To ensure high capacity with reasonable cycling stability using the conversion or K-alloy reaction, the anode should minimize or embrace the volume expansion during the discharge (potassiation) and charge (depotassia- tion) processes. Herein, we report on a Bi 2 Te 3 composite modified by nanosized carbon that boosts the highly reversible capacity with long cycling stability. The as-synthesized Bi 2 Te 3 powders and acetylene black carbon were compositized via high-energy ball-milling, resulting in improvement of the electrical conductivity to ∼10 − 4 S cm − 1 from ∼10 − 7 S cm − 1 . Electrochemical investigation revealed that the proposed Bi 2 Te 3 @C retained over 79.8% of its initial capacity (311 mAh g − 1 ) for 500 cycles at a current of 1000 mA g − 1 . We unveil the reac- tion chemistry behind the acceptable performance using operando X-ray diffraction, X-ray absorption near edge structure spectroscopy, and time-of-flight secondary-ion mass spectrometry. The Bi 2 Te 3 undergoes a conversion reaction to form Bi 0 metal and K 2 Te as a conversion byproduct, after which Bi is further potassiated to K 3 Bi. The highly reversible behavior is attributed to the enlargement of the active area along with the filling of voids among Bi 2 Te 3 particles by nanosized carbons in the composite electrode, enabling not only facile electron transport but also preservation of the electrode shape even after long-term cycling. Our approach highlights the feasibility of applying Bi 2 Te 3 @C as a sustainable anode material for potassium-ion batteries.