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C: Energy Conversion and Storage; Energy and Charge Transport
The Mechanism of the First Lithiation/ Delithiation Process in the Anode Material CoFeOPO@C for Li-ion Batteries 4
Hasna Aziam, Youssef Tamraoui, Lu Ma, Rachid Amine, Tianpin Wu, Bouchaib Manoun, Guiliang Xu, Khalil Amine, Jones Alami, and Ismael Saadoune J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01834 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018
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The Journal of Physical Chemistry
The Mechanism of the First Lithiation/ Delithiation Process in the Anode Material CoFeOPO4@C for Li-ion Batteries H. Aziama, b, c, Y. Tamraouib, L. Mac, R. Aminee, T. Wuc, B. Manounb,d, G. Xue, K. Aminee, J. Alami b, I. Saadoune*a,b a
LCME, Faculty of Science and Technology- Cadi Ayyad University, Av. A. El Khattabi, P.B..549 Marrakesh, Morocco b Materials Science and Nano-engineering, Mohammed VI Polytechnic University, Lot 660 Hay Moulay Rachid, Ben Guerir, Morocco c X-ray Science Division, Advanced Photon Sources, Argonne National Laboratory, Argonne, Illinois 60439, United States d Université Hassan 1er, Laboratoire des Sciences des Matériaux, des Milieux et de la modélisation (LS3M), 25000, Khouribga, Morocco e Argonne National Lab., Chem Sci & Engn Div, 9700 S Cass Ave, Argonne IL, 60439, USA
ABSTRACT A cobalt iron oxyphosphate CoFeOPO4@C (abbreviated as CFP@C) anode was prepared via a solid-state route, and its electrochemical performance was investigated vs. Li+/Li over a wide voltage range of 0.01-3.0 V at different current rates C/n (n= 20, 10, 5, 2 and 1). This anode material is able to intercalate more than six lithium ions into the structure at the C/10 current rate, delivering a specific capacity of 748.23 mAh g-1, which is much higher than the theoretical capacity (593.7 mAh g-1) calculated when the insertion of a single lithium ion is considered. A reversible capacity of 200 mAhg-1 was maintained after 30 cycles. Raman spectroscopy confirmed the incorporation of carbon layers into the CoFeOPO4@C composite. Scanning electronic microscopy revealed that CFP@C particles have an angular-flake shape with particle sizes ranging between 1 and 5 µm. In situ X-ray absorption spectroscopy of Fe and Co at the K-edge showed that both transition metals are active during the whole discharge and charge. In operando high energy X-ray diffraction revealed that this material undergoes a gradual evolution of the structure with lower crystallinity after the first discharge. Correlating electrochemical performance to the structural and electronic features indicated that the cycling mechanism of the CFP@C anode material exhibits a combination of intercalation and conversion processes.
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INTRODUCTION In recent decades, energy production started to switch from fossil fuel burning to green renewable energy sources like solar, wind, and hydropower. The development of such renewable but intermittent energy requires improved energy storage technology. Among the many energy storage approaches, electrochemical storage is considered to be one of the most successful. An example is the lithium-ion battery (LIB), which after its commercialization in 1991 by Sony1 rapidly became the most efficient storage alternative. LIBs2 are light and compact and have a high voltage of ~4-5 V, delivering a specific capacity of between 100 and 150 Wh kg-1. The current LIBs use graphite as the anode material, which has the advantages of a long cycle life, high abundance in nature, and a low cost, but also has some drawbacks such as a low energy density
(375 mAh g-1) and safety issues resulting from lithium
deposition3. As a result, researchers have explored other safe and stable anode materials. Indeed, polyanionic compounds Mn(PO4)y, where M is a transition metal, have gained attention due to their excellent stability and long-term cycling in comparison to lithium transition-metal oxides. The advantage of Mn(PO4)y is that oxygen-phosphorous bonds are more covalent than polar oxygen-metal bonds, i.e., there will be no oxygen release in the structure and, therefore, no combustion reaction with the used electrolyte4. However, these phosphates suffer from a low intrinsic electronic conductivity and a slow lithium-ion diffusion, which causes a poor rate capability. NASICON
5-7
Other phosphate anode materials are
(Na Super Ionic Conductors) materials with the general formula AxMy(PO4)3,
metal pyrophosphates MP2O7 (M = Sn, Ge, Zr, Si, Ce, Ti)8-12, and metal transition oxyphosphates13-17 AxMyOPO4, where A and M are alkali and metal atoms, respectively. A new oxyphosphate Ni0.5TiOPO4 with a monoclinic crystal structure (S.G. P21/c) was also synthesized and characterized18,19. Later on Belharouak and Amine12 reported its use as anode material for LIBs. This material was prepared by a solid-state method and, surprisingly, was able to intercalate three lithium atoms during the first discharge. The delivered specific capacity was 415 mAh g-1, which is higher than the theoretical capacity (145 mAh g-1), estimated from the reduction of all Ti4+ cations to Ti3+. This was successfully confirmed by Xray absorption near edge spectroscopy (XANES), which detected the insertion of one lithium ion per formula unit of Ni0.5TiOPO4. Belharouak and Amine suggested that this high capacity is attributable to the redox couple Ni2+/Ni0 in addition to Ti4+/Ti3+, but they were not able to detect it via XANES measurements12. Later studies could, however, confirm the reduction of Ni2+ to its metallic form using hard X-ray photoelectron spectroscopy (HAXPES), XANES, and extended X-ray absorption fine structure (EXAFS)16. Many in situ and ex situ X-ray 2
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The Journal of Physical Chemistry
diffraction (XRD) studies agreed that an irreversible amorphization16,
20-26
of the pristine
crystal structure occurs after the first lithiation reaction along with the formation of oxides such as Li2O, FeO, and TiOx21,
23
. Lasri et al. investigated M0.5TiOPO4 (M= Co, Ni, Fe)
prepared by a simple sol-gel method by mixing iron, cobalt, and nickel nitrates with H3PO4 and TiCl4 diluted in ethanol. Due to the small particle size (≤ 200 nm), the M0.5TiOPO4/C composites delivered a high reversible capacity of more than 250 mAh g-1, even after 50 discharge/charge cycles at a 5C rate20,
27-29,
which is comparable or higher than results
15, 25, 26
published elsewhere
. LiTiOPO4 with an orthorhombic crystal structure (S.G. Pnma)
was first reported by Fu’s group, who measured high reversible capacities of around 285 mAh g-1 at 0.1C and 195 mAh g-1 at 2C. Fu et al. also demonstrated that the cycling mechanism is similar to that of transition metal oxides, i.e., formation and decomposition of Li2O take place during the conversion of LiTiOPO4 with structural amorphization after the first lithiation21. The present work explores new transition metal oxyphosphates as anode materials for LIBs: The material is MM’OPO4, where M is cobalt, and M’ is cost effective and abundant iron, instead of titanium. We synthesized CoFeOPO4 using a solid-state chemistry method and conducted galvanostatic tests in Li//CoFeOPO4@C half-cells at different current rates (C/10, C/5, C/2, and 1C) in a large voltage window (0.01 V to 3.0 V). The results demonstrated that this material is able to intercalate at the C/10 rate more than six lithium ions per formula unit during the first lithiation process, which is equivalent to a specific capacity of 764.62 mAh g1
. The reversible capacity was around 190 mAh g-1 after 40 cycles. In addition, the reaction
mechanism occurring during the first cycle was established by using high synchrotron energy spectroscopic techniques, namely, in situ X-ray absorption spectroscopy (XAS) of Fe and Co at K-edges and in operando high-energy X-ray diffraction (HEXRD).
EXPERIMENTAL SECTION 1. Material preparation Cobalt iron (III) oxyphosphate, CoFeOPO4, was synthesized by mixing stoichiometric Fe2O3, Co3O4 oxides and di-ammonium phosphate (NH4)2HPO4, using a solid-state route described elsewhere18, 30. The chemical reagents (Sigma-Aldrich and Scharlau) had a purity exceeding 98.99%. Different intermediate grindings and thermal treatments were performed at 400 °C for 12 h and then at 1000 °C for 24 h with 5 °C min-1 heating rate, resulting in a black powder. The prepared material (85 wt.%) was carbon coated by intimate grinding with sucrose (15 wt.%) in a pure acetone solution (purity > 99%), followed by a firing step at 600 3
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°C for 5 h in an argon flow at a 5 °C min-1 heating rate. The bare and the coated materials are abbreviated as CFP and CFP@C, respectively. 2. Material characterization and electrochemical measurements The purity and the crystal structure of the samples were determined by XRD (D8 Advance, Burker) using a Cu Kα radiation source. Bragg Bretano XRD scans with a step size of 0.02° and a constant counting time of 10 s were recorded in the range of 2θ =10–80°. Lattice parameters were refined by a typical Rietveld method implemented in the FullProf program. Morphology and element mapping of the samples were obtained by scanning electron microscopy (SEM, Hitachi, S-4700-II) at an accelerating voltage of 15 kV and energy dispersive X-ray spectroscopy (EDS, Burker, Germany), respectively. The Raman shift spectra of the samples were recorded at ambient conditions using a Renishaw inVia Raman microscope with a scanning range of 100-2000 cm-1. For the electrochemical tests, the working electrodes were prepared by mixing 75 wt.% of the active material and 15 wt.% Super-P carbon conductive additive with 10 wt.% polyvinylidene fluoride (PVDF) as binder and N-methyl-2-pyrrolidine (NMP) as solvent. These chemical products are all supplied by Sigma Aldrich with purities exceeding 99.9%. The obtained slurry was then spread on a copper current collector and dried at 60 °C for 4 h in a convection oven. The electrodes were cut into disks of 12.7 mm diameter by a precision perforator (Hohsen) and dried overnight at 120 °C in a vacuum oven. The resulting electrode materials have an average weight of 5.05 mg. Electrochemical cells were, subsequently, assembled as Swagelock cells in an argon-filled glove box (Model Mbraun, Germany, O2
