Understanding the Reversible Anionic Redox in New Layered Na-Ion Cathodes
Authors: LIU Jue , Song Bohang , Hu Enyuan , Zhang Yiman , Yang Xiao-Qing , Nanda Jagjit , Huq Ashfia , Page Katharine 
Published: February 2020

The large voltage hysteresis is one of the biggest barriers to realizing reversible lattice oxygen redox for Li/Na-ion cathodes with high energy density. Thus, great efforts have been devoted to exploring novel cathode systems where intrinsic reversible oxygen redox chemistry can be realized with small voltage hysteresis. More recently, in order to stabilize the cathode structure upon electrochemical cycling by tuning the covalency of metal oxygen bonds, 4d and 5d transition metal (such as Ir and Ru) have been introduced to both Li-ion batteries and Na-ion batteries.1,2 It is thus important and timely to extend these concepts to the low-cost 3d transtion metal-based cathodes.
Very recently, a few new P3-type layered oxides, such as Na0.6Li0.2Mn0.8O2 and Na2Mn3O7 have been reported to be able to dramatically suppress the voltage hysteresis when using lattice anionic redox.3-6 However, the structure and electronic structure origin of this small voltage hysteresis has not been well understood. Especially considering that relatively large voltage hysteresis has often been observed for other P3-type layered cathodes using lattice anionic redox, such as Na2/3Mg1/3Mn2/3O2.7 In this presentation, through systematic studies using electron paramagnetic resonance, in situ XRD and neutron pair distribution function analysis, a unified theory will be proposed to explain the observed small voltage hysteresis in the two compounds mentioned above. I will also briefly discuss the interesting structure evolution for these compounds during the initial charge and discharge. In all, this discovery will pave a new route to achieve every small voltage hysteresis using lattice oxygen redox in layered oxide cathodes.
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