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 stability and capacity to intercalate Li. LiCoO₂ is commonly employed in commercial batteries as an intercalation compound because of its easiness in synthesis, high theoretical specific capacity and good reversibility. However, high toxicity and high cost are associated with the use of Co. 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 the electrolyte when the temperature exceeds 55 °C, leading to dissolution of manganese and the loss of capacity [7]. In the case of LiNiO₂ cathodes, its capacity is irreversibly lost when nickel oxide transforms from the hexagonal to the 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) could enhance the stability while adding the benefits of higher reversibility and lower cost [9-10]. It is also found from literature that when x is approximately 0.2, this material demonstrates better cycling properties than other materials mentioned above [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 structural inhomogeneity, multiple time consuming steps, irregular morphology and poor control of the particle size [14-16]. Reactive spray deposition technology (RSDT), a one-step flame based deposition method, offers a convenient solution to electrode fabrication by direct deposition of LiNi_0.8Co_0.2O₂ nanoparticles. Films can be grown directly on the current collector. RSDT has the capability to tailor the electrochemical properties of synthesized cathode materials by manipulating the several key processing conditions. The size, morphology, and composition of particles can be controlled by changing 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 flame combustion is prepared by dissolving acetylacetonates of lithium, cobalt and nickel into organic solvents 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.

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