Introduction
As a natural graphite anode material manufacturer, you may frequently encounter this frustration: despite excellent physical indicators such as crystallinity and particle size distribution, the battery’s cycle life and initial coulombic efficiency (ICE) remain unsatisfactory. The problem often lies in an easily overlooked aspect—the chemical compatibility between graphite and the electrolyte.
This compatibility is like a precision chemical “dance”: graphite anodes operate at extremely low working potentials (0.01-0.25V), while the electrolyte begins to decompose at 0.8V. Theoretically, they are “incompatible,” but through careful control of interfacial chemical reactions to form a stable solid electrolyte interphase (SEI) film, batteries can operate stably for thousands of cycles.
This article will provide an in-depth yet accessible analysis of this critical technical field, helping you optimize production processes from a chemical compatibility perspective and enhance product competitiveness.
Executive Summary
The chemical compatibility between natural graphite anodes and electrolytes directly determines the cycle life and safety of lithium-ion batteries. This article systematically analyzes organic electrolyte systems, interfacial chemical reactions, SEI film stability, and chemical side-reaction mechanisms, providing practical optimization strategies from formulation design to process control for anode material manufacturers. Learn the mechanisms of VC and FEC additives, master key points of formation temperature control, and effectively extend battery life while reducing safety risks.
Organic Electrolyte Systems: Why Choose Carbonates?
Currently, commercial lithium-ion batteries universally employ organic carbonate-based electrolyte systems. A typical formulation dissolves lithium salts (such as LiPF₆) in a mixed solvent of cyclic carbonates (such as EC) and linear carbonates (such as DMC, DEC, EMC). This combination is not arbitrary but the result of over 30 years of research and optimization.
Synergistic Effects of Electrolyte Components
| Component Type | Representative Substance | Primary Function | Typical Ratio |
| Lithium Salt | LiPF₆ | Provides lithium-ion conduction | 1.0-1.2 M |
| Cyclic Carbonate | EC | High dielectric constant, promotes film formation | 20-30% |
| Linear Carbonate | DMC/EMC | Reduces viscosity, increases ionic conductivity | 70-80% |
| Functional Additives | VC/FEC | Optimizes SEI film performance | Total 1-5% |
EC has a high dielectric constant, enabling dissolution of more lithium salt and increasing ionic conductivity. Linear carbonates reduce electrolyte viscosity, allowing faster lithium-ion movement. In 1990, researchers discovered that when EC is used as a co-solvent, it can form a stable SEI film on the graphite surface, enabling reversible lithium intercalation reactions. This discovery became the cornerstone of modern lithium-ion battery technology.
Core Contradiction of Chemical Compatibility
Graphite anodes operate at extremely low potentials between 0.25 and 0.01V vs. Li/Li⁺, while organic carbonate electrolytes begin to decompose at approximately 0.8V.
Understanding this contradiction: The working potential of graphite anodes (0.01-0.25V) is far lower than the thermodynamic stability potential of the electrolyte (approximately 0.8V), creating a potential difference of about 0.6V. This means the electrolyte is in a thermodynamically unstable state at the graphite surface and will continuously undergo decomposition reactions.
From a thermodynamic perspective, the electrolyte is unstable at the graphite surface and will continuously decompose. However, this “instability” leads to SEI film formation—decomposition products deposit on the graphite surface, forming a passivation layer that prevents further electrolyte decomposition while allowing free passage of lithium ions.
Interfacial Chemical Reactions: Key Processes in SEI Film Formation
Electrolyte Decomposition and Temperature Control
During initial charging, when the potential drops below approximately 0.8V, solvents like EC begin to undergo reductive decomposition. Major reactions include: EC + 2e⁻ + 2Li⁺ → Li₂CO₃ + C₂H₄↑, and LiPF₆ decomposition generating LiF and HF.
Temperature has a tremendous impact on these reactions. Research has found that increasing formation temperature from 25°C to 45°C significantly increases gas generation, with ethylene production rising sharply. This is because elevated temperature changes the primary decomposition pathway of EC from generating LEDC to generating Li₂CO₃.
Critical Process Recommendation: Formation temperature should be strictly controlled within the 25-30°C range, with temperature fluctuations preferably maintained within ±2-3°C. According to the Arrhenius equation, chemical reaction rates increase exponentially with temperature; for every 10°C increase, side-reaction rates typically increase by 1.5-2 times, directly affecting SEI film quality and battery life.
Solvation Effects and Fast-Charging Bottleneck
Lithium ions in the electrolyte are surrounded by solvent molecules to form solvation structures. Lithium-ion desolvation and diffusion in the SEI are two critical steps limiting the fast-charging capability of graphite-based lithium-ion batteries. At the graphite surface, lithium ions must shed their solvent molecules before intercalating into the graphite interlayers—a process requiring energy and time that directly constrains charging speed.
Hazards of Co-intercalation Reactions and Solutions
Early research found that in PC-based electrolytes, natural graphite undergoes severe exfoliation failure. This is because solvated lithium-ion co-intercalation into graphite causes carbon layer expansion and delamination—like forcibly inserting cardboard between book pages, ultimately causing the book to fall apart.
The good news is that the use of VC additives can successfully suppress PC co-intercalation and graphite exfoliation, greatly expanding the range of electrolyte options.
Gas Evolution and Battery Swelling
Interfacial reactions produce multiple gases including H₂, C₂H₄, CO₂, and CO. Compared to synthetic graphite, natural graphite batteries exhibit lower coulombic efficiency, higher capacity fade, and greater gas generation. These gases cause battery swelling, affecting safety and lifespan, requiring battery designs to allow for expansion space while production processes must minimize gas generation.
SEI Film Stability: Key to Long Battery Life
Bilayer Structure and Composition of SEI Film
In LiPF₆/EC/DMC electrolytes, the SEI film exhibits a bilayer structure: the inner layer (adjacent to graphite) comprises dense inorganic components such as LiF and Li₂CO₃; the outer layer (adjacent to electrolyte) consists primarily of organic components like lithium alkyl carbonates and lithium alkoxides.
SEI film thickness typically ranges from a few nanometers to several tens of nanometers, and its complex multilayer heterogeneous structure is not yet fully understood. This is why the SEI film is called the “most important yet least understood component” in the lithium-ion battery field.
Dynamic Evolution of SEI Film
The SEI film is not static after initial formation but continuously evolves. Organic components at elevated temperatures transform into inorganic compounds, such as (CH₂OCO₂Li)₂ decomposing to generate Li₂CO₃, C₂H₄, CO₂, etc. This continuous transformation consumes active lithium, leading to capacity fade and is also the chemical reason why batteries age faster at high temperatures.
VC Additive: Protective Priority Film Former
VC forms a polymer film on the graphite anode through ring-opening reactions before EC decomposition, stabilizing the interface. It effectively passivates graphite defect sites and edge oxides, producing an extremely thin SEI (only a few nanometers).
Research shows that VC is an effective additive for extending the cycle life of graphite-based anodes. Its mechanism involves preferential consumption of VC during formation, preserving EC for subsequent cycles. However, note: when VC concentration exceeds 4 wt%, charge transfer resistance increases, reducing high-rate performance.
FEC Additive: Thermal Stability Enhancer
FEC is a mono-fluorinated derivative of EC with lower melting point and higher oxidative stability. FEC begins reduction at a relatively higher potential of 1.1-1.2V, above that of EC (0.7-0.8V). This “preferential reduction” characteristic produces a more stable SEI layer, significantly improving cycling performance. Recent research indicates that using FEC can improve cycle life by 30-50%.
More importantly, SEI films formed with FEC have thermal stability up to approximately 200°C, while traditional SEI is only about 153°C. This is significant for improving battery safety, especially under high-temperature operating conditions.
Synergistic Formulation of VC and FEC
Research shows that combinations of VC and FEC are superior to single additives in terms of cycle life. Both form the same SEI components but with different concentrations and distributions, creating a more stable composite structure.
Recommended Formulation Strategy:
- VC alone: 1-2 wt%
- FEC alone: 2-5 wt% (single additive can be used at higher levels)
- Combined use: VC 0.5-1 wt% + FEC 1-3 wt% (total controlled at 2-4 wt%)
Formulation Logic Explanation: When used in combination, total content needs more conservative control because simultaneous action of both additives may lead to excessive SEI thickness, affecting lithium-ion transport. FEC alone can reach 5% because without synergistic effects from other additives, it won’t cause excessive SEI film growth.
Note that total additive content should not exceed 5%. When VC content reaches 10 wt%, it leads to capacity fluctuations and gas swelling. Specific formulations should be determined through small-batch testing based on graphite type (flake, spherical, modified).
Chemical Side Reactions and Capacity Fade Mechanisms
Primary Pathways of Continuous Electrolyte Consumption
Even with SEI film protection, side reactions continue to occur. The aging process involves interactions of multiple electrochemical processes, with main consumption mechanisms including:
- SEI Film Repair and Growth: Under volume changes, the SEI film develops microcracks, exposing fresh graphite surfaces and causing continued electrolyte decomposition. Each repair gradually thickens the SEI film, increasing lithium-ion transport impedance.
- Continuous LiPF₆ Decomposition: LiPF₆ thermal decomposition produces corrosive substances like HF. HF further attacks electrode materials and SEI films, creating a vicious cycle.
- Solvent Side Reactions: Under high-temperature or overcharge conditions, solvent molecules may undergo deep oxidation-reduction reactions, accelerating electrolyte consumption.
- Transition Metal Dissolution and Migration: In full cells, transition metal ions (such as Ni²⁺, Mn²⁺, Co²⁺) from cathode materials may dissolve into the electrolyte, migrate to the anode surface, and deposit. These metal ions catalyze electrolyte decomposition, increase interfacial impedance, and in severe cases may trigger lithium dendrite growth, creating safety hazards. This phenomenon is particularly significant in high-nickel ternary cathode systems and requires suppression through cathode surface coating and electrolyte optimization.
Irreversible Loss of Active Lithium
Each SEI repair consumes active lithium from the cathode, which is non-renewable. Repeated formation and destruction of SEI layers is the core issue of capacity fade.
Active Lithium Loss Pathways:
- Initial irreversible loss from SEI film formation (typically 5-15% of initial capacity, corresponding to ICE of 85-95%)
- Progressive consumption during cycling
- Dead lithium formation (metallic lithium losing electrical contact)
- Slow but continuous electrolyte shuttle reactions
Regarding ICE of Different Graphite Types:
- Untreated natural flake graphite: ICE 84-88%
- Spheroidized natural graphite: ICE 90-93%
- Surface-coated modified natural graphite: ICE 93-95%
- Synthetic graphite: ICE 93-95%
Temperature Acceleration Effects and Control Strategies
SEI decomposition is a thermally activated process, following the Arrhenius equation. For every 10°C increase in temperature, side-reaction rates typically increase by 1.5-2 times. According to research data from 2021-2024, SEI decomposition temperature varies with electrolyte system: in standard LiPF₆/EC/DMC systems, significant decomposition begins at approximately 80-120°C, while in certain systems containing specific additives or new lithium salts, this temperature may be higher.
Temperature Control Recommendations:
- 25-30°C: Recommended optimal operating temperature with slow side-reaction rates and longest battery life
- 35-45°C: Side reactions begin to accelerate, cycle life starts to decline
- Above 45°C: Side reactions significantly accelerate, battery aging noticeably quickens
- Above 60°C: Battery life drastically shortens, safety risks increase
This is why electric vehicles require thermal management systems—maintaining battery temperature within the optimal range can dramatically extend lifespan.
Process Optimization Strategies
Based on understanding of chemical compatibility mechanisms, here are practice-proven process optimization recommendations.
Environmental Control Overview
Different processes have varying environmental requirements; below are key control parameters:
Process Stage | Temperature Control | Humidity Control | Critical Requirements |
Formation | 25-30°C (±2-3°C) | Dew point ≤-30°C (glovebox can reach -40°C) | Temperature stability most important |
Storage | 15-25°C | Relative humidity <30% | Avoid moisture absorption and oxidation |
Dry Room | 20-25°C | Dew point -30°C (RH ≤1%) | Complies with GB 51377-2019 standard |
Note: Dew point -40°C is required for gloveboxes and most critical processes; formation processes can be moderately relaxed to -30°C. Relationship between dew point and relative humidity: at 25°C, 1% RH corresponds to approximately -30°C dew point, while -40°C dew point corresponds to approximately 0.1-0.2% RH.
Precise Formation Control
Employ multi-stage low-current formation protocols (e.g., C/20 → C/10 → C/5) to gradually establish a stable SEI film. As mentioned, formation temperature should be strictly controlled within 25-30°C, with temperature fluctuations preferably maintained within ±2-3°C. Resting for 2-4 hours after initial charging to 0.8V facilitates preliminary SEI formation and stabilization. Formation environment should remain dry (dew point recommended below -30°C, critical processes can reach -40°C) to avoid moisture affecting SEI quality.
Note: Specific formation parameters should be adjusted based on graphite type (particle size, specific surface area) and electrolyte formulation.
Surface Pretreatment Technology
Introduce oxygen-containing functional groups on the graphite surface through mild oxidation (such as dilute nitric acid treatment) to improve electrolyte wettability and SEI nucleation site uniformity. After treatment, rinse multiple times with deionized water and thoroughly dry in vacuum environment (recommended 120°C for 4+ hours, or adjust based on material characteristics) to avoid subsequent corrosion from residual acidic substances.
Storage Environment Control
Store finished anode materials in dry, cool (15-25°C) environments, avoiding prolonged air exposure. Recommend vacuum packaging or nitrogen-filled packaging, with packaging materials using high-barrier aluminum-plastic composite films (water vapor transmission rate <1 g/m²·24h, high-end applications can use <0.5 g/m²·24h materials).
Post-Opening Usage Time: After opening, materials should be used quickly in humidity-controlled environments. In RH <30% environments, use within 12 hours is recommended; in high-humidity environments (RH >50%), shorten to 4-6 hours to avoid moisture absorption and surface oxidation.
Storage area relative humidity should be controlled below 30% to prevent material moisture absorption leading to excessive water content in the electrolyte.
Synergistic Electrolyte Formulation Optimization
Work closely with electrolyte suppliers to optimize additive formulations based on specific graphite material characteristics (particle size distribution, specific surface area, crystallinity, surface functional groups). Consider using novel lithium salts (such as LiFSI) to reduce HF generation and improve thermal stability.
Electrolyte water content should be strictly controlled below 20 ppm (preferably below 10 ppm). Regularly test key indicators such as electrolyte acidity (HF content), conductivity, and water content to ensure they remain within safe ranges.
Process Considerations for Different Graphite Types
Graphite Type | ICE | Main Challenges | Additive Recommendations | Formation Key Points |
Natural Flake Graphite (Untreated) | 84-88% | Large surface area, thick SEI, high gas generation | FEC 3-5% | C/20 low current, extend formation time |
Spheroidized Natural Graphite | 90-93% | Surface defects still present | VC 1-2% + FEC 2-3% | Standard multi-stage formation process |
Surface-Coated Natural Graphite | 93-95% | Requires optimized coating thickness | VC 1-2% | Standard process, can moderately increase formation current |
Synthetic Graphite | 93-95% | High cost but stable performance | VC 1-2% | Standard process |
Important Note: Above parameters are reference values; actual applications require optimization based on specific material characteristics, equipment conditions, and product positioning.
Balancing Cost and Performance
The above process optimization strategies require comprehensive consideration based on product positioning and cost budget:
Power Batteries: Require stricter formation control (temperature ±2°C, dew point -40°C), high-purity additive formulations (VC+FEC combination), surface coating treatment to achieve longest cycle life and highest safety.
Energy Storage Batteries: Can moderately relax standards (temperature ±3°C, dew point -30°C), simplify additive formulations (use FEC alone), reducing costs while ensuring basic performance.
Consumer Electronics Batteries: Fall between the two, adjusted according to specific application scenarios and competitive pressures.
Recommended Approach: Through small-batch testing (50-100 cells), systematically test different process parameter combinations for ICE, cycle life (100-500 cycles), rate performance, and cost, plotting “performance-cost curves” to find optimal cost-effectiveness solutions. Focus on optimization measures with high input-output ratios, such as formation temperature control (relatively small equipment investment with significant performance improvement).
Conclusion
The chemical compatibility between natural graphite anodes and electrolytes is a core factor determining battery performance. From electrolyte formulation to SEI film control, from additive formulation to formation processes, every link requires optimization based on deep understanding of chemical mechanisms.
Through precise control of formation temperature, optimized additive formulations, improved storage conditions, and synergistic electrolyte formulation optimization, you can significantly enhance product cycle life, safety, and market competitiveness. The core of these optimization strategies is understanding interfacial chemical reaction mechanisms, controlling SEI film formation quality at the source, and reducing negative impacts of side reactions on battery performance.
It is particularly important to emphasize that different types of natural graphite (flake, spherical, coated) require differentiated process solutions. Untreated flake graphite has lower ICE (84-88%), requiring stricter formation control and higher additive levels; after spheroidization and surface coating treatment, ICE can reach above 90%, with relatively relaxed process requirements. Therefore, process optimization must be combined with material modification to achieve optimal results.
Meanwhile, in pursuing performance improvements, don’t neglect cost control. Flexibly adjusting process parameters according to product positioning (power, energy storage, consumer electronics) and finding optimal cost-effectiveness solutions through small-batch testing are keys for anode material manufacturers to succeed in fierce market competition.
Need to optimize your natural graphite anode production line? We provide integrated EPC solutions from equipment to production line design, helping you achieve precise process parameter control based on deep understanding of chemical mechanisms. Contact us for customized technical solutions.