Extending insertion electrochemistry to soluble layered halides with superconcentrated electrolytes.

Insertion compounds provide the fundamental basis of today's commercialized Li-ion batteries. Throughout history, intense research has focused on the design of stellar electrodes mainly relying on layered oxides or sulfides, and leaving aside the corresponding halides because of solubility issues. This is no longer true. In this work, we show the feasibility of reversibly intercalating Li+ electrochemically into VX3 compounds (X = Cl, Br, I) via the use of superconcentrated electrolytes (5 M LiFSI in dimethyl carbonate), hence opening access to a family of LixVX3 phases. Moreover, through an electrolyte engineering approach, we unambiguously prove that the positive attribute of superconcentrated electrolytes against the solubility of inorganic compounds is rooted in a thermodynamic rather than a kinetic effect. The mechanism and corresponding impact of our findings enrich the fundamental understanding of superconcentrated electrolytes and constitute a crucial step in the design of novel insertion compounds with tunable properties for a wide range of applications including Li-ion batteries and beyond.

Unlocking anionic redox activity in O3-type sodium 3d layered oxides via Li substitution.

Sodium ion batteries, because of their sustainability attributes, could be an attractive alternative to Li-ion technology for specific applications. However, it remains challenging to design high energy density and moisture stable Na-based positive electrodes. Here, we report an O3-type NaLi1/3Mn2/3O2 phase showing anionic redox activity, obtained through a ceramic process by carefully adjusting synthesis conditions and stoichiometry. This phase shows a sustained reversible capacity of 190 mAh g-1 that is rooted in cumulative oxygen and manganese redox processes as deduced by combined spectroscopy techniques. Unlike many other anionic redox layered oxides so far reported, O3-NaLi1/3Mn2/3O2 electrodes do not show discernible voltage fade on cycling. This finding, rationalized by density functional theory, sheds light on the role of inter- versus intralayer 3d cationic migration in ruling voltage fade in anionic redox electrodes. Another practical asset of this material stems from its moisture stability, hence facilitating its handling and electrode processing. Overall, this work offers future directions towards designing highly performing sodium electrodes for advanced Na-ion batteries.

Hybrid Electrolytes Based on Optimized Ionic Liquid Quantity Tethered on ZrO2 Nanoparticles for Solid-State Lithium-Ion Conduction.

This paper describes the simple, highly reproducible, and robust synthesis of a new solid organic/inorganic electrolyte based on the ionic liquid (IL) 1-butyl-3-(carboxyundecyl)imidazolium bis(trifluoromethylsulfonyl)imide tethered to zirconia nanoparticles (15-25 nm) by coordination and named ZrO2@IL. The IL monolayer formation, ensured by two-dimensional solid-state NMR, at the nanoparticles' surface considerably reduces both the IL's consumption and the IL amount at the ZrO2 surface compared to the IL-based hybrid electrolytes reported in the literature. After LiTFSI, used as a lithium source, content optimization (26 wt %), the hybrid exhibits unprecedented stable conductivity passing from 0.6 × 10-4 S.cm-1 to 0.15 × 10-4 S.cm-1, respectively, from 85 °C to room temperature (25 °C). Unlike silica which is commonly adopted for this type of hybrid material, zirconia makes it possible to produce more impact-resistant pellets that are easier to compact, thus being favorable for accurate conductivity studies and battery development by electrode/composite/solid electrolyte layer stacking. The ZrO2@IL/LiTFSI solid hybrid electrolyte's thermal stability (up to 300 °C) and performance make this electrolyte suitable for lithium conduction in all-solid-state batteries.

Activation of anionic redox in d0 transition metal chalcogenides by anion doping.

Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2-/Sen-) and (S2-/Sn-) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.