Direct Dry Synthesis of LiNi0.8Co0.2O2 Thin Film for Lithium Ion Battery Cathodes

In recent years, a number of cathode materials have been investigated for lithium ion battery applications. Transition metal oxides like LiCoO₂ [1-2], LiNiO₂ [3-4] and LiMn₂O₄ [5-6] have attracted much attention from researchers due to their good recharge capabilities. LiCoO₂ is commonly employed in commercial batteries as intercalation compound because of its easiness in synthesis, high theoretical specific capacity and good reversibility. However, high toxicity and high cost are associated with this material. Lithium nickel oxide and lithium manganese oxide are two feasible replacement materials for lithium cobalt oxide, but they come with challenges and limitations as well. LiMn₂O₄ reacts with electrolyte when the temperature exceeds 55 °C, leading to the loss of capacity from the dissolution of manganese [7]. In the case of LiNiO₂ cathode, its capacity can be irreversibly lost when nickel oxide transforms from hexagonal to cubic phase during the lithium deintercalation process [8]. This phase transformation can be prevented by partially replacing nickel with cobalt, thus LiNixCo₁₋xO₂ (0 < x   < 1) is proposed with the aim of improving the stability of cathode material, while adding the benefits of high reversibility and low cost [9-10]. It is also found from literature that when x is approximately 0.2, this material performs the best electrochemically [11-13].

While solid state reactions and sol-gel methods have been widely used to synthesize LiNi_0.8Co_0.2O₂ powders, there are many problems associated with these processes, such as inhomogeneity, multiple time consuming steps, irregular morphology and poor control of the particle size [14-16]. Reactive spray deposition technology, a one-step flame based deposition method in ambient environment, offers a convenient solution to electrode fabrication by direct deposition of LiNi_0.8Co_0.2O₂ nanoparticles onto the current collector to form a thin film. This technique has the capability to tailor the electrochemical properties of synthesized cathode materials by manipulating the processing conditions. The size, morphology, and composition of particles can be controlled via deposition parameters, including the reactant concentration in the solvent, the heat enthalpy of solvent, the flow rate of precursor solution, and the flow rate of oxidant gases. Precursor solution for the flame combustion is prepared by dissolving acetylacetonates of lithium, cobalt and nickel into organic solvent like methanol. X-ray diffraction (XRD), transmission electron microscopy (TEM) and inductively coupled plasma (ICP) are used to characterize the chemical composition, crystal structure, and structural morphology of the synthesized LiNi_0.8Co_0.2O₂ nanoparticles. Electrochemical properties of as-deposited LiNi_0.8Co_0.2O₂ thin film are evaluated with typical 2032 coin half-cell using lithium foil as the counter electrode. As the particle size of cathode material is in the nanometer range, the diffusion length of lithium ions is small, which enables a fast charge and discharge rate. The large surface area of the nanoparticles can also reduce the resistance of charge transfer. In order to further investigate the electrochemical performance of the material, a coin full-cell using graphite as the anode is tested.

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