Article pubs.acs.org/cm
Reaction Heterogeneity in LiNi0.8Co0.15Al0.05O2 Induced by Surface Layer Antonin Grenier,† Hao Liu,† Kamila M. Wiaderek,† Zachary W. Lebens-Higgins,‡ Olaf J. Borkiewicz,† Louis F. J. Piper,‡ Peter J. Chupas,⊥ and Karena W. Chapman*,† †
X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ‡ Department of Physics, Applied Physics and Astronomy, Binghamton University, Binghamton, New York 13902, United States ⊥ Energy and Global Security Directorate, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States S Supporting Information *
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 surfaceinduced 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.
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INTRODUCTION
commercial Li-ion batteries. These layered transition metal oxides span a number of phases distinguished by the layer stacking-sequence and interlayer cation geometry. For O3-type phases (α-NaFeO 2 structure type, R-3m), MO 2 layers containing mixed-metal cations (M = Mn, Fe, Co, Ni, Al) stack perpendicular to the c-direction in an ABC sequence, separated by interlayer Li+ in octahedral O2− environments.2 Previous reports describe a coexistence of phases (often denoted H1 and H2) near the beginning of charge for a number of compositions including LiNi0.15 Co 0.80 Al 0.15O 2 (NCA),3,4 LiNi1−yFeyO2,5 LiMn0.5Ni0.5O2,6 LiNi1/3Mn1/3Co1/3O2,4,7 and LiCoO2.8,9 In the case of LixCoO2, a coexistence of two phases arises from a first-order metal− insulator phase transition for 0.75 > x > 0.95.10 However, for Ni-based layered oxides, the origin of the reported two-phase behavior remains an open question, even for the simplest case
Decoupling the energy storage behavior intrinsic to an electrode phase from variables related to material processing, battery architecture, and cycling parameters provides an important baseline from which we develop strategies to enhance battery performance. The kinetics and mechanism for Li extraction and insertion in a given electrode phase can be varied depending on the morphology of the active material, its surface chemistry (e.g., coatings), and the interface with the electrolyte. In the case of nanoscale LiFePO4 particles, fast rate cycling can be achieved whereby the reaction proceeds via a solid-solution mechanism rather than the two-phase mechanism intrinsic to the bulk phase.1 Accordingly, in probing the electrochemical reaction mechanism for an electrode phase, for example, through operando X-ray diffraction (XRD), it is imperative to differentiate intrinsic properties of the active material from extrinsic variables related to the material preparation and experimental protocol. Layered transition metal oxides, which provide high energy densities, are among the most important materials for © 2017 American Chemical Society
Received: May 31, 2017 Revised: July 14, 2017 Published: August 14, 2017 7345
DOI: 10.1021/acs.chemmater.7b02236 Chem. Mater. 2017, 29, 7345−7352
Article
Chemistry of Materials
Figure 1. (a) Evolution of selected peaks from operando XRD data recorded on NCA during the first cycle, (b) a and c lattice parameters obtained from Rietveld refinements, and (c) galvanostatic charge−discharge (4.1−2.7 V, C/15, RT) curves. The marker size of the lattice parameters determined for the NCA sample stored under ambient conditions is dependent on the phase fraction obtained from the refinement. The estimated standard deviations are within the data markers.
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of LiNiO2.11 Reports quote widely varying Lix compositional ranges in which these phases coexist, with large discrepancies even for electrodes of the same nominal composition. For example, for NCA,3,4 the c-lattice parameter variations of both phases follow different trends that cannot be accounted for, even when considering differences in cycling rate. An earlier study by Robert et al.3 suggests that this so-called “two-phase” behavior is due to an activation of the electrode, an irreversible process that occurs during the first charge involving irreversible changes in NCA’s electrical conductivity. However, the variability between observations within a single composition suggests that this apparent “two-phase” behavior during the early stage of charge may not be intrinsic to the NCA material. Through operando synchrotron XRD, for identically prepared and cycled electrodes of NCA subjected to different environments, we demonstrate that during the first charge, the intrinsic reaction mechanism for NCA is solid-solution, not two-phase as suggested previously. The so-called “two-phase” behavior is due to reaction heterogeneity between secondary electrode particles induced by nonuniform erosion of a Li2CO3 surface layer. This Li2CO3 layer was observed to form on the NCA surface during exposure to atmosphere using highresolution XRD, infrared spectroscopy, and X-ray photoemission spectroscopy (XPS).
EXPERIMENTAL SECTION
Material Treatment and Electrochemistry. LiNi0.8Co0.15Al0.05O2 (NCA) was obtained from Toda America (NCA, NAT-1050, median particle diameter D50 = 6.5 μm, surface area = 0.44 m2 g−1) and separated into several portions for storage under different conditions. One sample was stored under ambient conditions (referred to as “ambient”) for 2 years in a polyethylene container that was regularly exposed to the laboratory atmosphere. Another sample (referred to as “inert”) was stored in an argon-atmosphere glovebox (
