Thermodynamics and kinetics of lithium at the silver-lithium battery interface

TL;DR

The paper addresses the thermodynamics and kinetics of Li at the silver–lithium interface to understand why Ag interlayers promote smooth Li deposition in anode-free solid-state batteries. By benchmarking DFT against a machine-learned potential (MACE-MP-0), it identifies Li deposition as FCC on Ag, with FCC(111) Li–Ag interfaces being the most stable, and shows that vacancy formation energies increase at the interface while cross-interface vacancy migration is extremely fast, enabling rapid Ag–Li alloying. However, diffusion into deeper Li layers is slower, which can hinder alloying when multiple Li layers form, potentially limiting the rate at which interfacial mixing keeps up with Li deposition. The work also proposes Mg alloying to expand the Ag lattice and further improve alloying kinetics and dendrite suppression, with implications for sustaining high-cycle, high-capacity anode-free solid-state batteries.

Abstract

Silver interlayers have been shown to enable smooth lithium deposition and cycling in anode-free solid-state batteries. Here, we report the atomic structure of the Ag and Li interface, showing that Li preferentially plates as FCC on both the (111) and (100) Ag surfaces. This forms an energetically favorable coherent interface with Ag, while the BCC phase forms a semi-coherent interface due to large lattice mismatch. We also calculate vacancy formation energies and migration energies for Li diffusion through the interface. We show that vacancy formation energies increase at the interface, leading to an energetic driving force for vacancies to diffuse away from the interface. Additionally, the migration barriers for vacancies from the Ag to the Li are small (29 meV), and therefore promote rapid alloying between Ag and Li. Rapid Li diffusion kinetics directly at the interface leads to smooth deposition of Li, reducing the onset of dendrites. However, diffusion in the 2nd and 3rd Li layers is slower compared to bulk FCC or BCC Li, leading to kinetically hindered alloying when multiple layers of pure Li form. The diffusion kinetics for Ag nanoparticles may be improved by alloying with Mg to expand the Ag lattice constant while forming a solid solution with both Ag and Li.

Figures (4)

• Figure 1: Schematic showing the unrelaxed interface orientations: KS, NW, FCC (111), Pitsch, Bain, and FCC (100). The KS interface differs from the NW orientation by a rotation of 5.26$^{\circ}$, and involves the close-packed plane for both FCC Ag and BCC Li. For the FCC (111) and FCC (100) interface orientations, we align the surface directions, and there is no relative rotation. For all six interfaces, all the strain is present in the Li surface, and the Ag has its relaxed bulk lattice constant (4.17 Å).
• Figure 2: Difference between the per atom energies and the chemical potential calculated using MACE-MP-0. For all four interface orientations, we find that the first Li layer has smaller per atom energies, and larger per-atom energies for the first Ag layer respectively. The Bain interface has the most variation in the per-atom energies of the same layer due to increased atomic disorder after relaxation. The Pitsch interface has the least variation because a coherent interface is created after relaxation.
• Figure 3: Vacancy formation energies calculated with MACE-MP-0 for the two FCC Li ORs. Vacancy formation energies were not calculated for fixed Ag atoms or Li atoms at the interface. Some Li atoms within 3 layers of the interface diffuse to the free surface and thus have non-physical vacancy formation energies were not included. Similarly, for the 12 Å of Ag that we have fixed to mimic bulk, we did not calculate the vacancy formation energies.
• Figure 4: Migration barriers for vacancy migration in both the FCC (111) and FCC (100) interface. The black barriers are calculated with MACE-MP-0 while the gray one was calculated using DFT. While the migration barriers are very small for Li migration across the interface, the sub-interface migration barriers are significantly larger. For the (100) interface, the migration barriers are large even at the interface and are larger than the bulk Li migration barriers.