Striking a Balance between Carbon Mediated Cathode Degradation and SEI Engineering Towards Stable High-Performance Solid-State Batteries
Authors: Bhadra Abhirup , Kundu Dipan 
Published: February 2024

Solid-state batteries (SSBs) can render a paradigm shift in battery safety and energy density. The promise hinges on integrating high-capacity alkali metal electrodes with state-of-the-art inorganic solid electrolytes and high-energy Li-ion cathodes. However, the development has been restricted, mainly owing to the limited understanding and partial overcoming of the interfacial electro-chemo-mechanical issues at the electrode-electrolyte interfaces, leading to inferior power capability, energy efficiency, and long-term stability of the cell.1

Sulfide-type solid electrolytes (SEs) like Li6PS5Cl, Li10GeP2S12, and Li3PS4 are promising for commercial SSB development, but they seem to suffer from interfacial stability issues with the high-energy NMC (LiNixCoyMnzO2) cathodes, particularly when a conductive carbon additive is employed, which is required for improved capacity and high current density operations. Earlier reports suggest that the carbon enables a faster electronic percolation pathway, leading to a quicker NMC-SE interfacial degradation over cycling. Parallelly, the reactive functional groups on the carbon surface may aggravate the decomposition pathway by reacting with the thiophosphate SEs.2–4
,
5 To shut down this conductive additive-led decomposition and to stabilize NMC-sulfide SE-based SSBs, often a protective coating of LiNbO3, LiZrO3, Li3PO4, etc., is applied on the NMC particles in addition to avoiding the use of carbon in the cathode composite. Relatively lesser-known approaches looked into the effect of modifying the conductive additive surface itself with a non-conductive polymer or insulator-type material to tame the degradation mechanism led by the electron percolation pathway.1,6–9

In this context, by thorough experimentation, we have established an in-depth understanding of carbon’s role in the NMC performance degradation process. This led to the reckoning that one of the insulating degradation products can be in situ generated with functionalized carbon as the conducting additive, rendering stable solid electrolyte interphase (SEI) during electrochemical cycling. We found that the controlled degradation kinetics with an optimally functionalized carbon material help achieve the required conductivity (low impedance) at the cathode for improved energy and power densities and a stable cathode SEI for durable SSB cycling. In conclusion, we present a novel and facile idea of engineering the conductive carbon additive for achieving high-capacity, long-life NMC-based SSBs without the complication of expensive coating strategies.
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