Researchers develop hybrid flow battery for renewable energy sources

May 5, 2013, 11:25 p.m.

A team of Stanford researchers recently unveiled a new low-cost, high-capacity, hybrid flow battery that can stabilize fluctuations in wind and solar power to enable greater use of renewable energy sources.

The researchers published their findings in a study co-authored by Associate Professor of Materials Science and Engineering Yi Cui, Yuan Yang M.S. ’10 Ph.D. ’12 and third-year doctoral candidate Wesley Zheng.

Cui, who began teaching at Stanford eight years ago, put together a research team to create innovative methods of energy storage, such as batteries for cellphones, laptops and electrical cars, as well as stationary forms of storage like solar cells and wind farms.

According to Cui, the new hybrid battery differs significantly from the traditional flow battery, which Cui described as “battery electrodes that are liquid and can flow.”

The team’s hybrid battery uses a new cathode with higher energy density and a solid as the negative electrode so that the battery only “half flows,” according to Cui. The researchers also eliminated the need for an ion-selective membrane, which is typically found in flow batteries and can be expensive.

“We potentially have four-times-higher energy compared to the other flow batteries that have already been developed,” Cui said. “[The new battery] stores a lot of charges, which is so important for solar storage to reduce the cost.”

Zheng, Yang and Cui began collaborating on the battery’s creation last year. According to Zheng, the project was born out of a discussion about how to improve lithium-sulfur batteries so that the sulfur would not dissolve into the electrolytes.

Zheng said that the battery was specifically designed to fix some of the problems found in other types of flow batteries, such as the high operating temperature of the sodium-sulfur battery.

“Most technologies have been developed a while back and have a lot of disadvantages,” Zheng said. “The system that we have developed here is trying to adjust to those problems. We are trying to apply the system at room temperature and work without the ion-selective membrane. It’s pretty promising technology.”

The battery developed by Cui’s team uses solid lithium anodes and a polysulfide catholyte in liquid form that is stored in a separate tank. Yang said that the battery’s design is unique in that the voltage range of cycling is controlled, eliminating some of the detrimental discharge effects of traditional lithium-sulfur batteries.

According to Yang, the battery is also distinguished by its high-energy density and low cost, as the materials used to create it are relatively inexpensive. Though the battery has not yet been deployed in a real energy system, Zheng estimated that it will cost $45 per kilowatt hour—less than half of the price of competing flow batteries, which can cost upwards of $100 per kilowatt hour.

“In large-scale energy storage, the most important thing is the cost and the cycle life,” Yang said. “It will stabilize the power output for renewable energy, which is its major application.”

According to Zheng, the team plans to focus on implementing the battery in solar and wind energy systems, as it could be used to store excess energy as a backup in a time of an energy deficit.

“Solar and wind energy are intermittent in nature. The disadvantage of a lot of renewable energy sources is that they cannot produce energy all the time,” Zheng said. “If you can store the energy and then produce energy from the grid, you allow these renewable energy sources to be more applicable to a lot of situations and you can allow it to play a more important role.”

Cui’s team is currently working on improving the battery’s performance, and Cui estimated that within the next three years, the team will test the battery on a real electrical grid. Cui said that he eventually hopes to make the battery available in the commercial market.

“Our plan is to understand some of the logical constraints and potential issues and then to build a bigger system so we can plug it together with the solar cell or wind device and do the real demonstration,” he said.

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