Widespread adoption of renewable energy in the power grid requires the right kind of battery — one that is safe, sustainable, powerful, long-lasting, and made from materials that are plentiful and ethically sourced. Researchers have formulated a new type of cell chemistry for dual-ion batteries (DIB) called graphite||zinc metal aqueous dual-ion battery, which uses a zinc anode and a natural graphite cathode in an aqueous, or water-in-bisalt, electrolyte.
The use of aqueous electrolytes is not new, nor is the use of graphite. Lithium-ion (Li-ion) batteries use graphite as the anode component and non-aqueous DIBs use graphite as both the anode and the cathode. What’s new is combining the two in a new chemistry. To do that, the team gave the aqueous electrolyte an extra boost by using a highly concentrated water-in-bisalt solution. The solution widens the electrochemical stability window of the electrolyte and enables graphite as a cathode material in a practical aqueous system. This helps stabilize the electrolyte at high voltages, allowing the graphite to electrochemically oxidize before the aqueous electrolyte.
The battery showed promising performance during testing. At approximately 2.3 to 2.5 volts, it achieved one of the highest operating potentials of any aqueous battery. But the new cell chemistry doesn’t only improve battery performance — it’s also better for the environment.
Cathodes made of highly abundant carbon-based materials, like natural graphite, are less costly and more sustainable than environmentally harmful, scarce, and expensive metals, like nickel and cobalt, that are regularly used in Li-ion batteries. Using an aqueous electrolyte also makes DIBs safer as they are nonflammable compared to commercial Li-ion batteries, which use non-aqueous electrolytes exclusively.
In DIBs, both the positive cathode and negative electrode can be made of low-cost carbon-based materials like graphite. This makes DIBs a particularly promising solution to support the widespread adoption of renewable energy sources like wind and solar for the power grid. Until now, the use of graphite as a cathode has been limited by the narrow electrochemical stability of water, which caps out at 1.23 volts. The electrochemical stability window is the potential range between which the electrolyte is neither oxidized nor reduced (decomposed) and an important measuring stick for the efficiency of an electrolyte in contact with an electrode. Graphite would require a much wider stability window.
Each battery cell has three main parts: a positive electrode called a cathode, a negative electrode called an anode, and an electrolyte. In Li-ion batteries, power is generated when the Li-ions (positively charged ions or cations) flow from the cathode to the anode and back again in a rocking chair motion through the electrolyte. This balances the charge when electrons flow through an external circuit from the cathode to the anode, creating electricity.
In DIBs, both cations and anions (negatively charged ions) are active and move in parallel from the electrolyte to the anode and cathode, respectively, in an accordion-like fashion, allowing for potentially high-power applications, like supercapacitors, while still being able to use moderately high energy, like batteries. Furthermore, this mechanism renders the ions in the electrolyte active, allowing for further optimization of the battery.
DIBs still perform at only about a third of the capacity of Li-ion batteries and Li-ion batteries still have one of the highest energy densities of any comparable system, meaning they can provide a significant amount of energy and still stay small. This advantage is one of the main reasons they’re used in mobile applications such as smartphones and electric cars.
If the researchers can achieve a high enough voltage for the battery, even if performance is not on par with Li-ion batteries, DIBs can be made bigger and a suitable candidate for grid energy storage applications.