An alternative to lithium-ion batteries has been posed as a cheaper solution to help solve the range limitations of electric vehicles and to enable cheaper energy storage on the grid.
Redox flow, or flow, batteries were first invented in the 1980s.
They typically involve two flows of electrolytes that are held in separate hemispheres, creating an interface where electrons move freely between the electrolyte solutions.
University of Cincinnati researchers have now developed a new type of redox flow lithium-ion battery, omitting that costly membrane interface and that might nearly halve production expenses.
This could enable redox flow batteries to be used more cheaply and effectively, for example by national grids seeking to store renewable power from wind and solar farms to use when baseload power is in short supply.
History
The first redox flow battery used an electrolyte solution made from vanadium that was dissolved into sulfuric acid.
It was invented by Maria Skyllas-Kazacos, a chemical engineer who originally demonstrated a vanadium redox flow battery that was patented in 1986 in Australia.
Aside from vanadium, other types of electrolytes can be used in a redox flow battery, such as zinc-bromine.
Design
A typical redox flow battery uses two electrolytes stored in external tanks, which are pumped into an electrode.
In such a design, electrolyte convection takes place on either side of a core membrane.
Known as a separator, a porous membrane typically sits between a battery’s anode and the cathode to prevent a short circuit.
A separator allows ions to travel and deliver charge as part of the current of any traditional electrolyte battery.
There are two types of redox flow batteries, aqueous and non-aqueous redox flow batteries.
In aqueous redox flow batteries, water is used as the solvent for the anolyte and catholyte, which are the two types of electrolytes running through the battery.
Although water allows for good ionic conductivity, aqueous redox flow batteries have limited cell voltage and low energy density.
In non-aqueous redox flow batteries, a range of solvents can be used that allow for higher energy density.
However, the benefits of these non-aqueous batteries are limited because the ion-selective membranes needed for them are scarce and expensive.
The porous membrane in a non-aqueous redox flow battery typically comprises approximately 40% of the total battery cost.
In the past, micro membrane-free batteries have been developed that use a liquid interface to separate the catholyte and anolyte, but these cannot be effectively scaled up.
While nonaqueous biphasic systems have more potential scalability, there are few organic solvents and materials that are soluble enough to prevent a crossover of materials.
Patent-pending
The latest patent-pending redox flow battery design from researchers at the University of Cincinnati attempts to solve this problem because it does not require a membrane to separate positive and negative charges.
According to a paper by the researchers published in the journal Nature in August, ion-selective membranes constrain the scalability of redox flow batteries.
The researchers have developed a high-voltage and membrane-free redox battery, described as an “all-organic biphasic system”, which uses a metal anode and several chemical cathodes.
This system uses a Li metal ionic liquid as the anode electrolyte solvent (BMP-TFSI) and a set of metal-free organic compounds including a TEMPO derivative, phenothiazine derivative, or cyclopropenium derivative as the cathode electrolyte solvent.
Li metal was chosen as the anode material because it has a high energy density and low electrochemical potential.
The researchers said they used Li metal alongside a graphite felt as the cathode material, due to its porous structure and high electrical conductivity and inertness.
According to their experiments, the use of these materials results in faster redox reactions and improves battery performance, allowing for greater scalability at a lower cost.