The main scientific objective of Energy Caps is the development of a sustainable and safe hybrid supercapacitor with high specific energy of 100Wh/kg and maintained high specific power of 10kW/kg and cyclability of > 100000 cycles for electrical storage application in electric cars, plug-in hybrids and the smart grid.
For further information on the Testing Methodologies, please click on the following link:
FP7 EnergyCaps project: hybrid Li-ion capacitors (achievements at the prototype scale)
Needs for energy efficiency coupled with environmental concerns open new doors to the market for electrochemical double layer capacitors, otherwise known as ultracapacitors or supercapacitors (SC). Indeed, there is a tremendous opportunity for an energy storage device of high power capability that can be used either as a complement to conventional batteries or as a standalone system. In addition to early applications in consumer electronics, new powerful SC markets arise in the renewable energy and hybrid vehicle technologies. However, the SC market growth is still rather modest due to the lack of energy density.
The low energy density remains an obvious drawback of SC devices, and therefore, the Energy Cap project was aimed at developing the hybrid lithium ion capacitor (LIC) technology to combine SC and Li-ion components in electrode materials. Such a hybridization technique enables to increase the energy density by at least the order of magnitude keeping at the same time the power capability of hybrid LIC devices comparable with that for the best carbon-carbon SC devices available on the market.
After a thorough study of various electrochemical systems, the following ones have been selected for final prototyping:
- positive electrode – a mixture of Lithium iron phosphate (LFP) with YP50F active carbon (AC, produced by Kuraray, Japan);
- negative electrode – pre-lithiated hard carbon (HC) or Lithium titanate (LTO);
- electrolyte1 – LiTFSI (received from Solvay) in acetonitrile (AN);
- electrolyte2 – LiTFSI (received from Solvay)+ LiPF6 in AN;
- electrolyte3 – LiTFSI (received from Solvay) in mixture of fluorinated ethylene carbonate (F1EC, received from Solvay) and dimethyl carbonate (DMC);
- electrolyte4 – LiPF6 in EC+DMC (1:1 wt) – commercially available LP30 electrolyte, received from Novolyte Technologies Co.
Two series of 500F LIC prototypes have been assembled, namely, 8 cells based on LTO/LFP+AC electrochemical system, and also 8 cells based on HC/LFP+AC system. Specific energy values of 60 Wh/kg for LTO/LFP+AC and of 86 Wh/kg for HC/LFP+AC prototypes have been achieved, and it has been found that LTO/LFP+AC prototypes demonstrate the highest power density of all the systems tested (~4kW/kg at ~80% of efficiency). No prototype degradation has been observed during the tests.
Fig. 1 illustrates typical Ragone plots, i.e. available energy density vs. power density, for LIC cells based on LTO negative electrode.
Fig. 1. Ragone plots for LIC cells based on the LTO negative electrodes
and two electrolytes: 1 – LiTFSI in AN; 2 –LiTFSI + LiPF6 in AN.
So, the LIC devices developed in the framework of EnergyCaps project can be recommended for further development and use. Most promising areas cover the applications wherein the fast charge is critical and the power supply unit must deliver high power within tens or hundreds of seconds. In this range the LIC devices can provide a unique performance, since Li-ion batteries, as high-energy devices, cannot be charged/discharged very fast. Even Li-ion batteries specially designed for high-power application would overheat after a few repeated high power cycles and would face severe safety issues like fire or explosion. On the other hand, conventional SC, as high-power devices, may compete in a time range up to 10 or maximum 20 seconds. Beyond this time range, SC’s are limited by their low-energy density.
The LIC devices fill efficiently the gap between Li-ion batteries and supercapacitors. Some examples of possible application are:
- hybrid vehicles (cars, buses, trucks…);
- hybrid garbage trucks (refuse collection vehicles);
- cranes, earth moving machines, hybridforklifts;
- electric transport, in particular, public transport;
- uninterrupted power supply;
- power tools;
- kids’ toys.
Some of the results were also presented by F. Gauthy, S. Isikli, Y. Maletin, V. Khomenko and N. Stryzhakova (5 presentations) at the 6th International Conference on Carbon for Energy Storage/Conversion and Environment Protection (CESEP’2015) in Poznan, Poland, October 18-22.