The basic idea is straightforward - you deposit a thin carbon layer on battery materials to make them more conductive. LFP without carbon coating has conductivity around 10^-9 S/cm which is basically an insulator. Add 2-3 wt% carbon coating and you get 10^-3 S/cm, enough to make a functional battery.
We run both CVD systems and wet chemical coating lines at our facility. CVD gives better uniformity but costs more. Wet coating works fine for most applications and the equipment is simpler.
Check out our CVD coating capabilities for more details on the high-end option.
Why Coat Materials with Carbon?
Most phosphate-based cathodes need it. The electronic conductivity is terrible without coating. Iron phosphate, manganese phosphate - same story. Even some oxide cathodes benefit from coating if you're pushing high C-rates.
The coating also acts as a protective layer between the cathode and electrolyte. This matters more at elevated temperatures where side reactions accelerate. We've seen cycle life improve 40-50% just from coating, especially when cells run above 45°C.
Silicon anodes are a different animal. The volume expansion during cycling (300-400%) will crack most coatings. You need flexible carbon structures or the coating fails after a few cycles. We worked on this problem for three years before we got a formulation that actually held up past 200 cycles.
CVD Coating Process
Our CVD setup uses acetylene or methane gas at 650-750°C. Flow rates depend on batch size - typically 50-200 sccm for a 100 kg batch. The gas decomposes on the particle surface and forms the carbon layer.
Thickness control is by time and temperature. 30 minutes at 700°C gets you roughly 5-8 nm depending on the substrate. If you need thicker coating you run it longer, but watch out for pore blocking especially with high surface area materials.
The graphitic content of CVD carbon is higher than wet chemical methods, which means better conductivity. Acetylene gives more graphitic carbon than methane but it's also more expensive and kind of a pain to handle safely.
Batch size on our CVD line ranges from 10 kg up to 200 kg. Larger batches are possible but temperature uniformity becomes an issue. We've learned this the hard way - ran a 500 kg batch once and the coating on material from the center versus the edges was noticeably different.
Wet Chemical Approach
Mix the powder with glucose solution, dry it, then pyrolyze in nitrogen atmosphere. The sugar carbonizes and coats the particles. Simple concept but getting it uniform takes some trial and error.
pH of the glucose solution matters. For basic materials like LFP we keep pH around 4-5 so the glucose adheres better. The drying step is critical - if you dry too fast you get clumping. We use spray drying now which works much better than our old rotary dryer setup.
Pyrolysis temperature is usually 500-650°C for glucose. Higher temperatures give more graphitic carbon but you start burning off carbon yield. Citric acid is another option, gives similar results to glucose. Some customers prefer sucrose but honestly we haven't seen much performance difference.
The carbon from wet coating is mostly amorphous with some short-range graphitic domains. Conductivity is decent, not as good as CVD but good enough for most batteries. Cost is roughly 40% lower than CVD per kg of coated material.
Applications We've Worked On
LFP cathodes are probably 70% of our coating volume. Standard spec is 2.5 wt% carbon, 8-10 nm thickness. Some customers want 3% for higher rate applications.
We also coat LTO anodes, though less frequently. Usually 1-1.5 wt% carbon is sufficient since LTO conductivity isn't as bad as LFP. The coating helps with high rate charge capability which matters for fast charging applications.
NCM811 and other nickel-rich cathodes sometimes get coated for surface stability rather than conductivity. The coating thickness is thinner, maybe 3-5 nm, just enough to reduce direct contact between cathode and electrolyte. This reduces transition metal dissolution which is a failure mode for nickel-rich materials at high voltage.
Silicon composite anodes are challenging. Standard coating doesn't work because of the volume expansion issue. We developed a carbon coating formulation with some elasticity using polymer-derived carbon. It costs more but it's the only way we've found to get decent cycle life. Even then, you're looking at maybe 500-800 cycles before significant capacity fade.
One automotive company wanted us to coat their experimental manganese-rich cathode material. That project didn't work out - the material was chemically unstable during the coating process and we kept seeing phase changes. Sometimes coating isn't the solution.
Real Numbers from Production Batches
Last month we coated 3 tons of LFP for a customer in South Korea. Target was 2.8 wt% carbon. Batch results ranged from 2.65% to 2.95%, which is within our ±0.3% tolerance. Conductivity on pressed pellets averaged 8.2 x 10^-3 S/cm.
For comparison, the same material uncoated measured 2.1 x 10^-9 S/cm. That's about 4 million times improvement in conductivity, though comparing pressed pellet conductivity to particle conductivity isn't perfect methodology.
Cycle life testing on coin cells (C/3 charge, C/3 discharge, 2.5-3.8V range) showed capacity retention of 91% after 1000 cycles at 25°C. The customer's target was 90% so it passed.
| Material Type | Carbon Content | Our Typical Range | Notes |
|---|---|---|---|
| LFP cathode | 2-3 wt% | 2.3-2.9% | Most common application |
| LTO anode | 1-2 wt% | 1.2-1.7% | Less critical than LFP |
| NCM/NCA | 0.5-1.5 wt% | 0.8-1.3% | For surface protection mainly |
| Silicon composite | 5-10 wt% | 6-9% | Need flexible coating |
The ranges shown are what we actually achieve in production, not theoretical targets.
Coating Quality Issues We've Seen
Incomplete coverage is the most common problem, especially with wet coating. You end up with bare spots on the particle surface which creates local current concentration during cycling. This shows up as capacity fade after 200-300 cycles.
Too thick coating blocks lithium diffusion. We had one batch where coating was 25 nm instead of target 10 nm due to a temperature control issue. Rate capability was noticeably worse - cells couldn't handle 1C discharge without significant voltage drop.
Carbon oxidation during storage is another issue. Coated powder should be stored under dry conditions. We had a customer who stored material in a humid warehouse for six months and the carbon content dropped from 2.5% to 1.9%. The carbon slowly oxidizes in moist air.
Equipment and Capacity
Our main CVD furnace can handle 200 kg batches. We also have a smaller R&D furnace for 5-10 kg batches when customers want to test coating on new materials. Turnaround for R&D batches is usually 1-2 weeks. Production batches take 3-4 weeks from material receipt to shipment.
Wet coating line has higher throughput, up to 500 kg per batch. The limiting factor is usually the spray dryer capacity rather than the pyrolysis furnace.
We're expanding capacity next year with a new CVD system, should be online by Q2 2026. Target capacity is 300 kg batches which will help with some of our larger customers.
Development Work
If you have a material that might benefit from coating but you're not sure, we can run development tests. Minimum quantity is usually 200 grams. We'll test 2-3 different coating conditions and provide coated samples plus electrochemical data from coin cells.
Development cost depends on testing scope. Basic coating evaluation with coin cell testing runs around $3500. If you need more extensive testing like full cell builds or long-term cycling we can quote that separately.
One issue we run into with development work is that lab results don't always translate to production scale. We coated a material at 50 gram scale that looked great, but when we scaled to 50 kg the coating uniformity was terrible. Particle size distribution and surface area both affect coating behavior and sometimes what works small doesn't work big.
Cost Factors
CVD coating adds roughly $2-4 per kg to material cost depending on batch size and coating specification. Wet chemical coating is $1.50-2.50 per kg.
Minimum order for production coating is usually 50 kg. Below that the setup cost makes it inefficient. For development quantities under 50 kg we charge a setup fee.
If you're buying precursor material from us (uncoated powder) and having us coat it, we can usually get better pricing than if you ship us your own material. The logistics are simpler and we're already set up with the material suppliers.
Shipping coated material requires some care because the powder is more pyrophoric than uncoated material. We use UN-approved packaging and ship via ground transport only. Air freight isn't allowed for most carbon-coated materials due to fire risk.
Testing and Specifications
Standard testing we provide with every batch:
Carbon content by combustion analysis (±0.1 wt%)
Tap density
Particle size distribution (D10, D50, D90)
Moisture content
SEM images (provided on request)
Additional testing available:
Conductivity measurement on pressed pellets
BET surface area
XRD for crystal structure
TEM cross-sections for coating thickness verification
ICP-MS for impurity analysis
Coin cell testing (cycling performance, rate capability, impedance)
Most customers just want the basic testing plus conductivity measurement. Full characterization adds about a week to turnaround time and costs extra.
What We Don't Do
We don't coat electrode sheets. Our equipment is designed for powder coating. If you need coating on already-fabricated electrodes that's a different process entirely.
We also don't handle materials with serious safety concerns. No lithium metal powders, no highly air-sensitive materials. Standard battery materials are fine but if your material spontaneously combusts in air we can't work with it.
Ultra-high purity coating (semiconductor grade) isn't our focus. We're set up for battery materials which means good purity but not clean room level. If you need sub-ppm contamination control you need a different kind of facility.
Customer Examples
A battery company in Michigan sent us their silicon-graphite composite anode material. They were seeing capacity fade after 150 cycles. We coated it with our flexible carbon formulation and they got cycle life up to 600 cycles. Material cost increased by $3.50/kg but the performance improvement justified it for their application.
Another project involved coating NCM811 for a European automotive customer. They were worried about capacity fade at high voltage (4.3V cutoff). Standard NCM811 showed 15% capacity loss after 500 cycles. With 1 wt% carbon coating plus some surface treatment we got that down to 8% capacity loss. The coating wasn't the only factor - they also optimized their electrolyte - but it helped.
We worked with a research group developing a new cathode composition (lithium-rich NCM variant). The material had good capacity but terrible rate capability. After coating with 2% carbon the discharge capacity at 1C improved from 140 mAh/g to 168 mAh/g. The conductivity was the limiting factor for that material.
Sometimes coating doesn't solve the problem. We had a customer with rapid capacity fade in their cells and they thought coating would fix it. After investigating we found their fade was due to lithium plating on the anode during fast charging. Coating the cathode wasn't going to help with that. We recommended they look at their charging protocol instead.
Technical Resources
We've published some papers on carbon coating if you want more details on the science. Most are behind paywalls but we can send PDFs if you contact us.
If you're working with lithium iron phosphate materials and want to understand the battery chemistry side of things, this article on [lithium ion phosphate battery] covers the basics pretty well. Understanding the battery chemistry helps explain why coating makes such a difference for LFP specifically.
