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Abstract
Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li4Ti5O12. In addition for being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.
Highlights
The development of new and improved materials for rechargeable batteries requires analytical tools, which can provide insights into the composition and structure of the electrodes, electrolyte, and their interface
Solid-state NMR spectroscopy has been shown to be useful, enabling very detailed characterization of the electrochemical and chemical transformations within the bulk of the materials used as electrodes and solid electrolytes as well as the electrode−electrolyte interface/phase.[1−5] In this respect, the main advantages of solid-state NMR (ssNMR) are its high chemical sensitivity, which enables tracking the formation of phases and perturbations to the local environment of the detected nuclei, and the ability to determine proximity between phases and chemical environments
We have recently introduced an alternative approach for dynamic nuclear polarization (DNP) in inorganic materials, metal ion DNP (MIDNP), based on the use of paramagnetic metal ion dopants.[13,14]
Summary
The development of new and improved materials for rechargeable batteries requires analytical tools, which can provide insights into the composition and structure of the electrodes, electrolyte, and their interface. We have recently introduced an alternative approach for DNP in inorganic materials, metal ion DNP (MIDNP), based on the use of paramagnetic metal ion dopants.[13,14] We demonstrated up to 104 fold improvement in NMR sensitivity by endogenous DNP from Mn(II) dopants in oxides, including the anode material Li4Ti5O12 (LTO) Such a gain in sensitivity enabled the detection of the 17O nuclei in the bulk of micronsized particles at natural abundance as low as 0.038%, suggesting that endogenous DNP can become a viable characterization tool for inorganic materials in general and for battery materials in particular. We demonstrate that a carbon free electrode formulation is an excellent route for studies with MIDNP, providing sufficient sensitivity while maintaining good electrochemical performance
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