Origin of High Capacity and Poor Cycling Stability of Li-Rich Layered Oxides: A Long-Duration in Situ Synchrotron Powder Diffraction Study

Abstract

High-energy Li1.17Ni0.19Co0.10Mn0.54O2 (HE-NCM) is a lithium-rich layered oxide with alternating Li- and transition-metal (TM) layers in which excess lithium ions replace transition metals in the host structure. HE-NCM offers a capacity roughly 50 mAh g–1 higher compared to that of conventional layered oxides but suffers from capacity loss and voltage fade upon cycling. Differential capacity plots (taken over 100 cycles) show that the origin of the fading phenomenon is a bulk issue rather than a surface degradation. Although previous studies indicate only minor changes in the bulk material, long duration in situ synchrotron X-ray powder diffraction measurements, in combination with difference Fourier analysis of the data, revealed an irreversible transition-metal motion within the host structure. The extensive work provides new insights into the fading mechanism of the material.

Manganese oxidation as the origin of the anomalous capacity of Mn-containing Li-excess cathode materials

“…We have highlighted several mechanisms that appear to be consistent with available data (illustrated in Figure 4): the oxidation of Mn 4+ to Mn 7+ accompanied by a migration from octahedral to tetrahedral sites; the formation of peroxide ions or trapped molecular oxygen; and the formation of Mn 7+ followed by the spontaneous dimerization of oxygen. We have explored in depth the oxidation of Mn 4+ to Mn 7+ and found this hypothesis to be compatible with experimental and firstprinciples thermodynamics, supported by the observation of tetrahedral occupancy at the end of charge through XRD, 93 and consistent with the electrochemical behavior in Cr-based cathode materials which are known to undergo tetrahedral/octahedral migration. [85][86][87][88] The anomalous capacity may, however, involve contributions from multiple redox mechanisms.…”

“…11,26,28,30 Second, a recent in-situ diffraction study on a Li-excess cathode concluded that transition-metal ions migrate to tetrahedral sites in the Li layer during charge and return to octahedral sites on discharge. 93 Lastly, the Raman feature attributed to the reversible formation of peroxide ions is also consistent with tetrahedral Mn 7+ . 45,94 Figure 4(a) and (b) illustrates the two alternatives to the lattice oxygen redox hypothesis that we have explored so far.…”

Oxygen Release and Surface Degradation of Li- and Mn-Rich Layered Oxides in Variation of the Li2MnO3Content

“…Therefore, the oxygen release is a side reaction occurring at the HE-NCM nearsurface region, 25 while reversible and irreversible transition metal migration in the bulk material cause the main voltage fading and the high hysteresis. 35,45,46 Figure 5 depicts the dQ/dV plots for cycle 3, cycle 20, and cycle 48 extracted from the cycling data shown in Figure 4. Hereby, cycle 3 is the first C/5 cycle between 2.0 V and 4.7 V and therefore is subjected to the same cycling conditions as cycle 20 and 48.…”

“…[31][32][33][34] Therefore, it is suggested that high degrees of delithiation and reversible oxygen redox trigger reversible and irreversible transition metal migration within the bulk material, leading to voltage fading and to the large charge/discharge voltage hysteresis due to the hindered lithium diffusion within the bulk material. 14,[35][36][37][38] In contradiction to the hypothesis of bulk oxygen release and bulk structural transformation, recent studies give clear evidence that the bulk structure is preserved, while a relatively small fraction of transition metals (about 10% over 100 cycles) 35 migrate reversibly and over extended charge/discharge cycling irreversibly between the transition metal and the lithium layers, leading to changes of the bulk material thermodynamics like the charge and discharge potentials as well as to the observed voltage fading. 25,35 In this study, we will examine the effect of oxygen release onto the bulk and the surface structure of HE-NCM and correlate it with the macroscopic electrochemical performance of the material.…”

“…14,[35][36][37][38] In contradiction to the hypothesis of bulk oxygen release and bulk structural transformation, recent studies give clear evidence that the bulk structure is preserved, while a relatively small fraction of transition metals (about 10% over 100 cycles) 35 migrate reversibly and over extended charge/discharge cycling irreversibly between the transition metal and the lithium layers, leading to changes of the bulk material thermodynamics like the charge and discharge potentials as well as to the observed voltage fading. 25,35 In this study, we will examine the effect of oxygen release onto the bulk and the surface structure of HE-NCM and correlate it with the macroscopic electrochemical performance of the material. These studies will be conducted with HE-NCM materials with different amounts of the Li 2 MnO 3 phase (x = 0.33, 0.42 and 0.50 if referenced to x Li 2 MnO 3 • (1-x) LiMeO 2 ), comparing the materials in terms of their oxygen release, their half-and full-cell performance as well as their impedance growth.…”