"Battery grade" is the most expensive two words in the inorganic chemicals trade, and what a buyer pays for is worth pausing over. The purity headline — 99.9%, 99.99% — is the least informative line on the certificate. Two lots of the same lithium salt can both assay 99.99% and behave completely differently inside a cathode kiln or an electrolyte plant. The difference lives in the impurity table: which elements are listed, in what units, and by what method. This article walks through that impurity logic, from lithium hydroxide to lithium sulfide.

Read the impurity classes, not the purity headline

Battery-grade specifications sort contaminants into three families, and each family earns its line for a different reason. Magnetic metals — iron, chromium, nickel, zinc — are controlled to parts-per-billion on cathode materials because a single conductive particle can seed an internal short. Alkali and alkaline earth metals (sodium, potassium, calcium, magnesium) sit in the ppm range: they do not short cells, but they substitute into crystal lattices and degrade cycle life, and calcium in particular co-precipitates stubbornly wherever you are trying to grow a clean precursor. Everything else — chloride, sulfate, heavy metals, loss on ignition — is process fingerprint evidence of how the material was made and washed. When a quote arrives without the magnetic-substance line, the discount you are looking at is the cost of the measurement the supplier chose not to run.

Carbonate pickup: the slow enemy of lithium hydroxide

Lithium hydroxide (CAS 1310-65-2) is strongly alkaline and reacts with atmospheric carbon dioxide on contact, converting surface material into lithium carbonate. A bag opened in a humid plant and left overnight will measure measurably higher carbonate by morning. In cathode calcination, that carbonate releases CO₂ and leaves sodium-and-carbonate-rich residues that show up as haze and poor first-charge efficiency. Two specification consequences follow. First, check which basis the assay is quoted on — the monohydrate (MW 59.75) and the anhydrous salt (MW 41.96) differ by 30%, and a price compared across the wrong basis is wrong by that margin. Second, ask how the material is packed; foil-laminate liners with a heat seal keep the carbonate number where the certificate says it is.

Particle size is a specification, not a courtesy

For several of these salts, the D50 matters as much as the assay. Lithium fluoride (CAS 7789-24-4) is supplied to 99.99% in the cubic NaCl-type structure, and its particle size distribution decides how it behaves twice: as a cathode additive it must melt and react evenly during calcination, and as an SEI-forming component it dissolves and re-precipitates at a rate set by surface area — finer lithium fluoride is explicitly cited as improving SEI film stability and extending cycle life. Lithium phosphate (CAS 10377-52-3) is the phosphate source in LiFePO₄ synthesis and a precursor for NASICON-type solid electrolytes; coarse material leaves unreacted phosphate residuals after the same firing schedule. And in the precursor plant, cobalt hydroxide (CAS 21041-93-0, pink to red powder, density 3.6 g/cm³) is bought partly on flowability and tap density, because a co-precipitation line fed with an uneven powder makes an uneven precursor particle — and the cathode inherits every lump.

The oxalate and the phosphates: feeding the cathode

Lithium oxalate (CAS 553-91-3, MW about 101.9, density 2.12 g/cm³) has one job that no other lithium salt does as cleanly: it decomposes at about 410 °C, releasing lithium exactly where a lithiated anode needs it. That is the chemistry behind pre-lithiation additives and energy-storage dosing. The trade is gas — the decomposition is not silent, it evolves CO and CO₂, so electrode calendaring and degassing schedules must assume it. For cathode synthesis routes that go through solution, lithium dihydrogen phosphate (CAS 13453-80-0, MW about 103.9) brings a mildly acidic pH that keeps iron in solution until the moment you want it out, which is why LFP lines prefer it over neutral salts. Lithium sulfate monohydrate (CAS 10102-25-7) serves as a lithium source in precipitation and reagent routes where carbonate would foam; its stable hydrate is also the reason assays are quoted per monohydrate, and the water of crystallisation belongs in every price comparison.

Lithium sulfide: the most unforgiving material on the list

Every lithium salt above dislikes water; lithium sulfide (CAS 12136-58-2, MW 45.95) turns it into a safety event. Contact with moisture releases hydrogen sulfide, which is why it is drummed under inert gas and why a qualification audit should look at the supplier's nitrogen-blanket filling line, not only the certificate. The prize justifies the handling: Li₂S is the lithium feedstock for argyrodite sulfide solid electrolytes (the Li₆PS₅Cl family), the chemistry behind the next generation of all-solid-state cells. A cheaper lot with a wet history — visible as hydrolysed sulfate and sulfite in the anion scan — will cost a solid-electrolyte batch its ionic conductivity. On the electrolyte side, lithium hexafluoroarsenate (CAS 29935-35-1) remains a niche salt for high-voltage systems thanks to its oxidative stability, with an arsenic stewardship burden that makes waste-stream documentation part of the commercial negotiation.

Precursor economics: nickel sulfate and the co-precipitation line

Most of the nickel in the EV supply chain passes through nickel sulfate (CAS 7786-81-4, blue crystalline powder) on its way to NCM cathode. Battery grade here means far more than "nickel sulfate that is pure": electroplating-grade material routinely carries calcium and magnesium at levels a plating bath never notices but a precursor tank cannot tolerate, because they co-precipitate into the hydroxide and end up as inclusions in every particle grown that day. Oil, magnetic substances and chlorine each carry their own line and their own ppm ceiling. Buyers comparing a battery-grade quote against an industrial-grade price should read those two tables side by side — the gap is not margin, it is wash efficiency and the cost of proving it.

Keep the water out: drying and storage chemistry

The last impurity — water — is managed with equipment rather than certificates. Electrolyte plants dry their solvent streams through molecular sieve and zeolite beds (3A for methanol and polar solvents, 4A for general drying), pushing water into the low-ppm region where LiPF₆ hydrolysis stops mattering; capacity and regeneration schedule, not price per kilogram, drive the operating cost. In the warehouse, the hygroscopic salts in this article live under silica gel desiccant and sealed liners — cheap insurance against the carbonate pickup described above. Where conductive or functional additives are part of the formulation, graphene oxide adds its own specification axis: flake size distribution and oxidation degree, both of which shift dispersion behaviour more than total purity does.

What to put on the purchase order

Eight lines, in this order: assay basis written out (monohydrate, anhydrous, or as-is); the magnetic-substance result in ppb with the method; Na/K/Ca/Mg individually, not summed; a carbonate or anion scan for hydroxide and sulfide lots; D50 and full particle-size distribution for calcination and SEI grades; packaging — liner type, inert gas, and whether the bag can be resealed; the impurity elements the supplier did not list (absence of a line is a finding, not a pass); and for sulfides, the filling-line audit. A supplier who answers all eight without prompting has already told you something the price list cannot.