Acrylonitrile (AN, C₃H₃N, molar mass 53.06 g/mol, normal boiling point 77.3 °C) functions as the primary monomer for polyacrylonitrile (PAN) precursor synthesis in carbon fibre and hard carbon routes. Established downstream outlets include acrylic fibre, acrylonitrile-butadiene-styrene resin, adiponitrile, and carbon fibre precursor PAN. A more compact but technically distinct demand stream is emerging for PAN-derived hard carbon used as an anode material in sodium-ion cells. In that route, AN is polymerized into PAN, processed into a precursor fibre or powder, stabilized in air, and carbonized under inert atmosphere to form disordered carbon with sodium-ion storage sites distributed across ultramicropores, closed pores, heteroatom defects, and turbostratic domains. The exact demand pathway is described here as PAN-Derived Hard Carbon for Sodium-Ion Anodes: Acrylonitrile's New Demand Stream because the same monomer quality, polymerization selectivity, and stabilization control that govern carbon fibre grade PAN also determine sodium-ion anode yield and cycling behaviour.

Acrylonitrile Supply Quality Constraints in PAN Precursor Polymerization

Commercial AN for PAN precursor synthesis is supplied with minimum purity typically specified at 99.5 wt%. Water content is controlled because water participates in initiation and hydrolysis side reactions. Free inhibitor, commonly monomethyl ether hydroquinone at 35–50 mg/kg, is present to suppress spontaneous polymerization during storage. Residual inhibitor concentration below 10 mg/kg can create uncontrolled polymerization in feed tanks, while excess inhibitor above 70 mg/kg retards initiation and broadens molecular weight distribution. Water content is determined by ASTM E203 Karl Fischer titration, and supplier specifications routinely require water below 0.25 wt%. Acetone, acetaldehyde, and hydrogen cyanide are controlled as chain-transfer impurities because their presence increases the concentration of low-molecular-weight polymer chains that reduce precursor drawability.

In continuous solution polymerization for PAN precursor, the monomer is dissolved in dimethyl sulfoxide or dimethylacetamide at solids of 18–25 wt%. Azobisisobutyronitrile initiates free-radical propagation at temperatures between 55 °C and 65 °C in DMSO-based systems. Higher temperatures accelerate chain transfer to solvent and lower number-average molecular weight. Conversion is terminated at 70–85%; the reaction dope is then devolatilized to remove unreacted monomer. Residual AN in the dope is maintained below 50 mg/kg before spinning to limit bubble defects and local exotherm irregularities during later stabilization. Polymer molecular weight is characterized by intrinsic viscosity in dimethylformamide at 25 °C; a weight-average molecular weight range of 120,000–250,000 g/mol is common for PAN fibre precursors. Higher molecular weight improves filament strength but reduces dope filterability and increases pressure drop across sintered-metal spin packs.

Comonomer selection for hard carbon grade PAN is more constrained than for textile-grade acrylic fibre. Methyl acrylate or itaconic acid is added at 1–2 wt% to moderate stabilization exotherm and improve cyclization. A comonomer content above 2 wt% reduces cyclization onset temperature by 10–15 °C and broadens the stabilization window, but content above 5 wt% lowers carbon yield to below 45 wt%. Published data for this specific configuration, especially sodium-ion anode performance across different comonomer levels, is limited. The presence of comonomer changes nitrile index and oxygen uptake rate, which in turn alters the ratio of open to closed porosity in the carbonized product.

What Limits Spinnability of PAN Precursor Dope for Hard Carbon Applications?

Solution-spun PAN precursor for hard carbon is produced by wet spinning or dry-jet wet spinning into a coagulation bath containing DMSO/water or sodium thiocyanate/water. Dope viscosity at 30 °C and a shear rate of 10 s⁻¹ is typically maintained between 30 Pa·s and 80 Pa·s. Viscosity below this range produces filament breakage; higher values create die-swell and non-uniform coagulation. Jet stretch ratio in dry-jet wet spinning is limited to 1.5–3.0. Exceeding this range causes skin-core heterogeneity that persists through carbonization and creates two distinct sodium diffusion environments. Coagulation bath temperature is controlled between 5 °C and 15 °C for DMSO/water systems to slow solvent diffusion and produce dense cross-sections. Rapid coagulation at higher temperature forms macrovoids that reduce carbon particle density and lower first-cycle coulombic efficiency.

After coagulation, the filament is washed, drawn in hot water at 90–98 °C, and further drawn in steam at 120–160 °C. Total draw ratio between 6× and 10× orients molecular chains and increases tensile strength to 0.6–0.9 GPa. Fibre diameter after drawing is typically 10–15 μm for hard carbon precursor. Coarser filaments require longer stabilization times and can develop oxygen gradients across the fibre radius. Stabilization ovens with constrained transport rollers compensate shrinkage and maintain tension; loss of tension during stabilization reduces carbon yield and produces a more isotropic carbon structure with lower plateau capacity.

The thermal stabilization step is performed in a forced-air oven or continuous belt furnace with temperature staged from 200 °C to 300 °C at 1–3 °C/min heating rate under tension. Cyclization of nitrile groups, dehydrogenation, and oxygen uptake form a ladder polymer that resists melting during carbonization. Stabilization density increases from approximately 1.18 g/cm³ for pristine PAN to 1.35–1.40 g/cm³ after complete stabilization. The exothermic cyclization peak in differential scanning calorimetry at 10 °C/min appears near 280 °C. Overheating above 320 °C causes runaway crosslinking and filament fusion. Tension during stabilization is controlled at 0.5–2.0 MPa to maintain orientation without breaking individual filaments. Oxygen content in stabilized PAN reaches 8–12 wt%; insufficient oxygen uptake leads to low carbon yield, while excessive oxygen introduces carbonyl groups that generate CO and CO₂ during carbonization.

When Carbonization Heating Rate Alters Ultramicropore Closure and Sodium-Ion Accessibility

Stabilized PAN is pyrolyzed in a high-temperature furnace under flowing nitrogen or argon with ramp rates between 1 °C/min and 5 °C/min. Pyrolysis to 800–1000 °C removes hydrogen cyanide, ammonia, nitrogen, and hydrocarbons from the ladder structure. The product at 800 °C retains substantial nitrogen heteroatoms at 12–18 wt% and hydrogen at 1.5–2.5 wt%, with true density below 1.9 g/cm³. These heteroatoms contribute sloping sodium-ion storage but reduce initial coulombic efficiency through irreversible electrolyte reduction. Heating to 1200–1500 °C eliminates more heteroatoms and induces closed-pore formation with true density rising to 1.95–2.05 g/cm³. Ultramicropore volume measured by CO₂ adsorption at 273 K increases at intermediate temperatures, but excessive temperature above 1600 °C can collapse accessible ultramicropores and reduce plateau capacity.

Ramp rate directly affects closed-pore development. Slow heating below 1 °C/min through 700–950 °C favours volatile escape before pore closure, increasing initial coulombic efficiency. Fast heating above 5 °C/min traps volatiles and creates internal porosity but lowers mechanical integrity of the carbon particle. Furnace atmosphere purity influences heteroatom removal; oxygen leakage above 10 ppm at 1200 °C causes surface burn-off and increases open surface area without producing sodium-accessible closed pores. Surface area measured by ISO 9277:2010 nitrogen adsorption is therefore kept below 5 m²/g for high-temperature hard carbon products intended for sodium-ion anodes.

PAN-derived hard carbon is wet-milled to a mean particle size D50 of 5–15 μm and mixed with polyvinylidene fluoride binder and carbon black in N-methyl-2-pyrrolidone. A typical electrode formulation uses 92 wt% active material, 4 wt% carbon black, and 4 wt% PVDF. The slurry is coated onto aluminium foil at a loading of 2–3 mg/cm² and vacuum-dried at 120 °C for 12 h. In a CR2032 half cell with sodium metal counter electrode and glass-fibre separator, electrolytes of 1 M NaClO₄ in propylene carbonate or 1 M NaPF₆ in ethylene carbonate/diethyl carbonate are used. Reversible capacity at 0.1C typically falls between 250 mAh/g and 320 mAh/g for PAN-derived hard carbon carbonized at 1200–1500 °C. Materials carbonized below 900 °C require pre-drying under vacuum at 150 °C before cell assembly because adsorbed water promotes NaPF₆ hydrolysis and HF formation.

How Does Solid Electrolyte Interphase Chemistry Shift with Residual Nitrile Functionality?

Residual nitrile and nitrogen sites in PAN-derived hard carbon influence the solid electrolyte interphase. Nitrogen-doped carbon surfaces show increased polarity and electrolyte adsorption; this enhances sodium-ion desolvation but also accelerates decomposition of carbonate solvents. Fluoroethylene carbonate at 2–5 wt% is added to the electrolyte to form a solid electrolyte interphase rich in sodium fluoride and polymeric species, raising initial coulombic efficiency to 80–85% for hard carbon pyrolyzed at 1200 °C. Extended cycling at 1C for 500 cycles shows capacity retention above 90% only when the electrode is calendered to 30–35% porosity. Lower porosity causes sodium-ion diffusion limitations; higher porosity increases electrolyte consumption and lowers volumetric capacity.

Electrolyte reduction on nitrogen-containing hard carbon is more pronounced when the carbonization temperature remains below 1000 °C. The first cycle irreversible capacity includes sodium trapping in closed pores that are not accessible at lower cut-off potentials, decomposition of surface oxygen groups, and SEI formation on ultramicropore walls. Galvanostatic intermittent titration analysis shows apparent sodium-ion diffusion coefficients in PAN-derived hard carbon in the range of 10⁻¹²–10⁻¹⁰ cm²/s for the sloping voltage region, while diffusion slows by one to two orders of magnitude in the low-voltage plateau region. Electrode sheets containing carbonized PAN with D50 below 5 μm show lower internal resistance but excessive surface area; D50 above 20 μm creates particle cracking during calendering and increases cell impedance.

Operational Boundaries for Acrylonitrile Storage and Carbonization Off-Gas Systems

Acrylonitrile is a flammable liquid with lower and upper explosive limits in air of 3.0 vol% and 17.0 vol%. Storage vessels require nitrogen blanketing. Polymer-scale supplier specifications typically require inhibitor content sufficient to maintain a 10 h induction period at 50 °C as measured by isothermal differential scanning calorimetry. The nitrile group can hydrolyze to acrylamide and ammonium acrylate in the presence of water and heat; stainless steel storage is preferred over carbon steel because iron-catalyzed polymerization can occur in the vapour space. Transfer lines are designed with relief capacity for bulk polymerization events, and equipment is rated for a maximum monomer temperature below 40 °C during normal operation.

Carbonization off-gas from PAN-derived hard carbon contains hydrogen cyanide, ammonia, nitrogen, hydrogen, and minor hydrocarbons. In a rotary tube furnace with 2 m heated length and 150 mm inner diameter, throughput of stabilized PAN precursor is limited by gas evolution rates and carbon bed temperature uniformity. HCN concentration in the off-gas can exceed 1000 mg/m³ before treatment; direct discharge is prevented by routing the stream through a natural-gas-fired thermal oxidizer with residence time above 0.5 s at 850 °C, followed by wet scrubbing with caustic solution to absorb nitrogen oxides and residual acid gases. Furnace tube material must withstand alkali metal residuals and nitrogen-containing radicals; Inconel or silicon carbide liners are used where carbon at temperatures above 1200 °C would react with stainless steel. The same off-gas handling boundary applies to batch box furnaces when precursor charge mass exceeds approximately 10 kg and local exhaust capacity is below 50 m³/h per charge.