QotW: Why do electric car batteries wear out?

Dr Chloe Coates from the University of Cambridge spoke to Evelyna Wang about the answers to this battery brain baffler...

Evelyna - The batteries that power your electric car are lithium ion batteries. How you use that car and its battery has an impact on its lifetime. Dr. Chloe Coates from the University of Cambridge is here to drive us to the bottom of this:

Chloe - For a consumer, let’s say an electric vehicle driver, there are 3 main considerations that impact battery degradation. These are the temperature of the system, state of charge (fully charged/discharged/in between), and the usage profile (how fast/slow charge/discharge).

Evelyna - When discharging a LiB, Li ions move from the negative graphite electrode to the positive cathode - which produces electrical energy we can use to power a car. And on charge, the Li ions are moved the opposite direction.

Chloe - On charge and discharge, both electrodes undergo large volume changes as the lithium is inserted or removed. This can cause stress on the particles and lead to cracking which exposes more surface area and loss of active material. (1:58) The graphite anodes are also affected by Li plating. At low temperatures or at fast rates, the Li can’t intercalate into the graphite layers fast enough so Li metal accumulates at the surface. (2:25) In extreme cases, the Li plates unevenly and forms dendrites which can short circuit the cell.

Evelyna - Another component of the LIBs is the liquid electrolyte, which conducts the Li ions in between the graphite to the cathode…

Chloe - The molecules that make up the electrolyte can decompose at high voltages on cycling or on reaction with trace water in the system, and can in turn react with the surfaces of the particles. This can cause surface layers that can increase the resistance in a battery. (2:38) The degradation mechanisms described here are only a sample of degradation that can occur in a battery. The interplay between the mechanisms and the order in which they occur remains an important question…

Evelyna - So how do we improve on this?

Chloe - In most cases, moderation is key. (0:38) Perhaps more unexpectedly, more degradation happens at high states of charge or discharge. So ideally, you would want your car to charge extremely slowly and keep it at about 50% charged. But this isn’t practical when driving long distances and charging often… (2:57) Since we can’t wait 10 years to study the battery degradation in a car under typical usage, we instead mimic failure modes in the lab..and look at specific parts of the battery to try and understand what is happening.

Evelyna - So what’s next in battery technology? Research into the specific conditions and reactions that cause degradation will continue to help improve current LIB technology. Industry research is also focused on increasing safety or decreasing cost, such as the recently discussed LFP systems (although these chemistries would decrease how far you can drive your electric car). There is also ongoing research into solid electrolytes or moving beyond Li ions, however many of them need further study.