Excellent Effect of Betaine in Dilute Electrolytes and Application to High Performance Aqueous Li‐Ion Batteries

The energy density and operating voltage of aqueous batteries are limited by the narrow electrochemical stability window and side reactions of traditional aqueous electrolytes. Considering that the optimization of electrolyte composition can effectively improve the electrochemical performance of aqueous batteries, and it has the advantages of simple and easy large‐scale preparation, betaine with rich polar groups and zwitterions is added to the electrolyte as a functional material to reduce the binding degree of water molecules and adsorbed in the electrolyte‐electrode interface to prevent the contact of water molecules with the interface. A wide electrolyte operating window (2.91 V) and low salt concentration (2 M) were achieved in the betaine aqueous electrolyte. The interactions between water molecules, lithium salts and betaine in the electrolyte were explored by ATR‐FITR, NMR and Raman spectroscopy. Aqueous TiO2/LiFePO4 full cells with stable specific energies 69 Wh kg−1 were demonstrated over 500 cycles on such low non‐fluorinated lithium salt concentration electrolyte. Betaine based aqueous electrolyte was initially developed by combining betaine with conventional lithium salt (LiCl) solution, which was for achieving a higher theoretical voltage. Three similar materials (carnitine, proline, choline) were chosen to further explore the effect of functional groups on electrochemical performance of electrolyte. Specially, betaine was first successfully used in aqueous electrolytes. Meanwhile, aqueous TiO2/LiFePO4 full cells with stable specific energy density 69 Wh ⋅ kg−1 were demonstrated over 500 cycles on low non‐fluorinated lithium salt concentration electrolyte.