At Levy we’re always evaluating new lithium-ion battery cells for use in our swappable battery packs. Upgrading the battery technology used in these packs means being able to offer extended range and power without our customers having to purchase an entirely new scooter.
That being said, here are some of the most promising trends in battery technology for electric vehicles we're looking forward to in 2020. But first let’s start with an overview of the current state of the market, focusing first on the industry's best Panasonic 18650 lithium-cobalt-acid battery - currently being used in our electric scooter.
These popular battery cells are named based on their cylindrical dimensions, having a diameter of 18 mm and length of 65 mm. Tesla made these batteries even more famous in their partnership with Panasonic, and by assembling over 7000 of them together in the Model S battery back. By comparison, the Levy electric scooter only uses 25 of these cells. Each cell has up to 3.2 amp-hours of charge at 3.6 Volts (or ~11.5 watt-hours of capacity).
All said, these cells give the Levy Electric Scooter battery pack a total power capacity of 274wH with a ~10 mile range.
Since the development of the lead-acid battery in the late 1800s, there have been only four or five major breakthroughs in battery technology, with energy density doubling roughly every 30 years. If this pattern holds, the next technological break-though is almost due. Lithium-ion batteries were first commercialized in 1991 by Sony for their digital camera.
So what's next?
The next step is likely to be an iterative improvement on the already popular lithium-ion batteries, which generate current through movement of lithium ions from the positive to negative electrodes of the battery via a liquid electrolyte. The basic technology of these batteries has stayed the same, while new elements are being tested that can increase energy density (and therefor capacity). Today the most popular set of elements in the cathode is a mix of nickel, cobalt, and aluminum - while graphite against a shield of copper is most commonly used in the anode.
Larger Battery Cells?
One might think that simply increasing the size of the battery cell could help yield more power compared to the overall weight of the battery. There have in fact been steps in this direction, and these larger cells were recently debuted in the Tesla Model 3. They're called 2170 cells (21mm diameter, 70mm length) and they are 25% larger than standard 18650 cells. While there are some efficiency gains in manufacturing and pack assembly costs with these, the energy density improvement is minimal. 2170 cells can get up to 247 Watt-hours / kg, compared to an average of 240 Wh / kg with the leading 18650 cells, yielding only a 3% improvement.
Solid State Battery Cells are the future
In modern lithium-ion batteries, ions move from one electrode to another across a liquid electrolyte medium, as we talked about in a previous post. In solid-state batteries, this liquid electrolyte is replaced by a special solid compound which still retains the ability to allow lithium ions to flow through. The result is being able to bring the anode and cathode closer together, resulting in a much higher energy density.
This concept isn't exactly new, but only recently have new solid electrolytes been discovered that have the very high ionic conductivity needed to make it work. Most recently, Samsung has begun testing a solid state battery built with a silver-carbon composite as the electrolyte layer.
Apart from higher energy density, estimated at up to 500 Wh / kg, the other advantage of these batteries is safety. Solid electrolytes are non-flammable when heated unlike liquid mediums. This means the use of new high-capacity materials can be used in the anode and cathode that were previously deemed too risky from a thermal and safety management standpoint.
Commercial grade solid state batteries could be available in the next 2 years, and could result in the doubling of battery cell capacity. To put it in perspective - this means the same size Levy battery pack could have a capacity of 540 watt-hours with a range of almost 20 miles on a single charge.
Lithium-Sulfur Batteries could be the next exponential step forward
These batteries would use pure lithium in the anode and sulfur in the cathode, resulting in theoretical energy density almost four times greater than standard lithium-ion batteries today. The technology would likely require a solid state electrolyte medium, otherwise the thermal energy released would be too high and unstable with current liquid electrolyte batteries.
Sulfur can hold many more lithium-ions during the transfer process, hence the high energy density, but the problem is that as lithium ions are transferred into the material it expands dramatically. This prevents the chemical exchange from happening in a standard 18650 cell container, which can't adjust its volume dynamically. Even if it were possible, the constant expansion / contraction of the sulfur cathode would cause the overall battery life to quickly degrade, only lasting 40 or 50 charging cycles based on current tests.
There are some solutions to this problem in sight, however. A research team in Australia has found a way to prevent the volume change by introducing carbon molecules into the structure that help hold the sulfur cathode at a constant volume during charging / discharging. The team thinks they can further perfect this technology which would result in a lithium-sulphur battery rated up to 700-800 charge / discharge cycles - on par with li-ion batteries on the market today.
With the electric scooter battery pack containing these types of cells, we could expect almost 1000 watt-hours and a range of 60 miles. While this is likely still years away, it's exciting to think about the impact this will have with the adoption of not just electric scooters, but all electric vehicles and the power grids of the world.
Thanks for reading!