Power/Performance Bits: Jan. 13

Glass instead of crystals
Since today’s lithium-ion batteries are not good enough if our future energy system is to rely on electrical power, researchers around the world are continually looking to improve capacity and energy density. To this end, ETH Zurich researchers have developed a type of glass that can be used as an electrode material in lithium-ion batteries.

ETH researchers discovered a material that may have the potential to double battery capacity: vanadate-borate glass, and they are using the glass as a cathode material.

Vanadate-borate glass is made of vanadium oxide (V2O5) and lithium-borate (LiBO2) precursors, then coated with reduced graphite oxide (RGO) to enhance the electrode properties of the material. The team used a vanadium-based compound because vanadium is a transition metal with various oxidation states, which can be exploited to reach higher capacities. In crystalline form, vanadium pentoxide can take three positively charged lithium ions – three times more than materials presently used in cathodes, such as lithium iron phosphate, they noted.

However, crystalline vanadium pentoxide cannot release all of the inserted Li-ions and only allows a few stable charge/discharge cycles because once the lithium ions penetrate the crystalline lattice during the loading process, the lattice expands. This means an electrode particle swells as a whole, i.e. it increases in volume only to shrink again once the charges leave the particle. And this may lead to instabilities in the electrode material in terms of structural changes and contact losses.

As a result, the researchers had to find a way to retain the structure of the initial material while maximizing the capacity and also maintaining its ability to “take” the charges, which is how they devised the idea of using vanadium as a glass rather than in its crystalline form. In glass, a so-called “amorphous” material, atoms do not arrange themselves in a regular lattice as they do when they are in a crystalline state. Instead, the atoms exist in a state of wild disarray.

These new material could almost double the capacity of lithium-ion batteries: Vanadate borate glass. (Source: ETH Zürich)