Strategies to Achieve Stable Manganese Oxyfluorides by Tuning the Reactivity of Pure Molecular Fluorine.

Thanks to their high initial electrochemical properties and broad compositional flexibility, lithium-rich disordered rocksalt cathode-active materials including high-performance manganese-only materials appear as a potential replacement to the cobalt-based current market leader "NMC" material. The main issue with these materials is their lack of stability. However, recent works have identified bulk fluorination as a potential solution to stabilize these compounds. There is, however, a clear lack of diversity in fluorination agents used to synthesize these disordered rocksalts, as most publications used LiF, a very stable compound. To achieve manganese-only materials, manganese oxyfluorides represent promising precursors, but the literature reports only MnO3F and Mn2O2F9, which are both unstable and hazardous. The present study develops several strategies for synthesis and a tailored characterization methodology to explore the chemical space of direct fluorination of manganese oxide MnO with molecular fluorine and shows how to tune its reactivity to achieve a range of novel, safe, and finely tunable manganese oxyfluorides of general formula MnOFx, with x going from 0 to 1 synthesized via a fluorine insertion mechanism.

Residual Li2O degrades PVdF during the preparation of NMC811 slurries for Li-ion batteries.

7Li MAS NMR quantification of lithiated species at the surface of aged NMC811 industrial powders and slurries shows that the electrode preparation process exacerbates Li extraction. A combination of 7Li MAS NMR and XPS suggest a new reaction for the PVdF binder degradation, involving in fact Li2O as the reagent and the formation of LiF.

Pristine Surface of Ni-Rich Layered Transition Metal Oxides as a Premise of Surface Reactivity.

The surface reactivity of Ni-rich layered transition metal oxides is instrumental to the performance of batteries based on these positive electrode materials. Most often, strong surface modifications are detailed with respect to a supposed ideal initial state. Here, we study the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material in its pristine state, hence before any contact with electrolyte or cycling, thanks to advanced microscopy and spectroscopy techniques to fully characterize its surface down to the nanometer scale. Scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS), solid-state nuclear magnetic resonance (SS-NMR), and X-ray photoelectron spectroscopy (XPS) are combined and correlated in an innovative manner. The results demonstrate that in usual storage conditions after synthesis, the extreme surface is already chemically different from the nominal values. In particular, nickel is found in a reduced state compared to the bulk value, and a Mn enrichment is determined in the first few nanometers of primary particles. Further exposition to humid air allows for quantifying the formed lithiated species per gram of active material, identifying their repartition and proposing a reaction path in relation with the instability of the surface.

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.

Probing Al Distribution in LiCo0.96Al0.04O2 Materials Using 7Li, 27Al, and 59Co MAS NMR Combined with Synchrotron X-ray Diffraction.

We prepared Al-doped LCO (LCA) powders with low Al content (4%) with a controlled Li/(Co + Al) stoichiometry by a solid-state reaction using Li2CO3 and two types of Co/Al precursors: simply mixed (Co3O4 and Al2O3) or heat-treated (Co3O4 and Al2O3). These samples were thereby used to propose a reliable protocol with the aim to discuss the homogeneity of the Al doping for LiCo1-yAlyO2 (LCA) prepared with low Al content by evidencing the distribution of Al within the powders, which clearly affects the electrochemical profiles of associated LCA//Li cells. For all samples we initially also characterized the Li/(Co + Al) stoichiometry by 7Li MAS NMR, to discard the possible effect of excess Li in the samples. Synchrotron XRD combined with 27Al and 59Co MAS NMR then provided a deep understanding of the doping homogeneity at the powder or particle scale. We showed that doping the Co3O4 spinel precursor by reacting it with Al2O3 may be avoided, as it most likely leads to an inhomogeneous mixture of Co3O4 and Co3-zAlzO4 as precursor, eventually reflecting in the final LiCo0.96Al0.04O2 powder, which shows a nonhomogeneous Al distribution. We believe that such a detailed characterization should be the first step toward a deeper understanding of the real beneficial effect(s) of Al doping on the high voltage performance of LCO.

Influence of the Initial Li/Co Ratio in LiCoO2 on the High-Voltage Phase-Transitions Mechanisms.

The influence of the initial Li/Co stoichiometry in LiCoO2 (LCO) (1.00 ≤ Li/Co ≤ 1.05) on the phase-transition mechanisms occurring at high voltage during lithium deintercalation ( V > 4.5 vs Li+/Li) was investigated by in situ X-ray diffraction. Even if the excess Li+ in Li1.024Co0.976O1.976 does not hinder the formation of the H1-3 and O1 phases, the latter are obtained at higher voltages and exhibit larger c parameters compared with their analogues formed from Li1.00CoO2. We also showed that for the stoichiometric Li1.00CoO2 the deintercalation process is more complex than already reported, with the formation of an intermediate structure between H1-3 and O1.