HAADF STEM and EELS Analysis of Li(Ni0.8Co0.15Al0.05)O2 Cathode Held at High Voltages

Abstract

Ni rich Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 commonly known as NCA is being used commercially as Li‐ion battery cathode material for its high discharge capacity. [1] The stability and related safety concerns at high charge voltages limit the use of NCA to 3.6 V charge voltage corresponding to 0.5 extracted Li. In order to extract a higher amount of Li, high charge voltages to 4.75 V are required. The bulk R‐3m layered structure of NCA does not change, however new surface phases are formed wich are induced by oxygen loss. To utilize the full potential of NCA, high‐voltage studies of surface phases, their chemical evolution and their mechanisms are needed. We present here the evolution of surface phases in NCA held at constant voltages up to 4.75V. The surface phases of NCA were observed in a cold cathode field emission aberration corrected JEOL ARM (at Lehigh University) for HAADF STEM imaging. The spatial resolution of the STEM images was 0.07 nm. EELS as well as HAADF STEM imaging were carried out using a cold cathode aberration corrected HITACHI HD2700C TEM and GATAN Enfina EELS spectrometer with an energy resolution of 0.5 eV. A HAADF STEM image of a NCA particle (held at 4.75V for 2 weeks) oriented along [010] zone axis of layered (R‐3m) structure is shown in Figure 1. Atomic–resolution images from different regions of the particle were obtained revealing structural inhomogeneity with two different surface phases. The HAADF STEM image from region 1 (Figure 2a) shows that the bulk layered structure (R‐3m) is maintained up to the surface, but with almost 1/3 of transition metal (TM) ions (mostly Ni) occupying to the Li layer as determined from the intensity profile (Figure 2b). In region 2 the surface shows a HAADF contrast corresponding to “rocksalt (RS) type” phase (Figure 2c), however with a higher O content than for stoichiometric NiO. The presence of two different phases within the same particle shows that the surface of NCA particles is highly inhomogeneous. Also, this large scale migration of TM ions (1/3 of TM moving to Li layer) from its original octahedral site to a tetrahedral Li site is driven by the presence of oxygen vacancies. This is consistent with the earlier report which shows that the diffusion barrier for TM migration is reduced when initial configuration of TM octahedron is five coordinated (MO 5, deficient in one O) instead of regular six coordinated (MO 6 ).[2] Oxygen loss as measured by EELS indeed accompanied these surface phase transformations. A surface with NiO chemistry is observed only within 1‐2 nm from the surface. A characteristic EELS feature of pristine NCA is the presence of O‐pre‐peak, 12.0 eV from the main O peak, which occurs due to a transition from O 1s to a hybridized state formed by O 2p and Ni 3d states. For NiO, this pre‐peak is smaller and located 7.5 eV from the main O peak. This pre‐peak is almost missing at the surface as shown for a sample held at 4.75V (Figure 3a). The O edge at this voltage resembles that of NiO phase (Figure 3b) with a lower pre‐peak intensity and average energy position of 7.5 eV. The O/TM atomic ratio of 1.6 measured for the surface phases and averaged over several particles show that the surface is oxygen deficient as compared to pristine NCA (2). Also, a reduction in Ni valence, measured from Ni L 3 /L 2 ratio is observed at the surface (Ni +2.4 ) as compared to pristine NCA (Ni +3 ). Both of these values are between pristine NCA and NiO (O/TM ratio of 1 and Ni 2+ valence state). So, the reduction in Ni observed at 4.75V is corroborated by a shift of Ni L edge with respect to bulk NCA (Figure 3c). [3]