UC Santa Barbara 'Bottles the Sun' With Liquid Battery That Stores Solar Heat at Nearly Twice Lithium Density

Chemists at the University of California, Santa Barbara have built a liquid material that can soak up sunlight, lock the energy away inside a single class of molecules for hours, and then release that energy on demand as heat — at an energy density that, by weight, is nearly twice what a commercial lithium-ion battery can store. The findings, published this week in Science under the title "Molecular solar thermal energy storage in Dewar pyrimidone beyond 1.6 megajoules per kilogram," mark a major advance in a long-pursued idea known as MOST, or molecular solar thermal energy storage.

The active ingredient is a chemically tweaked version of pyrimidone, an organic molecule that the team coaxed into a high-strain configuration called a Dewar isomer when it absorbs a photon of sunlight. The Dewar form behaves like a compressed spring: it is stable for long periods at room temperature, but releases its stored chemical energy as heat the instant a trigger — typically a small catalytic surface or a temperature shift — is applied. The molecule then snaps back to its starting shape, ready to absorb another photon. "We can effectively bottle the sun," said associate professor Grace Han, who led the work, in a UCSB statement.

The performance numbers are what have other energy researchers paying attention. Han's group measured a storage density of more than 1.6 megajoules per kilogram, which is roughly 1.7 to 1.8 times the energy density of a standard nickel-rich lithium-ion cell on a weight basis. Unlike a battery, however, the stored energy is released as heat rather than electricity, which makes the system best suited for applications such as space heating, industrial process heat, off-grid water heating, or charging the phase-change reservoirs used in district heating. Heat is also where the bulk of global energy consumption actually sits — accounting for roughly half of total final energy demand — and it is the hardest sector to decarbonize.

The Dewar pyrimidone system also addresses two long-standing weaknesses of earlier MOST candidates. Previous materials based on azobenzene or norbornadiene either degraded after a few hundred storage cycles or required ultraviolet light, which is a small slice of the solar spectrum. The new pyrimidone derivative absorbs across a broader band of visible light, and the team reported high cyclability through more than a thousand absorb–release cycles in the laboratory. Critically, the material is a flowable liquid, which means it can be pumped through a building's plumbing the way refrigerant is pumped through an air-conditioner.

Several practical hurdles remain. The molecule is currently synthesized in gram quantities, and scaling production to industrial volumes will require new catalytic routes and rigorous toxicity testing. The trigger temperature for heat release is also higher than would be ideal for many low-grade industrial uses. Han's group is collaborating with chemical engineers at Lawrence Berkeley National Laboratory and a Danish startup, Catcap, to test the material in benchtop solar collectors over the next year. If the cycle stability and economics hold up, MOST liquids could complement — though not replace — battery storage in a future grid, soaking up daytime solar heat and releasing it at night, when the sun goes down and demand peaks.