Unveil the Chemistry of Olivine FePO4 as Magnesium Battery Cathode

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Unveil the Chemistry of Olivine FePO4 as Magnesium Battery Cathode Ruigang Zhang, and Chen Ling ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03297 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on July 6, 2016

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ACS Applied Materials & Interfaces

Unveil the Chemistry of Olivine FePO4 as Magnesium Battery Cathode Ruigang Zhang 1 and Chen Ling1,* Toyota Research Institute of North America , 1555 Woodridge Avenue, Ann Arbor, Michigan, USA, 48105 *

Corresponding author: [email protected]

Abstract Despite growing interest in magnesium battery, the research is still challenged to find a cathode that fulfills requirements such as high capacity and good cyclability. Because of their positions in the periodic table and similar ionic sizes of lithium and magnesium, it was naturally postulated that classical intercalation-type Li-ion battery cathode may also accommodate the intercalation of Mg. On the contrary, many Li-ion battery cathodes performed very poorly in Mg cells, although the mechanism behind such phenomena is still unclear. Here we provide first-hand evidence about the chemistry of olivine FePO4 as Mg battery cathode using a combined theoretical and experimental approach. Although LiFePO4 is a commercial cathode with extraordinary good performance in Li-ion battery, the measured capacity of FePO4 in nonaqueous Mg cell was only ~13 mAh/g. Density functional theory calculations predicted sufficient mobility of Mg2+ in FePO4 lattice to support the insertion of Mg at reasonable rate, suggesting the poor performance cannot be simply attributed to the limitation of Mg2+ diffusion. Instead, the recorded low capacity was the result of surface amorphourization that prohibited the

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electrochemical reaction penetrating deeply into bulk phase. The amorphourization had a thermodynamic origin from the instability of intercalated product, which was predicted from DFT calculations and supported by the failure to synthesize magnesiated FePO4 in solid state reaction route. These results highlighted the importance of a thermodynamically preferred intercalation in order to achieve successful Mg battery cathode.

Keywords: magnesium battery, cathode, olivine, FePO4, interface, density functional theory

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1.

Introduction The success of Li-ion battery has revolutionized portable electronic devices in the past

decades. However, in large scale applications new chemistry is still urged to increase the energy density as well as to reduce the cost of battery. An alternative to current Li-ion technology is rechargeable batteries utilizing the transport of light multivalent ions. Magnesium battery, for example, is one of such candidates received wide attention recently because of the potential to provide greater amount of energy density and other advantages such as lower cost and safer operation with Mg-metal anodes.1-7 Surprisingly, the Chevrel phase Mo6S8 used in the first prototype Mg battery is still the most successful cathode to date after extensive research for 15 years. Despite its good cyclability, the application of Chevrel phase is limited by its low voltage and capacity.1 A crucial challenge lying at the center of Mg battery research is to find other suitable cathodes with improved electrochemical performance.2,

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Because of the so-called “diagonal relationship” in the

periodic table stating the chemical similarity between lithium and magnesium,8 significant amount of efforts have been attempted to use compounds analogous to classical Li-ion battery cathodes as Mg-intercalation hosts, including spinels,9-13 layered oxides,14,

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disulfides,16,

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polyanions,18 and open-channel compounds.19-22 It was hoped that these compounds, which exhibit nice performance in Li-ion battery, may also accommodate the intercalation of Mg2+ with similar ionic size to that of Li+. In contrast, this plausible postulation was strongly questioned by the poor performance of these compounds in Mg cells, although the mechanism that leads to the sharply different behaviors for the same cathode in Li and Mg cells still remains unclear. While olivine LiFePO4 is one of the commercialized Li-ion battery cathode with good energy density, stable cycling and high rate capability,23 the Mg-analogue of MgMSiO4, where


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M stands for transition metal Mn, Fe, Co and Ni, has been studied as possible cathodes for Mg battery.24-28 The physical properties of MgMSiO4 such as electronic structures and band gaps were predicted to be close to those of LiMPO4.20 However, the electrochemistry of MgMSiO4 differs greatly from that of LiMPO4. During the extraction of Mg, the transition metal M2+ in MgMSiO4 is first oxidized to M3+ in Mg0.5MSiO4, then to M4+ in the fully demagnesiated MSiO4,20 which is distinct with the extraction of Li from LiMPO4 generating only M3+. It was shown that the M3+/M4+ redox potential falls outside of the stability window of available Mg battery electrolytes, potentially leading to oxidation of electrolyte in the electrochemical demagnesiation.20 Although experimentally a high capacity of ~300 mAh/g was reported,24 the low charge voltage suggested the occurrence of side reactions during the electrochemical operation, such as the possible corrosion of cell parts caused by the electrolyte.29 Therefore it is of great interest to examine the performance of olivine compounds as Mg battery cathode with the redox process strictly limited at M2+/M3+. For this purpose we looked at olivine FePO4, in which the intercalation of Mg with the formation of compound Mg0.5FePO4 has a theoretical voltage positioned well within the stability window of electrolyte.20 However, up to our knowledge the electrochemistry of olivine MgxFePO4 in Mg cell has yet been careful examined. Here we report the first comprehensive study about the mechanism of olivine FePO4 as non-aqueous Mg battery cathode. With a combined theoretical and experimental approach we show that the intercalation of magnesium did not occur in the electrochemical operations. On the contrary to the common belief that the intercalation of magnesium is critically determined by the mobility of Mg2+ ions in the lattice, the poor performance of FePO4 cathode was caused by the passivation from surface amorphourization of active particle, similar to the phenomenon that we reported before for α-MnO2 cathode.30 Our study provides the first-hand evidence about the

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magnesiation mechanism of a classical cathode material and sheds light on future research in this field.

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Method

2.1

Computational Method The theoretical study was based on density functional theory (DFT) calculations

following our earlier work in olivine-based Mg battery cathode.20 DFT calculations were performed with the Vienna ab initio Simulation Package (VASP) using projector-augmented waves (PAW) pseudopotentials and the exchange-correlation functionals parametrized by Perdew, Burke, and Ernzerhof for the generalized gradient approximation (GGA).31-33 All ions were fully relaxed as well as the shape and the volume of the ferromagnetic supercell. Numerical convergence to about 5 meV per formula unit was ensured by using a cutoff energy 520.0 eV and appropriate Gamma centered k-point meshes. A GGA+U approach was used to correctly localize electrons on transition metal ions. A U value of 4.3 eV was used in the calculation, as recommended for the study of LiFePO4. Although it is slightly different with that in our previous work (U=5),20 the conclusion from our previous work was not affected. The energy barriers for the migration of Mg or vacancy were calculated with the climbing-image nudged elastic band (cNEB) method in 1×3×2 supercells. All ions were relaxed for calculations of the minimum energy pathways while the size and shape of the cell were fixed. We used GGA instead of GGA+U method in cNEB to improve the efficiency of the calculation. While GGA+U was proven to be crucial to accurately describe the band gap and redox energetics of this system, the ability to predict cation migration with GGA and GGA+U is not


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significantly different. Previous work to study Li diffusion in olivine LiFePO4 showed that the barriers calculated by GGA+U and GGA only varied by 0.01-0.02 eV.34

2.2

Experimental Section

Material Synthesis. To attempt the synthesis of Mg0.5FePO4 stoichiometric amount of MgCO3, Fe(CH3COO)2 and NH3H2PO4 powders (Sigma Aldrich) were mixed using a ball mill in acetone at 400 rpm for 2 hours. After ball milling, the slurry was dried at room temperature under vacuum and calcinated at 400 °C for 8 hours under argon atmosphere. The pre-treated powder was pelletized and calcined at fixed temperatures for 12 hours under argon flow. Materials Characterization. X-ray diffraction measurements were performed at room temperature on a Rigaku SmartLab diffractometer using Cu Kα radiation. The diffraction data were collected in a 2θ range of 10° to 70° with a step size of 0.05° and a scan rate of 0.04°/min. TEM measurements were performed with an HR-9000 (Hitachi) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) XPS spectra were collected on a PHIULVAC-PHIV ersa Probe II instrument for the samples after Ar+ sputtering for desired time. Fourier transform infrared (FTIR) spectroscopy was mesaured on a Bruker Vertex 70 spectrometer equipped with a liquid nitrogen-cooled MCT detector and a variable angle multireflection ATR accessory (ATRMaxII-Pike). Electrochemical Measurement: FePO4 was prepared by electrochemical delithiation of LiFePO4. LiFePO4 electrodes were composed of 80 wt% active material (MTI Corporation), 10 wt % Ketjen black and 10 wt% polytetrafluoroethylene binder. The mixture was ground using a mortar and pestle with the addition of 5 ml ethanol. After 30 minutes of grinding, the mixture became a soft block and was pressed into a 120 µm sheet via a press roller. The LiFePO4-PTFE

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sheet was dried at 120 °C under vacuum overnight. Then pellets with a diameter of 13 mm were cut out from the sheet. The loading content of each electrode was about 10.6 mg. The delithiation of LiFePO4 was conducted in 2325 coin cell using pure lithium foil as both the counter and reference electrode, 1M LiPF6 in Ethylene Carbonate (EC)/ propylene carbonate (PC)/ diethyl carbonate (DEC) as electrolyte (volumetric ratio, 1:1:3), and a glassy fiber paper as separator. The constant current density was 0.1 mA and the cut off voltage was 4.0 V vs Li/Li+. The electrochemical data were collected using a computer controlled Biologic VMP. After delithiatiation, the electrodes were thoroughly washed with DEC and tetrahydrofuran (THF), respectively. The performance of FePO4 in Mg cell was tested in a customized Tomcell (TJ-AC Tomcell Japan, see Supporting Information for detailed cell structure) using a 0.2 µm thick (28 mm diameter) standard glass filter (Sigma-Aldrich) as a separator and a Mg foil (19 mm diameter) as the counter and reference electrodes. Mg foil was polished by scraping each side of the foil with sandpaper and wiping clean with a Kimwipe (Kimberly-Clark). The magnesium mono-carborane (MMC) electrolyte was prepared using the method reported earlier.35 The water content in MMC was 2.2 ppm as determined by KarlFischer titration method. All cells were assembled under argon in a glove box (

Unveil the Chemistry of Olivine FePO4 as Magnesium Battery Cathode

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