Study on Interfacial Stability and Electrochemical Performance of LFP/Graphite Battery Regulated by Mechanical Pressure

1. Background: Why Mechanical Pressure Matters for LFP/Graphite Battery Cycle Life

LFP/graphite batteries — cells using LiFePO₄ (lithium iron phosphate) cathodes and graphite anodes — dominate commercial electric vehicle and grid energy storage deployments due to their high safety, long cycle life, and cost competitiveness. During real-world operation, these batteries are subject to continuous mechanical pressure arising from three sources: module assembly preloading, housing constraint, and the repeated volumetric expansion and contraction of electrodes during cycling.

Mechanical pressure in battery systems is defined as the compressive stress applied to a cell stack, typically expressed in MPa, that affects electrode particle contact geometry, electrode–electrolyte interfacial stability, lithium-ion transport behavior, and electrode structural evolution over cycling. Prior research has established that moderate external pressure improves electrode contact, reduces interfacial impedance, and stabilizes SEI film formation — but the coupled mechanisms by which pressure simultaneously regulates graphite anode SEI composition, cathode interfacial stability, aluminum current collector corrosion, and LiFePO₄/FePO₄ phase transformation reversibility in a full LFP/graphite cell had not been systematically characterized.

The SEI layer (solid electrolyte interphase) is defined as the passivation film that forms on the graphite anode surface during the first charge cycle from electrolyte reductive decomposition. Its composition — particularly the ratio of inorganic components (LiF, Li₂CO₃) to organic components (ROCO₂Li, ROLi) — determines ionic conductivity, mechanical stability, and the rate of ongoing electrolyte consumption during cycling. A LiF-rich SEI layer is mechanically harder and more ionically selective, providing superior long-term protection compared to organic-dominated SEI layers that are prone to cracking during volume change cycles.

2. Key Findings: Moderate Pressure Slashes Capacity Fade From 24.31% to 5.14%

The research team compared the electrochemical performance of LFP/graphite pouch cells under two conditions: no applied pressure (0 MPa) and moderate applied mechanical pressure (0.1 MPa). After cycling, the cells were analyzed using a comprehensive suite of characterization techniques — in-situ XRD, SEM, AFM, HRTEM, XPS, FTIR, TOF-SIMS, and DCIR measurement — to correlate electrochemical performance with structural and chemical evolution at both electrodes.

3. Mechanism 1 — Improved Electrochemical Performance and LFP Phase Transformation Uniformity

Electrochemical characterization confirms that 0.1 MPa mechanical pressure produces a measurable and significant improvement in LFP/graphite battery cycle stability. DCIR (direct current internal resistance) growth is substantially slower under pressure, indicating that interfacial stability is maintained more effectively throughout cycling. Reduced polarization in charge/discharge curves and more sharply defined features in dQ/dV analysis confirm that the electrochemical reaction kinetics are better preserved under pressure.

dQ/dV analysis (differential capacity analysis) — in which the derivative of capacity with respect to voltage is plotted against voltage to reveal phase transformation peaks — shows that the characteristic LiFePO₄ ↔ FePO₄ two-phase transition peaks remain more distinct and better-resolved under 0.1 MPa compared to the unpressurized condition, where peak broadening and shifting indicate progressive kinetic deterioration and reduced phase transformation reversibility.

In-situ XRD measurements during cycling reveal that 0.1 MPa pressure promotes more spatially uniform LiFePO₄/FePO₄ phase transformation — meaning lithium extraction and insertion occur more homogeneously across the cathode electrode rather than concentrating in localized regions. This uniformity directly reduces mechanical stress gradients within the cathode electrode and diminishes the driving force for crack formation and particle fracture. Concurrently, temperature measurements confirm lower thermal accumulation under pressure, consistent with reduced internal resistance and Joule heating.

Figure 1. Electrochemical performance of LFP/graphite batteries under 0 MPa and 0.1 MPa mechanical pressure: (a) capacity retention; (b) DCIR evolution; (c–d) charge/discharge curves and dQ/dV analysis. Capacity decay: 0 MPa → 24.31%; 0.1 MPa → 5.14%.

4. Mechanism 2 — Cathode Structural Integrity and Reduced Interfacial Side Reactions

Post-cycling characterization of the LFP cathode reveals that mechanical pressure substantially preserves cathode structural integrity and reduces parasitic interfacial reactions on the cathode side. SEM imaging shows that LFP cathode particles maintain more complete morphology after cycling under 0.1 MPa pressure compared to the 0 MPa condition, where greater particle fracture and surface roughening are evident.

Quantitative phase analysis using XRD shows that the irreversible FePO₄ phase content — residual non-lithiated cathode material that cannot be re-lithiated, representing permanently lost capacity — is reduced from 34.1% to 18.8% under 0.1 MPa pressure. This 45% reduction in irreversible phase content confirms that mechanical pressure promotes more complete and reversible LiFePO₄/FePO₄ two-phase cycling, directly explaining the improved capacity retention.

XPS (X-ray photoelectron spectroscopy) surface analysis indicates that pressure reduces the accumulation of electrolyte decomposition products on the cathode surface — including phosphate and fluoride species from PF₆⁻ anion decomposition — consistent with the lower interfacial side reaction rate enabled by more stable electrode–electrolyte contact under pressure. Reduced aluminum current collector corrosion under pressure also contributes to maintained cathode current collection efficiency throughout cycling.

Figure 2. In-situ XRD and temperature evolution of LFP cathode under 0 MPa and 0.1 MPa mechanical pressure: (a–b) lattice parameter evolution during LiFePO₄/FePO₄ two-phase transformation showing improved uniformity at 0.1 MPa; (c) temperature profiles confirming reduced thermal accumulation under pressure.

Figure 3. Post-cycling LFP cathode characterization: (a) SEM morphology; (b) XRD phase analysis — irreversible FePO₄: 34.1% (0 MPa) → 18.8% (0.1 MPa); (c) XPS surface chemistry showing reduced electrolyte decomposition product accumulation under pressure.

5. Mechanism 3 — LiF-Rich SEI Formation and Graphite Anode Protection

The most mechanistically significant finding of this study is that 0.1 MPa mechanical pressure fundamentally alters the composition and morphology of the SEI layer that forms on the graphite anode surface during cycling.

Under 0 MPa conditions, the graphite anode SEI layer is dominated by organic components — lithium alkyl carbonates (ROCO₂Li, ROLi) and lithium carbonate (Li₂CO₃) — which are mechanically soft and prone to cracking during the 10–15% volume expansion and contraction of graphite during lithiation/de-lithiation cycles. Cracks in the organic SEI expose fresh graphite surface, triggering further electrolyte decomposition and SEI regrowth in an ongoing capacity-consuming cycle that progressively thickens the SEI and increases interfacial impedance.

Under 0.1 MPa mechanical pressure, XPS, FTIR, HRTEM, and TOF-SIMS characterization consistently show that the graphite anode SEI layer becomes:

• LiF-rich: inorganic LiF content in the SEI is substantially higher under pressure. LiF has high mechanical strength (Young’s modulus ~65 GPa), high ionic conductivity for Li⁺ transport, and chemical stability against further reduction — making it an ideal SEI component for long-term cycling stability.
• More uniform: the LiF-rich SEI distributes more homogeneously across the graphite particle surface, eliminating the composition gradients and localized weak spots that initiate cracking under mechanical stress during cycling.
• More mechanically stable: the harder, denser LiF-rich SEI layer maintains integrity during graphite volume expansion/contraction cycles, suppressing ongoing electrolyte decomposition and graphite structural damage.

TOF-SIMS depth profiling confirms the compositional differences between pressurized and unpressurized SEI layers at nanometer resolution, showing that LiF species are enriched throughout the SEI thickness (not merely at the surface) under 0.1 MPa pressure. HRTEM imaging shows that the pressurized graphite anode retains a more ordered crystal structure with fewer structural defects after cycling — consistent with the protective effect of a mechanically stable SEI that prevents solvent co-intercalation and graphite exfoliation.

Figure 4. Post-cycling graphite anode characterization: (a) XPS C 1s and F 1s spectra showing LiF-rich SEI composition under 0.1 MPa; (b) AFM surface morphology — more uniform SEI under pressure; (c) HRTEM showing preserved graphite crystal lattice; (d) TOF-SIMS depth profiles confirming LiF enrichment throughout SEI at 0.1 MPa.

6. The Role of IEST PRCD3100 in This Research

Figure 5. IEST Instrument is acknowledged in the paper’s Acknowledgments section.

7. Conclusions and Implications for LFP Battery Module Design

This study provides the most comprehensive multi-scale characterization of mechanical pressure effects on LFP/graphite battery interfacial stability published to date, combining electrochemical testing (DCIR, dQ/dV), in-situ structural analysis (XRD), and surface/interface characterization (SEM, AFM, HRTEM, XPS, FTIR, TOF-SIMS) to establish a mechanistic framework linking applied pressure to battery performance.

The central conclusion is that moderate mechanical pressure (0.1 MPa) is not merely an external packaging constraint but an active regulator of LFP/graphite battery chemistry and degradation pathways:

• On the graphite anode: pressure promotes LiF-rich, uniform, mechanically stable SEI formation → suppresses electrolyte decomposition, graphite structural damage, and capacity-consuming SEI regrowth.
• On the LFP cathode: pressure promotes uniform LiFePO₄/FePO₄ two-phase transformation → reduces irreversible FePO₄ phase accumulation (34.1% → 18.8%), reduces Joule heating, and decreases aluminum current collector corrosion.
• At the system level: these combined effects reduce capacity decay from 24.31% to 5.14% — a nearly 5× improvement in cycling stability — with lower DCIR growth, reduced polarization, and lower temperature rise throughout cycling.

Key quantitative findings: 0.1 MPa mechanical pressure applied to LFP/graphite batteries reduces capacity decay from 24.31% to 5.14%; reduces irreversible FePO₄ phase from 34.1% to 18.8%; promotes LiF-rich, uniform SEI layer formation on graphite anodes; and suppresses aluminum current collector corrosion. These findings establish that stack pressure in battery modules is a primary design variable for interfacial stability and long-term LFP/graphite battery performance — not merely a mechanical packaging consideration — and provide quantitative targets (approximately 0.1 MPa) for optimizing module assembly preload to maximize cycle life in EV and energy storage applications.

8. Original Paper

Wang Y., Yan K., Dong J., Tang R., Guan Y., Zhao G., Lu Y., Hao J., Li B., Mo S., He X., Li N., Chen L., Wu F., Su Y. Pressurized vs. unpressurized LiFePO₄ batteries: A comparative study on interfacial stability and electrochemical performance. Nano Materials Science, 2025. DOI: 10.1016/j.nanoms.2025.11.020

9. Related IEST Testing Solutions for LFP Battery Research

IEST Instrument provides a comprehensive portfolio of testing systems directly applicable to the research methodologies used in this study:

• PRCD3100 Powder Resistivity and Compaction Density System: measures electrode powder electronic resistivity and compaction density under controlled pressures up to 350 MPa — the primary IEST instrument acknowledged in this paper, directly characterizing pressure-dependent electrode electronic transport properties.
• SWE Series In-Situ Cell Swelling Testing System: real-time measurement of cell swelling force and thickness during cycling under controlled preload — directly applicable to quantifying the mechanical pressure evolution studied in this research.
• ERT Series Electrochemical Performance Analyzer: DCIR measurement, dQ/dV analysis, in-situ EIS during cycling — providing the electrochemical characterization data complementary to the interfacial stability analysis performed in this study.

10. FAQ: Mechanical Pressure Effects on LFP/Graphite Battery SEI and Interfacial Stability