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-state 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 instability of LATP when in contact with lithium metal anodes poses significant challenges, including reduction of Ti4+ to Ti3+, structural degradation, and introduction of electronic conductivity, which collectively undermine the performance of solid-state batteries. To address these issues, we developed an innovative interface modification strategy using Prussian blue (PB), a metal-organic framework (MOF) material, as a mixed conductive interlayer. This approach not only enhances the interfacial compatibility between LATP and lithium metal but also leverages the unique electrochemical potential of PB to block electron transfer while facilitating homogeneous lithium-ion flux. Our findings demonstrate that PB-modified LATP electrolytes enable stable cycling in both symmetric and full solid-state battery configurations, underscoring the potential of this method for developing high-performance energy storage devices.
The pursuit of advanced solid-state batteries has intensified due to the limitations of traditional lithium-ion batteries, such as safety concerns related to organic electrolytes and the growth of lithium dendrites. Solid-state batteries, which employ solid electrolytes instead of liquid ones, eliminate the risk of leakage and combustion, thereby enhancing safety. Moreover, the rigid nature of solid electrolytes can suppress dendrite formation, allowing for the use of high-capacity lithium metal anodes. The wider electrochemical window of solid electrolytes also enables compatibility with high-energy-density cathode materials, further boosting the energy density of solid-state batteries. Despite these advantages, the widespread adoption of solid-state batteries is hindered by challenges such as low ionic conductivity of solid electrolytes and poor interfacial contact with electrodes. NASICON-type LATP electrolytes exhibit promising ionic conductivity (e.g., >10–4 S·cm–1 at room temperature), but their susceptibility to reduction upon contact with lithium metal necessitates effective interface engineering. In this study, we focus on optimizing the LATP-lithium interface using PB, which serves as a multifunctional buffer layer to mitigate reduction reactions and promote uniform lithium deposition and stripping.
Our experimental methodology involved the synthesis of LATP solid electrolytes via a two-step solid-state reaction. Starting materials, including Li3PO4, Al2O3, TiO2, and (NH4)2HPO4, were ball-milled in ethanol, pre-sintered at 900 °C, and then pressed into pellets using polyvinyl alcohol (PVA) as a binder before final sintering. The resulting LATP pellets exhibited high density (92%) and a pure phase structure, as confirmed by X-ray diffraction (XRD). Prussian blue was synthesized by reacting Na4Fe(CN)6·10H2O with hydrochloric acid at 60 °C, yielding nanocubes with a well-defined MOF structure. The PB modification was applied to LATP surfaces through a simple blade-coating technique, denoted as PB@LATP. Additionally, an ionic liquid (IL) wetting agent, composed of LiTFSI in [EMIM]TFSI, was used to enhance interfacial contact. For electrochemical testing, symmetric Li/Li cells and full cells with LiFePO4 (LFP) or FeF3 cathodes were assembled in an argon-filled glovebox. The ionic conductivity of LATP was measured via electrochemical impedance spectroscopy (EIS), and the activation energy was derived from Arrhenius plots. Galvanostatic cycling tests were conducted at various current densities to evaluate the performance of the solid-state batteries.
The characterization of LATP revealed a rhombohedral crystal structure consistent with LiTi2(PO4)3 (PDF#35-0754), with no detectable impurities. Scanning electron microscopy (SEM) images showed dense microstructure with grain sizes of 2–5 μm, indicating effective sintering. The ionic conductivity of LATP was calculated using the formula:
$$ \sigma = \frac{d}{R \cdot S} $$
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). At 60 °C, the ionic conductivity reached 7.34 × 10–4 S·cm–1, while at 30 °C, it was 1.12 × 10–4 S·cm–1. The activation energy ((E_a)) for ion transport was determined from the Arrhenius equation:
$$ \ln\left(\frac{T}{R}\right) = \ln C – \frac{E_a}{kT} $$
where (T) is the temperature (K), (R) is the interfacial resistance, (C) is a pre-exponential constant, and (k) is the Boltzmann constant. The fitted activation energy for LATP was 0.411 eV, reflecting favorable ion mobility. These properties make LATP a suitable candidate for solid-state batteries, but its interfacial instability with lithium metal remains a critical issue.
Prussian blue, with its open framework structure, provides abundant channels for lithium-ion diffusion and exhibits mixed electronic-ionic conductivity. The redox potential of PB (approximately 3.07 V vs. Li/Li+) is higher than that of LATP reduction (around 2.34 V), creating an energy barrier that prevents electron transfer from lithium metal to LATP. This unique characteristic allows PB to act as an electron-blocking layer while facilitating lithium-ion transport. The lithiation of PB enhances its electronic conductivity and lithiophilicity, improving the intimacy of contact between LATP and lithium. Furthermore, the robust MOF structure of PB maintains mechanical stability during cycling, accommodating volume changes in the lithium anode and preventing phase separation. The integration of lithium-ion and electron flows within the PB layer promotes homogeneous current distribution, which is crucial for inhibiting dendrite growth and ensuring long-term cycle life in solid-state batteries.
To quantify the benefits of PB modification, we performed electrochemical tests on symmetric Li/Li cells. The cells were assembled with configurations including unmodified LATP (Li/LATP/Li), IL-wetted LATP (Li/IL@LATP/Li), and PB-modified LATP with IL wetting (Li/IL@PB@LATP/Li). The voltage profiles during lithium plating and stripping were recorded at current densities of 0.025, 0.05, and 0.1 mA·cm–2. The unmodified Li/LATP/Li cell exhibited large polarization and rapid failure within 150 hours at 0.05 mA·cm–2, attributed to severe interfacial reactions and poor contact. The IL-wetted cell showed improved initial performance but suffered from increasing polarization over time due to IL consumption and subsequent degradation. In contrast, the PB-modified cell demonstrated minimal polarization and stable cycling for over 800 hours at 0.05 mA·cm–2. At higher current densities, the superiority of PB modification became even more apparent, with stable operation at 0.1 mA·cm–2 for 300 hours. The polarization voltages for the PB-modified cell were consistently low, e.g., below 50 mV at 0.025 mA·cm–2 and around 150 mV at 0.1 mA·cm–2, indicating enhanced interfacial kinetics and stability.
The Arrhenius plots of interfacial resistance for symmetric cells provided further insights into the role of PB. For the unmodified Li/LATP/Li cell, the activation energy decreased with cycling time, suggesting increased electronic conduction due to reduction products. The IL-wetted cell initially showed lower activation energy due to improved wetting, but it increased with prolonged cycling as IL depletion led to interfacial passivation. The PB-modified cell maintained a stable activation energy, confirming that ion transport dominated the interface throughout cycling, with effective electron blocking. This stability is crucial for the long-term performance of solid-state batteries, as it prevents continuous degradation and maintains low impedance.
We extended our investigation to full solid-state batteries using LiFePO4 and FeF3 cathodes to evaluate the practical applicability of PB modification. The Li/IL@PB@LATP/LFP full cell delivered a high initial discharge capacity of nearly 200 mAh·g–1 at 0.025 mA·cm–2, exceeding the theoretical capacity of LFP (172 mAh·g–1) due to reversible interfacial lithium storage and minimal side reactions. After 160 cycles, the capacity retention was exceptional, with no significant decay and a Coulombic efficiency of 99%. At a higher current density of 0.1 mA·cm–2, the PB-modified cell retained a discharge capacity of 145 mAh·g–1 after 200 cycles, whereas the IL-wetted cell without PB suffered rapid capacity fade. The rate capability of the PB-modified cell was also impressive, with a discharge capacity of 176 mAh·g–1 at 0.2 mA·cm–2. These results highlight the effectiveness of PB in enabling high-performance solid-state batteries with excellent cycling stability and rate performance.
For conversion-type cathodes like FeF3, which undergo significant volume changes during cycling, the PB modification demonstrated remarkable tolerance. The Li/IL@PB@LATP/FeF3 full cell maintained a discharge capacity above 300 mAh·g–1 after 60 cycles at 0.025 mA·cm–2, underscoring the ability of the rigid LATP electrolyte and PB interlayer to constrain volume expansion and maintain electrode integrity. This is particularly important for next-generation solid-state batteries targeting high energy density, as conversion materials offer substantial capacity but often suffer from poor cycle life due to mechanical degradation.
Microstructural analysis of the PB-modified LATP interface revealed a conformal PB layer with a thickness of approximately 1 μm, adhering tightly to the LATP surface without gaps. After 50 cycles in a Li/IL@PB@LATP/LFP full cell, the LATP surface remained relatively smooth with uniform particle-like protrusions, indicating homogeneous lithium deposition. In contrast, the unmodified LATP surface developed micro-scale cracks and pores due to reduction-induced degradation. The cross-sectional SEM of cycled PB@LATP showed that the PB layer retained its thickness and structural integrity, confirming the stability of the modification. X-ray photoelectron spectroscopy (XPS) of the cycled interface detected elements such as C, F, Li, N, and O, with peaks corresponding to LiF, C–F, and nitrogen species from the IL. The presence of LiF, with its wide bandgap and electrochemical stability, contributes to dendrite suppression and interfacial protection. The absence of Ti signals in XPS confirmed that the PB layer effectively isolated LATP from lithium metal, preventing reduction reactions.
The following table summarizes the key electrochemical performance metrics of PB-modified solid-state batteries compared to unmodified and IL-wetted counterparts:
| Battery Configuration | Current Density (mA·cm–2) | Cycle Life (hours or cycles) | Polarization Voltage (mV) | Capacity Retention |
|---|---|---|---|---|
| Li/LATP/Li (unmodified) | 0.05 | 150 h | Large | Rapid failure |
| Li/IL@LATP/Li | 0.05 | 350 h | ~3000 | Gradual degradation |
| Li/IL@PB@LATP/Li | 0.05 | 800 h | <100 | Stable |
| Li/IL@PB@LATP/LFP | 0.025 | 160 cycles | Low | ~200 mAh·g–1 |
| Li/IL@PB@LATP/FeF3 | 0.025 | 60 cycles | Low | >300 mAh·g–1 |
The mechanism of PB interface regulation can be described by the following key equations, which model the ion and electron transport dynamics. The lithium-ion flux ((J_{Li^+})) through the PB layer is governed by Fick’s law and the Nernst-Planck equation:
$$ J_{Li^+} = -D \frac{\partial C}{\partial x} + \frac{zF}{RT} D C \frac{\partial \phi}{\partial x} $$
where (D) is the diffusion coefficient, (C) is the lithium-ion concentration, (x) is the spatial coordinate, (z) is the charge number, (F) is the Faraday constant, (R) is the gas constant, (T) is temperature, and (\phi) is the electrochemical potential. The electron flux ((J_e)) is blocked by the PB layer due to its higher redox potential, effectively reducing the electron transfer rate. The overall current density ((i)) in the solid-state battery can be expressed as:
$$ i = F \sum z_j J_j $$
where (j) denotes different charged species. The PB layer optimizes this by maximizing (J_{Li^+}) while minimizing (J_e), leading to enhanced efficiency and stability.
In conclusion, our study demonstrates that Prussian blue interface modification effectively addresses the interfacial challenges of NASICON-type LATP solid electrolytes in solid-state batteries. The PB layer serves as a multifunctional barrier that blocks electron transfer, prevents reduction of Ti4+, and promotes homogeneous lithium-ion transport. This results in significantly improved cycle life, reduced polarization, and high capacity retention in both symmetric and full solid-state battery configurations. The ability of PB to accommodate volume changes in conversion-type cathodes like FeF3 further highlights its versatility. These findings pave the way for the development of reliable and high-performance solid-state batteries, contributing to the advancement of next-generation energy storage systems. Future work will focus on optimizing the thickness and composition of the PB layer and exploring its application in other solid-state battery architectures to enhance scalability and commercial viability.
