Decoupling Parasitic Reactions at the Positive Electrode Interfaces in Argyrodite-Based Systems.

Li-ion batteries are the key stones of electric vehicles, but with the emergence of solid-state Li batteries for improving autonomy and fast charging, the need for mastering the solid electrolyte (SE)/electrode material interfaces is crucial. All-solid-state-batteries (ASSBs) suffer from long-term capacity fading with enhanced decomposition reactions. So far, these reactions have not been extensively studied in Li6PS5Cl-based systems because of the complexity of overlapping degradation mechanisms. Herein, those reactions are studied in depth. We investigated their effects under various operating conditions (temperature, C-rate, voltage window), types of active materials, and with or without carbon additives. From combined resistance monitoring and impedance spectroscopy measurements, we could decouple two reactions (NMC/SE and VGCF/SE) with an inflection dependent on the cutoff potential (3.6 or 3.9 V vs Li-In/In are studied) on charge and elucidate their distinct repercussions on cycling performances. The pernicious effect of carbon additives on both the first cycle and power performances is disclosed, so as its long-term effect on capacity retention. As a mean to resolve these issues, we scrutinized the benefits of a coating layer around NMC particles to prevent high potential interactions, minimize the drastic loss of capacity observed with bare NMC, and simply propose to get rid of carbon additives. Altogether, we hope these findings provide insights and novel methodologies for designing innovative performing solid-state batteries.

Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox.

Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 involves a long-lived intermediate Ni3+/4+ species, which can fully evolve to Ni2+ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni3+/4+ species goes through a dynamic LMCT process (Ni3+/4+-O2- → Ni2+-On-). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic-anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.