Nuclear magnetic resonance spectroscopy of rechargeable pouch cell batteries: beating the skin depth by excitation and detection via the casing

Rechargeable batteries are notoriously difficult to examine nondestructively, and the obscurity of many failure modes provides a strong motivation for developing efficient and detailed diagnostic techniques that can provide information during realistic operating conditions. In-situ NMR spectroscopy has become a powerful technique for the study of electrochemical processes, but has mostly been limited to laboratory cells. One significant challenge to applying this method to commercial cells has been that the radiofrequency, required for NMR excitation and detection, cannot easily penetrate the battery casing due to the skin depth. This complication has limited such studies to special research cell designs or to ‘inside-out’ measurement approaches. This article demonstrates that it is possible to use the battery cell as a resonator in a tuned circuit, thereby allowing signals to be excited inside the cell, and for them to subsequently be detected via the resonant circuit. Employing this approach, ⁷Li NMR signals from the electrolyte, as well as from intercalated and plated metallic lithium in a multilayer (rolled) commercial pouch cell battery were obtained. Therefore, it is anticipated that critical nondestructive device characterization can be performed with this technique in realistic and even commercial cell designs.