Multiply charged negative ions are ubiquitous in nature. Their study has a long history due to their importance in chemistry as the building blocks of ionic compounds. In crystals, they are stable due to charge compensating cations while in solutions, solvent molecules protect them; but they are rarely stable in the gas phase due to strong electrostatic repulsion between the extra electrons. However, their stability in the gas phase poses great challenge. Due to the electrostatic repulsion between the extra electrons, the molecules either fragment or autoeject the added electron(s). Because the repulsion is greater as the size gets smaller, multiply charged anions in the gas phase are possible mostly for large biomolecules.

Understanding their stability without the influence of the environment, therefore, has been of great interest to scientists for decades. While much of the past work has focused on the di-anions, work on triply charged negative ions is sparse and the search for the smallest stable tri-anion against spontaneous electron emission or fragmentation continues. However, small multiply charged anions can exist in metastable state due to the repulsive Coulomb barrier that results from the short-range attraction between electrons and the positive charge on the nuclei and long-range repulsion between the electrons. Most of the studies of multiply charged anions in the gas phase has concentrated on di-anions.

Functioning of sodium-ion batteries. The high cost and scarcity of lithium are driving research to develop alternatives to lithium-ion batteries.

In a recent study, researchers from Peking University showed that a special category of Beryllium tri-anions are all stable in the gas phase. This unusual stability of these tri-anions opens the door to a new class of super-pnictogens with potential applications in Al-ion batteries.

“This is very important in this field, nobody has ever found such a tri-anion. […] Not only can it keep three electrons but the third electron is extremely stable. The guiding principles we have used in this paper will help with the design of other tri-anions. The question is: What do we do with this knowledge?”

Prof. P. Jena, Virginia Commonwealth University

The tri-anion may have a number of industrial applications. The main hypothesis is that the particle may be used in the creation of an aluminum ion battery, which has distinct advantages over the widely used rechargeable lithium ion battery. Aluminum is in greater supply than lithium and is less reactive. During the chemical reaction that would power the battery, the tri-anion would make the battery conductive by moving from one of its electrodes to the other.

While a battery is the only demonstrated use so far, existing applications for other particles with one additional electron, called mono-anions, and two additional electrons, called di-anions, show the potential of these results. In fact, these anions are very important for many reasons. First, they make salts which are easy to manipulate. Secondly, they are used in all kinds of chemical compounds, such as those in floor cleaners as oxidizing agents that kill bacteria. Moreover, they can also be used to purify air. Their potential uses are very vast.

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