Researchers from Argonne National Laboratory have found that they can lower the charging time of a Li-ion battery (LIB) by a factor of two or more by directly shining white light during charging  on its cathode made of lithium manganese oxide (LiMn₂O₄ or LMO), a material already in use in industrial applications such as cathodes for electric vehicle batteries. They report their work in Nature Communications.

For LIBs, in general, the use of light–matter interactions to alter the active material characteristics is a new idea. Until now, efforts were dedicated to capture, convert, and store energy in one device. Photovoltaics have been incorporated for example in the electrodes, with the aim to produce electricity through photoelectric conversion. 

“The work of Lee et al. is very interesting as it is the first time that a LIB is shown to respond to light illumination without incorporating a light-absorbing component,” says George Demopoulos, a professor at the McGill University in Montreal, Canada who was not involved in the study. 

In this work, the research team was focused on the electrochemistry that happens at the interface of the electrode. “Our goal was to see if we could modify the chemistry at the interface of the electrode by an external stimuli, like light. We thought that putting light on the electrode’s chemical interface, while the battery was still functioning, would be an interesting experiment,” says team leader Chris Johnson, senior chemist at Argonne National Laboratory. 

To begin, the researchers designed and constructed a coin cell battery with a transparent quartz window, to allow light to enter the cell. The materials they mixed to prepare the cathode were LMO powder, a Teflon binder, and a conductive additive, like acetylene carbon black. The weight ratio of the three components was 75:20:5 wt%. LMO is the active material that acts as a lithium cation host in the electrochemical reaction; the carbon particles are used to improve the conductivity of the active oxide and the binder ensures the mixture will acquire a laminate form. The cathode laminate together with a lithium metal anode, a glass fiber separator, and carbonate GenII electrolyte (1.2M LiPF₆ in 3:7 ethylene carbonate -ethyl methyl carbonate solution) were assembled into the coin cell battery. 

In the electrochemical experiments, the “open” cell was kept in either a “light-off” or a “light-on” state. Light-on meant that the samples were irradiated during charging with white light from a Xe lamp (300 nm to 1100 nm,) fitted with an IR filter to avoid overheating.

The result was unusual: In addition to the expected electrochemical current inside the electrode, the researchers detected the presence of a photo-current at its surface. “In other words, we were able to get a higher amount of current out of the electrode as a result of shining light onto it. From the battery’s standpoint, the two oxidizing processes means we were charging it faster,” Johnson says.

At that point, the researchers decided to do more fundamental experiments in order to understand how the light changed the response of the electrode. UV-Vis absorption measurements revealed that the electrolyte was transparent over the visible region of the spectrum, with minimal absorption in the UV portion, and this helped them understand how the material interacts with light.

During charging, the bulk LMO is oxidized, with Mn oxidation state changing from Mn³⁺ to Mn⁴⁺. Li cations leave the material and are reduced at the anode (in this case lithium metal). Electron paramagnetic resonance (EPR) measurements at low temperature (10 K) were carried out, to determine the chemical changes induced by illuminating the samples. EPR showed that, when light was shone on the material, the material was additionally photooxidized, and Mn⁴⁺ buildup was observed as a result of a photochemical conversion of Mn³⁺.

The technique helped the researchers propose a mechanism by which they believe light assists the fast charging of LMO. They suggest that upon illumination, photoexcited Mn³⁺ (or [Mn³⁺]*) leads to the formation of Mn⁴⁺ (hole) and electron pair and subsequently causes the ejection of more Li cations from the electrode. “In other words, LMO is itself acting as a photon absorber due to its favorable bandgap (~2 eV),” Demopoulos says.  

The researchers also used transient absorption measurements, which indicated the presence of a long-lived excited state in LMO, when irradiated with light, with lifetime in the range of microseconds (~0.066 μs, 2.5 μs, and 10.3 μs). Similar to photovoltaic devices, such long-lifetimes may be beneficial for light-induced modification of a battery system. 

“This was more or less the proof of concept that light can affect battery chemistry and now we want to study the mechanism more deeply,” Johnson says. His team is now focusing among others on Si-anodes. “I am really interested to shine light on a Si-electrode and see if we can speed up the electrochemical reactions at the interface of the Si.”

Demopoulos thinks there are other challenges to overcome, like low nominal capacity, because of restricted light penetration and dependence of charge carrier extraction on state-of-charge. “In other words, this strategy cannot be generalized to a full range of state-of-charge from 0% to 100% SOC. From our experience, only a very narrow composition range allows the photocharge process to happen. Despite these challenges this work brings a new perspective largely neglected to date that should generate new impetus toward photo-chargeable devices,” he says.  

Read the article in Nature Communications.