A nitroxide-containing cathode material for organic radical batteries studied with pulsed EPR spectroscopy

An electron spin echo in a nitroxide-containing polymer cathode film for organic radical batteries is observed for various states of charge at cryogenic temperatures. The EPR-detected state of charge (ESOC), as inferred from the number of paramagnetic centers in the film, is compared to the results of Coulomb counting based on galvanostatic charging. Spin concentration, longitudinal relaxation times $T_1$ and phase memory times $T_m$ strongly correlate with the ESOC. In the discharged film, the spin concentration reaches $\left(5\pm 3\right)\times 10^{20}$ cm⁻³, causing a phase memory time $T_m\ll$ 100 ns (shorter than the resonator ring-down time) that hinders the detection of the spin echo. In the charged film, the decreased spin concentration results in a longer $T_m$ between 100 ns and 300 ns that enables spin-echo detection, yet limits the length of the microwave pulse sequence. The short, broad-band pulses cause instantaneous diffusion in the unoxidized domains across the oxidized film, affecting the relative peak intensities in the pulsed EPR spectrum. By simulating the spectral distortion caused by instantaneous diffusion, we obtain information on the local spin concentration, which complements the information on the ‘bulk’ spin concentration determined by electrochemistry and continuous-wave EPR spectroscopy.