All-Organic Battery Based on Deep Eutectic Solvent and Redox-Active Polymers.

Sustainable battery concepts are of great importance for the energy storage demands of the future. Organic batteries based on redox-active polymers are one class of promising storage systems to meet these demands, in particular when combined with environmentally friendly and safe electrolytes. Deep Eutectic Solvents (DESs) represent a class of electrolytes that can be produced from sustainable sources and exhibit in most cases no or only a small environmental impact. Because of their non-flammability, DESs are safe, while providing an electrochemical stability window almost comparable to established battery electrolytes and much broader than typical aqueous electrolytes. Here, we report the first all-organic battery cell based on a DES electrolyte, which in this case is composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and N-methylacetamide (NMA) alongside the electrode active materials poly(2,2,6,6-tetramethylpiperidin-1-yl-oxyl methacrylate) (PTMA) and crosslinked poly(vinylbenzylviologen) (X-PVBV2+ ). The resulting cell shows two voltage plateaus at 1.07 V and 1.58 V and achieves Coulombic efficiencies of 98 %. Surprisingly, the X-PVBV/X-PVBV+ redox couple turned out to be much more stable in NaTFSI : NMA 1 : 6 than the X-PVBV+ /X-PVBV2+ couple, leading to asymmetric capacity fading during cycling tests.

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM-D.

Rechargeable magnesium batteries could provide future energy storage systems with high energy density. One remaining challenge is the development of electrolytes compatible with the negative Mg electrode, enabling uniform plating and stripping with high Coulombic efficiencies. Often improvements are hindered by a lack of fundamental understanding of processes occurring during cycling, as well as the existence and structure of a formed interphase layer at the electrode/electrolyte interface. Here, a magnesium model electrolyte based on magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2 ) and MgCl2 with a borohydride as additive, dissolved in dimethoxyethane (DME), was used to investigate the initial galvanostatic plating and stripping cycles operando using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). We show that side reactions lead to the formation of an interphase of irreversibly deposited Mg during the initial cycles. EQCM-D based hydrodynamic spectroscopy reveals the growth of a porous layer during Mg stripping. After the first cycles, the interphase layer is in a dynamic equilibrium between the formation of the layer and its dissolution, resulting in a stable thickness upon further cycling. This study provides operando information of the interphase formation, its changes during cycling and the dynamic behavior, helping to rationally develop future electrolytes and electrode/electrolyte interfaces and interphases.

Combining Deep Eutectic Solvents with TEMPO-based Polymer Electrodes: Influence of Molar Ratio on Electrode Performance.

For sustainable energy storage, all-organic batteries based on redox-active polymers promise to become an alternative to lithium ion batteries. Yet, polymers contribute to the goal of an all-organic cell as electrodes or as solid electrolytes. Here, we replace the electrolyte with a deep eutectic solvent (DES) composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and N-methylacetamide (NMA), while using poly(2,2,6,6-tetramethylpiperidin-1-yl-oxyl methacrylate) (PTMA) as cathode. The successful combination of a DES with a polymer electrode is reported here for the first time. The electrochemical stability of PTMA electrodes in the DES at the eutectic molar ratio of 1 : 6 is comparable to conventional battery electrolytes. More viscous electrolytes with higher salt concentration can hinder cycling at high rates. Lower salt concentration leads to decreasing capacities and faster decomposition. The eutectic mixture of 1 : 6 is best suited uniting high stability and moderate viscosity.