In the pursuit of advanced energy storage systems, we have focused on developing solid-state batteries due to their potential to surpass traditional lithium-ion batteries in safety and energy density. A critical component of such batteries is the negative electrode, and hard carbon has emerged as a promising anode material owing to its minimal volume changes during lithiation and delithiation, which helps maintain interfacial contact with solid electrolytes. This study details our first-person exploration into the preparation and performance of hard carbon composite anodes for solid-state batteries, utilizing a composite of lithiated Nafion (Li-Nafion) resin and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) as both binder and internal solid electrolyte. We aim to optimize electrode composition and structure to enhance ionic and electronic conduction networks, thereby achieving stable cycling performance.
The evolution of solid-state battery technology hinges on addressing key challenges such as interfacial resistance and ionic conductivity. In our work, we emphasize the importance of hard carbon anodes, which exhibit near-zero strain characteristics, making them ideal for solid-state systems where volume changes can disrupt solid-solid contacts. By integrating single-ion conductive Li-Nafion with LLZTO ceramic fillers, we create a composite solid electrolyte (CSE) that serves dual roles in the electrode: as a binder to maintain mechanical integrity and as an ionic conductor to facilitate lithium-ion transport. This approach aligns with the broader goal of advancing solid-state battery performance through innovative material design.
To set the stage, we recall that solid-state batteries offer inherent safety advantages by eliminating flammable liquid electrolytes. However, their commercial viability depends on achieving high ionic conductivity and robust electrode-electrolyte interfaces. Our research contributes to this field by systematically investigating hard carbon composite anodes, with repeated emphasis on the term “solid-state battery” to underscore its relevance. Below, we present our methodologies, results, and discussions, enriched with tables and formulas to summarize key findings.
Introduction to Solid-State Battery Components
Solid-state batteries represent a paradigm shift in energy storage, leveraging solid electrolytes to mitigate risks associated with thermal runaway and leakage. The solid electrolyte acts as both separator and ion conductor, and its compatibility with electrode materials is crucial. We have identified hard carbon as a favorable anode due to its disordered structure with micropores that accommodate lithium ions with negligible expansion. The formula for lithium insertion in hard carbon can be described in terms of capacity contributions from various sites:
$$ Q_{total} = Q_{adsorption} + Q_{intercalation} + Q_{pore-filling} $$
where \( Q_{adsorption} \) refers to lithium adsorption on surface defects, \( Q_{intercalation} \) to interlayer insertion, and \( Q_{pore-filling} \) to storage in micropores. In solid-state batteries, these mechanisms must be optimized to minimize irreversible capacity losses.
Our composite electrolyte, Li-Nafion/LLZTO, combines the flexibility of polymers with the high ionic conductivity of ceramics. The ionic conductivity \( \sigma \) follows the Arrhenius equation:
$$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{k_B T}\right) $$
where \( \sigma_0 \) is the pre-exponential factor, \( E_a \) the activation energy, \( k_B \) Boltzmann’s constant, and \( T \) the temperature. By tuning the LLZTO content, we maximize conductivity for enhanced solid-state battery performance.
Experimental Methods and Material Preparation
We began by synthesizing the composite solid electrolyte. Li-Nafion membranes were prepared via ion exchange from protonated Nafion in LiOH solution, followed by drying. The CSE was fabricated by ball-mixing Li-Nafion sol with LLZTO powder at varying mass fractions. For the solid-state battery anode, hard carbon (HC) was used as the active material, Super P as conductive additive, and the CSE as binder. The electrode slurry was cast onto copper foil, dried, and punched into discs. We designed electrodes with different CSE contents to study composition effects, as summarized in Table 1.
| Electrode Designation | Hard Carbon (wt%) | Super P (wt%) | Li-Nafion/10%LLZTO (wt%) |
|---|---|---|---|
| HC-20CSE | 70 | 10 | 20 |
| HC-30CSE | 60 | 10 | 30 |
| HC-40CSE | 50 | 10 | 40 |
| HC-50CSE | 40 | 10 | 50 |
The porosity \( \phi \) of electrodes was calculated to assess compaction effects, using the formula derived from mass and thickness measurements:
$$ \phi = \left[1 – \frac{m_{HC}/\rho_{HC} + m_{Li-Nafion}/\rho_{Li-Nafion} + m_{LLZTO}/\rho_{LLZTO} + m_{Super P}/\rho_{Super P}}{S(\delta_{electrode} – \delta_{Cu})} \right] \times 100\% $$
where \( m \) denotes mass, \( \rho \) density, \( S \) electrode area, \( \delta_{electrode} \) electrode thickness, and \( \delta_{Cu} \) copper foil thickness. This porosity metric is vital for understanding ion transport in solid-state battery electrodes.
For electrochemical testing, we assembled CR2016 coin cells in an argon-filled glovebox. Half-cells paired composite anodes with lithium metal, using Li-Nafion membranes swollen in PC/EC as separators. Full-cells were built with pre-lithiated hard carbon anodes and LiFePO4 (LFP) or LiNi0.6Co0.1Mn0.3O2 (NCM613) cathodes. Characterization included scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS).
Optimization of Electrode Composition for Solid-State Battery Performance
We first evaluated the impact of CSE content on ionic conductivity. The Li-Nafion/10%LLZTO composite exhibited the highest conductivity, with an activation energy \( E_a \) of 22.65 kJ/mol, as fitted from Arrhenius plots. This optimized CSE was used in all subsequent solid-state battery experiments. XPS analysis revealed Li-F bonding at 685.2 eV in the F1s spectrum, indicating interaction between Li-Nafion and LLZTO that likely enhances ion transport by forming space-charge layers.
Electrochemical performance of half-cells is shown in Table 2, highlighting initial charge capacities. The HC-30CSE electrode delivered the highest capacity, suggesting a balance between ionic and electronic percolation networks. EIS data were modeled with an equivalent circuit comprising ohmic resistance \( R_e \), electrode/separator interface resistance \( R_1 \), internal electrode resistance \( R_2 \), and Li/separator resistance \( R_3 \). Fitted values demonstrate that increasing CSE content reduces \( R_1 \) and \( R_2 \) but raises \( R_e \), aligning with theory for composite electrodes in solid-state batteries.
| Electrode | Initial Charge Capacity (mAh/g) | Coulombic Efficiency (%) | \( R_e \) (Ω) | \( R_1 \) (Ω) | \( R_2 \) (Ω) |
|---|---|---|---|---|---|
| HC-20CSE | 239.0 | ~30 | 45.2 | 120.5 | 85.3 |
| HC-30CSE | 334.0 | ~35 | 50.1 | 95.8 | 60.4 |
| HC-40CSE | 311.6 | ~32 | 55.7 | 80.2 | 50.1 |
| HC-50CSE | 179.9 | ~25 | 65.3 | 70.5 | 40.8 |
The irreversible capacity in solid-state battery anodes was probed via differential capacity (dQ/dV) analysis and XPS. First-cycle dQ/dV curves show peaks at 1.2 V and 0.75 V, attributable to solvent reduction and lithium adsorption on hard carbon defects, respectively. Post-cycling XPS indicated LiF and Li2CO3 formation, with LiF content correlating inversely with coulombic efficiency. We attribute this to interfacial reactions between Li-Nafion and lithiated hard carbon (LiCx), a key irreversibility source in solid-state batteries. The reaction can be schematized as:
$$ \text{Li-Nafion} + \text{LiC}_x \rightarrow \text{LiF} + \text{other products} $$
This underscores the need for interface engineering in solid-state battery design.
Effects of Porosity and Temperature on Solid-State Battery Anodes
Porosity modulation through cold-pressing significantly influenced performance. Applying pressures from 0 to 3480 MPa reduced porosity from ~40% to ~15%, concurrently decreasing \( R_e \), \( R_1 \), and \( R_2 \). The HC-30CSE electrode pressed at 3480 MPa achieved a charge capacity of 334.0 mAh/g, nearing that in liquid electrolytes. This highlights the importance of dense electrode architectures for efficient ion transport in solid-state batteries.
Temperature studies revealed that 70°C is optimal for our solid-state battery system. While higher temperatures boost ionic conductivity, they exacerbate side reactions. At 90°C, initial capacity reached 490.1 mAh/g but faded rapidly, whereas at 70°C, the HC-30CSE half-cell cycled stably for 180 cycles with an average coulombic efficiency of 99.82%. The Arrhenius dependence of conductivity is given by:
$$ \ln(\sigma) = \ln(A) – \frac{E_a}{RT} $$
where \( R \) is the gas constant. Fitting our data yielded \( E_a \) values around 23 kJ/mol for the CSE, confirming thermally activated transport. We recommend operating solid-state batteries at moderate temperatures to balance kinetics and stability.
Rate Capability and Full-Cell Performance of Solid-State Batteries
The rate capability of HC-30CSE anodes was evaluated at currents from 22 to 220 mA/g. Discharge capacities retained 45.7 mAh/g at 220 mA/g, recovering to 261.0 mAh/g upon returning to 22 mA/g. This demonstrates reasonable rate performance for solid-state batteries, though further improvement is needed for high-power applications.
For full-cell validation, we assembled solid-state batteries with pre-lithiated hard carbon anodes and LFP cathodes. The N/P ratio was set to 1.25 to compensate for irreversible losses. Cycling at 70°C and 0.1C rate, the LFP capacity was 157.2 mAh/g initially, maintaining 69.6 mAh/g after 400 cycles with an average coulombic efficiency of 99.80%. This mirrors half-cell stability, affirming the viability of our composite anodes in practical solid-state battery configurations.
Additionally, a pouch cell with NCM613 cathode and hard carbon anode was fabricated. The initial discharge capacity was 154.4 mAh/g (based on NCM mass), with 84.66% coulombic efficiency. Over 30 cycles, the average efficiency was 98.51%, though capacity fade was observed, likely due to interfacial degradation. These results underscore the challenges in scaling solid-state battery technology.
Discussion on Ionic and Electronic Networks in Solid-State Battery Electrodes
The performance of solid-state batteries hinges on percolating networks for ions and electrons. In our composite anodes, the CSE forms a continuous ionic pathway, while Super P and hard carbon provide electronic conductivity. The effective conductivity \( \sigma_{eff} \) can be estimated using Bruggeman’s symmetric model:
$$ \phi_e \left( \frac{\sigma_e – \sigma_{eff}}{\sigma_e + 2\sigma_{eff}} \right) + \phi_i \left( \frac{\sigma_i – \sigma_{eff}}{\sigma_i + 2\sigma_{eff}} \right) = 0 $$
where \( \phi_e \) and \( \phi_i \) are volume fractions of electronic and ionic conductors, and \( \sigma_e \) and \( \sigma_i \) their respective conductivities. Optimizing these parameters is crucial for high-performance solid-state batteries.
Furthermore, the interface between hard carbon and CSE is critical. XPS data show that LiF formation during cycling may passivate surfaces, but excessive growth increases resistance. We propose that controlling the Li-Nafion/LLZTO ratio can mitigate this, as LLZTO may scavenge reactive species. Future work should explore surface coatings or alternative binders to enhance solid-state battery longevity.
Conclusion and Future Perspectives
Our investigation demonstrates that hard carbon composite anodes, utilizing Li-Nafion/LLZTO as a multifunctional binder, offer promising performance in solid-state batteries. Key findings include: (i) an optimal CSE content of 30 wt% balances ionic and electronic conduction; (ii) cold-pressing to reduce porosity improves interfacial contact; (iii) operation at 70°C yields stable cycling with high coulombic efficiency; and (iv) irreversible capacity stems mainly from Li-Nafion decomposition and solvent reduction. The successful assembly of full-cells and a pouch cell validates the practicality of this approach for solid-state battery development.
Looking ahead, we envision further optimization through advanced composites, such as incorporating sulfide solid electrolytes for higher conductivity, or employing 3D electrode architectures to enhance loading. The integration of hard carbon anodes with high-voltage cathodes could unlock energy densities exceeding 400 Wh/kg in solid-state batteries. Moreover, in-situ characterization techniques will be invaluable for probing interfacial dynamics in real-time.
In summary, solid-state batteries represent a transformative technology, and our work on hard carbon composite anodes contributes to overcoming material-level hurdles. By continuing to refine electrode design and electrolyte formulations, we can accelerate the commercialization of safe, high-energy solid-state battery systems for diverse applications.