Deciphering Metastable Structure Evolution in Voltage Hysteresis of Lithium-Rich Cathodes

Voltage hysteresis, as one of the main concerns in lithium-rich oxides (LROs), significantly deteriorates the energy conversion efficiency and hinders its application in high-energy-density Li-ion batteries. Nevertheless, the structural origin of voltage hysteresis has still not been clarified. Here we reveal the thermodynamically metastable structure evolution, particularly negative thermal expansion and correlated transition-metal (TM) octahedral (TMO₆) distortion release, during thermally driven voltage hysteresis recovery in LROs. Moreover, whole exothermic structure evolution and related voltage recovery are linked: (1) abnormal negative thermal expansion below 200 °C due to the relief of octahedral distortion, which leads to a voltage recovery by 0.26 V; (2) reconstruction of Li@Mn₆ superstructure units due to Li-vacancy-assisted TM migration at 300 °C, which further recovers the voltage by 0.16 V and the oxygen plateau. Our work provides a more elaborate picture of the structure-voltage correlation in anionic-redox systems and highlights that these two structural variations form a metastable phase, serving as a solid base after the first cycle for subsequent cycling in LROs.