With the fast growth of consumer electronics market and wide commercialization of electric vehicles, the demand for advanced energy storage devices is soaring. The lithium ion battery is considered one of the most promising candidates to satisfy this demand due to its high energy density, superior cyclability, and low cost [1-3]. Since its discovery in 1991, LiCoO₂ has been commercially adopted as the intercalation compound for li-ion batteries because it is easy to synthesize, has high theoretical specific capacity, and has good reversibility [4,5]. However, high cost and high toxicity are associated with the use of Co [6, 7]. LiNiO₂ and LiMn₂O₄ are two feasible replacement materials for LiCoO₂, but they come with challenges and limitations. Although LiNiO₂ demonstrates higher specific capacity than LiCoO_2, phase transition of LiNiO₂ from hexagonal to cubic crystals occurs during the deintercalation process of the li-ion, and this dramatically reduces the reversible capacity of the material [8-9]. On the other hand, LiMn₂O₄ reacts with the electrolyte when the cell temperature exceeds 55 °C, leading to dissolution of manganese and the subsequent loss of capacity [10,11]. A novel cathode material, LiNi_1/3Co_1/3Mn_1/3O₂, was initially proposed by Ohzuku et al. back in 2001 [12]. This ternary solid solution preserves the benefits of LiCoO₂, LiNiO₂ and LiMn₂O₄ by synergizing three transition metal elements all at once. By partially substituting Co with Ni in LiCoO₂, higher specific capacity can be achieved [13]. The presence of electrochemically inactive Mn⁴⁺ in the lattice effectively suppress the phase transition of LiNiO₂, which improves the structural stability during cycling and enables a higher charge voltage without compromising the thermal stability of the cathode at the highly delithiated states [14,15]. More li-ions are impelled to intercalate and deintercalate from the electrode if the upper cut-off voltage is elevated from 4.2 to 4.5V or even higher, translating into a noticeable gain in the specific capacity and, in turn, a significant increase in energy density [16]. The remaining Co plays an important role in enhancing the conductivity of the NMC compound [17].

Numerous synthesis techniques have been employed to prepare NMC, including solid state reaction, co-precipitation, sol-gel and hydrothermal reaction [18-21]. However, several shortcomings are associated with these processes, such as structural inhomogeneity, irregular morphology, a series of time-consuming steps, and poor control of the particle size. Reactive spray deposition technology (RSDT), a single-step flame based deposition method, is capable of synthesizing and depositing NMC nanoparticles directly onto a stainless steel current collector to form a thin film electrode. The as-deposited electrode is ready for electrochemical testing in 2032 coin-type cell assembly without further processing. The particle size distribution and surface morphology of the deposited film can be well controlled through modifying the reactant concentration in the solvent, the feed rate of the precursor solution, and the flow rate of oxidant gases. The nanostructure of the LiNi_1/3Mn_1/3Co_1/3O₂ cathode fabricated by RSDT significantly shortens the diffusion length for the li-ions, offering potential to improve rate capability [22].

In this study, X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy (TEM) are employed to investigate the crystal and surface structure of the NMC thin film electrode. Electrochemical properties of as-deposited electrode are evaluated with using lithium foil as the counter electrode.

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