Battery Chemistries: LFP, NMC, Sodium-Ion, Flow and the Rest

Nearly every grid battery being built in the U.S. is lithium-ion. "Lithium-ion" is a family, though, and the members burn differently. The batteries and storage lesson compares LFP and NMC on price, density and cycle life. This lesson adds the safety chemistry, the newer alternatives and how to read chemistry claims at a hearing.

What is inside a lithium-ion cell

Sandia's Energy Storage Handbook lists five key parts: a negative electrode (usually graphite), a positive electrode (the cathode, which names the chemistry), a liquid organic electrolyte, a thin plastic separator between the electrodes, and metal current collectors. The electrolyte is flammable. The separator is what keeps the two electrodes from touching; conventional polyolefin separators melt at about 130 to 170 degrees Celsius, and when they do, the cell shorts internally.

Sandia puts the fire problem plainly: a lithium-ion cell with a metal-oxide cathode carries fuel (the electrolyte), oxygen (released by the hot cathode) and, once it fails, its own ignition source. That is why smothering a lithium-ion fire works poorly. The fire is partly feeding itself.

The chemistries side by side

"Safer" is a comparison

LFP is harder to push into thermal runaway than NMC, and when it does run away it releases less heat and much less oxygen. That is real. It does not follow that LFP systems don't burn or explode.

A 2024 review in the Journal of Energy Storage by researchers at the University of Sheffield, Imperial College London and the University of St Andrews pooled results from 60 papers on gas released during thermal runaway. NMC cells released more gas. LFP gas had more hydrogen, and its lower flammability limit was 6.2 percent against 7.9 percent for NMC. Using median gas volumes, the authors calculated that LFP off-gas reaches its lower flammability limit at a volume 18 percent smaller than NMC off-gas. The authors concluded LFP presents a greater flammability hazard even though LFP cells show flames less often in tests. For a battery inside a closed container, the gas that piles up without burning is the explosion risk; the failure modes lesson returns to this.

The fire record has LFP entries too. EPRI's database lists the April 16, 2021 explosion at a 25 MWh LFP system in Beijing that killed two firefighters, the June 2023 failures at two LFP sites in Warwick, New York (one with an explosion), and an October 2023 fire at a 2 MW / 8 MWh LFP system at an Idaho Power substation south of Melba.

What EPRI's chemistry data can and cannot show

In EPRI's stationary table as of its September 2026 update, incidents since 2023 with a named chemistry break down as 15 NMC and 11 LFP, with 36 more listed only as "Li ion." That count alone means little, because the share of LFP in the fleet rose fast over the same years. EPRI's October 2026 report tried to account for this and still could not separate chemistry from design maturity, so it did not conclude that cell chemistry alone is a risk factor for system-level failure. Industry groups cite that to say design matters more than chemistry. Residents cite the same finding to say a developer cannot rely on "we use LFP" as a safety answer. Both readings are fair. The practical upshot is the same: ask for test data on the specific product.

Sodium-ion and flow: the alternatives

Sodium-ion swaps lithium for sodium, which is cheap and abundant. Datang Hubei Energy Development brought the first 50 MW / 100 MWh phase of what was billed as the world's largest sodium-ion BESS online in Qianjiang, China, in July 2024, using cells from HiNa Battery, with 100 MW / 200 MWh planned. HiNa claims better performance in cold and in abuse tests. Energy-Storage.news reported that sodium-ion still cost more than lithium-ion at that point because of smaller scale. Treat cold-weather and safety claims as manufacturer claims until independent test reports exist for a given product.

Flow batteries keep their energy in tanks of liquid electrolyte, pumped through a cell stack where the reaction happens. Tank size sets the hours; stack size sets the megawatts. Rongke Power completed a 175 MW / 700 MWh vanadium flow project in Xinjiang in December 2024, described as the largest of its kind. Flow batteries take more space per MWh than lithium-ion and the U.S. market is small, but they shift the hazard from fire toward spill containment.

Form Energy's iron-air battery, covered in the storage lesson, targets multi-day discharge rather than the four-hour market.

How long the cells last

Cells lose capacity with every cycle and with time. Sandia notes that cycle aging depends on depth of discharge, charge rate and temperature, and that calendar aging depends on state of charge and temperature, rising with heat. Charging too fast, especially when cold, can plate lithium metal on the graphite, which costs capacity and can grow dendrites, a safety risk.

Years to 80% from cycling alone:
  6,000 cycles / 365 per year = about 16.4 years

Energy left at 80%:
  400 MWh x 0.80 = 320 MWh

To keep selling 400 MWh, the owner must add:
  400 - 320 = 80 MWh of new modules (augmentation)

If the contract runs 20 years at one cycle a day:
  20 x 365 = 7,300 cycles, past the 6,000 rating