Lithium-ion Battery Cathode Material

Lithium ion battery,as the new generation of secondary battery with high energy density,has been widely welcomed by people.It presents many advantages such as high voltage,high energy density,long cycle life and high thermal stability.But with the advance of high-power consuming electric devices,a lot of efforts have been paid on looking for materials with excellent high-rate performances.LiCoO₂,as the first commercialized cathode material for lithium-ion batteries,is still the mainstream in the present market.It displays many favorable attributes including high capacity,stable discharge curve,excellent cycle life and high thermal stability.Herein in this dissertation we tried to improve the high-rate performance of LiCoO₂ using the method of carbon-coating.We synthesized carbon-coated LiCoO₂ by solid state method with ball milling.The feasibility of this method is proved by TEM and SEM.EIS results showed that conductive carbon markedly decreases the charge transfer resistance and enhances the lithium ion diffusion coefficient.In addition,the carbon-coated LiCoO₂ presents better high-rate performances,which is in accordance with our expectation.Though LiCoO₂ presents good performance,its high cost and the toxicity of Co limit its wide use.Layered LiCo_1/3Ni_1/3Mn_1/3O₂,using substitution of less expensive Ni and Mn for Co in LiCoO₂,lowers the cost of materials.In addition, LiCo_1/3Ni_1/3Mn_1/3O₂ also shows high discharge capacity,high thermal stability and excellent cyclic ability,which make it as a promising cathode material of lithium ion battery.In this dissertation,we synthesized LiCo_1/3Ni_1/3Mn_1/3O₂ with high performance by co-precipitation method,and analyzed it by all means of characterization methods.The results showed that the LiCo_1/3Ni_1/3Mn_1/3O₂ we synthesized has a layered structure ofα-NaFeO₂,with homogeneous cations-mixing. The size of the particle is around 200nm,with uniform distribution.The voltammetry in the region of 2.8～4.5 V displays typical redox peaks corresponding to the transition between Ni²⁺ and Ni⁴⁺ with Li⁺ intercalation/de-intercalation.During the charge/discharge process,LiCo_1/3Ni_1/3Mn_1/3O₂ showed excellent performances.Its first discharge capacity reached 206 mAh/g when we charged it to 4.6V.Even in the region of 2.5～4.3V,the discharge capacity remains 134 mAh/g after 20 cycles.Despite the positive effect of LiNi_1/3Co_1/3Mn_1/3O₂ for the electrochemical property,the delivered capacities have shown a capacity fading of long-term cycling. In order to overcome this problem,we tried to improve its property by the method of cation-doping.According to that during the electrochemical process,nickel and cobalt are the electro-active species,cycling between Ni²⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺,while manganese only plays the role of stabilizing the structure,remaining as Mn⁴⁺ through out charge and discharge,the doping will be focused on the position of Mn.We synthesized LiCo_1/3Ni_1/3Mn_1/3(1-x)M_1/3xO₂ with M=Cr or Zr by co-precipitation method, and studied the effects on cation-doping.The results showed that a small amount of Cr doping doesn't affect the layered structure of the material and 1%Cr doping showed better performances.It improves the lithium ion diffusion coefficient as well as the discharge capacity,which is much better than the sample with 3%Cr doping. This illustrates that doping can be effective only in a certain region;otherwise it will reduce the performance once exceeded.While the results of Zr doping showed no good to the material,it affected the structure of the material by lowering the intensity of XRD peaks and decreased the corresponding electrochemical performance.The possible reason is that the size difference between Zr and Mn is bigger than that between Cr and Mn,which could lead to a tiny distortion of the material and affected its electrochemical properties.Nanostructured materials have attracted great interest recently because of their unusual mechanical,electrical and optical properties endowed by confining the dimensions of such materials and the combination of bulk and surface properties to the overall behavior.They are now becoming increasingly important in LIBS due to the following reasons:(â…°) better accommodation of the strain of lithium intercalation/de-intercalation,improving cycle life;(â…±) higher electrode/electrolyte contact area leading to higher charge/discharge rates;(â…²) short path lengths for electronic transport.In this dissertation we also studied the cathode material of LiCoNiMnO with nanowire structure.We use LiCo_1/3Ni_1/3Mn_1/3O₂ as the precursor, reacted at 170℃for 36h and obtained the final nanowire material.The studies on the nanowire material showed that its discharge capacity is around 50mAh/g for the previous 15 cycles,and the first discharge capacity is higher than the charge capacity due to the loss of Li and Ni in the hydrothermal process.Its electrochemical properties still need further improvements.