Electrolyte is one of the key components of electrochemical capacitors (ECs) and gives the ionic conductivity and enhances charge storage process on electrodes. Currently, most of the existing ultracapacitors employ organic electrolytes that are electrochemically stable for applied voltages ranging from 2.5 V to 2.8 V. Electrolyte plays cardinal functions such as supply of ions, electron conduction and adhesion of electrode particles in determining the performance of ECs, such as solid-state devices. An appropriate ultracapacitor’s electrolyte should basically have: broad decomposition potential window, high electrochemical stability, high concentration of ions, low resistivity, low toxicity, etc. It is more effective to improve energy density by increasing the applied voltage window going by the fact that energy and power densities are proportional to voltage squared according to equations of energy and power densities. This is successively achievable by selecting suitable electrolytes that have broader potential window. The formulation or design of novel electrolytes components is taken as one of the significant approaches to achieving the next-generation ultracapacitors with enhanced energy. Advanced electrolytes for ultracapacitors with high electrochemical stability are important in enhancing device suitability for various usages. Investigation of ECs electrolytes that use propylene carbonate (PC) and butylene carbonate (BC) solvents have been carried out. A useful guidelines and requirements for determination of optimum ratios, proper organic electrolytes for optimal performance and the entire blueprint and fabrication of ultracapacitors of enhanced energy and power with a reduction in device mass and volume have been presented. Performances of asymmetric EC can strongly be enhanced with a reduction in cell mass and volume by employing suitable electrode mass and operating potential range ratios employing aqueous electrolytes, and those with suitable electrode mass, operating potential range ratios with organic electrolyte of appropriate operating potential range and specific capacitance. Storable and deliverable energies of the asymmetric EC using suitable electrode mass and operating potential range ratios employing proper organic electrolyte are improved by a factor of 12.9 with 1.73 reduction in cell mass and volume, compared to those of symmetric EDLCs employing aqueous electrolyte. Storable energy, energy density and power density of symmetric EDLCs can be enhanced 5.56 folds with 1.77 folds reduction in cell mass and volume by using suitable electrode mass, operating potential range ratios and proper organic electrolyte. Also, introduction of an asymmetric electrode and organic electrolyte was very successful in improving the performance of the EC with reduction in cell mass and volume.

These guidelines and requirements aid in development of asymmetric ultracapacitors with optimum battery-type mass ratio, potential range ratio, maximum potential range ratio and ratio of capacitance of capacitor type. Also, ECs models that incorporated various self-discharge mechanisms used to determine minimum impurity or redox species concentration and optimum total thickness of separator and anode for self-discharge suppression were presented. Numerous groups have presented lithium-ion capacitors (LICs) as the new-generation ultracapacitors because they have the capability to give higher energy density compared with the commercial organic ECs. LICs’ higher energy density is because of Li intercalation/de-intercalation phenomena. The crucial factors in development of new-generation ultracapacitors with high voltage and outstanding performance are: the design or formulation of electrolytes with enhanced features like high ionic conductivity, viscosity and electrochemical/chemical stability modified to permit high applied voltages, low equivalent series resistance and long cycling. The mass ratio of anode and cathode is shifted for balancing charges so as to obtain maximum applied voltage and long life; and the asymmetric arrangements are needed to obtain capacitors with elevated voltage, power and energy densities. Concerning blueprint and optimization of novel electrolyte, choice of electrolytes to enhance the electrochemical stability potential windows, ionic conductivity, thermal stability, reduced viscosity and clearer basic understanding via experiments and theories is essentially needed. Research and development by industries and academia showed that lithium-ion capacitors could function with an applied voltage of 3.7–4.1 V while maintaining high cycle-life and perfect power density, which are regarded as real electrochemical capacitors.

Keywords: Aqueous electrolytes; organic electrolytes; ionic liquids electrolytes; solid-state electrolytes; ionic conductivity; electrochemical stability; energy and power densities; decomposition potentials; electrolyte stability potential window and cell voltage.