Based on nanoporous carbon electrodes electrochemical double layer capacitors (EDLC), otherwise known as supercapacitors or ultracapacitors, are currently widely used in various energy storage technologies, wherein the EDLC low internal resistance and long cycle life are at an advantage. It is still a good challenge to further reduce the internal resistance of EDLC since this can result in higher power density and higher efficiency of these promising power supply units. In this work it has been found that the EDLC internal resistance depends strongly on the electrolyte diffusion in the carbon electrode nanopores, and two techniques to measure the in-pore diffusion coefficients, namely, those based on spin-echo NMR or cyclic voltammetry with the use of porous rotating disc electrode are described. Cyclic voltammetry, impedance spectroscopy and transmission electron microscopy have also been used to select the best EDLC components. As a result, EDLC devices of very low internal resistance and high power density have been developed.

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Electrochemical double layer capacitors (EDLCs) and hybrid devices (HDs) become more and more popular solutions in various green energy technologies, in particu lar, in hybrid transport and wind power stations. After many years of research in EDLC and in HD technology we have developed some approaches and methods aimed at improving the performance of those devices. The results for EDLCs with the lowest inner resistance and highest power density, as well the results for HDs with the largest energy density as compared with the best similar compet ing devices are presented.

Understanding the degradation pathways of electrode materials is a key to develop more reliable Li-ion technologies along with an increased energy density and power rate. This study aims to demonstrate the benefits of the combined use of X-ray based characterization techniques and electrochemical assessment for thorough multi-scale anlaysis to elucidate the aging mechanisms of a Li4Ti5O12/AC//LiMn2O4/AC parallel hybrid lithium-ion supercapacitor. Analyses performed on samples extracted from full stack representative of industrial battery application, show that irreversible modifications are observed at all length scales on both electrodes. At the negative electrode, the disaggregation and corrosion of the LTO active material, as well as AC particle cracking and electrode film delamination have been observed. In the meantime, drastic cracking of the AC and LMO active material along with important micro-strain increase at the crystallite level for LMO as well as Mn3+ dissolution are reported at the positive. The formation of a cathode electrolyte interface is also reported. These structural and chemical changes have been identified as precursors to important polarization increase, Li inventory loss and furthermore capacity fading leading thus to device failure.