Solid-state NMR and EPR methods for metal ion battery research

The rapid development of batteries, especially lithium-ion batteries, has dramatically changed our daily lives. From portable electronics to electric vehicles and smart grids, batteries are extensively used in many fields and are difficult to be replaced in terms of their excellent energy and power densities. The advancement of battery technology requires the thorough understanding of electrochemical reaction mechanisms, which strongly depends on the collaboration of researchers from different fields. Magnetic resonance spectroscopy includes the important techniques of nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), and the former is suitable for studying light elements commonly found in batteries including Li, Na, P and O, while the latter is suitable for studying heavier transition metals such as Co, Mn, Fe and V. In addition, NMR and EPR are capable of quantitatively analysis in a nondestructive manner regardless of sample crystallinity. Hence, NMR and EPR spectroscopies have allowed for significant research progress and have become increasingly important for battery research over the past three decades. Herein, we will provide our perspective of magnetic resonance methods and first summarize the main interactions and the Hamiltonian forms of solid-state NMR and EPR (dipole-dipole interaction, electric quadrupole interaction, chemical shift, and hyperfine interaction). Subsequently, we summarize the important and frequently-used methods of solid-state NMR and EPR spectroscopies and introduce their representative applications in metal ion battery research (mainly lithium-and sodium-ion batteries). Specifically, we introduce the basic principles and representative applications of (i) MQMAS (multiple-quantum magic angle spinning), (ii) pjMATPASS (MAT = magic-angle turning, PASS = phase-adjusted sideband separation, and pj = projection), (iii) WURST-CPMG (WURST = wide band uniform rate smooth truncation, CPMG = Carr-Purcell Meiboom-Gil), (iv) 2D homonuclear correlation and exchange (2D EXSY), (v) 2D homonuclear correlation based on dipole coupling (i.e. RFDR), (vi) perpendicular mode EPR, (vii) parallel mode EPR, (viii) in situ NMR, and (ix) in situ EPR. In addition, we briefly introduce representative applications of 2D heteronuclear correlation (i.e. CP-HETCOR), pulsed field gradient NMR, spin-lattice relaxation (SLR), spin alignment echo (SAE), DFT calculations, and dynamic nuclear polarization (DNP). Previous reviews regarding the application of magnetic resonance technology in battery research are almost all reported in terms of the classification of battery materials. In other words, they are written from the perspective of applications in cathode, anode, and electrolyte research. Herein, we summarize from the perspective of solid-state NMR and EPR methods, which may be beneficial for the readers to fully understand the value of these important technologies. We believe this review can serve as a guide to solve challenges related to using solid-state NMR and EPR spectroscopies in battery research.