I designed and evaluated a three-dimensional electron/ion mixed-conducting bilayer interface to stabilize the contact between a NASICON-type solid electrolyte and a lithium metal anode in a solid state battery. The central challenge I addressed is that NASICON-type solid electrolytes, especially Li1.3Al0.3Ti1.7(PO4)3 (LATP), offer high ionic conductivity, ambient stability, and low cost, but they are thermodynamically unstable against lithium metal. Direct contact between LATP and Li triggers continuous reduction of Ti4+ to Ti3+, formation of a mixed-conducting interphase, resistance growth, mechanical degradation, and eventually safety risks. In my solid state battery design, I separated the lithium metal from LATP using a bilayer architecture that combines a lithiophilic mixed-conducting layer (MCL) with an electronically insulating/ion-conducting layer (ICL). The MCL faces the lithium metal and acts as a three-dimensional lithium host, while the ICL faces LATP and blocks electron percolation. This architecture transforms the interface into an in-situ 3D anode during electrochemical activation. The resulting solid state battery exhibits low interfacial resistance, uniform lithium deposition, long cycling life, and improved thermal stability.
Design Principles and Material Selection
My design follows a simple but demanding set of requirements for a solid state battery interface. The interface must be chemically stable against lithium metal, electronically insulating on the solid electrolyte side, highly conductive for Li+, mechanically compliant, and capable of accommodating volume changes during plating and stripping. I selected a bilayer structure because a single layer cannot simultaneously satisfy these conflicting requirements. The MCL must be electron/ion mixed-conducting to ensure intimate contact with lithium and to provide abundant nucleation sites. The ICL must be ion-conducting but electronically insulating to protect LATP from reduction. I fabricated both layers by electrospinning, and I deposited gold nanoparticles by magnetron sputtering onto the MCL surface. The gold nanoparticles serve as lithiophilic seeds with low nucleation overpotential for lithium deposition. I also infiltrated a small amount of ionic liquid into the porous bilayer to enhance Li+ transport and interfacial wetting.
The polymer matrix I used is based on poly(acrylonitrile) (PAN) and poly(vinylidene fluoride) (PVDF). PAN provides high ionic conductivity, good chemical stability, and a nitrile coordination environment for Li+. PVDF enhances mechanical strength and, because of its fluorine atoms, promotes lithium salt dissociation. I added nano-LATP particles as conducting fillers to accelerate Li+ transport and suppress polymer crystallinity. For the MCL, I incorporated single-walled carbon nanotubes to introduce electronic conductivity. The ICL contains no carbon nanotubes, so it remains electronically insulating. The bilayer is porous, with a measured porosity of approximately 89%, which allows rapid ionic liquid uptake and provides free volume for lithium deposition. The ionic liquid uptake reaches about 825 wt% relative to the bilayer, which is consistent with the high porosity and fibrous architecture.
| Component | Composition | Function in solid state battery |
|---|---|---|
| MCL | PAN:PVDF:nano-LATP:CNT = 9:1:5:1 with LiClO4 | Electron/ion mixed conduction, 3D Li host, lithiophilic Au seed support, uniform Li deposition |
| ICL | PAN:PVDF:nano-LATP = 9:1:5 with LiClO4 | Ion conduction, electron blocking, chemical protection of LATP |
| Au nanoparticles | Magnetron sputtered on MCL | Lithiophilic nucleation seeds, reduced nucleation barrier |
| Ionic liquid | PYR14-FSI/LiFSI/LiTFSI = 85:10:5 molar ratio | Interfacial wetting, Li+ transport, SEI formation |
| LATP pellet | Li1.3Al0.3Ti1.7(PO4)3 | Solid electrolyte, mechanical separator, dendrite suppression |
Fabrication and Structural Characterization
I prepared two electrospinning dispersions. For the ICL, I ultrasonicated nano-LATP in N,N-dimethylformamide, then added PAN, PVDF, and LiClO4 under stirring. The mass ratio of PAN:PVDF:nano-LATP was 9:1:5, and the LiClO4 content was set to achieve a [Li+]:[CN] molar ratio of 1:10, where [CN] refers to the nitrile groups of PAN. The total solid content was 12.5 wt%. For the MCL, I ultrasonicated nano-LATP and single-walled carbon nanotubes together in DMF, then added PAN, PVDF, and LiClO4. The mass ratio of PAN:PVDF:nano-LATP:CNT was 9:1:5:1, with the same [Li+]:[CN] ratio. The total solid content was 10.5 wt%. I electrospun the two dispersions sequentially for 2 h each, producing a bilayer with a total thickness of about 30 µm, with roughly equal thickness from each layer. I then sputtered Au nanoparticles onto the MCL under argon at 0.5 Pa, 30 mA for 10 s. The final interlayer was cut into discs, vacuum-dried at 50 °C for 72 h, and stored in an argon-filled glovebox.
The scanning electron microscopy images showed that the MCL consists of randomly oriented fibers with abundant inter-fiber space. Gold nanoparticles and nano-LATP fillers are uniformly distributed on the fiber surfaces. Energy-dispersive X-ray spectroscopy confirmed the homogeneous distribution of LATP fillers and gold on the MCL. The ICL also has a fibrous structure with nano-LATP embedded inside the fibers, but no gold or carbon nanotubes. The cross-sectional image revealed a clear bilayer with a total thickness near 30 µm. The electronic conductivity of the MCL, measured by a four-point probe, is \(7.0 \times 10^{-3}\) S cm−1, while the ICL shows no detectable electronic conductivity. X-ray diffraction confirmed that the nano-LATP retains its NASICON structure, and the characteristic peaks of PAN and PVDF are weakened after filler incorporation, indicating reduced polymer crystallinity. This structural change favors Li+ migration.
| Property | MCL | ICL |
|---|---|---|
| Electronic conductivity | \(7.0 \times 10^{-3}\) S cm−1 | Not detectable |
| Ionic conduction | High, assisted by nano-LATP and IL | High, assisted by nano-LATP and IL |
| Porosity | Contributing to 89% total bilayer porosity | Contributing to 89% total bilayer porosity |
| Thickness | About 15 µm | About 15 µm |
| Ionic liquid uptake | About 825 wt% for the bilayer | About 825 wt% for the bilayer |
Interfacial Stability and Symmetric Cell Performance
I assembled symmetric Li||Li cells with LATP pellets and the bilayer interface, with the MCL facing the lithium metal. For comparison, I also assembled cells wetted with ionic liquid (LATP/IL) and cells wetted with a conventional liquid electrolyte (LATP/LE). After a short activation step, the lithium nucleation overpotential decreased rapidly, and a stable voltage plateau formed. The activated MCL fibers increased in diameter by about 100 nm because of lithium embryo formation, while the ICL showed only slight swelling. In long-term cycling at 0.1 mA cm−2 and 0.1 mAh cm−2, the bilayer-equipped solid state battery cycled for more than 1400 h. The LATP/IL and LATP/LE cells failed after 842 h and 683 h, respectively. The bilayer cell maintained an ultralow overpotential of about 10 mV, while the control cells started with overpotentials of about 75 mV and 80 mV and deteriorated rapidly.
I retrieved the LATP pellets after cycling and examined their morphology. The pellets protected by the bilayer remained intact and white, with smooth surfaces, even after 500 h and 1000 h. In contrast, pellets from LATP/IL and LATP/LE cells fragmented and developed dark-purple or black reaction products. X-ray photoelectron spectroscopy of the Ti 2p region showed only Ti4+ peaks at 459.7 eV and 465.2 eV for the pristine and bilayer-protected pellets. The LATP/IL and LATP/LE pellets showed additional Ti3+ peaks at 458.6 eV and 463.5 eV, confirming reduction of Ti4+. The LATP/LE pellet had stronger Ti3+ signals, indicating more severe degradation. After 1000 h of cycling, the bilayer-protected pellet still showed no Ti3+ signal.
| Cell configuration | Cycling life at 0.1 mA cm−2 | Initial overpotential | LATP degradation |
|---|---|---|---|
| Bilayer interface | >1400 h | About 10 mV | No Ti3+ signal |
| LATP/IL | 842 h | About 75 mV | Ti3+ detected |
| LATP/LE | 683 h | About 80 mV | Strong Ti3+ signal |
I used electrochemical impedance spectroscopy to quantify interfacial resistance. In the equivalent circuit, \(R_2\) and \(R_3\) represent the resistance of Li+ migration through the SEI layer and through the interface between the interlayer and LATP, respectively. For cells wetted only with IL or LE, both \(R_2\) and \(R_3\) increased rapidly with cycling. In the bilayer cell, \(R_2\) remained low and stable at about \(7\ \Omega\) cm2 per side, and \(R_3\) increased only gradually from about 13 to 20 \(\Omega\) cm2. This indicates a highly conductive SEI and a chemically stable interlayer. I also measured the Li+ transference number using the Bruce–Vincent method. The bilayer interface reached 0.60, while the IL and LE systems reached only 0.21 and 0.20, respectively. The high transference number arises from several effects: confinement of the ionic liquid within the fibrous network restricts long-range anion migration; embedded nano-LATP provides solid-assisted Li+ pathways; the mixed-conducting MCL directs Li+ flux toward lithiophilic Au nanoparticles; and the interconnected structure shortens diffusion paths while anions are anchored to the polymer backbone or side chains.
$$ t_{\mathrm{Li}^+} = \frac{I_s(\Delta V – I_0 R_0)}{I_0(\Delta V – I_s R_s)} $$
In this expression, \(I_0\) and \(I_s\) are the initial and steady-state currents, \(\Delta V\) is the applied potential, and \(R_0\) and \(R_s\) are the charge-transfer resistances before and after polarization. The high \(t_{\mathrm{Li}^+}\) of my bilayer interface directly supports uniform Li+ flux and suppresses anion depletion near the lithium surface. The interfacial evolution I observed can be summarized as follows: the LATP/LE interface loses liquid electrolyte because of cell pressure, side reactions cause pellet fracture and discoloration, and lithium protrusions roughen the lithium surface. In contrast, the bilayer provides free space for capillary-driven ionic liquid infiltration and stable retention. During activation, the MCL offers abundant nucleation sites and lithium accommodation space, while its mixed conductivity guides uniform deposition. The ICL isolates lithium from LATP and acts as a buffer reservoir. The interface evolves into a robust 3D mixed-conducting lithium anode, enabling cooperative Li+ plating and stripping across the MCL and lithium surface.
Lithium Deposition and SEI Evolution
I examined the surface morphology of lithium anodes retrieved from different cells. After 500 h of cycling, the bilayer-protected lithium anode retained a smooth and intact metallic surface, nearly identical to pristine lithium. Scanning electron microscopy revealed a dense, moss-like morphology with uniform lithium deposition along MCL fibers. Energy-dispersive X-ray spectroscopy showed a homogeneous fluorine signal, indicating uniform SEI coverage. Carbon and oxygen signals originated mainly from surface lithium byproducts formed during brief air exposure, but their distribution confirmed uniform lithium deposition on the MCL side. The ICL side showed no lithium deposition and only a slight thickness increase due to swelling. Even after 1000 h of cycling, the 3D anode retained a flat surface and SEI integrity. In contrast, the LE-wetted lithium anode showed severe roughening, protrusion growth, and substantial dead lithium accumulation after 500 h. The SEI was unevenly reconstructed, and LATP-derived fragments were present, as indicated by titanium signals. Dendritic growth was also observed in IL-wetted cells. These results show that unregulated and spatially non-uniform Li+ flux inevitably triggers local plating.
I performed depth-profiling X-ray photoelectron spectroscopy to determine the SEI composition on the lithium anode as a function of etching time. The breakdown of the ionic liquid and lithium salts generated abundant LiF at 685 eV throughout the SEI on the MCL side of the 3D anode, which facilitated the formation of a highly ion-conductive SEI. Comparison of the results after 500 h and 1000 h showed that the elemental composition remained relatively stable, reflecting long-term compositional stability. In cells wetted by LE or IL, LiF was also detected, but its content was limited and its distribution was uneven, so it failed to provide effective interfacial protection. The SEI on my 3D anode therefore combines uniformity, high LiF content, and long-term stability.
| SEI feature | Bilayer-built 3D anode | LE-wetted Li anode | IL-wetted Li anode |
|---|---|---|---|
| Surface morphology | Smooth, moss-like Li | Rough, protrusions, dead Li | Dendritic growth |
| LiF distribution | Uniform, abundant | Limited, uneven | Limited, uneven |
| SEI stability after 1000 h | Stable composition | Degraded | Degraded |
| LATP fragments | Absent | Present | Possible |
Finite Element Simulation of Deposition
I used finite element simulations to visualize the electric field distribution, Li+ concentration field, and thickness evolution during lithium deposition. At the LE-wetted interface, surface roughness of the lithium anode induces a non-uniform local electric field, which drives Li+ accumulation at microscopic tips and forms steep electric field gradients. Sluggish Li+ transport in the liquid electrolyte leads to gradual ion depletion near the anode and significant concentration gradients. These gradients elevate the local current density and trigger non-uniform lithium deposition. The combined effects of electric field and concentration gradients ultimately produce lithium protrusions. These simulation results agree with my experimental observations of surface roughening and dendritic features at the LE/Li interface.
In contrast, the interlayer-built 3D anode benefits from the synergistic facilitation of Li+ transport by the solid–ionic liquid composite interlayer, which mitigates concentration gradients. Lithium deposition occurs exclusively on the MCL, and the in-situ 3D anode develops a uniform moss-like morphology. This explains the low overpotential observed in symmetric cell measurements. Analysis of surface thickness evolution further shows that at the LE-wetted interface, the lithium anode undergoes pronounced volumetric expansion, with some regions stretched and prone to SEI rupture, while other regions thicken markedly because of lithium deposition. In the 3D anode, lithium deposits cooperatively across the large effective surface area of the MCL fibers and the lithium anode, producing evenly distributed thickness changes with only slight surface thickening. This minimizes local volume variations and preserves SEI integrity.
$$ j = j_0 \left[ \exp\left(\frac{\alpha_a F \eta}{RT}\right) – \exp\left(-\frac{\alpha_c F \eta}{RT}\right) \right] $$
For the simple single-electron reaction \(\mathrm{Li}^+ + e^- \rightleftharpoons \mathrm{Li}\), the electrodeposition kinetics are mainly determined by the overpotential at the electrode. In this Butler–Volmer expression, \(j_0\) is the exchange current density, \(\alpha_a\) and \(\alpha_c\) are the anodic and cathodic charge-transfer coefficients, \(F\) is the Faraday constant, \(R\) is the universal gas constant, and \(T\) is the absolute temperature. Under lithium deposition conditions, the electrode is negatively polarized, and the cathodic reduction dominates. The anodic term can therefore be neglected:
$$ j \approx j_0 \exp\left(-\frac{\alpha_c F \eta}{RT}\right) $$
Because the reduction of lithium ions at the electrode exhibits fast electrochemical kinetics, the overall deposition process is predominantly governed by Li+ diffusion from the bulk electrolyte to the electrode surface, which follows Fick’s second law:
$$ \frac{\partial C_{\mathrm{Li}}(x,t)}{\partial t} = D_{\mathrm{Li}} \frac{\partial^2 C_{\mathrm{Li}}(x,t)}{\partial x^2} $$
The concentration overpotential arising from local Li+ depletion near the electrode interface can be expressed using the modified Nernst equation:
$$ \eta = \frac{RT}{F} \ln\left(\frac{C_{\mathrm{Li}}(0,t)}{C_{\mathrm{Li,bulk}}}\right) $$
I solved ionic current conservation and concentration-dependent transport using a tertiary current distribution module to capture the coupled effects of electrochemical reaction kinetics and ion transport under non-uniform interfacial conditions. The model parameters, including the initial Li+ concentration, exchange current density, and Li+ diffusion coefficient, were obtained from experimental measurements. The simulations confirm that the bilayer interface smooths the electric field, reduces Li+ depletion, and distributes deposition over a large 3D surface.
| Parameter | LE-wetted interface | Bilayer-built 3D anode |
|---|---|---|
| Electric field uniformity | Non-uniform, tip enhancement | Uniform, fiber-mediated |
| Li+ concentration gradient | Steep, local depletion | Mild, continuous supply |
| Deposition morphology | Protrusions and dendrites | Moss-like, uniform |
| SEI integrity | Prone to rupture | Preserved |
| Volume change | Localized thickening | Evenly distributed |
Puncture and Thermal Stability Tests
To evaluate the protective role of the interlayer more directly, I introduced a micropuncture on one side of the interlayer in a symmetric cell while leaving the rest intact. After cycling at 0.1 mA cm−2 and 0.1 mAh cm−2, pronounced asymmetric overpotential fluctuations appeared within 200 h. The LATP pellet showed a purple degraded region only on the punctured side. Scanning electron microscopy revealed pronounced surface roughening in the degraded region, while the interlayer-protected regions remained white and structurally intact. Energy-dispersive X-ray spectroscopy showed substantially higher carbon content in the degraded region than in the protected area, which resulted from air exposure of lithium and indirectly confirmed lithium dendrite penetration in the unprotected region. This experiment demonstrates that the bilayer must remain continuous to protect the solid electrolyte, and that a local defect can initiate degradation.
I also tested thermal stability at 300 °C in an argon-filled box, which is above the melting point of lithium and above the onset temperature of the violent self-heating reaction between LATP and lithium. The unprotected LATP pellet in direct contact with lithium began to crack at 31 s, fragmented severely at 36 s, and underwent violent reaction with thermal runaway and spark emission at 37 s. By 39 s, the fragments remained red-hot, but the flames gradually extinguished, and the pellet ultimately turned dark purple. In contrast, the LATP pellet protected by the interlayer showed no violent reaction throughout the heating process. Black reaction products formed around the pellet, likely amorphous carbon from decomposition of the ionic liquid at elevated temperatures, which insulated the LATP pellet from molten lithium and mitigated thermal runaway. Importantly, the interlayer-protected LATP retained structural integrity and functionality after heating, without short circuiting. I cleaned the retrieved LATP pellet with ethanol and reassembled it into a lithium symmetric cell with new interlayers, and the cell still exhibited stable cycling at 0.1 mA cm−2. This thermal test shows that the bilayer interface provides a robust safety barrier for a solid state battery under extreme thermal abuse.
| Test condition | Unprotected LATP + Li | LATP + bilayer + Li |
|---|---|---|
| 300 °C exposure | Cracking at 31 s, fragmentation, spark emission | No violent reaction, carbonaceous insulation |
| Post-heating structure | Fragmented, dark purple | Intact, functional |
| Short circuit | Severe risk | No short circuit observed |
| Reassembly cycling | Not possible | Stable at 0.1 mA cm−2 |
Role of Each Interfacial Component
I evaluated symmetric cells with different interface configurations to clarify the role of each component. When the bilayer lacked Au nanoparticles, the interface could not be activated efficiently, and unstable lithium plating and stripping occurred with noticeable voltage fluctuations. When only the MCL was used, the activation process was unstable, and the overpotential increased rapidly because of the lack of ICL protection, indicating severe LATP degradation. When only the ICL was used, the cell failed to achieve effective activation and exhibited a relatively high initial overpotential. Because the MCL was absent, lithium deposition was not regulated, and sudden drops in overpotential appeared during cycling, suggesting growth of lithium protrusions within the ICL and soft short circuits. These comparisons show that the MCL, ICL, and Au nanoparticles serve distinct but complementary roles. The MCL regulates lithium deposition and provides a 3D host. The ICL protects LATP and blocks electrons. The Au nanoparticles promote interfacial wetting and activation. Their synergistic effect is essential for stable interfacial kinetics in the solid state battery.
| Interface configuration | Activation behavior | Cycling stability | Main failure mode |
|---|---|---|---|
| Bilayer without Au | Inefficient activation | Unstable voltage fluctuations | Poor nucleation and wetting |
| MCL only | Unstable activation | Rapid overpotential increase | LATP degradation without ICL |
| ICL only | No effective activation | Sudden overpotential drops | Li protrusion and soft short |
| Full bilayer with Au | Fast and stable activation | Long-term stable cycling | None observed in tested period |
Electrochemical Performance of Solid State Battery Full Cells
I determined the critical current density by galvanostatic cycling with stepwise increasing current density from 0.1 to 2.0 mA cm−2. For the IL-wetted interface, the voltage increased with current and dropped abruptly at 0.5 mA cm−2. For the LE-wetted interface, a similar drop occurred at 0.4 mA cm−2. These values define the critical current densities for the control configurations. In contrast, the cell with the bilayer interface displayed stable plating and stripping up to 2.0 mA cm−2. Before activation, the bilayer cell had a critical current density of only 1.0 mA cm−2 and a relatively high polarization voltage, indicating that activation is essential for establishing favorable interfacial kinetics. After activation, long-term cycling at 1.0 mA cm−2 was stable for more than 650 h. This operating current density approaches the benchmark of about 3.5 mA cm−2 at 1 C for high-energy-density lithium metal batteries, which highlights the practical significance of my interface design.
I also analyzed Li+ transport across the different interfacial systems. The bilayer interface exhibits 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 and LATP/IL interfaces, which are \(7.74 \times 10^{-11}\) cm2 s−1 and \(8.52 \times 10^{-11}\) cm2 s−1, respectively. This result further highlights the critical role of the bilayer interface in facilitating Li+ transport.
$$ D_{\mathrm{Li}} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma_W^2} $$
In this diffusion coefficient expression, \(R\) 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 \(\sigma_W\) is the Warburg coefficient. The Warburg impedance in the low-frequency region is described by:
$$ Z_W = \sigma_W \omega^{-1/2} $$
I evaluated the electrochemical stability window by linear sweep voltammetry in a Li||interlayer||stainless steel asymmetric cell. The bilayer-equipped cell exhibited well-defined lithium plating and stripping redox peaks near 0 V, while anodic current was observed only above 5.5 V. No additional side-reaction peaks were detected within this voltage range. This indicates that the interlayer maintains a wide and stable electrochemical stability window from 0 to 5.5 V. The schematic of the interlayer-equipped Li||LiFePO4 full cell illustrates the electron and ion transport mechanism enabled by the bilayer architecture. During charging, Li+ migrates freely through the interlayer to reach the MCL, which acts as the 3D lithium host, while 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 electrochemical stability window fully encompasses the working potential range of the cell. This demonstrates its stability and potential for pairing lithium anodes with high-voltage cathodes.
| Interfacial system | Critical current density | Li+ diffusion coefficient (cm2 s−1) | Li+ transference number |
|---|---|---|---|
| Bilayer interface | Up to 2.0 mA cm−2 after activation | \(1.23 \times 10^{-9}\) | 0.60 |
| LATP/IL | 0.5 mA cm−2 | \(8.52 \times 10^{-11}\) | 0.21 |
| LATP/LE | 0.4 mA cm−2 | \(7.74 \times 10^{-11}\) | 0.20 |
In full cells composed of a lithium metal anode, LATP solid electrolyte, and LiFePO4 cathode, the interlayer exhibited excellent electrochemical performance. The cell with the interlayer displayed outstanding rate capability, achieving a high specific capacity of 166 mAh g−1 at 0.1 C and maintaining 126 mAh g−1 even at 1.5 C, with low polarization. Long-term cycling at 0.2 C delivered an initial capacity of 162 mAh g−1 and retained 95.5% of its capacity after 100 cycles, with Coulombic efficiency approaching 99.9%. In contrast, cells with the LE-wetted interface experienced drastic capacity loss, reaching 60.0% after 100 cycles. The evolution of voltage hysteresis confirmed the interlayer’s stability: hysteresis increased only gradually with minimal fluctuations, which I attribute to a stable lithium metal interface and low accumulation of inactive side products. Control cells showed a rapid increase in hysteresis and interfacial resistance, indicative of severe interface degradation. At 0.6 C, the interlayer-equipped full cell delivered an initial capacity of 150.0 mAh g−1 and retained 95.0% after 100 cycles. The control cell with the LE-wetted interface short-circuited after only 41 cycles.
| Full cell condition | Initial capacity | Capacity retention after 100 cycles | Coulombic efficiency |
|---|---|---|---|
| Bilayer at 0.2 C | 162 mAh g−1 | 95.5% | 99.9% |
| LE-wetted at 0.2 C | Lower and unstable | 60.0% | Lower |
| Bilayer at 0.6 C | 150.0 mAh g−1 | 95.0% | High |
| LE-wetted at 0.6 C | Failed early | Short circuit after 41 cycles | Not applicable |
Comparison with Reported NASICON-Type Interfaces
I compared the performance of my symmetric and full cells with recently reported NASICON-type solid state battery interfaces. The cumulative plated capacity and maximum achievable current density of symmetric cells, together with the capacity utilization of full cells at 0.1 C, are key metrics for evaluating solid state battery efficiency and practical applicability. My bilayer interface delivers superior performance across these metrics. The low-cost and scalable fabrication process, based on electrospinning and magnetron sputtering, provides practical feasibility for industrial applications. The ultra-low content of gold nanoparticles and the minimal amount of ionic liquid also contribute to improved sustainability and reduced environmental impact. I believe 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 battery systems, enhancing safety, and promoting practical implementation.
| Metric | My bilayer interface | Typical reported NASICON interlayers |
|---|---|---|
| Maximum current density | Up to 2.0 mA cm−2 | Often below 1.0 mA cm−2 |
| Symmetric cell cycling | >1400 h at 0.1 mA cm−2 | Typically hundreds of hours |
| Overpotential | About 10 mV | Often 50–100 mV |
| Full cell retention at 0.2 C | 95.5% after 100 cycles | Variable, often lower |
| Thermal stability at 300 °C | No violent reaction, no short circuit | Rarely demonstrated |
| Fabrication scalability | Electrospinning and sputtering | Often complex or costly |
Mechanistic Summary and Design Rules
My results support a set of design rules for stabilizing a NASICON-based solid state battery. First, the interface must decouple electronic and ionic transport. The MCL provides mixed conductivity for intimate lithium contact and uniform deposition, while the ICL provides pure ionic conductivity and blocks electron percolation into LATP. Second, the interface must provide a 3D host rather than a planar coating. The fibrous, porous architecture accommodates lithium deposition inside the pores, distributes current over a large area, and reduces local volume changes. Third, lithiophilic seeds are essential for activation. Gold nanoparticles lower the nucleation barrier and guide lithium deposition toward the MCL. Fourth, a small amount of ionic liquid improves wetting and Li+ transport, but it must be confined within the porous structure to avoid leakage and parasitic reactions. Fifth, the interface must remain chemically stable against both lithium metal and the solid electrolyte. The ICL prevents direct contact between lithium and LATP, suppressing Ti4+ reduction. Sixth, the interface must survive thermal abuse. The bilayer suppresses violent reactions between LATP and molten lithium up to 300 °C, which is a critical safety advantage for a solid state battery.
The electrochemical activation process transforms the bilayer into an in-situ 3D anode. Initially, the MCL fibers are coated with Au nanoparticles. During activation, lithium deposits preferentially on the Au seeds and along the MCL fibers, forming a continuous mixed-conducting network. The ICL remains free of lithium deposition and acts as an ionic bridge to LATP. After activation, the interface achieves a low interfacial resistance of about \(7\ \Omega\) cm2 per side, a high Li+ transference number of 0.60, and a Li+ diffusion coefficient of \(1.23 \times 10^{-9}\) cm2 s−1. These properties enable stable cycling at 1.0 mA cm−2 for more than 650 h and at 0.1 mA cm−2 for more than 1400 h. The full solid state battery with a LiFePO4 cathode retains 95.5% of its capacity after 100 cycles at 0.2 C and 95.0% at 0.6 C, with Coulombic efficiency of 99.9%.
$$ R_{\mathrm{interface}} \approx 7\ \Omega\ \mathrm{cm}^2 $$
$$ t_{\mathrm{Li}^+} = 0.60 $$
$$ D_{\mathrm{Li}} = 1.23 \times 10^{-9}\ \mathrm{cm}^2\ \mathrm{s}^{-1} $$
| Design rule | Implementation in my bilayer | Benefit for solid state battery |
|---|---|---|
| Decouple electron and ion transport | MCL with CNTs; ICL without CNTs | Uniform Li deposition; LATP protection |
| Use a 3D host | Electrospun fibrous bilayer with 89% porosity | Stress relief; high-capacity Li storage |
| Introduce lithiophilic seeds | Au nanoparticles on MCL | Low nucleation overpotential; uniform plating |
| Confine ionic liquid | IL infiltrated into porous bilayer | Fast Li+ transport; stable interface |
| Block direct Li/LATP contact | ICL facing LATP | No Ti4+ reduction; stable interface |
| Enhance thermal safety | Bilayer as thermal barrier | Suppresses violent Li/LATP reaction |
Conclusions
I have integrated a 3D lithium host design with solid electrolyte interfacial engineering to construct a bilayer electron/ion mixed-conducting interface between LATP and a lithium metal anode. The bilayer combines high ionic conductivity with electrochemical and thermal stability and can be fabricated by scalable electrospinning and magnetron sputtering. The interlayer consists 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 prevents the reduction of Ti4+ in LATP by lithium, ensuring chemical stability of the solid electrolyte. The interlayer alleviates interfacial stress during cycling and enables reversible lithium plating and stripping within the porous structure under high-capacity operation. Consequently, lithium symmetric cells exhibit extremely low overpotential of about 10 mV at 0.1 mAh cm−2, and Li||LiFePO4 full cells demonstrate excellent cyclability, with 95.5% retention at 0.2 C and 95.0% at 0.6 C after 100 cycles, and high Coulombic efficiency of 99.9%. Moreover, the interlayer exhibits excellent thermal stability, effectively suppressing the violent reaction of LATP with molten lithium. This work underscores that the rational design of multifunctional interfaces must comprehensively account for multiple physicochemical factors. Transitioning from conventional planar configurations to advanced 3D architectures represents a promising direction for future development of interfacial design in a solid state battery. 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 battery systems.
| Performance summary | Result |
|---|---|
| Symmetric cell cycling at 0.1 mA cm−2 | >1400 h |
| Symmetric cell overpotential | About 10 mV |
| Critical current density after activation | Up to 2.0 mA cm−2 |
| Interfacial resistance | About \(7\ \Omega\) cm2 per side |
| Li+ transference number | 0.60 |
| Li+ diffusion coefficient | \(1.23 \times 10^{-9}\) cm2 s−1 |
| Full cell retention at 0.2 C | 95.5% after 100 cycles |
| Full cell retention at 0.6 C | 95.0% after 100 cycles |
| Coulombic efficiency | 99.9% |
| Thermal stability at 300 °C | No violent reaction, no short circuit |