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Fluorine, Solvents and the Electrolyte

The most electronegative element appears in four places in a cell, each for a different consequence of the same property — and it generates hydrofluoric acid as a parting gift.

The Periodic Table of the EV · Part 3 — The Anode and the Electrolyte · Chapter 7 · 22 min read

All chapters
Weakly coordinating anionsDielectric constant vs viscosityDonor numberTransference numberHF generation

The electrolyte is where a battery stops being a materials problem and becomes a chemistry problem. It has to dissolve a salt, move a cation quickly, survive 4.2 V at one electrode and 0.1 V at the other, and decompose into something useful rather than something fatal.

Two elements do almost all of that work: fluorine, appearing in four separate places for four different consequences of one property, and carbon, arranged into solvents whose two most important properties are in direct conflict.

Cu6–10 µmAl12–20 µmANODE — GRAPHITESEPARATORCATHODE — LiFePO₄ / NMCLi⁺ — DISCHARGE →Me⁻ through the loadSEIELECTROLYTELiPF₆ in EC/DMC/EMCELEMENTS PRESENT:LiCO F PFe Ni Co MnCu Al— five regions of the table, five distinct jobs, one device.
Figure 7.1 — Every layer is an element choice. Copper on the left because it is noble enough to survive 0.1 V; aluminium on the right because it passivates at 4.2 V. Graphite because sp² carbon builds a conductive host with a weak interlayer gap. A d-block oxide on the right because only d orbitals cycle reversibly. Fluorine in the salt because it is the most electronegative element there is, and therefore the hardest to oxidise.

3.98

Fluorine electronegativity — the highest

~10 mS/cm

Typical electrolyte conductivity at 25 °C

< 1 %

Dry room relative humidity

0.2–0.4

Transference number — most current is anions

7.1 — Fluorine — one property, four jobs

Fluorine has the highest electronegativity in the periodic table at χ = 3.98, is the smallest halogen at 133 pm as F⁻, and forms the strongest single bond to carbon of any element at roughly 485 kJ/mol. It appears at four separate places in a lithium-ion cell, each exploiting a different consequence of that same extreme electronegativity.

7.1.1 — 1 — The salt anion

A salt dissolves and dissociates only if the anion is weakly coordinating — meaning its negative charge is spread over a large volume so it does not grip the Li⁺ cation.

Fluorine’s high electronegativity delocalises negative charge across six fluorine atoms in PF₆⁻, and across the –SO₂CF₃ groups in TFSI⁻. The result is a high degree of dissociation and therefore high ionic conductivity. A simple anion such as Cl⁻ would grip Li⁺ tightly and give a poorly conducting solution.

7.1.2 — 2 — Oxidative stability

Because fluorine holds its electrons so tightly, fluorinated anions are extremely hard to oxidise. That pushes the electrolyte’s HOMO down and widens the electrochemical stability window to roughly 4.5 to 5 V — a prerequisite for any high-voltage cathode.

7.1.3 — 3 — The SEI

LiF is a major SEI component: an excellent electronic insulator, mechanically hard and chemically inert. Fluoroethylene carbonate (FEC) is added specifically to generate more LiF in the SEI, and is essentially mandatory in silicon-containing anodes — because a silicon anode expanding 280 per cent needs an SEI that can survive being rebuilt constantly.

7.1.4 — 4 — The binder

PVDF, polyvinylidene fluoride. Its C–F bonds make it electrochemically inert at cathode potentials, chemically resistant to the electrolyte, and mechanically tough. It requires the solvent NMP, which is itself an environmental cost — hence the industry push toward aqueous CMC/SBR binders on the anode side and toward dry-coating processes.

7.2 — The dark side — HF generation

LiPF₆ ⇌ LiF + PF₅
PF₅ + H₂O → POF₃ + 2 HF

Trace moisture converts the salt into hydrofluoric acid. HF then attacks the cathode — accelerating the manganese and nickel dissolution described in chapter 4 — degrades the SEI, and corrodes aluminium.

Important

This is why cell manufacturing demands dry rooms below 1 per cent relative humidity, with dew points around −40 °C. The requirement traces directly to LiPF₆’s hydrolytic instability, not to general cleanliness.

LiFSI is considerably more hydrolytically stable, which is a large part of why it is displacing LiPF₆ despite the higher cost.

ELECTRON ENERGY — THE GOODENOUGH PICTUREENERGY ↑ / POTENTIAL vs Li ↓LUMO — reduction limit ≈ 1.0 VHOMO — oxidation limit ≈ 4.5 V≈ 3.5 V WINDOWcarbonate electrolyteORGANICGraphite Fermi level 0.1 V — ABOVE the LUMOCathode Fermi level 3.9 Velectrons spill into the electrolyte → it reduces→ but the products are the SEI, and the SEI stops itAQUEOUS1.23 V2H₂O → O₂ + 2H₂a hard thermodynamic ceilingIf the anode's Fermi level sits ABOVE the electrolyte LUMO, electrons flow into the electrolyte and reduce it.If the cathode's Fermi level sits BELOW the HOMO, electrons are pulled out of the electrolyte and it oxidises.Lithium-ion works because the reduction products happen to be ionically conductive and electronically insulating. That accident is the SEI.
Figure 7.2 — This is the fortunate accident on which the whole industry rests. Carbonate electrolytes are thermodynamically unstable at graphite potentials — they must decompose. What makes lithium-ion commercially possible is that the decomposition products (LiF, Li₂CO₃, Li₂O, alkyl carbonates) pass Li⁺ but block electrons, so the reaction self-limits after a few nanometres. Magnesium's products block everything, including magnesium. That, not capacity or voltage, is why magnesium batteries are still in laboratories after forty years.

7.3 — Why carbonate solvents, and why a mixture

You need two contradictory things from a solvent, and no single molecule provides both.

7.3.1 — High dielectric constant

The dielectric constant (ε) is the factor by which a medium reduces the electrostatic force between charges.

F = q₁q₂ / (4πε₀ε_r r²)

Coulomb’s law in a medium — a high ε_r screens the attraction between Li⁺ and its anion, allowing the salt to dissociate.

But high-ε solvents are strongly polar, which makes them viscous.

7.3.2 — Low viscosity

D = k_B T / (6πηr)

The Stokes–Einstein relation: diffusion coefficient is inversely proportional to viscosity.

But low-viscosity solvents are weakly polar and will not dissociate the salt. The two requirements pull in opposite directions.

In plain English

The solution is to stop looking for one solvent. Use a mixture: one component with a high dielectric constant to break the salt apart, and another with low viscosity to let the resulting ions move.

The carbonate solvents and what each is for

Solventε_rViscosity (cP)Role
EC (ethylene carbonate)89.81.90Dissociates the salt and forms a good SEI. Solid at room temperature (mp 36 °C) so cannot be used alone.
PC (propylene carbonate)64.92.53High ε and liquid to −49 °C, but co-intercalates into graphite and exfoliates it — usable with hard carbon, not graphite.
DMC (dimethyl carbonate)3.10.59Thins the mixture.
DEC (diethyl carbonate)2.80.75Thins the mixture.
EMC (ethyl methyl carbonate)2.90.65Thins; better low-temperature range.

A typical formulation is 1.0 to 1.2 M LiPF₆ in EC:DMC:EMC, giving about 10 mS/cm ionic conductivity at 25 °C.

Technical framing

Note the propylene carbonate entry. It looks superior to EC on paper — comparable dielectric constant, liquid far below zero — and it is unusable with graphite because it co-intercalates and exfoliates the sheets. This is a good reminder that electrolyte selection is constrained by the anode as much as by the numbers in the table.

7.4 — Donor number, desolvation and transference

Donor number (DN) measures a solvent’s ability to donate electron density to a cation — how strongly it solvates. High DN means strong solvation, which helps dissociation but makes desolvation at the electrode interface slow.

Why this matters

Desolvation is a major contributor to charge-transfer resistance and is strongly temperature-dependent. It is another reason cold charging is hard — before a lithium ion can enter the anode it has to shed its solvent shell, and in the cold that step slows down along with everything else, adding to the overpotential eating the 100 mV margin from chapter 6.

Transference number (t₊) is the fraction of total ionic current carried by the cation. In conventional liquid electrolytes it is only about 0.2 to 0.4.

Important

That means 60 to 80 per cent of the current is carried by anions moving the wrong way. They build a concentration gradient across the cell and add a concentration overpotential on top of the ohmic and charge-transfer terms. Single-ion-conducting and solid electrolytes aim for t₊ near 1, which is one of the main reasons they are pursued.

7.5 — Phosphorus — three separate jobs

Phosphorus sits in period 3, group 15. Unlike nitrogen above it, phosphorus has accessible 3d orbitals and can expand its coordination beyond four, forming PF₆⁻ and PO₄³⁻ readily. It earns three separate places in an EV.

  • •PO₄³⁻ in LFP — the inductive effect, covered in chapter 5.
  • •PF₆⁻ in the salt — the weakly coordinating anion above.
  • •Phosphate and phosphazene flame retardants — phosphorus compounds interrupt combustion by scavenging free radicals in the gas phase, the same mechanism used in polymer flame retardants.

7.6 — Carbon beyond the anode

  • •Conductive additive — carbon black, acetylene black, carbon nanotubes, graphene. Typically 1 to 3 weight per cent, providing the electron percolation network in a cathode that is otherwise a semiconductor or insulator. The percolation threshold is the critical volume fraction at which a connected path first spans the electrode.
  • •Carbon coating on LFP — a few nanometres, compensating for 10⁻⁹ S/cm intrinsic conductivity.
  • •Binder backbone — CMC (carboxymethyl cellulose) and SBR (styrene-butadiene rubber), both carbon polymers, used as an aqueous binder system on the anode side.

Quick check: test yourself

1.Why can’t a battery use a simple lithium chloride electrolyte?

Show answer
Because Cl⁻ is not weakly coordinating. Its charge is concentrated on one small ion, so it grips Li⁺ tightly, the salt dissociates poorly and the solution conducts badly. PF₆⁻ works because fluorine’s electronegativity delocalises the charge across six atoms.

2.Propylene carbonate has a high dielectric constant and stays liquid to −49 °C. Why is it not used in graphite cells?

Show answer
Because it co-intercalates into graphite along with lithium and exfoliates the sheets, destroying the anode. It is usable with hard carbon, which is why it appears in some sodium-ion formulations.

3.Why does a transference number of 0.3 hurt fast charging?

Show answer
Because 70 per cent of the ionic current is anions travelling the wrong way. They pile up as a concentration gradient across the cell, which adds a concentration overpotential on top of ohmic and charge-transfer losses — and at the anode, overpotential is what consumes the plating margin.

Chapter summary

  • ✓Fluorine appears in the salt, the stability window, the SEI and the binder — four jobs, all downstream of χ = 3.98.
  • ✓LiPF₆ hydrolyses to HF in the presence of trace water, which attacks the cathode, the SEI and aluminium. That single reaction dictates dry-room manufacturing below 1 per cent RH.
  • ✓A solvent must dissociate the salt (high ε) and let ions move (low η), and those are contradictory. Hence a mixture: EC for dissociation, DMC and EMC for fluidity.
  • ✓Strong solvation helps dissociation but slows desolvation at the interface, which is a major charge-transfer resistance term and another reason cold charging is hard.
  • ✓Only 20–40 per cent of ionic current is carried by lithium. The rest is anions moving the wrong way, building a concentration overpotential — which is what solid electrolytes are trying to eliminate.

Frequently asked questions

Why is fluorine used so heavily in lithium-ion cells?+

Because one property — the highest electronegativity in the periodic table at χ = 3.98 — produces four different useful consequences. In the salt, fluorine delocalises negative charge across six atoms in PF₆⁻ so the anion is weakly coordinating and the salt dissociates well. In stability terms, fluorinated anions are extremely hard to oxidise, pushing the electrolyte HOMO down and widening the stability window to 4.5–5 V. In the SEI, LiF is an excellent electronic insulator, mechanically hard and chemically inert. And in the binder, PVDF’s C–F bonds make it electrochemically inert at cathode potentials.

Why does lithium-ion manufacturing need a dry room?+

Because LiPF₆ hydrolyses. It decomposes to LiF and PF₅, and PF₅ reacts with water to give POF₃ and two molecules of hydrofluoric acid. HF then attacks the cathode — accelerating manganese and nickel dissolution — degrades the SEI and corrodes aluminium. Trace moisture is therefore not a cosmetic quality issue but a direct route to cell failure, which is why cell manufacturing demands relative humidity below 1 per cent and dew points around −40 °C. LiFSI is more hydrolytically stable, which is a large part of why it is displacing LiPF₆ despite the cost.

Why is battery electrolyte a mixture of solvents rather than one?+

Because it must satisfy two contradictory requirements. A high dielectric constant screens the attraction between Li⁺ and its anion so the salt dissociates — but high-ε solvents are strongly polar and therefore viscous. Low viscosity is needed for fast ion transport, since the Stokes–Einstein relation makes diffusion inversely proportional to viscosity — but low-viscosity solvents are weakly polar and will not dissociate the salt. The solution is a mixture: ethylene carbonate at ε = 89.8 dissociates the salt and forms a good SEI, while DMC and EMC at ε ≈ 3 thin it out. A typical formulation is 1.0 to 1.2 M LiPF₆ in EC:DMC:EMC, giving about 10 mS/cm at 25 °C.

What is the transference number and why does it matter?+

It is the fraction of total ionic current carried by the cation. In conventional liquid electrolytes it is only about 0.2 to 0.4, meaning 60 to 80 per cent of the current is carried by anions moving the wrong way. That builds a concentration gradient across the cell and adds a concentration overpotential on top of the ohmic and charge-transfer terms. Single-ion-conducting and solid electrolytes aim for a transference number near 1, which is one of the main reasons they are pursued.

Why is FEC added to electrolytes with silicon anodes?+

Fluoroethylene carbonate decomposes preferentially to generate more LiF in the SEI. Because silicon expands and contracts by hundreds of per cent and continually exposes fresh surface, its SEI must reform constantly, and a mechanically hard, chemically inert LiF-rich film survives that far better than one dominated by organic carbonates. FEC is essentially mandatory in silicon-containing anodes for that reason.

Reviewed by

SG

Sahil Goyal

Co-founder, Wingzman

LinkedIn
SG

Sourabh Goyal

Co-founder, Wingzman

LinkedIn

The Periodic Table of the EV is an original educational series on the materials science of electric vehicles. All values are standard-condition literature figures for representative materials, not measured data from a specific product, and sources differ on several of them. Always verify against the specific material datasheet in use before making design, purchasing or certification decisions.