One of the biggest challenges facing scientists and engineers in the clean energy era isn't how to make energy using sun and wind, it's how to store it.
Traditionally, lithium-ion batteries have been used in vehicles because they can store a large amount of power in a small space, waste only a small amount of energy during charging and discharging and respond quickly to shifts in power demand. Despite these advantages, lithium-ion batteries pose flammability risks, are prone to supply chain shortages and are expensive, prompting research into other material technologies for the batteries of the future.
One technology researchers have investigated for use in battery applications is aqueous zinc-ion batteries (AZIBs). In contrast to traditional lithium-ion batteries, AZIBs use divalent zinc ions (Zn²⁺, a positively charged metal atom that has lost two electrons) instead of monovalent lithium ions (Li⁺) as a charge carrier, or the moving particle that carries an electric charge to create a flow of electricity. Further, AZIBs use safe, water-based mild acid or neutral salt solutions instead of volatile, highly flammable organic liquid solvents as an electrolyte, making them non-flammable. The anodes of AZIBs, or source of electrons and the place where chemical oxidation occurs, are also made of cheap, abundant zinc rather than lithium compounds, making them more cost effective.
Finding an ideal cathode, or positively charged electrode that acquires electrons during discharge, for AZIBs has been the major bottleneck for developing the technology. To address this issue, a team of researchers from Chongqing University, Guizhou Education University, Hainan University and Taizhou University explored the use of manufactured electrolytic manganese dioxide (MnO2) as an AZIB cathode material due to its low cost, abundance, high capacity for energy storage and safety.
The team published their paper , "Unveiling the action mechanism of synergistic d/p-band center modulation on the zinc storage capability of industrial-grade MnO2 cathode," on July 15 in Nano Research Energy, published by Tsinghua University Press.
"The practical application of industrial manufactured electrolytic MnO2 (EMD) is hampered by its poor intrinsic conductivity ability to conduct electricity, which slows reaction kinetics, and its structural instability during cycling charging and discharging. The latter leads to irreversible phase transitions thermodynamic shifts that cannot return the system to its initial state without adding outside energy, structural degradation and consequent manganese dissolution, causing irreversible capacity loss. Therefore, the exploration of EMD cathodes represents a critical step in bridging the gap between fundamental research and the practical implementation of AZIBs… and commercially viable grid-scale energy storage solutions," said Zhenyue Xing, associate professor in the School of Materials Science and Engineering at Hainan University in Haikou, China and an author of the research study.
One way to enhance the electronic conductivity, weaken the electrostatic trapping of Zn²⁺ ions and accelerate the ion diffusion kinetics of EMD is to engineer oxygen defects, or missing oxygen atoms in the crystal structure of a solid material, like MnO2, in commercial EMD. These defects leave extra electrons behind and allow electrons to move freely through the material.
The researchers used ball milling, a process using the mechanical tumbling or vibrating of heavy balls inside a container to grind powders, as a cost-effective way to engineer oxygen defects in EMD. Through this process, mechanical shear striking energy allows lattice oxygen in EMD to escape.
The team measured the basic properties of ball-milled EMD compared to native EMD, such as adsorption energies (the binding strength between a molecule and a surface) and kinetic barriers, reflected in Zn2+ and H+ ion migration energies, or the energy barriers that charged atoms or molecules must overcome to move through a solid material or crystal lattice.
"Following ball-milling treatment, the oxygen content of EMD decreases from 68.93% to 61.17%, while the manganese content correspondingly increases from 31.07% to 38.83%. This compositional shift indicates the formation of oxygen vacancies (Ov) within the MnO2 structure. The presence of Ov promotes electron transport via the induced half-metallicity (a property where electrons of one spin direction conduct electricity like a metal, while electrons of the opposite spin behave like an insulator) that improves conductivity, which can improve MnO2 cathode electrochemical performance," said Xing.
The researchers discovered that the oxygen defects in ball-milled EMD shift the energy levels of metal (d)-bands and non-metal (p)-bands, which refer to the atomic orbitals that confer conductive metal or nonmetal properties, to balance ion binding energies, preventing trapping or retention of Zn2+ and H+ ions, one of the disadvantages of native EMD. This shift in energy levels also lowers diffusion barriers (the energy required for ions to move through a lattice structure) and limits the structural distortions observed in EMD during cycling.
"This study not only provides a high-performance electrode material for advanced ZIBs but also offers profound mechanistic insights into the critical role of defect chemistry in regulating reaction kinetics, presenting a promising and scalable pathway for the development of next-generation energy storage materials," said Xing.
Pengyang Xia, Zutao Zhu, Liang Luo, Linfang Hu and Bin Xiang from the College of Chemistry and Chemical Engineering at Chongqing University in Chongqing, China; Xuefeng Zou, Kaiwen Zhang and Yao Zhang from the Guizhou Provincial Key Laboratory of Critical Materials and Devices for Solid-State Batteries in the Guizhou Provincial Key Laboratory of Computational Nano-Material Science at Guizhou Education University in Guiyang, China; Mingyang Chen and Xiaodong Shi from the State Key Laboratory of Tropic Ocean Engineering Materials and Materials Evaluation in the School of Materials Science and Engineering at Hainan University in Haikou, China; Yang Zhou from the Analytical and Testing Center of Chongqing University in Chongqing University; and Lijin Yan from the School of Pharmaceutical and Chemical Engineering at Taizhou University in Zhejiang, China also contributed to this research.
This work was supported by the National Natural Science Foundation of China (22569005, 22562010), the Science and Technology Project of Guizhou Province (QKHJC-ZK[2023]ZD030), the Natural Science Foundation of Chongqing
(CSTB2025NSCQ-GPX0762), the Major Science and Technology Project of Guizhou Province (QKHZDZXZ[2024]022), the National Defense Science and Technology Key Laboratory Foundation (61420052022WD008), the Doctoral Program of Guizhou Education University (X2024071), and Guizhou Provincial Key Laboratory of Critical Materials and Devices for Solid-State Batteries (No.ZSYS(2025)036).
DOI Link:
https://doi.org/10.26599/NRE.2026.9120255
