(Invited) Electrode and Electrolyte Design for Low-Temperature Supercapacitors
Authors: Wang Xuehang , Gogotsi Yury 
Published: February 2020

An extension of reliable operation temperature for an electrical energy storage device is required by specialized application, such as oil industry, military, space exploration and electric vehicles.1-2 Supercapacitors, which stores charges electrostatically, are more suitable for low temperature applications, since the operation of batteries below -30 oC is limited by its faradic charge storage mechanism. Ionic liquids are attractive electrolytes for supercapacitors, because their good (electro-)chemical stability provides safety, large voltage window and large energy density. However, supercapacitors using ionic liquid electrolytes, suffer a substantial drop of capacitance at low temperatures. This is because a large fraction of small pores is not accessible by electrolyte ions, especially for ionic liquid with low ionic mobility and large ion size.3

To enhance the performance of a supercapacitor with ionic liquid electrolyte at low temperature, studies have been focusing on facilitating ion accessibility to the electrode porous structure. Mitigating the Coulomb ion pairing (anion-cation attraction), such as adding organic solvent and binary mixture of ionic liquid or eutectic electrolyte, has been used to establish sufficient fluidity and improve ions mobility of electrolytes at low temperatures.4 Combining two different species of cations and only one anion mixtures (or vice versa) can lower the melting point of ionic mixture to -80 oC.5 In addition, adjusting the morphology of the electrode material is another way to enhance the ion transportation dynamically. For example, with larger inter-tube distance and more open porous structure, carbon nanotube showed a much better rate performance than that of the onion-like carbon at the temperature of -50 oC. Similarly accessible pore structures ae required for operation of organic electrolytes at low temperatures.
With a fast development of electrode materials for supercapacitor, the choice of material extends from activated carbons, carbide-derived carbons, graphene, carbon onions and nanotubes, to many pseudocapacitive materials (carbides, nitrides, oxides, etc.), and the morphology of the material also varies in dimensions and confinement. Many unique electrolyte ion behaviors have been found or are waiting to be found in various electrode material with specific physical and chemical properties. Hence, a proper design of electrode material in conjunction with an electrolyte formulation will offer many opportunities for enhancing the supercapacitor performance at low temperatures.
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