New Energy and Fuel
Battery So Good and Its Real
Japan's AIST released information about their lithium air cell battery that can be configured both as a battery or fuel cell. The device offers huge capacity.
August 6, 2009
Japan’s National Institute of Advanced Industrial Science and Technology (AIST) researchers are developing a lithium-air cell with a new structure (a set of three different electrolytes) to avoid the degradation and performance problems of conventional lithium-air cells. But the big news is in the discharge capacity.
The newly developed lithium-air cell has shown a continuous cathode discharge capacity of 50,000 mAh g-1 per unit mass of the carbon, catalyst and binder. Compare that to conventional Li-ion batteries that offer 120-150 mAh g-1 active material + conduction assisting carbon + binder, and conventional lithium-air cells that offer 700-3,000 mAh g-1. This is a massive improvement in total power.
Lithium-air cells, dispense with the intercalation cathode of lithium-ion batteries and use a catalytic air cathode in combination with an electrolyte and a lithium anode instead. Lithium-air cells are attractive because of their theoretically very high energy capacity, but one of the serious problems with lithium-air cells reported to date is that a solid reaction product (Li2O or Li2O2), which is not soluble in organic electrolyte, clogs the air electrode (cathode) in the discharge process. If the air electrode is fully clogged, O2 from atmosphere cannot be reduced any more.
The AIST researchers use an organic electrolyte on the anode (metallic lithium) side and an aqueous electrolyte on the cathode (air) side. The two electrolytes are separated by a solid state electrolyte lithium super-ion conductor glass film, LISICON so that the two electrolytic solutions do not intermix. Only lithium ions pass through the solid electrolyte, and the battery reactions proceed smoothly.
AIST confirmed that the discharge reaction product is not a solid substance like lithium oxide (Li2O) seen in conventional batteries, but lithium hydroxide (LiOH). LiOH dissolves in the aqueous electrolyte so clogging of the pores does not occur at the carbon cathode. Water and nitrogen do not pass through the solid electrolyte at the partition wall, there are no unwanted reactions with the metallic lithium anode. During charging, corrosion and degradation of the air electrode is prevented by using another cathode electrode exclusively for charging. Very clever, dividing the cells into sections where the dividers only allow the communication for optimum performance.
Metallic lithium is used as the anode, and an organic electrolyte containing lithium salt is used on the anode side. A lithium-ion solid electrolyte is placed in between the two electrolytic solutions as a partition wall to separate the cathode and anode sides. An alkaline water-soluble gel is used as the aqueous electrolyte for the cathode side and the cathode consists of porous carbon and an inexpensive oxide catalyst. This information does lead one to wonder about the operating temperatures.
The discharging reactions proceed as follows:
1. Reaction at the anode: Li? Li+ + e-
Lithium ions dissolve into the organic electrolyte as lithium ions (Li+) and the electrons are fed into the conductor wire. The dissolved lithium ions (Li+) pass through the solid electrolyte into the aqueous electrolyte on the cathode side.
2. Reaction at the cathode: O2 + 2H2O + 4e- ? 4OH-
Electrons are fed from the conductor wire, and oxygen from the air and the reduction reacts on the surface of catalyst in the porous carbon to produce hydroxyl ions (OH-). They meet with lithium ions (Li+) in the aqueous electrolyte and produce water-soluble lithium hydroxide (LiOH).
AIST Lithium Air Cell in Discharge and Charge
The charging reactions proceed as follows:
1. Reaction at the anode: Li+ + e- ? Li
Electrons are fed from the conductor wire, and lithium ions (Li+) in the aqueous electrolyte of the cathode side pass through the solid electrolyte and reach the surface of the anode where metallic lithium precipitates.
2. Reaction at the cathode: 4OH- ? O2 + 2H2O + 4e-
Oxygen gas is generated. Generated electrons are fed to the conductor wire.
The new lithium-air batteries allow for continuous operation if the aqueous electrolyte on the cathode side is exchanged and metallic lithium is resupplied to the anode, say by means of exchangeable cassettes. The researchers say that this concept could then be taken as a “lithium fuel cell.” By retrieving LiOH from the aqueous electrolyte in the air electrode, metallic lithium can be recovered easily and reused as the fuel.
The opportunity for automotive use is huge; the researchers suggest that the technology holds great potential. For example at a filling station, the driver of a vehicle so equipped could exchange the aqueous electrolyte for the air electrode and refill the metallic lithium for the anode in the form of cassettes, and then continue driving without waiting for batteries to be recharged. That could be very quick indeed.
AIST says that the new lithium-air battery needs further technical improvement toward practical use.
Generally, there are two directions in this new lithium-air battery research, one is for a rechargeable lithium air battery and the other is for a lithium fuel cell. Understanding the difference might not seem important, but a battery would only need to be plugged in to a charger whereas a fuel cell would only need a change out of the modules or cassettes. Time will tell which would be most customer friendly and how they might be priced.
The discharge rate seems nearly feasible for automotive use. There would need to be some fast discharge capability for acceleration suggesting a capacitor. But .1A/g is not so low as to be impractical. What is important is the air cell design is at least three times high than the very best LithIon batteries of today.
The lifespan or the total cycles is also of interest, but not addressed in the report. Even so, the module or cassette exchange might well solve that matter a well with a pattern of a charges followed by and exchange of modules. Its easy to see this might work in a big way. Moreover the supply of lithium would eb greatly extended and subject to endless recycling.
This is worthwhile news. There is some speculation that its on Toyota’s mind as well, when representative were talking some months ago when they mentioned that current lithium batteries would be replaced by greatly superior technology units within a few years.
EEStor it seems won’t be alone in the high capacity electron storage market.
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