Hybrid supercapacitors have been developed in the pursuit of increasing the energy density of conventional supercapacitors without affecting the power density or the lifespan. Potassium-ion hybrid supercapacitors (KIC) consist of an activated carbon capacitor-type positive electrode and a graphitic battery-type negative one working in an electrolyte based on potassium salt. Overcoming the inherent potassium problems (irreversible capacity, extensive volume expansion, dendrites formation), the non-reproducibility of the results was a major obstacle to the development of this KIC technology. To remedy this, the development of an adequate formation protocol was necessary. However, this revealed a cell-swelling phenomenon, a well-known issue whether for supercapacitors or Li-ion batteries. This phenomenon in the case of the KIC technology has been investigated through constant voltage (CV) tests and volume measurements. The responsible phenomena seem to be the solid electrolyte interphase (SEI) formation at the negative electrode during the first use of the system and the perpetual decomposition of the electrolyte solvent at high voltage. Thanks to these results, a proper formation protocol for KICs, which offers good energy density (14 Wh·kgelectrochemical core−1) with an excellent stability at fast charging rate, was developed.

A correlation between the evolution of the KF content in the SEI and the evolution of cycling performance of the non-aqueous potassium-ion hybrid supercapacitor (KIC).

With the increase of energy storage demand for many applications, sustainable technologies must be developed. On one hand, Li-ion batteries (LIBs) offer high energy density but high cost and safety issues for high-power density applications, especially at low temperature. On the other hand, supercapacitors (SCs) exhibit high power and cyclability but insufficient energy density and self-discharge problems. The aforementioned properties result in a performance gap between LIBs and SCs. They are unable to meet the requirements for specific applications for storing/delivering large amount of energy within a short time during many cycles like regenerative braking in vehicles, memory backup in electronics or high voltage pulse delivery in industrial and medical sectors. Therefore, devices with a good energy density, an excellent power density and a long cycling stability are urgently required. This is the purpose of a hybrid capacitor by combining the electrostatic phenomena at a positive capacitor-type electrode and the faradic process at an oversized negative battery-type electrode. Fast charging is then possible while ensuring a higher energy density than conventional EDLCs. This higher energy density is possible because of the widen operating voltage range in hybrid configuration. Thanks to the deployment of Li-ion battery, the use of lithium is generally applied on hybrid capacitors. However, technologies using lithium chemistries (LIC and LIB) are dependent on critical and strategic materials (copper, lithium), use carbonate electrolytes and give rise to safety problems and a limited lifetime linked to the formation of dendrites. The idea of replacing lithium with other alkali elements is then gradually emerging. For example, potassium benefits of abundant resources, low standard electrode potential as well as low costs.The potassium-ion capacitor (KIC) presented here consist of an activated carbon positive electrode and a graphite negative one immersed in an acetonitrile based-non-aqueous electrolyte and a potassium salt. The hybrid configuration avoids specific problems inherent in the use of potassium. Indeed, the system is designed so that a high stage of graphite intercalation compound (GIC) is obtained in order to avoid critical volume expansion. Furthermore, with the spontaneous reaction between acetonitrile and metallic alkali elements, dendrites formation is not possible.However, the technology still faces obstacles that are being studied to be removed. Among them, a non-monotonous cycle ageing, with discharged capacities which decrease then increase drastically in a few thousand cycles, as well as a non-reproducibility of the results for identical cells were observed with the system initially developed. These results will be presented in order to show how they could be improved. In addition, the operation of KIC systems results in gas generation that induces cell swelling. This is a well-known issue whether for supercapacitors or Li-ion batteries. In the case of KIC technology, it has been proven to ...