In recent years, the demand for high-energy-density and safe energy storage systems has driven research into solid-state batteries as a promising alternative to conventional lithium-ion batteries. Solid-state batteries offer enhanced safety, higher mechanical stability, and a wider voltage window compared to their liquid electrolyte counterparts. However, the development of high-performance anode materials that are compatible with solid electrolytes remains a critical challenge. In this study, we explore the use of phosphorus-based composite anodes, specifically phosphorus/carbon/lithium (P/C/Li) composites, for application in solid-state batteries. Phosphorus exhibits a high theoretical specific capacity of approximately 2600 mAh/g, while carbon provides excellent chemical stability and interface compatibility, and lithium contributes superior mechanical properties and a high theoretical capacity. By combining these materials through high-energy ball milling and pre-lithiation, we aim to leverage their synergistic effects to achieve improved electrochemical performance in solid-state battery systems.
The preparation of P/C composites involved high-energy ball milling of red phosphorus and graphite in various mass ratios, followed by pre-lithiation with metallic lithium to form ternary P/C/Li composites. The composites were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD) to analyze their morphology and structure. Electrochemical evaluations included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge tests in both half-cell and full-cell configurations. The solid-state electrolyte used was a composite of lithium lanthanum zirconium tantalum oxide (LLZTO) and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), which was prepared via a solution-casting method. To mitigate interface impedance at room temperature, a minimal amount of liquid electrolyte (1 μL of LiPF6) was added to the electrolyte surface, without altering the lithium-ion transport mechanism in the solid-state electrolyte.
The structural analysis revealed that high-energy ball milling resulted in amorphous P/C composites, as evidenced by the diminished and broadened XRD peaks. SEM images showed that the composite powders achieved nanoscale dimensions, with more uniform and finer particles observed in the pre-lithiated samples. For instance, the P/C28-10% composite (with a P/C mass ratio of 2:8 and 10% pre-lithiation) exhibited a homogeneous distribution without significant agglomeration, which is crucial for enhancing ionic conductivity and mitigating volume expansion during cycling. The XRD patterns confirmed the amorphous nature of both binary P/C and ternary P/C/Li composites, indicating that the ball milling process effectively disrupted the crystalline structures of the raw materials.
Electrochemical performance was evaluated through CV curves, which displayed distinct redox peaks corresponding to lithium insertion and extraction processes. For the P/C28-10% composite, the CV curves showed a broad reduction peak between 0.3 V and 0.5 V and a main reduction peak at 0.14 V during the first cycle, shifting to 0.01 V in subsequent cycles. The oxidation process involved a broad peak at 0.3–0.6 V and a main peak at 1.03 V, indicating a two-step mechanism for lithium storage. This behavior is consistent with the alloying reaction of phosphorus with lithium, which can be represented by the following equation:
$$ \text{P} + x\text{Li}^+ + x\text{e}^- \leftrightarrow \text{Li}_x\text{P} $$
where \( x \) varies depending on the lithiation degree. The theoretical capacity of phosphorus can be calculated using the formula:
$$ C_{\text{theoretical}} = \frac{nF}{3.6M} $$
where \( n \) is the number of electrons transferred, \( F \) is Faraday’s constant, and \( M \) is the molar mass. For phosphorus, with \( n = 3 \) and \( M = 30.97 \, \text{g/mol} \), the theoretical capacity is approximately 2596 mAh/g.
The cycling performance of the P/C composites was assessed in half-cells with a voltage range of 0.01–2.80 V and a current density of 400 mA/g. The results demonstrated that the P/C28 composite (P/C mass ratio of 2:8) delivered the highest discharge capacity among the binary composites, with an initial capacity of 704.1 mAh/g and a retention of 204.2 mAh/g after 100 cycles, corresponding to a capacity retention rate of 29%. In contrast, the pre-lithiated P/C28-10% composite exhibited a significantly improved initial discharge capacity of 608.3 mAh/g and a first-cycle coulombic efficiency of 98.22%. After 100 cycles, it maintained a capacity of 211.1 mAh/g with a retention rate of 35% and an average coulombic efficiency of 99.54%. The enhanced performance is attributed to the pre-lithiation process, which compensates for active lithium loss and stabilizes the solid electrolyte interphase (SEI) formation. The table below summarizes the electrochemical properties of different P/C composites:
| Composite | Initial Capacity (mAh/g) | Capacity after 100 cycles (mAh/g) | Retention Rate (%) |
|---|---|---|---|
| P/C19 | 595.8 | 132.2 | 22.2 |
| P/C28 | 704.1 | 204.2 | 29.0 |
| P/C37 | 752.3 | 180.5 | 24.0 |
| P/C46 | 801.6 | 165.8 | 20.7 |
| P/C55 | 850.9 | 140.3 | 16.5 |
| P/C28-5% | 580.4 | 174.8 | 30.1 |
| P/C28-10% | 608.3 | 211.1 | 35.0 |
| P/C28-20% | 550.7 | 125.5 | 22.8 |
| P/C28-30% | 500.2 | 54.1 | 10.8 |
Rate capability tests were conducted at current densities ranging from 40 mA/g to 1000 mA/g. The P/C28-10% composite demonstrated superior rate performance, with discharge capacities of 771.0 mAh/g at 40 mA/g, 628.8 mAh/g at 100 mA/g, 545.3 mAh/g at 200 mA/g, 386.7 mAh/g at 400 mA/g, and 184.0 mAh/g at 1000 mA/g. When the current density was returned to 40 mA/g, the capacity recovered to 551.8 mAh/g, indicating good reversibility and structural stability. In comparison, the binary P/C28 composite showed lower capacities across all current densities, highlighting the beneficial role of pre-lithiation in enhancing the kinetics of lithium-ion transport. The impedance analysis via EIS revealed that the P/C28-10% composite had a lower interface resistance (11.2 Ω) compared to the binary P/C28 composite (35.4 Ω), further confirming the improved interface compatibility in solid-state batteries.
The full-cell configuration using LiFePO4 as the cathode, LLZTO/PVDF-HFP as the solid electrolyte, and P/C28-10% as the anode was assembled to evaluate practical application in solid-state batteries. The CV curves of the full-cell exhibited redox peaks corresponding to the lithiation/delithiation of both electrodes, with operating voltages between 1.0 V and 4.2 V. The symmetric cell P/C28-10% | LLZTO/PVDF-HFP | P/C28-10% was tested at a current density of 0.5 mA/cm² for lithium deposition/stripping cycles, showing stable voltage profiles over 200 hours without short-circuiting, which underscores the dendrite-suppression capability of the composite anode. The discharge potential plateau of the P/C28-10% anode was observed at around 0.7 V, which is higher than that of metallic lithium (0 V), thereby reducing the risk of lithium dendrite formation and enhancing the safety of solid-state batteries.
The rate performance of the full-cell was evaluated at various C-rates (0.1 C to 2 C), where 1 C corresponds to 170 mA/g based on the LiFePO4 capacity. The discharge capacities were 152.9 mAh/g at 0.1 C, 138.6 mAh/g at 0.2 C, 119.4 mAh/g at 0.5 C, 105.7 mAh/g at 1 C, and 79.9 mAh/g at 2 C. Upon returning to 0.1 C, the capacity recovered to 146.6 mAh/g, demonstrating excellent rate capability and stability. The cycling performance at 1 C showed an initial discharge capacity of 133.7 mAh/g, which retained 107.6 mAh/g after 100 cycles, with a capacity retention rate of 80.5%. The coulombic efficiency remained above 99% throughout the cycling test, indicating minimal side reactions and efficient lithium utilization. The following equation represents the overall reaction in the full-cell:
$$ \text{LiFePO4} + \text{Li}_x\text{P} \leftrightarrow \text{FePO4} + \text{Li}_{x+1}\text{P} $$
The volume change during cycling was calculated using the formula for volume expansion:
$$ \Delta V = \frac{V_{\text{lithiated}} – V_{\text{unlithiated}}}{V_{\text{unlithiated}}} \times 100\% $$
For phosphorus, the volume expansion can reach up to 300%, but in the P/C28-10% composite, the carbon matrix effectively buffered this expansion, as confirmed by post-cycling SEM analysis that showed minimal cracking and particle disintegration. The table below compares the key parameters of the solid-state battery with P/C28-10% anode:
| Parameter | Value |
|---|---|
| Initial Capacity (1 C) | 133.7 mAh/g |
| Capacity after 100 cycles | 107.6 mAh/g |
| Capacity Retention Rate | 80.5% |
| Average Coulombic Efficiency | >99% |
| Interface Resistance | 11.2 Ω |
| Stable Cycling Time (symmetric cell) | >200 hours |
In conclusion, the phosphorus/carbon/lithium composite anode developed through high-energy ball milling and pre-lithiation demonstrates significant potential for application in solid-state batteries. The optimal composition with a P/C mass ratio of 2:8 and 10% pre-lithiation (P/C28-10%) exhibits high capacity, excellent cycling stability, and superior rate performance. The composite anode’s discharge potential above that of metallic lithium reduces dendrite formation, enhancing the safety of solid-state batteries. These findings provide a new pathway for designing high-energy-density and safe anode materials for advanced solid-state battery systems. Future work will focus on optimizing the pre-lithiation degree and exploring scalable manufacturing processes to further improve the performance and practicality of solid-state batteries.