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Ca₃Co₄O₉, a p-type thermoelectric material based on transition-metal oxides, has garnered significant interest due to its potential in thermoelectric applications. Its unique misfit-layered crystal structure contributes to low thermal conductivity and a high Seebeck coefficient, leading to a thermoelectric figure of merit (zT) of ≥1 at 1000 K. Conventionally, it has been believed that thermopower reaches its upper limit above 200 K. However, our thermopower measurements on polycrystalline Ca₃Co₄O₉ samples have revealed an unexpected increase in thermopower above 380 K. In this study, we investigate the effects of high oxygen pressure annealing on Ca₃Co₄O₉ and provide an explanation based on the mixed oxide states of cobalt and carrier hopping. Our results demonstrate that annealing induces modifications in the defect chemistry of Ca₃Co₄O₉, leading to a decrease in electron hopping probability and the emergence of a thermal activation-like behavior in thermopower. These findings carry significant implications for the design and optimization of thermoelectric materials based on misfit cobaltates, opening new avenues for enhanced thermoelectric performance.