Lithium-Ion Batteries: How They Work Conceptually and Organic Electrode Materials Analyzed by Solid-State NMR

The study of the thermodynamics of lithium-ion batteries provides us with important insight into chemical bonding and convincingly accounts for the spontaneous, energy-releasing process of the movement of lithium ions and electrons out of the negative electrode into the positive electrode. In this work, we analyze a discharging battery with a two-phase LiFePO₄/FePO₄ positive electrode and quantitatively show that lithium in the positive electrode is more strongly bonded than in the negative electrode. Thus, the discharge process is energetically downhill, and the lithium remains stuck in the positive electrode, unless it is driven back to the negative electrode with a sufficiently high voltage. We extend this thermodynamic analysis to other common positive inorganic electrode materials, such as LiMnO₂ and LiCoO₂.

Although inorganic electrode materials have been the gold standard for several decades, they are expensive in cost, including labor. Hence, organic electrode materials have been at the cutting edge of materials science for some time. Due to their insolubility in many battery and NMR solvents, these materials are best studied by solid-state NMR. In this work, we analyze the chemical structure of the proposed organic electrode materials bis-tetraaminobenzoquinone (TAQ, also called TDT or BTABQ) and tetraamino-phenazine-1,4,6,9-tetrone (TAPT), which are made from the same precursor, tetraaminobenzoquinone (TABQ). We elucidate their structures via quantitative direct polarization ¹³C and ¹⁵N NMR spectra, long-range ¹³C{¹H} dipolar dephasing, ¹⁵N{¹H} dephasing, as well as two-dimensional heteronuclear correlation and exchange NMR experiments. The structures of TAQ and TAPT differ by two hydrogen atoms. Through this structural analysis, we quantitatively show that molecules denoted as “TAQ” are in fact TAPT.