Change battery pulse cd6/6/2023 ![]() The relaxation time constants become sufficiently long to allow room temperature pulse EPR (pEPR) experiments, which surpass the acquisition rate of previously demonstrated continuous-wave EPR 14, 20, 21. Thus, the apparent spin relaxation time is increased in comparison to a bulk sample. Within porous lithium structures, electrons are located comparatively long in the surface region sensitive to EPR excitation because the diameter of these structures is on the order of or below the skin depth and the interconnected lithium metal structure confines diffusion away from the surface. ![]() If conduction electrons leave the surface region with a thickness of about 1.1 µm in our experiments, they are not excited or detected anymore. The resonance signal linewidth depends on the apparent relaxation time of the electrons, which is largely affected by their mobility in different environments 17, 18. EPR observes spin transitions of lithium conduction electrons in an externally applied magnetic field 17, 18, 19. In comparison to the closely related nuclear magnetic resonance 15, 16 technique, EPR is more selective regarding surface detection due to a lower skin depth at higher frequency and shows higher sensitivity due to the larger gyromagnetic ratio of electron spins compared to nuclear spins 14. Using microwave (MW) excitation, EPR enables low-energy probing of realistic cell designs 14. Electron paramagnetic resonance (EPR) is one suitable technique. To actively monitor the emergence and evolution of microstructures, non-invasive diagnostics during operation are crucial. Post-testing revealed smoothed structures upon using large CDs of 10–15 mA cm −2, which induce extensive surface diffusion through high local temperatures. A promising perspective is the recent qualitative finding of self-heating-induced anode healing 12, 13. Towards safe application of lithium metal anodes, the interplay of cell characteristics and CD has to be investigated to optimise homogeneous plating and suppression of dendrites. The morphology and defective nature of the SEI contribute to irregular lithium growth 11. Decomposition products are found to be inorganic species covering the metallic surface 10. The exposure of freshly formed metallic lithium to electrolyte results in decomposition products referred to as solid electrolyte interphase (SEI). For example, high CDs result in a large number of deposition nucleation sites, facilitating smooth lithium plating 8, 9 but may also cause needle-like dendrites, which are responsible for rapid cell degradation and ultimately short-circuiting. Their appearances can be strongly affected by the applied current density (CD). Depending on the cell chemistry, cycle age, and charging protocol, microstructures of different morphologies are found whiskers, moss, and dendrites are discriminated when using liquid carbonate-based electrolytes 7. Lithium microstructures are formed intrinsically during electrochemical deposition and removal from a metallic surface 4, denoted as plating and stripping, respectively. So far, the formation of lithium microstructures upon cycling has prevented its widespread use because of a limited lifetime due to Coulombic efficiency loss and considerable likelihood of short circuits 5, 6. A strategy to further increase the stored energy is to use lithium metal anodes, which promise a tenfold increase of specific energy density compared to state-of-the-art graphite anodes 4. ![]() ![]() Rechargeable lithium-ion batteries are a key enabling technology for electric vehicles due to their high gravimetric and volumetric capacity 1, 2, 3. It was observed that the generated morphology continued to evolve after the end of a charging pulse, whereby surface features were fusing with a time constant that was slower than their formation. Sampling timescales of 100 ms enable real-time monitoring of the formation and evolution of porous lithium during and after charging pulses. Here, we demonstrate in operando pulse electron paramagnetic resonance to observe transient processes during pulsed fast charging in cells with metallic lithium anodes. To understand the underlying mechanisms and develop counter-measures, non-invasive online detection techniques providing satisfactory time resolution are crucial. Particularly for fast charging, inhomogeneous deposition of metallic lithium, for example on commercial graphite or metallic lithium anodes, leads to cell degradation and safety issues. Enhancing lithium-ion battery technology in terms of specific capacity and charging time is key for the advancement of the electrification of transportation.
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