Meaningful Metrics for an Efficient Solvent-Free Formulation of Polymer – Argyrodite Hybrid Solid Electrolyte

Abstract

Solid-state batteries generate huge excitement with the promise of higher energy density than current Li-ion technology, thanks to the use of lithium metal at the anode1. Recent advancements in ceramic electrolytes demonstrate comparable conductivity with liquid ones2. However, the brittleness of ceramics results in mechanical limitations, during both assembly and cycling3, constraining the scale-up of pure-ceramic batteries. Hybrid solid electrolytes (HSE) can overcome this hurdle by combining the superior ionic conductivity of inorganic fillers with the scalable process of polymer electrolytes in a unique material. Depending on the amount of ceramic electrolyte, two types of HSE can be prepared: a ceramic-in-polymer approach, in which the inorganic filler disturbs the polymer crystallinity; and a polymer-in-ceramic system where the organic electrolyte mainly acts as a binder. Many parameters such as polymer molar mass, type and concentration of Li salt, chemistry and size of ceramic particles and the mixing route, make the HSE formulation an intricate process. As a result, there is a large disparity in the reported formulations and performances of HSE4. In particular, most approaches focus on a slurry-assisted preparation, which becomes a limiting factor when using sulfide-based electrolytes, as these ceramics degrade in common solvents used for binder processing5.To circumvent these issues, we propose a novel, solvent-free HSE preparation, combining an organic matrix based on poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO:LiTFSI), with the highly conductive argyrodite-type Li6PS5Cl. The optimization of the HSE formulation is driven by key metrics, defined as sufficient ionic conductivity (σion ≥ 10-4 S.cm-1 @25°C) and adequate mechanical characteristics (self-standing property and ability to process as thin membrane <100 µm). Through rational screening, we elucidate the impact of formulation parameters and find the organic-to-inorganic ratio, the polymer molar mass and the ratio of mixed polymer lengths to be factors of paramount importance. Our methodology leads to an optimized formulation of the HSE with high ceramic content (75 wt.%) that meets the fixed criteria. Furthermore, we introduce an activation mechanism fitting (Arrhenius or Vulger-Tammann-Fulcher) as a new and effective metric to unravel the ionic pathway within the HSE. A shift in both ionic conductivity, mechanical cohesion and type of activation mechanism is observed at a unique threshold of 75 wt.% ceramic content. We show that our optimized polymer-in-ceramic HSE provides enhanced conduction through the ceramic network (10-4 S.cm-1 @25°C), displaying an Arrhenius activation, compared to ceramic-in-polymer HSEs, which behave as conventional PEO:LiTFSI following a VTF model. This gain in conductivity coincides with a higher mechanical resistance, as confirmed by tensile test on HSE membranes. Probing the compatibility of the organic and inorganic phases, using electrochemical impedance spectroscopy (EIS) alongside solid-state nuclear magnetic resonance (ssNMR), reveals the formation of an interphase, the quantity and resistivity of which grow with time and temperature. Finally, electrochemical performances are evaluated by assembling an HSE-based battery, which displays comparable stability as pure-ceramic ones (> 100 cycles before reaching 20% capacity loss at C/5) but still suffers from higher polarization and thus lower capacity (130 vs 150 mAh.g-1 at C/20). Janek, J. et al., Nat Energy 1, 16141 (2016).Abakumov, A. et al., Nat Commun 11, 4976 (2020).Doux, J.-M. et al., J. Mater. Chem. A 8, 5049–5055 (2020).Horowitz, Y. et al., J. Electrochem. Soc. 167, 160514 (2020).Ruhl, J. et al., Adv. Ener. Sust. Res. 2, 2000077 (2021).