Design of a fast broadband RF switch for multi-nuclear NMR in lithium-ion batteries

To address the need for observing the electrolyte influenced by dissolved metal ions during the charge and discharge process of lithium-ion battery, a fast-switching broadband radio frequency (RF) switch for broadband nuclear magnetic resonance (NMR) systems was designed. This design specifically aims to overcome the challenge where signals exhibiting fast relaxation-often containing critical information-are lost due to the long "dead time" of conventional NMR instruments. The switch utilizes a differential architecture paired with a single-ended drive circuit. Its innovative feature is the inclusion of common-mode chokes in the bias circuit, which suppresses switching spikes and accelerates the transition speed. Electrical testing results confirm its superior performance: within a 3 MHz-100 MHz bandwidth, the switch achieves an insertion loss of less than 0.86 dB and isolation greater than 85 dB, coupled with an input third-order intercept point (IIP3) linearity of 55.50 dBm. Crucially, the measured switching time is only 175.3 ns, and the switching spike is limited to 6.09 mV. Experimental validation with paramagnetic additives (Mn²⁺, Fe²⁺) demonstrated that the switch's short dead time preserves the signal-to-noise ratio (SNR) of short transverse relaxation time (T ₂) signals while a 5 μs delay reduced the SNR by over 45%, underscoring the practical importance of fast switching. In practical NMR experiments using LiPF₆ electrolyte, the switch successfully detected the free induction decay (FID) signals of ¹H, ⁷Li, and ¹⁹F nuclei. The resultant short switching time minimizes the NMR front-end dead time, which is highly advantageous for detecting samples with short T ₂ relaxation and broad spectra.