Flywheel energy storage system (FESS) becomes increasingly popular in dynamic uninterruptable power supplies (UPSs) and smart grids. In order to fully explore the advantageous features of FESS, fast dynamic response and wide speed range operation are required. Nonetheless, most of the existing control approaches for FESS are heavily dependent on machine parameters, indicating poor control robustness in wide speed range. Moreover, many of these approaches are based on field-oriented control (FOC) that requires knowledge of accurate rotor position. This increases the difficulty of realizing the reliable sensorless drives for FESS. To solve the above problems, this paper proposes a robust field-weakening method for sensorless direct torque and flux controlled FESS. It contributes to achieve smooth sensorless operation by employing direct torque and flux control. Moreover, it ensures the robust drives of FESS in full speed range with the presence of parameter variations. Performance of the proposed method is verified by simulations.