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Memory effect now also found in lithium-ion batteries

Professor Petr Novak, Head of the Electrochemical Energy Storage Section, and co-author of this study. Credit: Scanderbeg Sauer Photography

Lithium-ion batteries are high performance energy storage devices used in many commercial electronic appliances. Certainly, they can store a large amount of energy in a relatively small volume. They have also previously been widely believed to exhibit no memory effect. That’s how experts call a deviation in the working voltage of the battery, caused by incomplete charging or discharging, that can lead to only part of the stored energy being available and an inability to determine the charge level of the battery reliably. Scientists at the Paul Scherrer Institute PSI, together with colleagues from the Toyota Research Laboratories in Japan have now however discovered that a widely-used type of lithium-ion battery has a memory effect. This discovery is of particularly high relevance for advances towards using lithium-ion batteries in the electric vehicle market. The work was published today in the scientific journal Nature Materials.
Many of our everyday devices that get their energy supply from a battery, whilst not always being as “smart” as they are described in the adverts, often come equipped with a kind of memory. For example, a battery powered shaver or electric toothbrush that is recharged before the battery runs out, can later take revenge on the prudent user. The battery appears to remember that you have only taken part of its storage capacity– and eventually no longer supplies its full energy. Experts refer to this as a “memory effect”, which comes about because the working voltage of the battery drops over time because of incomplete charging-discharging cycles. This means that despite the battery still being discharged, the voltage it supplies is sometimes too low to drive the device in question. The memory effect therefore has two negative consequences: firstly, the usable capacity of the battery is reduced, and secondly the correlation between the voltage and the charge status is shifted, so the latter cannot be determined reliably on the basis of voltage. The memory effect has long been known to exist in Nickel-Cadmium- and Nickel-metal hydride batteries. Ever since lithium-ion batteries started to be successfully marketed in the 1990s, the existence of the memory effect in this type of battery had been ruled out. Incorrectly, as this new study indicates.



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