I.Introduction 

With the explosive growth of the electric vehicle and energy storage markets, the energy density, cycle life, and safety of lithium batteries have become the focus of industry competition. As the mainstream choice for anode materials—artificial graphite and natural graphite—their electrochemical performance is highly dependent on the quality of the graphitization process. Graphitization is not only the process of converting amorphous carbon into ordered graphite crystals, but it also determines key indicators such as the degree of graphitization, defect density, and impurity content of the final anode material, thus directly affecting the battery's cycle life.

In actual production, two core process parameters—maximum processing temperature and furnace vacuum—are often the key to success or failure. Is the temperature high enough and uniform? Can the vacuum effectively remove impurities? This article will start from the mechanism, deeply analyzing how temperature and vacuum affect the cycle life of anode materials, and, combined with our company's advanced control capabilities of high-temperature graphitization furnaces and vacuum sintering furnaces, demonstrating how precise thermal processes can improve product consistency.

Anode Material of Lithium-ion Battery

II. Basic Principles of Graphitization Process

Graphitization is essentially the process of rearranging carbon atoms from a disordered, random layered structure into a three-dimensional, ordered graphite layered structure (ABAB stacking) through high temperature.

A typical process flow includes: charging → vacuuming/atmosphere replacement → heating (degassing) → high-temperature graphitization → holding → cooling → unloading.

Key process parameters include:

Maximum processing temperature: directly determines the degree of graphitization.
Heating rate: affects thermal stress and microcracks.
Holding time: ensures sufficient crystal development.
Atmosphere/vacuum level: controls impurity volatilization and surface oxidation.
Cooling rate: affects internal stress and yield.

Core indicators for evaluating graphitization effectiveness include: degree of graphitization (converted from d002 interlayer spacing measured by XRD), specific surface area (BET), ash/impurity content, micro-defect density, and initial coulombic efficiency and cycle life in coin cell testing.

Graphitization Lattice Transformation Diagram

III. The Influence of Temperature on the Cycle Life of Anode Materials

3.1 Relationship between Graphitization Degree and Cycle Stability

At insufficient temperatures, the carbon layers are disordered, resulting in a large interlayer spacing (d002) (>0.337 nm). During lithium insertion/extraction, the graphite layer undergoes drastic volume expansion/contraction, easily leading to delamination and pulverization after repeated cycles, resulting in rapid capacity decay. Conversely, when the temperature reaches 2800~3000℃, the graphitization degree can reach over 95%, and the d002 approaches the ideal graphite size of 0.3354 nm, significantly improving structural stability and reducing irreversible capacity.

Empirical Data: After graphitization at 2950℃, a certain anode manufacturer achieved a 12~15 percentage point increase in capacity retention after 500 cycles compared to products grown at 2600℃.

3.2 The Role of High Temperature in Defect Healing

Dangling bonds, vacancies, dislocations, and other defects at the carbon layer edges are "nucleation points" for excessive SEI film growth. High-temperature heat treatment provides energy, causing carbon atoms to diffuse and rearrange, significantly reducing defect concentration. Reduced defects mean fewer lithium ions consumed during the first charge and discharge cycle, improving initial efficiency. Simultaneously, side reactions are reduced during cycling, interfacial impedance rises more slowly, and cycle life is naturally extended.

3.3 Localized Overheating or Underheating Due to Uneven Temperature

If the temperature field uniformity of an industrial graphitization furnace is poor, the temperature in the material zone near the heating element will be too high (overheating), potentially generating silicon carbide (SiC) byproducts or causing graphite lattice distortion; the temperature in areas far from the heat source will be too low (underheating), resulting in insufficient graphitization. Poor batch-to-batch consistency will lead to difficulties in cell assembly for customers.

Our graphitization furnace solution: Utilizing multi-zone independently temperature-controlled heating elements, optimized furnace structure, and high-performance heat shields, the effective working temperature difference within the furnace can be controlled within ±15℃, ensuring uniform heat treatment results for every furnace of anode material.

IV. The Influence of Vacuum Degree on the Cycle Life of Anode Materials

4.1 Vacuum Removal: Reducing Impurity Content

Raw coke or needle coke often contains impurities such as Na, Ca, Fe, S, and Cl. Under high-temperature, high-vacuum conditions (e.g., vacuum degree <100 Pa), the vapor pressure of these impurity elements is much higher than that of carbon, causing them to volatilize from the solid and escape through the vacuum pump. Metallic impurities such as Fe and Co catalyze the decomposition of the electrolyte and deposit on the anode surface, accelerating SEI film damage; alkali metals such as Na and Ca can cause localized overpotential anomalies. High-vacuum graphitization can reduce ash content from hundreds of ppm to below 50 ppm, increasing cycle life by more than 30%.

4.2 Preventing Oxidation and Controlling Surface Defects

Even at high temperatures, trace amounts of oxygen or water vapor can oxidize the graphite surface, generating oxygen-containing functional groups such as –OH and –COOH. These functional groups undergo irreversible reactions during the initial lithium insertion, forming a thicker SEI film, consuming active lithium, and increasing charge transfer resistance. Oxidation can be completely avoided by maintaining a high vacuum (≤10 Pa) or a slightly positive pressure high-purity argon atmosphere. Our vacuum sintering furnace is equipped with a high vacuum pumping system (ultimate vacuum ≤1 Pa) and can flexibly switch between vacuum and atmosphere to meet the process requirements at different stages.

4.3 Influence of Vacuum and Degassing Rate on Layered Structures

It is worth noting that when the vacuum is too low and the pumping rate is too fast, water vapor and organic matter adsorbed between carbon layers will desorb instantly, potentially causing localized pressure shocks that lead to particle cracking or interlayer delamination. Therefore, segmented vacuum control is crucial: slow vacuuming during the low-temperature stage (<500℃), maintaining high vacuum degassing from 500~1200℃, and switching to a slightly positive pressure inert atmosphere during the high-temperature stage. Our equipment supports multi-segment vacuum/atmosphere program settings and can customize pumping curves for different raw materials.

V. Key Parameter Control: From Raw Materials to Finished Products

5.1 Raw Material Selection and Pretreatment

Coke/Needle Coke: Volatile matter controlled at 8%~12%, ash content <0.5%, sulfur content <0.3%.

Pre-carbonization: Raw materials are pre-carbonized in a vacuum sintering furnace at 800~1200℃ to remove most of the volatile matter and moisture, avoiding violent exhaust in the graphitization furnace.

Particle Size Control: D50 = 10~20μm; excessively fine powder is prone to flying under vacuum.

5.2 Sintering Process Curve Design (Recommended Scheme)

5.3 Quality Inspection and Feedback

Graphitization Degree: Determined by XRD method, requirement ≥93% (superior grade ≥96%).
Specific Surface Area: BET method, controlled within 1~3 m²/g; excessively high surface area leads to more side reactions.
Impurity Content: ICP-OES, Fe <20ppm, other metals <10ppm.
Electrochemical Evaluation: For coin cells, capacity retention >85% after 500 cycles at 0.2C/0.5C is considered qualified.
Data Closed Loop: Test results are fed back to the process database to optimize the temperature-vacuum combination.

VI. Our Equipment Advantages: Designed for Long-Cycle Anode Materials

6.1 Ultra-High Temperature Graphitization Furnace 

Ultra-high Temperature Graphitization Furnace

Maximum temperature: Up to 3000℃~3200℃, meeting the graphitization requirements of high-end artificial graphite and silicon-carbon composite materials.

Temperature field uniformity: Multi-zone heating + high-purity graphite heating element, effective temperature difference ≤ ±15℃.

Energy-saving design: Employing multi-layer composite insulation structures, it balances high temperature resistance, low outgassing, high vacuum sealing, and high-efficiency insulation, improving thermal efficiency by over 30%. Using IGBT power supplies, it saves approximately 15% more energy than traditional thyristor power supplies. It also features fewer high-order harmonics, less grid pollution, and a high power factor, ensuring it does not interfere with the operation of electronic equipment.

Multi-furnace design: Multiple furnaces can be configured with a single power supply according to the time requirements of the high-temperature sintering process. Multiple furnaces can be sequentially energized for heating and de-energized for cooling, enabling continuous operation and improving production efficiency.

6.2 Vacuum Sintering Furnace

One-to-Two Vacuum Sintering Furnace

Ultimate vacuum: ≤1 Pa, working vacuum can reach 5 Pa.

Multi-segment process control: More than 32 preset temperature control + vacuum/atmosphere switching programs are available.

Safety protection: PLC-based automated control and protection for water, electricity, and gas, including protection against over-temperature, water pressure, water flow, furnace over-pressure, and water temperature issues.

Energy-Saving Design: Employing multi-layer composite insulation structures, it balances high temperature resistance, low outgassing, high vacuum sealing, and high-efficiency insulation, improving thermal efficiency by over 30%.

Application example: After using our vacuum sintering furnace for pre-carbonization, a negative electrode material company saw its post-graphitization ash content decrease from 180 ppm to 46 ppm, and its cycle life increased by 28%.

6.3 Intelligent Control System

Real-time recording of temperature, vacuum, pressure, and power curves.

Equipped with a programmable controller and human-machine interface, process recipes are stored in real time, enabling fully automated operation and monitoring, and supporting batch traceability.

VII. Conclusion

Temperature and vacuum level are the two vital elements of the graphitization process. Sufficiently high and uniform temperature ensures the ordered arrangement of graphite crystals and reduces defects; a properly controlled vacuum level achieves deep impurity removal and surface protection, suppressing side reactions. Only through their synergistic effect can anode materials with low ash content, high graphitization degree, and low defect density be obtained, ultimately giving the battery excellent cycle life.

Our company has been deeply involved in the field of industrial vacuum heat treatment furnaces for many years, providing a full range of thermal equipment from pre-carbonization to high-temperature graphitization. If you are troubled by poor cycle life of your anode materials, or wish to optimize your existing graphitization process, please contact us for material testing or technical exchange.