Energy storage has always involved a trade-off. Capacitors charge in seconds and cycle almost indefinitely but store little; batteries store far more but charge slowly and wear out. A team from Zhejiang University and the National University of Singapore has built an electrode material that combines both modes: a molecular cage that stores energy capacitively at low voltage and through a redox reaction at high voltage. In an aqueous zinc-bromine battery, the design delivered 481.8 mAh/g at a current of 0.83 A/g and retained 89.4% of its capacity after 1,500 cycles at ten times that current. The work appears in Nature Communications.
Zinc-bromine cells run on the Br2/Br- redox couple, with a theoretical capacity of 335.5 mAh/g and a reaction potential near 1.82 volts against zinc. The ingredients are cheap, abundant, and water-based, which is why zinc-bromine has long been a candidate for grid-scale storage, particularly in flow-battery form. The chemistry has known drawbacks. The intermediate polybromides, Br3- and its relatives, drift through the electrolyte, a phenomenon called the shuttle effect, corroding the zinc anode and draining the cell. The conversion between bromide and bromine is slow, and carbon-based electrodes that avoid the corrosion problem by relying on capacitive storage never reach high capacity. The field has been stuck between a chemistry that stores well but misbehaves and a material that behaves but stores little.
The design that breaks the deadlock is a pillararene, a class of pillar-shaped macrocyclic molecules with a hollow cavity at the center. The team, with Yongxing Ding and Mingming Han as co-first authors and Hao Cheng of Zhejiang University, Siyuan Li of the National University of Singapore, and Yingying Lu of Zhejiang University as corresponding authors, used 1,4-bis(ethoxy)pillar[6]arene, abbreviated BEP6A, treating the cavity as a microscopic reactor rather than a passive scaffold. At low voltage, the cavity’s electrostatic landscape acts as an ion-sequestration matrix, storing charge capacitively. At high voltage, spatially confined noncovalent interactions anchor the polybromide intermediates and promote their conversion to bromine, doing the work of a redox catalyst. The same molecule switches modes according to the applied voltage. X-ray photoelectron spectroscopy showed bromine species evolving inside the cavities as the voltage changed, and density functional theory mapped the cavity’s electrostatic landscape, confirming the dual-mode picture.
The measurements support the mechanism. Bromine accumulates inside the cavities to 6.73 atomic percent, far above what equivalent non-cavity materials capture, and the equilibrium adsorption capacity reaches 7.1 g of polybromide per gram of host. Calculations give the confined Br3- a binding energy of -58.4 kcal/mol, unusually strong for a noncovalent interaction, which keeps the shuttling intermediates in place. Because the capacitive mode contributes at low voltage and the redox mode at high voltage, the combined capacity of 481.8 mAh/g exceeds the 335.5 mAh/g theoretical ceiling of the bromine redox couple alone, showing that the two storage mechanisms are additive rather than competing.
The approach extends beyond this one chemistry. The trade-off between fast capacitive storage and high-capacity redox reactions has been treated as an intrinsic property of electrode materials; the authors argue it is a design problem. By regulating ions at the molecular scale inside a confined cavity, a single material can participate in both modes without sacrificing either. They frame the concept as general, applicable to systems with multivalent ions or conversion reactions, where ion transport and reaction kinetics are often the primary limiting factors, and point to static bromine batteries and flow-battery bromine complexing agents as immediate targets.
The results are electrode-level demonstrations, not full commercial cells. Capacity is normalized to the mass of the electrode material, and practical batteries must also solve separator, electrolyte, and zinc-anode engineering. The cycling test at 8.3 A/g is demanding, but 1,500 cycles remains far short of the tens of thousands of cycles grid storage expects. The manuscript is an early-access version, published in unedited form before final copyediting.
The work presents the power-versus-capacity trade-off as a design constraint rather than a law of nature: an ion inside a cavity of the right shape can store energy capacitively and through redox reactions at once, whichever mode the grid needs at that moment.
Sources: Ding, Y., Han, M. et al. Cavity-enabled supramolecular regulation of capacitive-redox bromine electrochemistry in zinc-bromine batteries. Nature Communications (2026). DOI: 10.1038/s41467-026-76539-8.