Electrochemistry: Mapping a battery electrode

12/22/2014

Microscopy technique tracks the flow of lithium ions in a common cathode material

The scanning electrochemical cell microscope mapped the topography of a lithium iron phosphate particle in a battery electrode (left), as well as measuring the current that flows into it (right; red corresponds to the maximum current of 143 picoamperes).
The scanning electrochemical cell microscope mapped the topography of a lithium iron phosphate particle in a battery electrode (left), as well as measuring the current that flows into it (right; red corresponds to the maximum current of 143 picoamperes).

Modified, with permission, from Ref. 1 © Y. Takahashi et al.

A powerful new imaging technique has mapped the reactivities related to lithium ion transport in an electrode for the first time, which will lead to battery designs with higher performances.

Lithium-ion batteries are widely used to power mobile devices, but some aspects of their operation are still veiled. Studying the interaction between lithium ions and electrodes has been particularly challenging due to a lack of suitable analytical techniques for measuring reactivities.

Now, Tomokazu Matsue, Yasufumi Takahashi and Akichika Kumatani of the AIMR at Tohoku University and colleagues have developed a nano-scanning electrochemical cell microscope that can record the electrochemical properties of a lithium iron phosphate (LiFePO4) electrode1. “This is the first time anyone has visualized the reactivity of an electrode surface,” says Takahashi.

The microscope uses a pipette probe whose aperture diameter is roughly 50 nanometers. The probe contains a solution of lithium ions and scans slowly across the electrode surface. Whenever the solution within the nanopipette touches the electrode, lithium ions flow between the two, creating an electrical current. This allowed the researchers to map an electrode’s landscape to a scale of about 100 nanometers and to investigate its electrical properties at specific points.

The team tested their device on a typical electrode containing a blend of LiFePO4 and acetylene black. While being cheap and environmentally benign, LiFePO4 suffers from a low electrical conductivity. Acetylene black improves the electrode’s conductivity, but it reduces the battery’s overall energy capacity since it cannot store lithium ions.

The nano-scanning electrochemical cell microscope confirmed that lithium ions flowed readily into micrometer-sized LiFePO4 particles that were embedded in the electrode, but did not enter regions made of acetylene black. The probe was sensitive enough to target individual particles, without interference from neighboring materials (see image).

By altering the concentration of lithium ions in the electrode, the nanopipette probe could also reveal how charging or discharging affected the voltage at specific points of the electrode. Stringing together snapshots of the process, taken every 10 milliseconds, produced a movie of an individual LiFePO4 particle’s electrochemical behavior.

The researchers also analyzed individual nanoparticles of LiFePO4 on a platinum plate to discover how the crystal structure and particle orientation affected the way lithium ions flowed through it. They are now carrying out further experiments on these properties, which might help to optimize the position and structure of LiFePO4 particles in electrodes to boost battery performance.

References

  1. Takahashi, Y., Kumatani, A., Munakata, H., Inomata, H., Ito, K., Ino, K., Shiku, H., Unwin, P. R., Korchev, Y. E., Kanamura, K. & Matsue, T. Nanoscale visualization of redox activity at lithium-ion battery cathodes. Nature Communications 5, 5450 (2014). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.