The quest for safer and higher-energy-density energy storage solutions has driven significant research into solid-state batteries. These systems, which replace the flammable organic liquid electrolytes of conventional lithium-ion batteries with solid alternatives, offer a promising path toward mitigating thermal runaway risks while potentially enabling the use of high-capacity electrodes like lithium metal. The development of high-performance electrodes compatible with solid electrolytes is therefore of paramount importance for realizing the full potential of solid-state battery technology.
Among anode materials, hard carbon (HC) presents distinct advantages for integration into solid-state battery architectures. Its mechanism of lithium storage, involving adsorption in micropores and intercalation between disordered graphene layers, results in minimal volumetric change during charge and discharge cycles. This near-zero strain characteristic is crucial for maintaining intimate physical contact at the solid-solid interface between the active material and the surrounding solid electrolyte matrix during prolonged cycling, a common failure point in solid-state battery electrodes. The performance of any solid-state electrode is fundamentally governed by the efficiency of ionic and electronic transport networks within its composite structure. Unlike liquid electrolytes that permeate porous electrodes, solid electrolytes require meticulously engineered percolation pathways. In this work, we explore the design and fabrication of a composite anode for solid-state batteries using hard carbon as the active material, integrated with a dual-functional binder that also serves as the ion-conducting phase.
Solid electrolytes are broadly categorized into inorganic, polymeric, and organic-inorganic composites. Inorganic types, such as sulfides (e.g., Li6PS5Cl) and oxides (e.g., garnet-type Li7La3Zr2O12), offer high ionic conductivity but face challenges like poor air stability (sulfides) or high rigidity leading to poor interfacial contact (oxides). Polymer electrolytes, like poly(ethylene oxide) (PEO), provide good flexibility but suffer from low room-temperature conductivity and narrow electrochemical stability windows. Composite solid electrolytes (CSEs) aim to synergize the benefits of both, combining the mechanical flexibility and processability of polymers with the enhanced ionic conductivity and stability of ceramic fillers.
A critical advancement in solid-state battery safety involves the use of single-ion conductors. In conventional dual-ion conductors (e.g., PEO-LiTFSI), both cations and anions are mobile, which can lead to concentration polarization at the electrode/electrolyte interface during plating/stripping, promoting dendritic lithium growth. Single-ion conductors, where anions are covalently tethered to the polymer backbone or immobilized in a lattice, ensure a uniform Li+ flux, suppressing concentration gradients and promoting homogeneous lithium deposition. Lithium-exchanged Nafion (Li-Nafion) is one such single-ion conducting polymer, where sulfonate groups (-SO3–Li+) provide the Li+ transport pathway. Our prior research demonstrated that Li-Nafion membranes could enable uniform Li deposition and inhibit dendrite formation. Furthermore, composites of Li-Nafion with garnet-type oxide ceramics (e.g., Li6.25La3Zr2Al0.25O12, LLZAO) showed significantly enhanced ionic conductivity and improved rate capability in solid-state cells, attributed to favorable interfacial interactions creating space-charge layers with lower activation energy for ion transport.
Building on this foundation, this work employs a composite of Li-Nafion and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) garnet ceramic as both the binder and the ionic conductor within the hard carbon electrode. This approach aims to construct a continuous and resilient ionic network throughout the anode composite. We systematically investigate the influence of electrode composition, porosity (via calendaring pressure), and operating temperature on the electrochemical performance of the hard carbon composite anode in a solid-state battery configuration. The sources of irreversible capacity are elucidated through electrochemical analysis and X-ray photoelectron spectroscopy (XPS). Finally, the practicality of the optimized anode is validated in full solid-state battery cells paired with LiFePO4 and single-crystal LiNi0.6Co0.1Mn0.3O2 (NCM613) cathodes.
Experimental Design and Methodology
The core strategy involves fabricating a composite electrode where the ionic transport medium is integral to the electrode structure. The materials synthesis, electrode preparation, and cell assembly were conducted under an argon atmosphere in a glovebox (H2O, O2 < 0.5 ppm).
1. Synthesis of Composite Solid Electrolyte (CSE) Binder
Proton-form Nafion 211 membrane was lithiated by immersion in 1 M LiOH solution at 80°C for 12 hours, followed by thorough washing and drying to obtain Li-Nafion. A 5 wt.% solution of Li-Nafion in N-methyl-2-pyrrolidinone (NMP) was prepared. Li6.4La3Zr1.4Ta0.6O12 (LLZTO) powder was synthesized via a solid-state reaction. The composite electrolyte binder was prepared by ball-mixing the Li-Nafion solution with LLZTO powder in specific mass ratios. The ionic conductivity ($\sigma$) was determined from electrochemical impedance spectroscopy (EIS) measurements on symmetric stainless steel | CSE | stainless steel cells, using the equation:
$$ \sigma = \frac{l}{R_b \cdot A} $$
where $l$ is the thickness of the electrolyte, $A$ is the contact area, and $R_b$ is the bulk resistance obtained from the high-frequency intercept on the real axis of the Nyquist plot. The activation energy ($E_a$) for ion conduction was derived from the Arrhenius equation:
$$ \sigma T = A_0 \exp\left(-\frac{E_a}{k_B T}\right) $$
where $A_0$ is the pre-exponential factor, $k_B$ is the Boltzmann constant, and $T$ is the absolute temperature.
2. Fabrication of Hard Carbon Composite Anodes
The hard carbon composite anodes were prepared by ball-mixing hard carbon powder, Super P carbon conductive additive, and the as-prepared Li-Nafion/LLZTO CSE binder in NMP solvent. The slurry was cast onto copper foil and dried. The composition was varied to study its impact, as summarized in Table 1.
| Electrode Designation | Hard Carbon (wt.%) | Super P (wt.%) | CSE Binder (wt.%) |
|---|---|---|---|
| HC-20CSE | 70 | 10 | 20 |
| HC-30CSE | 60 | 10 | 30 |
| HC-40CSE | 50 | 10 | 40 |
| HC-50CSE | 40 | 10 | 50 |
After drying, the electrodes were calendared at different pressures (0 to 3480 MPa) to control porosity ($\phi$). The porosity was calculated based on the mass and thickness of the electrode components:
$$ \phi = \left[1 – \frac{\frac{m_\text{HC}}{\rho_\text{HC}} + \frac{m_\text{SP}}{\rho_\text{SP}} + \frac{m_\text{CSE}}{\rho_\text{CSE}}}{S \cdot (\delta_\text{electrode} – \delta_\text{Cu})}\right] \times 100\% $$
where $m$ and $\rho$ denote mass and true density of each component (HC, Super P, CSE), $S$ is the electrode area, and $\delta$ denotes thickness.
3. Cell Assembly and Electrochemical Characterization
For half-cell tests, CR2032 coin cells were assembled with the HC composite anode as the working electrode, a PC/EC-swollen Li-Nafion membrane as the separator/solid electrolyte, and lithium metal as the counter/reference electrode. Full cells were assembled using a pre-lithiated HC composite anode (to compensate for initial irreversible capacity loss) and a LiFePO4 or NCM613 cathode with Li-Nafion as the binder. Galvanostatic charge-discharge cycling and rate capability tests were performed. EIS was conducted over a frequency range of 0.1 Hz to 100 kHz. Differential capacity (dQ/dV) analysis was used to identify electrochemical reactions. XPS was employed for surface chemical analysis of cycled electrodes.
Results and Discussion
1. Optimization of the Composite Electrolyte Binder
The ionic conductivity of the Li-Nafion/LLZTO composite was first optimized by varying the LLZTO content. The Arrhenius plots for composites with 5, 10, 20, and 30 wt.% LLZTO are shown below. The composite containing 10 wt.% LLZTO exhibited the highest ionic conductivity across the measured temperature range (30-90°C) and the lowest activation energy for ion migration ($E_a$ = 22.65 kJ/mol), as detailed in Table 2.
| Sample | Activation Energy, $E_a$ (kJ/mol) |
|---|---|
| Li-Nafion / 5% LLZTO | 28.70 |
| Li-Nafion / 10% LLZTO | 22.65 |
| Li-Nafion / 20% LLZTO | 23.16 |
| Li-Nafion / 30% LLZTO | 26.62 |
XPS analysis of the F 1s region revealed a distinct peak at ~685.2 eV in the Li-Nafion/10%LLZTO composite, attributable to Li-F species, which was negligible in pure Li-Nafion. This suggests a chemical interaction at the polymer-ceramic interface, likely similar to the space-charge layer formation previously reported for Li-Nafion/LLZAO composites. This interfacial modification is believed to lower the energy barrier for Li+ hopping, contributing to the enhanced conductivity. Consequently, the Li-Nafion/10%LLZTO composition was selected as the optimal CSE binder for subsequent solid-state battery electrode fabrication.
2. Impact of Electrode Composition on Solid-State Battery Performance
The composition of the composite anode critically balances electronic percolation (via HC and Super P) and ionic percolation (via the CSE binder). The electrochemical impedance spectra of HC/Li-Nafion/Li half-cells with different CSE contents were fitted using an equivalent circuit model: $R_e$ (ohmic resistance), $R_1$ (anode|separator interface resistance), $R_2$ (internal anode composite resistance), and $R_3$ (Li|separator interface resistance). The fitted values and the corresponding first-cycle charge capacity are consolidated in Table 3.
| Electrode | $R_e$ (Ω) | $R_1$ (Ω) | $R_2$ (Ω) | 1st Charge Capacity (mAh/g) |
|---|---|---|---|---|
| HC-20CSE | 85 | 420 | 280 | 239.0 |
| HC-30CSE | 110 | 250 | 180 | 334.0 |
| HC-40CSE | 135 | 200 | 150 | 311.6 |
| HC-50CSE | 170 | 180 | 120 | 179.9 |
The data reveals a clear trend: as the CSE content increases from 20% to 50%, the interfacial resistances ($R_1$ and $R_2$) decrease due to improved ionic contact and more extensive ion-conducting pathways. However, the ohmic resistance ($R_e$) increases, likely due to the dilution of the electronically conductive network. HC-30CSE and HC-40CSE achieve an optimal balance, delivering the highest reversible capacities close to the HC capacity in liquid electrolyte. HC-20CSE suffers from high interfacial impedance limiting Li+ access, while HC-50CSE suffers from high electronic resistance, leading to significant polarization and capacity loss. This underscores the critical need for bicontinuous networks in a solid-state battery electrode.
3. Origins of Irreversible Capacity in the Solid-State Battery Anode
The initial coulombic efficiency (ICE) of the hard carbon composite anodes was low (15-40%), indicating substantial irreversible capacity. Cyclic voltammetry and dQ/dV analysis identified reduction peaks at ~1.2 V and ~0.75 V vs. Li+/Li only in the first cycle. The 1.2 V peak is attributed to the reductive decomposition of the PC/EC solvent used to swell the Li-Nafion separator. The 0.75 V feature is associated with irreversible lithium adsorption onto surface defects/functional groups of hard carbon.
XPS analysis of electrodes after the first cycle provided deeper insight. The relative amounts of LiF and Li2CO3—common solid electrolyte interphase (SEI) components—were quantified from the F 1s and C 1s spectra, respectively. A strong correlation was observed: as the CSE binder content increased, the LiF content on the electrode surface increased significantly, while Li2CO3 decreased, coinciding with a lower ICE. Li2CO3 primarily forms from solvent (PC/EC) reduction. The LiF, however, has a different origin. Control experiments with Li-Nafion binder (without LLZTO) confirmed that LiF forms during discharge and remains after charging. This points to an interfacial reaction between the sulfonate groups in Li-Nafion and the lithiated hard carbon (LiCx) formed during discharge:
$$ \text{LiC}_x + \text{R-SO}_3\text{Li} \rightarrow \text{LiF} + \text{Other Products} $$
This reaction occurs not only at the outer electrode/separator interface but also internally at the HC/CSE binder interfaces, consuming active Li+ and contributing dominantly to the irreversible capacity. Managing this interface is a key challenge for this specific solid-state battery chemistry.
4. Porosity and Calendaring Effects on the Solid-State Battery Electrode
In liquid cells, electrolyte fills pores, ensuring good interfacial contact. In solid-state batteries, reducing porosity via calendaring is essential to improve solid-solid contact points. HC-30CSE electrodes were calendared at different pressures, and their performance was evaluated. The results are summarized in Table 4.
| Pressure (MPa) | Porosity, $\phi$ (%) | $R_e$ (Ω) | $R_1$ (Ω) | $R_2$ (Ω) | 1st Charge Capacity (mAh/g) |
|---|---|---|---|---|---|
| 0 | ~45 | 210 | 550 | 350 | 210.5 |
| 870 | ~32 | 165 | 380 | 270 | 268.3 |
| 1740 | ~25 | 135 | 300 | 220 | 298.7 |
| 3480 | ~18 | 110 | 250 | 180 | 334.0 |
Increasing calendaring pressure monotonically decreased porosity, which in turn reduced all impedance components ($R_e$, $R_1$, $R_2$) and increased the achievable capacity. This demonstrates that densification is a vital processing step to minimize interfacial resistances and maximize active material utilization in this solid-state battery configuration.
5. Temperature-Dependent Performance of the Solid-State Battery
The ionic conductivity of polymer-based solid electrolytes is thermally activated. The performance of HC-30CSE/Li half-cells was evaluated at 50, 70, and 90°C. As expected, increasing temperature lowered all impedance parameters and increased initial capacity. However, long-term cycling stability revealed an optimum. While the cell at 90°C delivered the highest initial capacity (~490 mAh/g), it showed continuous decay and a sharp drop after 55 cycles. The cell at 70°C offered the best compromise, with a high initial capacity (~344 mAh/g) and exceptional cycling stability, maintaining an average coulombic efficiency of 99.82% over 180 cycles. The cell at 50°C had lower capacity and poorer stability. This indicates that while higher temperatures benefit kinetics, they may accelerate detrimental interfacial side reactions in the solid-state battery. Therefore, 70°C was identified as the optimal operating temperature for this system.
6. Rate Capability and Full Solid-State Battery Performance
The optimized HC-30CSE anode (calendared at 3480 MPa) demonstrated good rate capability at 70°C. Specific capacities of 273.6, 198.5, 163.5, 112.2, and 45.7 mAh/g were obtained at current densities of 22, 44, 66, 110, and 220 mA/g, respectively. Capacity recovered to 261.0 mAh/g upon returning to 22 mA/g, indicating the robustness of the internal conductive network.
To validate practical utility, full solid-state battery cells were constructed. A pre-lithiated HC-30CSE anode was paired with an LiFePO4 cathode. The full cell cycled stably for 400 cycles at 0.3C and 70°C, with an average coulombic efficiency of 99.80%. Furthermore, a practical solid-state pouch cell was fabricated using a pre-lithiated HC composite anode and a single-crystal NCM613 cathode. The cell delivered an initial discharge capacity of 154.4 mAh/g (based on NCM mass) with an ICE of 84.66%, and cycled for 30 cycles with an average coulombic efficiency of 98.51%. These results affirm the viability of the developed hard carbon composite anode in functional solid-state battery devices.
Conclusion
This work systematically investigates the design principles for high-performance hard carbon composite anodes in solid-state battery applications. By employing a Li-Nafion/LLZTO composite as a dual-functional ion-conducting binder, we successfully constructed a resilient ionic network within the electrode. The study establishes that electrode composition must strike a precise balance between ionic and electronic percolation, with a CSE content of 30-40 wt.% yielding optimal capacity. Furthermore, electrode densification through calendaring is crucial to minimize interfacial resistances inherent to solid-state battery systems. Operating temperature is a critical parameter, with 70°C providing an optimal balance between enhanced ionic conductivity and interfacial stability.
A key finding is the identification of the primary source of initial irreversible capacity: a chemical reaction between the lithiated hard carbon and the Li-Nafion component of the binder, leading to LiF formation. This insight is crucial for future interface engineering in similar solid-state battery chemistries.
Ultimately, the optimized hard carbon composite anode exhibited excellent cycling stability and rate capability. Its successful integration into both coin-type and pouch-type full cells with lithium metal oxide cathodes demonstrates a significant step toward practical, high-energy-density solid-state batteries. Future work will focus on mitigating the irreversible interfacial reaction through surface modification of hard carbon or the development of alternative single-ion conductors, further pushing the boundaries of solid-state battery technology.