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Lithium-ionbatteries are undergoing rapid development to meetthe energy demands of the transportation and renewable energy-generationsectors. The capacity of a lithium-ion battery is dependent on theamount of lithium that can be reversibly incorporated into the cathode.This work directly quantifies the time- and current-dependent lithiumtransfer within a cathode functioning under conventional charge–dischargecycling. We examine Li1+yMn2O4under real working conditions using in situ neutronpowder diffraction and link the atomic-scale structure to the batteryperformance. The lithium location and content, oxygen positional parameter,and lattice parameter of the cathode are measured and linked to thebattery’s charge/discharge characteristics. Lithium insertion(discharge) differs from extraction (charge), a feature that may explainthe relative ease of discharge (compared with charge) of this material.An atomic-scale understanding of cathode functionality, such as revealedhere, will direct improvements in battery performance at both thepractical and the fundamental level. [ABSTRACT FROM AUTHOR]