Put two graphite spec sheets side by side, and both list carbon content at 99.95% C. One supplier’s material sails through a cell maker’s qualification. The other’s gets rejected when the customer retests it. What went wrong?
Often, the answer is hiding in that number: which method produced it, on what basis it was reported, and what it leaves out. For companies new to anode materials, fixed carbon content is often the first quality metric they meet, and one of the easiest to misread. It tells you where a graphite product sits in the value chain, but on its own it can’t tell you whether a batch is battery grade.
Fixed carbon content is the mass percentage of carbon left in a graphite sample after moisture, volatile matter, and ash are subtracted: Fixed carbon (%) = 100 − moisture − volatile matter − ash. It’s a calculated value rather than a direct measurement, and it’s a standard indicator of natural graphite purity and grade.
Key takeaways:
- Fixed carbon is calculated, not measured directly. In purified graphite, it mostly reflects ash content.
- EV-grade spherical purified graphite (SPG) is commonly specified at 99.95% carbon or higher.
- Carbon content shows total impurities, not which elements they are. Battery-grade specs also limit metals, magnetic impurities, and physical properties.
- Fixed carbon, LOI, and total carbon are different reporting bases. Compare like with like.
What Fixed Carbon Content Actually Measures
The concept of fixed carbon comes from proximate analysis, a method originally developed for fuels such as coal. Proximate analysis splits a sample into four parts: moisture, volatile matter, ash, and fixed carbon. The first three are measured by heating and ignition. Fixed carbon is calculated by subtracting those three values from 100, not measured on its own.
The graphite industry adopted the same approach. In purified natural graphite, moisture and volatile matter are usually very low, so fixed carbon depends mostly on ash, the noncombustible inorganic residue left after ignition. The lower the ash, the higher the fixed carbon.
There’s a catch. On graphite spec sheets and test reports, “carbon content” doesn’t always mean the same thing. You’ll commonly see four different terms:
Table 1. Common ways graphite carbon content is reported
| Term | What it means | Where you’ll usually see it |
| Fixed carbon (FC) | Calculated as 100 minus moisture, volatile matter, and ash | Product spec sheets, purchase contracts |
| LOI carbon content | Loss on ignition: the share of mass lost when the sample is burned in air at high temperature | Concentrate and purification test reports |
| Total carbon, C(t) | Carbon measured directly with a combustion analyzer, including non-graphitic carbon | Mine and beneficiation test reports |
| Graphitic carbon, Cg | Graphitic carbon only, excluding carbonate and organic carbon | Resource estimates, beneficiation reports |
At high purity, these values often land close together, but the methods and reporting bases differ. Before you compare two reports, make sure they use the same one.
Typical Carbon Content at Each Stage of the Natural Graphite Value Chain
Between the mine and the battery anode, natural graphite’s impurity content falls by several orders of magnitude. The table below shows typical carbon levels at each stage. It doesn’t imply a fixed process sequence, because production lines arrange their steps differently. ZDZN’s natural graphite anode lines, for example, purify first and shape second.
Table 2. Typical carbon content by stage
| Stage | Typical carbon content | Notes |
| Run-of-mine ore | Anywhere from single digits to tens of percent | One low-grade ore from eastern India, for example, contained just 8.59% fixed carbon, with the rest mostly quartz and minor mica |
| Flotation concentrate | About 90%–95%+, depending on ore and flotation circuit | Feedstock for lithium-ion batteries is known as “-195” grade, with at least 80% passing 100 mesh and a minimum of 95% fixed carbon |
| High-purity graphite (HPG) | 99.9% and up | Chemically or thermally purified, with particle shape unchanged; in academic classification, graphite at 99.9% carbon or higher counts as high purity |
| Spherical purified graphite (SPG) | 99.95% and up | The price benchmark specification is 99.95% C, 15 µm, uncoated |
Going from 95% to 99.95% looks like a gain of less than five percentage points. In reality, impurities have to drop from about 5% to 0.05% or less, a 100-fold reduction. That’s what makes purification the hard part. A concentrate’s carbon number also says little about how hard its impurities will be to remove. For how to evaluate a feedstock before you commit to it, see How to Qualify Natural Graphite Concentrate for Battery Anode Production. For how the main purification routes work, see Natural Graphite Purification Methods Compared. For the differences between HPG, SPG, and CSPG, see SPG vs. CSPG vs. HPG.
How Is Fixed Carbon Content in Graphite Measured?
Ash and LOI Methods
The most basic method is to burn the sample in air at high temperature until all the carbon is gone, then weigh the ash that remains. A standard ash method for graphite is ASTM C561.
This method has a limitation that’s easy to overlook. In its own scope, ASTM C561 states that it isn’t intended for measuring ash in purified graphites such as nuclear materials, and that it doesn’t establish how ash content relates to a sample’s mineral content. That doesn’t make ash or LOI testing irrelevant at high purity. Battery-grade carbon content is still commonly reported as LOI, and labs adapt their procedures to very low ash levels. But at the 99.95% level, a total impurity figure alone can’t tell you whether the material is fit for a battery. It has to be paired with elemental analysis.
Thermogravimetric Analysis (TGA)
A thermogravimetric analyzer weighs the sample continuously under a programmed temperature profile, so it can determine moisture and ash automatically. For graphite’s low ash levels, instrument makers use test methods based on modified versions of ASTM C561 and C562. TGA is automated and repeatable, but it still reports a total. It can’t tell you which elements are in the residue.
Trace Element Analysis (ICP-OES and ICP-MS)
At battery grade, customers want to know exactly how much iron, chromium, copper, nickel, and zinc is present. That calls for inductively coupled plasma optical emission spectrometry (ICP-OES) or mass spectrometry (ICP-MS). ICP-MS is the more sensitive of the two and can measure elemental impurities in graphite anode materials down to the ppb level.
Table 3. Common test methods for carbon content and purity
| Method | What it measures | Strengths and limits |
| Ash / LOI | Residue left after ignition, or mass lost on ignition | Simple and low cost; the usual way carbon content is reported; at high purity needs adapted procedures and must be paired with elemental analysis |
| Thermogravimetric analysis (TGA) | Mass change under programmed heating, yielding moisture and ash | Automated, repeatable; reports only totals |
| Combustion carbon analyzer | Total carbon, measured directly | Gives C(t); doesn’t distinguish graphitic carbon from other forms |
| ICP-OES / ICP-MS | Content of individual metallic elements | Essential at battery grade; reaches ppm and even ppb levels |
A Real-World Example: When 0.01% Decides Pass or Fail
During lab-scale testing for one North American graphite project, a batch of purified spherical graphite left a white residue after ashing and graded only 99.94% LOI, short of the 99.95% LOI minimum for EV-grade SPG. After further treatment, the material reached 99.99%, and its impurities were checked element by element with ICP.
At high purity, a hundredth of a percentage point can decide whether material passes, and only elemental analysis tells you what’s in the residue.
Why Fixed Carbon Alone Doesn’t Define Battery-Grade Graphite
Fixed carbon tells you how much impurity there is in total, but cell makers care about far more than that one number. Beyond carbon content of at least 99.95%, a typical battery-grade spherical graphite spec sheet also sets limits on iron, moisture, particle size distribution, tap density, and specific surface area.
Table 4. Common items on a battery-grade spherical graphite spec sheet
| Category | Typical items | What it tells you |
| Chemical purity | Carbon content (fixed carbon or LOI), ash | Total impurities |
| Trace impurities | Metals such as Fe, Cr, Cu, Ni, and Zn; magnetic impurities | Whether specific elements exceed limits, typically at ppm level or below |
| Physical properties | Particle size distribution (D10/D50/D90), tap density, specific surface area | Effects on electrode processing and electrochemical performance |
| Other | Moisture, morphology | Stability in storage and processing |
Two batches can both assay at 99.95% carbon while their remaining 0.05% looks completely different. In one, it might be mostly silica. In the other, it might contain a good deal of iron. For a battery, the second batch carries far more risk. That’s why spec sheets set limits for individual elements and magnetic impurities, not just carbon content. For how magnetic impurities are removed during production, see What Is Natural Graphite Magnetic Separation for Anode Material Production?
How Fixed Carbon and Impurities Affect Battery Performance
No single metric determines how a graphite anode performs. One study of battery-grade graphite supply sums up the key requirements: crystallinity sets lithium storage capacity, purity supports cycle life, particle size governs rate performance, sphericity drives electrode packing density, and surface area controls first-cycle losses from SEI formation. Within that framework, purity matters in three ways.
Metallic impurities are a safety risk. Metal particle contamination is a top concern in lithium-ion manufacturing because it can cause abnormal self-discharge in cells and even lead to thermal runaway. Research backed by the U.S. Department of Energy also identifies foreign metal particles as a leading cause of internal short circuits in cells. That’s why spec sheets limit iron, other metals, and magnetic impurities separately instead of relying on carbon content alone.
Purity affects cycle life and batch consistency. Impurities can trigger side reactions that gradually consume active lithium and electrolyte over many cycles. Just as important to cell makers is batch-to-batch consistency. Material whose carbon content and impurity profile swing widely has a hard time passing long-term qualification.
First-cycle efficiency depends on more than purity. A common assumption is that higher purity automatically means higher first-cycle coulombic efficiency (ICE). In practice, ICE depends largely on surface area, surface defects, and coating quality. A well-known study compared natural graphite purified by chemical and thermal methods: the chemically purified material reached a first-cycle efficiency of 91%, versus 84% for material treated thermally at 2,400°C, even though later work noted that the thermally treated samples actually had less silicon. The study dates from the early 2000s, but its lesson still holds: meeting the purity spec is a prerequisite, while ICE also depends on morphology, surface condition, and coating.
Two Common Misconceptions About Fixed Carbon
“Results from different labs and methods can be compared directly.” Fixed carbon, LOI, and total carbon are reported on different bases, and drying, ignition temperature, and ignition time all affect the result. Align the method and basis before comparing supplier data with your own. When in doubt, send both samples to the same lab.
“If carbon content meets spec, impurities are under control.” Carbon content reflects only total impurities. Specific elements and magnetic impurities have to be tested separately. And hitting the spec after purification is only the first step: later grinding, shaping, coating, and conveying can reintroduce metallic impurities through equipment wear and piping. For how to reach battery-grade purity and hold it across an entire line, see our complete guide to purity control for natural graphite anode materials.
FAQ
How is fixed carbon content in graphite calculated? Fixed carbon (%) = 100 − moisture (%) − volatile matter (%) − ash (%), with all values on the same basis. Moisture, volatile matter, and ash are measured first, and fixed carbon is what remains.
What carbon content does battery-grade graphite require? Spherical purified graphite (SPG) for EV batteries is commonly specified at a minimum of 99.95% carbon, though requirements vary by application and anode grade, so go by your target customer’s spec sheet. Spec sheets usually also limit iron and other metals, magnetic impurities, particle size, tap density, and specific surface area.
Is LOI the same as fixed carbon? Not exactly. LOI is the percentage of mass a sample loses on ignition, while fixed carbon is calculated by subtracting moisture, volatile matter, and ash from 100. In high-purity graphite the two numbers are usually close, but always note which one you’re comparing.
What’s the difference between fixed carbon and total carbon? Fixed carbon is calculated by subtracting moisture, volatile matter, and ash from 100. Total carbon, C(t), is measured directly with a combustion analyzer and includes all forms of carbon, not just graphitic carbon.
Why can graphite at 99.95% carbon still fail a cell maker’s qualification? Carbon content reflects only the total amount of impurities, not which ones are present. If the remaining 0.05% contains too much iron or another metal, or too many magnetic impurities, or if particle size or surface area is off spec, the material will still be rejected.
Read the Number Before You Plan the Line
Fixed carbon tells you which tier of the value chain a material belongs to, but it can’t prove on its own that the material is ready for the battery supply chain. For new entrants, understanding how every number on a spec sheet was measured matters more than chasing a higher figure.
Depending on where your project stands, ZDZN can support you in several ways:
- Have samples in hand? Send them to ZDZN for anode material performance testing, and use the results to see where your material stands against your target specs.
- Still comparing feedstocks and process routes? Talk with our process engineers about your concentrate, your purification route, and the purity control points in the downstream steps.
- Ready to build or upgrade a line? Request a complete production line assessment. We’ll coordinate equipment configuration and process integration from purification and milling through spheroidization, coating, and carbonization, and lay out a phased EPC plan.