An Innovative Semi-Solid Lithium Redox Flow Air (O2) Battery Concept
Authors: Ruggeri Irene , Arbizzani Catia , Soavi Francesca 
Published: February 2020

Worldwide, national energy policies are based on fossil fuels, with subsequent concerns about CO2-related global warming.
Efforts are hence devoted to promote an efficient use of renewable energy sources and a sustainable electric transportation. This in turns requires high efficiency, energy storage/conversion systems, like batteries.
Li-ion batteries (LIBs) and Redox flow batteries (RFBs) are playing a key role for the development of electric vehicles and the exploitation of intermittent and decentralized sustainable energy sources. In order to increase energy and power of both LIBs and RFBs, advancements in materials, design and concepts with attention to costs, safety and reliability are hence required.
Coupling an O2-cathode with a lithium anode gives one of the highest performing metal/air battery chemistry towards a next generation of battery. Furthermore, the need to decouple energy from power and to decrease the amount of inactive components is triggering the research in the development of semi-solid, fluidic electrodes where electroactive species are dispersed in the electrolytes instead to be included in a solid electrode.
In Li/O2 batteries, O2 redox reaction (ORR) takes place at the solid electrode/electrolyte interface, involving the formation of insulating superoxide and peroxide of lithium that clog the electrode, highly increasing recharge overpotentials and determining battery losses. Cathode passivation by discharge products is one of the most serious drawbacks of such batteries, along with the slow O2mass transport, which in the case of air breathing cells, limits current densities to one order of magnitude lower than the values for commercial LIBs [1].
In order to obviate these issues, novel configurations of Li/O2 battery, like the Li Redox Flow Air Battery (LRFAB), are under development [2, 3]. The use of an O2- based catholyte that is continuously fed with O2, from air or from an external O2 tank, renders cell discharge capacity less dependent on catholyte volume with respect to conventional RFBs. Indeed, the electrolyte is only a carrier for O2and this is beneficial to size and weight reduction.
The use of semi-solid electrodes has been shown to be successful for Li-ion, Li/polysulfides, Na-ion flow batteries and for flow supercapacitors [4-7]. These studies pointed to the demonstration that changing cell configuration by substituting solid electrodes with semi-solid, fluidic slurries is an effective strategy that improves battery rate response.
Here, we report a new battery concept, a non-aqueous Semi-Solid Lithium Redox Flow Air (O2) Battery (SLRFAB) [8-9]. The catholyte is a suspension of high surface area carbon in O2-saturated non-aqueous electrolyte. Oxygen reduction takes place on the semi-solid electroactive particles dispersed in the catholyte, avoiding the electrode passivation, enhancing the capacity and, in turn, the delivered energy.
Exceptionally high capacity is achieved at voltages >2.6 V vs Li/Li+ and high discharge rates (> 2.5 mA cm-2) of interest for practical applications. The results of the galvanostatic tests at different currents are here reported as well as the strategies to reach and overcome practical specific energies of 500 Wh kg-1are discussed.
Aknowledgements
Alma Mater Studiorum –Università di Bologna is acknowledged for financial support (RFO, Ricerca Fondamentale Orientata).
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