As you might suspect, a lithium-ion capacitor is a kind of ultracapacitor. But you won't see any headlines about lithium-ion capacitor fires. Unlike the battery chemistry, lithium-ion caps don't heat up.

Devised by JSR Micro Inc., the lithium-ion capacitor's claim to fame is an energy density four times higher than that of conventional electric double layer capacitors (EDLC), i.e. supercaps. It also has a maximum cell voltage of 3.8 V, about 50% higher than ordinary supercaps. This lets about one-third fewer cells connected in series provide a target voltage.

All in all, the high energy density and cell voltage gives a significantly smaller foot print than what's possible with ordinary supercaps, JSR claims.

Right now, the company markets 1,100 and 2,200 F laminate versions and 2,300 and 3,300 F prismatic versions. The 1,100 F version has a gravimetric energy density of 10 Wh/kg, the 2,200 version 8 Wh/kg. Related figures for the 2,300 and 2,300 F versions are 8 and 12 Wh/kg respectively. JSR lists maximum discharge currents for the four caps as 800, 1,350, 1,200, and 1,100 A respectively.

It looks as though lithium-ion caps could find a role in energy harvesting applications because they have a high charge retention. Self-discharge is said to be less than 5% over three months at room temperature. And like ordinary supercaps, the lithium-ion versions charge and discharge quickly. JSR says the cycle life of its capacitor, dubbed the Ultimo, can be in the millions of cycles.

JSR calls its design an asymmetric ultracapacitor. Its construction is a hybrid between a lithium ion battery and an EDLC capacitor. As with lithium-ion batteries, the Ultimo contains a lithium-doped electrolyte and a lithium-doped anode. And as in EDLCs, the cathode is activated carbon and contains no lithium.

But unlike lithium-ion batteries, Ultimo does not contain any lithium metal salts and won't experience thermal runaway. JSR says the lithium-doped anode is the key to LIC technology. During charge and discharge, the voltage is nearly constant resulting in higher maximum voltage and twice the capacitance of an ordinary ultracap.

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