Reaction Heterogeneity in LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Induced by Surface Layer. Grenier, A., Liu, H., Wiaderek, K., M., Lebens-Higgins, Z., W., Borkiewicz, O., J., Piper, L., F., J., Chupas, P., J., & Chapman, K., W. Chemistry of Materials, 29(17):7345-7352, 9, 2017.
Reaction Heterogeneity in LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Induced by Surface Layer [link]Website  doi  abstract   bibtex   
Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi 0.8 Co 0.15 Al 0.05 O 2 (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 Li 2 CO 3 forms on the electrode particle surface during exposure to air through reaction with atmospheric CO 2 . 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 Li 2 CO 3 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. ■ INTRODUCTION 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 LiFePO 4 particles, fast rate cycling can be achieved whereby the reaction proceeds via a solid-solution mechanism rather than the two-phase mecha-nism 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 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 O 2− environments.
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 title = {Reaction Heterogeneity in LiNi<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Induced by Surface Layer},
 type = {article},
 year = {2017},
 pages = {7345-7352},
 volume = {29},
 websites = {http://pubs.acs.org/doi/pdfplus/10.1021/acs.chemmater.7b02236,http://pubs.acs.org/doi/10.1021/acs.chemmater.7b02236,https://pubs.acs.org/doi/10.1021/acs.chemmater.7b02236},
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 abstract = {Through operando synchrotron powder X-ray diffraction (XRD) analysis of layered transition metal oxide electrodes of composition LiNi 0.8 Co 0.15 Al 0.05 O 2 (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 Li 2 CO 3 forms on the electrode particle surface during exposure to air through reaction with atmospheric CO 2 . 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 Li 2 CO 3 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. ■ INTRODUCTION 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 LiFePO 4 particles, fast rate cycling can be achieved whereby the reaction proceeds via a solid-solution mechanism rather than the two-phase mecha-nism 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 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 O 2− environments.},
 bibtype = {article},
 author = {Grenier, Antonin and Liu, Hao and Wiaderek, Kamila M and Lebens-Higgins, Zachary W and Borkiewicz, Olaf J and Piper, Louis F J and Chupas, Peter J and Chapman, Karena W},
 doi = {10.1021/acs.chemmater.7b02236},
 journal = {Chemistry of Materials},
 number = {17}
}

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