Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43 (15, 1966—1969
Liberated Lithium Ions
Additive speeds the efficiency of lithium-ion accumulators
Electric motors will be driving the environmentally friendly vehicles of tomorrow. Whether the power is directly "pumped" from an outlet or is generated on board by a fuel cell, the heart of such a propulsion system will be a high-capacity accumulator that efficiently stores and releases electricity. It must also immediately satisfy extreme peaks in energy demand upon acceleration. The accumulator must be able to be emptied and reloaded countless times without loss of energy. It should also naturally be as small and lightweight as possible. Japanese chemists have now developed a method by which the efficiency of lithium ion accumulators can be increased by the use of an additive.
Lithium-based accumulators are the leaders when it comes to energy density, that is, they store an especially large amount of energy in a small volume. However, conventional lithium accumulators suffer from a problem; the electrolytic solution they contain is sensitive, which makes production expensive and, in extreme cases, can cause the accumulator to explode if it is overloaded or short-circuited. The alternative is a solid electrolyte, in which the electrolytic salt is bound within the network of a polymer. The disadvantage of these solid electrolytes is their comparatively low ionic conductivity. The team led by Masataka Wakihara has now achieved a simple but effective trick to accelerate charge transfer between the electrolyte and the electrode.
It goes like this: positively charged lithium ions in the electrolyte have to deposit themselves onto the electrode and give up their positive charge, which makes the current flow. The rate of this process depends on the number of free lithium ions swirling around in the solid electrolyte. Only a certain fraction of the lithium ions are unbound, the rest are bound to their negatively charged counterpart, the anion of the lithium salt. In order to raise the number of free lithium ions, they must be liberated from their anions. It is thus necessary to otherwise "occupy" the anion’s charge, which is so attractive to the lithium ions, without disrupting the electrochemical processes in the accumulator. The solution of choice is a polymeric boric acid ester. Boric acid esters have a kind of "gap" in their electronic systems, which the "extra" electrons of the anions sometimes like to "borrow". This interaction forms loose adducts between the anions and the boric acid ester.
In a model system, the boric acid ester proved to be a highly promising additive for solid electrolytes, even bringing the efficiency of large lithium accumulators with high energy density up to speed.