Ternary Laves Phase Cu₃ZnLi₂ in Li‑Metal Batteries

• Cu₃ZnLi₂ is a metastable ternary intermetallic formed in situ during repeated Li plating and stripping on nanocrystalline brass collectors.
• Its formation relies on rapid Zn diffusion, stress-induced dynamic recrystallization, and the Kirkendall effect, which together modulate Li transport and dendrite suppression.
• The partial reversibility of the phase results in trapped, inactive Li that leads to capacity fade, highlighting the need for optimized current collector designs.

The ternary Laves phase Cu₃ZnLi₂ is a metastable intermetallic compound that emerges in situ during the electrochemical cycling of nanocrystalline α-brass (Cu 63%–Zn 37%) current collectors in anode-free Li-metal battery architectures. Its formation, microstructural evolution, and impact on lithium transport are central to recent advances and challenges in optimizing battery performance, particularly with respect to dendrite suppression and capacity retention.

1. Electrochemical Formation Pathway

The genesis of Cu₃ZnLi₂ is intrinsically linked to repetitive Li plating and stripping processes on a mechanically polished, nanocrystalline brass current collector. Initially, brass surfaces possess an approximately 80 nm thick deformation-induced nanocrystalline layer. Early electrochemical cycles facilitate preferential Li deposition along grain boundaries, initiating rapid diffusion along these pathways. Zinc exhibits diffusion kinetics roughly three times faster than copper in brass, enabling significant Zn migration toward the interface during cycling.

The resulting non-equilibrium interdiffusion, accentuated by high Li activity, concentration gradients, and stress/vacancy fluxes (the Kirkendall effect), drives the formation of Cu₃ZnLi₂ according to:

$3Cu + Zn + 2Li \rightarrow Cu_3ZnLi_2$

As cycling progresses, up to 100 cycles, the deformation layer expands to 200–250 nm and portions are converted into the Laves phase within the original brass matrix.

2. Microstructural Evolution Mechanism

The transformation of the collector surface involves dynamic recrystallization and phase conversion processes:

• Li initially infiltrates grain boundaries in the nanocrystalline α-brass, enhancing local Li activity and stress.
• Zn migrates upward from the bulk brass, accelerated by grain boundary diffusion ($D_{Zn}^{GB} \approx \Delta \cdot D_{Zn}^{bulk}$), and a conversion front forms where Li concentration drops.
• Unbalanced atomic fluxes—fast Zn outflow, slower Cu migration—contribute to the nucleation of the ternary phase, facilitating local volume expansion and generating tensile stress, further stimulating grain coarsening.
• Vacancy counter-flux ($J_{vac} = -D_{vac} \nabla C_{vac}$) promotes dynamic recrystallization and creates Zn-depleted regions beneath the conversion front.

Over repeated cycles, this mechanism leads to a discrete stratification: an upper nanostructured layer rich in Cu₃ZnLi₂, a conversion front, and a lower region reverting toward the original brass composition.

3. Influence on Lithium Transport and Battery Performance

The presence of Cu₃ZnLi₂ modulates lithium transport with dual effects. In regions where the Laves phase is pronounced, it enhances homogeneity of Li plating. The phase offers additional rapid Li incorporation sites, potentially suppressing dendritic growth—a key factor in extending cell lifetime.

However, Cu₃ZnLi₂ exhibits partial reversibility. During Li stripping, the phase does not fully decompose, retaining substantial fractions of lithium. This trapped, electrochemically inactive Li—termed "dead Li"—does not participate in subsequent cycles. After 100 cycles, approximately 10–15 at% Li is irreversibly retained. The consequence is gradual capacity fade and diminished transport efficiency, as Li mobility through the collector becomes locally hindered.

4. Implications for Anode-Free Current Collector Design

The dynamic formation and partial irreversibility of Cu₃ZnLi₂ necessitate new paradigms in battery component design:

• Microstructural engineering of binary alloy collectors (e.g., adjusting grain size, texture, and nanostructuring) becomes critical to control or avoid detrimental phase conversion.
• Cycle protocols and electrolyte compositions may require optimization to promote complete electrochemical stripping and restrict persistent Li sequestration.
• Additives or composite architectures could stabilize the beneficial aspects of the Laves phase—such as uniform plating—while curbing its overgrowth and associated Li entrapment.

Comprehensive assessment of metastable ternary phase emergence is essential when selecting and processing current collectors for next-generation Li-metal batteries.

5. Context within Alloy Systems and Future Directions

Historically, binary alloy-based current collectors have been assessed primarily for bulk properties and phase stability under thermal conditions. The emergence of metastable ternary phases like Cu₃ZnLi₂ during purely electrochemical cycling introduces a new axis of complexity. This necessitates the expansion of phase diagrams to include non-equilibrium compositions and in situ structural transformations under operational conditions.

A plausible implication is that future high-performance batteries will require integrated strategies balancing uniform Li plating/dendrite suppression against minimization of irreversible Li sequestration. Explicit consideration of dynamic phase evolution—previously overlooked in “binary” system design—is now essential.

6. Summary and Research Outlook

The electrochemically-driven emergence of Cu₃ZnLi₂ in nanocrystalline brass collectors is a central, dynamic phenomenon affecting both mechanical and ionic properties of Li-metal battery devices. Initially, the phase formation promotes improved Li plating uniformity and dendrite suppression, but its tendency toward partial decomposition and persistent Li sequestration ultimately contributes to non-trivial capacity loss.

Understanding, predicting, and controlling this phase’s formation, growth, and reversibility will be critical to future advances in anode-free battery design—such control strategies may include tailored microstructures, optimized cycle protocols, and comprehensive phase stability analysis under cycling conditions. The fundamental recognition is that battery component microstructures are neither static nor immune to dynamic phase evolution during operation, and these phenomena can decisively impact device lifetime and reliability.