Pressure-Dependent Electrochemical Behavior of Di-Lithium Rhodizonate Cathodes

Herein, we investigate the electrochemical properties of the high-capacity organic cathode material di-lithium rhodizonate (Li2C6O6) under different applied mechanical loads. We demonstrate, through a combination of pressure-dependent voltammetry and electrochemical impedance spectroscopy, that the charge-transfer kinetics at the cathode/electrolyte interface is strongly impacted by the magnitude of the load applied to the cathode. At low pressures, lithium rhodizonate displays untenably high charge-transfer impedances toward lithiation and delithiation. As the load applied to the cathode material is increased, the charge-transfer impedance decreases, reflecting a reduction in the overpotential associated with lithiation and delithiation. Furthermore, pressure-dependent galvanostatic cycling reveals that cells cycled at high pressures exhibit improvements in their overall capacity retention when compared with their low-pressure counterparts. Using a combination of postmortem X-ray diffraction and first-principles calculations, we show that, in the absence of sufficient external load, lithium rhodizonate converts from its redox-active structure to a redox-inactive structure, resulting in the observed rapid capacity fade. As the pressure applied to the electrode is increased, this phase transition is suppressed, resulting in improvements in both long-term stability and electrochemical kinetics.