The pursuit of higher energy density has positioned lithium metal as the ultimate anode for next-generation batteries. However, its integration into conventional liquid electrolyte systems is plagued by dendrite growth, unstable solid electrolyte interphase (SEI), and severe safety hazards. The transition to solid-state batteries represents a paradigm shift, promising enhanced safety, superior energy density, and extended cycle life by replacing flammable organic liquids with solid-state electrolytes (SSEs). Among various SSEs, NASICON-type ceramics, particularly Li1.3Al0.3Ti1.7(PO4)3 (LATP), stand out due to their high ionic conductivity (often >10–4 S·cm–1 at room temperature), excellent chemical stability against air/moisture, and high mechanical strength (shear modulus ~40–60 GPa). These properties make LATP a compelling candidate for constructing robust solid-state batteries.
Despite these advantages, a critical roadblock impedes the direct application of LATP with lithium metal: interfacial instability. The tetravalent titanium (Ti4+) in the Lattice is thermodynamically unstable against reduction by lithium metal. Upon contact, a spontaneous reaction occurs, reducing Ti4+ to Ti3+. This process not only degrades the electrolyte structure, leading to the formation of resistive phases like Li3PO4, Li3P, and Li2O, but also introduces electronic conductivity within the interfacial region. The resulting mixed conductive interphase (MCI) can accelerate further reduction, cause structural collapse, and severely increase interfacial resistance, leading to rapid performance decay and failure of the solid-state battery. Therefore, engineering a stable, ion-conductive, and electron-blocking interface between Li and LATP is paramount for realizing practical NASICON-based solid-state batteries.
Our research addresses this fundamental challenge by implementing a novel interfacial modification strategy. We employ Prussian blue (PB), a metal-organic framework (MOF) with the formula Fe4[Fe(CN)6]3, as a conformal buffer layer on the LATP surface. This approach is distinct from common polymer coatings or in-situ formed conversion layers. The PB interlayer, after an initial lithiation process, transforms into a multifunctional interface with unique advantages for solid-state battery operation:
- Electrochemical Potential Regulation: The most distinctive feature is its redox potential positioning. The oxidation potential of lithiated PB is higher than the reduction potential of LATP’s Ti4+/Ti3+ couple. This creates an electronic energy barrier, effectively blocking electron transfer from the anode to the LATP bulk, thereby preventing its reduction.
- Dual (Ionic/Electronic) Conductivity with Structural Integrity: Unlike conversion reactions that lead to phase separation and porous, discontinuous interfaces, PB undergoes a topotactic Li+ insertion/extraction reaction. Its open, rigid framework remains intact, providing continuous, high-speed pathways for Li+ transport while maintaining sufficient electronic conductivity to ensure uniform current distribution.
- Interfacial Lithium Flux Homogenization: The three-dimensional ion diffusion channels in PB help to homogenize the Li+ flux at the interface, promoting uniform lithium deposition and stripping and suppressing dendrite initiation.
- Mechanical and Chemical Stability: The robust MOF structure offers mechanical resilience to accommodate volume changes at the lithium anode, while its chemical stability helps maintain a constant interface during cycling.
In this comprehensive study, we detail the synthesis and characterization of LATP electrolytes, the fabrication and application of the PB modification layer, and a thorough electrochemical evaluation. We demonstrate that this interface engineering strategy enables highly stable Li/Li symmetric cell cycling and significantly improves the performance of full solid-state batteries with both intercalation (LiFePO4) and conversion (FeF3) cathodes. The mechanisms behind the performance enhancement are elucidated through post-cycled interface morphology and composition analysis.
Materials Synthesis and Electrochemical Characterization of LATP Solid-State Electrolyte
The Li1.3Al0.3Ti1.7(PO4)3 solid-state electrolyte was synthesized via a conventional solid-state reaction route. Stoichiometric amounts of Li3PO4, Al2O3, TiO2, and (NH4)2HPO4 were ball-milled, pre-sintered at 900 °C, milled again, and finally pressed into pellets followed by sintering at 900 °C. The resulting ceramic pellets were dense, with a relative density exceeding 92% as measured by the Archimedes method.
X-ray diffraction confirmed the formation of a pure NASICON phase. Scanning electron microscopy revealed a well-sintered microstructure with grain sizes ranging from 2 to 5 μm and minimal porosity, consistent with the high density measured.
The ionic conductivity, the most critical parameter for a solid-state battery electrolyte, was determined by electrochemical impedance spectroscopy (EIS). The typical Nyquist plot consists of a depressed semicircle in the high-frequency region, corresponding to the grain boundary resistance, and a low-frequency tail representing electrode polarization. The total resistance (R) was extracted from the intercept of the semicircle with the real axis. The ionic conductivity (σ) was calculated using the formula:
$$ \sigma = \frac{d}{R \cdot S} $$
where d is the pellet thickness and S is the electrode area. The temperature-dependent conductivity followed the Arrhenius relationship:
$$ \ln(\sigma T) = \ln A – \frac{E_a}{k_B T} $$
where A is the pre-exponential factor, Ea is the activation energy for ion migration, kB is the Boltzmann constant, and T is the absolute temperature. The calculated values are summarized in the table below.
| Temperature (°C) | Ionic Conductivity (S·cm-1) | Activation Energy, Ea (eV) |
|---|---|---|
| 30 | 1.12 × 10-4 | 0.411 |
| 60 | 7.34 × 10-4 |
The high conductivity at 60 °C and moderate activation energy confirm LATP’s suitability as a fast ionic conductor for solid-state battery applications operated at mild temperatures.
Prussian Blue Interlayer: Rationale and Function Mechanism
Prussian blue analogues are a class of coordination polymers with an open framework structure. The PB used here, Fe4[Fe(CN)6]3, features a cubic lattice where FeII and FeIII ions are bridged by cyanide (C≡N) ligands, creating large interstitial sites and three-dimensional channels. These channels are ideal for rapid alkali ion transport. Furthermore, the extended π-conjugation through the cyanide bridges imparts a degree of electronic conductivity, making PB a mixed conductor.
The core innovation of using PB as an interlayer lies in its electrochemical potential alignment. To understand this, we consider the redox potentials of the involved components relative to Li/Li+:
- Lithium Metal Anode: ~0 V (Oxidation: Li → Li+ + e–)
- LATP Reduction: Ti4+ + e– → Ti3+ (Occurs at ~2.34 V vs. Li/Li+)
- Prussian Blue Redox: FeIII/FeII couple in the framework (Occurs at ~3.07 V vs. Li/Li+ for lithiated form)
This creates a unique potential profile at the interface: Li (0 V) | Lithiated PB (~3.07 V) | LATP (Ti4+/Ti3+ at ~2.34 V). During battery operation, lithium is oxidized at the anode, and Li+ ions travel through the PB layer into the LATP. However, electrons face a critical barrier. The lithiated PB, having a redox potential of ~3.07 V, is thermodynamically incapable of being oxidized by the LATP’s Ti4+ ions, which have a lower reduction potential of ~2.34 V. Consequently, electron transfer from the PB layer into the LATP bulk is effectively blocked. The PB layer thus acts as an “electron filter,” permitting the passage of Li+ ions while electronically insulating the LATP from the reducing environment of the lithium metal. This mechanism fundamentally suppresses the reduction degradation of LATP, which is the root cause of instability in unmodified Li/LATP interfaces for solid-state batteries.
Additionally, the rigid and porous MOF structure of PB ensures intimate contact with both the rigid LATP ceramic and the soft lithium metal. Upon initial lithiation, the PB layer becomes more lithophilic, further improving wettability and contact. Its structural stability upon cycling prevents pore formation and maintains a continuous ion-conduction pathway.
Electrochemical Performance Evaluation in Symmetric and Full Solid-State Battery Cells
To evaluate the efficacy of the PB modification, we assembled and tested three types of cells. A viscous ion liquid (IL, 1 M LiTFSI in [EMIM]TFSI) was applied as a wetting agent on both sides of the LATP pellet in all cells involving PB or direct comparison to ensure initial interfacial contact.
| Cell Configuration | Description | Key Test Conditions |
|---|---|---|
| Li/LATP/Li | Unmodified LATP, no wetting agent | Temperature: 60 °C Pressure: 25 MPa |
| Li/IL@LATP/Li | LATP with IL wetting, no PB | |
| Li/IL@PB@LATP/Li | LATP with PB layer and IL wetting |
1. Li/Li Symmetric Cell Cycling
Symmetric cell testing directly probes the stability of the Li/SSE interface during repeated lithium plating and stripping. The voltage hysteresis (overpotential) reflects the interfacial resistance and stability.
- Unmodified Cell (Li/LATP/Li): Exhibited large initial polarization and failed after only ~150 hours at 0.05 mA·cm–2, indicating severe and irreversible interfacial degradation.
- IL-wetted Cell (Li/IL@LATP/Li): Showed improved initial contact, leading to lower starting polarization. However, the polarization steadily increased over 350 hours before failure, suggesting gradual consumption of the IL and eventual exposure of LATP to Li.
- PB-modified Cell (Li/IL@PB@LATP/Li): Demonstrated exceptional stability. At 0.05 mA·cm–2, the cell cycled for over 800 hours with minimal and stable polarization (~60 mV single-side). Even at a higher current density of 0.1 mA·cm–2, stable cycling for 300 hours was achieved. The cell also showed good rate capability, with polarization smoothly adapting to changes in current density from 0.025 to 0.1 mA·cm–2 and back.
Analysis of the interfacial activation energy (Ea, interface) derived from Arrhenius plots of interface resistance provided further insight. While the Ea, interface for the IL-wetted cell fluctuated with cycling due to IL consumption and side product formation, the Ea, interface for the PB-modified cell remained relatively constant. This indicates that the PB-established interface provides a stable, ionically dominated transport channel throughout cycling, which is crucial for long-term operation of a solid-state battery.
2. Full Solid-State Battery Performance
The ultimate test of the interface is in a complete solid-state battery. We evaluated cells with LiFePO4 (LFP, intercalation cathode) and FeF3 (conversion cathode).
Li/IL@PB@LATP/LFP Cell:
The PB-modified solid-state battery delivered outstanding performance. At a current density of 0.025 mA·cm–2, the cell exhibited a high initial discharge capacity close to the theoretical value of LFP and, remarkably, retained nearly 200 mAh·g–1 after 160 cycles with an average Coulombic efficiency of 99%. This signifies extremely low parasitic side reactions at the anode interface. At a higher rate of 0.1 mA·cm–2, the capacity remained at 145 mAh·g–1 after 200 cycles. In stark contrast, the cell without the PB layer (Li/IL@LATP/LFP) suffered from rapid capacity fade under identical conditions. The PB layer’s role in stabilizing the interface is clearly the differentiating factor enabling such high-performance, long-cycling solid-state batteries.
Li/IL@PB@LATP/FeF3 Cell:
Conversion cathodes like FeF3 offer very high theoretical capacity but undergo large volume changes during cycling, posing a significant challenge for solid-state configurations. The PB-modified interface demonstrated excellent tolerance to this stress. The solid-state battery maintained a reversible discharge capacity above 300 mAh·g–1 for 60 cycles at 0.025 mA·cm–2. The rigid LATP electrolyte mechanically constrains the cathode expansion, while the stable PB interface on the anode side remains intact, allowing the cell to harness the high capacity of conversion materials in a solid-state battery architecture.
| Cell Type | Current Density (mA·cm-2) | Initial Discharge Capacity (mAh·g-1) | Capacity Retention / Cycle Life | Key Feature Demonstrated |
|---|---|---|---|---|
| Li/IL@PB@LATP/LFP | 0.025 | ~200 | ~200 mAh·g-1 after 160 cycles | Long-term interfacial stability |
| Li/IL@PB@LATP/LFP | 0.1 | ~160 | 145 mAh·g-1 after 200 cycles | Rate capability & cycling |
| Li/IL@PB@LATP/FeF3 | 0.025 | >400 | >300 mAh·g-1 after 60 cycles | Tolerance to cathode volume change |
Post-Cycling Interface Morphology and Composition Analysis
Direct observation and chemical analysis of the cycled interfaces provide conclusive evidence for the proposed mechanism.
Morphology: Cross-sectional SEM of a freshly prepared PB@LATP pellet showed a conformal PB layer, approximately 1 μm thick, adhering well to the LATP surface. After 50 cycles in a Li/LFP full solid-state battery, the surface of the PB-modified LATP remained relatively flat and dense, with only minor granular features. Crucially, the PB interlayer retained its thickness and cohesion. In contrast, the surface of cycled LATP from a cell without PB modification was severely damaged, showing micron-scale cracks and pores. This morphological degradation is a direct consequence of the uncontrolled reduction reaction and structural collapse of LATP, highlighting the protective role of the PB layer in a solid-state battery.
Composition (XPS): X-ray photoelectron spectroscopy on the LATP surface from a cycled PB-modified symmetric cell revealed key elements from the stabilized interface. Signals corresponding to Li-F species were detected, which are beneficial for a stable SEI due to LiF’s high mechanical and electrochemical stability. Nitrogen species from the residual IL and possibly cyanide groups were present. Significantly, no titanium signals were detected from the LATP substrate, confirming that the PB/SEI layer was sufficiently thick and continuous to completely shield the underlying LATP from side reactions and from the XPS probe itself. This compositional analysis corroborates that the PB interlayer successfully prevented the exposure and reduction of LATP, fulfilling its primary function in the solid-state battery.
Conclusion
In summary, we have successfully developed and demonstrated an effective interfacial engineering strategy to overcome the critical Li/LATP instability in NASICON-based solid-state batteries. The integration of a Prussian blue metal-organic framework as a buffer layer operates on the principle of electrochemical potential regulation. Its strategically higher redox potential electronically decouples the reducing lithium metal from the reducible Ti4+ in LATP, while its open framework ensures efficient and homogeneous Li+ transport. This dual functionality transforms a highly unstable interface into a robust and conductive one.
The electrochemical results are compelling: PB-modified Li/Li symmetric cells achieve unprecedented cycling stability over 800 hours; LiFePO4 full solid-state batteries exhibit excellent capacity retention over hundreds of cycles with near-perfect Coulombic efficiency; and FeF3 conversion-based solid-state batteries demonstrate feasible cycling, showcasing the interface’s tolerance to volume changes. Post-cycling characterization confirms the preservation of the LATP surface morphology and the formation of a stable, protective interfacial composition.
This work provides a fundamental and practical solution to a major bottleneck in ceramic-based solid-state batteries. The concept of using a potential-regulating MOF interlayer can be extended to other unstable solid electrolyte/anode pairs, paving the way for the development of high-energy-density, safe, and long-lasting all-solid-state lithium metal batteries.