Reaction Heterogeneity in LiNi_0.8Co_0.15Al_0.05O₂ Induced by Surface Layer

Abstract

Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi0.8Co0.15Al0.05O2 (NCA), we decouple the intrinsic bulk reaction mechanism from surface-induced effects. For identically prepared and cycled electrodes stored in different environments, we demonstrate that the intrinsic bulk reaction for pristine NCA follows solid-solution mechanism, not a two-phase as suggested previously. By combining high resolution powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and surface sensitive X-ray photoelectron spectroscopy (XPS), we demonstrate that adventitious Li2CO3 forms on the electrode particle surface during exposure to air through reaction with atmospheric CO2. This surface impedes ionic and electronic transport to the underlying electrode, with progressive erosion of this layer during cycling giving rise to different reaction states in particles with an intact versus an eroded Li2CO3 surface-coating. This reaction heterogeneity, with a bimodal distribution of reaction states, has previously been interpreted as a “two-phase” reaction mechanism for NCA, as an activation step that only occurs during the first cycle. Similar surface layers may impact the reaction mechanism observed in other electrode materials using bulk probes such as operando powder XRD.

Operando Observations and First‐Principles Calculations of Reduced Lithium Insertion in Au‐Coated LiMn2O4

“…The behavior during delithiation and lithiation seen for the bare LMO is consistent with previous reports . Other persistent peaks include a polytetrafluoroethylene (PTFE) peak at 8.49°, conductive carbon (graphite) peak at 12.38°, and the strong cubic Au peak at 17.78° . Based on XRD, graphite and Au do not undergo any significant changes during cycling, as is expected at these positive potentials.…”

Surface/Interface Structure Degradation of Ni‐Rich Layered Oxide Cathodes toward Lithium‐Ion Batteries: Fundamental Mechanisms and Remedying Strategies

“…Strikingly, the nearly identical result is achieved from the investigation of NCM523 after stored at 55 °C and 80% relative humidity for 4 weeks . Consistently, the adventitious Li 2 CO 3 , which will impede ionic and electronic transport, also forms on the NCA particle surface during exposure to air through the reaction with atmospheric CO 2 . Specifically, the thickness and structure of the Li 2 CO 3 layer, which enormously depends on the exposure time, temperature, and CO 2 /H 2 O partial pressures, determine the electrochemical cycle, lattice parameter, and Li composition distribution in the underlying NCA during the first charge (Figure c) .…”

“…Copyright 2017, Elsevier B.V. c) Schematic of the first delithiation mechanism for the NCA sample stored under ambient conditions. Reproduced with permission . Copyright 2017, American Chemical Society.…”

Surface and Interfacial Chemistry in the Nickel‐Rich Cathode Materials

“…To date, the detailed mechanisms regarding the reformation of the Li 2 CO 3 have not been clearly revealed. However, the existence of the Li 2 CO 3 on the cathode results in the sluggish kinetics for the lithium ions and electrochemical reaction heterogeneity during the electrochemical test . In addition, the residual lithium compounds could react with the PVdF during electrochemical cycling, which results in the LiF formation .…”

“…However, the existence of the Li 2 CO 3 on the cathode results in the sluggish kinetics for the lithium ions and electrochemical reaction heterogeneity during the electrochemical test. [110,117] In addition, the residual lithium compounds could react with the PVdF during electrochemical cycling, which results in the LiF formation. [26,[118][119] Recently, Cho et al investigated the correlations between the PVdF binder and residual lithium compounds in the LiNi 0.7 Mn 0.3 O 2 (LNM) cathode by artificially coating the lithium oxide (Li 2 O).…”