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In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery’s energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone^{, , , , , , , , –}. There is a high voltage associated with oxidation of O²⁻ during the first charge, but this is not recovered on discharge, resulting in reduced energy density. Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis)^{,, , , –}. By comparing two closely related intercalation cathodes, Na_0.75[Li_0.25Mn_0.75]O₂ and Na_0.6[Li_0.2Mn_0.8]O₂, here we show that the first-cycle voltage hysteresis is determined by the …