MXenes, synthesized from MAX, have emerged as new energy-storage materials for a good combination of metallic conductivity and rich surface chemistry. The reported MXenes are synthesized mostly from Al-based MAX. It is still a big challenge to synthesize MXenes from abundant Si-based MAX because of its strong Ti–Si bonds. Here, we report for the first time a high-energy ultrasonic cell-crushing extraction method to successfully prepare Ti3C2T x MXenes from Si-based MAX using a single low-concentration etchant. This novel strategy for preparing MXenes has a high extraction efficiency and is a fast preparation process of less than 2 h for selective etching of Si. Furthermore, through the high-energy ball-milling technology, unique P–O–Ti bonded red phosphorus nanodot/Ti3C2T x (PTCT) composites were successfully prepared, which enable superior electrochemical performance in lithium- and sodium-ion batteries because of the double-morphology structure, where the amorphous nano red phosphorus particles were strongly absorbed to Ti3C2T x MXene sheets, facilitating the transport of alkali ions during cycling processes. This novel synthesis method of Ti3C2T x MXenes from Si-based MAX and unique P–O–Ti bonded PTCT composites opens a new door for preparing high-performance MXene-based materials and facilitating the development of low-cost MXenes and other two-dimensional materials for next-generation energy storage.