Will glass electrodes double lithium-ion battery capacities?
The ETH research team led by Afyon and Reinhard Nesper, professor emeritus of chemistry, are using the glass as a cathode material and have described its applications in Scientific Reports, a journal from the publishers of Nature.
The material is made of vanadium oxide (V2O5) and lithium-borate (LiBO2) precursors, and was coated with reduced graphite oxide (RGO) to enhance the electrode properties of the material. The researchers 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.
Crystalline vanadium pentoxide cannot release all of the inserted Li-ions and only allows a few stable charge/discharge cycles. This is because once the lithium ions penetrate the crystalline lattice during the loading process, the lattice expands. As a result, an electrode particle swells as a whole, i.e. it increases in volume only to shrink again once the charges leave the particle. The process may lead to instabilities in the electrode material in terms of structural changes and contact losses.
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.
To produce the cathode material, Afyon and his colleagues blended powdered vanadium pentoxide with borate compounds. “Borate is a glass former; that’s why the borate compounds were used, and the resulting glass compound is a new kind of material, neither V2O5 nor LiBO2 at the end,” explained Afyon. The materials scientists melted the powder at 900°C and cooled the melt as quickly as possible to form glass. The resulting paper-thin sheets were then crushed into a powder before use, as this increases their surface area and creates pore space. “One major advantage of vanadate-borate glass is that it is simple and inexpensive to manufacture.”
To produce an efficient electrode, the researcher coated the vanadate-borate powder with reduced graphite oxide (RGO) which increases conductivity while at the same time protecting the electrode particles. However, it does not impede electrons and lithium ions as they are transported through the electrodes.
Afyon used the vanadate-borate glass powder for the battery cathodes, which he then placed in prototypes for coin cell batteries to undergo numerous charge/discharge cycles.
During initial trials with vanadate-borate electrodes, which were not made with material coated in RGO, the discharge capacity dropped drastically after 30
charge/discharge cycles, when the current rate was increased to 400 milliamp per gram. In contrast, when the RGO coating was used, the capacity was quite stable at high rates and it remained at a consistently high level after more than 100 charge/discharge cycles.
One battery with an RGO-coated vanadate-borate glass electrode exhibited an energy density of around 1000 watt-hours per kilogram. The battery achieved a discharge capacity that exceeded 300 mAh/g. Initially, the figure even reached 400 mAh/g, but dropped over the course of the charge/discharge cycles.
“This would be enough energy to power a mobile phone between 1.5 and two times longer than today’s lithium-ion batteries,” estimated Afyon. The range of electric cars could be improved by one and a half times the standard amount.
The researchers have already applied for a patent for the new material.
Afyon S, Krumeich F, Mensing C, Borgschulte A, Nesper R (2014): New High Capacity Cathode Materials for Rechargeable Li-ion Batteries: Vanadate-Borate Glasses.
Scientific Reports 4, Article number: 7113. doi: 10.1038/srep07113
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