Degradation…what is it? A phenomenon where something “disappears”? Well, not quite. When we think of something degrading, our instinctual mind immediately thinks of something going from good to bad. However, just because something degrades doesn’t necessarily mean it’s going to be a bad product. There are instances when degradation is actually needed in order to improve the performance or physical property of a material. Before getting to these particular situations, though, we need to first understand degradation from a fundamental standpoint.
According to Webster, degradation is defined as “the act or process of damaging or ruining something”. There are many mechanisms that cause degradation to occur. These include chemical (e.g. acidification), thermal (e.g. oxidation), and physical (e.g. shearing). Each of these mechanisms themselves has degrees by which one can assess degradation. For example, in chemical degradation, the degree to which a chemical reaction proceeds is typically used to assess how bad a material has degraded. Should a mechanism proceed too far, the degradation can lead to severe damage of the material rendering it useless. Therefore, a person typically uses controlled methods to invoke a degradation scheme to ensure the desired result is obtained.
So how does degradation impact an application like battery systems? The answer resides on the inside of the battery where the cathode, anode, and electrolyte reside. As ions are shuttled back and forth from the cathode to the anode, a degradation process is initiated. This process tends to cause the crystal matrices on the cathode and/or anode to shift, leading to an amorphous (i.e. degraded) state. As each cycle occurs, one can see the performance of the overall battery decrease. This is normal with any battery system. As the saying goes…nothing lives forever. The challenge is developing materials that withstand severe degradation and last thousands of cycles. Scientists continue to investigate innovative materials that can achieve long-term stability in battery systems. Efforts have been focused on all three areas of the battery: cathode, anode, and electrolyte. It is understood that it will not be a single area where long-term stability will be achieved. It will require improvements in all three areas.
Can degradation be suppressed to the point of eliminating it? Not quite. Nature likes disorder so it will almost be impossible to totally eliminate degradation. The best course of action is to slow down the process. This has been the goal of much research lately. Extending the life of many systems will be the way to counteract the effects of degradation. However, work must continue to be done to find novel systems that achieve the goal of minimum degradation. Recall from the beginning that I said that not all degradation was bad. That’s true. Sometimes, a material needs to be degraded in order to enhance its physical properties to promote better performance. You will find in some cases that a battery will be put through a few cycles to pre-lithiate the anode in order to minimize capacity loss associated with the formation of a solid electrolyte interface. This enhances the performance of the battery leading to improved cycle life of the battery system.
Degradation is both good and bad. In developing new technologies, one must carefully assess how degradation is either helping or hurting the cause. Nature dictates how degradation will proceed. It is not enough to think that one can completely eliminate this phenomenon. The best that we could ever do is slow down the process to allow for extended life of materials for desired applications. In the end, a full understanding of degradation must be achieved before designing and assessing any system of interest.