Paper TF2-MoM3
Radical Enhanced Atomic Layer Deposition of Cobalt Oxide Based Electrodes for 3D Lithium-ion Battery Applications

Monday, October 22, 2018, 9:00 am, Room 104B

Lithium-ion batteries have been an enabling factor in the success of consumer electronics and have the potential to offer energy storage solutions for microelectromechanical systems (MEMS). Current thin film battery technology consists of a two-dimensional planar stack of materials characterized by poor volumetric utilization where large areal footprints are required to supply the needed energy and power for device operation. Moving away from these traditional two-dimensional batteries towards next generation three-dimensional battery architectures (e.g. cylindrical arrays, interdigitated plates, etc.) allows for an effective decoupling of the areal energy and power density resulting in improved areal footprint utilization. In 3D architectures, the short distances between the anode and cathode improve the transport properties allowing for high areal power densities and the high aspect-ratio nature of the electrodes promotes high areal energy densities. Integration with these 3D architectures presents a challenge—requiring the synthesis of conformal thin films of both the electrolyte and counter-electrode, where optimization of the solid electrode/electrolyte interface is crucial for optimal device performance. Cobalt oxide is a potential candidate as a high capacity thin film anode material demonstrating lithiation capacities of 716 and 891 mAh/g for CoO and Co₃O₄, respectively.

The atomic layer deposition of cobalt oxide thin films was explored via a radical enhanced process employing the use of the metalorganic precursor, cobalt(II) (tmhd=2,2,6,6-tetramethylheptane-3,5 dione) and atomic oxygen, in addition to a thermal process involving H₂O. The growth rate of the radical enhanced CoO_x films was determined to be 0.3Å/cycle demonstrating a stable ALD temperature window from 190-230°C. The as-deposited cobalt oxide thin films demonstrated polycrystalline character on Pt(111)-Si substrates and a post deposition annealing treatment was utilized to further promote crystallization of the Co₃O₄ phase. A 75 nm CoO_x thin film demonstrated lithiation capacities ranging from 3,200 to 2,500 mAh/cm³ at C/6 to 2C rates with a voltage cut-off of 0.4V vs. Li/Li⁺. The discharge capacity and rate-ability were explored as a function of film thickness and post deposition annealing treatment conditions. Crucial in the realization of the all solid state 3D lithium-ion batteries is the optimization of the solid electrode/electrolyte interface. A solid electrolyte material, Li_xAl_ySi_zO, synthesized via a thermal ALD process utilizing H₂O as the oxidant source was integrated with the cobalt oxide electrode materials and their electrochemical properties explored.