Will perpetual mobile phone batteries become a reality?

Replacing tired, worn out batteries in mobile phones and laptops is a pain but when batteries start to fail in your electric car it's a financial disaster. Not surprisingly then the Holy Grail for battery technologists is how do we create the never-ending battery? That is, a battery system that will still need charging but will not become increasingly inefficient as time goes by.

A possible breakthrough that may herald the era of the perpetual battery has been made at the Technical University of Munich.

But before charging (sorry) ahead with that story, why does lithium eventually fail?

When being charged lithium naturally moves toward the graphite or negative anode. A layer of lithium atoms forms on the surface of the anode called solid electrolyte interface. This film gets thicker and eventually forms a barrier that obstructs interaction with graphite.

Just as the anode can get covered with a lamina of material by the charging process, the cathode can also develop a similar covering because of electrolyte oxidation. And the hotter a battery gets the faster and more damaging the reaction is. The cathode's reactive qualities are immediately impeded, causing a rapid and irreversible capacity loss.

As a consequence the lithium-ion reaction required to deliver power can no longer happen adequately and the battery won't retain or deliver as much power as it did when new.

Heat is a well-documented demon when it comes to battery degradation. One example of this in the United Sates several years ago involved drivers of the Nissan Leaf car who took legal action against the manufacturer. Their action sought to prove that a design inadequacy in their cars' systems caused them to prematurely lose battery life and driving range.

Guess what was blamed? That old battery devil called heat. As it turned out the battery design in Leaf cars did not have active thermal management to keep the cells cool and, because of this, batteries in those cars were losing nearly a third of capacity after approximately 18 months.

Closer to understanding

Currently very little is understood about how ageing directly effects battery performance but scientists from the Department of Technical Electrochemistry and the Research Neutron Source FRM II at the Technical University of Munich (TUM) have taken a technological step closer to identifying the causes.

Batteries with graphite anodes have a significant loss of capacity of up to 10% during the initial charging cycle, often referred to as the formation step. And as mentioned earlier each subsequent charge-discharge cycle reduces battery capacity further. In addition to that, capacity can also deteriorate when batteries are stored above room temperature.

To investigate the aging mechanism and to understand the reasons behind them, the TUM scientists used electrochemical investigation techniques and measurement methodologies such as X-ray diffraction, impedance measurements and gamma activation analysis.

By doing this the scientific team was able to analyse the behaviour of batteries with graphite anodes and nickel-manganese-cobalt cathodes (NMC) cells at various temperatures. NMC cells typically have a large capacity and can handle charging voltages up to slightly below 5V. However, above 4.4V aging effects increase measurably.

Using X-ray diffraction the scientists investigated the loss of active lithium over multiple charging cycles and used impedance measurements to register the increasing resistance in the battery cells. Neutron activation analysis allowed the accurate determination of miniscule quantities of transition metals on the graphite electrodes.

Capacity and temperature conundrum

The proven capacity loss in the formation step is caused by the build-up of a pacifying layer on the anode. This consumes active lithium, but also protects the electrolyte from decomposition at the anode.

The research group determined two key mechanisms for the loss of capacity during operation. The active lithium in the cell is slowly used up in various side reactions and is therefore no longer available and this process is very temperature dependent. At 25 °C the effect is weak but becomes stronger at 60 °C.

When charging and discharging cells with a cut off potential up to 4.6 V, cell resistance increases rapidly. The transition metals deposited on the anode may increase the conductivity of the pacifying layer and thereby speed up the decomposition of the electrolyte.

Irmgard Buchberger, PhD student at the Department of Electrochemistry at the Technical University of Munich believes that because of the University's research, battery processes can be improved and it may be possible to include additives that improve performance.