Synthesis And Properties Of Lithium-Rich Manganese-Based Cathode Materials

Lithium ion batteries are widely used as rechargeable energy storage devices in the world and their demands is growing rapidly owing to the development of portable devices as well as electrical vehicles. While the energy density of commercialized lithium ion batteries is still not very satisfactory, which is mainly constraints by the properties of cathode materials. Layered oxides such as LiCoO2 and LiNii/3Mni/3Coi/3O2 compose the majority of cathode materials used in LIB. However, the reversible capacity of these materials has almost reached their limits (200 mAh g-1). Cathode materials delivering higher capacity are needed to meet the demand of ever growing energy density for transportation, portable devices and grid.Lithium-rich layered oxides are of extreme interests as potential cathode materials due to their large reversible capacity. These materials can be described in two ways:for examples, as a two-component "composite" structure xLi2MnO3-(1-x)LiMO2 (M represents transition metals) or as a "solid-solution" Li[LixM1-x]O2 with a homogeneous long-rang order. Both notations have the same material and have been extensively used in published literature.Lithium-rich manganese-based cathode materials Li[Li0.2Ni0.16Co0.1Mn0.54]O2 were synthesized through co-precipitation method. Precursor Ni0.200Co0.125Mn0.675(OH)2 was synthesized by using 2 L continuous stirred tank reactor. Metal sulfates were used as metal sources. Sodium hydroxide and ammonium hydroxide were used as precipitant and complexing agent separately. By controlling temperature and pH value in the co- precipitation process, a homogeneous distribution of metal cations is ensured. The morphology and size distribution of the secondary particles of Ni0.20oCo0.125Mno.675(OH)2 can be controlled by the regulation of reaction time, stirring speed, and concentration of metal solution during the co-precipitation. Under optimal conditions (reaction time of 5 h, stirring speed of 1000 rpm, and metal solution of 2 M), uniform and spherical metal hydroxide precipitates can be obtained, with size of 7-11 μm. Calcined the as-prepared precursor with LiOH, Li[Li0.2Ni0.16Co0.1Mn0.54]O2 powders can be obtained. The final product Li[Li0.2Ni0.16Co0.1Mn0.54]O2 shows a hexagonal-layer, well-ordered structure as confirmed by Rietveld refinement of X-ray diffraction pattern. Li[Li0.2Ni0.16Co0.1Mn0.54]O2 powders have an average diameter of about 10 μm, and the corresponding tap density is about 2.25 g cm-3. The specific discharge capacity of Li[Li0.2Ni0.16Co0.1Mn0.54]O2 electrode is 278 mAh g-1 at 0.03 C, in the voltage range of 2.0-4.7 V. This electrode can retain a reversible capacity of 215 mAh g-1 after 50 cycles at 0.1 C, corresponding to 95% retention capacity. Li[Li0.2Ni0.16Co0.1Mn0.54]O2 electrode was assembled to full cells with Si anode which can deliver an extremely specific energy of 590 Wh kg-1 based on the weight of cathode or anode (including active materials and binder). Specific energy of these full cells is much higher than current commercial lithium-ion batteries.A series of samples with varied lithium content and transition metal composition in the Li-Mn-Ni-O oxide pseudoternary were synthesized for the sake of understanding the influence of the lithium composition and transition metal on the structure and electrochemical performance. These samples were made by same precursors which was obtained through co-precipitation method. It has been found that the samples near the composition line which connected Li2MnO3 and LiNio.5Mno.5O2 still are hexagonal-layered phase. Scanning electron microscope was used to characterize the morphology of samples. It can be find that secondary particles of all samples have the size of 7-11μm and spherical morphology while the morphology of primary particles is different from each other. Samples with deficient lithium have small plate-like crystallites and larger hexagonal-like particles, while samples with excess lithium have plate- and needle-like shaped primary particles. When lithium content of the composition was decreased to a certain value, the structure will convert to hexagonal-layered and layered-rocksalt two-phase co-existence which was confirmed by the X-ray diffraction patterns. According to the metal compositions and the oxidation states versus atomic occupancy rules, it can be found that there are transition metal site vacancy existed in the samples with deficiency of lithium in composition as well as oxygen vacancy existed in the samples with excess of lithium in composition, respectively. The final chemical composition of these samples is Li[Li0.128□0.051Ni0.205Mn0.615]O2, Li[Li0.164□0.025Ni0.202Mn0.607]O2, Li[Li0.2Ni0.2Mn0.6]O2, and Li[Li0.234□-0.024Ni0.197Mn0.592]O2 respectively. The samples which possess transition metal site vacancy have low IRC comparing to the samples have no metal vacancy. This phenomenon may be caused by the avenues for enhanced atomic diffusion when charge to 4.45 V plateaus. Samples possessing oxygen vacancy displayed a large charge capacity. It can extract more lithium ion from the structure as compared to the others at 4.45 V plateaus because of activating the Mn sites as the redox center in the lithium extraction process. Lithium rich oxides materials exhibit amazing properties when transition metal or oxygen vacancy exist.