New secondary battery design uses porous silicon instead of graphite
By using a novel electrode material, South Korean scientists have managed to improve the capacity of lithium-ion batteries, though it will take a little while longer before they reach the market. Their key innovation was to replace the standard graphite electrode with a porous silicon one. The research team at Hanyang University, led by Jaephil Cho, reports that it has created three-dimensional, highly porous silicon structures that allow greater memory capacity to be achieved than is available from existing Li-ion cells using graphite. This holds out the promise of a new generation of batteries with a noticeably longer lifetime, very desirable not only for mobile digital devices like mobile phones, music players, PDAs and notebooks, but also for electric vehicles – in other words, anywhere conventional Li-ion cells are used. Along with lithium-polymer (Li-po) cells, these now constitute the highest class of batteries: relatively expensive and demanding in terms of charging equipment, but with a very good ratio of power delivery relative to their low space requirement and weight.
When a lithium battery is charged, ions are drawn from the cathode to the anode. Silicon possesses the correct electrochemical affinity to lithium, so it has already long been seen as an interesting anode material. It seems to have been just too efficient, however: silicon absorbs so many ions during the charging process that the anode can swell to four times its original size. The consequence of that is that the material can break. The silicon anodes previously made survived only a few charging cycles.
The Korean scientists solved this problem with a special manufacturing process using nanoparticles of silicon dioxide and a gel based on silicon as the initial basis. This yielded miniature carbon-coated silicon crystals having a three-dimensional, highly porous structure. Anodes made from this new material show a high charging capacity and, say the researchers, accept rapid charging and discharging.
Whether more powerful anodes on their own yield improved batteries is something to be proven in practice. Researchers elsewhere however are already also working on alternative materials for both anode and cathode. Last year, for example, a team of materials scientists at Stanford University showed how the capacity of lithium batteries could be at least tripled with the aid of nanowire electrodes.
Although Stanford's silicon nanowires form an excellent anode, the cathode also contained in lithium batteries could do with an upgrade. A battery with a "super-anode" is of no use without a cathode that can emit correspondingly more charge.
For that reason, a number of laboratories are working assiduously and in parallel on new cathode materials. One of the researchers involved said these were the Holy Grail in his field, and that anyone who managed to achieve a significantly greater cathode capacity would be making "a gigantic breakthrough for lithium battery technology".