As a potential competitor of the "next generation battery", fluorine ion battery research and development has attracted increasing attention. Japan's Toyota and Honda, Germany's Helmholtz-Ulm Institute, NASA's Jet Propulsion Laboratory and some Chinese universities have launched research.

Experts believe that the research and development of fluoride ion batteries is still in the "extremely preliminary stage", and there are still many problems to be overcome when entering the application. However, fluorine ion batteries have great potential and may replace lithium ion batteries as the mainstream battery in the future, especially the all-solid-state fluorine ion batteries at room temperature. Once the technology is mature, it is likely to completely replace lithium ion batteries.

Four advantages

Among the many directions of "next generation battery", fluorine ion battery has attracted much attention because of a series of research breakthroughs in recent years. It works in a similar way to lithium-ion batteries, which are widely used today, using fluoride ions to shuttle between positive and negative electrodes to store energy. Experts believe that compared with lithium-ion batteries, fluoride ion batteries have significant advantages in four aspects: energy density, safety, raw material supply and cost.

The pursuit of higher energy density is an important goal for charge-discharge batteries because it means greater storage capacity. The literature shows that the theoretical energy density of all-solid fluoride ion batteries can approach 5000 watt-hours per liter, which is 8 times the theoretical limit of lithium-ion batteries.

Ma Cheng, a professor at the Department of Materials Science and Engineering at the University of Science and Technology of China, recently told Xinhua News Agency that fluoro-ion batteries use compounds such as copper fluoride and calcium fluoride as electrode materials, and their energy density is much higher than lithium-ion batteries because of the amount of charge provided by a specific quality of active electrode material.

In terms of safety, the growth of lithium dendrites is one of the main reasons affecting the safety of lithium-ion batteries, and fluorine ions are extremely difficult to be oxidized into fluorine elemental, which can avoid problems similar to the growth of lithium dendrites.

In terms of raw materials, fluorine is much more abundant in the Earth's crust than lithium, and the current global annual production of fluorine is about two orders of magnitude higher than lithium. In addition, mining lithium requires a lot of water, while mining fluorine has a much smaller environmental impact.

In terms of cost, the fine chemistry department of Daikin Industrial Corporation of Japan released data showing that cobalt, the raw material commonly used in lithium batteries, is expensive, while the cost of other anode and cathode materials except silver in fluorine ion batteries is low. Theoretically, the cost of fluorine ion batteries per watt hour is only 20% to 25% of that of lithium ion batteries.

Three routes

As early as the 1970s, scientists began to study fluorine ion batteries, but there has been no substantial progress. In 2011, German scientists took the lead in developing all-solid fluoride ion batteries using barium lanthanum fluoride as electrolyte, and the development of fluoride ion batteries received more attention.

At present, the main technological routes of fluorine ion battery research and development roughly include room temperature liquid fluorine ion battery, high temperature all-solid fluorine ion battery and room temperature all-solid fluorine ion battery. Among them, the room temperature liquid fluoride ion battery uses flammable and fluorine-containing organic solution as electrolyte, which has safety and environmental risks. However, high temperature all-solid-state fluoride ion batteries need to operate at high temperature and can only be used in energy storage or other specific scenarios.

The room temperature all-solid fluorine ion battery is considered the most valuable of the three technological routes. Theoretically, the room-temperature solid-state fluoride ion battery can be used in all the applications of current lithium-ion batteries, and is likely to replace lithium-ion batteries once the technology is mature.

Japan attaches great importance to the research and development of fluoride ion batteries and has made a series of important progress in recent years. In December 2018, Japan's Honda Research Institute, NASA's Jet Propulsion Laboratory, California Institute of Technology and other institutions jointly published a paper in the American journal Science, saying that the team prepared the first fluorine ion battery using liquid electrolyte, reversible charge and discharge at room temperature.

In 2020, Kyoto University and Toyota announced the successful trial production of a prototype all-solid-state fluoride ion battery. Japanese media reported at the time that fluorine-ion batteries could provide longer battery life than lithium-ion batteries for the same size or weight, and that a range of 1,000 kilometers on a single charge of an electric car would be "the future within reach."

Professor Ma Cheng's research group is engaged in the research of all solid-state fluoride ion batteries at room temperature. In November 2021, our research group published a paper in the German magazine Small announcing the design and synthesis of a new fluorine ion solid electrolyte, which achieved the stable long cycle of all-solid fluorine ion battery at room temperature for the first time in the world. After 4,581 hours of continuous charging and discharging at 25 degrees Celsius, the battery capacity did not significantly decrease. Before this, the room temperature solid-state fluoride ion batteries reported in the literature do not have more than 20 cycles of charge and discharge, which is generally considered to be a difficult technical route to achieve.

Prospects and challenges

Lithium-ion batteries are also possible for electric cars that can run more than 1,000 kilometers on a single charge, but to achieve satisfactory range for more powerful vehicles such as trucks, ships and planes, Mr. Ma said, you need to look for energy storage technologies that are much more energy-dense than lithium-ion batteries. Fluoride ion battery is a promising direction in this kind of technology.

"Research and development of fluoride ion batteries is at an extremely early stage. Researchers are still searching for suitable material systems. Practical and commercial systems have yet to emerge." Ma stressed that the basic research phase of fluoride ion batteries still faces many challenges, including the fact that researchers have yet to find positive and negative electrode materials with good enough cycling performance, and electrolytes with both commercial value and good performance.

Ma believes that in order to realize the application value of fluoride ion battery technology as soon as possible, there is still a need to increase the investment in basic research and solve a series of problems related to basic research, such as electrode materials and electrolyte materials. (finish)