Introduction
As the core raw material for lithium battery anode materials, the chemical reactivity of natural graphite directly impacts production safety, product quality, and process stability. Many anode material manufacturers have encountered similar challenges in actual operations: Why does slight temperature elevation during coating processes lead to oxidation defects? How do we determine the safety upper limit for process temperatures? How can dust explosions and static accumulation be effectively prevented?
The answers to these questions lie within the chemical reaction characteristics of natural graphite. Understanding graphite’s chemical behavior under different conditions is not only a prerequisite for ensuring production safety but also key to achieving material modification, optimizing process parameters, and enhancing product performance.
This article systematically analyzes the major chemical reaction types of natural graphite, translating complex chemical principles into actionable engineering parameters and equipment design requirements, helping you establish a complete knowledge system from chemical mechanisms to production practices.
Executive Summary
Natural graphite exhibits complex chemical reactivity in lithium battery anode material production. This article provides an in-depth analysis of graphite’s oxidation reaction characteristics (400°C onset temperature, three-stage oxidation mechanism), reaction risks with strong oxidizing agents (concentrated nitric acid, potassium permanganate systems), halogen intercalation chemistry, and alkali metal intercalation mechanisms. The article details critical temperature control points in production processes (400°C threshold, 600-800°C sensitive zone), electrostatic protection measures (grounding resistance <4Ω, humidity >40%), and oxidation risk control strategies (oxygen content <10ppm, multi-point temperature monitoring). These chemical principles provide scientific foundations for equipment design, process optimization, and production safety management in anode material production lines.
Graphite Oxidation Reactions: Scientific Basis for Temperature Control
Three-Stage Oxidation Reaction Mechanism
Although natural graphite exhibits good chemical stability at room temperature, it undergoes oxidation reactions under specific conditions. According to thermogravimetric analysis studies of Tsinghua University’s HTR-10 high-temperature gas-cooled reactor, graphite oxidation behavior in dry air exhibits three distinct temperature zones.
Stage One (400-600°C): Chemical Reaction Control Zone
Within this temperature range, oxidation is controlled by chemical reaction kinetics, with an activation energy (the energy barrier the reaction must overcome) of 158.56 kJ/mol and relatively slow reaction rates. Large-scale thermal oxidation treatment studies of nuclear graphite show that oxidation rates at 400°C and 500°C are too low to measure accurately and are considered negligible, with measurable oxidation rates only observed starting from 600°C. This means that within this temperature range, as long as oxygen concentration is strictly controlled, oxidation risk is relatively manageable, making 400°C the safe threshold for process temperatures.
Stage Two (600-800°C): Pore Diffusion Control Zone
When temperature rises to this range, oxidation kinetics shift to pore diffusion control, with activation energy dropping to 72.01 kJ/mol and oxidation rates significantly accelerating. This temperature range requires stricter protective atmosphere, as even trace amounts of oxygen may cause significant oxidation.
Stage Three (>800°C): Boundary Layer Diffusion Control Zone
Above 800°C, activation energy becomes very low, with the oxidation process controlled by boundary layer diffusion. High-temperature oxidation studies in nanomaterials and energy fields found that in the boundary layer control zone (1073-1273 K), average activation energies for different graphite materials range from 41.8-58.5 kJ/mol, with graphitization degree significantly affecting oxidation rates.
Oxidation Products and Crystal Plane Differences
The reaction of natural graphite with oxygen primarily produces two gaseous products. At low temperatures, complete oxidation dominates, producing carbon dioxide (C + O₂ → CO₂). At high temperatures, incomplete oxidation becomes more significant, mainly producing carbon monoxide (2C + O₂ → 2CO).
In-depth research on high-temperature oxidation kinetics performed detailed characterization of graphite surface oxidation in the 1100-2300 K range, finding that carbon monoxide (CO) is the primary reaction product. Notably, carbon dioxide (CO₂) can only be detected when surface temperature is below 1400 K. This means that in high-temperature processes like graphitization, CO gas detection and emission systems must be equipped.
Different crystal planes of graphite exhibit significantly different oxidation activities. The basal plane (the planar portion of graphite layers), with its complete sp² hybridization (stable planar structure formed by carbon atoms bonding with three surrounding carbon atoms), shows strong chemical inertness and the lowest oxidation rate. The edge plane (edges of graphite layers), due to numerous dangling bonds, exhibits oxidation rates 100-1000 times higher than the basal plane. Lattice defect sites have oxidation rates between the two, approximately 10-100 times the basal plane. Related research points out that at the crystalline level, intrinsic oxidation rates depend on the availability of active sites, namely the edges of graphene sheets in the graphite matrix.
Real Production Case: A company’s coating process experienced monitoring failure, with local temperatures reaching 480°C combined with insufficient nitrogen purity (only 99.5%), resulting in preferential edge plane oxidation, 15% decrease in product specific surface area, and 3% reduction in first-cycle coulombic efficiency. This case fully demonstrates the direct impact of edge plane oxidation sensitivity on product performance.
📌 Production Insights: This characteristic requires the use of inert atmosphere protection in high-temperature processes (such as graphitization, coating) to prevent preferential oxidation of edge planes and defects that would degrade material performance. Temperature control thresholds should be set below 400°C to ensure the safety of conventional processes.
Strong Oxidizing Agent Reactions: Functionalization and Safety Risks
Industrial Oxidation Methods and Dual Risks
Strong oxidizing agents can oxidize natural graphite at lower temperatures, which is an important pathway for graphite functionalization and purification, but also represents a risk point requiring strict control in production. Industrial graphite oxidation methods mainly include the Brodie method (nitric acid + potassium chlorate system) and Hummers method (sulfuric acid + potassium permanganate system).
A review study published in ACS Nano notes that the classic Brodie method uses fuming nitric acid (HNO₃) and potassium chlorate (KClO₃) to oxidize graphite, with the original method requiring 3-4 days at 60°C. Modern research has significantly shortened reaction times through optimized conditions. The Staudenmaier improved method introduces concentrated sulfuric acid, achieving optimal oxidation in a single treatment, with its effectiveness attributed to nitronium ions (NO₂⁺) produced from the reaction between sulfuric acid and nitric acid.
Oxidation mechanism research published in PMC revealed the dual oxidation mechanism of the modified Hummers method through nuclear magnetic resonance, thermogravimetric analysis, and X-ray photoelectron spectroscopy: The first stage is intercalation oxidation (the process of other atoms or molecules inserting between graphite layers), with electrically neutral species acting as oxidizing agents; the second stage is diffusion oxidation, with MnO₃⁺ ions as the primary oxidizing agent. The research particularly emphasizes that the generated oxide species are highly reactive and have been known to explode at high temperatures or when in contact with organic compounds, necessitating strict control of reaction conditions.
Thermal Decomposition Risks of Hydrogen Peroxide Treatment
Research on the thermal decomposition safety of graphite oxide revealed a safety issue crucial to large-scale manufacturing. The study found that when graphite oxide is pretreated with OH⁻ in suspension, the onset temperature of its thermal decomposition reaction can decrease by as much as 50°C. This effect may cause the decomposition exothermic onset temperature to overlap with conventional drying operation temperatures (100-120°C), potentially resulting in self-heating and thermal runaway during processing. Under adiabatic self-heating conditions, theoretical temperatures can reach >2000°C.
⚠️ Safety Alert: When handling strong oxidizing agents, note that: fuming nitric acid releases highly toxic gases (NOₓ) at 60-100°C, requiring closed systems and tail gas treatment; KMnO₄/H₂SO₄ mixtures may explode when in contact with organic materials at 40-50°C, requiring strict temperature control and slow addition; oxides treated with H₂O₂ may trigger self-heating runaway in the normal to 120°C range, necessitating pH control and temperature monitoring.
Special Requirements for Strong Oxidizing Agent Processing Equipment
Equipment for handling strong oxidizing agents must use corrosion-resistant materials (such as PTFE, glass lining), be equipped with precise temperature control systems (accuracy ±2°C) and emergency cooling devices, and have complete tail gas absorption and treatment systems. Reactors should be equipped with pressure relief devices and temperature interlock protection systems to ensure rapid response in abnormal situations. For large-scale production, detailed thermochemical and kinetic studies are required to determine safe operating conditions that avoid explosive events during storage and processing.
Alkali Metal Intercalation Mechanisms: Chemical Foundation of Lithium Battery Operation
Halogen Intercalation: Reference for Understanding Side Reactions
Before delving into alkali metal intercalation, let’s briefly understand halogen intercalation chemistry. Halogens (F₂, Cl₂, Br₂, I₂) can form intercalation compounds with graphite. Research on room-temperature fluorinated graphite intercalation compounds and microwave-assisted halogenation research published in Scientific Reports indicate that highly halogenated graphite can be achieved through specific methods, with chlorine atom percentages as high as 21% and bromine atom percentages reaching 4%.
Research reported in Nature demonstrated the application prospects of halogen intercalation chemistry in energy storage, where halogen conversion-intercalation chemistry in graphite can produce composite electrodes with a capacity of 243 mAh/g and average potential of 4.2 V vs Li/Li⁺. Although halogen intercalation technology is not directly used in traditional lithium battery anodes, its revealed intercalation mechanisms and interlayer spacing change patterns provide important references for understanding lithium ion insertion/extraction processes and side reactions such as electrolyte decomposition.
Compared to halogen intercalation primarily used for material modification research, alkali metal intercalation is the core mechanism of actual lithium battery operation, requiring deeper understanding.
Lithium Intercalation: Core Reaction of Anode Materials
Lithium intercalation mechanism is the most core electrochemical process of anode materials, involving reversible Li⁺ ion insertion and extraction, forming LiC₆ compounds with a theoretical capacity of 372 mAh/g. A PMC review on graphite intercalation compounds points out that lithium intercalation follows a staged mechanism, from fourth stage → second stage → first stage, with interlayer spacing gradually expanding from 3.35 Å to approximately 3.70 Å.
The smaller Li ion (radius 0.76 Å) is an exception among alkali metals because its bond with C atoms contains covalent components, resulting in negative formation energy (meaning the reaction proceeds spontaneously and stably). This unique chemical bonding characteristic makes graphite an ideal material for lithium-ion battery anodes. During charging, Li⁺ deintercalates from the positive electrode, travels through the electrolyte to the negative electrode, and intercalates into graphite layers to form LiC₆; during discharge, the reverse occurs as Li⁺ deintercalates from graphite and returns to the positive electrode.
Sodium Intercalation Peculiarities: Why Graphite Isn’t Suitable for Sodium-Ion Batteries?
In stark contrast to lithium, sodium intercalation amount is very small, a phenomenon that has attracted widespread attention. First-principles calculation research in RSC Advances revealed the essence of this phenomenon: the main reason is the change in chemical bonding between alkali metal ions and C atoms. Calculations found that the formation energy of NaC₆ becomes positive (meaning the reaction is unstable and cannot proceed spontaneously).
Batteries & Supercaps review further clarifies: this phenomenon stems from Na atom’s unique electronic structure, making its chemical binding force with graphite layers weaker than Li and K. Specifically, there exists an unfavorable competitive relationship between sodium’s ionization energy and its coupling energy with graphene.
ScienceDirect review emphasizes that classical graphite anodes cannot effectively intercalate Na⁺, with physical factors also contributing: Na⁺ has a larger ionic radius (1.02 Å vs. 0.76 Å for Li⁺) and higher atomic weight (22.99 g/mol for Na vs. 6.94 g/mol for Li), leading to slower diffusion kinetics in solids and lower energy density at the battery level.
However, when ions are wrapped in solvent shells, considerable sodium intercalation can be achieved through co-intercalation processes (the process where solvent molecules insert into graphite layers together with ions). A breakthrough reported in Nature Communications shows that while traditional intercalation chemistry cannot store sodium in graphite, co-intercalation chemistry changes this situation, enabling reversible and ultrafast sodium storage in graphite. Optimized electrolyte enables sodium-ion batteries to achieve 3.1 V improved voltage, high power density of 3863 W kg⁻¹, and extremely low capacity decay rate of 0.007% per cycle over 1000 cycles.
Potassium Intercalation Compounds: Emerging Energy Storage Technology
Potassium intercalation compound KC₈ is one of the most extensively studied intercalation compounds. Comprehensive research on graphite intercalation compounds describes that KC₈ can be prepared by melting potassium on graphite powder. After potassium is absorbed into graphite, the material color changes from black to bronze. Chemistry blog article notes that this deep golden-copper colored solid is pyrophoric and must be handled under inert atmosphere. KC₈ has higher electrical conductivity than pure graphite and is a superconductor with critical temperature Tc = 0.14 K.
Potassium-ion batteries are receiving significant attention as next-generation power sources for large-scale energy storage systems. Potassium-ion battery research reveals that potassium’s standard electrode potential is -2.94 V vs. SHE, exhibiting electrochemical properties similar to lithium, with more abundant potassium resources. Potassium’s electrochemical potassiation/depotassiation curves primarily display unique voltage segments consisting of sloping voltage plateaus (approximately 0.3-0.6 V) and low voltage plateaus (<0.3 V vs. K⁺/K). Pioneering research first demonstrated highly reversible intercalation of potassium in graphite at room temperature in carbonate solutions, achieved through electrochemical reduction near the K⁺/K standard potential, which is lower than that of Li⁺/Li, with intercalation leading to formation of first-stage KC₈ compound, providing reversible capacity of 244 mAh g⁻¹.
Calcium Intercalation Compounds: Academic Research Value
Calcium intercalation compound CaC₆ is prepared by immersing highly oriented pyrolytic graphite in liquid Li-Ca alloy at 350°C for 10 days. After calcium intercalation, graphite interlayer spacing significantly increases from 3.35 Å to 4.524 Å, with carbon-carbon distance increasing from 1.42 Å to 1.444 Å. Among superconducting graphite intercalation compounds, CaC₆ exhibits a relatively high critical temperature Tc = 11.5 K, which further increases under applied pressure (15.1 K at 8 GPa). Although calcium intercalation compounds are primarily used for fundamental research, their superconducting properties and substantial interlayer spacing changes provide important references for understanding intercalation mechanisms.
Chemical Safety Control in Production Processes
Dust Explosion Risk Identification
During raw material storage and processing stages, dust explosions are the primary safety hazard. MoldMaking Technology’s special article on graphite dust handling notes that combustible dust is defined as “any finely divided solid material with a diameter of 420 microns or smaller that, when dispersed in air and ignited, presents a fire or explosion hazard.” According to dust explosion theory, when dust concentration reaches a certain range (typically 20-6000 g/m³ for combustible dust, with specific values varying by material) and an ignition source is present, explosions may occur.
Research on graphite dust explosibility using standard methods with a 20-liter spherical apparatus found that pure graphite dust with particle sizes from 4 to 40 μm and very low volatile content can explode over a wide concentration range. Dust particle size is a very important characteristic of the explosion hazard that graphite dust may pose to safety.
The five elements of fire and explosion include: fuel (graphite dust), oxygen, ignition source, dispersion, and confinement. Controlling any one of these elements will essentially eliminate the possibility of dust explosion. Effective production safety prevention measures include: installing dust collection systems, real-time monitoring of dust concentrations, strict control of ignition sources, maintaining good cleaning practices, and efficient preventive maintenance programs.
Oxidation Risk Control in Various Processes
Ball milling or crushing processes typically operate below 100°C, but mechanical friction may cause local heat generation. Mill chamber temperature must be monitored to ensure it does not exceed 80°C. Dust accumulation and static electricity generation must also be prevented.
Coating treatment processes are critical links in production safety control. Coating temperatures range from 400-1000°C, precisely in graphite’s oxidation-sensitive zone. According to the aforementioned oxidation mechanism research, measurable oxidation of graphite begins above 400°C, with oxidation rates significantly increasing above 600°C.
In the 400-600°C range, oxidation is in the chemical reaction control zone with activation energy of 158.56 kJ/mol. This means that within this temperature range, reaction rates are relatively controllable, but oxygen concentration (<100 ppm, ppm being parts per million by volume, i.e., the volume ratio of oxygen per cubic meter of gas) and temperature uniformity (temperature difference <±10°C) must be strictly monitored.
When temperature reaches 600-800°C, oxidation enters the pore diffusion control zone with activation energy dropping to 72.01 kJ/mol, making oxidation more likely. This temperature range requires strict oxygen concentration control below 10 ppm while ensuring uniform temperature distribution to avoid local overheating.
Graphitization processes reach temperatures of 2800-3000°C. Although overall oxidation rate is controlled by the boundary layer, high purity of inert atmosphere must be ensured (O₂ < 1 ppm), as any trace oxygen may trigger rapid oxidation at such high temperatures. Since carbon monoxide (CO) is primarily generated at high temperatures, good gas displacement and emission systems are needed to prevent CO accumulation that could cause safety incidents.
Chemical Principles and Measures for Electrostatic Protection
Dust Safety Science article on static electricity points out that static electricity is a common ignition source in dust explosions. Natural graphite powder easily accumulates static electricity during conveying and mixing processes. Primary mechanisms of static accumulation include friction (contact and separation between materials) and induction (influence of nearby charged objects on other materials). When static electricity accumulates to a certain level (typically several thousand volts), and discharge energy exceeds the minimum ignition energy of dust clouds (typically tens to hundreds of millijoules for combustible dust, depending on dust type and particle size), dust explosions may be triggered.
Stonehouse article on electrostatic hazard analysis emphasizes that static electricity most commonly occurs when any two materials contact and then separate, resulting in electron imbalance. This charge imbalance can lead to electrostatic discharge (ESD), which can ignite flammable atmospheres depending on the specific discharge type and energy content. This risk is particularly high in industries handling flammable gases, vapors, and dusts. However, the dangers of static electricity are often overlooked or underestimated because its effects are not always immediately apparent.
Effective production safety prevention measures include:
Grounding systems: Ensure all equipment is reliably grounded (grounding resistance <4 Ω), with ground rods inserted at least 8 feet deep underground. This is the fundamental measure for preventing static accumulation.
Bonding systems: Use conductive wires or metal straps to connect all conductive components, ensuring good electrical continuity so the entire system has the same potential.
Material selection: Use antistatic materials to construct conveying systems (surface resistivity <10⁶ Ω·m), reducing the source of static generation.
Environmental control: Control workshop relative humidity >40% to improve air conductivity, helping static charges dissipate naturally and reducing static accumulation.
Process control: Avoid high-velocity gas flow conveying (wind speed <15 m/s) to reduce frictional charging, appropriately reducing conveying speed when necessary.
RoboVent’s technical article on grounding and bonding for dust collection systems details that bonding and grounding of dust collection systems are critical safety measures to prevent static charge accumulation within dust collectors and ductwork. Several key regulations and standards apply, including NFPA 70 (National Electrical Code), NFPA 654 (Combustible Dust), OSHA 29 CFR 1910.307, and IEC/ATEX (European Directives).
Overall Anti-Oxidation Design for Production Lines
In response to natural graphite’s oxidation sensitivity, integrated anode material production line equipment design should focus on the following aspects:
Atmosphere control systems: Equipped with high-purity nitrogen or argon gas supply systems (purity >99.999%), online oxygen analyzers for real-time monitoring (detection accuracy <1 ppm, response time <30 seconds), multi-stage gas purification devices to remove residual oxygen and moisture (including deoxidizer columns and molecular sieve drying towers). The system should be designed for positive pressure operation to prevent air infiltration (internal pressure 50-200 Pa higher than atmospheric pressure). This multi-layer protection ensures full-process high purity from gas source to point of use.
Sealing systems: High-temperature areas use water-cooled seal structures to prevent high-temperature deformation, dynamic sealing components use wear-resistant ceramics or graphite materials (typical service life exceeds 5000 hours), pressure balance design prevents air infiltration from the low-pressure side. Key locations use double seal design with protective gas flowing between, forming a gas barrier such that even if the main seal slightly leaks, the secondary seal and protective gas prevent air entry.
Temperature control: Achieve multi-zone independent temperature control (accuracy ±5°C per zone), equipped with rapid cooling systems for emergency cooling (cooling rate up to 50°C/min), optimize furnace temperature gradient distribution through CFD (Computational Fluid Dynamics) simulation (gradient <10°C/m). Use multi-point temperature monitoring (at least 2 measurement points per square meter), equipped with temperature data recording systems for full traceability. This precise temperature control ensures process stability and repeatability.
Emergency protection: Install automatic fire suppression systems (CO₂ or FM-200 gas, response time <10 seconds), set over-temperature alarms and interlocking shutdown devices (automatic alarm when temperature exceeds set value by 5°C, automatic heating cutoff and cooling activation when exceeding 10°C), equipped with emergency pressure relief devices to prevent over-pressure explosions (pressure relief valve set at 1.1-1.2 times working pressure). Additionally, CO gas leak detection systems and combustible gas alarms should be installed for multi-level safety protection.
Industry practice shows that after implementing the above production process optimization and equipment improvement measures based on chemical principles, oxidation defect rates in coating processes can be significantly reduced, equipment failure rates notably decreased, and production safety incidents effectively prevented. This fully demonstrates the effectiveness of translating chemical reaction mechanisms into engineering design parameters and validates the important value of scientific, systematic safety management for anode material production.
Conclusion
The chemical reactivity of natural graphite exhibits distinct duality. On one hand, it has good tolerance to most chemical reagents at room temperature, with sp² hybridization structure endowing it with chemical inertness, which is the stability foundation enabling lithium batteries to operate through long-term cycling. On the other hand, under specific conditions (high temperature >400°C, strong oxidizing agents, alkali metals, etc.), it exhibits controllable chemical reaction selectivity. These reactions are both important pathways for material functionalization and modification and risk points requiring strict prevention in anode material production processes.
In-depth understanding of graphite oxidation mechanisms at different temperature ranges is key to ensuring production safety: the 400-600°C chemical reaction control zone (activation energy 158.56 kJ/mol, relatively controllable oxidation risk), the 600-800°C pore diffusion control zone (activation energy 72.01 kJ/mol, accelerated oxidation rate), and the boundary layer control zone above 800°C (oxidation rate mainly dependent on oxygen supply). Understanding the conditions and hazards of reactions with strong oxidizing agents is equally important, particularly the thermal decomposition risks of graphite oxide, where OH⁻ presence can lower onset temperature by 50°C, potentially overlapping with conventional drying temperatures.
Understanding intercalation chemistry—from halogens to alkali metals’ reaction mechanisms—provides a complete theoretical foundation for understanding lithium battery operating principles. Lithium ion’s unique covalent bonding characteristics make it an ideal anode material, while sodium’s weak chemical bonding explains why graphite isn’t suitable for traditional sodium-ion batteries, and potassium and calcium intercalation properties open new directions for novel energy storage technologies and fundamental research.
For anode material manufacturers, translating chemical principles into equipment design parameters and process control points is the only way to truly achieve high-quality transformation from raw materials to products. Temperature segmented control (<400°C safety zone, 400-600°C strict protection zone, >800°C ultra-high purity zone), atmosphere protection (purity >99.999%, O₂ <10 ppm), multi-point monitoring (temperature, oxygen concentration, CO concentration), emergency interlocking (over-temperature alarm, automatic shutdown, rapid cooling), electrostatic protection (grounding <4Ω, surface resistivity <10⁶ Ω·m, humidity >40%)—these specific production process control measures all stem from deep understanding of chemical reaction mechanisms and are the scientific foundation for ensuring production safety, enhancing product quality, and optimizing process efficiency.
As an EPC contractor focused on anode material production equipment and solutions, our integrated production line solutions fully consider graphite chemical reaction characteristics, providing precise temperature control based on oxidation mechanisms, ultra-high purity atmosphere protection design, electrostatic protection systems compliant with international standards, and comprehensive production safety management systems.