(Invited) Novel Approaches for the Study of Disordered Rocksalt Oxyfluoride Intercalation Cathodes

Abstract

Novel approaches for the study of disordered rocksalt oxyfluoride intercalation cathodes Raynald Giovine,a Yuefan Ji,a Ashlea Patterson,a Emily Foley,a Zhengyan Lun,c Daniil Kitchaev,a,b Bin Ouyang,d Jinhyuk Lee,c,e Yuan Yue,f Yang Ha,g Wanli Yang,g Wei Tong,f Gerbrand Ceder,c,d Raphaële Clémenta a. Materials Department, University of California, Santa Barbara, CA 93106.b. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.c. Department of Materials Science and Engineering, University of California, Berkeley, CA 94720.d. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.e. Department of Mining and Materials Engineering, McGill University, Montreal, QC, H3A 0C5, Canada.f. Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.g. Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.Disordered rocksalt oxides and oxyfluorides (DRX) have recently emerged as a promising class of lithium-ion cathodes, with initial capacities up to 300 mAh/g and initial energy densities up to 1000 Wh/kg.[1] The presence of extensive disorder and the ability to partially replace O by F result in unique Li+ diffusion and redox properties. Yet, the structure of such compounds is particularly difficult to characterize, as a result of the intrinsic disorder on the cation lattice, the small particle size required for reasonable rate performance, and the inability of x-ray and neutron diffraction to distinguish between O and F.In this presentation, I will show that high resolution ex situ solid-state NMR is uniquely suited for the study of cation short-range order[2] and structural degradation mechanisms resulting in rapid capacity fade upon cycling, and to assess the solubility limit of F into the rocksalt structure. Our findings indicate that the limited F solubility in disordered rocksalts depends on cathode composition and does not impede their performance, that one major source of impedance build up during cycling is degradation of common Li-ion electrolyte salts at high voltage,[3] as well as metal dissolution, and that cation short-range order can be eliminated through the formation of ‘high entropy’ compounds.[4] These insights allow us to establish links between composition and electrochemical properties and establish design rules for next-generation intercalation-type Li-ion cathodes.