The polyoxometalates-based electrochromic energy storage devices (POMs-EESDs) were constructed using P₂W₁₇O₆₁¹¹⁻ coated TiO₂ as the working electrode and MnO₂ film as the counter electrode. The MnO₂ films with different thicknesses acted as the charge balancing layer. The device showed bifunctional of enhanced EC and energy storage properties along with high durability and cycle stabilities after the introduction of MnO₂ film. The optimistic thickness of MnO₂ was found to be 345 nm through balancing the optical modulation and the energy storage properties. For the EC aspect, the optical contrast was enhanced by four times under the potential step of −2.0/1.0 V. The MnO₂ film also introduces high capacitance leading to the device can be used as a transparent capacitor. It is the first time to construct high-performance POMs-based EESD only by introducing a simple charge balancing (MnO₂) layer in the device. Moreover, the mechanism of the MnO₂ layer towards the performance improvement of the POMs-based EESDs was also explained.