Molten Air – A new, highest energy class of rechargeable batteries with up to 4.5 times the energy capacity of lithium air batteries

Arxiv – Molten Air – A new, highest energy class of rechargeable batteries

Where are the high energy capacity, cost effective batteries urgently needed for a range of medical, transportation and power generation devices, including in greenhouse gas reduction applications such as overcoming the battery driven “range anxiety” of electric vehicles, and increased capacity energy storage for the electric grid? This study introduces the principles of a new class of batteries, rechargeable molten air batteries, and several battery chemistry examples are demonstrated. The new battery class uses a molten electrolyte, are quasi-reversible (rechargeable), and have amongst the highest intrinsic battery electric energy storage capacities. Three examples of the new batteries are demonstrated. These are the iron, carbon and VB2 molten air batteries with respective intrinsic volumetric energy capacities of 10,000, 19,000 and 27,000 Wh liter-1. These compare favorably to the intrinsic capacity of the well known lithium air battery (6,200 Wh liter-1) due to the latter’s single electron transfer and low density limits.

Higher energy capacity, cost effective batteries are needed for a range of electronic, transportation and greenhouse gas reduction power generation devices. Needed greenhouse gas battery reduction applications include overcoming the battery driven “range anxiety” of electric vehicles, and increased capacity energy storage for the electric grid.

The molten iron air battery. Charging (in this case from dissolved LiFeO2, dissolving 1:1 Li2O to Fe2
O3) forms a thick iron layer on the cathode as described in the text. Rights side: Discharge polarization (following electrochemical charge to form iron) of the air and iron electrodes in 730°C molten lithium carbonate with LiFeO2.

A proof of principle of the rechargeable VB2 molten air system has been presented, starting with a cell in the discharged (molten borate and vanadate) condition. Impediments to efficiency are recombination of the solid product formed at the cathode with solution phase oxygen, and the poor conductivity of the cathode product inhibiting discharge.

The foundations and experimental demonstration of a new class of molten air batteries is extablished. The iron molten and carbon molten air batteries exhibit high rate charging capability and quasi reversibility (rechargeability). An extensive range of experimental parameters can be investigated to start the optimization process of these batteries. For example, while these experiments have been conducted in the 700°C temperature range, the molten carbonate electrolyte has a wide range of electrolyte opportunities, and mixed alkali carbonate eutectics have a minimum melting point below 400°C. A range of cell configurations with lower polarization (with similar discharge potentials, but supporting significantly higher current density) will be reported on in a future study.

“There have been rechargeable batteries that use molten electrolytes, but not air. For example, molten-sulfur batteries have been widely studied for electric car and grid applications. However, sulfur is twice as massive as oxygen (per electron stored) and its mass needs to be carried as part of the battery (whereas air is freely available). The molten-air batteries are the first rechargeable batteries to use a molten salt to store energy using ‘free’ oxygen from the air and multi-electron storage molecules.”

This ability to store multiple electrons in a single molecule is one of the biggest advantages of the molten-air battery. By their nature, multiple-electron-per-molecule batteries usually have higher storage capacities compared to single-electron-per-molecule batteries, such as Li-ion batteries. The battery with the highest energy capacity to date, the vanadium boride (VB2)-air battery, can store 11 electrons per molecule. However, the VB2-air battery and many other high-capacity batteries have a serious drawback: they are not rechargeable.

SOURCES- Physorg, arxiv, royal society chemistry

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