Energy storage system powered by forest waste retains 60% capacity after 10,000 cycles

In a study published in the Journal of Power Sources, researchers at the University of the Basque Country in Spain (UPV/EHU) presented an energy storage system made using electrodes derived from wood biomass.

The wood biomass they used comes from a variety of pine tree, and it is widely available as a waste product from sawmills, making it both a cost-effective and sustainable method according to the Solid State and Materials Research Group at the UPV/EHU.

The group created the system using electrodes resulting from the discarded wood chips that they combined into a lithium-ion capacitor (LIC), a hybrid system combining batteries and supercapacitors. The negative electrode is made of hard carbon and achieves high capacity values of up to 112 mAh g⁻¹ at 10C without complex doping procedures, the use of expensive additives or complex processing.

The positive electrode uses an activated carbon derived from the same hard carbon. The system’s energy density is up to 105 Wh kg⁻¹ at 700 W kg⁻¹, retaining 60% capacity after 10,000 cycles at 10C.

The hard carbon electrodes achieved 112 mAh g⁻¹ at 10C, while the activated carbon electrodes achieved 71 mAh g⁻¹ at 10 A g⁻¹.

For the full cell, the cell tension and electrode mass ratios were studied to optimize the performance of each electrode and prevent issues like lithium plating or electrolyte decomposition. The best performing full cell used 1:1 mass ratio and operated in a cell voltage of 1.5–4.0 V. This system achieved energy density values of 111 Wh kg⁻¹ at 51 W kg⁻¹ and 52 Wh kg⁻¹ at 24.4 kW kg⁻¹.

It demonstrated a capacity retention of 70% after 5,000 cycles and 60 % after 10,000 cycles at 10C (calculated relative to the anode).

Although less common than the lithium-ion batteries and supercapacitors, LICs are being recognized in research due to their ability to deliver a long cycle life and a source of high power. LICs combine electrodes from both LIB and supercapacitor technologies to leverage the advantages of each.

In an interview with UPV/EHU’s campus magazine, the study’s co-author Eider Goikolea said, the approach “offers the advantages of both systems: high-power energy can be stored (as in batteries), it can operate at high-power levels and is able to withstand many charge-discharge cycles (like supercapacitors).” LICs enjoy a wide potential window, provide high energies (150–200 Wh kg) and present a very low self-discharge rate.

Idoia Ruiz de Larramendi, also a co-author of the study, pointed out that not all biomass provides the right carbon for this application. However, “very satisfactory results can be obtained from the biomass” of the particular pine they used, the pinus radiata.

“It constitutes a cost-effective, sustainable alternative for improving conventional lithium-ion capacitors. Materials originating from biomass offer great opportunities for developing eco-friendly, cost-effective high-power energy storage systems. It is important to further this line of research”, the UPV/EHU researchers said.

They prioritized using cost-effective, sustainable processes to produce the electrodes for the system. “The process to produce the electrodes was energy-efficient. The synthesis temperatures did not exceed 700 °C,” and economical additives were used.