(312b) Model-Assisted Development of Microfabricated 3D Ni(OH)2 Electrodes with Rapid Charging Capabilities

Recently, several studies have focused on the development of materials that realize batteries with improved power densities, and thus, rapid charging and discharging capabilities. Yet, they are primarily based on non-deterministic fabrication techniques which limit the control over critical electrode dimensions affecting the power performance of the battery, such as the electrode surface area and the active material thickness. The study reported herein involves rationally designed and deterministically engineered 3D structures with nearly precise control over the optimized dimensions that address the aforementioned critical factors affecting the high-power capability and ultimately determine the performance of the electrochemical system.

A series of fabrication methods encompassing electrochemical techniques and microfabrication technologies have been utilized for the formation and characterization of the scalable, well-ordered, and high-surface-area 3D architectures that can be potentially used as high-power electrodes in a variety of applications, ranging from autonomous microsystems to macroscale portable electronics. These versatile structures were based on a large number of thin layers of two different materials, i.e., Cu and Ni, the former being sacrificial while the latter was structural, fabricated through a temporary photoresist mold in an alternating sequence via an automated robotic system. The final structure was obtained by the selective removal of the sacrificial Cu layers, leaving behind a large number of separate Ni layers to be coated with the electrochemically active material. Nickel hydroxide (Ni(OH)₂), one of the most well-studied active material in secondary (i.e., rechargeable) battery systems along with Li-ion chemistry, was selected as the active material to be deposited onto the 3D multilayer structure. Electrodes with areal capacities as high as 2.43 mAh cm^-2 were realized, which were able to deliver more than 50% of their capacities when charged at ultrafast rate of 150C. The resultant electrodes also featured remarkable cycling stability when charged and discharged at high rates for more than 80 cycles.

For the optimization of the characteristic dimensions of the electrodes, a two-dimensional mathematical model has been developed in COMSOL 5.2 by employing fundamental mass transport and reaction kinetics principles. The geometry of a repeating unit of the multilayer electrode structure was studied. Fickâ??s second law, Nernst-Planck equation, and Butler-Volmerâ??s equation were used to describe the solid phase diffusion, the electrolyte phase transport, and the electrode/electrolyte interfacial kinetics, respectively. The model was shown to successfully predict the energy/power tradeoff of the fabricated multilayer electrodes. Diffusion coefficient of protons within the Ni(OH)₂ active material, a critical parameter that determines the power performance of the Ni(OH)₂ electrode, was experimentally measured using cyclic voltammetry. The obtained value of 2~3*10^-10 cm²/s is within the range reported from existing literatures and close to the best-fit value (3.4*10^-10 cm²/s) obtained from the model. Accordingly, effort was directed towards finding the optimal electrode dimensions that lead to enhanced power performance based on the model. Geometrical parameters that were taken into account included the active material thickness, the interlayer spacing between two individual layers, and the nickel current collector thickness. Projections showed that multilayer structures based on the optimized dimensions exhibited superior performance than the experimentally fabricated non-optimized structures. Depending on the energy and power needs, this model opened up the possibility for the realization of deterministically engineered electrodes for both micro- and macroscale applications.