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Francesca Soavi1 Irene Ruggeri1 Federico Poli1 Catia Arbizzani1 Stefania Rapino1 Francesca De Giorgio2 1 Alessandro Brilloni1 2

1, University of Bologna, Bologna, , Italy
2, Bettery srl, Massafra, , Italy

Lithium/O2 (air) batteries are promising exceptionally high specific energy, that can be theoretically 2-3 times higher than today Li-ion batteries.
Low cycling efficiency, low cycle life and slow discharge kinetics are still considered the main issues to be addressed for the development of high performance Li/O2 batteries. The discharge product, lithium peroxide (Li2O2) which is formed on the cathode surface is an insulator that affects battery capacity and causes high overpotential during recharge and, thus, low recharge efficiency.
This process can be controlled by tailoring the electrolyte environment. Glyme-based solvent-in-salt (SIS) electrolytes have been recently proposed for Li-O2 batteries. In SIS glyme-Li+ complexes are formed. They improve the electrochemical stability of the electrolyte and stabilize superoxide ion with a positive effect on cathode passivation.
An additional strategy for improving Li/O2 battery performannce is the use of a flowable catholyte ink. It is an organic dispersion of carbon particles which also acts as oxygen carrier, therefore improving Li/O2 battery rate response. In this catholyte, ORR takes place on the solid phase of the carbon particles. Consequently, cathode surface passivation is limited, the battery cell death is delayed and the Li/O2 battery energy increased. Such approach enabled areal capacity of 180 mAh cm-2, energy of 500 mWh cm-2 and current of 4 mA cm-2
Here, we discuss about three strategies to improve semi-solid Li-O2 battery cycling performance. Specifically, we report on i) a scanning electrochemical microscopy (SECM) investigation carried out to investigate the Li2O2 formation mechanism in salt-in-solvent and SIS electrolytes based on TEGDME and LiTFSI and in ionic liquids, iii) a semi-empirical modeling study that enables to predict the best cell design that improves the power gain of the flow Li/O2 battery and iii) the study of the carbon composition of the semisolid catholyte that permits to achieve the exceptional high specific energy of 1 kWh kg-1 .

Acknowledgments
Alma Mater Studiorum –Università di Bologna is acknowledged for financial support (RFO, Ricerca Fondamentale Orientata). We also thank the Italian Ministry of Education, University and Research (MIUR) for the “Dipartimento di Eccellenza 2018-2022” grant to the Department of Chemistry “Giacomo Ciamician” of the University of Bologna.

References

[1] F. Messaggi, I. Ruggeri, D. Genovese, N. Zaccheroni, C. Arbizzani, F. Soavi. Oxygen Redox Reaction in lithium-based electrolytes from salt-in-solvent to solvent-in-salt. Electrochim. Acta,
[2] I. Ruggeri; C. Arbizzani, F. Soavi. A novel concept of Semi-solid, Li Redox Flow Air (O2) Battery: a breakthrough towards high energy and power batteries. Electrochim. Acta, 206 (2016) 291-300.
[3] F. Soavi, Ruggeri I., and C. Arbizzani, Design Study of a Novel, Semi-Solid Li/O2 Redox Flow Battery. ECS Trans., 72 (2016) 1-9.
[4 ] I. Ruggeri, C. Arbizzani, F. Soavi, Carbonaceous catholyte for high energy density semi-solid Li/O2 flow battery, Carbon, 130 (2018) 749e757
[5] F. Poli, L. K. Ghadikolaei, F. Soavi, Semi-empirical modeling of the power balance of flow lithium/oxygen batteries, Applied Energy, 248 (2019) 383–389
[6] I. Ruggeri, C. Arbizzani, S. Rapino, F. Soavi, Oxygen redox reaction in Ionic Liquid and Ionic Liquid-like based electrolytes: a scanning electrochemical microscopy study, J. Phys. Chem. Lett., Just Accepted Manuscript, DOI: 10.1021/acs.jpclett.9b00774

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