Improving the Cyclic Reversibility of Layered Li-Rich Cathodes by Combining Oxygen Vacancies Introduction and Surface Fluorination

Abstract

Layered-type Li-rich cathode materials have attracted significant attention for the next generation Li-ion batteries, but their poor reversibility in terms of both voltage and capacity upon cycling are as prominent as their high capacity. Irreversible oxygen redox activity and surface deterioration have been deemed as the root cause and direct cause for their poor performance, respectively. To mitigate those issues, we introduce substantial amounts of oxygen vacancies into layered-type Li1.2Ni0.2Co0.2Mn0.4O2 by using CaH2 as a reduction agent. Reduced samples show higher reversible capacity due to the increased cation redox participation, but still sustain poor cyclic performance. With further fluorination to reduced Li1.2Ni0.2Co0.2Mn0.4O2-x by using NH4HF2, the reversible capacity reached 270 mAh/g and maintained its 99% after 100 cycles. STEM-EELS detects deeper F signals rather than just on surface as direct fluorination methods show, and HAXPES patterns indicate there are both ionic and covalent fluorine coordination. These results demonstrate that by the combination of oxygen deficiency introduction and surface fluorination, some F- ions could occupy the sites of oxygen vacancies near the surface rather than sole LiF generation on the surface, which illuminates a new strategy to modify the cathode materials for Li-ion batteries. The Ni0.2Co0.2Mn0.4(OH)1.6 precursor was prepared by the coprecipitation of NiSO4, CoSO4 and MnSO4, and then mixed with LiOH. The mixture was calcined with oxygen flow at 900 ℃ to obtain pristine Li1.2Ni0.2Co0.2Mn0.4O2. The pristine Li1.2Ni0.2Co0.2Mn0.4O2 was reduced by the appropriate amount of CaH2 in a vacuumed glass tube at 260 ℃, then the reduced Li1.2Ni0.2Co0.2Mn0.4O1.85 was mixed with various amounts of NH4HF2 and sintered at 450 ℃ with argon flow. Hard X-ray photoemission spectroscopy (HAXPES, at BL46XU in SPring-8) was used to investigate the coordination state of fluorine ions. The electrodes of different samples were prepared by slurrying the active material, acetylene black, and PVDF with a weight ratio of 8:1:1, then coating on the Al foil. The 2032-type coin cells were assembled for the electrochemical test. The counter electrode was lithium metal and the electrolyte was 1 M LiPF6 in EC and EMC solvent (3:7 by volume). The charge-discharge test was operated on an automatic cycling and data recording system (HJ1001SD8, Hokuto Denko). Fig. 1a shows the F 1s HAXPES spectra of fluorinated samples which were synthesized by subjecting reduced Li1.2Ni0.2Co0.2Mn0.4O1.85 to chemical fluorination using NH4HF2. When F is 0.1 stoichiometry, F ions are more covalent which implies they could mainly occupy the oxygen vacancy sites near surface. Further fluorination increased the intensity of LiF on surface. The cyclic performance of four samples is shown in Fig. 1b. The pristine sample suffered from the severe capacity decay whose drastic oxygen redox activity is deemed to cause various deterioration [1]. Direct fluorination of pristine sample had some positive effect to the cyclic performance because of the reported interface stabilization [2]. The initial capacity of the reduced sample is higher than pristine sample owing to the increased cation redox contribution, but its cyclability is still poor especially after long cycles. The important point to note is the combination of reduction and fluorination significantly improved both the capacity and cyclic performance upon 100 cycles, and the coordination and role of fluorine ions could be different from the conventional fluorination strategy. Figure 1