Solid-state lithium metal batteries represent a transformative advancement in electrochemical energy storage, promising energy densities exceeding 500 Wh/kg, which is critical for extending the driving range of electric vehicles beyond 800 km. However, the practical implementation of these batteries is hindered by poor interfacial contact stability between the lithium metal anode and solid electrolytes, leading to increased impedance, dendrite growth, and rapid capacity degradation. In this study, we address this challenge by introducing a copper thin film modification layer at the interface between the Li7La3Zr2O12 solid electrolyte and the lithium metal anode. Unlike conventional modification strategies employing highly electrochemically active materials like silver, copper exhibits low electrochemical activity towards lithium, minimizing volume strain at the interface and enhancing mechanical stability. Through electrochemical impedance spectroscopy, we demonstrate that the copper modification reduces the interfacial area-specific resistance from 2,030 Ω/cm² to 65 Ω/cm². Furthermore, the specific capacity of lithium cobalt oxide cathodes increases from 118 mAh/g to 145 mAh/g, and the solid-state battery exhibits a capacity retention of 93.5% after 500 cycles at 0.33 C, compared to 46.3% for unmodified cells. This work underscores the efficacy of electrochemically inert modification layers in stabilizing interfaces for high-performance solid-state batteries.
The pursuit of higher energy densities in batteries has driven research towards lithium metal anodes, which offer a theoretical capacity of 3,860 mAh/g and a low electrochemical potential of -3.04 V versus the standard hydrogen electrode. Solid-state batteries, utilizing solid electrolytes instead of liquid counterparts, mitigate issues like dendrite formation and side reactions, thereby improving safety and cycle life. Despite these advantages, the rigid solid-solid contact between the lithium anode and solid electrolytes often results in high interfacial resistance and mechanical failure due to volume changes during cycling. Common modification layers, such as silver, form alloys with lithium that undergo significant volume expansion and contraction, exacerbating interface degradation over time. In contrast, copper’s low reactivity with lithium minimizes such volume variations, providing a stable interface that enhances long-term performance. Our approach focuses on leveraging this property to achieve superior interfacial stability in solid-state batteries.
To fabricate the solid electrolyte, we employed a solid-state synthesis method using lithium carbonate, lanthanum oxide, and zirconium oxide as precursors. These materials were ball-milled at 500 rpm for 48 hours, followed by sintering at 900°C for 24 hours to obtain Li7La3Zr2O12 powder. The powder was then pressed into pellets under 400 MPa and sintered at 1,200°C for 24 hours in an argon atmosphere, with additional powder covering the pellets to prevent lithium loss. The resulting pellets were polished and stored in an argon-filled glove box. For the copper modification layer, we used magnetron sputtering with a copper target at a power of 100 W, a base pressure below 5×10-4 Pa, and an argon pressure of 0.1 Pa. The sputtering duration was 5 minutes, yielding a thin film of 2–4 μm thickness, which was subsequently annealed at 800°C for 5 minutes to enhance adhesion and uniformity. A similar process was applied for silver modification as a comparative study.
We assembled symmetric lithium metal batteries by attaching 50 μm thick lithium foils to both sides of the modified and unmodified Li7La3Zr2O12 pellets, followed by heat treatment at 200°C for 1 hour to ensure intimate contact. For full-cell evaluations, we prepared LiCoO2 cathodes with a mass ratio of 8:1:1 for active material, Super-P conductive agent, and polyvinylidene fluoride binder, respectively. The cathode was wetted with 5 μL of ester-based electrolyte to improve interfacial ion transport, while the anode interface was treated identically to the symmetric cells. The assembled soft-pack batteries had a nominal capacity of 500 mAh and were tested under a pressure of 0.5 MPa at 25°C. Characterization included scanning electron microscopy for morphological analysis, electrochemical impedance spectroscopy for interfacial resistance measurement, and galvanostatic cycling for performance assessment.
The interfacial morphology revealed significant gaps of approximately 400 nm in unmodified cells, which expanded to 550 nm after 50 cycles, indicating progressive contact loss. In contrast, copper-modified interfaces showed no visible gaps initially or after cycling, attributed to the formation of a lithium-copper alloy that enhanced wettability. Elemental mapping confirmed a sharp interface between copper and the solid electrolyte without interdiffusion, highlighting the chemical stability of the modification layer. The wetting behavior was quantitatively assessed through contact angle measurements; unmodified surfaces exhibited contact angles greater than 90°, indicating poor lithium wettability, whereas copper-modified surfaces had angles below 90° due to alloy-induced stress that promoted spreading.
Electrochemical impedance spectroscopy data for the solid electrolyte pellets showed a bulk ionic conductivity of 0.35 mS/cm, corresponding to an area-specific resistance of 373 Ω/cm². The symmetric cell with an unmodified interface exhibited a high interfacial resistance of 2,030 Ω/cm², which was reduced to 65 Ω/cm² with copper modification. This drastic reduction facilitates efficient lithium-ion transport across the interface, crucial for high-rate performance. The impedance spectra were modeled using an equivalent circuit comprising a resistor for bulk electrolyte resistance and a constant phase element for the interface, with the impedance $Z$ given by:
$$Z = R_{\text{bulk}} + \frac{1}{(Q(j\omega)^n)}$$
where $R_{\text{bulk}}$ is the bulk resistance, $Q$ is the constant phase element parameter, $j$ is the imaginary unit, $\omega$ is the angular frequency, and $n$ is an exponent related to surface heterogeneity. The low $n$ values for modified interfaces (close to 1) indicate near-ideal capacitive behavior, underscoring the homogeneity introduced by copper.
Galvanostatic cycling of symmetric cells at 0.1 mA/cm² demonstrated the stability of copper-modified interfaces, with polarization voltages remaining stable around 50 mV for over 2,000 hours, compared to unmodified cells that short-circuited within 130 hours due to dendrite penetration. Silver-modified cells showed intermediate performance, with polarization increasing gradually over 1,000 hours, likely due to volume changes in lithium-silver alloys. The critical current density for dendrite initiation, determined through stepwise current increases, was 0.4 mA/cm² for unmodified cells, 0.8 mA/cm² for silver-modified, and 1.2 mA/cm² for copper-modified cells. This improvement aligns with the reduced volume strain $\Delta V$ at the interface, which can be expressed as:
$$\Delta V = \frac{V_{\text{alloy}} – V_{\text{Li}}}{V_{\text{Li}}} \times 100\%$$
For copper, the volume change upon alloying is minimal compared to silver, as copper does not form extensive intermetallic phases with lithium under operational conditions. This mechanical stability is vital for suppressing dendrites in solid-state batteries.
Full-cell evaluations with LiCoO2 cathodes revealed that copper modification enhanced the specific capacity to 145.3 mAh/g in the first cycle,接近 the theoretical limit, while unmodified cells achieved only 116.2 mAh/g due to high polarization. The rate capability test showed that copper-modified cells maintained higher capacities at increased C-rates, as summarized in Table 1. The capacity retention after 500 cycles at 0.33 C was 93.5% for copper-modified cells, versus 81.6% for silver-modified and 46.3% for unmodified cells. This long-term stability is attributed to the sustained interfacial contact, which minimizes resistance growth and prevents active material loss.
| Modification Type | Initial Specific Capacity (mAh/g) | Interfacial ASR (Ω/cm²) | Critical Current Density (mA/cm²) | Capacity Retention after 500 Cycles (%) |
|---|---|---|---|---|
| Unmodified | 116.2 | 2030 | 0.4 | 46.3 |
| Silver-Modified | 130.2 | 66 | 0.8 | 81.6 |
| Copper-Modified | 145.3 | 65 | 1.2 | 93.5 |
The interfacial stability can be further analyzed using the strain energy density $U$ stored in the modification layer during cycling, given by:
$$U = \frac{1}{2} \sigma \epsilon$$
where $\sigma$ is the stress and $\epsilon$ is the strain. For copper, the low electrochemical activity results in smaller $\epsilon$ values, reducing $U$ and mitigating fatigue failure. In contrast, highly active materials like silver experience larger $\epsilon$ due to repeated alloying/dealloying, leading to crack formation and increased resistance. This mechanistic insight explains the superior performance of copper in solid-state batteries.
In summary, our study demonstrates that copper thin film modification effectively enhances the interfacial stability of solid-state lithium metal batteries by reducing volume strain and impedance. The electrochemically inert nature of copper minimizes interfacial degradation, leading to improved capacity retention and dendrite suppression. These findings highlight the importance of selecting modification materials based on their volume change characteristics, rather than solely on wetting properties. Future work will explore other low-activity materials and their scalability for industrial applications. This strategy paves the way for developing reliable high-energy-density solid-state batteries that meet the demands of next-generation electric vehicles.
The development of solid-state batteries is crucial for advancing energy storage technologies, and interfacial engineering plays a pivotal role in overcoming current limitations. By prioritizing mechanical stability over high reactivity, we can achieve longer-lasting and safer batteries. The copper modification approach presented here offers a scalable solution that could accelerate the commercialization of solid-state batteries, contributing to a sustainable energy future.