Ternary Laves Phase Cu3ZnLi2 in Li Batteries

• The paper shows that rapid grain-boundary diffusion of Li and Zn in a deformed brass current collector enables Cu3ZnLi2 formation, confirmed by DFT and GIXRD analyses.
• Ternary Laves Phase Cu3ZnLi2 is a metastable intermetallic with a distinct Cu:Zn:Li ratio of 3:1:2, characterized by close-packed arrangements and significant microstructural evolution.
• The formation of Cu3ZnLi2 promotes uniform Li plating by enhancing kinetics but leads to irreversible Li sequestration, thereby reducing the active Li inventory over cycles.

Ternary Laves Phase Cu₃ZnLi₂ is a metastable intermetallic compound that forms in situ during the electrochemical cycling of nanocrystalline brass (Cu 63% Zn 37%) current collectors in anode-free Li-metal batteries. Characterized as a Laves phase with a nominal composition of Cu₃ZnLi₂, it emerges from dynamic microstructural transformations induced by repeated lithium plating and stripping, directly influencing lithium transport and overall battery performance. This phase’s formation, stability, and decomposition are intricately linked to grain-boundary diffusion mechanisms, Li–Zn alloying, and the evolving stresses during battery operation.

1. Structural and Thermodynamic Description

Cu₃ZnLi₂ crystallizes as a ternary Laves phase, an intermetallic structure class characterized by a specific stoichiometry and close-packed arrangements, here with a Cu:Zn:Li ratio of 3:1:2. During cycling, the formation is facilitated by non-equilibrium nanoscale conditions and a defect-rich environment in the nanocrystalline brass collector, initially possessing an ~80 nm deformation layer. Density functional theory (DFT) calculations demonstrate that Cu₃ZnLi₂ rests on the convex hull and has a negative formation enthalpy under these conditions, confirming its thermodynamic stability at room temperature given sufficient chemical driving force from the electrochemical environment (Woods et al., 29 Jul 2025).

The essential reaction for phase genesis is:

$$3\,\text{Cu} + \text{Zn} + 2\,\text{Li} \rightarrow \text{Cu}_{3}\text{ZnLi}_{2}$$

This transformation proceeds via rapid Li and Zn diffusion along grain boundaries, where Li mobilities may exceed the bulk by up to three orders of magnitude, catalyzing the incorporation of Li into the brass alloy and the subsequent nucleation of the ternary Laves phase.

2. Mechanisms of Formation During Battery Cycling

Repeated plating/stripping of Li onto a nanocrystalline brass (α-brass) current collector initiates a multi-step sequence leading to Cu₃ZnLi₂ formation:

• Initial nanocrystalline state: The polishes surface exposes an ~80 nm thick deformation layer composed of nanograins, offering extensive grain-boundary pathways.
• Li incorporation: Electrochemical plating drives Li into this layer, rapidly diffusing via grain boundaries. Simultaneously, Zn atoms migrate upwards, creating a compositional gradient.
• Reaction front propagation: After ~100 cycles, a distinct interlayer forms where the original deformed brass is partially replaced by Cu₃ZnLi₂, now occupying 200–250 nm thickness. This transition is confirmed by grazing-incidence X-ray diffraction (GIXRD) and scanning transmission electron microscopy (STEM), indicating the phase’s presence within a nanocrystalline brass matrix.
• Dynamic recrystallization: The Zn-depleted zone below the conversion front, created by continued cycling and compositional gradients, experiences tensile stresses. Coupled with a Kirkendall vacancy counter-flux, these stresses drive recrystallization, coarsening grains and modifying microstructure substantially.

3. Lithium Transport Dynamics

The Laves phase exerts a twofold influence on lithium transport:

• Enhanced plating uniformity: Cu₃ZnLi₂ accommodates Li at higher rates than the surrounding brass, leading to rapid and more uniform Li incorporation upon plating. This uniformity improves nucleation conditions and is associated with suppressed dendrite formation.
• Irreversible Li sequestration: The phase exhibits incomplete reversibility during stripping. Not all incorporated Li is removed electrochemically; residual Li remains sequestered within the intermetallic structure and in associated Li-rich pockets (including those at grain boundaries or Kirkendall voids). This persistent retention of Li, commonly termed "dead Li," contributes to the progressive loss of electrochemically active lithium over cycling.

The effective diffusivity of Li, especially during initial plating, can be approximated as:

$$D_{\mathrm{eff}} \approx D_{\mathrm{bulk}} + \delta D_{\mathrm{GB}}$$

with $D_{\mathrm{GB}} \gg D_{\mathrm{bulk}}$, and $\delta$ quantifying the contribution of the high-diffusivity grain-boundary network.

4. Microstructural Evolution and Reaction Fronts

Microstructural development is marked by sequential phenomena:

Cycling Stage	Layer Composition	Dominant Process
As-prepared	~80 nm nanocrystalline brass	Grain-boundary-facilitated Li-Zn alloying begins
After ~100 cycles	200–250 nm Cu₃ZnLi₂ interlayer	Laves phase formation, reaction front propagation
Continued cycling	Zn-depleted/coarsened matrix	Tensile stress, dynamic recrystallization, Kirkendall effect

Dynamic recrystallization emerges as a consequence of interdiffusion and local stress fields, where vacancy flows (Kirkendall effect) generate nucleation sites for new grains, particularly in Zn-depleted regions. The result is a conversion front where Li concentration drops abruptly, sharply delineating the transformed Cu₃ZnLi₂ layer from unreacted brass.

5. Implications for Anode-Free Li Metal Batteries

The behavior of the Cu₃ZnLi₂ phase yields both advantageous and adverse outcomes for battery performance:

• Dendrite suppression and uniform Li plating: The rapid incorporation of Li into Cu₃ZnLi₂ and the resultant uniform nucleation act to suppress dendritic Li growth, which is a primary safety concern in Li-metal batteries.
• Capacity fade via dead Li: Incomplete dealloying upon stripping means residual Li remains trapped, diminishing the inventory of active Li and leading directly to capacity loss over repeated cycling.
• Microstructural heterogeneity: Dynamic recrystallization exacerbates heterogeneity in the current collector, potentially altering Li transport properties in subsequent cycles and possibly degrading long-term cell performance.

A plausible implication is that the emergence and (ir)reversibility of such metastable ternary phases must be carefully evaluated in both the design and cycling protocols of bimetallic current collectors for advanced battery architectures. Notably, strategies must be developed either to stabilize and exploit the initial beneficial properties of ternary Laves phases, or to ensure their complete electrochemical removal during operation to avoid dead Li accumulation.

6. Broader Relevance and Research Directions

The discovery that Cu₃ZnLi₂ is generated exclusively through electrochemical cycling—rather than traditional alloy synthesis—suggests that metastable ternary intermetallics may be a broader phenomenon in alloy-based current collectors under far-from-equilibrium conditions. Assessment of such phase evolution is thus critical for the rational selection and optimization of current collector materials in next-generation batteries. The interplay among phase stability, grain-boundary diffusion, and microstructural evolution identified for Cu₃ZnLi₂ has direct corollaries in other bimetallic systems and interfacial alloy phases.

Future research will likely probe underlying reaction mechanisms at higher time and spatial resolution, seek to quantify the rates and extent of (ir)reversible Li trapping, and explore alloy design strategies that either promote beneficial intermetallic formation or suppress phases with detrimental irreversibility. The microstructural concepts established for Cu₃ZnLi₂ thus set a precedent for comprehensive studies into dynamic phase transformation phenomena in nanostructured electrochemical interfaces (Woods et al., 29 Jul 2025).