Beefing up the batteries

Delta, 31 March 2010

Lithium-ion batteries hold promise for substantial improvements. Researchers at the faculty of Applied Sciences are in the race for lightweight power with nanoparticles.

“Powering a laptop or a phone is easy,” says dr. Wouter Borghols, who last Monday defended his PhD thesis on lithium-ion batteries. “They use only a little power and there is plenty of time to recharge.” What motivated Borghols to conduct his PhD research of fundamental aspects of materials and energy (fame) at Applied Sciences was his wish to develop batteries for applications that demand higher charge and discharge rates (more power), more stored energy per unit of weight and faster charging. Automobiles would be a typical application for those improved batteries, as would satellites.
Asked shortly before the thesis defence what the main achievement of Borghols’ research is, professor Fokko Mulder, his PhD supervisor, mentions the surprising doubling of the amount of energy stored. Borghols himself doesn’t agree. He thinks the fifty to hundred times quicker charge rate is more important, because this enables higher currents, which is what you need if you want a stiff acceleration. “Well, in fact it’s both”, says dr. Marnix Wagemaker, who co-supervised Borghols’ work. “He switched from micro to nanoparticles in the batteries to enhance the reactivity and to speed up charging. But he experienced a lot of unexpected side-effects as well. Some of those are in fact very promising.”
Some basics first: Lithium-ion batteries (first introduced by Sony in 1992) have two electrodes with an electrolyte in between, through which lithium ions can move easily. The negative electrode (anode) is usually made of carbon, but in the most modern ones titanium oxides are introduced.
When charged, lithium nestles in the crystal lattice of the titanium oxide. But a cobalt or manganese oxide electrode in the vicinity attracts the positively charged lithium ions through the electrolyte layer, leaving behind a surplus of electrons.
An external circuit transports the electrons to the positive electrode (cathode). The amount of energy involved equals the product of the electric charge of the amount of lithium and the voltage difference between the poles.
“Because the lithium ions diffuse into titanium oxide lattices, we thought we’d speed up the process by using smaller particles”, Borghols explains. Overall, that proved right, but some details were puzzling. “It was hard to find an explanation for some of our measurements”, Borghols admits. “At first we wanted to chuck them in the bin.”
Borghols discovered that at smaller particle sizes, lithium diffused less deeply into the lattice. Instead, it forms a crust around the particle, inhibiting further lithium uptake in the whole particle. The thickness of the crust depends on the crystal form. Overall the amorphous (no crystal structure) nanostrucured titanium oxide seemed to perform best, meaning it has an excellent lithium-uptake in terms of both quantity and speed. Alas, after about fifty cycles of charging and discharging, only a quarter of the initial capacity is left, which, by the way, is still more than ordinary titanium oxide.
The processes taking place between titanium, lithium, oxygen and the crystal surface are not very well understood yet. Still, amorphous TiO2 beats other forms of titanium oxide in storage capacity and certainly in discharge rate. Borghols concludes the material is “very appealing for cheap, high rate, high capacity Li-ion anodes.” While Wagemaker adds: “With the right ingredients, you can make them within five minutes.”
Meanwhile, another promising researcher is at work in this renowned laboratory. Frits Klaver recently received the Young Wild Idea award (worth 10,000 euro) for his project on ‘Efficient Li-ion batteries’.
He is not working on nanoparticles, but rather instead aims to optimise micro particle sized batteries by improving the contact between the titanium oxide electrode and an ion-conducting polymer. Experimenting with various sizes and shapes of the electrode particles, Klaver hopes to present his findings next July.
Wouter Borghols, Lithium insertion in nanostructured titanates, defence Monday, 29 March 2010. PhD supervisor: Prof. Fokko Mulder (AS). This PhD project was financed by the Delft Institute for Sustainable Energy.

Weblink to Delta