Storing electrical energy locally has been a key focus for several industries and has acquired significant research focus as it provides for the efficient use of energy generated locally (microgrids) as well as its transport. For example, a hybrid or an all-electric vehicle requires a battery for supplying a constant source of power. Currently, the addition of batteries leads to additional costs thus making the hybrid technology less profitable. Other important applications of batteries can be the storage of power generated by renewable sources such as solar and wind power plants (or domestic solar panels) to store excess energy during the peak supply and utilize it later when there is no sunlight or wind power. It is important to note here that there are other ways to store excess energy to be utilized later. One such method is the production of hydrogen which can be used as fuel for a fuel cell engine but not discussed here for brevity.
Various solutions have been proposed to store the energy locally but only a few have seen the light of day owing to the constraints posed by commercial viability. Ideally a successful technology should be simple, highly efficient, safe, quiet and modular for its commercial viability. Some of the competing technologies are fuel cells, Li-ion batteries and more recent lithium-air batteries or Li-air batteries for short. A Li-air battery in comparison to a Li-ion battery draws oxygen from air as the cathodes active material and uses lithium metal at the anode thus providing it with a more dense structure. Theoretical energy density for gasoline is in the order of 13,000 Wh/kg, whereas for PEM fuel cells it’s in the order of 3000-9000 Wh/kg and Li-ion batteries is ~ 900 Wh/kg. The theoretical energy density reported for Li-air batteries, first demonstrated by Abraham and Jiang (1996), is in the order of 11,400 Wh/kg (3623 Wh/kg, based on Li2O2) which is one order higher than that of Li-ion batteries.
In practice, a system demonstrating better performance than Li-ion battery has not been demonstrated owing to the various challenges and limitations. Some of the challenges for the Li-air battery include instability of materials, incomplete reversibility, dendrite formation at Li surface and incomplete utilization of cathode pores. For non-aqueous batteries, the discharge products are generally insoluble and block the pores thus limiting the performance and utilization. Also, the reaction of Li with electrolytes and dissolved gases (CO2) leads to further irreversibility. Hence, a large part of the reported research has focused on identifying new electrolyte materials and air electrodes for non-aqueous batteries. Various researchers have shifted their focus from Li-ion batteries to the development of Li-air batteries, along with certain industries such as Bosch and IBM owing to the promise of a higher energy density. Overall, the success of the technology depends on identifying better electrolytes and attaining higher reversibility for it to be commercially viable. This might be critical to defining the path towards a greener planet.
References:
[1] Y. Wang and S.C. Chao, Journal of Electrochemical Society, 162(1), A114-A124 (2015)
[2] Y. Wang and S.C. Chao, Journal of Electrochemical Society, 160(10), A1847-A1855 (2013)
Photo courtesy of extremetech.com, IBM battery 500 research lab at IBM Research Almaden
