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Synthesis, Structure, and Electrochemical Performance of High Capacity Li2Cu0.5Ni0.5O2 Cathodes Rose E. Ruther, Hui Zhou, Chetan Dhital, Saravanan Kuppan, Andrew K. Kercher , Guoying Chen, Ashfia Huq, Frank M. Delnick, and Jagjit Nanda Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02843 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015
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Synthesis, Structure, and Electrochemical Performance of High Capacity Li2Cu0.5Ni0.5O2 Cathodes Rose E. Ruther1, Hui Zhou1, Chetan Dhital1,2, Kuppan Saravanan3, Andrew K. Kercher1, Guoying Chen3, Ashfia Huq2, Frank M. Delnick1, and Jagjit Nanda1,* 1
3
Materials Science and Technology Division and 2Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
*Correspondence:
[email protected] This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).
Abstract Orthorhombic Li2NiO2, Li2CuO2, and solid solutions thereof have been studied as potential cathode materials for lithium-ion batteries due to their high theoretical capacity and relatively low cost. While neither endmember shows good cycling stability, the intermediate composition, Li2Cu0.5Ni0.5O2, yields reasonably high reversible capacities. A new synthetic approach and detailed characterization of this phase and the parent Li2CuO2 are presented. The cycle life of Li2Cu0.5Ni0.5O2 is shown to depend critically on the voltage window. The formation of Cu1+ at low voltage and oxygen evolution at high voltage limit the electrochemical reversibility. In situ X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and gas evolution measurements are used to follow the chemical and structural changes that occur as a function of cell voltage. Introduction Over the last two decades there has been intense research activity directed towards stable, highcapacity, low-cost cathode materials for lithium-ion batteries. Copper-based chemistries have garnered interest mainly as anode materials in conversion-type electrodes1-6 or as cathodes for lithium primary batteries.7-12 This is mostly due to the relatively low potentials of the Cu0/Cu1+/Cu2+ redox couples which use the common oxidation states of copper. Several cathode materials for rechargeable batteries have been proposed which take advantage of the relatively rare, high-voltage Cu2+/Cu3+ redox couple including Na2/3Cu1/3Mn2/3O2,13-14 LiCu0.5Mn1.5O4,15-18 Li2Cu2O(SO4)2,19 and Li2CuO2.20-25 Of all these compounds, Li2CuO2 has the highest capacity 1 ACS Paragon Plus Environment
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achieved experimentally (>200 mAh/g) and operates at voltages which fall within the stability window of current generation electrolytes (250 mAh/g discharge capacity after 10 cycles). They attributed the 2 ACS Paragon Plus Environment
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increased stability to the difference in the phases that form after electrochemical cycling. According to their report, Li2CuO2 and copper-rich solid solutions change from orthorhombic to monoclinic during cycling (Figure 1), whereas nickel-rich compositions transform into a layered phase (Figure 2). To fully understand the role nickel plays in improving the cycle life, the redox chemistry that occurs in the nickel-substituted samples needs to be understood.
Figure 2. Phase transformations that occur during electrochemical cycling of orthorhombic Li2NiO2
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The formal valence of copper in LiCuO2 is Cu3+. However, for many copper oxides in high oxidations states, it is actually energetically favorable for a majority of the holes to reside in the oxygen band.19, 24, 30-33 In other words, the holes are localized to peroxide-like oxygen states and copper remains as Cu2+. Density functional theory (DFT) calculations of the electronic structure of LixCuO2 also show that more electron density is lost from oxygen than copper during charging.24 The participation of oxygen in the redox chemistry destabilizes the lattice and oxygen gas can be evolved during charge at high voltage (V > 3.7 V vs. Li/Li+).20, 24, 34 Figure 3 shows the Li-Cu-O ternary phase diagram.35-37 Li1.5CuO2 and LiCuO2 (indicated with stars in the diagram) form when lithium is deintercalated from Li2CuO2 and are metastable phases.21, 32-33, 3841 Clearly, removal of lithium from Li2CuO2 occurs in a region of the phase diagram where the transition metal oxides are in equilibrium with oxygen.
Figure 3. Li-Cu-O ternary phase diagram at 298 K adapted from references 35-37. The locations of Li1.5CuO2 and LiCuO2 have been added to the reference data and marked with stars. Dashed line indicates the trajectory through phase space for the intercalation/deintercalation of lithium from Li2CuO2. Li2NiO2 also evolves oxygen gas when charged above 4.1 V vs. Li/Li+.26 Ni3+ and Ni4+ are accessible oxidation states for nickel, and the extent of oxygen participation in charge compensation in Li2NiO2 is not known. However, some oxygen involvement in redox chemistry has been proposed to occur in other lithium nickel oxides, especially near the surface.42-43 In this contribution, we clarify some of the questions regarding the charge compensation mechanism in Li2Cu0.5Ni0.5O2 solid solutions using in situ XAS and gas evolution measurements. 4 ACS Paragon Plus Environment
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We show the impact of different voltage windows on cycling performance and discuss some of the reaction mechanisms which limit electrochemical reversibility in Li2Cu0.5Ni0.5O2. Experimental: Synthesis: Li2Cu0.5Ni0.5O2 was synthesized following a modified version of the procedures reported by Imanishi et al.22 and Arachi et al.44 First, the transition metal hydroxides were synthesized via co-precipitation. Aqueous solutions of the nitrate salts (
