Improvement of thin-film Ni-rich ALD cathode for microbatteries

Thin-film Li-ion batteries produced by precise thickness control atomic layer deposition (ALD) method are promising, safe, and high-energy density sources for miniature devices. The paper presents a detailed analysis of the multilayer thin film cathode for microbatteries. Structures were obtained by combining several technological parameters to increase capacity, uniformity, and lifetime. The protective function of a nano-sized coating was demonstrated. The influence of coating application on cathode capacity was explained. The studied structure consists of Ni-rich cathode layer modified with an amorphous Li-Ta-O functional layer on a steel substrate with a Cr coating. The cathode and the functional layer were obtained by the ALD multilayer approach followed by heat treatment at 800 °C for a minute. According to XPS and TEM data, the composition of the cathode after annealing was LiNi0.7Co0.3O2 well-defined structure without separation into Ni-rich and Co-rich layers, with texturing of crystallite pillars and with the absence of Fe. The Li-Ta-O films have a slight chemical composition gradient from Li3Ta1.2O4 on the surface to Ta1.7O5 in bulk. The electrochemical characterization showed that combining the functional layer and cathode heat treatment preserves the electrochemical capacity (32 µAh·µm−1·cm−2) and increases Coulomb efficiency.