Lithium-ion batteries: Recharge with solar energy

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2015

A new process will enable electric vehicles to be recharged without a network of charging stations.

KEY CONCEPTS
• The limited number of charging stations available to service electric vehicles means that an alternative technology must be developed.
• Placement of perovskite solar cells in series with a lithium- ion battery means that solar energy may be a viable option for charging electric vehicles.
• When illuminated with solar light for nearly 18 hours, the photocharged lithium-ion battery demonstrates a high photoelectric conversion, good cycling stability and durability.

THE GROWING USE OF LITHIUM-ION BATTERIES AS A VIABLE, nonhydrocarbon-based energy is highlighted by the commercial introduction of Tesla electric vehicles. The battery pack used by Tesla contains 7,000 cylindrical lithium- ion batteries that are each 18 mm in diameter and 65 mm in length. The batteries are connected in series and parallel.

But lithium-ion batteries have been faced with stability concerns that can lead to flammability problems. In a previous TLT article, the source of the instability in the lithium-ion battery is traced to the liquid electrolyte where dendrites formed from metal fibers of lithium or other metals can initially start on the anode and can grow into a network of fern-like structures that may eventually cause a short circuit leading to failure (1). Researchers developed a new electrolyte based on aramid nanofibers and poly(ethylene oxide) that stops dendrites from growing and improves lithium-ion battery stability.

With all of the problems faced in using lithium-ion batteries, one challenge that needs to be overcome is the very limited number of charging stations available to service electric vehicles. Dr. Jiantie Xu, macromolecular science and engineering research associate at Case Western Reserve University in Cleveland, Ohio, says, “The growing use of electric vehicles in the U.S. cannot be sustained unless a cross-country network of charging stations is built to provide service.”

Building such a network will be costly and time consuming. An option to consider is finding a second energy generating technology that can be used in combination with the lithium- ion battery. Such an approach was described in a previous TLT article to improve the capabilities of a microbial fuel cell and reverse electrodialysis (2). Both technologies were combined leading to a more effective way to produce electric power from wastewater.

Using this approach, a new process has now been developed that will enable electric vehicles to be recharged without the need for a network of charging stations.

PEROVSKITE SOLAR CELLS
Liming Dai, Kent Hale Smith professor of macromolecular science and engineering at Case Western Reserve University, in collaboration with Xu and Dr. Yonghua Chen, have found that solar cells can be used to efficiently photocharge lithium-ion batteries leading to the possibility of electric vehicles operating without the need for using charging stations.

Xu says, “Perovskite solar cells (PSCs) are used in this application because they demonstrate higher current density and higher voltage and storage efficiency compared to other types.”

A lamellar fabrication technology is used to prepare the PSCs by systematically applying lead iodide into a methylammonium iodide solution. The resulting process is repeated three times to form an interface-free uniform methylammonium lead iodide or perovskite layer with a thickness of 125 nanometers.

Four of these single PSCs each of which are 0.1 cm² are prepared and placed in series with a lithium-ion battery. Lithium iron phosphate and lithium titanium oxide are used as the lithium-ion battery’s cathode and anode, respectively. Xu says, “Both of these materials are utilized because they exhibit higher energy density and cycling stability than other options.”

These two lithium-based materials have been determined to be suitable for the next generation of high-performance lithium-ion batteries. Figure 3 shows a diagram of a photocharged lithium-ion battery. Photogeneration leads to the production of free holes (positive charge) and electrons that flow from the PSCs into the cathode and anode, respectively. Charging takes place through oxidative extraction of lithium ions from the cathode followed by reductive insertion into the anode.


Figure 3. Four perovskite solar cells placed in series with a lithium-ion battery can effectively recharge the battery using solar energy. (Figure courtesy of Case Western Reserve University.)

Turning off S1 and turning on S2 in Figure 3 leads to the battery releasing charge, which leads to the back flow of lithium ions from the anode to the cathode. The researchers found that the photocharged lithium-ion battery displayed an excellent solar-to-electric power-conversion efficiency of 12.65%.

When illuminated with solar light, the photocharged lithium-ion battery showed a high photoelectric conversion and storage efficiency of 7.8% during the discharge-charge process. Moreover, the durability of the unit appears to be good over the long illumination timeframe of nearly 18 hours. Xu says, “Ten photocharge/galvanostatic (steady current) discharge cycles occurred during this period exhibited identical discharge/charge curves. This result indicates that the components are compatible with each other and display good cycling stability and durability.”

For the future, Xu indicates that the researchers will be looking to improve the performance of the photocharged lithium-ion battery. He says, “We also are looking to reduce the cost of fabricating this battery and improving its performance.”

The researchers know this is a big challenge but believe the photocharged lithium-ion battery has the potential to not only power electric vehicles but also be used in portable electronics. Additional information can be found in a recent article (3) or by contacting Dai at liming.dai@case.edu.

REFERENCES
1. Canter, N. (2015), “Dendrite-suppressing battery technology,” TLT, 71 (4), pp. 14-15.
2. Canter, N. (2012), “Synergy from combining two energy-generating technologies,” TLT, 68 (7), pp. 14-15.
3. Xu, J., Chen, Y. and Dai, L. (2015), “Efficiently photo-charging lithium-ion battery by perovskite solar cell,” Nature Communications, 6, Article # 8103.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.