Rechargeable lithium ion batteries were, with their high energy densities, a significant improvement for most applications over the earlier nickel-cadmium Ni-Cad batteries, but they do also have their limitations such as limited storage capacity. Now, researchers at the University of Illinois at Chicago have demonstrated that the eponymous lithium ions can be replaced with doubly charged magnesium ions, which means twice the number of electrons moving per ion when compared with lithium and so higher charge and, thus, energy, stored in the device. Cabana et al, Adv Mater, 2015, online; DOI: 10.1002/adma.201500083]

All batteries require a positive and a negative electrode and an electrolyte via which electrons and ions can be exchanged. Unfortunately, the chemistry can never be one hundred percent efficient and current designs fall far short of this through the charge-discharge cycles. The batteries gradually become unusable as the process becomes less and less effectively reversed.

"The more times you can do this back and forth, the more times you will be able to recharge your battery and still get the use of it between charges," chemist Jordi Cabana of UIC explains. "In our case, we want to maximize the number of electrons moved per ion, because ions distort the structure of the electrode material when they enter or leave. The more the structure is distorted, the greater the energy cost of moving the ions back, the harder it becomes to recharge the battery."

The team has intercalated magnesium ions into a spinel-type manganese oxide material. They demonstrated that the ions reversibly occupy the tetrahedral voids of the structure as the manganese is reduced electrochemically. Lithium-intercalated manganese(IV) oxide has previously been used in batteries for electric vehicles. Now, the team has used analytical techniques of different sensitivity - atomic resolution X-ray spectroscopy to bulk X-ray diffraction - to study the process of intercalation of magnesium ions into this oxide. This process happens at high potential, making it a candidate as a cathode material in a Mg battery. Theoretically, intercalation of one magnesium ion per two manganese(IV) oxide units should give a capacity of about 270 milliamp hours per gram, whereas even the best Li-ion battery cathodes is limited to 200 mAh per gram.

The team has not built a complete battery yet, just a piece of a battery, but having established that magnesium can be reversibly inserted into the electrode material's structure they are a step closer to a prototype. "We hope that this work will open a credible design path for a new class of high-voltage, high-energy batteries," explains Cabana.

In their paper, the team concludes that, "This work highlights that fully functional, high voltage, high capacity oxide cathodes for multivalent batteries could be forthcoming, which would bring this technology several steps closer to fulfilling the promise of the concept."

"We are already a few steps forward from the published work,"   Cabana told Materials Today. "Our efforts right now are directed at assembling a battery prototype that is based on this oxide, a Mg anode and a non-aqueous electrolyte. This would be a true battery and, thus, give us an indication as to whether this concept can beat Li-ion."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".