Deliberate Short Circuits and Nail Torture as a Means of Enhancing Security: Battery research

The scientists place the battery cells in the test rooms from outside the buildiThe scientists place the battery cells in the test rooms from outside the building. © WWU - Peter Lessmann
Most of the people who stroll across the Leonardo campus are unlikely to notice a narrow, single-storey building. The construction in question is about 25 metres long and five metres wide. It is adorned with red and white stripes, has a corrugated iron roof, and is surrounded by a wire mesh fence which is about 1.5 metres high. In Markus Börner’s view, however, this is not just any building: ‘The fact that this building has been placed at our disposal is a dream come true.’

What is so special about this small, rather nondescript complex set in a green meadow? A brief walk around the building will rapidly unravel the mystery. Markus Börner enthuses about the ‘battery safety laboratory’, where specialists like himself cause short circuits by overcharging batteries and ‘torturing’ them with nails. They do this in order to detect flaws and find out how safe batteries are. Why do they get a kick out of this? ‘At the University’s MEET Battery Centre we produce the cells ourselves. As a result, we know exactly how much of each element is in a cell, and we know precisely where these elements are. We can model the entire process chain from the production of individual components (e.g. active material, electrodes, or electrolytes) to the manufacture of reproductible cells endowed with a capacity which makes them industrially viable. After making the cells, we just have to go next door to carry out our tests. These shortcuts enable us to develop new kinds of batteries,’ says Markus Börner, who studied nanostructure technology in Würzburg and has been working at the MEET since 2012.

Since batteries are ubiquitous at home and in the workplace, users take it for granted that they are safe. However, the subject of battery safety has recently assumed much greater importance owing to the planned introduction of electric-powered vehicles on a massive scale. In a matter of seconds, a battery can accelerate a car weighing over a ton - occupants and all - to a speed of 100 km/h and more. The fact that such batteries can store an enormous amount of energy in a very small space is automatically fraught with risks and uncertainties.

Of course, the same goes for fuel tanks and combustion engines. Over the past few decades, however, motorists have got used to driving around with several dozen litres of inflammable fuel in a plastic tank that is only a few millimetres thick. Nonetheless, a single photograph of a burning electric car makes some people wonder how safe batteries are, especially since it is far from easy to extinguish a burning battery.

Statistics suggest that worries about batteries catching fire are unfounded. According to studies relating to cars with combustion engines in the USA, says Markus Börner, there were 90 fires per billion driven miles. After carrying out a similar study bearing on a battery-driven car fleet belonging to the US company Tesla, the authorities reported only one fire for the same mileage. ‘It’s the prevailing fear of new technologies that is at the root of our initial misgivings about safety,’ says the MEET expert. ‘A further complicating factor is undoubtedly the fact that we haven’t got enough figures based on actual experience.’

The battery safety laboratory was set up in 2015. The people who work here spend most of their time conducting tests on cells that are bigger than those used in mobile phones or laptops. To what extent is a battery’s safety determined by its age? What happens when a cell is overcharged, and which parts of the cell are affected? Which component reacts most violently when a nail is hammered into a cell? In the MEET, scientists work in three big groups: ‘Materials’, ‘Analytics’, and ‘Cell Systems’. The latter group also deals with safety issues. What requires most skill and acumen is devising ways to ensure that batteries remain safe in the event of a failure of all the security components, which are mainly steered by electronics. Summing up, Markus Börner says: ‘The battery ought to be intrinsically safe.’

The battery safety laboratory has four small rooms where tests can be carried out. Each room is protected by a pane of armoured glass which is approximately one metre high and 50 centimetres broad. From outside the building each pane is fitted with test cells. Rooms 3 and 4 bear the inscription ‘Overcharging/Short Circuit’. These rooms are for testing battery safety in cases of electronic malfunction. In room 2 (‘Compactor’) the researchers test cells which have suffered mechanical damage. The first room is labelled ‘Calorimeter’. This is where the impact of high temperatures on cells is studied. By looking through the glass panels, the scientists can observe their tests. It remains to add that the laboratory has a kind of outbuilding with a gas absorber which serves to clean exhaust fumes before they can escape. The outbuilding is also equipped with a cooling system and a cyclization room. If need be, the scientists can therefore examine the batteries’ electrochemical performance under a single roof before and after a safety test.

In the first room you can see exactly what happens when a firmly held nail is slowly bored into a lithium-ion battery cell measuring thirteen by eight centimetres. There is a short circuit! This is a kind of standard test which enables scientists to observe how a cell behaves under constantly changing conditions. ‘Considered from a scientific viewpoint, this test is not particularly instructive, yet it is included in many standards,’ says Markus Börner. ‘Fortunately, in this laboratory we can conduct a whole lot of tests that are much more instructive.’

Norbert Robers