Lithium (Li)

Most elements have an atomic weight you can look up as a single number. Lithium's is published as an interval, and the interval is unusually wide — wide enough that IUPAC advises against quoting a single value for material of unknown origin. The variation is not natural. It is the residue of a weapons programme.

Someone took the lighter atoms

Lithium-6 absorbs a neutron and breaks into tritium and helium, which makes it the breeding material for the fusion stage of a thermonuclear weapon. Producing it in quantity meant separating it from lithium-7, and during the 1950s and 60s the Oak Ridge Y-12 plant did so at industrial scale using the COLEX process, in which lithium exchanges between an aqueous solution and a mercury amalgam with a slight isotopic preference at each step, compounded through a long column.

The programme consumed enormous quantities of lithium and left behind thousands of tonnes of the depleted remainder — lithium with much of its lithium-6 removed. Some of that stock was subsequently sold into ordinary chemical commerce. A bottle of lithium reagent may therefore contain material whose isotopic composition was set by a Cold War enrichment cascade rather than by geology, and its atomic weight can differ from the natural value in the third significant figure. IUPAC formally moved lithium to an interval in 2009 for exactly this reason.

The same separation problem persists in reverse. Pressurised water reactors use lithium hydroxide, enriched in lithium-7, to control the pH of their primary coolant; ordinary lithium would generate tritium in the core. Very few facilities in the world make it, and Western utilities have depended on Russian and Chinese supply for a material their reactors cannot run without.

Found in a stone, and named for it

Johan August Arfwedson was working in Berzelius's Stockholm laboratory in 1817, analysing petalite from the Utö mine, and could not make his numbers add up: around four per cent of the sample's mass belonged to something he could not identify. It behaved like an alkali and matched neither sodium nor potassium.

Berzelius supplied the name. Potassium had come out of plant ashes and sodium out of soda and natron, both associated with organic or evaporitic sources; this alkali had turned up in a hard mineral. Lithos is Greek for stone, and the name records the contrast rather than any property of the element.

Arfwedson never had the metal. Humphry Davy and William Brande obtained traces by electrolysis around 1821, and Bunsen and Matthiessen produced usable quantities in 1855.

The best possible battery metal, on both counts at once

A battery's energy per kilogram depends on two things: the voltage of the cell and the mass of the material you have to carry to produce it. Lithium is exceptional on both.

It has the most negative standard electrode potential of any element, at about −3.04 volts, which sets the ceiling on cell voltage. It is also the lightest metal in existence and the least dense solid element at room temperature — light enough to float on water, which it also reacts with. No other element combines a very high potential with a very low atomic mass, and that is why lithium has no substitute at the top end of energy density. Sodium and other alternatives are pursued for cost and supply reasons, never because they store more.

The markets that came before the batteries

Lithium's association with batteries is recent and it is easy to forget that the element had a substantial industry for most of the twentieth century.

• Glass and ceramics took the largest share for decades. Lithium lowers the melting point of a melt, cutting furnace energy, and lithium aluminosilicate glass-ceramics have a coefficient of thermal expansion near zero — the basis of ceramic cooktops, telescope mirror blanks and flame-proof cookware.
• Lubricating greases thickened with lithium soaps remain serviceable across an unusually wide temperature range and resist water washout, which made them the default automotive and industrial grease.
• Aluminum smelting used lithium carbonate in the electrolyte to lower the bath temperature and raise conductivity.

Batteries only overtook these combined in the middle of the 2010s.

The chemistry behind that shift was assembled over fifteen years by three people who shared the 2019 Nobel prize for it: Stanley Whittingham at Exxon in the 1970s demonstrated reversible intercalation into titanium disulfide; John Goodenough at Oxford in 1980 identified lithium cobalt oxide as a far better cathode; Akira Yoshino at Asahi Kasei in 1985 replaced the hazardous lithium metal anode with carbon, which made the cell safe enough to sell. Sony commercialised it in 1991. Goodenough was ninety-seven at the award, the oldest Nobel laureate in any field.

Ponds in a desert, and rock in Australia

Lithium is produced by two quite different routes with different economics.

Brine operations pump lithium-bearing groundwater from beneath salt flats into shallow evaporation ponds and let the sun concentrate it over twelve to eighteen months. The Salar de Atacama in Chile is the flagship, and the method is cheap because the energy is free. It is also water-intensive in one of the driest places on Earth, in direct competition with copper mining and with the water rights of Atacameño communities, and that conflict is the central political fact of Chilean lithium.

Hard-rock mining crushes and processes spodumene ore, principally in Western Australia, where Greenbushes and Pilgangoora have made the country the largest producer by volume. It costs more per tonne and responds far faster to price, which is why Australian supply expanded so sharply during the price spike of 2021-22.

Bolivia holds one of the world's largest lithium resources in the Salar de Uyuni and produces almost none of it, because that brine is high in magnesium and sits in a wetter basin — a reminder that a resource figure is not a reserve.

Discovered by a wrong hypothesis, still unexplained

John Cade, working in a disused kitchen at a repatriation hospital outside Melbourne in 1949, suspected that mania was caused by a circulating toxin. He injected guinea pigs with urine from manic patients, and needed a soluble salt for one of his comparisons, so he used lithium urate. The animals became notably placid.

Almost every step of his reasoning was wrong — there is no such toxin, and the sedation he saw in guinea pigs was probably lithium toxicity rather than the therapeutic effect. The conclusion was nevertheless correct. Lithium remains the most effective long-term treatment for bipolar disorder and the only psychiatric medication with reasonable evidence for reducing suicide risk specifically.

How it works is genuinely unknown after seventy-five years. Inhibition of inositol monophosphatase and of GSK-3β are the leading candidates and neither is established. Part of the reason the question stays open is structural: lithium is an element, it cannot be patented, and there is no commercial sponsor with an incentive to fund the trials.

The element had a brief consumer career too. The soft drink launched in 1929 as Bib-Label Lithiated Lemon-Lime Soda contained lithium citrate, was later renamed 7 Up, and lost the lithium in 1948.

Three times too little in the oldest stars

Lithium-7 is the heaviest nuclide produced in any quantity by Big Bang nucleosynthesis, and it is the one that does not fit.

Standard nucleosynthesis theory takes a single input — the density of ordinary matter, now measured independently and precisely from the cosmic microwave background — and predicts the primordial abundances of deuterium, helium-3, helium-4 and lithium-7. The first three match observation extremely well. Lithium-7 does not: the abundance measured in the atmospheres of the oldest, most metal-poor stars in the galactic halo is roughly a factor of three below the prediction.

This is the cosmological lithium problem, and it has resisted resolution for decades. Explanations divide into stellar ones, in which the lithium was there originally and has been mixed down into the stars' interiors and destroyed, and exotic ones invoking new physics in the early universe. Neither camp has won, and it remains the one significant discrepancy in an otherwise extraordinarily successful theory.