In sulfide all-solid-state lithium batteries, the composite positive electrode mixes an oxide active material with an argyrodite solid electrolyte (Li6PS5Cl) and a small amount of electronically conductive carbon. Quemin et al. (2022) discovered a counterintuitive consequence of this formulation: although vapor-grown carbon fibers (VGCF) collapse the composite's electronic resistance, they simultaneously raise its ionic resistance, degrading rate capability and long-term cycling. The paper left the mechanism unresolved, proposing two competing hypotheses - chemical reactivity between Li6PS5Cl and surface species on carbon, or a geometric increase in the tortuosity of Li+ transport pathways. The open problem is to determine and quantify the mechanism(s) of this carbon-induced ionic resistance increase in NMC/Li6PS5Cl composites; establishing alternative conductive additives or additive treatments that provide electronic percolation without degrading ionic transport is a neighboring engineering thread that a mechanism adjudication would inform, not as part of the problem core.

Background

An all-solid-state battery (ASSB) replaces the flammable liquid electrolyte of a lithium-ion cell with a solid Li+ conductor. The most conductive family, sulfide solid electrolytes, includes the argyrodite Li6PS5Cl (a crystalline chloride-doped thiophosphate with room-temperature ionic conductivity near 10−310^{-3} S/cm). Because the oxide cathode active material (CAM; typically a layered LiNix_xMny_yCoz_zO2_2 oxide, abbreviated 'NMC') is a poor electronic conductor, the positive electrode is a three-component composite of CAM particles, solid electrolyte particles, and typically 1-5 wt% of an electronically conductive carbon additive such as vapor-grown carbon fibers (VGCF); the carbon network ensures every CAM particle is electronically wired to the current collector, while the solid electrolyte network carries Li+ to the CAM. Unlike in liquid cells, both networks are physically constrained: Li+ must percolate through the solid-electrolyte phase, whose effective transport is degraded by tortuosity (the ratio of the effective ion path length through the electrolyte phase to the straight-line electrode thickness, arising from how electrolyte particles pack around CAM and additive particles) and by insulating interfacial reaction products. Partial ionic and electronic conductivities of such composites are commonly separated by impedance spectroscopy (measuring the frequency-dependent resistance) fitted with transmission line models (TLMs), which represent the composite as coupled ionic and electronic rails with interfacial elements between them. Quemin et al. (2022) used DC polarization and potentiostatic impedance in three-electrode cells to decouple the parasitic reactions of Li6PS5Cl-based composites and found, for a 70/30 wt% NMC622/Li6PS5Cl composite (approx. 70 micrometers, 16 mg/cm2), that the ionic resistance of about 120 ohm rose unexpectedly as VGCF content was increased (2 wt% VGCF dropped the electronic resistance from about 60 ohm to below 1 ohm), that VGCF/Li6PS5Cl interfaces form a continuously growing insulating phase without passivation at 3.6 V vs Li-In/In (a lithium-indium alloy reference electrode, the standard potential scale in sulfide solid-state cells), and that 5 wt% VGCF severely limits rate capability. Prior work (Randau et al. 2021) had already shown that the microstructure of the carbon additive matters and that coated carbon microwires can improve ASSB cathodes. Subsequent studies have reproduced the effect and pursued both mechanism threads - interfacial decomposition on one side, tortuosity measurement on the other - but the origin remains unadjudicated, and recent modeling and review studies still describe the specific effects of carbon additives on ionic and electronic conductivities as insufficiently understood (Choi et al. 2024; König et al. 2023; Hong et al. 2025; Han et al. 2025). The mechanistic question left open is stated in the first author's thesis (reference 2) as two competing hypotheses: 'A plausible hypothesis lies in the increasing tortuosity of Li+ pathway when carbon is added. Another one is the chemical reactivity of Li6PS5Cl with surface species existing on carbon additive surfaces.'

Problem Statement

Determine the physical and chemical origin of the ionic-resistance increase that electronically conductive carbon additives induce in argyrodite-based composite cathodes. Specifically, for NMC/Li6PS5Cl composites of the type studied by Quemin et al. (2022) (70/30 wt% NMC622/Li6PS5Cl, approx. 70 micrometers, 16 mg/cm2, VGCF content varied up to about 5 wt%, at 3.6 V vs Li-In/In), adjudicate and quantify the contributions of the two hypotheses stated in the source - (i) chemical reactivity between Li6PS5Cl and surface species/functional groups on the carbon additive, and (ii) a geometrically increased tortuosity of the Li+ pathways caused by the carbon phase - including whether one dominates, both contribute simultaneously, or an additional mechanism (e.g., carbon-catalyzed interfacial decomposition) is required. (Establishing electronically conductive additives or additive treatments that provide electronic percolation without the carbon-typical increase in ionic resistance, protective coatings for carbon-containing composites, and same-protocol halide-electrolyte comparisons are neighboring engineering threads, not part of this problem's completion boundary.)

References

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