Abstract: The proton transport in fuel cell ionomer membranes is intimately related to the dynamical properties of water molecules confined in the interconnected network of ionic domains. The confinement of a fluid at the nanoscale profoundly affects the molecular dynamics and results in important deviations with respect to the bulk. The rotational and translational motions can be uniquely investigated by means of Quasi Elastic Neutron Scattering experiments, especially in the case of protonated systems due to the high incoherent cross section of hydrogen. Insights into the nature of the motions, i.e. diffusive or confined, can be gained by analysing the scattering function S(Q,w) measured on the ps-ns time-scale, Q being the momentum transfer and w the energy transfer of the neutrons. We will present here a comprehensive QENS study of water/proton dynamics in PFSA membranes [1,2] and perfluorinated surfactants used as a model soft confining system [3.4] . The Gaussian model for localized translational diffusion [5] was used to obtain quantitative parameters as the relaxation times, confinement sizes and local/nanometric diffusion coefficients of protons and water molecules. The comparison between state-of-the art fuel cell membranes and self-assembled well-defined surfactant phases as host confining matrices will be used to highlight the subtle interplay between spatial confinement and local interactions, resulting in the presence of slow hydronium motions and faster water motions localized in nanometric droplets. Additionally, we will show that the QENS technique can be invaluably combined with PFG-NMR to obtain a multi-scale comprehensive description of the ionic diffusion and water behaviour [1,2] . These experimental results are also cross-fertilized by Molecular Dynamics simulations performed on the same materials using a coarse-grained backbone and explicit solvent molecules [6]. [1] J-C. Perrin, S. Lyonnard and F. Volino; Quasielastic neutron scattering study of water dynamics in hydrated nafion membranes , Journal of Physical Chemistry C, 111 (2007), 3393-3404. [2] S. Lyonnard and G. Gebel, Neutrons for Fuel Cells Membranes: structure, sorption and transport properties, European Physical Journal  213 (1) (2012), 195-211;S. Lyonnard, Structure and Transport Properties in Polymer Electrolyte Membranes Probed at Microscopic Scales , Springer-Verlag, New Energies, Ed. German Antonio Ferreira, 2013. [3] S. Lyonnard, Q. Berrod, B-A. Bruning, G. Gebel, A. Guillermo, H. Ftouni, J. Ollivier and B. Frick, Perfluorinated surfactants as model charged systems for understanding the effect of confinement on proton transport and water mobility in fuel cell membranes. A study by QENS ., Eur. Phys. Journal Special Topics, 189 (1), 205-216 (2010). [4] Q. Berrod; S. Lyonnard ; A. Guillermo.;J.Ollivier.; B. Frick; G.Gebel, QENS investigation of proton confined motions in hydrated perfluorinated sulfonic membranes and self-assembled surfactants , The European Physical Journal 2014, in press. [5] F. Volino, J-C. Perrin and S. Lyonnard; Gaussian Model for Localized Translational Motion: Application to Incoherent Neutron Scattering, Journal of Physical Chemistry B, 110 (2006), 11217-11223. [6] S. Hanot, S. Lyonnard and S. Mossa, Water confined in self-assembled ionic surfactants nanostructures , under review (2014).

Abstract: The high-temperature proton exchange membrane fuel cell (HT-PEMFC) conducts protons through the hydrogen bond network established by the polymer and phosphoric acid (PA), which reduces the dependence on humidity and allows its operating temperature to be higher than 100°C [1] . A higher operating temperature is conducive to improve catalyst activity, reducing carbon dioxide adsorption on the catalyst thus reducing the requirement for hydrogen purity, and convenient water management [2] . As the most widely commercialized HT-PEMFC proton exchange membrane material, the performance and durability of Polybenzimidazole (PBI) still need to be improved. In particular, it has insufficient proton conductivity, insufficient mechanical properties, and phosphoric acid leaching issues under high acid doping level [3] [4] [5] . The doping of functionalized graphene oxide in the PBI membrane can build additional proton transfer channels, promote proton hopping and act as a trap for PA to reduce its leaching by virtue of abundant functional groups of functionalized GO [6] [7] [8] . Among the groups that can be used for functionalization, the phosphoric acid group has become one of the most promising due to its strong hydrogen bonding and water retention ability [9] [10] [11] . Phosphonated graphene oxide (PGO) is usually synthesized by further phosphonation of GO obtained by chemical exfoliation [6] [7] . Chemical exfoliation methods usually require the long-term action of strong acids and strong oxidants [12] . The safety and environmental issues caused by those methods can not be underestimated. And the two-step synthesis method of PGO has a long reaction period. This work achieved the rapid, safe, and large-yield production of electrochemically exfoliated PGO by using a 3D printed reactor, ammonium dihydrogen phosphate as the electrolyte and natural graphite flakes as the raw material. The two-step electrochemical exfoliation method of producing GIC with concentrated sulfuric acid as the first electrolyte is also used to synthesize electrochemical exfoliated (E)GO. 1.5wt% EGO or PGO was doped in the PBI membrane to explore the effect of different GO on the performance and durability of the PBI- membrane-based HT-PEMFC. Compared with pure PBI, the doping of EGO and PGO increases the peak power density of HT-PEMFC by 17.4% and 35.4%, respectively. [1] Y.-L. Ma, J.S. Wainright, M.H. Litt, R.F. Savinell, Journal of The Electrochemical Society 2004 , 151 , A8. [2] H. Su, S. Pasupathi, B. Bladergroen, V. Linkov, B.G. Pollet, International Journal of Hydrogen Energy 2013 , 38 , 11370. [3] S. Galbiati, A. Baricci, A. Casalegno, R. Marchesi, International Journal of Hydrogen Energy 2013 , 38 , 6469. [4] S.H. Eberhardt, F. Marone, M. Stampanoni, F.N. Büchi, T.J. Schmidt, Journal of Synchrotron Radiation 2014 , 21 , 1319. [5] Q. He, X. Yang, W. Chen, S. Mukerjee, B. Koel, S. Chen, Physical Chemistry Chemical Physics 2010 , 12 , 12544. [6] J. Yang, C. Liu, L. Gao, J. Wang, Y. Xu, R. He, RSC Advances 2015 , 5 , 101049. [7] C. Xu, Y. Cao, R. Kumar, X. Wu, X. Wang, K. Scott, Journal of Materials Chemistry 2011 , 21 , 11359. [8] Y. Cai, Z. Yue, S.X.-J. of A.P. Science, undefined 2017, Wiley Online Library 2017 , 134 , 44986. [9] E. Abouzari-Lotf, H. Ghassemi, A. Shockravi, T. Zawodzinski, D. Schiraldi, Polymer 2011 , 52 , 4709. [10] E. Abouzari-Lotf, M. Zakeri, M.M. Nasef, M. Miyake, P. Mozarmnia, N.A. Bazilah, N.F. Emelin, A. Ahmad, Journal of Power Sources 2019 , 412 , 238. [11] S. Some, I. Shackery, S.J. Kim, S.C. Jun, Chemistry - A European Journal 2015 , 21 , 15480. [12] C. Xu, Y. Cao, R. Kumar, X. Wu, X. Wang, K. Scott, Journal of Materials Chemistry 2011 , 21 , 11359.

Abstract: Alkali and alkaline earth metal anodes are gaining increasing research attention because they could provide high energy densities and theoretical specific capacities. Among them, lithium metal is the most investigated anode candidate for next generation batteries. However, due to some challenges, the commercialization of lithium metal anodes is still delayed. Reactions between lithium metal and the electrolyte lead to the formation of an inhomogeneous solid electrolyte interphase (SEI). Consequently, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, resulting in high surface area lithium and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also causes safety issues due to an increased risk of short-circuits and thermal runaway. 1,2 To enable increased safety in high energy batteries with lithium metal anodes an effective SEI is required to limit lithium dendrite growth. There is a variety of approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods and chemical modification. 3-5 Recently, alkaline earth metal anodes, such as magnesium or calcium, are also of considerable interest for next generation anodes. Their main advantage is the significantly lower susceptibility to dendrite formation, while still offering high theoretical specific capacities. However, their oxide passivation layer does not allow significant mobility for their divalent ions, leading to large overvoltage and slow reaction kinetics which hinders the cycling. Therefore, electrolytes which enable a non-passivating SEI have to be applied and additives are investigated on their ability to remove the oxide layers. 6,7 Herein, we present a novel mechanochemical approach to form an effective SEI on the lithium metal surface by combining mechanical and chemical modification utilizing ionic liquids (ILs) prior to cell assembly. This method suppresses dendrite growth even at high current densities of 10 mA cm -2 in lithium metal batteries with liquid electrolytes. To further highlight the importance of surface treatment and SEI formation, we investigate electrolytes for calcium metal batteries as well as mechanical modification of calcium metal anodes. Acknowledgements: The authors would like to acknowledge financial support from the European Union through the Horizon 2020 framework program for research and innovation within the project “VIDICAT” (829145). References: Peled, E.; Menkin, S., Journal of The Electrochemical Society 2017, 164 (7), A1703-A1719. Bieker, G.; Winter, M.; Bieker, P., Physical Chemistry Chemical Physics 2015, 17 (14), 8670-8679. Peled, E., Journal of The Electrochemical Society 1979, 126 (12), 2047-2051. Josef, E.; Yan, Y.; Stan, M. C.; Wellmann, J.; Vizintin, A.; Winter, M.; Johansson, P.; Dominko, R.; Guterman, R., Israel Journal of Chemistry 2019 , 59 (9), 832-842. Wellmann, J.; Brinkmann, J.-P.; Wankmiller, B.; Neuhaus, K.; Rodehorst, U.; Hansen, M. R.; Winter, M.; Paillard, E., ACS Applied Matererials & Interfaces 2021 , 13 (29), 34227-34237. Shterenberg, I.; Salama, M.; Yoo, H. D.; Gofer, Y.; Park, J.-B.; Sun, Y.-K.; Aurbach, D., Journal of The Electrochemical Society 2015, 162 (13), A7118-A7128. Stievano, L.; de Meatza, I.; Bitenc, J.; Cavallo, C.; Brutti, S.; Navarra, M. A., Journal of Power Sources 2021, 482 , 228875.

Abstract: Thermoelectric materials are of interest for converting waste heat to electricity and could be useful in applications such as power generation. 1 Currently, bulk bismuth telluride (Bi 2 Te 3 ) is the most commonly used material for thermoelectric coolers and waste heat recovery. However, theoretical calculation 2 predicts that the thermoelectric performance of Bi 2 Te 3 can be dramatically improved by confining its dimensions to the nanometre scale. Electrodeposition provides a bottom-up technique compared to conventional coating methods, such as physical vapor deposition (PVD). It is of advantage in the efficient use of the starting material and can be used to plate curved surfaces and even inside of topologically demanding surfaces. 3 The use of electrodeposition to fill nano-structures is also achievable, which is exploited in the Damascene process to grow 20 nm diameter Cu wire. 4 In electrochemical experiments water is the most commonly used solvent but its limitation includes narrow potential window, which is crucial in electrodeposition as water reduction will be a competing process for metal reduction/deposition and introduces complexity to experiments. Here we employ an electrolyte system based on tetrabutylammonium chlorometallate metal sources with compatible tetraalkyl halide supporting electrolytes in non-aqueous, weakly coordinating solvents, 5,6 which provides the ability to deposit alloys with desired structures and properties. In this work we report the deposition of Bi 2 Te 3 films from dichloromethane. Bi 2 Te 3 is a narrow band gap layered semi-conductor and it is demonstrated that the composition and structure of the BiTe films can be controlled by tuning the electrochemical parameters, such as the electrolyte concentration and deposition potential (Figure 1). 7 We then investigate Bi 2 Te 3 nucleation and growth using transmission electron microscopy (TEM) with a boron doped diamond electrode, acting as the electron transparent substrate. 8 We observe the initial stage of Bi, Te and BiTe nuclei after electrodeposition (from individual atoms to atom arrays and nanocrystals) This work is conducted as part of the ADEPT project funded by EPSRC (EP/N035437/1). [1] Materials 7.4 (2014): 2577-2592. [2] Physical review B 47.24 (1993): 16631. [3] Proceedings of the National Academy of Sciences 106.35 (2009): 14768-14772. [4] IBM Journal of Research and Development 42.5 (1998): 567-574. [5] Chemistry-A European Journal 22.1 (2016): 302-309. [6] RSC Advances 3.36 (2013): 15645-15654. [7] Journal of Electroanalytical Chemistry 839 (2019): 134-140. [8] ACS nano 12.7 (2018), 7388-7396 Figure 1

Abstract: The materials currently attracting most interest as positive electrodes for Lithium-ion batteries are Li and Mn-rich layered oxides that exhibit outstanding energy densities at an affordable cost. 1 A common feature for all these layered oxides is a high capacity “plateau”, observed only at the end of the first charge, once all the transition metal ions are already at the tetravalent state. That behavior has been explained by the reversible participation of oxygen anions in the redox processes, thanks to hybridization between their p levels and the d levels of the transition metals. This reaction is reversible within the bulk, occurring without any major structural modification, while oxidized oxygen ions are lost at the surface causing irreversible structural reorganizations at the outer part of the particles, those being at the origin of a continuous voltage decay upon cycling. 2,3 We will show how we tried to stabilize concentration gradients and core-shell composites with Li and Mn-rich layered oxides in the bulk and stoichiometric layered oxides at the outer part of the spherical aggregates, 4 with the goal to combine high energy density and chemical stability respectively. We will also highlight that Tavorite-type compositions offer a very rich crystal chemistry, among which LiVPO 4 F has the highest theoretical energy density (i.e. 655 Wh/kg). 5 New Tavorite-type compositions were recently obtained: LiVPO 4 OH and LiVPO 4 F 1-y O y , for these latter by direct syntheses or by aging of LiVPO 4 F upon oxidation in air. 6-8 We will show how we can tailor the structure, the potential and the reaction mechanism involved, playing with the composition of the Tavorite-type phases. We will discuss how detrimental/positive the defects can be on the electrochemical properties of the mixed oxy-fluorophosphates LiVPO 4 F 1-y O y . Acknowledgements: These researches are funded by Région Nouvelle Aquitaine for layered oxides and by the French National Research Agency ANR (Labex STORE EX and project HIPOLITE) for polyanionic materials. The authors thank also the French network RS2E (http://www.energie-rs2e.com), the European network ALISTORE-ERI (http://www.alistore.eu), FEDER and Région Haut-de-France. Reference s : [1] Croguennec, L.; Palacin, M. R., Journal of the American Chemical Society 2015 , 137, 3140-3156 [2] Koga, H.; Croguennec, L.; Ménétrier, M.; Douhil, K.; Belin, S.; Bourgeois, L.; Suard, E.; Weill, F.; Delmas, C., , Journal of the Electrochemical Society 2013 , 160, A786-A792 [3] Genevois, C. ; Koga, H. ; Croguennec, L. ; Ménétrier, M. ; Delmas, C. ; Weill, F., Journal of Physical Chemistry C 2015 , 119, 75-83 [4] Pajot et al. , in preparation [5] C. Masquelier and L. Croguennec, Chemical Reviews 2013 , 113, 6552−6591 [6] Boivin, E.; Chotard, J.-N.; Ménétrier, M.; Bourgeois, L.; Bamine, T.; Carlier, D.; Fauth, F.; Suard, E.; Masquelier, C.; Croguennec, L., J ournal of Mater ial Chem istry A 2016 , 4 , 11030–11045. [7] Boivin, E.; Chotard, J.-N., Ménétrier, M.; Bourgeois, L.; Bamine, T.; Carlier, D.; Fauth, F.; Masquelier, C.; Croguennec, L., Journal of Physical Chemistry C 2016 , 120(46), 26187-26198 [8] Boivin et al., in preparation

Abstract: Tin oxide (SnO 2 ) is considered as a promising material for both Li- and Na-ion batteries due to its high theoretical capacities (1494 mAh/g with Li and 1378 mAh/g with Na; 2~3 times higher than common carbon-based anode) 1-3 . However, the initial irreversible capacity loss induced by inactive Li 2 O/Na 2 O formation and volume change (260% with Li and 420% with Na) during charge/discharge process need to be addressed in order to improve cycling performance 4 . In addition, the poor electronic conductivity of above mentioned alkali metal oxides and gradual aggregation of Sn particles in the electrode structure during operation lead to poor rate capability and rapid capacity fading. Many studies have strived to address these issues and result in the development of diverse types of SnO 2 -based nanocomposites with carbonaceous materials including reduced graphene oxides 5-7 , carbon nanofibers 8,9 , carbon nanotubes 10,11 , and disordered carbons 12,13 to enhance initial coulombic efficiency and electronic conductivity in the electrode structure as well as to prohibit Sn particle aggregation by introducing physical barriers between active materials. However, the cycling performance and initial irreversible capacity loss of the currently reported composites still remain insufficient to be adopted in practical cells. In this work, carbon-coated porous Sn/SnO 2 composite (Sn/SnO 2 @C) is synthesized via inorganic CO 2 reduction route with magnesium stannide (Mg 2 Sn) for Li- and Na-ion batteries. High purity Mg 2 Sn powder is prepared by solid-state reaction and then thermally treated under CO 2 flow environment. During the second heat treatment, gaseous CO 2 molecules becomes reduced down to elemental C via interaction with Mg which is known to be highly reductive in nature (Mg 2 Sn + CO 2 à 2MgO + Sn + C, ΔG = -690 kJ/mol). The resultant Sn is partially oxidized to form SnO 2 which eventually results in Sn/SnO 2 @C composite. Electrodes with this composition and structure exhibit enhanced initial coulombic efficiency and stable cycling performance. It appears that a nanocomposite matrix in which active materials are distributed without aggregation in intimate contact with C is essential for realizing SnO 2 -based anodes for Li- and Na-ion batteries with long cycle life. 1 Lee, J.-I. et al. Multifunctional SnO2/3D graphene hybrid materials for sodium-ion and lithium-ion batteries with excellent rate capability and long cycle life. Nano Research , doi:10.1007/s12274-017-1756-3 (2017). 2 Kim, Y., Yoon, Y. & Shin, D. Fabrication of Sn/SnO2 composite powder for anode of lithium ion battery by aerosol flame deposition. Journal of Analytical and Applied Pyrolysis 85 , 557-560, doi:https://doi.org/10.1016/j.jaap.2008.06.005 (2009). 3 Lee, Y. et al. Hollow Sn–SnO2 Nanocrystal/Graphite Composites and Their Lithium Storage Properties. ACS Applied Materials & Interfaces 4 , 3459-3464, doi:10.1021/am3005237 (2012). 4 Sivashanmugam, A. et al. Electrochemical behavior of Sn/SnO2 mixtures for use as anode in lithium rechargeable batteries. Journal of Power Sources 144 , 197-203, doi:https://doi.org/10.1016/j.jpowsour.2004.12.047 (2005). 5 Hu, X., Zeng, G., Chen, J., Lu, C. & Wen, Z. 3D graphene network encapsulating SnO2 hollow spheres as a high-performance anode material for lithium-ion batteries. Journal of Materials Chemistry A 5 , 4535-4542, doi:10.1039/C6TA10301D (2017). 6 Fan, L. et al. Controlled SnO2 Crystallinity Effectively Dominating Sodium Storage Performance. Advanced Energy Materials 6 , 1502057-n/a, doi:10.1002/aenm.201502057 (2016). 7 Tian, R. et al. The effect of annealing on a 3D SnO2/graphene foam as an advanced lithium-ion battery anode. Scientific Reports 6 , 19195, doi:10.1038/srep19195 https://www.nature.com/articles/srep19195#supplementary-information (2016). 8 Wang, M., Li, S., Zhang, Y. & Huang, J. Hierarchical SnO2/Carbon Nanofibrous Composite Derived from Cellulose Substance as Anode Material for Lithium-Ion Batteries. Chemistry – A European Journal 21 , 16195-16202, doi:10.1002/chem.201502833 (2015). 9 Liu, Y. et al. Enhanced electrochemical performance of hybrid SnO2@MOx (M = Ni, Co, Mn) core-shell nanostructures grown on flexible carbon fibers as the supercapacitor electrode materials. Journal of Materials Chemistry A 3 , 3676-3682, doi:10.1039/C4TA06339B (2015). 10 Cui, J. et al. Enhanced conversion reaction kinetics in low crystallinity SnO2/CNT anodes for Na-ion batteries. Journal of Materials Chemistry A 4 , 10964-10973, doi:10.1039/C6TA03541H (2016). 11 Chen, S. et al. Branched CNT@SnO2 nanorods@carbon hierarchical heterostructures for lithium ion batteries with high reversibility and rate capability. Journal of Materials Chemistry A 2 , 15582-15589, doi:10.1039/C4TA03218G (2014). 12 Fan, J. et al. Ordered, Nanostructured Tin-Based Oxides/Carbon Composite as the Negative-Electrode Material for Lithium-Ion Batteries. Advanced Materials 16 , 1432-1436, doi:10.1002/adma.200400106 (2004). 13 Pol, V. G., Wen, J., Miller, D. J. & Thackeray, M. M. Sonochemical Deposition of Sn, SnO2 and Sb on Spherical Hard Carbon Electrodes for Li-Ion Batteries. Journal of The Electrochemical Society 161 , A777-A782, doi:10.1149/2.064405jes (2014).

Abstract: Low volatility, thermal and electrochemical stability and good conductivity of ionic liquids (ILs) make them “key electrolytes” for the development of Li/O 2 batteries which are considered the most promising high-energy systems for a market success of electric vehicles of long driving range. Particularly, hydrophobic pyrrolidinium salts of bis(trifluoromethanesulfonyl)imide feature a high stability toward superoxide that is formed during the discharge/recharge of Li/O 2 batteries. Under the European LABOHR Project, the oxygen redox reaction (ORR) and O 2 diffusion coefficient and solubility, which are crucial parameters for Li/O 2 battery operation, have been investigated in such ILs and related to the physical-chemistry properties of the electrolytes. It has also been demonstrated that ILs provide a real possibility for the development of rechargeable Li/O 2 batteries that can safely operate even above room temperature. The main achievements of this study are here reported and discussed Acknowledgement Work funded by the European Commission in the 7 th Framework Programme FP7-2010-GC-ELECTROCHEMICAL STORAGE, under contract no. 265971 “Lithium-Air Batteries with split Oxygen Harvesting and Redox processes” (LABOHR) . All the partners of the LABOHR Project are acknowledged for the fruitful discussions on Li/O 2 batteries with ionic liquids. References [1] www.labohr.eu [2] S. Monaco, A. M. Arangio, F. Soavi, M. Mastragostino, E. Paillard, S. Passerini, Electrochimica Acta 83 ( 2012 ) 94–104. [3] F. Soavi, S. Monaco, M. Mastragostino, Journal of Power Sources 224 ( 2013 ) 115 – 119. [4] S. Monaco, F. Soavi, M. Mastragostino, J. Phys. Chem. Lett. 4 ( 2013 ) 1379−1382.

Abstract: Lithium metal has the most negative deposition potential of all metals (-3.04 V vs. SHE) and a very high theoretical specific capacity of 3861 mA h g -1 , which makes it a promising anode material for next generation batteries. However, the commercialization of lithium metal anodes is still impaired by several drawbacks. 1 Lithium metal reacts with the electrolyte, forming a solid electrolyte interphase (SEI). Due to the inhomogeneity of this SEI, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, leading to protrusions and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also raises the risk of short circuits and thermal runaway. 2-3 Therefore, an effective SEI is required to enable safe high energy batteries with lithium metal anodes by limiting lithium protrusions. An ideal SEI is electronically insulating and highly conductive for Li + but blocking for other ionic species in the electrolyte. Furthermore, it does not react with the electrolyte, is homogeneous in terms of Li + transport and mechanically stable. 4 To enable those characteristics, there are different approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods (roll-pressing, micro-patterning) and chemical modification (immersion). 5-7 Herein, we present a novel approach to form an effective SEI on the lithium metal surface by combining mechanical (roll-pressing) and chemical modification utilizing various ionic liquids (ILs) and salts prior to cell assembly for application in high voltage, low temperature lithium metal batteries with liquid electrolytes. Applying this mechanochemical method leads to significantly decreased impedance and low overvoltage during electrodissolution/-deposition, even at high current densities of 10 mA cm -2 . In addition to electrochemical tests, X-Ray photoelectron spectroscopy (XPS) was utilized to shed light on the correlation between improved electrochemical performance and the composition of the artificial SEI layer. Acknowledgements: The research presented is part of the ‘VIDICAT’ project funded by the European Union's Horizon 2020 research and innovation program under grant agreement n° 829145. The authors would like to thank Dr. Uta Rodehorst for conducting the XPS measurements. References: Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S., Chemical Reviews 2014, 114 (23), 11751-11787. Peled, E.; Menkin, S., Journal of The Electrochemical Society 2017, 164 (7), A1703-A1719. Bieker, G.; Winter, M.; Bieker, P., Physical Chemistry Chemical Physics 2015, 17 (14), 8670-8679. Peled, E., Journal of The Electrochemical Society 1979, 126 (12), 2047-2051. Josef, E.; Yan, Y.; Stan, M. C.; Wellmann, J.; Vizintin, A.; Winter, M.; Johansson, P.; Dominko, R.; Guterman, R., Israel Journal of Chemistry 2019 . Basile, A.; Bhatt, A. I.; O’Mullane, A. P., Nature Communications 2016, 7 , 11794. Becking, J.; Gröbmeyer, A.; Kolek, M.; Rodehorst, U.; Schulze, S.; Winter, M.; Bieker, P.; Stan, M. C., Advanced Materials Interfaces 2017, 4 (16), 1700166.

Abstract: Li-air batteries are an exciting class of energy storage devices with exceptional theoretical capacities which could facilitate longer-range electrical vehicles if fully optimised systems are realised. 1 Energy storage in Li-air batteries proceeds via different mechanisms to those associated with conventional Li-ion batteries, necessitating detailed studies into the fundamental processes associated with discharge and charge. Recently, increasing attention has been devoted to understanding the role of the electrolyte in determining the performance of Li-air batteries. It has been conclusively shown that carbonate based electrolytes are not suitable for rechargeable systems due to the rapid accumulation of carbonate based species on the cathode. 2 These carbonates are particularly prevalent during the charging process due to decomposition of the electrolyte and reactions between the electrolyte, the primary discharge product Li 2 O 2 and its intermediates. Numerous reports have revealed that characteristic Li 2 O 2 toroids often form on the cathode surface during discharge in a variety of cathode/electrolyte systems. 3 , 4 Recently, Nazar et al. showed that the formation of these toroids is strongly related to the applied current for a given system (TEGDME/LiTFSI electrolyte and Super P carbon cathode). 5 Their results show that at low applied currents, large crystalline Li 2 O 2 toroids form on the cathode surface with a clear change to quasi-amorphous Li 2 O 2 films at higher applied currents. They also found that the toroids were much more difficult to decompose during charge than the thin films, indicating that the nature of the Li 2 O 2 formed on discharge plays a key role in determining rechargeability. In this work we have investigated the morphology and composition of discharge products formed on Super P cathodes using various different electrolyte solvent and salt combinations. We demonstrate that even at high applied currents (250 μA), electrolytes containing sulfolane as the electrolyte solvent show preferential Li 2 O 2 toroid formation (Figure 1 a,b). In comparison, electrolytes using TEGDME (Figure 1 c,d) as the electrolyte solvent do not lead to the formation of Li 2 O 2 toroids at the same applied currents. The comparative performance of cells using the various electrolytes determined using galvanostatic charge-discharge methods were demonstrated to be linked to the morphology and areal coverage of the Li 2 O 2 formed on the cathodes. The decomposition of these particles and films is visualized by conducting ex-situ SEM analysis at various stages of the charge process. The influence of catalyst addition (Pd, MnO 2, Co 3 O 4 ) to Super P carbon cathodes on the morphology of Li 2 O 2 formed on cathodes and thus their electrochemical performance was also probed. This report gives insight into the importance of understanding the formation and decomposition of Li 2 O 2 on cathodes for realizing rechargeable high capacity Li-O 2 battery systems. Figure 1: SEM images of Super P cathodes discharged with an applied current of 250 μA using different electrolyte solvent/salt combinations. 1. Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Nature materials 2011, 11, (1), 19-29. 2. McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, A. C. The Journal of Physical Chemistry Letters 2011, 2, (10), 1161-1166. 3. Fan, W.; Cui, Z.; Guo, X. The Journal of Physical Chemistry C 2013 . 117 (6), pp 2623–2627 4. Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. The Journal of Physical Chemistry Letters 2013 , 1060-1064. 5. Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Energy & Environmental Science 2013, 6, (6), 1772-1778. Acknowledgements: Financial support was provided by the European Union Seventh Framework Programme (FP7/2007-2013) project STABLE.

Abstract: Redox flow batteries (RFBs) are re-emerging as a safe, scalable, efficient and versatile means of large-scale energy storage. Growing adoption of renewable energy generation has ushered a new optimism regarding future reduction in fossil fuel dependency; limiting further harm dealt to the climate and wider environment. However, this momentum demands more efficient energy storage solutions to reconcile power demand and the intermittent nature of wind, solar and tidal energy generation [1]. Without proper storage, reliable back-up generation – largely provided by fossil fuels – will continue to be an unavoidable reality [2] . RFBs utilise the electrochemical properties of dissolved metal ions to store and release energy. Independent sizing of electrolyte storage tanks and flexible power delivery contributes to the flexibility of RFBs compared to alternative battery technologies [3] . RFB reactions occur at the electrode-electrolyte interface, and thus the mass transport at this interface is a critical factor in determining the overall RFB performance. Most RFBs use porous electrodes, with a popular choice of material being carbon felt (CF) due to its low cost, chemical stability and high conductivity [4], although the hydrophobic nature of some CF can cause poor electrode ‘wettability’- decreasing the contact area between the electrode and the electrolyte. Further, electrolyte depletion and asymmetric flow can lead to the presence of ‘dead spots’ where the interface is inactive. Many studies have been conducted on CF to improve the electrochemical performance, using surface treatments, compression and channel flow to improve active area, wettability, and species transport [5-7] . These each have direct impact on current density, pressure drop, overpotentials, and energy efficiency. While the flow of electrolyte in the RFB porous electrodes can be modelled using computational fluid dynamics (CFD) [8], and experimentally assessed using x-ray tomography [9,10] and some optical visualisation methods [11], there are limited experimental methods which can be used on entire RFB stacks. In this study, we explore the use of electrical resistive tomography (ERT) to probe the flow of electrolyte through RFB electrodes. ERT has the advantage that it can provide a non-intrusive means of investigating the hydrodynamics of the otherwise opaque cell stack. Measurements were performed using an array of electrodes place around the perimeter of an RFB electrode chamber which contained conventional carbon felt electrodes. Sensitivity maps were generated using the COMSOL Multiphysics platform and compared with experimental measurements. The flow distribution was evaluated by using injections of concentrated KCl solutions into the background electrolyte. Overall, ERT demonstrated promise as a technique for characterising real-time flow dynamics in RFB stacks and for future research into porous electrode and flow field modifications. References [1] Ambec, S., and Crampes, C., Electricity Provision with Intermittent Sources of Energy. Resource and Energy Economics, Elsevier, 2012. 34 : p.320-331. [2] Wagner, F., Electricity by intermittent sources: An analysis based on the German situation 2012 . The European Physical Journal Plus, Springer, 2014. 129 : 20. [3] Wang, W., et al., Recent Progress in Redox Flow Battery Research and Development. Advanced Functional Materials, Wiley, 2013. 23 : p. 970-986. [4] Gonzalez-Garcia, J., et al., Characterization of a carbon felt electrode: structural and physical properties . Journal of materials Chemistry, 1999. 9 : p. 419-426. [5] Wang, Q., et al., Experimental study on the performance of a vanadium redox flow battery with non-uniformly compressed carbon felt electrode. Applied Energy, Elsevier, 2018. 213 : p. 293-305. [6] Wang, S., et al., Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries. The Journal of Physical Chemistry Letters, ACS Publications, 2012. 3 : p. 2164-2167. [7] Kim, K., et al., The effects of surface modification on carbon felt electrodes for use in vanadium redox flow batteries . Materials Chemistry and Physics, 2011. 131 : p. 547-553. [8] Oh, k., et al., Three-dimensional, transient, non-isothermal model of all-vanadium redox flow batteries. Energy, Elsevier, 2015. 81 : p. 3-14. [9] Eifert, L., et al., Synchrotron X-ray Radiography and Tomography of Vanadium Redox Flow Batteries—Cell Design, Electrolyte Flow Geometry, and Gas Bubble Formation. ChemSusChem, Wiley, 2020. 13 : 3154-3165. [10] Trogadas, P., et al., X-ray micro-tomography as a diagnostic tool for the electrode degradation in vanadium redox flow batteries. Electrochemistry Communications, Elsevier, 2014. 48 : p. 155-159 [11] Bhattarai, A., et al., Study of flow behaviour in all-vanadium redox flow battery using spatially resolved voltage distribution. Journal of Power Sources, Elsevier, 2017. 360 : p. 443- 452.