Charging your ebike battery can be unpleasant, especially if you don’t have a garage of your own. Moreover, if you enjoy long rides, range anxiety is not irrelevant, since pushing back home a 26 kg ebike is definitively annoying if not painful. Consequently, for ebikes, like for electro mobility in general, battery performance is paramount. For the time being, batteries energy density is their weak point. They deliver only 300 Wh per kilogram in ebike batteries, and generally for all lithium-ion batteries, versus 12.5 Wh/kg for gasoline. This results in a limited range. Their lifespan is sufficient, one can reckon at least 20,000 km for ebike batteries, depending on usage conditions, whereas car batteries are normally guaranteed to retain at least 70% of their capacity for at least 180,000 km.
Electro mobility has many advantages for bicycles
Just think how burdensome it would be to assist your pedaling with an internal combustion engine, or ICE. Should it be mounted as a mid-motor, you would be compelled to breathe toxic gases every time you stop. It would certainly be a two-stroke engine, consuming oil, becoming clogged and weaker after a few thousand kilometers because of combustion residues especially in the exhaust pipe, but also in the cylinder.
They tried to sell some bicycles with ICE in the early 60s, like Italy’s Atala Mosquito. It was quite a flop. It was rather difficult to harmonize the motor power output with pedaling, ICE engines being much less flexible than electric ones. Connecting the engine with torque, speed or cadence sensors would have been too complicated, although nowadays they do it on cars with cruise controls. They were also expensive. In the end, a failure, discontinued after a few years. Assuredly, motor assisted bicycles will have to rely on batteries for many years to come. Other solutions, like hydrogen fuel cells ebikes, are way down the road, at least for mass diffusion.
Motor assisted bicycle Atala Mosquito – courtesy Autobelle.it
How batteries work
Lithium-ion and solid state batteries
The four components of a battery
- A positive electrode, called the cathode, normally made of graphite in lithium-ion batteries;
- A negative electrode, called the anode, also made of graphite;
- An electrolyte solution, normally a liquid with lithium salts for lithium-ion batteries;
- A separator.
Energy is created by the cyclic and reversible movement of ions between the two electrodes through the electrolyte. The separator prevents short circuits between the anode and cathode.
How a battery delivers electric energy
A battery can generate energy through a chemical reaction: oxidation-reduction. When electricity is demanded, the anode oxidises. It releases ions, which are positive, and electrons, which are negative. Positive particles move towards the cathode through the electrolyte. However, the separator allows the passage of ions, but prevents the passage of electrons. In order to maintain chemical equilibrium within the battery, the electrons must nevertheless move towards the cathode. This flow of electrons, which is nothing other than an electric current, reaches the positive pole via the external circuit connected to your appliance. The battery will deliver electricity until the reserve of ions in the cathode is exhausted.
How a battery recharges
Feeding the battery with electricity will cause a reduction reaction at the cathode. The anode components will be able to regenerate by recovering the ions released at the cathode during the discharge. The electrons will flow to the negative pole via the charger circuit. To balance the flow, the ions accumulated at the cathode during discharge will flow towards the anode through the separator. Charging is complete when all the ions have returned to the negative pole.
Increasing batteries range, what ebike manufacturers aim the most at
The sacred graal, i.e. the most wanted technology is therefore one who would enable batteries to considerably extend the ebike range. Now, promising technologies are being developed to achieve that goal. We already wrote here about solid-state lithium-ion batteries, which are already operational, for instance on about 100 Mercedes eCitaro buses. They have a solid electrolyte and provide 50% more range than conventional liquid lithium-ion batteries, and a duration twice as long.
Silicone anode batteries are a promising newcomer
French and Canadian universities, and also the San Diego University (California) in the USA are researching the way to improve this technology. Quantumscape, a Californian company developing batteries among others for its investor Volkswagen, uses silicone anodes too (see header image). A crystalline silicon anode has a theoretical specific capacity of 3600 mAh/g, about ten times that of commonly used graphite anodes (limited to 372 mAh/g). Each silicon atom can bind up to 3.75 lithium atoms in its fully lithiated (= when recovering ions released by the cathode) state, compared to one lithium atom for every 6 carbon atoms in fully lithiated graphite (LiC6). The volumetric energy density of silicone is three times higher than that of graphite, which is the material actual lithium-ion batteries anodes consist of. Just consider that silicone anodes can be combined with the above-mentioned solid-state electrolytes, exponentially increasing batteries range. The problem: silicone anodes wear off after a few recharges, and their volume increases threefold as they combine with lithium ions during discharge, when they deliver electricity to appliances. See the table below.
Comparison of volume change and capacities of 5 anodes after lithiation (discharge)
| Anode | Lithiated Anode | Capacity
(mAh/g) |
Volume change after lithiation |
| Pure lithium metal | Li | 3862 | – |
| Aluminum anode | Li9Al4 | 2235 | 604% |
| Tin anode | Li13Sn5 | 990
|
252% |
| Silicon anode | Li15Si4 | 3600
|
320% |
| Graphite | LiC6 | 372 |
10%
|
Source: https://wikibattery.org/de/akademie/silizium-anode-silizium-komposite-in-siliziumbatterien/
Conclusion
The latest scientifical developments, although not yet applicable, can well fuel our hopes for much better ebike batteries. Some companies producing prototypes of silicone anode batteries are already listed on the US Stock Exchange (Nasdaq).