Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4792−4800
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Nonstoichiometry and Defects in Hydrothermally Synthesized ε‑LiVOPO4 Youngmin Chung,†,‡ Ellen Cassidy,† Krystal Lee,† Carrie Siu,† Yiqing Huang,†,‡ Fredrick Omenya,‡ Jatinkumar Rana,† Kamila M. Wiaderek,†,§ Natasha A. Chernova,†,‡ Karena W. Chapman,†,§,∥ Louis F. J. Piper,† and M. Stanley Whittingham*,†,‡ †
NorthEast Center for Chemical Energy Storage, Binghamton University, Binghamton, New York 13902-6000, United States Materials Science & Engineering, Binghamton University, Binghamton, New York 13902-6000, United States § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
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‡
S Supporting Information *
ABSTRACT: ε-LiVOPO4 has been synthesized through the hydrothermal method by adjusting the pH of the hydrothermal solution and the reaction temperature. This phase is formed between 180 and 220 °C, as diamond-like crystals around 10−15 μm in size. X-ray diffraction (XRD) analysis shows that hydrothermal ε-LiVOPO4 lattice parameters a and b linearly decrease, while c linearly increases when the synthesis temperature increases. Thermogravimetric analysis with mass spectroscopy reveals 1.5 to 0.5% water loss at about 350 °C for ε-LiVOPO4 synthesized at 180 and 220 °C, suggesting water or protons incorporation into the structure. Magnetic studies reveal ferrimagnetism in hydrothermal ε-LiVOPO4 below 10 K, as opposed to antiferromagnetic ordering below 14 K found in samples synthesized at high temperature. In-situ XRD upon heating of the hydrothermal ε-LiVOPO4 synthesized at 180, 200, and 220 °C reveals that the temperature dependences of their lattice parameters merge at about 500 °C; furthermore, at the same temperature the structure reversibly changes from triclinic to monoclinic. The lattice parameters and the magnetic properties of the hydrothermal samples heated to 750 °C are similar to those of solid-state synthesized ε-LiVOPO4. Based on structure and composition analysis, we suggest that hydrothermal samples can be described as an ε-Li1+xHyV1−zOPO4 (x, y, z < 0.1) solid solution. The electrochemical characterization of hydrothermal ε-LiVOPO4 reveals the first cycle capacity of about 300 mAh/g, which holds for about five cycles, gradually decreasing thereafter. The lowvoltage region does not reveal voltage plateaus corresponding to Li1.5VOPO4 and Li1.75VOPO4 phases found in the solid-state material, further suggesting structural disorder in the low-temperature samples evidenced from the lattice parameters and the magnetic properties. KEYWORDS: lithium-ion batteries, cathode, LiVOPO4, hydrothermal synthesis, structural disorder, high-energy density
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INTRODUCTION Lithium-ion batteries (LIBs) are considered the most important energy storage tool and power source for mobile electronic devices such as cell phones and laptops.1,2 To this point, polyanion compounds, specifically transition metal phosphates, have been noted as very attractive cathode materials.3,4 These materials have lithium ions and redox transition metal centers within a strong phosphate structure resulting in increased redox potentials, improved thermal stability, and cycling performance. The olivine LiFePO4 is commercialized and the most studied material among polyanion cathodes.5,6 Mixed transition metal phosphates LiMPO45,7−9 and fluorophosphates, such as LiVPO4F,10 have also been investigated. However, LiFePO4 has limited energy density due to its relatively low operating voltage (3.4 V) and ordinary capacity of 170 mAh/g.2,6 Therefore, alternative polyanion cathodes have been studied with a focus on those capable of transferring more than one electron per transition © 2019 American Chemical Society
metal, such as silicates and sulfates Li2MnSiO4, Li2FeSiO4, and mixed Li2Fe0.5Mn0.5SiO4.11−13 Recently, vanadyl phosphate cathode materials such as LiVOPO4 have become a research focus, since an additional lithium ion can be inserted per vanadium ion. The LiVOPO4 exists in several phases: α1-LiVOPO4 (tetragonal), β-LiVOPO4 (orthorhombic), and ε-LiVOPO4 (triclinic).14−25 Here we use nomenclature introduced by Whittingham,2 where LixVOPO4 phases are named after the corresponding VOPO4 phases to avoid the confusion; i.e., the product of ε-VOPO4 lithiation is ε-LiVOPO4, not α-LiVOPO4 as it would have been if the historic names were used. ε-LiVOPO4 is a multielectron intercalation cathode with a theoretical capacity of 305 mAh/g based on Li2VOPO4 composition. It could be practically Received: March 1, 2019 Accepted: June 7, 2019 Published: June 7, 2019 4792
DOI: 10.1021/acsaem.9b00448 ACS Appl. Energy Mater. 2019, 2, 4792−4800
Article
ACS Applied Energy Materials
rate was 5 °C/min and cooling naturally by room temperature conditions, and data were measured at each setting temperature; the stability of the temperature set was dependent on each powder’s properties. XAS data treatment and analysis utilized the IFEFFIT package.32 To test these materials as cathodes in a lithium-ion battery, the εLiVOPO4 powders were mixed with carbon black (weight ratio 8:1) and high-energy ball-milled (HEBM) for 30 min. To prepare the electrodes the ball-milled product was mixed with PVDF (10 wt %) and 1-methyl-2-pyrrolidinone (NMP, Aldrich) to make a slurry which was cast on aluminum foil and vacuum-dried at 80 °C overnight. A pure lithium foil (Aldrich, thickness 0.38 mm) was used as an anode, the electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, BASF), and the separator was Celgard 2400 (Hoechst Celanese). CR2032 coin cells were assembled in a heliumfilled glovebox. The coin cells were cycled at a current density of C/ 50 (about 33 μA/cm2) and C/20 (about 78 μA/cm2) over the voltage range 1.6 to 4.5 V at room temperature using a VMP multichannel potentiostat (Biologic). Cyclic voltammetry (CV) was performed at a rate of 0.05 mV/s between 1.6 and 4.4 V.
realized if both high-voltage Li removal at about 4.0 V involving V4+/V5+ redox couple and low-voltage Li insertion processes (2.3 V, V4+/V3+) could run reversibly over many cycles. We and other groups have demonstrated that more than one Li can, in fact, be reversibly cycled in ε-LiVOPO4 and identified intermediate Li1.5VOPO4 and Li1.75VOPO4 phases formed upon Li insertion.17,21,23 This work was done using LiVOPO4 synthesized by a solid-state method with further ball-milling to reduce particle size. The solid-state method widely used in the synthesis of cathode materials requires a high temperature to overcome diffusion barriers, leading to increasing cost of materials. The hydrothermal reactions offer advantages of mild conditions (below 240 °C), particle size, and morphology control along with reaction homogeneity.26,27 However, low synthesis temperature may result in structural defects. We have previously investigated hydrothermally synthesized LiFePO4 and found that Fe ions tend to go on Li sites at low hydrothermal synthesis temperatures (
