Chemical and Electrochemical Lithiation of LiVOPO 4 Cathodes for Lithium-Ion Batteries Katharine L. Harrison, † Craig A. Bridges, ‡ Carlo U. Segre, § C. Daniel Varnado Jr., ∥ Danielle Applestone, ⊥ Christopher W. Bielawski, ∥ Mariappan Parans Paranthaman, ‡ and Arumugam Manthiram* ,† † Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, United States ‡ Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Physics & CSRRI, Illinois Institute of Technology, Chicago, Illinois 60616, United States ∥ Department of Chemistry and ⊥ Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States * S Supporting Information ABSTRACT: The theoretical capacity of LiVOPO 4 could be increased from 159 to 318 mAh/g with the insertion of a second Li + ion into the lattice to form Li 2 VOPO 4 , significantly enhancing the energy density of lithium-ion batteries. The phase changes accompanying the second Li + insertion into α-LiVOPO 4 and β-LiVOPO 4 are presented here at various degrees of lithiation, employing both electrochemical and chemical lithiation. Inductively coupled plasma, X-ray absorption spectroscopy, and Fourier transform infrared spectroscopy measurements indicate that a composition of Li 2 VOPO 4 can be realized with an oxidation state of V 3+ by the chemical lithiation process. The accompanying structural changes are evidenced by X-ray and neutron powder diffraction. Spectroscopic and diffraction data collected with the chemically lithiated samples as well as diffraction data on the electrochemically lithiated samples reveal that a significant amount of lithium can be inserted into α-LiVOPO 4 before a phase change occurs. In contrast, lithiation of β-LiVOPO 4 is more consistent with the formation of a two-phase mixture throughout most of the lithiation range. The phases observed with the ambient-temperature lithiation processes presented here are significantly different from those reported in the literature. ■ INTRODUCTION There has been considerable interest recently in developing alternative cathode materials that are safer and less expensive than the currently used layered Li[Mn,Ni,Co]O 2 for lithium- ion batteries, particularly for large-scale applications like electric vehicles and grid energy storage. Following the initial investigation of the polyanion cathodes Fe 2 (SO 4 ) 3 , Fe 2 (MoO 4 ) 3 , and Fe 2 (WO 4 ) 3 by Manthiram and Good- enough, 1,2 many polyanion cathodes have since then been investigated as potential candidates. Among them, olivine LiFePO 4 is the most extensively investigated cathode, but its energy density is limited due to the relatively low operating voltage of 3.45 V, reversible extraction/insertion of only one Li + ion per transition metal ion, and the low packing density of the LiFePO 4 nanoparticles. Very few polyanion cathodes exhibit the ability to reversibly insert/extract more than one Li + ion per transition metal ion. One such material is LiVOPO 4 , which offers the ability to reversibly insert/extract two Li + ions per vanadium ion, involving two voltage plateaus around 4 and 2 V with a theoretical capacity of 318 mAh/g. Even with an extraction/insertion of one Li + ion, LiVOPO 4 offers higher energy density than LiFePO 4 , with a theoretical capacity of 159 mAh/g at 4 V. However, relatively few studies have focused on understanding and optimizing the insertion/extraction of two Li + ions into/from LiVOPO 4 , compared to the large body of literature for LiFePO 4 . Received: May 2, 2014 Published: May 20, 2014 Article pubs.acs.org/cm © 2014 American Chemical Society 3849 dx.doi.org/10.1021/cm501588j | Chem. Mater. 2014, 26, 3849−3861