Li−O₂ batteries are based on the oxidation of lithium at the (lithium) metal electrode and reduction of oxygen at the air electrode to induce current flow. Li−O₂ systems require an open system to obtain oxygen from the air; Li metal must also be used as the metal electrode to provide the lithium source.

On the basis of the oxidation of 1 kg of lithium metal, the theoretical energy density of a Li−O₂ cell is calculated to be 11,680 Wh/kg—not much lower than that of gasoline (13,000 Wh/kg). Thus, Li-air batteries are of great interest (as evidenced by more than 300 research papers on the topic in the past 3 years). (Earlier post.)

However, there are a number of formidable challenges facing their commercialization. One of these is tackling the issue of the high overpotential that leads to low cycle electrical energy efficiency—currently on the order of 60-70%. (Practical propulsion batteries should exhibit “round-trip” energy efficiencies of 90%.)

The lithium–oxygen (Li–O₂) batteries are recently attracting increasing research attention because of their higher specific energy density compared with conventional Li-ion batteries. In a typical non-aqueous rechargeable Li–O₂ battery, lithium peroxide (Li₂O₂) is the discharging product formed on the oxygen electrode surface and is oxidized back to O₂ and Li⁺ in a following charging process. However, the insulating property of bulk Li₂O₂ and the sluggish kinetics of the Li₂O₂ oxidation reaction make it difficult to electrochemically decompose Li₂O₂ efficiently.

This leads to a severe charging overpotential issue, which causes not only a very low battery round-trip efficiency, but also the decompositions of the oxygen electrode and electrolyte. Recently, redox shuttles, such as tetrathiafulvalene (TTF⁺/TTF), have been introduced in the battery electrolyte to address this poor charge transport issue. During the charging process, the reduced form of the redox shuttle, M^red, is first converted to M^ox on the oxygen electrode, which in turn oxidizes the Li₂O₂. By efficiently shuttling charges between Li₂O₂ particles and the oxygen electrode surface, it facilitates the oxidation of Li₂O₂ and therefore reduces the overpotential.

In this work, we introduce the approach of using a redox shuttle to couple a photoelectrode with the oxygen electrode in non-aqueous Li–O₂ batteries, which enables the photo-assisted charging process. The photovoltage generated on the photoelectrode compensates the battery’s charging voltage. By utilizing solar energy, the device can be charged with a ‘negative’ overpotential, which is otherwise thermodynamically impossible. This concept of ‘photo-assisted charging process’ offers a novel strategy to address the overpotential issue of non-aqueous Li–O₂ batteries and also a distinct approach for integrating solar cells and batteries.

—Yu et al.

The solar battery contains three electrodes: a Li-metal anode, an oxygen electrode made from carbon paper and a photoelectrode. The discharging process remains the same as that of a conventional Li–O₂ battery, including the formation of Li₂O₂ at the oxygen electrode.

The charging process is different; charging voltage is applied on the Li-metal anode and the photoelectrode. Under illumination, photoelectrochemical oxidation occurs on the photoelectrode first, generating triiodide ions, which in turn diffuse to the oxygen electrode and oxidize lithium peroxide (Li₂O₂). With the contribution of the photovoltage, the charging overpotential is greatly reduced.

Photo-assisted charging. (a) The solar battery consists of a Li anode, an oxygen electrode and a photoelectrode. On charging, the photoelectrode and Li anode are connected to the outside circuit; on discharging, the oxygen electrode and Li anode are connected to the outside circuit. (b) The proposed photoelectrochemical mechanism of the photo-assisted charging process: on charging under illumination, the redox shuttle in its reduced form (Mred) first gets oxidized to Mox on the photoelectrode and then diffuses to the Li2O2 particles that are deposited on the oxygen electrode. By oxidizing the Li2O2 into O2 and Li+ , the Mox is reduced back to Mred. (c) The energy diagram of a solar battery which integrates a dye-sensitized semiconductor photoelectrode. (‘SC’ stands for semiconductor and ‘S’ stands for sensitizer.) The photoassisted charging voltage is determined by the energy difference between the Li+/Li redox potential and the quasi-Fermi level of electrons in the semiconductor electrode (at best close to its conduction band (CB) edge.) Yu et al. Click to enlarge.

The design takes some cues from a battery previously developed by Wu and doctoral student Xiaodi Ren. They invented a high-efficiency air-powered battery that discharges by chemically reacting potassium with oxygen. The design won the $100,000 clean energy prize from the US Department of Energy in 2014, and the researchers formed a technology spinoff called KAir (potassium air) Energy Systems, LLC to develop it.

For this new study, the researchers wanted to combine a solar panel with a battery similar to the KAir. The challenge was that solar cells are normally made of solid semiconductor panels, which would block air from entering the battery.

Doctoral student Mingzhe Yu designed a permeable mesh solar panel from titanium gauze, a flexible fabric upon which he grew vertical rods of titanium dioxide. Air passes freely through the gauze while the rods capture sunlight.

Normally, connecting a solar cell to a battery would require the use of four electrodes, the researchers noted. Their hybrid design uses only three.

The state of the art is to use a solar panel to capture the light, and then use a cheap battery to store the energy. We’ve integrated both functions into one device. Any time you can do that, you reduce cost.

—YiYing Wu

The mesh solar panel forms the first electrode. Beneath, the researchers placed a thin sheet of porous carbon (the second electrode) and a lithium plate (the third electrode). Between the electrodes, they sandwiched layers of electrolyte to carry electrons back and forth.

An iodide additive in the electrolyte acts as a “shuttle” that carries electrons, and transports them between the battery electrode and the mesh solar panel. The use of the additive represents a distinct approach on improving the battery performance and efficiency.

In tests, they charged and discharged the battery repeatedly, while doctoral student Lu Ma used X-ray photoelectron spectroscopy to analyze how well the electrode materials survived.

The proposed photoassisted charging process has been proved to be efficient. The performance of the solar battery will be further improved by optimizing the device configuration and exploring more stable photoelectrode materials. With this concept demonstrated, our work provides a novel and generic strategy to address issues of low energy efficiency and side reactions brought by the high charging overpotential for non-aqueous Li–O₂ batteries. Furthermore, by using a redox shuttle to couple a photoelectrode and a battery into one device, this work also suggests a distinct approach for integrating solar cells and batteries, which represents an important direction for the future applications in solar energy conversion and storage.

—Yu et al.

The US Department of Energy funded the project, which will continue as the researchers explore ways to enhance the solar battery’s performance with new materials.

The university will license the new “solar battery” to industry. Professor Wu says it will help tame the costs of renewable energy. Wu and his students believe that their device brings down costs by 25%.

Resources

• Mingzhe Yu, Xiaodi Ren, Lu Ma & Yiying Wu (2014) “Integrating a redox-coupled dye-sensitized photoelectrode into a lithium–oxygen battery for photoassisted charging,” Nature Communications 5, Article number: 5111 doi: 10.1038/ncomms6111