Abstract
Understanding the materials design features that lead to high power electrochemical energy storage is important for applications from electric vehicles to smart grids. Electrochemical capacitors offer a highly attractive solution for these applications, with energy and power densities between those of batteries and dielectric capacitors. To date, the most common approach to increase the capacitance of electrochemical capacitor materials is to increase their surface area by nanostructuring. However, nanostructured materials have several drawbacks including lower volumetric capacitance. In this work, we present a scalable “top-down” strategy for the synthesis of EC electrode materials by electrochemically expanding micron-scale high temperature-derived layered sodium manganese-rich oxides. We hypothesize that the electrochemical expansion induces two changes to the oxide that result in a promising electrochemical capacitor material: (1) interlayer hydration, which improves the interlayer diffusion kinetics and buffers intercalation-induced structural changes, and (2) particle expansion, which significantly improves electrode integrity and volumetric capacitance. When compared with a commercially available activated carbon for electrochemical capacitors, the expanded materials have higher volumetric capacitance at charge/discharge timescales of up to 40 s. This shows that expanded and hydrated manganese-rich oxide powders are viable candidates for electrochemical capacitor electrodes.
Highlights
Electrochemical energy storage plays ever-increasing roles in our daily lives by powering everything from portable electronics to electric vehicles
On the subsequent cathodic scan, the overall current decreases and is relatively constant as a function of potential. These electrochemical changes suggest a change in the structure and energy storage mechanism in the material, which we attribute to expansion and interlayer water insertion as shown in Figure 1B and described in our previous work (Boyd et al, 2018)
This work describes a promising new material for Electrochemical capacitors (ECs) electrodes by detailing how electrochemically expanding micron-sized layered oxides and hydrating the interlayer leads to EC electrodes with high volumetric capacitance and cyclability in aqueous electrolytes
Summary
Electrochemical energy storage plays ever-increasing roles in our daily lives by powering everything from portable electronics to electric vehicles. Commercialized ECs are highly attractive for diverse applications requiring reliability, large currents, and high energy efficiency. They exhibit specific energy of up to 10 Wh/kg, specific power of up to 30 kW/kg, typical charge/discharge timescales on the order of seconds, and lifetimes of up to ∼1,000,000 cycles (Conway, 1999; Nybeck et al, 2019). Most commercialized ECs store energy by the rapid (within seconds) formation of the electric double layer at the electrode/electrolyte interface This mechanism can give rise to specific capacitance values of up to 150 F/g when the electrolyte ion size matches the average pore size of the electrode material (Largeot et al, 2008). Another capacitive energy storage mechanism, termed pseudocapacitance, Electrochemically Expanded Hydrated Manganese-Rich Oxides uses fast and reversible redox reactions in materials such as transition metal oxides and can lead to capacitances >150 F/g (Fleischmann et al, 2020)
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