NSRRC Activity Report 2022

Energy Science 057 L i-rich layered transition-metal-oxide cathodes with a chemical formula of Li 1+x (Ni,Co,Mn) 1−x O 2 have attracted great attention, mainly due to their substantially high capacities for advanced Li-ion batteries (LIB). However, Li-rich cathodes are limited by oxygen release, voltage fade/hysteresis, and poor electrochemical kinetics during cycling, possibly resulting from the oxygen redox reaction. To mitigate the unfavorable electrochemical properties of Li-rich cathodes, it is imperative to establish the oxygen redox mechanism. Recently, Heng-Liang Wu (National Taiwan University), Maw-Kuen Wu (Academia Sinica) and their collaborators from the NSRRC investigated the charge- storage mechanism of Li-rich cathodes with different amounts of the Li 2 MnO 3 phase and Ni using operando quick-scanning X-ray absorption spectroscopy (XAS) at TPS 44A , soft XAS at TLS 20A1 , and operando X-ray diffraction at TLS 01C2 . The unfavorable electrochemical properties of the Li-rich cathodes could be limited by optimizing the ratio of Li 2 MnO 3 phase. The effect of excess Li on the reversibility of the cation and anion redox of Li-rich cathodes was discussed. Both the as-synthesized Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 (HLC) and Li 1.08 Ni 0.34 Co 0.08 Mn 0.5 O 2 (LLC) materials exhibit similar crystal structures, albeit different electrochemical behaviors. LLC materials reveal that reversible cation/ anion redox reactions and voltage decay can be suppressed during cycling. The cation oxidation of the LLC cathode is kinetically slower than that of the HLC cathode, suggesting that the local coordination structure exerts a considerable impact on the reaction kinetics. The mechanistic insights into the Li 2 MnO 3 phase and electrochemical kinetics of cation and anion redox are indispensable for improving Li-rich cathodes. Fig. 1 : dQ/dV plots of (a) HLC and (b) LLC cathode materials. (c) Electrochemical performance of LLC and HLC cathode materials obtained at a rate of 20 mA/g in a potential range of 4.6 to 2.2 V. [Reproduced from Ref. 1] Modulating the Voltage Decay and Cationic Redox Kinetics of Li-Rich Cathodes Li-rich layered transition-metal-oxide cathodes with a lower fraction of the Li 2 MnO 3 phase exhibit improved electrochemical performance in Li-ion batteries. Figures 1(a–c) show the differential capacity (dQ/dV) plots and electrochemical performance of HLC and LLC cathode materials. As compared to the intensity of peak B for the HLC cathode, the peak D with a weaker intensity possibly results from the low content of Li in the transition metal (TM) layer of LLC materials ( Figs. 1(a) and 1(b) ). The corresponding reduction peaks are shifted drastically toward the low-potential region for the HLC cathode (marked by the arrows), suggesting that voltage fading is more severe in HLC cathode than in the LLC cathode during cycling. Figure 1(c) shows that the electrochemical stability of the LLC cathode is improved. Figures 2(a) and 2(b) (see next page) show the representative operando Ni K-edge X-ray absorption spectra of the HLC and LLC cathodes in the first cycle. The Ni K-edge absorption energies of the HLC cathode and the corresponding charge-discharge curves are shown in Figs. 2(c) and 2(d) , respectively. At a potential of greater than 4.4 V, the oxidation state of Ni remains the same, which is consistent with the fact that the oxidation process is dominated by the anion (O 2− ) oxidation process rather than the TM oxidation process at a high- potential region (> 4.4 V). During the discharge process, the Ni K-edge absorption energy shifts to a lower energy as a result of Ni reduction. The changes in the Ni K-edge absorption energy are monotonic during the discharge process, suggesting that anion and TM reduction processes could be performed at similar potentials. Figures 2(e) and 2(f) show the Ni K-edge absorption energies of the LLC cathode and the corresponding charge–discharge curves in the first cycle. Interestingly, the changes in Ni K-edge absorption energies yield two different slopes, which are indicated by dashed lines during the discharge process ( Fig.