According to Niklas Thom, the idea for the ReBiCell project – i.e. regeneration and reactivation of battery modules for electric bicycles – was more like one spontaneous thing
. The idea surrounding the use of batteries that have lost capacity has been with the EMLE (Electromobility, Power Electronics and Decentralized Energy Supply) science center employee for a long time. Within two weeks, Thom wrote the project application for the EK.SH funding call – with success.
The starting point for the application was initially the idea of reusing old e-bike batteries in a second-life approach, for example as stationary energy storage devices. But then Thom came across an interesting publication from Stanford University. Instead of continuing to use batteries with decreasing capacity, the aim was to recover or reactivate part of the lost power.
TH Lübeck: From theory to practice
A theoretical approach on a laboratory scale with small coin cells – albeit successful. Because the results show: The lithium, which was actually believed to be dead, reacts to applied electric fields. It has been shown in the laboratory that up to 30 percent of the original capacity can be restored.
One of the main reasons for the loss of capacity is the formation of inactive lithium. Normally, lithium ions are exchanged between the anode and cathode
explains Thom. However, it can happen that metallic lithium is deposited – these deposits are called dendrites and can be seen in microscopic images as fine, needle-like structures.
Lithium that is thought to be dead is not inactive after all
These structures form during loading and are broken down again during discharging. However, this happens irregularly because in a cell the field distribution between anode and cathode is not the same: This means that there are places in the battery that discharge faster than others, which in turn causes the dendrites to also break down irregularly
said the scientist. This can result in fragments of lithium remaining behind – the lithium that was previously thought to be inactive.
These particles often float in the electrolyte a few nanometers away from the anode. Theoretical studies at Stanford University have shown that these very particles respond to the application of electric fields. As soon as they move again and the metallic fragment touches the anode, they split back into lithium ions and electrons. This means that more particles are available for charge exchange and the capacity increases.
Thom points out: But just because something works on a laboratory scale doesn’t mean it’s commercially viable.
That’s why he wants to investigate the transferability of the laboratory theory to real-world applications in the form of bicycle batteries.
ReCiBell: Studies on the lifespan of batteries
The first step into reality is the transition from the coin cell to a commercially available lithium cell, before moving on to the series and parallel connections of a bicycle battery. A bicycle battery has 13 individual cells connected in series and another strand in parallel. Unlike in the laboratory situation, compensatory effects can occur when cells are connected to one another.
I have a great interest in understanding why things work the way they do. That’s why I’m fascinated by the way to move from theory step by step to a real application and see whether it can actually be implemented
says Thom. The goal of the ReCiBell project is to find out whether the service life of batteries can be increased and how great this benefit actually is in the end.