For more than a century, manganese dioxides (MDOs) have been used as electrochemically active materials in energy storage and conversion applications. To reduce the cost and hazardous impact of synthesis, a redox method was used to prepare MnO2 using extracts of green (GT-MnO2) and black (BT-MnO2) tea as biological reducing agents. Polyphenols present in the extracts of tea not only reduce Mn7+ ions but also act as “capping agents” to stabilize and prevent the produced MnO2 nanoparticles from degradation and aggregation. Elemental, structural properties and morphology of nanosized α-KxMnO2 were studied by thermogravimetric analysis, X-ray powder diffraction and Raman spectroscopy, which revealed the different crystallinity between GT-MnO2 and BT-MnO2 due to the strength of the antioxidant species. Electrochemical investigations highlight the beneficial presence of K+ ions in (2 × 2) tunnels; with a higher concentration, α-K0.135MnO2 exhibits a discharge specific capacity of 155 mAh g−1 after 20 cycles at C/26 rate.