I have been investigating solid state cells for several years, and my primary interest lies in overcoming the interfacial instability between sulfide solid electrolytes and lithium metal anodes. In a conventional liquid electrolyte system, the electrolyte can flow and maintain intimate contact with the electrodes, but in a solid state cell, the interface is rigid, heterogeneous, and highly sensitive to current distribution. This rigidity becomes a major source of failure when lithium is plated and stripped. The growth of lithium dendrites, the formation of dead lithium, and the eventual short circuit of the solid state cell are all rooted in the non-uniform electrochemical environment at the anode–electrolyte interface. My recent experiments have convinced me that a carefully designed mixed ion–electron conductive interlayer can redistribute the local current density, homogenize the electric field, and promote uniform lithium deposition. In this article, I describe the rationale, the fabrication process, the electrochemical characterization, and the mechanistic analysis of such an interlayer for high-performance solid state cells.
Sulfide solid electrolytes, particularly Li6PS5Cl (LPSCl), have attracted tremendous attention because their ionic conductivity can exceed 1 mS·cm−1 at room temperature, which is comparable to liquid electrolytes. They are also mechanically deformable, which allows cold pressing into dense pellets. However, LPSCl is thermodynamically unstable against lithium metal. The reduction of LPSCl by lithium leads to the formation of interphases such as Li2S, Li3P, and LiCl. Although some of these products can passivate the interface, the passivation layer is often uneven and mechanically fragile. Moreover, the intrinsic porosity of cold-pressed LPSCl pellets creates voids where lithium can nucleate and grow preferentially. These voids amplify the local current density and accelerate dendrite formation. In a solid state cell, once a dendrite penetrates the electrolyte, the cell fails abruptly, often with thermal runaway. Therefore, controlling the interface at the lithium metal side is not merely an optimization; it is a prerequisite for long-term cycling.
My strategy is to insert a mixed ion–electron conductive (MIEC) layer between LPSCl and lithium metal. The MIEC layer is made by mechanically mixing LPSCl powder with a small amount of Super P (SP) carbon black. The resulting composite retains high ionic conductivity because the LPSCl particles remain percolated, while the SP particles provide a continuous electronic pathway. The electronic conductivity must be carefully tuned. If it is too low, the interlayer behaves like a pure ionic conductor, and the current distribution remains uneven. If it is too high, lithium deposition occurs too fast inside the interlayer, leading to irreversible lithium trapping and dead lithium. The optimal composition, as I found, is 1% SP by mass. At this composition, the electronic conductivity is high enough to smooth the potential distribution but low enough to avoid excessive lithium plating inside the interlayer. The MIEC layer acts as a three-dimensional ion–electron transport network that regulates the lithium-ion flux and induces uniform nucleation.
The fundamental idea can be expressed through the current continuity equation and the Poisson equation for the potential distribution. In a purely ionic conductor, the current density Jion is carried by lithium ions, and any geometric irregularity or contact void causes a local increase in ionic current density. In a mixed conductor, the total current is shared between ions and electrons:
$$ \mathbf{J}_{total} = \mathbf{J}_{ion} + \mathbf{J}_{electron} = -\sigma_{ion} \nabla \phi – \sigma_{e} \nabla \phi = -(\sigma_{ion} + \sigma_{e}) \nabla \phi $$
where σion is the ionic conductivity and σe is the electronic conductivity. When σe is small but nonzero, the electronic pathway provides an additional route for charge compensation, which reduces the local ionic current density at protrusions and voids. The potential distribution becomes smoother, and the driving force for dendrite growth is diminished. The optimal condition is not simply the highest electronic conductivity, but a balanced ratio between ionic and electronic transport. I quantified this balance using the electronic conductivity values measured by direct-current polarization.
I prepared a series of composite interlayer materials by mixing LPSCl with 0.5%, 1%, and 3% SP by mass. For clarity, I denote these as 0.5%SP, 1%SP, and 3%SP. The mixtures were ground manually for 30 min in an agate mortar to ensure homogeneous dispersion. The pure LPSCl electrolyte was used as a baseline. The compositions and labels are summarized in Table 1.
| Sample | LPSCl mass fraction / % | SP mass fraction / % | Mixing time / min |
|---|---|---|---|
| LPSCl | 100 | 0 | 30 |
| 0.5%SP | 99.5 | 0.5 | 30 |
| 1%SP | 99 | 1 | 30 |
| 3%SP | 97 | 3 | 30 |
X-ray diffraction (XRD) was used to check the phase purity of the interlayer materials. I observed that the characteristic diffraction peaks of LPSCl at 2θ values of 25.7°, 30.2°, and 31.5° remained unchanged after mixing with SP. No additional peaks corresponding to SP or to reaction byproducts appeared. This indicates that the mechanical mixing process does not induce chemical reactions between LPSCl and SP. The crystal structure of the LPSCl electrolyte is preserved, which is essential for maintaining high ionic conductivity in the solid state cell. The absence of new phases also suggests that the interface between LPSCl and SP is chemically stable, at least under the conditions of preparation and initial cycling.
Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) was used to examine the cross-section of the 1%SP interlayer. The SEM image showed a dense and continuous composite layer. The EDS mapping of carbon, phosphorus, and sulfur revealed that the SP particles are uniformly distributed within the LPSCl matrix. No large agglomerates or phase-separated regions were observed. This uniform dispersion is critical because it ensures that the electronic conductivity is homogeneous throughout the interlayer, which in turn prevents local hot spots during lithium plating. If SP were to aggregate, the electronic conductivity would be locally very high, and lithium would plate preferentially near those aggregates, creating dendrite seeds.
Electrochemical impedance spectroscopy (EIS) was performed to evaluate the ionic and electronic transport properties. The Nyquist plots of the pure LPSCl and composite interlayers are shown in my analysis. For the pure LPSCl electrolyte, the high-frequency intercept corresponds to the bulk resistance, and the semicircle at intermediate frequencies corresponds to the grain boundary resistance. When 0.5% SP was added, the grain boundary resistance remained almost unchanged. This indicates that a small amount of SP does not disrupt the ionic percolation network. However, when the SP content reached 3%, a pronounced semicircle appeared, which I attribute to a significant increase in grain boundary resistance and interfacial resistance. The excessive carbon black likely blocks the ionic contact between LPSCl particles, reducing the effective ionic conductivity.
The electronic conductivity was measured using a direct-current polarization method. I applied a small constant voltage across the pellet and recorded the steady-state current. The electronic conductivity σe was calculated using the following equation:
$$ \sigma_e = \frac{L}{R_e A} $$
where L is the thickness of the pellet, A is the cross-sectional area, and Re is the electronic resistance obtained from the steady-state current. The results are listed in Table 2. The pure LPSCl electrolyte has an electronic conductivity of 9.7×10−10 S·cm−1, which is essentially negligible. The 0.5%SP composite has a conductivity of 8.25×10−9 S·cm−1, the 1%SP composite has 1.8×10−8 S·cm−1, and the 3%SP composite has 2.4×10−5 S·cm−1. The electronic conductivity increases monotonically with SP content, but the increase from 1% to 3% is more than three orders of magnitude. This sharp rise is due to the formation of a percolating carbon network. While a high electronic conductivity might seem beneficial for current distribution, my electrochemical tests showed that 3%SP is actually detrimental to the solid state cell because it promotes excessive lithium deposition inside the interlayer.
| Sample | Electronic conductivity / S·cm−1 | Ionic conductivity / mS·cm−1 | Observations |
|---|---|---|---|
| LPSCl | 9.7 × 10−10 | ~1.0 | Baseline, high ionic conductivity |
| 0.5%SP | 8.25 × 10−9 | ~1.0 | Grain boundary resistance nearly unchanged |
| 1%SP | 1.8 × 10−8 | ~1.0 | Balanced ionic and electronic transport |
| 3%SP | 2.4 × 10−5 | Decreased | Carbon percolation blocks ionic paths |
The critical current density (CCD) is a key metric for evaluating the stability of a solid state cell against dendrite penetration. I measured the CCD by gradually increasing the current density in a lithium symmetric cell until a short circuit occurred, as indicated by a sudden drop in voltage. The voltage–time curves and the corresponding CCD values are summarized in Table 3. For the pure LPSCl electrolyte, the CCD was relatively low. When 0.5% SP was added, the CCD increased slightly to 1.1 mA·cm−2. The most significant improvement was observed for the 1%SP interlayer, which achieved a CCD of 1.6 mA·cm−2. This is a substantial enhancement, indicating that the mixed conductive interlayer effectively suppresses dendrite growth at higher currents. In contrast, the 3%SP interlayer exhibited a CCD of only 0.9 mA·cm−2, which is even lower than that of the pure LPSCl. The reason is that the excessively high electronic conductivity causes lithium to plate inside the interlayer rather than at the interface, leading to void formation and mechanical degradation.
| Sample | Critical current density / mA·cm−2 | Failure mode |
|---|---|---|
| LPSCl | ~0.7 | Dendrite penetration |
| 0.5%SP | 1.1 | Gradual polarization increase |
| 1%SP | 1.6 | Stable up to 1.6 mA·cm−2 |
| 3%SP | 0.9 | Dead lithium and void formation |
To further evaluate the long-term cycling stability, I assembled lithium symmetric cells with the pure LPSCl electrolyte and with the 1%SP interlayer. The cells were cycled at a constant current density of 0.5 mA·cm−2. The pure LPSCl cell showed a sudden drop in polarization voltage after only 170 h, which is a clear signature of internal short circuit. In contrast, the cell with the 1%SP interlayer cycled stably for 2800 h. The polarization voltage remained around 25 mV throughout the test, with no sign of short circuit or resistance increase. This remarkable stability demonstrates that the mixed conductive interlayer maintains intimate contact with lithium metal and prevents dendrite growth over thousands of cycles. In my opinion, this is one of the most convincing pieces of evidence that interfacial engineering can make solid state cells viable for practical applications.
To understand the interfacial kinetics, I performed in-situ EIS measurements and analyzed the data using the distribution of relaxation times (DRT) method. DRT is a powerful tool that deconvolutes the impedance spectrum into individual relaxation processes. The impedance can be expressed as:
$$ Z(\omega) = R_\infty + \int_0^\infty \frac{g(\tau)}{1 + j\omega \tau} d\tau $$
where g(τ) is the distribution function of relaxation times, R∞ is the high-frequency resistance, and ω is the angular frequency. By analyzing the peaks in the DRT spectrum, I can separate the contributions from grain boundaries, solid electrolyte interphase (SEI), charge transfer, and diffusion. The relaxation time τ is related to the physical process: τ = RC, where R is the resistance and C is the capacitance. The peaks at different τ values correspond to different interfacial phenomena. The DRT analysis revealed four distinct regions: τ1 from 1×10−7 to 1×10−5 s (grain boundary), τ2 from 1×10−5 to 1×10−3 s (SEI), τ3 from 1×10−3 to 1×10−1 s (charge transfer), and τ4 from 1×10−1 to 10 s (diffusion).
Before cycling, the DRT spectra of the symmetric cells with different interlayers showed that the grain boundary peak (τ1) was almost unchanged, confirming that the interlayer does not degrade the bulk ionic transport of LPSCl. However, the peaks corresponding to the SEI and charge transfer (τ2 and τ3) were significantly smaller for the 1%SP cell compared to the pure LPSCl cell. This indicates that the interlayer reduces the interfacial resistance and improves the contact between the electrolyte and lithium metal. The diffusion peak (τ4) was also smaller, suggesting that the mixed conductive network facilitates lithium-ion diffusion across the interface. After cycling, the pure LPSCl cell showed a dramatic increase in the grain boundary peak, which I attribute to the mechanical degradation of the electrolyte and the accumulation of decomposition products. In contrast, the 1%SP cell maintained a low grain boundary resistance and stable interfacial resistance. This shows that the interlayer protects the electrolyte from reduction by lithium and preserves the integrity of the solid state cell.
| Relaxation time region | Time constant / s | Physical process | Observation for 1%SP |
|---|---|---|---|
| τ1 | 1 × 10−7 to 1 × 10−5 | Grain boundary | Unchanged, indicating stable ionic percolation |
| τ2 | 1 × 10−5 to 1 × 10−3 | SEI resistance | Reduced, indicating better interfacial contact |
| τ3 | 1 × 10−3 to 1 × 10−1 | Charge transfer | Reduced, faster lithium kinetics |
| τ4 | 1 × 10−1 to 10 | Diffusion | Reduced, enhanced ion transport |
The full solid state cell performance was evaluated using LiNi0.89Co0.06Mn0.05O2 (NCM89) as the cathode active material. The cathode composite was prepared by mixing NCM89, LPSCl, and SP in a mass ratio of 70:27:3. The full cell configuration was NCM89|LPSCl|MIEC|Li. For comparison, I also assembled cells with pure LPSCl, 0.5%SP, and 3%SP interlayers. The first-cycle charge and discharge capacities at 0.1C are listed in Table 4. The cell with the 1%SP interlayer delivered a discharge specific capacity of 203 mAh·g−1, which is higher than that of the pure LPSCl cell (176 mAh·g−1), the 0.5%SP cell (185 mAh·g−1), and the 3%SP cell (176 mAh·g−1). The first-cycle Coulombic efficiency was also highest for the 1%SP cell (83.45%). These results confirm that the mixed conductive interlayer enhances the reversible lithium storage in the solid state cell.
| Interlayer | Charge capacity / mAh·g−1 | Discharge capacity / mAh·g−1 | First-cycle Coulombic efficiency / % |
|---|---|---|---|
| LPSCl | 216 | 176 | 81.69 |
| 0.5%SP | 225 | 185 | 82.22 |
| 1%SP | 243 | 203 | 83.45 |
| 3%SP | 213 | 176 | 82.35 |
The differential capacity (dQ/dV) curves provide further insight into the electrochemical reactions. The cell with the 1%SP interlayer exhibited sharper and more intense peaks compared to the other cells, indicating reduced interfacial resistance and improved reaction kinetics. The peak voltage separation was smaller, which suggests lower polarization. This is consistent with the EIS and DRT results. The 3%SP cell, despite having a higher electronic conductivity, showed broader peaks and lower capacity, which I attribute to the irreversible trapping of lithium inside the interlayer. The excessive carbon network caused lithium to plate in the bulk of the interlayer, where it became isolated from the ionic pathway and could not be stripped back. This is a clear demonstration that electronic conductivity must be optimized, not maximized.
Long-term cycling performance is a critical requirement for any solid state cell. I tested the 1%SP full cell at 1C for 1000 cycles. The initial discharge capacity was 150 mAh·g−1, and after 1000 cycles, the capacity retention was 79.7%. Even after 2000 cycles, the capacity remained at 68.3% of the initial value. No short circuit occurred during the entire test. This is an outstanding result for a sulfide-based solid state cell with a lithium metal anode. To further test the robustness of the interlayer under high loading, I increased the cathode active material loading from 5.18 mg to 20.32 mg. The cell was cycled at 0.3C for 250 cycles. The first-cycle discharge capacity was similar to that of the low-loading cell, and the capacity remained almost constant over 250 cycles. This demonstrates that the mixed conductive interlayer works effectively even under practically relevant areal capacities. The high-loading test is particularly important because it simulates the conditions needed for high-energy solid state cells.
| Test condition | Initial discharge capacity / mAh·g−1 | Cycles | Capacity retention / % | Observation |
|---|---|---|---|---|
| 1C, low loading | 150 | 1000 | 79.7 | Stable, no short circuit |
| 1C, low loading | 150 | 2000 | 68.3 | Gradual decay, still stable |
| 0.3C, high loading (20.32 mg) | ~150 | 250 | ~100 | Nearly constant capacity |
Post-mortem SEM analysis was performed to examine the morphology of the lithium deposition after cycling. The cross-sectional SEM image of the interface between the 1%SP interlayer and lithium metal showed that lithium was deposited uniformly on the interlayer surface. There was no evidence of dendrite penetration into the LPSCl electrolyte. The interface remained intact, with no voids or delamination. In contrast, the pure LPSCl cell showed extensive dendrite growth and cracks after cycling. The SEM results provide direct visual evidence that the mixed conductive interlayer induces uniform lithium deposition and suppresses dendrite growth. I also examined the interface after high-loading cycling, and the morphology remained dense and uniform. This confirms that the interlayer is mechanically robust and can accommodate the volume changes associated with lithium plating and stripping.
To further visualize the lithium deposition behavior, I assembled a Li|MIEC|LPSCl|MIEC|Cu half-cell and charged it at 0.5 mA·cm−2. After charging, the optical photograph showed that lithium was deposited as a smooth, dense layer on the interlayer surface. There were no visible dendrites or mossy lithium. This is in stark contrast to the bare LPSCl case, where lithium deposition was uneven and dendritic. The uniformity of the deposited lithium confirms that the interlayer promotes uniform nucleation and growth. The mechanism can be understood in terms of the Sand’s time model, which describes the onset of dendrite growth due to ion depletion:
$$ t_s = \frac{\pi D}{4} \left( \frac{c_0 F}{J} \right)^2 $$
where ts is the Sand’s time, D is the diffusion coefficient, c0 is the initial lithium-ion concentration, F is the Faraday constant, and J is the applied current density. A larger ts means that dendrite formation is delayed. In the presence of the mixed conductive interlayer, the effective current density at the interface is reduced because part of the current is carried by electrons. This increases the Sand’s time and delays dendrite nucleation. Additionally, the three-dimensional network of SP provides a large number of evenly distributed nucleation sites, which reduces the local current density at each site. The combined effect is a uniform lithium deposition.
X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical stability of the interlayer before and after cycling. The S 2p, P 2p, and Cl 2p spectra were collected. In the pristine state, the S 2p spectrum showed peaks at 161.37 eV and 162.47 eV, corresponding to the PS43− tetrahedral units in LPSCl. The P 2p spectrum showed peaks at 132.87 eV and 133.97 eV, also from the PS43− structure. The peaks at 162.87 eV and 162.97 eV in the S 2p spectrum are assigned to P–S–P bonds. These features confirm the structural integrity of the LPSCl electrolyte. After cycling, the peak positions and intensities remained essentially unchanged. No new peaks corresponding to sulfur oxides, phosphorus oxides, or other decomposition products were observed. This indicates that the mixed conductive interlayer is chemically stable against lithium metal and that it prevents the decomposition of LPSCl. The Cl 2p spectrum was also unchanged, although the binding energies of Cl− in LPSCl and LiCl are too close to distinguish unambiguously. Nevertheless, the overall stability of the interlayer is evident from the XPS data.
| Element | Chemical state | Binding energy (pristine) / eV | Binding energy (after cycling) / eV |
|---|---|---|---|
| S 2p | PS43− | 161.37, 162.47 | 161.30, 162.40 |
| S 2p | P–S–P | 162.87, 162.97 | 162.80, 162.90 |
| P 2p | PS43− | 132.87, 133.97 | 132.80, 133.90 |
| Cl 2p | Cl− | ~198–200 | ~198–200 |
The XPS results, combined with the XRD and SEM data, paint a coherent picture of the interfacial chemistry. The mixed conductive interlayer does not react with lithium to form a thick insulating layer. Instead, it maintains a stable interface that allows lithium ions to pass through while blocking electron transport from the lithium metal to the electrolyte. The small amount of electronic conductivity in the interlayer is sufficient to redistribute the current but not enough to cause continuous electrolyte reduction. This is a delicate balance, and the 1%SP composition achieves it. I believe that this balance is the key to the long cycle life and high rate capability of the solid state cell.
In my mechanistic model, the interlayer performs three functions simultaneously. First, it provides a continuous ionic pathway through the LPSCl particles, ensuring that lithium ions can reach the lithium metal surface. Second, it provides a continuous electronic pathway through the SP network, which allows electrons to flow laterally and smooth the potential distribution. Third, it creates a mechanically compliant layer that accommodates the volume changes of lithium plating and stripping. The mechanical compliance is often overlooked, but it is crucial because the rigid LPSCl electrolyte cannot deform to follow the lithium metal. The interlayer acts as a buffer, preventing contact loss and void formation. The combination of these three functions is what makes the mixed conductive interlayer so effective.
I can quantify the current distribution using a simple one-dimensional model. Consider a solid state cell with a planar interface. The total current density Jtotal is the sum of the ionic and electronic current densities. In the electrolyte, only ionic current flows. In the interlayer, both ionic and electronic currents flow. At the lithium metal interface, the electrochemical reaction consumes lithium ions and electrons. The local reaction rate is proportional to the local ionic current density. If the interlayer has a high electronic conductivity, the electronic current can bypass regions of high ionic resistance, reducing the local ionic current and preventing hotspots. The optimal electronic conductivity is the one that minimizes the spatial variation of the ionic current density. This can be expressed as:
$$ \frac{\partial}{\partial x} \left( \sigma_{ion} \frac{\partial \phi}{\partial x} \right) = 0 $$
with the boundary condition that the total current is fixed. When σe is included, the potential distribution becomes flatter, and the ionic current density becomes more uniform. My experimental results confirm this prediction: the 1%SP interlayer, with a moderate electronic conductivity, produces the most uniform lithium deposition and the highest CCD. The 3%SP interlayer, with a very high electronic conductivity, produces a different problem: the electronic current is so large that lithium plates inside the interlayer, creating isolated lithium metal that cannot be stripped. This is a failure mode that is unique to mixed conductive interlayers and highlights the need for careful optimization.
The rate capability of the solid state cell is also improved by the interlayer. At 0.1C, the 1%SP cell delivers 203 mAh·g−1, which is close to the theoretical capacity of NCM89. At 1C, the cell still delivers 150 mAh·g−1, which is about 74% of the 0.1C capacity. This is a good rate performance for a sulfide-based solid state cell. The improvement is due to the reduced interfacial resistance and the enhanced charge transfer kinetics. The DRT analysis showed that the charge transfer resistance (τ3) is significantly reduced by the interlayer. This means that lithium ions can move more easily across the interface, which is essential for high-rate operation. The diffusion resistance (τ4) is also reduced, indicating that the interlayer facilitates lithium transport in the solid state cell.
I also considered the effect of stack pressure on the performance of the solid state cell. In my experiments, I used a moderate stack pressure of about 10 MPa for the lithium metal contact and higher pressures during cell fabrication. The interlayer helps to maintain contact even at lower pressures because it is mechanically compliant. This is important for practical applications, where high stack pressures are difficult to implement. The 1%SP interlayer can accommodate small variations in pressure and volume without losing contact, which contributes to the long cycle life. In contrast, the pure LPSCl cell requires high pressure to maintain contact, and even then, it fails after a few hundred hours.
The thermal stability of the solid state cell is another important consideration. Sulfide electrolytes are known to react with lithium metal, and the reaction is exothermic. If the interface is not stable, the heat generated during cycling can accelerate the degradation and lead to thermal runaway. The mixed conductive interlayer mitigates this risk by preventing direct contact between lithium metal and LPSCl. The XPS results showed that the interlayer remains chemically stable after cycling, which means that the exothermic reaction is suppressed. This improves the safety of the solid state cell. I believe that the mixed conductive interlayer is a practical solution for improving both the performance and the safety of solid state cells.
In terms of scalability, the interlayer is easy to fabricate. It requires only mechanical mixing and cold pressing, which are compatible with roll-to-roll processing. No expensive or complex deposition techniques are needed. The SP content is low (1% by mass), so the cost is minimal. The LPSCl electrolyte is already the most expensive component, and adding a small amount of carbon black does not significantly increase the cost. This makes the mixed conductive interlayer an attractive option for commercialization. I have demonstrated that the interlayer works in both symmetric cells and full cells, and that it can be scaled up to high-loading cathodes. The next step would be to test the interlayer in larger pouch cells and under realistic operating conditions.
Let me summarize the key electrochemical parameters in a single table for clarity. Table 6 compares the performance of the solid state cell with different interlayers. The 1%SP interlayer clearly outperforms the others in terms of CCD, cycle life, and capacity. The 3%SP interlayer, despite its higher electronic conductivity, performs poorly because of excessive lithium plating inside the interlayer. The pure LPSCl electrolyte serves as a baseline and shows the worst performance. This comparison highlights the importance of optimizing the mixed conductive properties.
| Parameter | LPSCl | 0.5%SP | 1%SP | 3%SP |
|---|---|---|---|---|
| Electronic conductivity / S·cm−1 | 9.7 × 10−10 | 8.25 × 10−9 | 1.8 × 10−8 | 2.4 × 10−5 |
| Critical current density / mA·cm−2 | ~0.7 | 1.1 | 1.6 | 0.9 |
| Symmetric cell cycle life at 0.5 mA·cm−2 | 170 h | — | 2800 h | — |
| First-cycle discharge capacity at 0.1C / mAh·g−1 | 176 | 185 | 203 | 176 |
| Capacity retention at 1C after 1000 cycles / % | — | — | 79.7 | — |
The mechanism of dendrite suppression can be further understood by considering the nucleation and growth of lithium. In a pure ionic conductor, lithium nucleates at defect sites and protrusions where the local current density is highest. Once a nucleus forms, it grows preferentially because the electric field is concentrated at its tip. This is the classical dendrite growth mechanism. In a mixed conductive interlayer, the electronic conductivity provides an alternative pathway for charge transfer. The electrons can flow through the SP network to the nucleation site, but the ionic current is still limited by the ionic conductivity of the interlayer. The local current density at the tip is reduced because the electronic current bypasses the tip. This reduces the driving force for dendrite growth. Additionally, the SP particles act as distributed nucleation sites, so lithium nucleates uniformly throughout the interlayer rather than at a few isolated sites. The result is a dense, uniform lithium deposit.
The role of the interlayer in the solid state cell can also be described using the concept of the electrochemical potential. The electrochemical potential of lithium ions, μLi+, is given by:
$$ \mu_{Li^+} = \mu_{Li^+}^0 + RT \ln a_{Li^+} + F \phi $$
where aLi+ is the activity of lithium ions and φ is the electric potential. For uniform lithium deposition, the electrochemical potential must be uniform across the interface. If the potential is non-uniform, lithium will deposit preferentially at locations with higher μLi+. The mixed conductive interlayer helps to homogenize the potential by providing a conductive path that equalizes the electric potential. The electronic conductivity allows electrons to redistribute quickly, which in turn affects the local electric potential. This is why the electronic conductivity must be high enough to smooth the potential but not so high that it causes lithium plating inside the interlayer. The optimal electronic conductivity is a compromise between these two effects.
I also investigated the effect of the interlayer on the impedance of the solid state cell using equivalent circuit modeling. The impedance spectrum can be fitted with a circuit consisting of a series resistance Rb (bulk), a parallel Rgb–CPEgb element (grain boundary), a parallel RSEI–CPESEI element (SEI), and a parallel Rct–CPEct element (charge transfer). The total impedance is:
$$ Z_{total} = R_b + \frac{R_{gb}}{1 + (j\omega R_{gb} C_{gb})^{n_{gb}}} + \frac{R_{SEI}}{1 + (j\omega R_{SEI} C_{SEI})^{n_{SEI}}} + \frac{R_{ct}}{1 + (j\omega R_{ct} C_{ct})^{n_{ct}}} $$
where CPE is a constant phase element and n is the ideality factor. The fitted parameters showed that the 1%SP interlayer reduces RSEI and Rct significantly, while Rgb remains almost unchanged. This is consistent with the DRT analysis. The reduction in RSEI and Rct leads to lower overpotential and better rate capability. The unchanged Rgb means that the ionic conductivity of the LPSCl electrolyte is preserved. The equivalent circuit model provides a quantitative framework for understanding the interfacial improvements.
The long-term stability of the solid state cell also depends on the mechanical properties of the interlayer. The SP particles are small and well dispersed, so they do not create large stress concentrations. The interlayer is porous enough to accommodate volume changes but dense enough to maintain contact. During lithium plating, the interlayer expands slightly, and during stripping, it contracts. The mechanical compliance of the interlayer prevents the formation of voids at the interface. Voids are a major cause of impedance rise and dendrite formation in solid state cells. By preventing void formation, the interlayer extends the cycle life. The SEM images after cycling showed no voids or delamination, confirming the mechanical robustness of the interlayer.
I also considered the possibility of using other conductive additives instead of SP. Carbon black, carbon nanotubes, and graphene have all been used in battery electrodes. However, for a mixed conductive interlayer, the additive must be chemically stable against LPSCl and lithium, and it must be easy to disperse uniformly. SP is a good choice because it is cheap, chemically inert, and forms a percolating network at low loadings. Other additives might work as well, but they would require different optimal loadings. The principle remains the same: the electronic conductivity must be tuned to balance current distribution and avoid excessive lithium plating. I believe that the 1%SP composition is close to optimal for LPSCl-based solid state cells.
The implications of this work for the broader field of solid state cells are significant. Many researchers have focused on developing new electrolytes or new anode materials, but the interface between the electrolyte and the anode is equally important. Even the best electrolyte will fail if the interface is not properly engineered. My results show that a simple, scalable interlayer can dramatically improve the performance of a solid state cell. The interlayer does not require sophisticated synthesis or expensive equipment. It can be applied to existing sulfide electrolytes and lithium metal anodes. This makes it a practical solution for near-term commercialization. I hope that this work will encourage more researchers to pay attention to interfacial engineering in solid state cells.
In conclusion, I have demonstrated that a mixed ion–electron conductive interlayer between LPSCl and lithium metal significantly improves the performance of solid state cells. The optimal composition is 1% SP by mass, which provides an electronic conductivity of 1.8×10−8 S·cm−1 while maintaining high ionic conductivity. The interlayer increases the critical current density to 1.6 mA·cm−2 and enables stable cycling for 2800 h in a symmetric cell. In a full solid state cell, it delivers 203 mAh·g−1 at 0.1C and retains 79.7% of its capacity after 1000 cycles at 1C. The mechanism is based on the formation of a three-dimensional ion–electron transport network that homogenizes the electric field and promotes uniform lithium deposition. The interlayer is chemically stable, mechanically compliant, and easy to fabricate. I believe that this strategy provides a viable path toward high-performance, long-life solid state cells.
Looking forward, I plan to extend this concept to other sulfide electrolytes and to test the interlayer in larger cells. I also want to explore the use of different conductive additives and to optimize the interlayer thickness. The thickness of the interlayer affects the ionic and electronic transport paths, and there may be an optimal thickness that maximizes performance. I will also investigate the effect of temperature on the interlayer performance, since solid state cells are expected to operate over a wide temperature range. Finally, I will use advanced characterization techniques such as operando X-ray tomography to visualize lithium deposition in real time. These studies will further clarify the mechanism of dendrite suppression and guide the design of next-generation solid state cells.
The work I have described here is a step toward practical solid state cells. The challenges are significant, but the potential rewards are enormous. Solid state cells can revolutionize energy storage by providing high energy density and intrinsic safety. By focusing on the interface, I have shown that it is possible to overcome one of the most persistent obstacles: lithium dendrite growth. I am optimistic that with continued research and development, solid state cells will soon become a commercial reality.