Understanding Lithium Plating Kinetics in Anode-Free Lithium Metal Batteries Using Operando Multi-Modal Characterization

Abstract

Anode-free lithium-metal batteries (LMBs) represent a promising path toward next-generation energy storage, offering 40–60% higher energy density than conventional lithium-ion batteries by eliminating the graphite anode and excess lithium. Realizing their potential is essential to meeting the U.S. Department of Energy’s 2030 targets for electric vehicles. However, their practical deployment is currently limited by low Coulombic efficiency, unstable cycling, and lithium dendrite formation. Electrolyte engineering has emerged as a key strategy to mitigate these limitations. In this work, we systematically compare three commercially available electrolytes, a conventional LP57 electrolyte, a dual-salt electrolyte, and a localized high-concentration electrolyte, in anode-free pouch cells using a combination of operando synchrotron X-ray diffraction (XRD), operando optical imaging, and complementary ex-situ techniques. The two advanced electrolytes significantly outperform LP57 in Coulombic efficiency, cycling stability, and overpotential. XRD mapping of aged cells reveals that capacity fade is largely driven by the accumulation of electrochemically isolated crystalline lithium (“dead” Li) on the copper anode. The advanced electrolytes promote the formation of dense, uniform Li deposits with reduced surface area, suppressing side reactions and dead Li formation. Operando characterization further shows that the advanced electrolytes encourage the initial formation of non-crystalline Li species, which provide abundant nucleation sites and facilitate uniform Li growth. These insights highlight the importance of early-stage plating dynamics in dictating long-term performance. High-energy, high-flux synchrotron X-rays enable non-destructive, spatially resolved tracking of Li morphology and phase evolution inside realistic pouch cells, overcoming the inherent challenges of weak Li scattering and cell complexity. Overall, this study demonstrates how multi-modal operando and ex-situ tools can reveal electrolyte-dependent Li plating and stripping mechanisms at the microstructural level. These insights provide design rules for next-generation electrolytes and offer a robust metrology framework to accelerate the development of high-performance, safe, and long-cycle-life anode-free LMBs.