Carbonization temperature is the main lever that shifts hard carbon from high-voltage adsorption toward low-voltage sodium storage. At approximately 650–950 °C, residual oxygen, nitrogen, defects, and disordered surface sites favor Na⁺ adsorption and produce mainly sloping capacity above 1.0 V. From roughly 1000–2000 °C, heteroatoms are removed and carbon domains become more ordered, increasing the contribution from low-voltage pore filling and interlayer storage. At still higher temperatures, often 2000–2800 °C, stronger graphitic ordering and mesopore development can produce a more pronounced low-voltage plateau, although the final result depends strongly on the precursor and atmosphere.
The central effect of temperature is a redistribution of capacity: lower temperatures preserve defect- and surface-based storage, while higher temperatures promote a low-potential plateau associated with sodium storage in nanopores and expanded, turbostratic carbon layers. The optimum temperature balances plateau capacity, accessible porosity, conductivity, initial Coulombic efficiency, and rate capability rather than simply maximizing graphitization.
How Temperature Reshapes Hard Carbon
Low-temperature carbonization preserves reactive storage sites
Carbonization in the approximate 650–950 °C range leaves more oxygen- and nitrogen-containing groups, structural defects, and disordered carbon.
These features provide abundant sites for Na⁺ adsorption, including surface, edge, defect, and nanovoid environments. Because these sites operate over a broad range of potentials, the resulting charge-discharge curve is dominated by a sloping region, generally above about 1.0 V and extending toward lower voltage.
Intermediate temperatures develop a mixed storage mechanism
Between approximately 1000 and 2000 °C, progressive deoxygenation and denitrogenation reduce the number of surface adsorption sites. At the same time, turbostratic carbon domains grow and the carbon framework becomes more electrically conductive.
Sodium storage therefore becomes mixed: adsorption remains important in the higher-voltage slope, while low-voltage pore filling and interlayer storage contribute increasingly to a plateau near 0.1 V.
High temperatures intensify low-voltage plateau behavior
At approximately 2000–2800 °C, the carbon becomes substantially more ordered, with larger graphitic microdomains and, depending on the precursor, increased mesoporosity.
These structural changes favor a more distinct low-potential plateau. The plateau is commonly associated with sodium occupying nanopores or entering suitably expanded turbostratic interlayers; its precise origin depends on pore size, interlayer spacing, and the degree of graphitic ordering.
How the Capacity Profile Changes
The sloping capacity comes from defects and surfaces
The high-voltage slope primarily reflects sodium adsorption at energetically diverse sites. These include exposed surfaces, edge planes, heteroatom-containing groups, defects, and small disordered voids.
Lower-temperature carbons generally contain more of these sites. They can therefore show substantial sloping capacity, but they may also exhibit greater irreversible electrolyte decomposition and a lower initial Coulombic efficiency.
The plateau reflects more confined sodium storage
The low-voltage plateau is associated with a narrower range of sodium-storage environments. Depending on the material, these include pore filling and sodium insertion into expanded interlayer regions within turbostratic carbon.
As carbonization temperature rises, the reduction in surface defects and the development of suitable graphitic domains generally shift capacity from the slope toward the plateau. This often improves energy density because more capacity is delivered at a lower average anode potential.
The slope-to-plateau ratio is more informative than total capacity alone
Two hard carbons can have similar total capacities but very different voltage profiles. One may store most sodium through high-voltage adsorption, while another delivers more capacity through a low-voltage plateau.
The ratio between these regions helps reveal whether temperature treatment created the intended balance of surface defects, accessible pores, interlayer spacing, and electronic conductivity.
Which Structural Changes Control Sodium Storage?
Heteroatom removal reduces adsorption-dominated capacity
Increasing the carbonization temperature generally removes oxygen- and nitrogen-containing functional groups. This lowers the density of polar and chemically active adsorption sites.
The result is usually a smaller high-voltage sloping contribution and improved structural and electronic stability. However, removing too many sites can reduce accessible capacity if the material lacks sufficient pore or interlayer storage to replace them.
Graphitic-domain growth improves conductivity and insertion pathways
Higher temperatures enlarge turbostratic nanocrystallites and improve electrical conductivity. These changes support more efficient electron transport and can facilitate sodium storage in appropriately spaced carbon layers.
The interlayer spacing must remain sufficiently large for sodium insertion. A spacing above approximately 0.37 nm is identified in the supplied references as important for lowering the insertion energy barrier, while excessive contraction can make interlayer insertion less favorable.
Porosity determines whether the plateau is accessible
Temperature changes not only graphitic order but also pore structure. Micropore volume often decreases as heat treatment progresses, while mesoporous and macroporous features may become more prominent, depending on the precursor and thermal history.
This evolution can improve ion transport and promote plateau storage, but excessive pore loss or pore closure can remove active storage volume. Furnace temperature must therefore be optimized together with precursor chemistry and heating schedule.
Why an Intermediate Temperature Is Often Optimal
Moderate temperatures balance disorder and order
Many biomass-derived hard carbons perform well after treatment in the approximate 1100–1500 °C range. This window can provide enough structural order for conductivity and low-voltage storage while preserving sufficient interlayer spacing and accessible porosity.
The frequently cited example of treatment near 1300 °C illustrates this balance: the material can retain an interlayer spacing in a sodium-compatible range while developing larger turbostratic domains.
Higher temperature does not guarantee higher capacity
Increasing temperature can improve the plateau contribution, but it can also reduce active surface area, eliminate useful defects, narrow interlayer spacing, and close pores.
Consequently, the total reversible capacity may peak at an intermediate temperature rather than at the highest available furnace temperature. The optimum is material-specific and must be established experimentally.
Capacity and initial efficiency may improve together—but not automatically
Low-temperature carbons with high surface area and abundant functional groups can consume more electrolyte during the first cycle. This increases irreversible capacity and lowers the initial Coulombic efficiency.
A controlled increase in carbonization temperature commonly reduces these parasitic reactions. Nevertheless, excessive ordering or pore closure can lower reversible capacity, so initial efficiency and capacity must be optimized together.
Understanding the Trade-offs
A slope-dominated profile can support fast surface storage
Surface and defect adsorption can provide comparatively accessible sodium-storage sites and contribute to rate capability. The drawback is that this capacity occurs at a higher potential and may be accompanied by greater electrolyte reactivity.
A strongly sloping profile can therefore be useful for kinetics but less favorable for maximum full-cell energy density.
A plateau-dominated profile improves energy density
Low-voltage plateau capacity increases the average operating voltage of a sodium-ion full cell relative to a high-potential anode slope. It is therefore generally desirable when the objective is higher energy density.
The plateau can be less tolerant of unsuitable pore sizes or excessively narrow interlayers, however. Poorly designed structures may deliver limited rate performance or lose capacity when sodium-ion transport becomes constrained.
Over-graphitization can reduce sodium accessibility
Higher temperature increases structural order, but hard carbon is not improved by graphitization without limit. Excessive ordering can reduce disorder, shrink interlayer spacing, and eliminate pores that were needed for sodium storage.
The practical target is controlled turbostratic order, not complete graphite-like crystallinity.
Furnace conditions can change the apparent temperature effect
Temperature cannot be interpreted independently of atmosphere, heating rate, dwell time, sample loading, and precursor composition. An inert argon or nitrogen atmosphere is required to limit oxidation, while poor thermal uniformity can create different structures within the same batch.
Reproducible temperature control and gas management are therefore essential when comparing capacity profiles across samples.
How to Apply This to Your Project
The correct carbonization temperature should be selected from the desired storage mechanism and verified through both structural characterization and electrochemical testing.
- If your primary focus is high rate capability: Retain a controlled amount of surface defects, heteroatoms, and accessible porosity so that adsorption contributes to the high-voltage slope, while avoiding excessive surface area that lowers initial Coulombic efficiency.
- If your primary focus is maximum energy density: Increase carbonization temperature enough to develop suitable graphitic domains, interlayer storage, and low-voltage plateau capacity, but stop before pore closure or excessive interlayer contraction reduces reversible capacity.
- If your primary focus is high initial Coulombic efficiency: Use sufficiently high-temperature treatment to remove unstable functional groups and reduce excessive surface area, while preserving the pore and interlayer structures needed for reversible sodium storage.
- If your primary focus is process reproducibility: Use a controlled-atmosphere tube or box furnace with uniform temperature, defined dwell time, and regulated inert-gas flow, then compare slope and plateau capacities under identical electrode and testing conditions.
The best hard carbon is not the most graphitized one, but the material whose temperature-controlled structure provides the right balance of adsorption, pore filling, interlayer storage, conductivity, and reversible capacity.
Summary Table:
| Temperature Range | Dominant Storage Mechanism | Key Structural Changes | Capacity Profile |
|---|---|---|---|
| 650–950 °C | Na⁺ adsorption at defects, surfaces, heteroatoms | High heteroatom content, disordered carbon, many defects | Mostly sloping capacity above 1.0 V |
| 1000–2000 °C | Mixed adsorption and interlayer/pore filling | Progressive deoxygenation, growth of turbostratic domains, better conductivity | Blend of slope and plateau (near 0.1 V) |
| 2000–2800 °C | Pore filling and interlayer insertion | More graphitic order, larger microdomains, mesopore development | Pronounced low-voltage plateau |
Key Takeaway: Higher temperatures shift capacity from high-voltage slope (adsorption) to low-voltage plateau (pore/interlayer storage). The optimum depends on balancing defects, porosity, conductivity, and Coulombic efficiency.
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