|Long-term discharge curve of the newly developed lithium-air cell. Source: AIST. Click to enlarge.|
Researchers at Japan’s AIST (National Institute of Advanced Industrial Science and Technology) are developing a lithium-air cell with a new structure (a set of three different electrolytes) to avoid degradation and performance problems of conventional lithium-air cells.
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). By comparison, conventional Li-ion batteries offer 120-150 mAh g-1 (active material + conduction assisting carbon + binder), and conventional lithium-air cells offer 700-3,000 mAh g-1. The research was presented earlier this year at an Electrochemical Society of Japan meeting held in Kyoto.
Lithium-air cells, which 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, are attractive because of their theoretically very high energy capacity. (Earlier post.) However, 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 used 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.
|Schematic diagram of a conventional lithium-air (oxygen) battery. Source: AIST. Click to enlarge.||Configuration of new AIST lithium-air cell. Source: AIST. Click to enlarge.|
AIST confirmed that the discharge reaction product is not a solid substance like lithium oxide (Li2O), but lithium hydroxide (LiOH), which dissolves in the aqueous electrolyte; clogging of the pores does not occur at the carbon cathode. Furthermore, as water and nitrogen do not pass through the solid electrolyte (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.
|The new cell uses an exclusive cathode for charging. Source: AIST. Click to enlarge.|
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.
The discharging reactions proceed as follows:
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.
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).
The charging reactions proceed as follows:
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.
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, e.g., by means of cassettes. The researchers say that this concept can 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 fuel.
The researchers suggest that the technology holds great potential for automotive applications. At a filling station, the driver of a vehicle thus 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.
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.