A new type of lithium-oxygen battery, which uses lithium oxide glass nanoparticles, will be able to provide more energy, as well as higher energy efficiency and stability, according to the researchers.
![In the new approach, solid nanoscale particles composed of compounds of lithium and oxygen (in red and white) are immobilized within a sponge-like structure of cobalt oxide (yellow) that keeps them stable. [Courtesy of the researchers]](https://www.hayadan.org.il/images/content3/2016/08/MIT-Nano-Cathode_01-500x333.jpg)
Lithium-air batteries are considered particularly promising technologies for electric cars and portable electronic devices, thanks to their ability to provide high energy relative to their weight. At the same time, batteries of this type have a number of serious disadvantages: they use a considerable amount of the energy invested in them as wasted heat and they break down relatively quickly. In addition, they also require particularly expensive components to suck the oxygen gas in and out, in an open cell configuration that is very different from normal sealed batteries. However, a new version of an electrochemical system, which can be used inside a normal, completely sealed battery, promises theoretical performance at the level of lithium-air batteries, while overcoming all these shortcomings.
The new type of battery, known as a cathode nanolithium battery, is described in the scientific journal Nature Energy in an article written by Ju Li, a professor of nuclear science and engineering at MIT, along with colleagues from Peking University in China. One of the disadvantages of lithium-air batteries, the researcher explains, is the lack of correlation between the voltage involved in charging and discharging the battery. The output battery voltage is 1.2 volts lower than the voltage used to charge it, which presents a significant energy loss in each charge cycle. "You lose 30% of the electrical energy as heat emitted during charging. The battery gets really hot if you charge it for too long," he adds. Lithium-air batteries suck oxygen from the surrounding air to accelerate the chemical reaction with the lithium in the battery during the discharge cycle, and this oxygen is then released back into the atmosphere during the reverse reaction of the charge cycle.
In the new version, the electrochemical reactions that occur between the lithium and the oxygen during the charging and discharging phases are carried out without the oxygen returning to its gaseous state. Instead, the oxygen remains within the solid and moves between its three possible oxidation states, while bound to three different types of solid chemical compounds: Li2O, Li2O2, and LiO2 mixed together as glass. This mode reduces the voltage loss by a factor of five, from 1.2 volts to just 0.24 volts, so that only 8% of the electrical energy is converted to heat. "The meaning is faster charging in vehicles, this is because the emission of heat from the battery case reduces the safety risk, as well as achieving higher energy efficiency," explains the chief researcher.
This approach helps to overcome another problem in lithium-air type batteries: when the chemical reaction involved in charging and discharging converts the oxygen from its gaseous state to its solid state, the material undergoes very large volume changes that may interfere with the electrical charge transfer pathways in the structure, a result that may reduce the lifespan of the battery . The secret to the new mixture lies in the production of tiny particles, at the nanometer level, which includes both lithium and oxygen in the form of glass, firmly held within a surface of cobalt oxide. The researchers call these particles 'nanolithia'. In this situation, the transition between Li2O, Li2O2 and LiO2 can occur entirely within the solid material, explains the researcher. Normally, such particles are supposed to be very unstable, so the researchers fixed them inside the cobalt oxide surface, a sponge-like material full of pores only a few nanometers in size. This surface stabilizes the particles and also serves as a catalyst for the transition between their different forms.
Normal lithium-air batteries, the researcher explains, are actually "oxygen and dry lithium batteries since these components cannot deal with moisture or carbon dioxide", so these should be properly separated from the air entering the battery. "In normal batteries, you need bulky auxiliary systems in order to keep the water and carbon dioxide away from the system, and this is quite challenging." On the other hand, the new battery, which does not use air suction from the environment, avoids this problem. The new battery is also inherently protected from overcharging, says the research team, since the chemical reaction in this case is self-limited - when overcharging begins, the reaction switches to another state that prevents further activity. "In normal batteries, the condition of overcharging may cause irreversible structural damage or even an explosion," explains the lead researcher. However, with a nano-lithium battery, "we overcharged the battery for 15 days, to hundreds of times its capacity, and we still don't see any structural damage."
A laboratory version of the new battery went through 120 charge-discharge cycles, and showed a capacity loss of less than 2%, a result indicating that the new technology could be stable for a long time. Since the entire system is solid, it will be possible to easily assemble it into existing systems and into commercial battery cases in the automotive, electronics and energy storage industries. In light of the fact that the "solid oxygen" type cathodes are very light-weight compared to the cathodes of ordinary lithium-ion batteries, the new design will be able to store double the amount of energy (per unit of weight) compared to existing batteries. All this progress was achieved without the addition of expensive components or materials, explains the researcher. The carbon they use as the liquid electrolyte in the battery is the cheapest electrolyte available; And the cobalt oxide surface weighs less than half the weight of the nanolithium component. All in all, the new system is "cheaper and very fast and easy to assemble," than conventional lithium-air batteries, the researcher notes.
In the new approach, solid nanoscale particles composed of compounds of lithium and oxygen (in red and white) are immobilized within a sponge-like structure of cobalt oxide (yellow) that keeps them stable. [Courtesy of the researchers]
