Investigating all-manganese flow batteries

Flow batteries present an attractive alternative to lithium-ion in stationary storage, offering longer lifetimes and lower degradation. Since the batteries aren’t suitable for electric vehicles or consumer electronics, the market size is much smaller than lithium-ion. Flow batteries, however, are gaining market share in stationary storage projects designed to support regional grids and boost consumption of renewables generated power.

To date, most of these projects have relied on vanadium or zinc-bromine flow batteries. There is, however, a wealth of other materials with the potential to be used in flow batteries, and scientists are hard at work narrowing down those that offer the best potential for the highest performance, the lowest cost and the least environmental damage.

Manganese is a relatively cheap and abundant element, already to set to play an increasing role in lithium-ion batteries amid growing concerns over the supply chain and toxicity of cobalt and nickel. A group of scientists led by Germany’s University of Freiburg decided to investigate manganese for flow batteries, achieving some impressive results.

A new and impressive setup

The group fabricated all-manganese flow batteries in a variety of configurations with different electrode materials, solvents and membranes. The best of these demonstrated an energy density of 74 watt-hours per liter and a cell voltage of 2.59 V. “Although further optimizations are necessary,” the group states, “this system represents a new and promising setup toward sustainable energy storage.”

The batteries are described in the paper Investigations toward a Non-aqueous Hybrid Redox-Flow Battery with a Manganese-based Anolyte and Catholyte, published in Advanced Energy Materials. The group notes that its ‘first try’ with manganese already exceeds the energy density of commercial vanadium redox-flow batteries, which have several decades of research behind them.

The promising performance comes with several caveats. In observing the batteries the group noted issues including high overpotential for manganese deposition, leaching toward the negative electrode, capacity fade and low energy efficiency, as well as what they describe as “cauliflower like” deposition, leading to the formation of an unstable manganese film. They also note that the battery’s area specific resistance would have to be dramatically reduced to develop a commercially viable battery.

With further investigation into different materials combinations, additives, and pre-treatment,s the group expects most of these challenges to be overcome, and states that performance should improve alongside this, with the tweaking of various parameters in the battery design.

“Compared to the benchmark vanadium redox flow battery system, the all-manganese flow battery has a higher energy density and is based on the cheap and abundant element manganese,” the researchers conclude. “Additionally, there is still a lot of room for improvements, making the all-MFB presented in this work an interesting field for further research.”