Aqueous Dual-Ion Batteries Gain Momentum as Safe, Scalable Energy Storage Solution
In the rapidly evolving landscape of energy storage, a new contender is emerging from the laboratories of Shenzhen with the potential to reshape how we think about grid-scale and consumer battery systems. Aqueous dual-ion batteries (ADIBs)—a class of electrochemical devices that simultaneously utilize both cations and anions from a water-based electrolyte as active charge carriers—are drawing increasing attention for their unique combination of safety, cost-effectiveness, and environmental sustainability. As the global push toward decarbonization intensifies, researchers argue that ADIBs could fill a critical niche where conventional lithium-ion batteries fall short, particularly in large-scale stationary storage applications.
Unlike traditional lithium-ion batteries (LIBs), which rely solely on the shuttling of lithium ions between electrodes, ADIBs operate on a fundamentally different principle. During charging, anions from the electrolyte—such as sulfate (SO₄²⁻), perchlorate (ClO₄⁻), or bis(trifluoromethanesulfonyl)imide (TFSI⁻)—intercalate into the cathode (often graphite), while cations like Zn²⁺ or Mg²⁺ deposit onto the anode. This dual-carrier mechanism not only enhances the theoretical capacity but also enables higher power density due to the rapid ion transport inherent in aqueous media.
The safety advantages are perhaps the most compelling. LIBs, despite their dominance in electric vehicles and portable electronics, carry well-documented risks: flammable organic electrolytes, thermal runaway, and sensitivity to mechanical damage. In contrast, water-based electrolytes are non-flammable, thermally stable, and far less reactive—making ADIBs inherently safer, especially in densely populated or critical infrastructure settings like data centers, hospitals, or urban energy grids.
“Safety isn’t just a feature—it’s a prerequisite for mass deployment,” says Xiuli Guo, a doctoral researcher at the Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. “When you’re talking about gigawatt-hour-scale storage for renewable integration, you can’t afford catastrophic failures. That’s where aqueous systems like ADIBs offer a game-changing proposition.”
Yet, for years, the promise of aqueous batteries was hampered by a fundamental limitation: the narrow electrochemical stability window of water—just 1.23 volts under standard conditions. Beyond this, water decomposes into hydrogen and oxygen gases, leading to efficiency loss, pressure buildup, and electrode degradation. For a battery technology aiming to compete with LIBs, which routinely operate above 3 volts, this was a seemingly insurmountable barrier.
The breakthrough came with the advent of “water-in-salt” electrolytes. Pioneered around 2015, this approach involves dissolving extremely high concentrations of salts—sometimes exceeding 20 mol/L—into water. At such concentrations, the solvation structure of water molecules is dramatically altered. Free water molecules, which are prone to decomposition, become scarce as they are tightly bound within the ionic network. This effectively suppresses both hydrogen and oxygen evolution, pushing the practical voltage window to over 3 volts—and in some cases, even 4 volts.
Recent work by Guo and her colleagues demonstrates just how far the field has come. Using a dual-salt “water-in-bisalt” electrolyte composed of lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), they constructed an ADIB that operates stably between 1.6 and 4.8 volts versus Li/Li⁺—a range that rivals many organic-electrolyte systems. More impressively, the battery retained over 95% Coulombic efficiency after 600 cycles, with minimal gas evolution or structural degradation.
But electrolyte innovation is only half the story. Equally critical is the development of electrode materials capable of reversibly hosting large anions without collapsing under repeated insertion and extraction. Graphite remains the cathode of choice due to its layered structure, high conductivity, and low cost. However, conventional graphite suffers from severe volume expansion (>130%) during anion intercalation, leading to exfoliation and capacity fade.
To address this, researchers are engineering advanced carbon architectures—such as nitrogen-doped few-layer graphene or porous self-standing graphite electrodes—that accommodate strain more effectively. In one notable example, a zinc||nitrogen-doped graphene ADIB delivered a discharge capacity of 134 mAh/g, with one-third attributed to reversible anion intercalation into turbostratic carbon layers. Such hybrid mechanisms—combining double-layer capacitance, surface adsorption, and bulk intercalation—offer a pathway to higher energy densities without sacrificing cycle life.
Beyond carbon, organic cathodes are emerging as a versatile and tunable alternative. Polymers like polyaniline (PANI), polytriphenylamine (PTPA), and polyindole can undergo reversible p-doping, where oxidation of the polymer backbone is balanced by anion uptake. These materials are not only synthesized from abundant elements (C, H, N, O) but also offer molecular-level design flexibility. By tailoring functional groups or backbone conjugation, scientists can fine-tune redox potentials, ionic affinity, and solubility—critical factors for long-term stability.
One standout example is a cathode based on a conjugated microporous polytriphenylamine (m-PTPA), which achieved an energy density of 236 Wh/kg in a zinc-based ADIB—comparable to some commercial LIBs. The material’s rigid 3D network prevents dissolution, while its abundant amine groups provide high-density binding sites for chloride anions. After 1,000 cycles at 6 A/g, it retained 87.6% of its initial capacity, showcasing the potential of organic electrodes for high-rate, long-life applications.
On the anode side, metallic zinc dominates due to its low redox potential (−0.76 V vs. SHE), high theoretical capacity (820 mAh/g), and compatibility with aqueous environments. However, zinc anodes are not without challenges. Dendrite formation, hydrogen evolution, and passivation can still plague performance. Recent strategies include electrolyte additives, surface coatings, and three-dimensional current collectors to homogenize ion flux and suppress side reactions.
Interestingly, the ADIB architecture also enables unconventional chemistries. In so-called “reverse” dual-ion batteries, the roles of anion and cation storage are flipped: the anode hosts anions (e.g., OH⁻ or F⁻), while the cathode stores cations. Materials like Fe₂O₃ with exposed {104} facets or BiF₃ have demonstrated reversible anion storage through redox-coupled conversion reactions. While still in early stages, these systems expand the design space for pH-asymmetric or multi-ion batteries.
Another frontier is the integration of deep eutectic solvents (DESs)—mixtures of salts and hydrogen-bond donors that form low-melting-point liquids. When water is added to DESs like choline chloride and ZnCl₂, the resulting aqueous DES electrolytes can stabilize exotic species such as [ZnCl₃(H₂O)]⁻ or [LiCl₂]⁻ superhalides. These anionic complexes can intercalate into graphite at lower voltages than conventional anions, enabling high-capacity cathodes with reduced oxidative stress on the electrolyte.
In one remarkable demonstration, a DES-based ADIB using a self-standing graphene cathode and graphene fiber fabric anode achieved a specific capacity of 605.7 mAh/g and an energy density of 908.5 Wh/kg—figures that surpass most current LIBs. Although practical energy density (accounting for full-cell packaging) would be lower, the result underscores the untapped potential of unconventional electrolyte chemistries.
Despite these advances, ADIBs remain largely in the research and prototyping phase. Commercialization hurdles include scaling up high-concentration electrolytes (which can be viscous and expensive), ensuring long-term interfacial stability, and competing with the entrenched LIB supply chain. Yet, the economics may shift as lithium prices fluctuate and geopolitical concerns over critical minerals grow. Zinc, magnesium, and sodium—key components in many ADIB formulations—are orders of magnitude more abundant and geographically widespread than lithium.
Moreover, the environmental footprint of ADIBs is significantly lower. Water-based processing eliminates the need for dry rooms and inert atmospheres, slashing manufacturing costs and energy use. End-of-life recycling is also simpler, as there are no toxic fluorinated solvents or cobalt-containing cathodes to manage.
Industry observers note that the first commercial applications are likely to appear in stationary storage, where weight and volume are less critical than safety, lifetime, and levelized cost of storage (LCOS). Pilot projects in China and Europe are already testing aqueous battery systems for solar and wind integration, frequency regulation, and backup power.
“The grid doesn’t need the highest energy density—it needs reliability, safety, and 20-year lifespans,” explains Yongbing Tang, a senior researcher at the Shenzhen Institutes of Advanced Technology and co-author of the recent review. “ADIBs aren’t trying to replace LIBs in electric vehicles. They’re solving a different problem: how to store terawatt-hours of clean energy without risking communities or ecosystems.”
Looking ahead, the research community is focusing on three key directions: first, developing ultra-low-cost, high-solubility salts to make “water-in-salt” electrolytes economically viable at scale; second, designing cathode materials with expanded interlayer spacing and tailored surface chemistry to accommodate larger anions more reversibly; and third, engineering full-cell architectures that minimize side reactions and maximize energy efficiency.
As nations race to meet net-zero targets, the need for diverse, resilient energy storage technologies has never been greater. Aqueous dual-ion batteries may not capture headlines like solid-state lithium or sodium-ion systems, but their quiet progress could prove pivotal in building a safer, more sustainable energy future.
Reference:
Xiuli Guo, Xiaolong Zhou, Caineng Zou, Yongbing Tang. Research progress and perspectives of aqueous dual-ions batteries. Energy Storage Science and Technology, 2024, 13(2): 462–479. DOI: 10.19799/j.cnki.2095-4239.2023.0614
Affiliations: 1 Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, Guangdong, China; 2 Petro China Shenzhen New Energy Research Institute, Shenzhen 518118, Guangdong, China.