One of the drawbacks of the widespread installation of renewable energy sources, such as wind and solar, is the lack of solutions for stationary energy storage. Unlike rechargeable batteries for mobile equipment, such as laptops and telephones, the operating parameters for large-scale batteries are different. For these, priority is given to low cost, very long life and sustainability. Especially because of the sustainability issue, large systems like these cannot afford the use of rare chemicals, such as cobalt and nickel, as they are scarce and involve high manufacturing costs.

On the otherside, due to the use of non-aqueous electrolytes and transition metal oxides in current lithium-ion battery technologies, safety, cost and environmental issues are a major concern. A promising alternative, even at an early stage, is dual-ion technology, as it is competitive in terms of cost and safety, and also uses renewable materials.

In a collaboration between Pacific Northwest National Laboratory and the MEET Battery Research Center, the scientists developed a new type of dual-ion battery, a graphite and zinc metal chemical cell with an aqueous electrolyte, which is safer, cheaper and more sustainable than other energy storage systems and showed promising electrochemical performance. The breakthrough was published in Volume 10 of the November issue of the journal Advanced Energy Material. The paper presents a double ion aqueous graphite || metallic zinc battery from ≈2.3-2.5 V, a remarkably high voltage for aqueous zinc batteries, which reaches more than 80% capacity retention after 200 cycles, enabling natural graphite as a reversible cathode using a lithium-free aqueous bisalt electrolyte.

These dual-charge ion (DIB) batteries can be very competitive in terms of cost and material abundance, resulting in a sustainable system for large-scale stationary storage because both the positive (cathode) and negative (anode) electrodes can be made of low-cost redox-amphoteric carbonaceous materials. Another important aspect of this technology is the charging and discharging mechanism, since instead of a single type of ion - such as lithium ions - the anions of the electrolyte also participate in the storage of energy in the double-ion battery, which is a clear advantage for research into this system. The problem for its development is that very few non-aqueous electrolytes have been reported to withstand such high potentials without oxidation and decomposition, and the aqueous electrolytes used are very expensive.

Promising prototype

The research team developed a novel prototype dual-ion battery, consisting of a graphite || zinc metal chemical cell with the specially developed aqueous electrolyte, a 20 M NaFSI solution, so the electrolyte is free of expensive lithium salts (e.g., LiPF₆ or LiTFSI). Natural graphite as an anion intercalation host was paired with a metallic Zn anode, recording a full cell voltage of ≈2.3-2.5 V, a remarkably high voltage for an aqueous Zn system.

Schematic representation of the prototype. Image designed by Hive account@emiliomoron in powerpoint.

The cell developed has clear advantages over systems using lithium, especially with respect to sustainability and safety due to the materials used, for example, the cathode of the energy storage device is based on graphite carbons, which can be produced from renewable raw materials, and the metal-based zinc anode offers better material availability compared to other metals used to build batteries of this type.

Although the researchers said that further advances are needed to develop a more stable electrolyte and reduce side reactions, thus improving energy density, this work shows a promising, safe, cost-effective and especially sustainable system for large-scale energy storage.

Although lithium-ion batteries are a mature technology that plays an important role in modern society, the scarcity of cobalt and lithium sources limits their future deployment to the scale needed to supply fossil fuels. Therefore, the emerging dual-ion battery (DIB) technology based on a different combination of more available chemicals represents a great alternative for energy storage systems.

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