There are a number of factors which affect the power capacity and lifespan of the rechargeable battery packs that are found in hybrid electric vehicle [HEV] or electric vehicle [EV]. Battery chemistry, of course, plays a significant role, and currently, lithium-ion [Li-ion] batteries offer the industry’s best power transfer characteristics.
Charging and discharging must also be precisely controlled to maximize battery life. Anode and cathode material are another important aspect of battery life. Until now, though, battery researchers had never considered that something, besides the charging system, might have an effect on battery life.
During normal battery operation, lithium ions collect on the anode during charging, and on the cathode when discharging. Researchers at Ohio State University [OSU] were investigating the accumulation of lithium on the anode, which shortens battery life, and accidentally discovered lithium accumulation elsewhere.
“We didn’t set out to find lithium in the current collector, so you could say we accidentally discovered it, and how it got there is a bit of a mystery. As far as we know, nobody has ever expected active lithium to migrate inside the current collector,” explained Bharat Bhushan, Ohio Eminent Scholar and Howard D. Winbigler Professor of Mechanical Engineering.
In order for the potential energy contained in an EV battery to be put to any use, it needs to be connected to the vehicle’s electrical systems, from accessories to drive systems. This is accomplished using a copper strip known as a “current collector,” where lithium accumulation was found, and could affect battery life in the same way as the accumulation in the anode.
Using the neutron depth profiling [NDP] technique, researchers fire a neutron stream into the material being tested, and capture the resulting charged particles for analysis. The reaction between lithium and neutrons gives off a different energy signature than the copper alone, so researchers were able to determine that even in the current collector, there was up to 0.08% lithium that had migrated from the electrolyte and anodes.
This miniscule amount of lithium, found in the current collectors of six used Li-ion EV batteries, may not seem significant, but is certainly part of the overall battery system. The new data will help battery manufacturers to build more efficient and long lasting batteries, including the copper current collector.