In recent years, the demand for high-energy-density and safe energy storage systems has driven research toward solid-state batteries as a promising alternative to conventional lithium-ion batteries. Traditional lithium-ion batteries, while widely used, face challenges such as electrolyte leakage and safety risks due to the flammable organic liquid electrolytes. Solid-state batteries, which employ solid electrolytes, offer enhanced safety, mechanical stability, and a wider voltage window. However, the performance of solid-state batteries is heavily influenced by the electrode materials, particularly the anode, which must exhibit high capacity, good interfacial compatibility, and minimal volume changes during cycling. Among various anode materials, phosphorus-based composites have garnered attention due to their high theoretical specific capacity, but issues like poor conductivity and significant volume expansion hinder their practical application. In this study, we developed a phosphorus/carbon/lithium composite anode via high-energy ball milling combined with pre-lithiation to address these challenges and enhance the performance of solid-state batteries.
The preparation of the composite anode involved high-energy ball milling of red phosphorus and graphite in different mass ratios, followed by pre-lithiation with metallic lithium to form a ternary amorphous structure. We investigated various phosphorus-to-carbon ratios (e.g., 1:9, 2:8, 3:7, 4:6, and 5:5) and pre-lithiation levels (5%, 10%, 20%, and 30%) to optimize the electrochemical properties. The composite powders were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD), which confirmed the amorphous nature and uniform particle distribution. For electrochemical evaluation, we assembled half-cells with liquid electrolytes and full solid-state batteries using a LiFePO4 cathode and a LLZTO/PVDF-HFP composite solid electrolyte. Tests included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge cycling, and rate capability assessments.
The SEM analysis revealed that the ball-milled phosphorus/carbon composites exhibited nanoscale particles, with the ternary phosphorus/carbon/lithium sample showing more uniform and finer morphology compared to the binary counterparts. This improved structure facilitates better ion transport and reduces volume changes during lithiation and delithiation. The XRD patterns indicated a loss of crystallinity in the ball-milled samples, confirming the formation of an amorphous phase, which is beneficial for enhancing ionic conductivity and mitigating structural degradation. The electrochemical performance was evaluated through CV curves, which displayed distinct redox peaks corresponding to lithium insertion and extraction processes. For instance, the ternary composite showed reduction peaks around 0.3–0.5 V and 0.14 V, and oxidation peaks at 0.3–0.6 V and 1.03 V, indicating a multi-step reaction mechanism.
To quantify the performance, we conducted cycling tests on half-cells at a current density of 400 mA/g. The binary phosphorus/carbon composite with a 2:8 ratio (P/C28) delivered a discharge capacity of 204.2 mAh/g after 100 cycles, while the pre-lithiated ternary composite (P/C28-10%) achieved a higher initial discharge capacity of 608.3 mAh/g with a first-cycle coulombic efficiency of 98.22%. After 100 cycles, the P/C28-10% sample retained a capacity of 211.1 mAh/g, demonstrating improved cycling stability. The rate capability tests further highlighted the superiority of the pre-lithiated anode, with capacities of 771.0, 628.8, 545.3, 386.7, and 184.0 mAh/g at current densities of 40, 100, 200, 400, and 1000 mA/g, respectively. EIS measurements revealed lower interfacial resistance for the P/C28-10% composite (11.2 Ω) compared to the non-pre-lithiated sample (35.4 Ω), indicating enhanced ion transport kinetics.
In solid-state battery configurations, the P/C28-10% anode paired with a LiFePO4 cathode and LLZTO/PVDF-HFP electrolyte exhibited excellent performance. The symmetric cell P/C28-10% | LLZTO/PVDF-HFP | P/C28-10% maintained stable voltage profiles for over 200 hours at 0.5 mA/cm2, suggesting minimal lithium dendrite formation and good interfacial stability. The full cell delivered a discharge capacity of 133.7 mAh/g at 1 C rate initially, with a capacity retention of 107.6 mAh/g after 100 cycles, corresponding to 80.5% capacity retention. The rate performance showed capacities of 152.9, 138.6, 119.4, 105.7, and 79.9 mAh/g at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, underscoring the robustness of the composite anode in solid-state batteries.
The enhanced performance can be attributed to the synergistic effects of the composite design. Phosphorus provides high theoretical capacity, graphite offers structural stability and conductivity, and pre-lithiated lithium compensates for irreversible lithium loss and improves mechanical properties. The amorphous structure from ball milling facilitates better stress distribution during volume changes, while the pre-lithiation process optimizes the initial coulombic efficiency. These factors collectively contribute to the development of high-energy-density and safe solid-state batteries. Below, we summarize key parameters and theoretical aspects using tables and equations to provide a comprehensive overview.
| Anode Material | Initial Discharge Capacity (mAh/g) | Capacity after 100 Cycles (mAh/g) | Capacity Retention (%) | First-Cycle Coulombic Efficiency (%) |
|---|---|---|---|---|
| P/C19 | ~600 | 132.2 | 22.2 | ~95 |
| P/C28 | ~700 | 204.2 | 29.0 | ~96 |
| P/C37 | ~750 | ~180 | ~24 | ~95 |
| P/C46 | ~800 | ~150 | ~19 | ~94 |
| P/C55 | ~850 | ~100 | ~12 | ~93 |
| P/C28-5% | ~550 | 174.8 | ~32 | ~97 |
| P/C28-10% | 608.3 | 211.1 | 35.0 | 98.22 |
| P/C28-20% | ~500 | 125.5 | ~25 | ~96 |
| P/C28-30% | ~450 | 54.1 | ~12 | ~95 |
The theoretical specific capacity of phosphorus can be expressed using the formula:
$$C_{\text{theoretical}} = \frac{nF}{M}$$
where (n) is the number of electrons transferred per mole (for phosphorus, (n = 3) in the reaction (P + 3Li^+ + 3e^- \leftrightarrow Li_3P)), (F) is the Faraday constant (96485 C/mol), and (M) is the molar mass of phosphorus (30.97 g/mol). This gives:
$$C_{\text{theoretical}} = \frac{3 \times 96485}{30.97} \approx 2596 \, \text{mAh/g}$$
For graphite, the theoretical capacity is based on the formation of LiC6:
$$C_{\text{graphite}} = \frac{1 \times 96485}{72.06} \approx 372 \, \text{mAh/g}$$
The composite capacity can be modeled as a weighted average, but in practice, it is influenced by the synergistic effects. The volume change during lithiation for phosphorus is significant, and the stress ((\sigma)) generated can be approximated by:
$$\sigma = E \cdot \frac{\Delta V}{V}$$
where (E) is the Young’s modulus and (\Delta V/V) is the volume strain. For phosphorus, (\Delta V/V \approx 3), leading to high stress, which is mitigated by the carbon matrix in the composite.
| Current Rate (C) | Discharge Capacity (mAh/g) | Capacity Retention (%) | Average Coulombic Efficiency (%) |
|---|---|---|---|
| 0.1 | 152.9 | 100 | ~99 |
| 0.2 | 138.6 | 90.6 | ~99 |
| 0.5 | 119.4 | 78.1 | ~98 |
| 1.0 | 105.7 | 69.1 | ~98 |
| 2.0 | 79.9 | 52.2 | ~97 |
The electrochemical reaction in phosphorus-based anodes involves alloying and dealloying processes, which can be described by the following equation:
$$P + xLi^+ + xe^- \leftrightarrow Li_xP$$
where (x) varies depending on the state of charge, with (x = 3) for full lithiation. The voltage profile of phosphorus shows a plateau around 0.7 V vs. Li/Li+, which is higher than that of graphite (0.1 V) and silicon (0.2 V), reducing the risk of lithium dendrite formation in solid-state batteries. This is crucial for enhancing the safety of solid-state batteries, as dendrites can cause short circuits and failure.
In terms of interfacial properties, the solid electrolyte interface (SEI) formation consumes lithium ions, leading to initial capacity loss. Pre-lithiation helps compensate for this by providing excess lithium, which can be quantified by the irreversible capacity loss ((C_{\text{irr}})):
$$C_{\text{irr}} = C_{\text{first discharge}} – C_{\text{first charge}}$$
For the P/C28-10% composite, (C_{\text{irr}}) was minimal, indicating effective pre-lithiation. The overall energy density of a solid-state battery can be estimated using:
$$E_{\text{density}} = \frac{C_{\text{anode}} \times V_{\text{anode}} \times C_{\text{cathode}} \times V_{\text{cathode}}}{C_{\text{anode}} + C_{\text{cathode}}}$$
where (C) represents capacity and (V) represents voltage. With the high capacity of the phosphorus-based anode, the energy density of solid-state batteries can be significantly improved.
In conclusion, our development of a phosphorus/carbon/lithium composite anode via high-energy ball milling and pre-lithiation demonstrates a viable approach for enhancing the performance of solid-state batteries. The optimized composite exhibits high capacity, excellent cycling stability, and good interfacial compatibility with solid electrolytes. These findings pave the way for next-generation solid-state batteries with high energy density and safety, suitable for applications in electric vehicles and large-scale energy storage. Future work will focus on scaling up the synthesis and further optimizing the composite for commercial solid-state battery systems.