Synthesis And Modification And Its Lithium Storage Performance Of Spinel LiMn₂O₄, AND LiNi_0.5Mn_1.5O₄

Rechargeable lithium ion batteries are regarded as promising energy storage devices for portable electronic devices as well as electric vehicles (EV) and hybrid electric vehicles (HEV), due to their high energy density, high power density, low cost, superior safety, and stable cycling lifespan. Among various lithium-ion batteries-being developed, the spinel lithium manganese oxide(LiMn2O4) seems to be one of the most promising cathode materials because of its intrinsic advantages such as the abundant and cheap resources, environmental benignity, better safety, high voltage and good rate of capability. However, along with these exceptional advantages, capacity decay and poor cycling stability of spinel LiMn2O4 cathode materials hamper its application in commercial lithium-ion batteries. To solve this problem, various strategies have been attempted by researchers.(1) MnCO3 powder with a particle size of about 1 mm can be obtained by controlling the experimental conditions accurately. In general, the LiMn2O4 powder composed of spherical particles could lead to a higher tap density, which would enhance the volumetric energy density compared with the irregularly shaped nanoparticles. Nanomaterials facilitate rapid ionic diffusion and electronic transport due to drastically shortened transport distance, thus enabling better power performance as compared to the bulk electrode. When the microsphere LiMn2O4 are applied as cathode materials for rechargeable lithium-ion batteries, its first discharge capacity is 116 mAh g-1 at 0.2 C rate and the retention rate of capacity is 98.3% after 50 cycles, and discharge capacity is over 54.9 mAh g-1 at 5 C. However, it still remains a challenge to synthesize LiMn2O4 microspheres in large scale because of the complicated technology for preparation and low production yield.(2) Surface modification is one effective method to prevent the dissolution of Mn2+. Spinel lithium manganese oxide microspheres have been prepared by solid state reaction between Li2CO3 and MnCO3 microspheres obtained by co-precipitation method. Polypyrrole is a kind of conductive polymer, has low toxicity, high specific capacity, cycle characteristics and good stability. Chemical oxidation method.has been used to obtain the surface coated lithium manganese microspheres (PPy@LiMn2O4). Polypyrrole is formed on the surface of double electron layer has a charge capacity, makes the PPy@LiMn2O4 microspheres at the rate of 0.2 C during the first discharge capacity reached 118.4 mAh g-1. In addition, the polypyrrole coated layer which has a high conductivity can be used as the cathode material of the conductive agent and binder, reducing the charge transfer resistance LiMnaO4 interface, improving the surface of lithium ion and electron transfer rate, and then improve the rate performance of electrode. Its discharge capacity remains 104.5 mAh g-1 at the rate 5 C.(3) The dissolution of Mn2+ ions are reported to have a tendency to dissolve into the electrolyte and further deposit on the surface of the anode, and the deposition subsequently increases the impedance of the battery and causes capacity fading. A potential approach to overcome this problem is to modify the surface of LiNi0.5Mn1.5O4 with a thin layer of coating material, which is a strategy that has been successfully used on a number of cathode and anode materials. The formation of Li3PO4@LiNi0.5Mn1.5O4 microspheres coated lithium phosphate is carried out by mixing ammonium hydrogen phosphate with different proportion of the precursor and lithium manganese carbonate with a heat treatment. Discharge capacity of primary LiNi0.5Mn1.5O4 and Li3PO4@LiNi0.5Mn1.5O4 with 4% ratio of coating material were 77.4 mAh g-1 and 102.6 mAh g-1 at 2 C. Though the rate discharge volume of Li3PO4@LiNi0.5Mn1.5O4 is lower than primary LiNio.5Mn1.5O4 at low charge rate, the coating material discharge specific capacity improved obviously at high rate. After 10 charge/discharge cycles at high rate, the capacity retention rate was 95% LiNi0.5Mn1.5O4 and the proportion of Li3PO4@LiNi0.5Mn1.5O44% coating material capacity retention rate as high as 97%.