Lithium Ion Capacitors

The most common negative electrode or anode used in lithium ion batteries is made of graphite, the same material used for pencil lead. The electrodes are comparatively cheap, have high roundtrip charging/discharging efficiency, and have very good cycle life. The positive electrode or cathode is substantially more expensive and has a shorter cycle life. A new class of electrochemical storage devices has emerged called lithium ion capacitors. They are not true capacitors because the anode is the same graphite anode used in lithium ion batteries and undergoes a chemical reaction with lithium. However, the cathode is replaced by the same high surface activated carbon used in ultracapacitors. This electrode does not interact chemically with lithium but stores energy in the electric field between two layers of charges.

Ultracapacitors using high surface area activated carbon have very high power density (they can be charge and discharged very quickly) and very long lifetimes. Maxwell Technologies, a prominent manufacturer of ultra-capacitors claims that their capacitors can last up to one million duty cycles. Lithium ion capacitors are intermediate in performance between lithium ion batteries and ultra-capacitors. They have 4 to 10 times higher energy density than ultra-capacitors (at the high end the approach the energy density of lead-acid batteries) and they have higher power density and longer cycle life (100,000 duty cycles) than lithium ion batteries. They also have a higher fully charged voltage than ultra-capacitors (3.7V as opposed to 2.7V) which is an advantage for some applications. In addition lithium-ion capacitors have much lower self discharge rates than ultracapacitors; Even after 2000 hours nearly 98% of the original charge remains in the capacitor.

Among the manufacturers and developers of lithium ion capacitors are Taiyo Yuden, JSRMicro, and AFEC (Asahi Kaisei FDK Energy Device Co).

A key question relating to the application potential of these devices is cost. If the higher energy density relative to ultra-capacitors translates into lower cost then the field of application might be very large. I looked on line for lithium ion capacitor and ultracapacitor prices. I found an offer for a 350F 2.7V Maxwell technologies capacitor for $10.75. I also found an offer for Taiyo Yuden 200F 3.8V lithium ion capacitor for $46.09. The energy stored on a capacitor is ˝CV2. Therefore the energy storage capacity of the Maxwell Technologies capacitor is somewhat less than the Taiyo Yuden capacitor in spite of the higher capacitance. If I scale the price of the Taiyo Yuden capacitor by the ratio of the energy storage capacities I get a price of $40.72 for equivalent energy storage.

There may be certain application for which the higher voltage, the smaller size, or the longer charge retention time make lithium ion capacitors the preferred solution, but if the price quote I found above is reflective of real costs then lithium ion capacitors then the field of applications which they can take over from ultracapacitors may not be that large.

Dec 23 2014

Energy Storage News

rogerkb [at] energystoragenews [dot] com