People have assumed that these particles just sit there and slowly degrade, the researcher said. What we found instead is that they move around a lot. This change in perspective is important because battery life was previously considered to be the gradual decay of the battery’s chemistry within an unchanging structure. The new studies, however, indicate that the battery’s interior is actually a much more active place, with the particles in the electrode moving, colliding, swelling, and cracking as the battery is repeatedly charged and discharged. The implications are important because they indicate that the battery is not only failing as the chemistry within the battery gradually decays, but also as the battery’s structure changes over time, eventually becoming incapable of performing its function.
The most important change, however, is conceptual. Rather than thinking of the active particles as grains that decay over time, the researchers are now thinking of the battery as a changing structure. As the particles move over time as the battery is charged and discharged, stress is built up, and eventually, cracks form, changing the battery’s structure and ultimately its capacity, often well before the chemistry is exhausted.
Evidence is now being gathered to demonstrate that this is, in fact, the case. In nickel-rich lithium-ion battery cathodes, submicron CT scans of cycled pouch cells have shown that there was a considerable amount of micro-cracking, hollowing, swelling, and damage in the areas where current collectors were. The damage, however, does not stop there. The damage begins in the individual particles and then progresses outwards to the entire electrode, affecting geometry and ion conductivity. In extreme cases, the increase in thickness of the cathode structure, up to 30% to 40% after 2.5 years, was attributed to significant structural changes.
This, however, does not mean that all cracks are necessarily negative. The University of Michigan has found that there was a reduction in charge time that was made possible because of the presence of cracks in the cathode particles, allowing for a maximum amount of surface area to be made available to lithium ions. As uncomfortable a notion as this is, the reality remains that a design meant to be longer-lasting will result in a reduction in fast-charging capabilities, and a design meant to allow ions access to the active materials will result in a quick degradation of the structure of the battery cell.
Another issue with the structure of the lithium-ion battery comes from the world of lithium metal batteries, where scientists have noted the creation of brittle structures known as dendrites, which have the potential to puncture the separator and cause an internal short circuit in the battery. While the movement of the particles of the cathode is different from the creation of these structures, the reality of the situation is the same. The degradation of the battery is a mechanical issue just as much as it is an issue of the materials used and the chemistry of the reaction. This is an important issue to note. The degradation of the battery is a mechanical issue just as much as it is an issue of the materials used and the chemistry of the reaction.
As a consequence of these findings, the key thing to learn from all of these, which manufacturers need to take into consideration, is the fact that the extended lifespan of the cells will not simply depend on the formula they are using, nor the materials they are implementing, but will depend on the structure of the cell, as well as the ability of the materials to move, change, and not degrade in the process. In fact, one of the research studies into the problem of degradation has already indicated the direction in which the solution to the problem can be found: the use of single-crystal cathodes has indicated “significantly fewer cracks” than the use of polycrystalline materials. In other words, battery life is being redefined from the inside out. The more researchers study the battery, the less it looks like a container of chemistry, the more it looks like a machine whose parts are in constant motion.