Lithium-ion technology: lots of life left

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The true potential of rechargeable lithium-ion batteries is still waiting to be tapped. A doubling of the battery’s capacity appears to be a real possibility. A special measuring process is expected to produce the breakthrough.

The rechargeable lithium-ion technology is, without a doubt, a success story. From smartphones and self-optimizing boosters to electric cars – the battery keeps all mobile electronic devices alive and kicking. Unfortunately, not always long enough. For this reason, researchers around the world are testing completely new battery concepts. At the same time, they are trying to “squeeze out” higher capacities by tweaking the lithium-ion architecture.

The most obvious approach: enabling negative electrodes (anodes) to store more lithium ions during the charging process. After all, the number of these ions determines the length of time that a battery can supply power. The anodes used in today’s rechargeable lithium-ion batteries are made of graphite.

Six carbon atoms per anode form a ring here. One electron and one lithium ion are found in the center of the ring. But this is not the most efficient solution. Small silicon crystals could store a lithium ion per atom – a change that, in theory, would represent a sixfold increase in storage capacity.

Unlike graphite, however, the silicon crystals can expand by up to four times while absorbing lithium and can simply crumble in the process. The practical solution to this problem is to partially charge the silicon or to use architectures made of stabilizing nano structures, among other approaches. But one problem still remains, something that occurs when the material encounters liquid electrolytes.

Lithium-ion technology
The solid/liquid barrier surface on graphite electrodes remains stable during the discharging/charging process due to limited volume expansion. On silicon, this layer that measures just a few nanometers cracks as a result of high volume changes and restricted flexibility. (Image: Technical University of Vienna).

Complex chemical decomposition reactions that occur during the charging and discharging process result in a nanometer-thin, ion-conducting film that forms on the surface of the electrodes. The film also forms on graphite – much like the passive layer on stainless steel. But it remains stable after just a few charging cycles and ceases to expand. By contrast, the drastic expansion of the silicon crystal causes the barrier layer to continuously crack. The result: Fissures form on a new layer. Every charging cycle uses some electrolyte liquid, a process that results in short battery life.

Lithium-ion technology with improved electrolytes

For this reason, scientists are searching for electrolytes that would facilitate elastic barrier layers. One typical work area is materials science. Researchers at the Technical University of Vienna have developed a special atomic force microscope for this purpose. The microscope is used to analyze the elastic properties of this layer during the charging and discharging process by measuring the growth and elasticity on a minute scale while applying force. This process enables a range of material variations to be studied and the right electrolyte for silicon-based rechargeable lithium-ion batteries to be found.

As you can see, “old” batteries still have lots of hidden life in them. Researchers at the Technical University of Vienna think that a doubling of capacity is certainly a realistic possibility. Initial results already show that the energy density of silicon-based rechargeable batteries could be increased by 15 to 50 percent. This technology is expected to be introduced to the market in three to five years.

Knowledge base

Original publication:

Moeremans et al., In Situ Mechanical Analysis of the Nanoscopic Solid Electrolyte Interphase on Anodes of Li‐Ion Batteries, Advanced Science 6, 1900190 (2019).

 

 

 

Lithium-ion technology (Image: pixabay)

Researchers think that a new measuring method will help push the storage capacity of lithium-ion rechargeable batteries to unexpected heights. (Image: pixabay).