Lithium ion batteries – they’re everywhere, and their name has become mainstream enough that non-scientists know they’re in our laptops and electronics. These types of batteries are so popular because lithium boasts a high energy density and they don’t lose charge very quickly when it’s not being used. But then, you may ask, why are we still doing battery research? Why haven’t we already hooked up a bunch of Li-ion batteries to solar cells and created our sustainable, green energy future already?
There are several reasons for this, but probably the major one is that the energy density of these batteries just isn’t good enough. Comparisons are all relative, and this especially applies to batteries. Li-ion batteries have high energy density compared to other battery technologies out there – you can see comparison in the graph above, where the bottom left indicates low energy density and specific energy and upper right indicates high values of both. But when we’re talking about batteries operating as energy storage devices for solar cell-based electricity at the scale of homes, buildings, etc., batteries need to compete with natural gas, coal, and oil. Oil has nine times the energy density of a Li-ion battery! So there’s much work to do if we want to apply this technology as a replacement for fossil fuels.
When the battery is charged, all the lithium is stored up in the anode. When we want energy from the battery, lithium discharges and moves though the electrolyte between the anode and cathode. The electrolyte is usually liquid and requires an environment in which Li is highly mobile. As the Li moves across the electrolyte, a freed electron exits the battery through an external circuit to do the work we want. The cathode, then, is where all the Li and the electrons meet up again to complete the circuit. The opposite trajectories occur upon charging.
So the key to a good anode is that it can store as much lithium as possible. Why don’t we just make anodes out of pure lithium, then, as that would give us the most Li per volume (highest energy density)? Manufacturers would love to do this, but unfortunately there are several problems with this setup. First, without a ‘host’ structure in the anode into which the Li deposits (intercalation), the Li-only anode would increase in volume without limit, causing a massive increase in anode size that doesn’t quite fly given the finite battery space. For this reason, we use layered structures like graphite that have a smaller, finite volume expansion upon lithium intercalation (shown above).
Also, lithium has this nasty habit of reacting with the electrolyte. This is actually a good thing in that it creates a solid electrolyte interface (SEI) that protects the anode from further reactions with the electrolyte. However, when Li metal reacts at the electrolyte, it tends to create these long, dendritic spines that stretch out from the anode (a great image of one above, where the black is the anode surface).
These dendrites can grow so long that they connect the anode and cathode and short-circuit the battery! This is very bad – there will no longer be a potential between the anode and cathode and therefore no more electric current to do work. Li-only anodes are especially susceptible to this happening – another drawback that makes them less feasible.
That’s where this new study comes in. Stanford researchers have designed a new Li-ion anode that uses hollow, amorphous carbon nanospheres as a layer between the Li metal and the electrolyte. This monolayer prevents the creation of the long dendrites and prevents rapid volume expansion of the Li-only anode. Both problems solved! This is a truly exciting finding that could revolutionize Li-ion technology and push the energy density up quickly.Above is a nice diagram from the paper demonstrating the clear differences between this new technology (bottom row) and the status quo (top row). As you can see along the top, volumetric changes in the Li (blue) during deposition lead to the SEI (yellow) breaking, and the thin dendrites grow outward (rightmost diagram). In contrast, the carbon nanospheres (black, bottom row) limit volume expansion during deposition, keep the SEI intact and preventing any dendritic formation. Here’s a great cross-sectional image of the nanospheres deposited on the Li anode (this is an SEM image, for those who care!): The nanospheres are created by depositing carbon on an array of ordered polystyrene nanoparticles resting on a copper substrate (copper is used as the current collector in the batteries). The array is then heated up to 400 C, which completely removes the polystyrene and leaves the carbon spheres deposited on the copper (plastics like polystyrene have a much lower melting point than carbon or copper!) Performance is what finally matters, and that’s the true home run for this paper. The results above show the Coulombic efficiency for increasing number of cycles of the nanosphere anode (solid squares) versus a traditional Li-only anode (open squares). Coulombic efficiency is the ratio of the energy gained upon discharge to the energy required to charge the battery. A cycle is just one period of discharging and charging. In a traditional Li anode, cycles create the dendrites and loss of available Li to discharge in later cycles, leading to much reduced Coulombic efficiency. This can be clearly seen in the sudden drop in open squares in all three examples once enough cycles have occurred. On the other hand, the hollow nanosphere anode shows an extremely stable Coulombic efficiency around 99% over 150 cycles! This is amazing performance and one of the most crucial parameters required for any rechargeable battery to be competitive on the market, since we all charge our devices for hundreds of cycles over their lifetime. In actuality, this performance needs to be improved even further, as market-ready batteries show Coulombic efficiency of over 99.9% over this number of cycles.
So good news on the battery front! Hopefully this research will lead to more ideas of how to use all-lithium anodes and set some new records in terms of energy density and performance. This technology could also be applied to other types of Li batteries, such as Li-S and Li-oxygen batteries, which already have a higher energy density. Exciting possibilities!
Zheng, G., Lee, S., Liang, Z., Lee, H., Yan, K., Yao, H., Wang, H., Li, W., Chu, S., & Cui, Y. (2014). Interconnected hollow carbon nanospheres for stable lithium metal anodes Nature Nanotechnology DOI: 10.1038/nnano.2014.152