a fast and efficient electronic conducting network andacting as a barrier to buffer the volume expansion/contraction of N-MnO during theLi⁺insertion/extraction process）.N-MnO/TiN composites with different molar ratios were first prepared by a hightemperature solid state method, and TEM, EDS, XRD, XPS and electrochemicaltechniques were employed to characterize and test the prepared composites. It isdemonstrated that nitrogen is doped into the MnO lattices and N-MnO form partialsolid solution with TiN. Through EIS analysis, it is shown that the charge transferresistance of the N-MnO/TiN electrodes becomes smaller with increasing the contentof TiN, which can be attributed to the high electronic conductivity of TiN.N-MnO/TiN electrodes exhibit high capacity, exellent cycling performance and ratecapability, which is superior to pristine N-MnO electrode. N-MnO/TiN^-11electrodedelivers the best electrochemical performance, at a current density of40mA g^-1, theinitial discharge specific capacity of N-MnO/TiN^-11and N-MnO is1075mAh g^-1and932mAh g^-1, respectively, with an initial coulombic efficiency of65.4%and54.2%, respectively. At a current density of400mA g^-1and2A g^-1, the reversible specificcapacities of N-MnO/TiN^-11are380mAh g^-1and210mAh g^-1, respectively, which isfar more than pristine N-MnO electrode.Furthermore, N-MnO/TiN composite with equal molar ratio was preparedthrough sol-gel method, and the composite are characterized by XRD, SEM, EDS,and electrochemical techniques. At a current density of40mA g^-1, the stablereversible discharge specific capacity of N-MnO/TiN is431mAh g^-1, which is twicethan that of pristine N-MnO electrode. Even at a high current density of2A g^-1. Thereversible specifc capacity of N-MnO/TiN can maintain at75mAh g^-1, which istenfold that of the pristine N-MnO.Li-ion batteries have been applied widely in various field, however, even whenfully developed, the highest energy storage that Li-ion batteries can deliver is too low（driving distance limited at200km） to meet the demands of future electric vehicles.The energy density of Li-O₂batteries is comparable with gasline, if successfullydeveloped, it will offer a new energy sources for future electric vehicles. However,there are many scientific and engineering challenges need to be overcome beforethe application of Li-O₂batteries. There is a consensus that the selection ofcathode electrocatalysts and solvents is critical to develop Li-O₂batteries.Inspired by previous research, the rechargeable Li-O₂and Li-ion hybridbatteries with carbon supported spherical LiNi_1/3Co_1/3Mn_1/3O₂as cathodes wereoperated in DME-based electrolytes. The spherical LiNi_1/3Co_1/3Mn_1/3O₂serves asboth a cathode catalyst and a lithiation/delithiation host. The hybrid battery showsremarkable performance by combining both the characteristics of Li-O₂batteryand Li-ion battery. Through ex-situ XRD and SEM characterization, it isdemonstrated that the main discharge products of the hybrid battery are Li2O₂toroids, and some LiOH cubes are also observed after deep discharge. The hybridbatteries delivered specific capacities （voltage plateaus） of2300mAh g^-1（2.73V）,1360mAh g^-1（2.68V）, and1165mAh g^-1（2.62V） at current densities of0.05mAcm^-2,0.10mA cm^-2, and0.15mA cm^-2, respectively. The excellent rate capabilitycan be ascribed to the high electrocatalytic activities of the catalyst and the exellent O₂solubility in DME. The cycling performance of the hybrid battery werestudied by a constant capacity approach and a constant potential approach, and theformer exhibits better performance. By restricting the specific capacity to1335mAhg^-1, the hybrid battery exhibits a good reversibility of8cycles. Furthermore, thefactors that deteriorate the cycling performance were also discussed.