Accident yields anodes that offers 4x battery lifespan
Scientists at Tsinghua University, Beijing, and the Massachusetts Institute of Technology, Boston have been working on techniques to extend the working life of Li-ion batteries. The scientists were working methodically on making the anodes of the batteries more durable, as the decay of the anode material is a major cause of the drop off in Li-ion battery life after a few hundred charge cycles. In the course of their carefully timed experimental process one batch of anodes were left under chemical treatment for several hours more than they should have been, by accident. Instead of throwing away the forgotten batch, the scientists decided to test them and the results were great – an anode 4x more durable than the current best available technology.
Dr Wang Changan at Tsinghua and Dr Li Ju at MIT were behind the experiments. These materials scientists had been working on using aluminium for the anode of a Li-ion battery in preference to graphite. Today’s Li-ion batteries use carbon anodes because aluminium expands and contracts too much during each charge/discharge cycle. Aluminum is a low-cost option with theoretical capacity of 2 Ah/g. But aluminum and other high-capacity materials, expand a lot when they get to high capacity, when they absorb lithium. And then they shrink, when releasing lithium.
This expansion and contraction of aluminum particles generates great mechanical stress, which can cause electrical contacts to disconnect. Also, the liquid electrolyte in contact with aluminum will always decompose at the required charge/discharge voltages, forming a skin called solid-electrolyte interphase (SEI) layer, which would be ok if not for the repeated large volume expansion and shrinkage that cause SEI particles to shed. As a result, previous attempts to develop an aluminum electrode for lithium-ion batteries had failed.
That’s where the idea of using confined aluminum in the form of a yolk-shell nanoparticle came in. In the nanotechnology business, there is a big difference between what are called “core-shell” and “yolk-shell” nanoparticles. The former have a shell that is bonded directly to the core, but yolk-shell particles feature a void between the two — equivalent to where the white of an egg would be. As a result, the “yolk” material can expand and contract freely, with little effect on the dimensions and stability of the “shell.”
The scientists were working on stopping the oxide coating forming on aluminium nanoparticles when they are exposed to air by soaking the material in sulphuric acid and titanium oxysulphate. That replaces the aluminium oxide with a more conductive titanium oxide. As mentioned in the intro, timed soaking sessions were undertaken trying to get the timing right for creating the most durable anode.
One batch got accidentally soaked and left in the chemicals for several hours longer than intended. Luckily the scientists decided to test these samples – as they found they had nearly all the advantageous properties they had been looking for in a new anode. The new anode formed with a solid nanoparticle shell and an inner ‘yolk’ of aluminium material that could expand and contract without damaging the coating. Dr Wang and Dr Li created some batteries using the ‘accidental anodes’ with the nanoparticle coating and ran them through 500 charge/discharge cycles. Compared to graphite-electrode equivalents put through the same charging cycle these new batteries retained as much as four times the capacity. The manufacture of these nanoparticle imbued shell, plus aluminium yolk, anodes is reasonably likely to be commercialised as the process seems to be both simple and scalable.
Pokdepinion: This is a tech that doesn’t seem so out-of-this-world, which means that it may actually be marketed soon.