A Dash of Sodium Carbonate Supercharges Five-Metal Oxide Supercapacitors

Supercapacitors occupy a tantalizing middle ground in the world of energy storage. They charge in seconds, survive hundreds of thousands of cycles, and deliver bursts of power that batteries simply cannot match. Yet their Achilles heel has always been energy density: pound for pound, they store far less energy than the lithium-ion cells in our phones and cars. Now, a team of researchers at Dalian Jiaotong University in China has shown that a surprisingly humble knob, the amount of precipitating agent used during synthesis, can transform the performance of a cutting-edge electrode material, pushing an asymmetric supercapacitor to an energy density of 46.94 watt-hours per kilogram, a figure that begins to encroach on battery territory while retaining the lightning-fast charge behavior that defines the technology.

The material at the heart of the study is a high-entropy oxide with the formula (FeCoMnCuZn)3O4, a spinel in which five different metal cations, iron, cobalt, manganese, copper, and zinc, share a single crystal lattice in roughly equimolar proportions. High-entropy materials have become one of the most exciting frontiers in materials chemistry precisely because of this compositional chaos. When four or five elements are forced to coexist in one phase, the resulting compound often exhibits properties that no single-component analogue can match: enhanced structural stability, tunable electronic states, and a wealth of synergistic interactions between neighboring cations. For supercapacitor electrodes, where charge is stored through fast, reversible Faradaic reactions at the surface, having five electroactive metals working in concert is a considerable advantage.

Writing in the journal Ionics, Zijing Tian, Fanen Zeng, Yuhan Liang, Zilong Zhou, and Bing Xu describe a coprecipitation route followed by calcination in air as their synthesis platform. The chemistry is conceptually simple: dissolve salts of the five metals, add a base to precipitate a mixed hydroxide or carbonate precursor, and then heat the precipitate to drive the formation of the mixed oxide spinel. But the devil, as so often in synthesis, lies in the details. The team systematically varied both the type of precipitant and, crucially, the dosage of sodium carbonate, the precipitant that ultimately proved decisive. The dosage determines how quickly and completely the metal ions are pulled out of solution, which in turn governs the nucleation and growth of the precursor particles and, after calcination, the phase purity, crystallinity, and morphology of the final oxide.

The experiments revealed a clear optimum. When 10 millimoles of sodium carbonate were used, the resulting powder was a single-phase spinel with relatively high crystallinity, uniform nanoparticle morphology, and a homogeneous distribution of all five metals across the particles. Samples prepared with other dosages deviated from this ideal, underscoring how sensitive high-entropy phase formation is to the kinetics of precipitation. In a five-component system, any compositional inhomogeneity introduced at the precipitation stage can seed secondary phases or elemental segregation during calcination, and both are detrimental to electrochemical performance. The 10 millimole condition effectively threaded the needle, producing the disordered yet single-phase structure that high-entropy design principles call for.

X-ray photoelectron spectroscopy added a deeper layer of insight into why the optimized material performs so well. The analysis confirmed the coexistence of multivalent species of iron, cobalt, manganese, and copper on the surface, alongside oxygen-vacancy-related defects. This redox-rich, defect-laden surface chemistry is exactly what an electrode for pseudocapacitive charge storage needs. Multivalent cations provide multiple accessible oxidation states for reversible Faradaic reactions, while oxygen vacancies act as charge carriers and active sites that facilitate electron transfer. In combination, they shorten diffusion paths for electrolyte ions and lower the kinetic barriers for charge transfer, allowing the electrode to store charge quickly without sacrificing capacity.

The electrochemical results speak for themselves. The 10 millimole electrode delivered a specific capacitance of 416.4 farads per gram at a current density of 1 ampere per gram, an excellent value for a five-metal oxide. More telling is what happened under stress: even when the current density was increased twentyfold to 20 amperes per gram, the electrode still delivered 207.4 farads per gram, retaining roughly half of its low-rate capacity. Rate capability of this quality indicates that ions and electrons can move in and out of the material fast enough to keep up with aggressive charging, a direct benefit of the uniform nanoparticle morphology and the conductive, defect-rich lattice engineered through the precipitation step.

A single electrode, however, does not make a device. To demonstrate practical relevance, the researchers assembled an asymmetric supercapacitor pairing the (FeCoMnCuZn)3O4 spinel as the positive electrode with activated carbon as the negative electrode. This architecture is the standard strategy for widening the operating voltage of aqueous supercapacitors: the two electrodes have complementary potential windows, so the full cell can charge and discharge across a combined window without electrolyte decomposition. The device operated stably within a 1.4-volt window and achieved an energy density of 46.94 watt-hours per kilogram at a power density of 750 watts per kilogram. Even when pushed to a punishing 15,000 watts per kilogram, a power level associated with rapid-delivery applications, the device still delivered 12.06 watt-hours per kilogram, demonstrating that the energy advantage does not evaporate at high rates.

Durability, the other half of the practical equation, also held up. After 10,000 charge-discharge cycles at 5 amperes per gram, the asymmetric device retained 85.41 percent of its initial capacitance. For an oxide electrode undergoing repeated Faradaic reactions, where volume changes and dissolution can slowly erode performance, this level of stability reflects the robust spinel framework that high-entropy design provides. The five cations mutually stabilize the lattice, and the structural rigidity of the spinel resists the degradation mechanisms that plague simpler oxides over long cycling campaigns.

What makes this study resonate beyond its impressive numbers is the simplicity of the control lever. Precipitant dosage is a one-line change in a synthesis recipe, requiring no exotic equipment, no templates, and no rare reagents. Yet it dictated phase purity, crystallinity, particle uniformity, elemental distribution, surface valence states, and ultimately device-level energy and power metrics. For a field racing to translate high-entropy oxides from laboratory curiosities into deployable electrode materials, the message is that processing chemistry deserves as much attention as composition itself. The same five-metal formula can be mediocre or outstanding depending on how its precursor is coaxed out of solution.

The broader context is a global push toward energy storage technologies that complement batteries rather than compete with them head-on. Supercapacitors excel where batteries struggle: regenerative braking, grid frequency regulation, backup power for wind turbine pitch systems, and any application demanding rapid charge-discharge cycles and long service life. If high-entropy spinel oxides synthesized by tunable, scalable coprecipitation can keep pushing energy densities upward while preserving cycle lives measured in tens of thousands of cycles, the gap between supercapacitors and batteries will continue to narrow. This work, published in Ionics with a reported energy density approaching 47 watt-hours per kilogram and capacitance retention above 85 percent after 10,000 cycles, suggests that sometimes the most powerful optimization in advanced materials is also the most ordinary: getting the dose right.

Subject of Research: Precipitant-dosage optimization of spinel-type (FeCoMnCuZn)3O4 high-entropy oxide electrodes for asymmetric supercapacitors

Article Title: Precipitant-dosage regulation of spinel-type (FeCoMnCuZn)3O4 high-entropy oxide for high-performance asymmetric supercapacitors

Article References: Tian, Z., Zeng, F., Liang, Y., Zhou, Z., & Xu, B. (2026). Precipitant-dosage regulation of spinel-type (FeCoMnCuZn)3O4 high-entropy oxide for high-performance asymmetric supercapacitors. Ionics. https://doi.org/10.1007/s11581-026-07549-1

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07549-1

Keywords: supercapacitors, high-entropy oxides, spinel structure, coprecipitation, sodium carbonate, asymmetric supercapacitor, electrode materials, energy density, oxygen vacancies, transition metal oxides, energy storage, Dalian Jiaotong University