Memory effect now also found in lithium-ion batteries

[OKIsItJustMe]

http://www.psi.ch/media/memory-effect-now-also-found-in-lithium-ion-batteries

[font face=Serif][font size=5] Memory effect now also found in lithium-ion batteries[/font]

14. April 2013

[font size=4]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[/font]

[font size=3]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.

Consequences of the memory effect for electric and hybrid vehicles

The memory effect and its associated abnormal working voltage deviation have now been confirmed for one of the most common materials used as the positive electrode in lithium-ion batteries, lithium-iron phosphate (LiFePO4). With lithium-iron phosphate, the voltage remains practically unchanged over a large range of the state of charge. This means that even a small anomaly in the operating voltage could be misinterpreted as a major change in the state of charge. Or, to put it another way: when the state of charge is determined from the voltage a large error can be caused by a small deviation in the voltage. The existence of a memory effect is particularly relevant in the context of the anticipated steps towards using lithium-ion batteries in the electric mobility sector. In hybrid cars in particular, the effect can arise during the many cycles of charging/discharging that occur during their normal operation. In such vehicles, the battery is partially recharged during each braking operation by the engine running in a generator mode. It is in turn discharged, and usually only partially, to assist the engine during acceleration phases. The numerous successive cycles of partial charging and discharging lead to individual small memory effects adding up to a large memory effect, as this new study demonstrates. This leads to an error in the estimate of the current state of charge of the battery, in cases where the state of charge is calculated by software on the basis of the current value of the voltage.

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For the future use of lithium-ion batteries in vehicles however, this recent discovery is not the final word. It is indeed absolutely possible that the effect could be detected and taken into account through clever adaptation of the software in battery management systems, Novak pointed out. Should that prove successful, the memory effect would not stand in the way of a reliable and safe use of lithium-ion batteries in electric vehicles. So now, engineers face the challenge of finding the correct way of handling the peculiar memory of batteries. [/font][/font]

http://dx.doi.org/10.1038/NMAT3623

[BlueStreak]

1. That's very interesting. I don't think lithium is the answer

We need large scale batteries, not just for transportation, but as storage to buffer the renewable sources, when the sun goes down, when the wind speed drops, etc. Lithium if a very limited resource and will always remain expensive.

Some of the most interesting happenings are on the sodium-air front. These batteries can react with the oxygen in air, so they don't have to carry the weight of a reactant around. That greatly increases energy density.

You can do that same trick with lithium. Lithium-air has a very high POTENTIAL energy density, but it turns out that once you account for the packaging required for a safe battery, the practical energy density of lithium-air is not that much greater than sodium-air.

Sodium is virtually unlimited and cheap. And the early indications are that it may have better safety factors and better recharge times.

It is still early in the basic research phase, but I would not invest in any lithium operations personally.

http://www.extremetech.com/electronics/149779-sodium-air-batteries-could-replace-lithium-air-as-the-battery-of-the-future

[JimDandy]

2. K & R for wider exposure. n/t