Magali Gauthier

Highly concentrated aqueous electrolytes have revitalized the field of aqueous batteries. Herein, we investigated the influence of anions on the physicochemical characteristics, solvation structure, and electrochemical behavior of magnesium aqueous electrolytes. Four salts, featuring acetate, nitrate, perchlorate, and imide anions, were selected to span a wide range of chaotropic and kosmotropic behaviors according to the Hofmeister classification. By combining ¹ H, ¹⁷ O NMR, and infrared spectroscopy with viscosity measurements, a strong correlation emerges between the anion’s nature and its ability to weaken (chaotropicity) or reinforce (kosmotropicity) the water hydrogen‐bond network. For the nitrate, at the boundary between chaotropic and kosmotropic behavior, and the weakly chaotropic perchlorate, results suggest a uniform solvation around [Mg(H ₂ O) _x ] ²⁺ clusters; NO ₃ ⁻ may retain some interaction with water around Mg ²⁺ , while ClO ₄ ⁻ appears more decoupled from Mg ²⁺ . In contrast, the strongly chaotropic TFSI ⁻ (bis(trifluoromethanesulfonyl)imide anion) induces a more pronounced perturbation of the hydrogen‐bond network, with TFSI ⁻ largely outside the Mg ²⁺ hydration shell. Finally, the anion‐dependent solvation contributes to electrochemical performance. A 4 m magnesium perchlorate electrolyte exhibits markedly enhanced capacity retention and Coulombic efficiency than a 4 m magnesium acetate electrolyte. However, the overall electrochemical behavior cannot be described by a simple monotonic relationship with the Hofmeister classification.

The solvation properties of electrolytes are one of the primary drivers of the performance of concentrated aqueous-based batteries. In this study, we provide an understanding of the lithium solvation and its dynamics in an electrolyte derived from a deep eutectic solvent (comprising LiClO₄, urea and water) by effectively correlating multinuclear (¹H, ¹⁷O, ³⁵Cl, and ⁷Li) diffusion nuclear magnetic resonance (NMR) spectroscopy with molecular dynamics (MD) simulations. The freely accessible static field gradient from a commercial NMR magnet was used to determine diffusion coefficients for all population-averaged constituents─Li⁺, ClO₄^–, urea, and water─at various concentrations. The diffusion ratios D_Li⁺/D_urea ≈ 1 and D_Li⁺/DClO₄^– ≈ 1 for the concentrated 1:3:2 electrolyte suggest correlated ionic motion and transport dominated by a vehicular character rather than hopping mechanisms. Ab initio MD simulations reveal mixed Li solvation shell configurations, with substantial urea coordination (∼66% of Li⁺ with one or two urea molecules in the 1:3:2 system) and an increasing occurrence of contact ion pairs with concentration. This results in relatively slow ionic mobility with a cation transference number around 0.5. Ion cluster analysis reveals a systematic increase in the occurrence of contact ion pairs with concentration, yet cluster sizes remain far below percolation thresholds, consistent with vehicular transport. Residence time analysis further quantifies this picture: the characteristic diffusional length exceeds the inner-sphere solvation shell radius (L_c/L_s > 1) across all concentrations, providing evidence for vehicular-dominated transport. ¹⁷O diffusion NMR measurements (D_¹⁷O/D_¹H > 1.2 in a concentrated system) provide insights into water dynamics despite significant experimental challenges from quadrupolar relaxation. These findings provide molecular-level insights into aqueous electrolytes based on deep eutectic solvents to further guide the rational design of aqueous battery systems.

Magnesium-based batteries hold great promise for energy storage, owing to the abundance of magnesium and its theoretical high energy densities. However, the use of organic electrolytes in these batteries presents significant safety concerns, particularly regarding corrosion, flammability and toxic degradation products. Water-in-salt electrolytes (WiSE) offer a safer solution, expanding the electrochemical stability window of conventional aqueous solutions through high salt concentrations. Among the various salts, magnesium perchlorate stands out due to its good solubility, low cost, and chaotropic properties, which modify the water network significantly. To investigate the aging mechanisms of Mg(ClO₄)₂ WiSE electrolytes, radiolysis-induced degradation processes were studied using picosecond pulse radiolysis experiments, with a focus on the effects of salt molality on the formation of reactive species. These experiments, in conjunction with quantum chemical calculations, provide insights into the formation of transient species and their evolution during electrolyte degradation. Moreover, the radiolytic yields of stable species, including dihydrogen (H₂), dioxygen (O₂), chlorate (ClO₃^-), and chloride (Cl^-), were measured. O₂ is exclusively derived from perchlorate anions, while H₂ originates from water. As the concentration of Mg(ClO₄)₂ increases, the radiolytic yields of chlorinated species rise, in particular for ClO₃^-. The results obtained highlight that excited perchlorate anions in concentrated solutions generate a range of reactive species, namely ClO₃^-, ClO₂^-, as well as ClO₄^• and ClO₃^• radicals. Notably, ClO₃^- anions are formed from excited perchlorate anions. On the other hand, ClO₄^•, also directly generated through ionizing radiation/matter interaction, leads to the formation of O₂ and ClO₂^•. The ClO₂^• radical quickly loses O₂, resulting in the formation of the chlorine radical, and eventually of the chloride anion. The study also reveals that hydrogen peroxide formation is consistent across varying molalities, indicating a balance between contributions from both water and perchlorate anions. These findings provide critical insights into the radiolysis processes in Mg(ClO₄)₂ electrolytes, contributing to the understanding of electrolytes aging to further develop more efficient magnesium-ion batteries.

(Text in French) Radiolysis and concentrated electrolytes for aqueous batteries: an accelerated aging tool to elucidate degradation mechanisms. Aqueous batteries have long been constrained by the electrochemical stability of water, limiting their theoretical operating voltage window to 1.23 V due to hydrogen and oxygen evolution reactions. The emergence of highly concentrated aqueous electrolytes, commonly referred to as water-in-salt electrolytes (WISEs), has significantly broadened this window. This breakthrough has paved the way for safer and more sustainable aqueous batteries, though most developments still rely on costly and toxic lithium salts. Magnesium emerges as a promising alternative due to its abundance, low toxicity, and low cost. However, studies on magnesium-based concentrated aqueous electrolytes have so far achieved a limited voltage window of around 2 V, without a clear understanding of the mechanisms limiting this performance. The objective of this Ph.D. work was to gain fundamental insight into the reactivity of magnesium-based concentrated aqueous electrolytes, with the aim of identifying strategies to expand their electrochemical stability window. To this end, radiolysis, the set of chemical reactions induced by ionizing radiation, was employed as a tool for accelerated aging. This innovative approach makes it possible to simulate in just a few hours the cumulative effects of several weeks of electrochemical cycling. Using this methodology, we were able to identify the major degradation pathways of the electrolytes, detect transient radical species, and distinguish the respective roles of water and anions in the overall reactivity of the system, depending on the salt concentration. The study was conducted first on lithium-based electrolytes, then extended to magnesium-based systems, laying the groundwork for the rational design of next-generation aqueous battery electrolytes.

Manufacturing aqueous batteries based on the magnesium cations is an important step towards more sustainable and safer energy storage solutions. Thus, it is important to understand how these systems age and which species are formed throughout numerous charge/discharge cycles. To this end, we have used radiolysis to induce accelerated ageing in concentrated aqueous solutions of magnesium bistriflimide Mg(TFSI)₂ (also called “water‐in‐salt electrolytes” or WISEs). We demonstrate in this work that the degradation products formed, whether in the gas or liquid phase, are very similar to those formed in concentrated LiTFSI aqueous solutions. In fact, the behavior under ionizing radiation is driven by the anion/water molar ratio regardless of whether the cation is Li⁺ or Mg²⁺. This is because both cations are non‐reactive, and the bond strengths in the TFSI⁻ anion do not vary with the nature of the cation. Reaction mechanisms are proposed to explain the formation of several species under ionizing radiation.

The concept of water-in-salt electrolytes (WISEs)¹, implying highly concentrated aqueous solutions, has completely rejuvenated the aqueous systems. These electrolytes can be depicted as few water molecules surrounded by cations and anions forming ion pairs. This change of solvation structure compared to standard electrolytes increases the electrochemical stability window. For example, using 20 molal (molality: m, moles per kg) lithium (bistrifluoromethanesulfonyl(imide) (LiTFSI) aqueous solutions, a stability window of 3 V is reached.¹ While WISEs open the way for sustainable systems, these solutions only use expensive and toxic lithium salts. To move towards more sustainable elements, one can envision magnesium and associated salts: safer, less expensive and more abundant than lithium. Yet, the state-of-the-art on water-in-salt aqueous Mg batteries is scarce,^2,3 with potential windows reaching only 2-2.3 V. There is no clear understanding of the mechanisms at work but the low solubility of Mg salts could account for the narrow potential window. Rationalizing reactivity of water-in-salt solutions on a fundamental level is critical to design efficient strategies to increase the voltage window of concentrated aqueous Mg batteries. We investigated the Mg(TFSI)₂/H₂O system with radiolysis, a technique which provides a chemical approach⁴ to exacerbate in a short time the degradation mechanisms. The aging process with radiolysis is much shorter than the electrochemical one and mimics the products of salt and solvent degradation observed during long battery operation. We performed radiolysis to study and compare Mg(TFSI)₂ and LiTFSI-based electrolytes with different molalities. Degradation products formed after irradiation were identified using Nuclear Magnetic Resonance spectroscopy (NMR) for the liquid phase, and with gas chromatography coupled to mass spectrometry (GC-MS) and micro-gas chromatography (μ-GC) for the gas phase. For example, the evolution of the amount of H₂ and CO₂ gases in the case of LiTFSI or Mg(TFSI)₂/H₂O electrolytes show large discrepancies, hinting at a preferential anion degradation in the Mg system. Overall, the nature of the degradation species strongly depends on the molality, with degradation routes driven by the water or the anion at low or high molalities. Finally, radiolysis allows to identify minor species, providing a view of the long-term (un)stability of these electrolytes. References 1. L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 2015, 350, 938–943. 2. F. Wang, X. Fan, T. Gao, W. Sun, Z. Ma, C. Yang, F. Han, K. Xu, C. Wang, ACS Cent. Sci. 2017, 3, 1121–1128. 3. X. Tang, D. Zhou, B. Zhang, S. Wang, P. Li, H. Liu, X. Guo, P. Jaumaux, X. Gao, Y. Fu, C. Wang, C. Wang, G. Wang, Nat. Commun. 2021, 12, 2857. 4. D. Ortiz, I. Jiménez Gordon, J.-P. Baltaze, O. Hernandez‐Alba, S. Legand, V. Dauvois, G. Si Larbi, U. Schmidhammer, J.-L. Marignier, J.-F. Martin, J. Belloni, M. Mostafavi, S. Le Caër, ChemSusChem 2015, 8, 3605–3616.

A multi‐tetrazine compound tailored for grafting on graphene sheets has been designed and investigated. It is composed of hydrogen substituted tetrazines involved in inverse Diels‐Alder demand cycloaddition reactions for grafting and a central dialkoxytetrazine to confer redox properties to the final material. The successful incorporation of this compound in reduced graphene oxide was demonstrated by elemental analysis, XPS, Raman spectroscopy, AFM, SEM and XRD. Cyclic voltammetry shows the redox activity of the remaining central tetrazine unit in graphene and allows to assess the specific capacity of the material. A significant increase in the specific capacity was found compared to pristine graphene and another tetrazine model compound bearing only one grafting unit. This result was assigned to an increase in the specific area in relation to bridges created between graphene sheets, as evidenced by AFM and XRD. The performance in energy storage were assessed in symmetrical Swagelok cells confirming the high impact of the functionalization on capacitance values.

Operando and in situ techniques are becoming mandatory to study Li-ion, post Li-ion, and solid-state batteries. They are essential for monitoring the (electro)chemical and dynamic processes in the battery environment and for providing understanding at different spatial and temporal scales. While operando measurements are becoming more and more routine, scientists have to keep in mind that such experiments are not always harmless for the battery operation, especially when using synchrotron techniques. This is the case in the example described herein with Mg batteries. We show that the electrode reactivity in a InSb/organohaluminate electrolyte/Mg cell is strongly retarded during operando synchrotron X-ray absorption acquisition. Through comparison of ex situ, operando, and in situ data, we demonstrate that this effect occurred only on the samples' volumes exposed to the X-ray radiation. This study illustrates how incorrect conclusions might be drawn from operando measurements, especially when looking at new battery chemistries, and calls for extreme caution when designing and interpreting operando data.