Flexible thin film all-solid-state Li-ion batteries are considered as promising candidates to power a multitude of flexible and miniaturized electronic devices. The production of crystalline battery active materials generally involves high process temperatures above 500 °C. One current challenge in mechanically flexible thin film electrode fabrication is the direct deposition of such crystalline active materials onto temperature sensitive substrates. In the current work we have made a paradigm shift depositing highly pure crystalline Li4Ti5O12 nanoparticles onto a flexible polyimide foil in a single step using flame spray pyrolysis technique. The Li4Ti5O12 films were mechanically compressed at room temperature to 0.55 µm thin layers, to enhance their adhesion to the substrates, i.e. to increase mechanical stability. The smooth Li4Ti5O12 electrodes were covered with a solid electrolyte and tested against lithium metal electrodes. Stable electrochemical cycling behavior of the battery cells demonstrated the feasibility of the proposed technique for LTO thin film electrode fabrication on temperature sensitive and mechanically flexible polyimide substrates. Fundamental data on possible electrode cyclability upon electrode bending was obtained by successful cycling of LTO flex-TFBs in statically bent condition. This study could initialize a new branch for facile manufacturing of flexible thin film battery cells.