New aqueous battery without electrodes may be the kind of energy storage the modern electric grid needs

The goal of creating very inexpensive, energy-dense, safe, and durable batteries to store excess electricity to support power grids during shortages took a big step forward in research recently reported by a team of scientists at Stanford University and SLAC National Accelerator Laboratory.

Two inventions created the advance. The battery the team created does not have permanent electrodes, the first such battery like this, though some batteries have only one permanent electrode. Instead, the charge-carrying metals – zinc and manganese dioxide – in the water-based electrolyte self-assemble into temporary electrodes during charging, which dissolve while discharging. This reduces the weight and space of the batteries, increasing the amount of electricity stored per unit of volume and mass, which are the key energy density metrics for batteries. The researchers estimate that dual-electrode-free batteries, which also do not need other components like separators, could achieve energy densities six times higher than existing zinc-manganese dioxide batteries.

The self-assembly in liquid crystal form catalyzed by a surfactant – a substance chemically active at the surface, like soap – also resulted in a much longer cycle life for the batteries. The test batteries retained 80% capacity after about 950 charge-discharge cycles, the researchers report in a study in Nature Energy. Without the surfactant, the batteries had similar declines after about 250 cycles.

“Because we don’t use active metals for permanent electrodes and the electrolyte is water-based, this design should be easy and cheap to manufacture,” said Yuqi Li, a postdoctoral researcher with Professor Yi Cui in Stanford’s Department of Materials Science & Engineering.

“Zinc manganese batteries today are limited to use in devices that don’t need a lot of electricity, like smoke detectors. Plus, they’re not rechargeable, because they can’t recharge much after discharging a handful of times. So, this is a big advance for these batteries in backing up the grid, charging and discharging hopefully thousands of times,” said Li, the lead author of the study.

This study is the first supported by the Aqueous Battery Consortium, a Stanford and SLAC-led group of 12 universities and three federal-government laboratories pursuing aqueous batteries powerful enough to support the electric grid, as well as reliable, safe, environmentally sustainable, and inexpensive. The U.S. Department of Energy funds the five-year project.

Liquid crystal formation on the electrode surface

Rechargeable aqueous batteries, which have water-based electrolytes, have been around for 200 years and are used today extensively for the batteries that start gasoline and diesel cars. The key to unlocking broader applications is increasing energy density and cycle life. The focus in accomplishing this has homed in on zinc-manganese dioxide chemistries. Zinc and manganese are eco-friendly, abundant, and inexpensive, but progress in overcoming the two main barriers has been slow.

For all the attractiveness of the zinc and manganese ions crystallizing to form electrodes as needed, to date the chemicals created nonuniform crystals, leading the zinc to form dendrites that do not dissolve back into the electrolyte. The dendrites build up over time, and the zinc becomes chemically inactive. Also, hydrogen separates from the water, which corrodes the device. The reversibility of electrode formation during charging and discharging diminishes quickly. Furthermore, the manganese dioxide produced exhibits poor electronic conductivity, exacerbating the issue and contributing to poor reversibility.

The key to success in the new approach was the addition of a surfactant to the electrolyte, which facilitates the formation of well-organized liquid crystal structures on the electrode surface during deposition. Liquid crystal materials can flow like a liquid, but the molecules’ crystal formation acts like a solid, at least temporarily. In fact, liquid crystals are more common in our daily lives than we might think. For example, the thin films of soap bubbles can exhibit liquid crystal-like behavior, although the energy that creates the bubbles—such as sloshing your hand in a bucket of soapy water—eventually dissipates, potentially disrupting these ordered structures. However, under the influence of an electric field, surfactant molecules can self-assemble into even more ordered liquid crystal structures and maintain their stability. This phenomenon is also the foundation of Liquid Crystal Display (LCD) technology, which underpins most modern televisions and monitors, where liquid crystals are manipulated to control light and produce images.

“First, we developed a kind of non-ionic surfactant that created templates for the start of orderly self-assembly and orientation of strong hexagonal crystals, which smoothly dissolve upon discharge, avoiding corrosion and dendrite formation,” said Michal Bajdich, a staff scientist at SLAC and co-author of the study.

“The additive’s molecules, which are composed of carbon, hydrogen, and oxygen, serve as templates for both metals’ crystalline structure starting at the current collectors, which has been used in other technologies,” Bajdich added. “The surfactant also reduces leakage when the battery is idle, or self-discharge. As an added bonus, you don’t need to add much surfactant to the electrolyte for all of this to work.”

The researchers aimed for specific crystal orientations. The zinc orientation results in more uniform and stable deposits. The manganese-oxide orientation also results in more conductive formations. The liquid crystal structures transport positive ions more efficiently while hindering the movement of negative ions, preventing unwanted side reactions.

Looking ahead

Nevertheless, the researchers seek to extend their battery’s lifespan. The formation of temporary liquid crystal structures they found may be generalizable, the researchers report. Various surfactants, not just the surfactant they invented, may also work under the right conditions. This may even lead to aqueous batteries not only for the grid but for other applications, including electric vehicles, though their battery’s voltage would have to be doubled for that. They suggest some potentially key criteria in the study to guide other researchers.

“This liquid crystal chemistry is very promising for controlling self-assembly in other crystal systems,” said Cui, the study's senior author. “Maybe we have opened exciting research opportunities to develop next-generation high-energy-density and long-duration batteries with water-based electrolytes.”

Co-authors of the study not mentioned are postdoctoral scholars in the Department of Materials Science & Engineering: Xueli Zheng (now associate scientist at SLAC), Xin Xiao, Xiwen Chi, Ge Zhang, Guangxia Feng, Xun Guan, Yecun Wu, '23; PhD students in the same department: Evan Z. Carlson, Yi Cui, Louisa C. Greenburg, Elizabeth Zhang, Chenwei Liu, Yufei Yang, Mun Sek Kim '24 (now a senior scientist at SES AI Corp.), Pu "Riley" Zhang, Hance "Jerry" Su, Jiawei Zhou, '24, Weiyu Li, '24, (now an assistantpProfessor at the University of Wisconsin-Madison); and Zhichen Xue, Stanford Institute for Materials and Energy Sciences, SLAC

Yi Cui is also a faculty member in the departments of Energy Science & Engineering of the Stanford Doerr School of Sustainability and of Photon Science at SLAC, director of the Doerr School’s Sustainability Accelerator, director of the Aqueous Battery Consortium, and a senior fellow at the Precourt Institute for Energy and the Stanford Woods Institute for the Environment. This work was funded by the Aqueous Battery Consortium, which the U.S. Department of Energy supports. The researchers also benefited from the use and support of the Stanford Nano Shared Facilities and the Stanford Nanofabrication Facility.