In Operando Visualization of Cation Disorder Unravels Voltage Decay in Ni-Rich Cathodes

Despite the high energy density of Ni-rich layered-oxide electrodes, their real-world implementation in batteries is hindered by the substantial voltage decay on cycling, which mainly originates from bulk and surface structural degradation. Here, in operando observation of cation disorder, a major origin of structural degradation, reveals the voltage decay mechanism in Ni-rich cathode. Viewed along [1 1-0] and [110] orientations by scanning transmission electron microscopy, it is demonstrated that transition metal (TM) migration gives rise to the drastic fluctuation of interlamellar spacing and Ni-O bond length, but almost exerts no influence on atom site in ab plane. Density functional theory calculations reveal that the fluctuation of the Ni-O bond length triggers voltage decay via lifting the energy level of the antibonding (3dz(2)-2p)* orbits. Broadening bands by a shorter Ni-O bond increase the voltage slope of battery, which will reduce the accessible Li capacity within the stable voltage range of the electrolyte. Furthermore, a collaborative path of TM migration triggered by oxygen vacancy is verified to account for the TM migration. The finding provides insights into new chemistry to be explored for developing high-capacity layered electrodes that evade voltage decay.