A Novel, Semi-Solid Li/O2 Redox Flow Battery
Authors: Soavi Francesca , Ruggeri Irene , Arbizzani Catia 
Published: February 2020

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 renewable energy sources.
Efforts are being devoted to advancements in materials, concepts, and design for the increase of energy and power of both lithium batteries and RFBs with attention to costs, safety and reliability.
Coupling an O2-cathode with a Lithium anode gives one of the highest performing metal/air battery chemistry and the need to decouple energy from power and to decrease the amount of inactive components has triggered research in the development of semi-solid, fluidic electrodes.
Cathode passivation by discharge products is one of the most serious drawbacks of Li/O2 batteries, along with the slow O2 mass 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 flow Li/O2 [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/cm2) 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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