The capping agent is the key: Structural alterations of Ag NPs during CO2 electrolysis probed in a zero-gap gas-flow configuration

We apply silver nanoparticles (Ag NPs) as catalysts of CO2reduction in a zero-gap gas-flow electrolyser.Ag NPs stabilized by different ligands —branched polyethylenimine (BPEI), polyvinylpyrrolidone (PVP),polyethylene glycol (PEG), and citrate— are used in the experiments. The as-prepared NPs have almostidentical initial size distributions, yet their catalytic performance, in terms of achievable current andCO selectivity, is different. During electrolysis all Ag NPs exhibit unambiguous morphology changes;the degradation pathway they follow, however, markedly depends on the chemical nature of the cappingagent stabilizing them. Scanning electron micrographs obtained before and after constant-charge elec-trolyses carried out at different potentials reveal that amongst the studied ligands, BPEI seems to bethe most effective stabilizer of Ag NPs; in turn, however, BPEI also limits CO formation the most. In caseof PVP, mostly corrosion (particle shrinkage) is observed at practically relevant electrolysing potentials,while the application of PEG leads more to particle coalescence. Ostwald ripening seems to appear only athigh applied (H2forming) potentials in case of the three afore-mentioned ligands while in case of citrateit becomes significant already at mild (CO forming) voltages. By studying the effects of capping agentremoval and exchange we demonstrate that apart from ligands directly attached to the Ag NPs, alsothe excess of capping agents (adsorbed on the electrode surface) plays a decisive role in determiningthe extent and mode of catalyst degradation. The results of SEM-based particle sizing are also confirmedby synchrotron based wide-angle X-ray scattering measurements that provide further insight into theevolution of crystallite size and lattice strain in the applied Ag NPs during electrolysis

Multiple scales post mortem and operando XPS, XAS and XPEEM investigations of reactivity at interfaces in Li-ion batteries

The understanding of the electrode-electrolyte interface in general and of the degradation reaction mechanisms at the surface of the active materials in particular remain crucial topics in the search for high energy density and longevity next generation of Li-ion batteries. For both liquid and solid-state electrolytes, the interface stability challenges associated with the electrolytes and the surface structure of the active materials arises when the cell operates at high working voltages. In this thesis, I highlight how the combination of X-ray photoelectron spectroscopy (XPS), X-ray photoemission electron microscopy (XPEEM) and X-ray absorption spectroscopy (XAS) can provide a reliable platform to investigate the electrode-electrolyte interface and the surface modifications of the active materials. The complementarity of these techniques allows an in-depth analysis from the surface (few nm) to the bulk (few hundreds of nm) of the active material particles and with different lateral resolutions, thus probing from single particles to the overall electrode. The first part of the thesis is dedicated to the combination of post mortem measurements to investigate electrodes cycled in liquid electrolytes. The LiNi0.8Co0.15Al0.05O2 (NCA) cathode is thoroughly investigated at different states of charge, clarifying the role during cycling of the adventitious carbonate on the pristine NCA powder and the charge compensation mechanisms occurring during (de-)lithiation. In particular, at the high potential of 4.9 V vs. Li+/Li, oxygen redox activity is observed on the NCA surface, accompanied with partial dissolution of the transition metals, which are subsequently detected after long-term cycling on the Li4Ti5O12 (LTO) counter electrode. In the second part of the thesis, I discuss the development of electrochemical cells for operando and in-situ analysis, using a solid-state electrolyte to adhere to the vacuum requirement of XPS, XAS and XPEEM techniques. The combination of in-situ XAS and operando XPS analysis during (de )lithiation of SnO2 conversion-alloy anode material allows us to identify with remarkable precision the potential at which the different redox reactions take place. The versatility of the electrochemical cell for operando XPS measurements allows also the in-situ XAS monitoring of the transition metals oxidation state in the case of the cathode LiNi1/3Co1/3Mn1/3O2 (NCM111). Finally, a newly developed electrochemical cell for in-situ XPEEM is presented, permitting one to localise with high lateral resolution the reactions occurring on the surface of the active materials and the solid-electrolyte. The establishment and validation of this platform of techniques opens the door to the investigation of complex materials for the next generation of Li-ion batteries for both liquid and solid-state electrolytes, with a particular focus on the surface reactions often responsible for the Li-ion battery capacity fading during prolonged cycling

Investigating LiFePO4 electrode degradation in water-in-salt electrolyte

International audienc

Synthesis and characterization of Na3PS4 and Na 2.85 P 0.85 W 0.15 S 4 as solid electrolytes

International audienceThe next generations of postlithium systems are already under development. These hold the promise of overcoming some of the remaining challenges associated with current lithium-ion battery technology related to safety, the need for scarce resources and their environmental impacts. Na₃PS₄ (NPS) is a sodium superionic conductor that crystallizes in the I-43m space group and reaches ionic conductivities in the range of the mS/cm. • X-ray diffractograms show a cubic crystalline phase of Na3PS4 and a mixture of Na3PS4 and WS2 for the Na2.85P0.85W0.15S4 sample.• SEM-EDX results confirmed that a reaction occurs between the sample holder (Macor®) and the solid electrolytes during the heat treatments.• Electrochemical results show that a heat treatment under Ar slightly improves the ionic conductivity for the NPS sample, while the HTs in presence of Macor® spacer turn the materials into ionic insulants

Synthesis and characterization of Na3PS4 and Na 2.85 P 0.85 W 0.15 S 4 as solid electrolytes

International audienceThe next generations of postlithium systems are already under development. These hold the promise of overcoming some of the remaining challenges associated with current lithium-ion battery technology related to safety, the need for scarce resources and their environmental impacts. Na₃PS₄ (NPS) is a sodium superionic conductor that crystallizes in the I-43m space group and reaches ionic conductivities in the range of the mS/cm. • X-ray diffractograms show a cubic crystalline phase of Na3PS4 and a mixture of Na3PS4 and WS2 for the Na2.85P0.85W0.15S4 sample.• SEM-EDX results confirmed that a reaction occurs between the sample holder (Macor®) and the solid electrolytes during the heat treatments.• Electrochemical results show that a heat treatment under Ar slightly improves the ionic conductivity for the NPS sample, while the HTs in presence of Macor® spacer turn the materials into ionic insulants

Investigating LiFePO4 electrode degradation in water-in-salt electrolyte

International audienc

Deciphering the Roles of Chemistry and Structure in Iridium Oxide Oxygen Evolution Nanocatalysts

International audienc

Effect of pressure and time on the properties of amorphous electrolytes

International audienc

Effect of pressure and time on the properties of amorphous electrolytes

International audienc

Effect of pressure and time on the properties of amorphous electrolytes

International audienc