Herein we present a concept of a high-temperature, thermal energy storage (HT-TES) system for large-scale long duration energy storage (>10 hours) applications. The system relies on tunable composite ceramic materials with high electrical conductivity and can output the stored energy flexibly in the form of heat at 1100 degrees C or higher, and as electricity. We model the performance and cost of the system in a techno-economic analysis to identify key material and system properties influencing

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... viability. For applications with daily operation (12 hours storage duration), we find achieving levelized storage costs below US Department of Energy's 5 ₵/kWhe (1-2.5 ₵/kWhth equivalent) target by 2030 is possible. Candidate materials should have above 600-900 high-temperature cycle stability while offering at least 104 S/m of electrical conductivity. Our results suggest this system can be economical for longer storage durations (weeks to months) when coupled with intermittent charging using surplus renewable energy sources.