Dynamic Nuclear Polarization from Lithium Metal, a New Method to Study Lithium Batteries

Abstract

The massive development of portable electronics, electric vehicles, and the increased need for energy storage require higher battery energy density, improved efficiency, and safety. In this context, lithium metal is the ultimate high-energy-density battery anode material. However, its use as an anode material is limited due to the formation of lithium dendrites that could lead to low efficiency and potential safety issues. Therefore, understanding and controlling the formation of lithium dendrites is key to realise lithium metal batteries.Here we show a new approach combining Scanning Electron Microscopy (SEM) and Magic-Angle Spinning Dynamic Nuclear Polarization (MAS DNP) to study the interplay between lithium morphology and interface reactivity. We aim to understand how the Solid-Electrolyte-Interphase (SEI), formed at the surface of lithium metal, affects lithium morphology. 1,2. Whereas the morphology of lithium dendrites can easily be identified using SEM, information about the SEI is more challenging to access. Among the different techniques used to characterize battery material, Nuclear Magnetic Resonance (NMR), has emerged as a powerful method providing information about the chemical composition as well as dynamic properties of a given material, at the atomic scale. Yet, the main limitations of NMR are its low intrinsic sensitivity and lack of selectivity which highly complicate the study of thin interphase as the SEI. In the present work, we use MAS DNP to overcome the low sensitivity of NMR. MAS DNP is based on the transfer of the large electron spin polarization of unpaired electrons to the nuclear spins, leading to the enhancement of the NMR signal intensity. However, the addition of free organic radicals solution to the typical exogenous DNP may alter the nature of the SEI. Furthermore, exogenous DNP is not selective to the interfaces between the lithium metal and the SEI. In this respect, we use the recently developed Lithium metal DNP method which exploits lithium metal's conduction electrons to perform endogenous MAS DNP and achieve one order of magnitude hyperpolarization at room temperature3. In addition to the gain of sensitivity, this technique allows to selectively enhance the chemical species close to the lithium metal providing critical information about the chemical structure of the SEI. This approach, currently under development, provides a powerful tool in research to control and prevent the formation of lithium dendrites which typically result in rapid degradation and potential safety issues. Figure: A) Schematic representation of Lithium dendrites with SEM of lithium dendrites formed on a copper current collector. B) Schematic representation of SEI formed on lithium metal with 7Li MAS DNP spectra recorded with and without microwave irradiation on lithium dendrites