Space Charges in Solid State Batteries: Friend, Foe or Fantasy?

Abstract

Thermodynamics tells us that all materials have an internal energy that determines how they act and react with all matter in the universe. Furthermore, it forces all matter to strive towards equilibrium with its surroundings. For solid-state lithium-ion conductors, the chemical potential of lithium is an important parameter that determines stability and influences conductivity. However, it also implies something deeper. The chemical potential of an element in a solid is determined by the bonds that surround it as well as its concentration. Whereas the former is an intrinsic property of the material, the latter can be altered. When two solids with different chemical potentials are placed in contact, a driving force is created for atoms to migrate across the surface and establish thermodynamic equilibrium by altering their concentration. For semiconductors, in which either only a hole or electron can migrate, something interesting occurs. The migrating entity moves across but the counter-charge cannot, resulting in excess charge at the interface. The migrating partner, unable to return to its original material due to thermodynamics, is now unable to dissipate into the new material due to electrostatics. The result is a thin, charged layer across the interface. Because solid electrolytes conduct lithium cations but no anions or electrons, this ‘space-charge layer’ is also thought to occur in solid-state batteries between electrolyte and electrode. Whereas its size is thought to only be a few nanometres, its (negative) influence could be very large as it can act as barrier for lithium transport across the surface. Yet experimental evidence of its existence is rare and investigation of its behaviour even more so. The space charge between two materials is very thin and chemically almost identical to the bulk. But just like how water can carve the stone, we employ liquids to crack a solid-state problem.We find that when mixing an argyrodite solid electrolyte (Li6PS5Cl) with an ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethyls ulfonyl) imide or EMIN TFSI) , we observe a new chemical environment with NMR (see figure) that cannot be attributed to dissolved precursors. We see an increase in the mobility of lithium ions in the solid electrolyte itself with NMR and an increase in conductivity of the mixture with EIS. Furthermore, exchange of lithium has been confirmed between argyrodite and the mystery environment. What does this all mean? Are we observing the illusive space charge layer? Or are we just staring at an empty promise in the well-known shape of a mysterious peak in a spectrum? Figure 1