Solid electrolytes with fast ion kinetics and superior mechanical properties are critical to electrochemical energy devices; however, how to design low-cost, high-performance solid electrolytes has become a critical challenge in the energy field, and significant progress has not been achieved until now. Here, lake-water-based semisolid electrolytes with a low cost of 1.89 $ kg -1 have been put forward for the purpose of market promotion. By virtue of the palygorskite dopants and lake water source, the electrolytes display satisfying mechanical, electrical, and electrochemical properties as well as economic benefits. The application potential of electrolytes has been demonstrated by employing a polyelectrolyte with ionic conductivity of 0.82 × 10 -4 S cm -1 in flexible supercapacitors. The as-assembled devices give a high energy density of 54.72 Wh kg -1 and excellent cycling stability with a capacity retention of 94.8% over 20 000 cycles. The flexibility of devices has been verified through 5000 repetitive bending tests. Our work presents insight into the design of flexible solid electrolytes based on cheap and green raw materials.