Solid state batteries represent a transformative advancement in energy storage technology, offering enhanced safety, higher energy density, and improved cycle stability compared to conventional liquid electrolyte systems. Among various solid electrolytes, NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) stands out due to its high ionic conductivity, excellent chemical stability, and substantial shear modulus (40–60 GPa). However, the inherent reduction tendency of tetravalent titanium ions in LATP upon contact with lithium metal anodes leads to structural degradation and introduced electronic conductivity, posing significant challenges for practical applications. In this work, we address these issues by implementing a Prussian blue (PB) interfacial modification layer on LATP surfaces, which optimizes electrode-electrolyte contact and enhances the overall performance of solid state batteries. The PB layer, with its metal-organic framework (MOF) structure, provides abundant lithium-ion diffusion channels and unique redox properties that facilitate homogeneous ion flux while blocking electron transfer, thereby preventing LATP reduction. This approach significantly improves the cycling stability and kinetics of symmetric and full cells, underscoring the potential of interface engineering in advancing solid state battery technology.
The development of high-performance solid state batteries is critical for next-generation energy storage, as they eliminate flammable organic electrolytes and enable the use of high-capacity electrodes like lithium metal. NASICON-type LATP electrolytes exhibit room-temperature ionic conductivities exceeding 10–4 S·cm–1, making them promising candidates for solid state batteries. Nonetheless, the interfacial instability between LATP and lithium metal results in Ti4+ reduction to Ti3+, formation of resistive phases, and eventual cell failure. To mitigate these issues, we explored the use of PB as a multifunctional interlayer. PB’s open framework allows for rapid lithium-ion transport, and its electrochemical potential acts as a barrier to electron flow, thus preserving the integrity of the LATP electrolyte. Our investigations reveal that PB-modified interfaces not only enhance the wettability and mechanical stability but also promote uniform lithium deposition and stripping, leading to prolonged cycle life in solid state batteries.
In our experimental approach, we synthesized LATP solid electrolytes via a two-step solid-state reaction, ensuring high purity and density. The PB layer was applied through a simple doctor-blading technique, followed by infiltration with an ionic liquid (IL) wetting agent to improve interfacial contact. We assembled symmetric Li/Li cells and full cells with LiFePO4 (LFP) or FeF3 cathodes to evaluate the electrochemical performance. The ionic conductivity of LATP was determined using electrochemical impedance spectroscopy (EIS), and the interfacial properties were characterized through scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results demonstrate that PB modification effectively suppresses interfacial reactions and enhances the stability of solid state batteries under various cycling conditions.
The ionic conductivity of LATP was calculated using the formula: $$\sigma = \frac{d}{R \times S}$$ where \(\sigma\) is the ionic conductivity (S·cm–1), \(d\) is the thickness of the electrolyte (cm), \(R\) is the resistance obtained from EIS (\(\Omega\)), and \(S\) is the contact area (cm2). The Arrhenius equation was employed to determine the activation energy (\(E_a\)): $$\ln(\sigma T) = \ln C – \frac{E_a}{kT}$$ where \(T\) is the temperature (K), \(C\) is a pre-exponential constant, and \(k\) is the Boltzmann constant. Our measurements indicated that LATP exhibits an ionic conductivity of \(7.34 \times 10^{-4}\) S·cm–1 at 60°C with an activation energy of 0.411 eV, confirming its suitability for solid state battery applications.
| Temperature (°C) | Ionic Conductivity (S·cm–1) | Activation Energy (eV) |
|---|---|---|
| 30 | \(1.12 \times 10^{-4}\) | 0.411 |
| 40 | \(2.45 \times 10^{-4}\) | |
| 50 | \(4.78 \times 10^{-4}\) | |
| 60 | \(7.34 \times 10^{-4}\) |
The symmetric cell performance was evaluated to assess the impact of PB modification on interfacial stability. Cells configured as Li/IL@PB@LATP/Li demonstrated significantly reduced polarization and extended cycle life compared to unmodified cells. For instance, at a current density of 0.05 mA·cm–2, the PB-modified symmetric cell sustained stable cycling for over 800 hours with minimal voltage hysteresis, whereas cells without PB failed within 150 hours due to severe interfacial degradation. The enhanced performance is attributed to PB’s ability to homogenize lithium-ion flux and block electron transfer, as illustrated by the lower interfacial activation energy in modified cells. The Arrhenius plots derived from EIS data further confirmed that PB modification maintains consistent ion transport pathways, whereas unmodified cells exhibit increased activation energy over time due to parasitic reactions.
| Cell Configuration | Current Density (mA·cm–2) | Cycle Life (hours) | Polarization Voltage (mV) |
|---|---|---|---|
| Li/LATP/Li | 0.05 | 150 | >500 |
| Li/IL@LATP/Li | 0.05 | 350 | ~3000 |
| Li/IL@PB@LATP/Li | 0.05 | 800 | <100 |
| Li/IL@PB@LATP/Li | 0.1 | 300 | <150 |
Full cell evaluations with LFP and FeF3 cathodes further highlighted the benefits of PB modification in solid state batteries. The Li/IL@PB@LATP/LFP cell delivered a high discharge capacity of approximately 200 mAh·g–1 at 0.025 mA·cm–2, retaining nearly full capacity after 160 cycles with a Coulombic efficiency exceeding 99%. In contrast, cells without PB suffered from rapid capacity fade, achieving only 27.8 mAh·g–1 after 30 cycles. The superior performance is linked to PB’s role in facilitating reversible interfacial lithium storage and suppressing side reactions. Moreover, the Li/IL@PB@LATP/FeF3 cell maintained a discharge capacity above 300 mAh·g–1 for 60 cycles, demonstrating the tolerance of PB-modified interfaces to volume changes in conversion-type electrodes. These results underscore the versatility of PB in enhancing the durability and efficiency of solid state batteries.
The interfacial morphology and composition were analyzed using SEM and XPS to elucidate the mechanisms behind PB’s effectiveness. SEM images revealed that the PB layer, approximately 1 μm thick, adhered conformally to the LATP surface and remained intact after cycling, whereas unmodified LATP surfaces exhibited cracks and pores due to reduction reactions. XPS analysis of cycled PB-modified interfaces detected species such as LiF and Li–N compounds, which contribute to stable SEI formation and ion transport. The absence of Ti signals in XPS spectra confirmed that PB effectively isolates LATP from lithium metal, preventing reduction. The homogeneous structure of PB, even after lithiation, ensures continuous ion and electron integration, which is crucial for long-term stability in solid state batteries.
In conclusion, our study demonstrates that PB interfacial modification significantly enhances the performance of NASICON-type LATP-based solid state batteries. By leveraging PB’s unique redox potential and MOF structure, we achieved stable lithium plating/stripping, reduced interfacial resistance, and extended cycle life in both symmetric and full cells. The PB layer acts as an electron barrier while promoting uniform ion flux, addressing key challenges in solid state battery development. Future work will focus on optimizing the PB coating thickness and exploring other MOF materials to further improve the energy density and safety of solid state batteries. This strategy paves the way for practical high-energy-density storage systems, highlighting the importance of interface engineering in advancing solid state battery technology.
The electrochemical performance of solid state batteries is governed by the interplay between ionic and electronic conductivities at interfaces. The general equation for current density in a battery system can be expressed as: $$J = \sigma_i \nabla \phi_i + \sigma_e \nabla \phi_e$$ where \(J\) is the current density, \(\sigma_i\) and \(\sigma_e\) are the ionic and electronic conductivities, and \(\nabla \phi_i\) and \(\nabla \phi_e\) are the gradients of ionic and electronic potentials, respectively. In PB-modified interfaces, the high redox potential of PB creates a potential barrier that minimizes \(\sigma_e \nabla \phi_e\), thereby reducing electron leakage and enhancing the Coulombic efficiency of solid state batteries. This principle is essential for designing robust interfaces in next-generation energy storage devices.
| Parameter | Value | Description |
|---|---|---|
| PB Redox Potential | 3.07 V vs. Li/Li+ | Acts as electron barrier |
| LATP Reduction Potential | 2.34 V vs. Li/Li+ | Prevents Ti4+ reduction |
| Ionic Conductivity (60°C) | \(7.34 \times 10^{-4}\) S·cm–1 | Ensures efficient ion transport |
| Interfacial Activation Energy | 0.411 eV | Indicates stable ion pathways |
Overall, the integration of PB as an interfacial modifier in solid state batteries offers a promising solution to overcome the limitations of NASICON-type electrolytes. The ability to regulate electrochemical potentials while maintaining structural integrity makes PB an ideal candidate for enhancing the performance and longevity of solid state batteries. As research in solid state batteries progresses, such interface engineering strategies will be crucial for realizing their full potential in commercial applications, from electric vehicles to grid storage systems.