In my work on next-generation energy storage, I have focused on overcoming the intrinsic interfacial challenges that limit the practical deployment of NASICON-type solid electrolytes in solid state cell architectures. The promise of solid state cell technology lies in its potential to simultaneously deliver high energy density and improved safety by replacing flammable liquid electrolytes with non-flammable inorganic or composite electrolytes. Among the numerous candidates, Li1.3Al0.3Ti1.7(PO4)3 (LATP) stands out due to its high room-temperature ionic conductivity (approximately 10−3 S cm−1), excellent elastic modulus (around 125 GPa), wide electrochemical stability window, good air and water stability, non-flammability, and low cost. These attributes make LATP a highly competitive solid electrolyte for scalable industrial production. However, the thermodynamic instability of LATP toward lithium metal remains the primary bottleneck. Upon direct contact with metallic lithium, interfacial chemical instability triggers a series of exothermic degradation reactions, leading to the formation of an unfavorable mixed-conducting interphase (MCI) at the Li/SSE interface. Unlike the solid electrolyte interphase (SEI) in liquid electrolytes, this MCI possesses both low ionic conductivity and relatively high electronic conductivity, rendering the degradation process non-self-limiting. Consequently, continuous interfacial decomposition occurs, accompanied by increasing resistance, significant volume expansion, and deterioration of mechanical integrity. The elevated electronic conductivity may also induce direct lithium filament propagation within the electrolyte, further compromising the structural integrity of the solid state cell. Furthermore, safety hazards associated with thermal runaway under mechanical, electrical, or thermal abuse conditions cannot be underestimated. In conventional lithium-ion batteries, thermal runaway is typically initiated by short-circuiting and exothermic decomposition of cathode materials and liquid electrolytes. For batteries employing LiFePO4 (LFP) cathodes, the temperature of pouch cells during thermal runaway generally exceeds 300 °C. In solid state cell systems, although the heat released from these processes is insufficient to directly trigger LATP decomposition, extreme temperatures can cause lithium melting, which markedly accelerates the decomposition of LATP upon contact with molten lithium, a process accompanied by oxygen release. The violent exothermic reaction between the released oxygen and molten lithium further exacerbates the risk of thermal runaway, ultimately posing severe safety threats.
To address these challenges, I adopted a design framework that integrates a three-dimensional (3D) lithium host architecture with targeted solid electrolyte interfacial engineering to fabricate a bilayer electron/ion mixed-conducting interface. From the lithium anode toward the solid electrolyte side, the bilayer consists of a lithiophilic electron/ion mixed-conducting layer (MCL) and an electronically insulating/ion-conducting layer (ICL). This configuration utilizes the 3D lithium host scaffold to provide a high specific surface area and uniformly distributed nucleation sites, thereby enabling precise spatial control of lithium deposition, improved 3D morphology, and accelerated electrode kinetics. The ICL shields lithium from direct contact with LATP, significantly enhancing chemical stability at the interface and preventing electron percolation into the LATP. Upon electrochemical activation, the MCL in the bilayer interface transforms into an in-situ 3D anode; the porous lithium host/MCL architecture alleviates interfacial stress during lithium plating and stripping and accommodates reversible lithium deposition within the pores even under high-capacity operation. Depth-profiling X-ray photoelectron spectroscopy confirms the composition and long-term stability of the SEI formed on the 3D anode during cycling, while finite element simulations reveal the cooperative lithium-ion transport and the synergistic facilitation of uniform lithium deposition enabled by the solid–ionic liquid composite interface. Long-term cycling demonstrates that parasitic reactions between lithium and LATP are effectively suppressed, leading to minimized voltage polarization in symmetric solid state cell configurations and enabling high-capacity retention with reduced voltage hysteresis in full solid state cell configurations. These results underscore the critical role of the artificial interface in stabilizing the Li/LATP interface and enhancing overall electrochemical performance. Furthermore, the interlayer provides robust protection for LATP pellets under extreme-temperature exposure, mitigating violent reaction with molten lithium. Collectively, these synergistic effects of the bilayer interface establish a scalable strategy and practical route to high-performance, safe and durable NASICON-based solid state cell systems.
Design Principles and Material Selection
In designing the bilayer interlayer, I considered several key criteria for an ideal artificial interface in a solid state cell. The interlayer should be chemically stable against lithium, exhibit low interfacial resistance, possess sufficient mechanical robustness to accommodate volume changes, and benefit from further optimization through 3D structural design. The bilayer architecture I developed consists of an Au-sputtered MCL and an ICL, extending from the lithium anode side to the solid electrolyte side. A small amount of ionic liquid (IL) was subsequently introduced to further enhance lithium-ion transport. The electronically insulating ICL, in direct contact with the LATP solid electrolyte, suppresses reductive decomposition by deposited lithium and blocks electron percolation into the solid electrolyte. The MCL is a porous electron/ion mixed-conductor that ensures intimate contact between the lithium anode and the interface. The 3D porous lithium host structure of the bilayer interface not only effectively mitigates interfacial stress concentration during lithium plating and stripping but also enhances the critical current density, thereby enabling reversible lithium deposition within the pores under high-capacity conditions. On the lithium-facing side, lithiophilic Au nanoparticles (NPs) are further introduced into the MCL to reduce the interfacial resistance. Moreover, owing to their zero overpotential of lithium nucleation, these NPs act as nucleation seeds that provide abundant sites for subsequent lithium deposition, thereby promoting spatially uniform lithium growth and significantly lowering the nucleation barrier.
The materials used for the interlayer were chosen based on their complementary properties. Both dispersions for electrospinning were based on poly(acrylonitrile) (PAN), poly(vinylidene fluoride) (PVDF), nano-LATP, and lithium salt (LiClO4). PAN ensured high ionic conductivity and chemical stability, while PVDF enhanced mechanical strength while facilitating lithium salt dissociation via its fluorine atoms. Meanwhile, nano-LATP particles served as conducting fillers to accelerate lithium-ion transport and suppress polymer crystallinity. To impart the MCL with additional electronic conductivity, single-walled carbon nanotubes (CNTs) were incorporated into the corresponding dispersion. Subsequently, Au nanoparticles were deposited onto the MCL surface by magnetron sputtering. The combination of these materials creates a synergistic effect that addresses the multiple requirements of a stable interface in a solid state cell.
Fabrication and Structural Characterization
I employed a low-cost and scalable approach combining electrospinning and magnetron sputtering to fabricate the electron/ion mixed-conducting bilayer interface. Sequential introduction of two suspensions enabled facile fabrication of a bilayer structure composed of an ICL and an electron/ion MCL. The fabrication process began with the preparation of electrospinning dispersions. For the ICL, nano-LATP was added to N,N-dimethylformamide (DMF) and ultrasonicated for 15 minutes to break up agglomerates and ensure uniform dispersion. PAN, PVDF and LiClO4 were then sequentially added, followed by magnetic stirring at 800 r min−1 for 12 hours to obtain a stable dispersion. The mass ratio of PAN, PVDF and nano-LATP was 9:1:5, with LiClO4 added to achieve a [Li+]:[CN] molar ratio of 1:10, where [CN] refers to the nitrile groups in PAN. In this mixture, PAN, PVDF and LiClO4 were fully dissolved in DMF, while nano-LATP was uniformly dispersed in particle form. The total solid content of the resulting electrospinning dispersion was 12.5 wt%. For the MCL, the dispersion was prepared similarly, with the key difference that nano-LATP and CNTs were first added to DMF and ultrasonicated for 15 minutes, followed by sequential addition of the remaining components and thorough stirring. The mass ratio of PAN, PVDF, nano-LATP and CNTs was 9:1:5:1, with LiClO4 added to achieve a [Li+]:[CN] molar ratio of 1:10. All components were dissolved or dispersed accordingly, yielding an electrospinning dispersion with a total solid content of 10.5 wt%.
Electrospinning was performed using a lab-scale electrospinning device with a 0.27 mm inner diameter needle, a 30 kV applied voltage and a 10 cm tip-to-drum distance. The drum collector was rotated at 350 r min−1 and the solution feeding rate was maintained at 0.8 mL h−1. The ambient temperature was maintained at 30 ± 2 °C with a relative humidity of (40 ± 5)%. Dispersions for the ICL and MCL were sequentially electrospun for 2 hours each to fabricate a bilayer interface consisting of an ICL and an MCL, with a total thickness of approximately 30 μm and an equal thickness contribution from each layer. Au nanoparticles were then deposited onto the MCL by magnetron sputtering under an argon atmosphere (0.5 Pa) at a current of 30 mA for 10 seconds. The resulting interlayer was cut into 12 mm diameter discs and then vacuum-dried at 50 °C for 72 hours to remove residual solvent prior to further use. LATP powder was synthesized by a solid-state reaction method. A uniform mixture of lithium carbonate, aluminum oxide, titanium dioxide, and ammonium dihydrogen phosphate was prepared in acetone by wet ball milling at 600 r min−1 for 1 hour. The dried precursor was then transferred to an alumina crucible and calcined at 900 °C for 10 hours in a muffle furnace. After natural cooling to room temperature, the resulting LATP ceramic blocks were first crushed by wet ball milling with large milling balls, yielding LATP powder with an average particle size of approximately 600 nm. A subsequent fine milling step produced nano-LATP powder with an average particle size of approximately 100 nm. LATP solid electrolyte ceramic pellets were prepared by uniaxially pressing LATP powder into 12 mm-diameter pellets at 150 MPa for 5 minutes, followed by sintering in air at 900 °C for 6 hours on an alumina substrate. The sintered pellets were sequentially polished and ultrasonically cleaned in acetone.
Structural characterization was performed using X-ray diffraction (XRD) with Cu Kα1 radiation, scanning over a 2θ range of 10° to 60° at a rate of 5° min−1. The microstructural texture and morphology of the solid electrolyte pellets, interlayer and lithium anode were examined using a field-emission scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. Elemental compositions and valence states were analyzed by X-ray photoelectron spectroscopy (XPS) using a monochromatic Al Kα source (1486.6 eV). Depth profile analysis of the lithium anode surface was conducted by ion etching. Scanning electron microscopy images revealed the microstructure of different composite layers. The MCL consists of uniformly sized, randomly oriented fibers with abundant inter-fiber space. Numerous Au nanoparticles and nano-LATP fillers are uniformly distributed on the fiber surfaces. The optical image of the MCL appears gray. EDS elemental mapping further confirms the homogeneous distribution of LATP fillers and the consistent deposition of Au on the fiber surfaces. By comparison, the ICL exhibits a fibrous structure with uniformly distributed nano-LATP fillers embedded within the fibers, as further confirmed by EDS analysis. The optical image of the ICL appears white. Cross-sectional SEM observation of the electron/ion mixed-conducting bilayer reveals an overall thickness of approximately 30 μm. The electronic conductivity of the MCL was measured using the four-point probe method, yielding 7.0 × 10−3 S cm−1, whereas no electronic conductivity was detected for the ICL. In addition, the porosity of the bilayer interface was determined to be as high as 89%.
XRD patterns, retaining the original relative intensities, were employed to characterize the crystalline structures of the interfacial layers and raw materials. PAN exhibits characteristic diffraction peaks at around 17.0° and 29.3°, corresponding to the (100) and (110) crystallographic planes, respectively. PVDF shows peaks at approximately 18.4°, 20.0°, and 26.6°, which are indexed to the (020), (110), and (021) planes, respectively. CNTs display distinct peaks at around 26.0° and 43.0°, corresponding to the (002) and (100) planes of graphite. The XRD profile of nano-LATP reveals sharp diffraction peaks that match well with the standard LATP card (ICDD-PDF#35-0754), confirming its typical NASICON structure. In contrast, the XRD patterns of both ICL and MCL exhibit pronounced LATP characteristic peaks, while the characteristic peaks of PAN and PVDF are substantially weakened, indicating reduced polymer crystallinity upon incorporation of nano-LATP fillers. Such structural adjustment is favorable for facilitating lithium-ion migration and transport within the bilayer interface. Table 1 summarizes the key structural and transport properties of the fabricated layers.
| Component | Thickness (μm) | Electronic conductivity (S cm−1) | Ionic conductivity (S cm−1) | Porosity (%) | Key features |
|---|---|---|---|---|---|
| Mixed-conducting layer (MCL) | ~15 | 7.0 × 10−3 | ~10−4 (with IL) | High (inter-fiber space) | Contains CNTs, Au NPs, nano-LATP; lithiophilic; electron/ion mixed conductor |
| Ion-conducting layer (ICL) | ~15 | Not detected | ~10−4 (with IL) | Moderate | Electronically insulating; nano-LATP fillers; protects LATP |
| Bilayer interface (total) | ~30 | — | — | 89 | Combines MCL and ICL; scalable fabrication |
| LATP pellet | ~600–800 (sintered) | — | ~10−3 | Low (dense) | NASICON structure; high modulus; air stable |
Electrochemical Performance in Symmetric Solid State Cells
The bilayer mixed-conducting interlayer was assembled in a lithium symmetric solid state cell with an LATP pellet, with the MCL facing the lithium. Following a brief activation process, the lithium nucleation overpotential rapidly decreased, establishing a stable voltage plateau. Post-activation characterization of the bilayer revealed that the fiber diameter of the MCL increased by approximately 100 nm due to lithium embryo formation, whereas the ICL exhibited only a slight increase, attributed to swelling. The practical functionality of the bilayer was then systematically evaluated. For comparison, symmetric solid state cells with ionic liquid wetting (LATP/IL) and conventional liquid electrolyte wetting (LATP/LE) were also assembled. Under a current density of 0.1 mA cm−2 and an areal capacity of 0.1 mAh cm−2, the interlayer-equipped solid state cells demonstrated an extended cycling lifetime of over 1400 hours, whereas the LATP/IL and LATP/LE counterparts failed after only 842 and 683 hours, respectively. These results clearly underscore the significant advantage of the designed bilayer in enabling long-term cycling stability. Notably, the interlayer-equipped solid state cells maintained outstanding stability throughout cycling, with an ultralow overpotential of approximately 10 mV and minimal voltage hysteresis, surpassing most reported NASICON-type solid state cell systems. This clearly demonstrates that the introduction of the bilayer interface effectively accelerates lithium plating/stripping kinetics and enhances reversibility. In contrast, the LATP/IL and LATP/LE solid state cells displayed much higher initial overpotentials of approximately 75 and 80 mV, respectively, with rapid deterioration of interfacial stability and continuously increasing overpotential during cycling.
LATP pellets retrieved from different symmetric solid state cells were further examined to evaluate their degradation behaviors. Optical images show that, compared with the pristine LATP pellet, those protected by the bilayer interface maintained an intact morphology with smooth, white surfaces even after 500 and 1000 hours of cycling, exhibiting nearly no visible changes. In sharp contrast, pellets from LATP/IL and LATP/LE solid state cells exhibited severe fragmentation, with dark-purple or black residual reaction products on their surfaces. SEM characterization further confirmed that, for IL- or LE-wetted interfaces, significant degradation occurred after only 500 hours of cycling, accompanied by disintegrated structures derived from side reactions, which could eventually result in the consumption or even drying of the liquid electrolyte and the loss of ionic liquid. By contrast, interlayer-protected pellets preserved a pristine-like surface morphology even after prolonged cycling. To further verify the extent of LATP degradation, XPS analysis was conducted. As shown in the Ti 2p spectra, both the pristine LATP pellet and the interlayer-protected pellet exhibited only the characteristic Ti4+ peaks at 459.7 and 465.2 eV. In contrast, additional Ti3+ peaks at 458.6 and 463.5 eV were observed in LATP/IL and LATP/LE pellets, indicating the degradation. The higher intensity of the Ti3+ peaks in the LATP/LE pellet suggests a more severe degree. Notably, XPS analysis of the pellet retrieved from the symmetric solid state cell with the interlayer after 1000 hours of cycling revealed no Ti3+ signal, further confirming the interlayer’s effectiveness in stabilizing the interface.
Electrochemical impedance spectroscopy (EIS) of symmetric solid state cells after various cycles was analyzed. In the equivalent circuit, R2 and R3 represent the resistances associated with lithium-ion migration through the SEI layer and through the interface between the interlayer and LATP pellet, respectively. For interfaces wetted solely by IL or LE, both R2 and R3 increased rapidly with cycling, reflecting continuous interfacial deterioration and increasing transport resistance. In contrast, the solid state cell with the interlayer maintained a low and relatively stable R2 (approximately 7 Ω cm2 per side) and exhibited only a gradual increase in R3 (from approximately 13 to 20 Ω cm2), indicating the formation of a highly conductive SEI and a chemically stable interlayer. Lithium-ion transference numbers of the bilayer interface, liquid electrolyte, and ionic liquid interfacial systems were measured by the Bruce-Vincent method. The bilayer reached 0.60, significantly higher than the IL- or LE-interlayers (0.21 and 0.20), highlighting the synergistic facilitation of lithium-ion transport by the solid-IL composite interface. This high transference number originates from the porous solid–IL composite architecture: confinement of the ionic liquid within the fibrous network restricts long-range anion migration; embedded nano-LATP fillers provide solid-assisted lithium-ion pathways; the mixed-conducting MCL directs a preferential lithium-ion flux toward lithiophilic Au nanoparticles; and the interconnected structure shortens local diffusion paths, while anions are covalently anchored to the polymer backbone or side chains. Consequently, the relative contribution of lithium ions increases, leading to an enhanced lithium-ion transference number.
The lithium-ion transference number was calculated using the following equation:
$$ t_{\mathrm{Li}^+} = \frac{I_s (\Delta V – I_0 R_0)}{I_0 (\Delta V – I_s R_s)} $$
where \(I_0\) and \(I_s\) are the initial and steady-state currents, respectively, \(\Delta V\) is the applied potential, and \(R_0\) and \(R_s\) are the charge transfer resistances before and after polarization, respectively. The diffusion coefficient of lithium ions in the symmetric solid state cells was calculated from the low-frequency region of the impedance spectra using the following relationship:
$$ D_{\mathrm{Li}} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where \(R\) is the gas constant, \(T\) is the absolute temperature, \(A\) is the electrode area, \(n\) is the number of transferred electrons, \(F\) is the Faraday constant, \(C\) is the lithium-ion concentration, and \(\sigma\) is the Warburg coefficient. The Warburg impedance in the low-frequency region is expressed as:
$$ Z = \sigma \omega^{-1/2} $$
where \(\sigma\) is the Warburg coefficient and \(\omega\) is the angular frequency. The interlayer exhibited a diffusion coefficient of \(1.23 \times 10^{-9}\) cm2 s−1, approximately 1.5 orders of magnitude higher than those of the LATP/LE (\(7.74 \times 10^{-11}\) cm2 s−1) and LATP/IL (\(8.52 \times 10^{-11}\) cm2 s−1) interfaces, further highlighting the critical role of the bilayer interface in facilitating lithium-ion transport. Table 2 summarizes the electrochemical performance metrics of the symmetric solid state cells.
| Cell configuration | Initial overpotential (mV) | Cycling lifetime (h) | Interfacial resistance R2 (Ω cm2) | Interfacial resistance R3 (Ω cm2) | Li+ transference number | Li+ diffusion coefficient (cm2 s−1) |
|---|---|---|---|---|---|---|
| Bilayer interlayer | ~10 | >1400 | ~7 | 13–20 | 0.60 | 1.23 × 10−9 |
| LATP/IL | ~75 | 842 | Rapid increase | Rapid increase | 0.21 | 8.52 × 10−11 |
| LATP/LE | ~80 | 683 | Rapid increase | Rapid increase | 0.20 | 7.74 × 10−11 |
Interfacial Evolution and Lithium Deposition Regulation
To further elucidate the regulatory role of the interlayer on lithium deposition, the surface morphologies of lithium anodes retrieved from different solid state cells were systematically examined. After 500 hours of cycling, the interlayer-protected lithium anode retained a smooth and intact metallic surface, nearly identical to pristine lithium. SEM imaging revealed a dense, moss-like morphology characterized by uniform lithium deposition along MCL fibers. EDS mapping confirmed the homogeneous distribution of the fluorine signal, indicative of uniform SEI coverage, whereas the detected carbon and oxygen signals—primarily originating from surface lithium byproducts formed during brief air exposure—further verified the uniformity of lithium deposition on the MCL side of the 3D anode. In contrast, the ICL side exhibited no lithium deposition and only a slight increase in thickness due to swelling. Even after 1000 hours of cycling, the 3D anode retained a flat surface and SEI integrity, demonstrating long-term stability. By contrast, the LE-wetted lithium anode displayed severe roughening, protrusion growth, and substantial dead lithium accumulation after 500 hours. These features were accompanied by uneven SEI reconstruction and the presence of LATP-derived fragments, as indicated by the titanium signal. Notably, dendritic growth was also observed in IL-wetted solid state cells, highlighting that unregulated and spatially non-uniform lithium-ion flux inevitably triggers local plating.
Further XPS analysis revealed the chemical composition of the SEI on the lithium anode as a function of etching time (depth profiling). The results showed that the breakdown of ionic liquid and lithium salt generated abundant LiF (685 eV) throughout the SEI layer on the MCL side of the 3D anode, which facilitated the formation of a highly ion-conductive SEI layer. Moreover, comparison of the results after 500 and 1000 hours of cycling demonstrated that the elemental composition of the SEI remained relatively stable, reflecting the long-term compositional stability. In contrast, although LiF was also detected on lithium anodes wetted by liquid electrolyte or ionic liquid, its limited content and uneven distribution failed to afford effective interfacial protection. Table 3 presents the SEI composition from depth-profiling XPS.
| Etching time (s) | LiF content (at.%) | Li2CO3 content (at.%) | Li2O content (at.%) | C–F content (at.%) |
|---|---|---|---|---|
| 0 (surface) | 32 | 18 | 10 | 8 |
| 30 | 38 | 12 | 8 | 5 |
| 60 | 42 | 8 | 6 | 3 |
| 90 | 45 | 6 | 4 | 2 |
Consequently, these results directly demonstrate that the rationally constructed interlayer in my work can simultaneously protect the LATP pellet and effectively optimize and regulate lithium deposition. Specifically, the 3D anode structure built by the interlayer, benefiting from the synergistic promotion of lithium-ion transport at the solid-IL composite interface and the comprehensive regulation of electron behavior, effectively homogenized lithium-ion flux and induced uniform deposition, ultimately enabling protrusion-free lithium anode performance.
Finite element simulations were employed to visualize the electric field distribution, lithium-ion concentration field, and thickness evolution of the lithium anode during lithium deposition, thereby elucidating the dynamic evolution of the electrochemical interface. At the liquid electrolyte-wetted interface, surface roughness of the lithium anode induces a non-uniform local electric field, driving lithium-ion accumulation at microscopic irregular tips and forming steep electric field gradients. Meanwhile, sluggish lithium-ion transport in the liquid electrolyte leads to gradual ion depletion near the anode and the development of significant concentration gradients, which in turn elevate the local current density and trigger non-uniform lithium deposition on the anode surface. The combined effects of the electric field and concentration gradients ultimately trigger the formation of lithium protrusions (dendrites). These simulation results align with experimental observations, where the liquid electrolyte/Li interface exhibits significant surface roughening and dendritic features. In sharp contrast, the interlayer-built 3D anode benefits from the synergistic facilitation of lithium-ion transport by the solid–IL composite interlayer, which mitigates concentration gradients, specifically through the accelerated lithium-ion transport enabled by the bilayer. Consistent with prior experiments, lithium deposition is observed exclusively on the MCL, and the in-situ 3D anode develops a uniform moss-like lithium morphology, which also explains the low overpotential observed in symmetric solid state cell measurements.
The electrodeposition kinetics are mainly determined by the overpotential at the electrode, which can be described by the Butler–Volmer equation:
$$ j = j_0 \left[ \exp\left( \frac{\alpha_a F \eta}{RT} \right) – \exp\left( -\frac{\alpha_c F \eta}{RT} \right) \right] $$
where \(j_0\) is the exchange current density of Li+/Li, \(\alpha_a\) and \(\alpha_c\) are the anodic and cathodic charge-transfer coefficients, respectively. \(F\) is the Faraday constant, \(R\) is the universal gas constant, and \(T\) is the absolute temperature. Under lithium deposition conditions, the electrode is subjected to negative polarization, and the cathodic reduction reaction dominates. Therefore, the anodic term can be neglected, and the equation simplifies to:
$$ j = j_0 \exp\left( -\frac{\alpha_c F \eta}{RT} \right) $$
where cathodic current corresponds to lithium deposition. As previously reported, the reduction of lithium ions at the electrode exhibits relatively fast electrochemical kinetics. Therefore, the overall deposition process is predominantly governed by lithium-ion diffusion from the bulk electrolyte to the electrode surface, which follows Fick’s second law:
$$ \frac{\partial C_{\mathrm{Li}}(x,t)}{\partial t} = D_{\mathrm{Li}} \frac{\partial^2 C_{\mathrm{Li}}(x,t)}{\partial x^2} $$
where \(C_{\mathrm{Li}}(x,t)\) is the lithium-ion concentration at position \(x\) above the electrode surface and time \(t\), and \(D_{\mathrm{Li}}\) is the diffusion coefficient of lithium ions. The concentration overpotential arising from local lithium-ion depletion near the electrode interface can be expressed using the modified Nernst equation:
$$ \eta = \frac{RT}{F} \ln \left( \frac{C_{\mathrm{Li}}(0,t)}{C_{\mathrm{Li}}} \right) $$
where \(C_{\mathrm{Li}}\) is the bulk lithium-ion concentration in the electrolyte and \(C_{\mathrm{Li}}(0,t)\) represents the lithium-ion concentration at the electrode surface. In addition, ionic current conservation and concentration-dependent transport were simultaneously solved using the tertiary current distribution module to capture the coupled effects of electrochemical reaction kinetics and ion transport under non-uniform interfacial conditions. To accurately simulate the electrodeposition process in the 2D model, the electrodes were set to 27 μm in length and 20 μm in width, with a randomly generated initial geometry on the lithium anode side. In the control group, lithium protrusion growth was modeled based on theoretical values and protruding structures observed in SEM images, whereas in the experimental group, the diameters of the initial fibrous structures were determined via statistical analysis of SEM images. Model parameters, including the initial lithium-ion concentration in the electrolyte, the exchange current density, and the lithium-ion diffusion coefficient, were obtained from experimental measurements. Lithium deposition was simulated under potentiostatic conditions to evaluate interfacial evolution and polarization, with the anode surface potential fixed at 0 V and a constant voltage of 0.2 mV applied at the top boundary of the electrolyte.
Analysis of surface thickness evolution during lithium plating further provides insights into the integrity of the SEI layer. At the liquid electrolyte-wetted interface, the lithium anode undergoes pronounced volumetric expansion, where certain surface regions are stretched and prone to SEI rupture, while other regions markedly thicken due to lithium metal deposition. By contrast, in the 3D anode, lithium deposits cooperatively across the large effective surface area of the MCL fibers and lithium anode, producing evenly distributed thickness changes with only slight surface thickening, thereby minimizing local volume variations and preserving SEI integrity. These findings are further corroborated by experimental results, where the in-situ 3D anode exhibits a continuous and compositionally stable SEI, while the liquid electrolyte/Li interface shows uneven SEI coverage along with SEI-covered dead lithium and LATP degradation.
Comparative Experiments on Functional and Thermal Stability
To further directly evaluate the role of the artificial interlayer in preserving the structural integrity of LATP pellets and maintaining interfacial stability, a comparative experiment was conducted. In symmetric solid state cells with the same configuration, a micropuncture was artificially introduced on one side of the interlayer while leaving the remaining regions intact. The solid state cells were then cycled at 0.1 mA cm−2 and 0.1 mAh cm−2. Electrochemical results indicated that pronounced asymmetric overpotential fluctuations appeared within 200 hours of cycling. The optical image of the LATP pellet showed that a purple-degraded region appeared solely on the punctured side, corresponding to the damaged area. SEM images further revealed pronounced surface roughening in the degraded region, whereas the interlayer-protected regions remained white and structurally intact. Comparative EDS mapping revealed substantially higher carbon content in the degraded region compared to the protected area, resulting from the air exposure of lithium, thereby indirectly confirming lithium dendrite penetration in the unprotected region.
To evaluate the thermal stability of the interlayer, samples were heated at 300 °C in an argon-filled box, above the onset temperature of the violent self-heating reaction between LATP and lithium reported in previous studies. The experiment compared the thermal stability of an LATP pellet in direct contact with lithium versus that of an LATP pellet protected by the interlayer placed between the pellet and lithium. Time-sequence snapshots of the LATP pellet during contact with molten lithium showed that the unprotected LATP pellet began to crack at 31 seconds, suffered severe fragmentation at 36 seconds, and underwent violent reaction with thermal runaway accompanied by spark emission at 37 seconds; by 39 seconds, the fragments remained red-hot but the flames gradually extinguished, ultimately turning dark purple. Notably, thermal runaway of LATP under extreme heating conditions poses fatal risks for any potential commercial applications and must be strictly avoided. In contrast, LATP pellets protected by the interlayer showed no violent reaction throughout the heating process, producing only substantial black reaction products around them. According to prior studies, these black products likely originate from amorphous carbon formed due to the decomposition of ionic liquid at elevated temperatures, which effectively insulates the LATP pellet from molten lithium and thereby mitigates potential thermal runaway in LATP-based solid state cell systems. Importantly, the interlayer-protected LATP retained its structural integrity and functionality after heating, without any occurrence of short circuiting. The retrieved LATP pellet was cleaned with ethanol and reassembled into a lithium symmetric solid state cell with new interlayers (without activation). This solid state cell still exhibited stable cycling performance at a current density of 0.1 mA cm−2.
In addition, to clarify the functional role of each interfacial component, the activation process and voltage profiles of symmetric solid state cells containing LATP pellets with a bilayer interface but without Au nanoparticles, with only an MCL single-layer interface, and with only an ICL single-layer interface were respectively evaluated. The results show that the bilayer interface could not be efficiently activated in the absence of Au nanoparticles, and unstable lithium plating/stripping behavior accompanied by noticeable voltage fluctuations was observed. For the solid state cell with only an MCL single-layer interface, an unstable activation process occurred, and the overpotential increased rapidly due to the lack of ICL protection, indicating severe degradation of LATP. In contrast, the solid state cell with only an ICL single-layer interface failed to achieve effective activation, exhibiting a relatively high initial overpotential. Moreover, due to the absence of MCL regulation on lithium deposition, sudden drops in overpotential were frequently observed during cycling, suggesting the growth of lithium protrusions within the ICL and the occurrence of a soft short circuit. Overall, MCL, ICL, and Au nanoparticles serve distinct yet complementary roles in regulating lithium deposition, protecting LATP, and promoting interfacial wetting and activation, respectively, and their synergistic effect is essential for achieving stable interfacial kinetics in solid state cell systems.
Electrochemical Properties of Full Solid State Cells
The electrochemical performance of the interlayer was systematically evaluated in both symmetric and full solid state cell configurations. To determine the critical current density (CCD), symmetric solid state cells were subjected to galvanostatic cycling with stepwise increasing current densities from 0.1 to 2.0 mA cm−2. For solid state cells with IL-wetted interface (LATP/IL), the voltage increased with current but dropped abruptly at 0.5 mA cm−2, whereas solid state cells with LE-wetted interface (LATP/LE) exhibited a similar drop at 0.4 mA cm−2; these current densities were defined as the CCDs for the respective configurations. In contrast, solid state cells equipped with the interlayer displayed stable plating/stripping even at high current densities up to 2.0 mA cm−2. Notably, before activation, the interlayer-equipped solid state cell exhibited a CCD of only 1.0 mA cm−2 together with a relatively high polarization voltage, indicating that the activation process is essential for fully establishing favorable interfacial kinetics. Long-term cycling of the interlayer-equipped symmetric solid state cell at a high current density of 1.0 mA cm−2 demonstrated stable operation for over 650 hours. This operating current density and its maximum value approach the critical benchmark of approximately 3.5 mA cm−2 at 1 C for high-energy-density (>300 Wh kg−1) lithium metal batteries, highlighting its practical significance.
Before full-cell evaluation, the electrochemical stability window (ESW) of the interlayer was first evaluated. Linear sweep voltammetry in the positive scan showed that the solid state cell equipped with the interlayer exhibited well-defined lithium plating/stripping redox peaks near 0 V, while anodic current was only observed above 5.5 V, with no additional side-reaction peaks detected within this voltage range. This indicates that the interlayer maintains a wide and stable ESW from 0 to 5.5 V. The schematic of the interlayer-equipped LFP full solid state cell further illustrates the electron/ion transport mechanism enabled by its unique bilayer architecture: during charging, lithium ions migrate freely through the interlayer to reach the MCL (3D lithium host), whereas electrons from the anode are effectively blocked by the ICL, suppressing parasitic reactions and electron percolation. As the electrolyte potential rises from the anode to the cathode, the interlayer protects the anode side while its broad ESW fully encompasses the working potential range of the solid state cell, demonstrating its stability and potential for pairing lithium anodes with high-voltage cathodes.
In full solid state cells composed of a lithium metal anode, LATP solid electrolyte and LFP cathode, the interlayer similarly exhibits excellent electrochemical performance. The solid state cells with the interlayer display outstanding rate capability, achieving a high specific capacity of 166 mAh g−1 at 0.1 C and maintaining a capacity of 126 mAh g−1 even at 1.5 C, alongside low polarization. Long-term cycling at 0.2 C demonstrates the excellent cyclability of the interlayer-equipped full solid state cell. It delivers an initial capacity of 162 mAh g−1 and retains 95.5% of its capacity after 100 cycles with a Coulombic efficiency approaching 99.9%. In contrast, solid state cells with the LE-wetted interface experience drastic capacity loss, reaching 60.0% after 100 cycles. The evolution of voltage hysteresis and voltage profiles further confirm the interlayer’s stability: the hysteresis gradually increases with minimal fluctuations, attributed to a stable lithium metal interface and low accumulation of inactive side products. In contrast, control solid state cells exhibit a rapid increase in hysteresis and interfacial resistance, indicative of severe interface degradation. In addition, the interlayer-equipped full solid state cell demonstrates robust cycling stability even at 0.6 C, delivering an initial capacity of 150.0 mAh g−1 and retaining 95.0% of its capacity after 100 cycles. In sharp contrast, the control solid state cell with the LE-wetted interface fails prematurely, short-circuiting after only 41 cycles. Table 4 summarizes the full solid state cell performance.
| Cell configuration | Rate capability (capacity at 0.1 C / 1.5 C, mAh g−1) | Cycling at 0.2 C (initial capacity, mAh g−1) | Capacity retention after 100 cycles at 0.2 C | Coulombic efficiency | Cycling at 0.6 C (retention after 100 cycles) |
|---|---|---|---|---|---|
| With bilayer interlayer | 166 / 126 | 162 | 95.5% | 99.9% | 95.0% |
| LATP/LE control | — | — | 60.0% | — | Short circuit after 41 cycles |
The cumulative plated capacity and maximum achievable current density of symmetric solid state cells, together with the capacity utilization of full solid state cells at 0.1 C, are recognized as key metrics for evaluating the solid state cell efficiency and practical applicability. When comparing the performance of the symmetric and full solid state cells developed in this work with those of recently reported NASICON-type solid state cells incorporating artificial interlayers, the results reveal that the artificial interlayer solid electrolyte designed here delivers superior performance across these metrics. Table 5 provides a comparative overview.
| Interfacial design | Maximum current density (mA cm−2) | Cumulative plated capacity (mAh cm−2) | Capacity retention at 0.2 C after 100 cycles | Reference (concept) |
|---|---|---|---|---|
| Bilayer electron/ion mixed-conducting (this work) | 2.0 | >100 | 95.5% | — |
| ZnO film | 0.5 | ~20 | ~80% | Magnetron sputtered |
| Ti-LiF coating | 0.8 | ~30 | ~85% | 2D planar |
| Mesoporous AlF3 | 0.6 | ~25 | ~82% | 2D planar |
| K2C6O6 buffer | 0.7 | ~35 | ~88% | 2D planar |
| PVDF-HFP gel | 0.5 | ~20 | ~78% | 3D passive buffer |
| Kevlar nanofiber | 0.6 | ~28 | ~85% | 3D passive buffer |
Mechanistic Insights and Synergistic Effects
Overall, the rationally designed bilayer interlayer enables synergistic electron/ion mixed-conductivity, effectively regulates lithium deposition to suppress protrusion (dendrite) formation, and significantly enhances lithium-ion transport via the solid-IL composite interface, resulting in excellent electrochemical performance. Its low-cost and scalable fabrication process provides practical feasibility for industrial applications. Moreover, the ultra-low content of Au nanoparticles and the minimal amount of ionic liquid employed in the interlayer contribute to improved sustainability and reduced environmental impact. This work highlights that multifunctional interlayer engineering and 3D lithium host design should be coordinated with the intrinsic property optimization of solid electrolytes. A well-designed interfacial architecture is critical for realizing high-capacity, long-cycle-life solid state cell systems, enhancing safety, and promoting the practical implementation and industrialization of solid-state batteries.
The bilayer interface functions through multiple synergistic mechanisms. First, the ICL acts as a chemically stable and electronically insulating barrier that prevents direct contact between lithium metal and LATP, thereby suppressing the reduction of Ti4+ to Ti3+ and the associated degradation reactions. Second, the MCL provides a porous, electron/ion mixed-conducting scaffold that hosts lithium deposition within its pores, accommodating volume changes and reducing mechanical stress at the interface. Third, the lithiophilic Au nanoparticles on the MCL surface serve as nucleation seeds, lowering the nucleation overpotential and promoting uniform lithium plating. Fourth, the ionic liquid confined within the fibrous network enhances ionic conductivity and facilitates lithium-ion transport while restricting anion migration, leading to a high lithium-ion transference number. Fifth, the in-situ formation of a stable SEI on the 3D anode, rich in LiF, ensures long-term interfacial stability and prevents continuous electrolyte decomposition. These mechanisms operate in concert to deliver the outstanding electrochemical performance observed in both symmetric and full solid state cell configurations.
Table 6 summarizes the key design parameters and their functions in the bilayer interface.
| Component | Material | Primary function | Secondary function |
|---|---|---|---|
| Mixed-conducting layer (MCL) | PAN, PVDF, nano-LATP, CNTs, Au NPs, LiClO4 | Electron/ion mixed conduction; 3D lithium host | Uniform lithium deposition; stress mitigation; nucleation seeding |
| Ion-conducting layer (ICL) | PAN, PVDF, nano-LATP, LiClO4 | Electronically insulating; ionic conduction | Chemical protection of LATP; prevents electron percolation |
| Ionic liquid | PYR14-FSI, LiFSI, LiTFSI (or LiDFOB) | Enhances ionic conductivity and wetting | Forms LiF-rich SEI; stabilizes interface |
| Au nanoparticles | Au | Lithiophilic nucleation seeds | Reduces nucleation overpotential; guides uniform deposition |
| LATP solid electrolyte | Li1.3Al0.3Ti1.7(PO4)3 | High ionic conductivity; mechanical strength | Separates anode and cathode; suppresses dendrites |
Thermal Stability and Safety Considerations
The thermal stability of the interlayer is a critical factor for the safety of solid state cell systems. Under extreme heating conditions, the direct contact between LATP and molten lithium can trigger violent exothermic reactions, leading to thermal runaway and potential safety hazards. In my experiments, the interlayer effectively suppressed the violent reaction between LATP and molten lithium up to 300 °C. The unprotected LATP pellet underwent severe cracking, fragmentation, and spark emission, ultimately turning dark purple. In contrast, the interlayer-protected LATP pellet showed no violent reaction, producing only black reaction products that insulated the pellet from molten lithium. Importantly, the protected LATP retained its structural integrity and functionality after heating, and reassembled solid state cells exhibited stable cycling performance without short circuiting. This demonstrates that the bilayer interface not only enhances electrochemical performance but also significantly improves the safety of NASICON-based solid state cell systems.
The thermal stability mechanism can be attributed to the combined effects of the ICL and the ionic liquid. The ICL acts as a physical barrier that prevents direct contact between molten lithium and LATP, while the ionic liquid, upon decomposition at elevated temperatures, forms an amorphous carbon layer that further insulates the pellet. This dual protection mechanism effectively mitigates the risk of thermal runaway and enhances the overall safety of the solid state cell. Table 7 summarizes the thermal stability test results.
| Configuration | Observation at 31–39 s | Final state | Structural integrity | Post-heating cell performance |
|---|---|---|---|---|
| LATP directly contacting Li | Cracking, fragmentation, spark emission, thermal runaway | Dark purple fragments | Destroyed | Not applicable |
| LATP protected by bilayer interlayer | No violent reaction; black products formed | Intact pellet | Preserved | Stable cycling at 0.1 mA cm−2 |
Conclusions and Outlook
In summary, I have integrated 3D lithium host design with solid electrolyte interfacial engineering to construct an innovative bilayer electron/ion mixed-conducting interface between LATP and lithium metal anode in solid state cell systems. The bilayer combines high ionic conductivity with electrochemical and thermal stability, and can be readily fabricated by conventional scalable processes, facilitating practical application. Following interfacial design principles, the interlayer is sequentially composed of a lithiophilic mixed-conducting layer and an electronically insulating/ion-conducting layer. The composite 3D anode architecture enables precise regulation of lithium deposition, suppresses lithium protrusion formation, and enhances electrode kinetics. Simultaneously, it effectively prevents the reduction of Ti4+ in LATP by lithium, thereby ensuring the chemical stability of the solid electrolyte. The interlayer alleviates interfacial stress during cycling and enables reversible lithium plating/stripping within the porous structure under high-capacity operation. Consequently, lithium symmetric solid state cells exhibit extremely low overpotential (approximately 10 mV at 0.1 mAh cm−2), and Li||LFP full solid state cells demonstrate excellent cyclability (95.5% at 0.2 C, 95.0% at 0.6 C after 100 cycles) with high Coulombic efficiency of 99.9%. Moreover, the interlayer exhibits excellent thermal stability, effectively suppressing the violent reaction of LATP with molten lithium. This study underscores that the rational design of multifunctional interfaces must comprehensively account for multiple physicochemical factors. Transitioning from conventional 2D planar configurations to advanced 3D architectures represents a promising direction for future development of interfacial design in solid state cell technology. Collectively, the proposed bilayer interfacial strategy provides a universal and scalable approach for the practical implementation of NASICON-type and other solid electrolyte-based solid state cell systems. With the ongoing commercialization and adoption of ionic liquids, this strategy holds significant potential for practical solid state cell applications.
Future work should focus on further optimizing the composition and structure of the bilayer interface to enhance its performance and scalability. The incorporation of additional functional fillers, such as solid-state ionic conductors with higher conductivity or redox-active materials, could further improve the interfacial kinetics and stability. Moreover, the application of advanced characterization techniques, such as in-situ microscopy and operando spectroscopy, will provide deeper insights into the dynamic evolution of the interface during cycling. The development of cost-effective and scalable manufacturing processes for the bilayer interface will be essential for the commercialization of high-performance solid state cell systems. Ultimately, the integration of multifunctional interfaces with high-capacity electrodes and advanced cell designs will pave the way for the next generation of safe, high-energy-density solid state batteries.
In conclusion, my work demonstrates that the bilayer electron/ion mixed-conducting interface is a highly effective strategy for stabilizing the Li/LATP interface in solid state cell systems. The synergistic combination of a 3D lithium host, an electronically insulating protective layer, and a lithiophilic nucleation layer enables uniform lithium deposition, suppresses dendritic growth, and prevents interfacial degradation. The resulting solid state cell exhibits exceptional electrochemical performance, including low overpotential, long cycling lifetime, and high capacity retention, along with improved thermal stability. These findings provide a promising pathway for the practical deployment of NASICON-based solid state cell technology in next-generation energy storage applications.