Watching Inside A Battery During The Charge/Discharge Cycle

Energy storage is one of the most important aspects of our highly demanding society. In the last decades, the search for appropriate energy storage materials has become of paramount importance to fulfill the increasing needs of electrical power. As such, energy storage has become a key research topic, and political authorities have created dedicated programs for the development of sustainable energy production technologies as well as for developing efficient-energy storage devices.

The market of metal-ion batteries is dominated by ubiquitous Li-ion batteries, which constitute around 60% of the global market. However, in recent years, an increasing interest in the replacement of Li by other alkali and alkaline earth metals such as Na, K, Mg, and Ca has emerged. In this respect, due to the large natural abundance and lower cost of Na compared to Li, Na-ion batteries are considered as a convenient alternative, especially for grid energy storage.

Batteries are complex devices exhibiting structures at several length scales, from the atomic level, at which electrochemical reactions take place, to the micrometer range through the nanoscale. This accounts for the need to address the structural modifications that batteries undergo during operation, which are in part responsible for the loss of performance, limiting its lifetime at all the length scales involved.

On the other hand, the technology of Na-ion batteries is not as mature as that of Li-ion batteries, and a big research effort is being invested so as to optimize these devices. In recent years, TiO₂ nanoparticles have emerged as a promising anode material for Na-ion batteries, although the exact working mechanism remains elusive. In this sense, there is a need for conducting operando studies, i.e., investigating the complete assembled device during the charge-discharge cycles. These kinds of studies provide a much deeper knowledge of the exact working mechanism as well as on the changes occurring to the device. As such, they allow for the search of new strategies for optimizing the performance of Na-ion batteries as well as for extending its lifetime.

Recently, we have carried out an operando investigation on the morphological changes experienced by a Na-ion battery with TiO₂ nanoparticles as anode material. Most of the operando experiments performed to-date address the structural alteration of the crystalline structure of the active material. However, as previously mentioned, the batteries possess structures at several length scales. In particular, little attention has been paid to the nanometer-range morphological modifications of the nanoparticulate anode material, though it may have implications on the battery performance. This is the length scale we have focused on.

By employing synchrotron X-ray scattering, we have followed in real time the changes in the arrangement of the TiO₂ nanoparticles while cycling the battery. In particular, changes in the interparticle distance have been observed whereas the nanoparticle size remained constant. Furthermore, evidence of nanoparticle aggregation have been observed. Altogether, the results suggest that two different phenomena, not previously reported, are occurring simultaneously: swelling of the electrode and nanoparticle aggregation. Both phenomena irreversibly change the morphology of the battery at the nanoscale, which might have an impact on the battery capacity loss on cycling.

Finally, operando studies have emerged as an invaluable tool for deepening our knowledge of energy storage devices since they provide a much accurate picture of the physical phenomena involved during the charge-discharge cycles. In the near future, these will provide new insights into the physics of energy storage devices.