(381e) Are Mechanical Instabilities (fracture widening) Driven By Electro-Chemical Forces (Li penetration) in Anode-Free Solid-State Batteries?

Anode-free solid-state batteries (AF-SSBs) present a promising pathway toward sustainable transportation by eliminating the lithium-metal anode and using a bare current collector (CC) with a high-energy-density cathode and solid electrolyte (SE).¹ This design offers improved safety, recyclability, and energy performance—achieving specific energies >500 Wh/kg and projected costs <$100/kWh—compared to conventional Li-ion batteries.² Recent progress includes lithium-metal-free configurations; however, critical challenges related to interfacial stability limit their practical adoption.^3-7

A major limitation is the electro-chemo-mechanical instability of buried interfaces—particularly CC|SE, SE|Li, and CC|Li—during cycling.⁸ These interfaces undergo complex morphological changes during lithium plating yet remain difficult to probe due to their buried and highly localized nature. To establish precise structure–property–performance relationships, in situ and operando 3D investigations of lithium plating at these solid–solid interfaces are critical.⁹

This work investigates how lithium plating at the CC|SE interface correlates with mechanical instabilities—such as fracture widening and lithium intrusion into cracks—across current densities. We address two open questions critical to understanding failure modes in AF-SSBs:

Q1: Is lithium penetration into SE fractures at low and high current densities solely electrochemically driven, or is it partially due to lithium extrusion during assembly?

Q2: What is the origin of lithium observed within fractures—pre-existing or resulting from electrochemical cycling?

To answer these, we used simultaneous in situ neutron and X-ray micro-computed tomography (NeXT), as well as standalone in situ neutron tomography. NeXT experiments were performed at the Institut Laue-Langevin (ILL)¹⁰ using a custom-designed electrochemical cell with ~4–6 µm spatial resolution.¹¹Complementary neutron tomography was conducted at Oak Ridge National Laboratory (ORNL),¹² achieving an effective spatial resolution of 20–30 µm.

We imaged five batteries: three pristine, one cycled at low current density (0.5 µA), and one at high current density (5 µA). X-ray tomography revealed pre-existing cracks near the SE pellet edges in pristine cells and crack widening at the center in both cycled cells. Neutron tomography showed no lithium in one pristine cell (ILL), while two pristine cells (ORNL) displayed lithium near the start of fractures at the Li|SE interface—likely due to lithium extrusion during assembly. However, deep lithium penetration was not observed in these pristine cells. In plated cells (ILL), lithium accumulated within fractures—showing "filament-like" morphology at low current density and "flow-like" behavior at high current. We also observed interfacial void formation and contact loss at the CC|SE interface. Interestingly, no lithium plated directly on the stainless-steel CC at high current, likely due to lithium being redirected into SE fractures. Limited plating on the CC was observed at low current density.

Our findings suggest that fracture widening may be driven by electrochemical forces, especially under high current density. Moreover, lithium penetration into SE fractures likely results from a combination of initial extrusion and subsequent electrochemical driving forces. We hypothesize that visco-plastic deformation governs lithium intrusion during cycling, consistent with electro-chemo-mechanical phase-field modeling.¹³To conclusively determine the mechanisms behind lithium intrusion and its origin, operando time-resolved neutron and X-ray imaging studies are currently underway. This work advances mechanistic insight into buried solid–solid interfacial degradation in AF-SSBs and informs the design of more robust architectures for next-generation electric vehicle batteries.

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Keywords: Plated Lithium; In Situ 3D Neutron and X-ray imaging; Anode-free; Solid-state batteries