Solar-powered drone flies for 11 consecutive days using lithium-sulfur battery

Due to the widespread use of electronic products and the rapid popularization of new energy vehicles today, we seem to be able to say that there has never been an era in the past when we were more concerned about the energy density of batteries than we are now.

The energy density of the ternary lithium battery, which is more familiar to the public, can reach 300 watt-hours/kilogram. However, a lithium-sulfur battery, whose technology is still immature and has not yet been widely used, can easily achieve an energy density of 600 watt-hours/kilogram, and its theoretical energy density is astonishingly high!

The "Lithium-Sulfur Battery White Paper" released by the International Battery Materials Association points out that the theoretical energy density of lithium-sulfur batteries is 2600 watt-hours/kilogram!

Such an attractive energy density will inevitably attract technicians from all over the world to conduct research. On February 29 this year, the National Natural Science Foundation of China released the "Top Ten Advances in Chinese Science" in 2023, and a study on lithium-sulfur batteries was successfully selected. Today, let's take a closer look at this battery.

Similar elements

Among the various batteries under research, the one with the highest energy density is not the lithium-sulfur battery, but the lithium-air battery. Its theoretical energy density is as high as over 3,500 watt-hours/kilogram, which is much higher than that of lithium-sulfur batteries.

The principle is to use lithium as the negative electrode material and oxygen in the air as the positive electrode material. During discharge, oxygen reacts with lithium ions under the action of a catalyst to generate lithium peroxide; during charging, lithium oxide decomposes to generate oxygen and lithium ions .

There is no doubt that this type of battery still has a lot of technical difficulties, such as:

▶The lithium oxide generated during discharge will deposit, which will hinder the charging and discharging efficiency of the battery.

▶Moisture and impurities in the air will corrode the battery and shorten its life.

Therefore, when the laboratory is currently studying "lithium-air batteries", it is often carried out in a "pure oxygen" environment. Maybe it will be successful in the future, but at present, lithium-air batteries are not a mature technology that we can achieve.

Since using "oxygen" as the positive electrode of lithium batteries is too advanced, is there a slightly more realistic material? Of course there is.

Periodic table of elements, image from Wikipedia.

In the periodic table, lithium and sodium belong to the same group of elements and have similar chemical properties. Therefore, while lithium-ion batteries are widely used today, sodium-ion batteries are gradually becoming commercially available and are expected to have a bright future.

Periodic table of elements, image from Wikipedia.

In the periodic table, oxygen and sulfur also belong to the same group of elements, and they have similar chemical properties. Since "oxygen" can be used as the positive electrode in lithium-air batteries, sulfur, which is an element of the same group, can also be used as the positive electrode in batteries - this is the lithium-sulfur battery.

History of Lithium-Sulfur Batteries

Research on lithium-sulfur batteries began in the 1960s.

In 1967, Herbert and Ulam first proposed that sulfur could be used as the positive electrode material of lithium batteries. It should be noted that the "lithium-sulfur battery" proposed at that time was still a primary battery, that is, a disposable battery.

In the 1980s, Plichta et al. studied the charging and discharging mechanism of lithium-sulfur batteries.

Since the 1990s, research on lithium-sulfur batteries has made significant progress, and the energy density has been continuously improved. However, the safety and economic efficiency of lithium-sulfur batteries are relatively poor.

After 2014, lithium-sulfur batteries began to enter the trial application stage in small quantities.

Application on large solar drones

The picture shows the Zephyr series of large solar-powered drones designed and manufactured by European Airbus. The picture comes from Wikipedia.

In 2014, the large solar-powered drone Zephyr7, or West Wind 7, flew for 11 consecutive days using lithium-sulfur batteries. The lithium-sulfur batteries provided by Sion Power at the time had an energy density of up to 350 watt-hours per kilogram.

It seems that the energy density of 350 Wh/kg seems average, but it should be noted that this was 10 years ago in 2014. At that time, new energy vehicles were just starting, and the energy density of the lithium-ion batteries used at that time seemed pitifully low now.

Although Zephyr 7 uses lithium-sulfur batteries, the newer Zephyr S, also called Zephyr 8, achieved 64 days of continuous high-altitude flight in 2022. However, Zephyr 8 did not use lithium-sulfur batteries. This also indirectly shows that lithium-sulfur batteries are still in the stage of small-scale trial application.

In 2020, the high-altitude solar drone "EAV-3", equipped with lithium-sulfur batteries and developed by the Korea Aerospace Research Institute, successfully conducted a stratospheric flight test.

EAV-3 solar-powered drone, image from Wikipedia.

In this flight test in 2020, the EAV-3 reached a maximum altitude of 22 kilometers. During the 13-hour flight, the drone flew stably in the stratosphere at an altitude of 12 kilometers to 22 kilometers for 7 hours.

Advantages of lithium-sulfur batteries

In summary, we can see that lithium-sulfur batteries have been tried and applied on a small scale. Compared with traditional lithium-ion batteries, they have the following two core advantages:

1. The theoretical energy density of lithium-sulfur batteries far exceeds that of traditional lithium-ion batteries.

Ten years ago, lithium-sulfur batteries achieved 350 watt-hours/kilogram, and current traditional lithium-ion batteries have not surpassed this energy density.

Energy density is also called "mass energy density", which refers to the energy per unit mass. For example:

There are two groups of batteries with the same mass . The energy density of battery group A is 200 Wh/kg, and the energy density of battery group B is 400 Wh/kg. This means that under the same usage environment, the battery life of battery group B will be twice that of battery group A.

Large solar-powered drones flying in extremely thin air above 20,000 meters are extremely concerned about their own weight. Therefore, they tried to use lithium-sulfur batteries in the early days. The core purpose is to make the battery pack as light as possible while having as large an energy storage capacity as possible.

Yellow sulfur melts into a blood-red liquid with a blue flame when burned. Image from Wikipedia.

2. The "sulfur" material in lithium-sulfur batteries is extremely cheap and has abundant reserves around the world.

If lithium-sulfur battery technology really matures in the future and is used on a large scale, it will not be restricted by the supply and price of sulfur.

A man carries chunks of sulfur from a volcano in Indonesia. Image via Wikipedia.

Current difficulties of lithium-sulfur batteries

There is a huge difference in volume between elemental sulfur and lithium sulfide. During the reduction reaction of the battery, when elemental sulfur turns into "lithium monosulfide", the volume expands by about 80%.

▶Volume expansion

In other words, lithium-sulfur batteries are relatively large in size. This is not a big problem for the large solar drone mentioned above, because it is huge in size and has a lot of room to withstand the expansion of the battery.

But if it comes to the mobile phones or cars that we often use, it will be a bit of a headache because both of them have restrictions on the size of the battery, especially mobile phones.

▶Shuttle effect

Volume expansion is not the biggest difficulty. The biggest technical difficulty of lithium-sulfur batteries at present is the "lithium polysulfide shuttle effect."

During the charge and discharge process of lithium-sulfur batteries, the intermediate product lithium polysulfide will dissolve in the electrolyte and migrate to the negative electrode of the battery, and then react with lithium metal to generate new lithium sulfide. This process is called the "lithium polysulfide shuttle effect", which will cause the battery capacity to decay rapidly and shorten the cycle life.

The working principle of lithium-sulfur batteries and the "shuttle" effect. Image from Wikipedia.

Latest Developments

In order to solve the current technical difficulties, researchers need to understand the chemical reactions occurring inside lithium-sulfur batteries more clearly, and then they can solve the problems in a targeted manner.

However, this is difficult to achieve due to the low temporal and spatial resolution of traditional in situ microscopy research techniques and the instability of the lithium-sulfur system.

Among the "Top Ten Advances in Chinese Science" in 2023, Liao Honggang, Sun Shigang from Xiamen University and Chen Jianfeng from Beijing University of Chemical Technology developed high-resolution electrochemical in situ transmission electron microscopy technology, which achieved atomic-scale dynamic real-time observation and research on lithium-sulfur battery interface reactions.

More importantly, for nearly a hundred years, "electrochemical interfacial reactions" have generally been believed to exist only as single-molecule pathways of "inner-sphere reactions" and "outer-sphere reactions."

This time, the research of Chinese researchers revealed the existence of a third pathway, which is the "charge storage aggregation reaction."

Undoubtedly, this new discovery will provide guidance for the future design of lithium-sulfur batteries.

References:

[1]https://sionpower.com/2014/sion-powers-lithium-sulfur-batteries-power-high-altitude-pseudo-satellite-flight/

Author: Hanmu Diaomeng, a popular science writer and winner of the "National Excellent Popular Science Work Award" from the Ministry of Science and Technology

Reviewer : Zhang Haijun, Professor of the School of Safety Science and Engineering, Civil Aviation University of China, Deputy Secretary-General of Tianjin Emergency Management Society, Postdoctoral Fellow of the Department of Chemistry, University of Puerto Rico

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.