Identity
Alpha-lipoic acid (INCI: Thioctic Acid) is a sulphur-containing antioxidant the body also makes as a mitochondrial cofactor. Its defining cosmetic property is chemical: it is amphipathic — soluble in both water and oil — which is rare and useful for an antioxidant. It is used topically under the INCI name Thioctic Acid, and it is also a popular ingestible antioxidant/metabolic supplement (that use is outside this register's topical scope but drives much of its "cellular energy" marketing).
Mechanism (as proposed)
Most antioxidants work in one phase: vitamin C in water, vitamin E in lipid. Alpha-lipoic acid works in both, so it can quench radicals across the water-oil boundary, and in the lab it also regenerates other antioxidants — vitamins C and E, and glutathione — extending their action. This dual-phase, recycling mechanism is genuine and distinctive. The practical catches are formulation-side: ALA is unstable (light- and heat-sensitive), can carry a sulphurous odour, and stings or irritates at higher strengths — so a good result depends heavily on how it is formulated and dosed.
Reading the evidence honestly
For skin, alpha-lipoic acid has one decent controlled trial — a 5% facial cream reporting improved appearance of roughness and fine lines over about twelve weeks — plus a thinner surrounding literature. That is enough for an honest C: a real topical antioxidant with a modest, genuine human signal, not a deeply evidenced hero. The "cellular energy / mitochondrial" framing borrows from its ingestible biology and stays out of the cosmetic claims; topically it supports the look of skin and complements, never replaces, sunscreen.