© Volker Presser, INM

High stor­age capac­i­ty and short charg­ing time — The nanoworld between bat­tery and capac­i­tor opens up new perspectives

Super­ca­pac­i­tors and bat­ter­ies are ener­gy stor­age types with dif­fer­ent advan­tages. While bat­ter­ies score with high stor­age capac­i­ties, super­ca­pac­i­tors impress with their short charg­ing time. Are there inter­sec­tions in the under­ly­ing tech­nolo­gies? Can the advan­tages from both worlds be com­bined? The team of authors led by Prof. Volk­er Press­er from the Saar­brück­en Leib­niz Insti­tute for New Mate­ri­als (INM) and Dr. Simon Fleis­chmann, Helmholtz Insti­tute Ulm (HIU), address­es this issue in their per­spec­tive arti­cle in the renowned sci­en­tif­ic jour­nal Nature Energy.

Time is a pre­cious com­mod­i­ty. This is espe­cial­ly true for elec­tro­chem­i­cal ener­gy stor­age devices: the almost emp­ty cell phone bat­tery short­ly before leav­ing home or the e‑car that has to stay plugged in for a few more hours before you can set off to vis­it rel­a­tives. In such cas­es, you want recharg­ing times to be as short as pos­si­ble. How­ev­er, rapid charg­ing and dis­charg­ing process­es are extreme­ly stress­ful for the elec­trode mate­ri­als in bat­ter­ies and short­en their ser­vice life. Super­ca­pac­i­tors do not have this prob­lem: unlike bat­ter­ies, no ions are incor­po­rat­ed into crys­tal lat­tices here, but are mere­ly deposit­ed on the enor­mous­ly large sur­face of acti­vat­ed car­bon. This means they store sig­nif­i­cant­ly less ener­gy than bat­ter­ies, but a few sec­onds are enough to recharge the cell.

To com­bine the best of both worlds, sci­en­tists are con­duct­ing inten­sive research into so-called pseudo­ca­pac­i­tors. These are elec­tro­chem­i­cal ener­gy stor­age devices that behave elec­tri­cal­ly like a capac­i­tor and can there­fore be charged par­tic­u­lar­ly quick­ly. Their ener­gy stor­age mech­a­nism, on the oth­er hand, works like that of a bat­tery: ener­gy is stored by ion inter­ca­la­tion in crys­tal lat­tices. These spe­cial prop­er­ties can often be achieved by using 2D mate­ri­als as elec­trodes. Dr. Simon Fleis­chmann, a for­mer INM employ­ee and doc­tor­al stu­dent at Saar­land Uni­ver­si­ty and now a junior research group leader at the Helmholtz Insti­tute in Ulm, explains: “The spe­cial thing about 2D mate­ri­als is their flex­i­ble inter­lay­er space. By selec­tive­ly adjust­ing the inter­lay­er spac­ing in the range around 1 nanome­ter, we can observe inter­est­ing nanoef­fects in the so-called con­fine­ment.” What is meant by this is that ions and elec­trolytes, which are need­ed for ion trans­port, behave quite dif­fer­ent­ly in such small nano-spaces than they do in a large vol­ume or on a sur­face. Prop­er “match­ing” of ion size, elec­trolyte and nanospace of the elec­trode lat­tice can enable sig­nif­i­cant increas­es in ener­gy stor­age capac­i­ty and fast-charge capability.

The stor­age mech­a­nism of pseudo­ca­pac­i­tors has so far been assigned to either capac­i­tors or bat­ter­ies. Cur­rent research by an inter­na­tion­al team led by Prof. Veron­i­ca Augustyn of North Car­oli­na State Uni­ver­si­ty has now estab­lished a uni­fy­ing approach to this. “We see a con­tin­u­ous tran­si­tion from very clas­si­cal lithi­um-ion bat­tery mate­ri­als to ide­al acti­vat­ed car­bon,” explains Volk­er Press­er, head of the Ener­gy Mate­ri­als pro­gram area at INM. “It is impor­tant to under­stand this grad­ual tran­si­tion from elec­trosorp­tion to inter­ca­la­tion as a spec­trum. Depend­ing on the size and geom­e­try of the nanospace, ions will (par­tial­ly) shed their elec­trolyte shells and can under­go redox process­es.” Which brings us back to 2D mate­ri­als like MXenes or lay­er-struc­tured met­al oxides. “The inter­lay­er space of 2D mate­ri­als in par­tic­u­lar is a great play­ground for us in mate­ri­als sci­ence. Here we can com­bine fast ion trans­port and high ener­gy stor­age capac­i­ty through reversible redox process­es by means of tar­get­ed mate­r­i­al design,” adds Simon Fleischmann.

The Per­spec­tive Paper “Con­tin­u­ous tran­si­tion from dou­ble-lay­er to Farada­ic charge stor­age in con­fined elec­trolytes” in the cur­rent issue of Nature Ener­gy is part of the long-stand­ing Amer­i­can-Ger­man-French coop­er­a­tion of the INM and an impor­tant sub­ject of the BMBF-fund­ed NanoMat­Fu­tur project of Dr. Fleis­chmann at the Helmholtz Insti­tute Ulm.