“Our battery has a higher rate capability (a measure of how fast a battery operates) than current lithium-ion batteries and a higher energy density than capacitors or even supercapacitors,” explains team leader Guihua Yu. “More importantly, it retains its capacitance to a greater degree when compared with previous capacitive batteries (that is, those with capacitor-like fast charging/discharging capability).”

The team made its battery using passivated lithium metal as an anode and non-aqueous ferrocene as the “catholyte” (the ferrocene dissolved in a solvent electrolyte). It is "semi-liquid" because it contains a liquid ferrocene electrolyte, a liquid cathode, and a solid lithium anode. The device has a stable capacity retention up to 94% of the theoretical capacity of ferrocene itself (145 mAhg–1). It also boasts a diffusion coefficient of around 10–6cm2s–1 and a standard reaction constant of about 10–1cm s–1. These values (which are used to measure how quickly ions move through the liquid battery and the redox reactions in which the electrons are transferred between electrodes in the device) are much larger than those of solid-phase electrodes in lithium-ion batteries.

And that is not all: the battery has a power density of over 1400 WL–1 and an energy density of over 40 WhL–1. It can also charge/discharge for over 500 cycles at high rates without losing more than 20% of its capacitance.

Further improvements

“These good properties could be improved further by enhancing the solubility of the ferrocene in better solvents via chemical functionalization,” team member Yu Ding tells nanotechweb.org, “and these batteries might then have comparable or higher energy densities than even the best Li-ion batteries available today.”

Ferrocene is a good material for high-energy storage devices thanks to its prompt redox kinetics and low-cost, he adds. The activation energy required for redox reactions in the battery is very low (around 10 kJmol–1), leading to high rate constants, orders of magnitude higher than those of the redox couples employed in traditional redox-flow batteries.

The researchers reckon that the battery might find use in applications in which high-power output is needed, such as hybrid electric vehicles, and in high-power devices like electrical pulse buffers for harvesting energy from renewable sources such as wind and tides.

The team is now busy working on further improving the energy density of its ferrocene-based devices by chemically functionalizing the ferrocene. “We will also focus on finding better anode materials (anolytes) and electrolytes to potentially construct a cell that does not contain any lithium metal,” says Yu.

Lithium-free anodes?

The main drawback of this battery, when it comes to long-term stability and safety, is the lithium in the anode, he adds. "We need to better protect the lithium anode to fully suppress self-discharge. Alternatively, other metals like zinc and magnesium may also function as the anode for such a battery as long as they are compatible with the electrolyte employed. Other organometallic compounds with multivalence-state metal redox centres might work as anodes here too, and using these would allow us to make fully liquid batteries."

Yu says that his group is now collaborating with other research teams specializing in solid electrolytes to make Li-redox flow batteries that are robust and safer, and which could potentially be fabricated on a large scale.”

The research is detailed in Nano Letters DOI: 10.1021/acs.nanolett.5b01224.