Supercapacitors or electrochemical double layer capacitors (EDLCs) are energy storage devices with moderate energy and high power densities and can act complementarily to batteries which have high energy and low power densities, in applications like electric vehicles. Significant effort is spent on developing and improving supercapacitor materials like activated carbon, electrolytes, and separators. However, high-performing materials do not always translate to high performing devices. This work describes a theoretical framework coupled with experiments that establishes two necessary and sufficient conditions to build supercapacitors with optimized performance. Establishing lower bounds for the electrolyte conductivity when the device is at a fully charged state and the excess electrolyte volume automatically apply constraints on the material, process, and design variables. For a Type A electrode system (specific capacitance = 75 F cm(-3)), the lower bounds for conductivity and excess electrolyte volume were found to be 28 mS cm(-1) and 3 mL for a D-cell sized supercapacitor. The underlying philosophy of counting ions and matching it with the number of electrons to optimize materials can be modified for individual systems (for example, by introducing Faradaic reaction components for pseudocapacitors) and be broadly applied to energy storage technologies such as pseudocapacitors, lithium batteries and redox flow batteries.
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