Solvent-induced covalent bond softening boosts battery voltage

Abstract

Increasing cell voltage is a key strategy for enhancing the energy density of lithium batteries. Previously, this was mainly achieved by adjusting the redox potentials of transition-metal-based cathode materials through inductive effects that altered the covalency of metal—oxygen bonds. Here, we present a novel strategy for increasing battery voltage that consists of acting on the redox potential of the electrochemically active electrode through charge transfer with the electrolyte. To demonstrate this new concept, we used CFx-type electrodes, which are found in commercial primary batteries, and successfully achieved an impressive increase in redox potential of over 250 mV. This was done by increasing the ionicity of the C─F bond via a lactam-based electrolyte with high electron-donating capability. This finding, which was extended to other electrodes, namely I2, was rationalized through an array of analytical techniques and computational methods. Contrary to common belief, we clearly demonstrate that the electrolyte itself can significantly impact the bulk redox properties of electrodes, such as voltage. The new proposed inductive effect, driven by interactions between the solvent and the redox center, opens up new avenues of research in chemical bond regulation. It would also be highly valuable in energy-related systems, including electrocatalyst and beyond. The strong attractive interaction between the nucleophilic center of the solvent and the electrophilic region of the covalently bonded cathode fragment triggers charge transfer. This results in the electron population in the antibonding orbital of the covalent bond, leading to bond weakening and elongation, and consequently increasing the cathode potential. Graphical abstract: [Figure not available: see fulltext.]