Fabrication of thin film cathode materials are of huge importance to fulfil the criteria of high energy density Li-ion micro-batteries (LIMBs) with the size within the range of 1-10 mm³ volume including all components and packaging with energy densities of 100 Wh/L.¹. In this context, electrochemical deposition (ECD), is a versatile and straightforward approach compared to commonly applied techniques such as sputtering, pulse laser deposition (PLD), and atomic layer deposition (ALD)², enabling conformal and coherent deposition of films under mild conditions. Manganese oxides are being considered as the promising cathode materials for LIB/LIMBs owing to their high abundance, non-toxicity and rich electrochemistry arises from various oxidation states of Mn as well as high theoretical capacity. Among the vast variety of reported polymorphs of MnO₂, birnessite phase (δ-MnO₂) with 2D layered structure comprised from edge-sharing MnO₆ octahedra is of particular interest. To the best of our knowledge, there are no comprehensive publications focusing on the electrochemical performance of lithiated δ-MnO₂ as a promising cathode material of LIB/LIMBs. Although Popov et al. reported one-step ECD of birnessite Li_0.35MnO₂.H₂O via anodic potentiostatic method with applying 1.0 V vs. Ag/AgCl in 3-electrode cell in the bath containing an aqueous solution of 2 mM MnSO₄ and 50 mM LiClO₄. They reported the initial discharge capacity of 73 mAh.g^-1 for the electrodeposited Li_0.35MnO₂.H₂O which rapidly decayed rapidly after 8 cycles to 56 mAh g^-1.⁴

Herein, we report one-step cathodic electrodeposition of birnessite Li_0,17MnO₂.0.8H₂O via potentiostatic technique using 3-electrode cell from aqueous solution of 10 mM KMnO₄ and Li₂SO₄ (pH =7) on the surface of stainless steel 316 (SS-316) as 2D substrate. The crystalline structure of the electrodeposited Li_0,17MnO₂.0.8H₂O was assigned to birnessite layered phase with C2/m space group based on the PXRD patterns. The SEM images proved the microstructure of the electrodeposited δ-Li_0.17MnO₂ comprised of beneath rugged layer with columnar structures mounted on top of the latter layer with thickness of ~19 µm. The adherence of the electrodeposited film was excellent on the bare surface of SS-316 without any further treatment or sputtering of adhesion layers on the surface with no signs of peeling off after annealing/handling of the films. High resolution XPS spectra of Mn 2p elucidated the annealed deposited film dominantly comprised of Mn (III) with coexistence of Mn (III) and Mn (IV) species in the lattice as a result of trapped Li⁺ and K⁺ cations in the interlayer space. The electrodeposited δ-Li_0.17MnO₂ film on the surface of SS-316 after annealing at 200 °C under ambient air was tested as the cathode of Li-ion cell against metallic Li. To this end, CR-2032 coin cells were assembled inside Ar filled glove-box using LiPF₆ (1M) in EC/DMC (1:1) as the electrolyte. The electrodeposited film was able to render the first discharge capacity of 200 mAh g^-1 at 0.1 C accounting for ~70% of the theoretical capacity retaining ~100 mAhg^-1 after 60 cycles and 75 mAh g^-1 after 100 cycles at 0.1 C. In spite of the sever capacity fade after initial cycles, the delivered capacity became quite stable after 8^th cycle with delivering ~100 mAh g^-1. Also, δ-Li_0.17MnO₂ films showed good rate capability by delivering the discharge capacities of 113 (565 mAh cm^-3), 107 (535 mAh cm^-3), 97 (485 mAh cm^-3) mAh g^-1 at 1 C, 5 C and 10 C respectively which are comparable to the performance of LiMnO₂ thin films fabricated with r.f. magnetron sputtering.⁵

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