Although brittle, battery electrode material NaFePO4 can vastly swell and shrink when ions incorporate during charging and discharging. A research team around MIT professor Chiang discovered that the occurrence of a glass-like phase is the answer to the mystery.
On a microscopic level, a lot is going on when you charge or discharge a battery. Electrically charged atoms and molecules, called ions, travel between the battery electrodes. Once arrived at the electrode, the ions often incorporate into the electrode material, a process that is referred to as intercalation. For the electrode material, this means that it has to swell to accommodate the ions. When the ions leave, the electrode shrinks again to its previous size.
After too many charge and discharge cycles, the strain of changing the volume can become too much for the material. Then, the electrode material breaks and causes the battery to fail. Yet, some brittle materials can be used as electrode materials and it has so far been unclear, why.
Disorder instead of breaking
Now, researchers collaborating between MIT, the University of Southern Denmark, Rice University and Argonne National Laboratory may have found the answer for a phosphor-olivine material. More specifically, they looked at a sodium-iron-phosphate with the chemical formula NaFePO4. The compound is a potential candidate for the positive electrode, called cathode, of sodium-ion-batteries.
The scientists around MIT professor Yet-Min Chiang and graduate students Kai Xiang and Wenting Xing discovered that a glass-like phase prevents the material from cracking despite large volume changes. The findings, which can open up for novel battery designs using materials that have been initially discarded, were recently published in the journal Nano Letters .
NaFePO4 is a brittle material that easily cracks when subjected to mechanical stress. However, when used as a battery material it manages to accommodate rapid changes in its dimensions that are as large as 17%. “We know that brittle compounds like this would normally fracture with less than a one percent volume change” Chiang says. So, how does the phosphor-olivine compound do it? Chiang explains: “We found […] that the crystal gives up and forms a disordered glass”.
In other words, the material changes its structure. Before the ions intercalate, the phase of NaFePO4 is crystalline with its atoms neatly ordered in regular patterns.
When more and more ions incorporate (or intercalate), scientists have so far believed that a different crystalline phase forms with more space for the ions. In contrast to this, Chiang and his colleagues observed that a different kind of phase was created without any order or regular patterns. Such phases are called amorphous, or glass-like.
Revisiting battery materials
These findings are “a seminal contribution that links the electrochemical, mechanical, and crystallographic aspect of battery electrodes”, so assistant professor William Chueh from Stanford University, who is active in the field of materials science but not involved in the study.
Chiang suspects that the phase-change mechanism could apply to similar compounds as well. In addition, he points out that they might have found “a new way to create glassy materials that may be useful for batteries”.
This new generation of batteries could exhibit longer life times because with a glassy composition, the volume changes are slow instead of rapid. Stanford professor Chueh adds that the discovery “may lead scientists to revisit battery materials previously deemed unusable due to the large volume change during charging and discharging.”
Chiang can also think of other possible future applications, such as robotic actuators or pumps for the delivery of drugs from implantable devices. The research team now tries to find easier ways to synthesise the olivine compounds. They also want to investigate whether the phase transition to an amorphous phase is a general property of similar crystalline materials.
 K. Xiang et al. (2017) Accommodating High Transformation Strains in Battery Electrodes
via the Formation of Nanoscale Intermediate Phases: Operando Investigation of Olivine NaFePO4. Nano Lett. 17:1696, DOI: 10.1021/acs.nanolett.6b04971.
 D. L. Chandler (2017) How some battery materials expand without cracking, Phys.org, available at: https://phys.org/news/2017-04-battery-materials.html?utm_source=nwletter&utm_medium=email&utm_campaign=daily-nwletter (assessed on 13/04/2017).
 Image Credit: Shutterstock.com/sdcoret