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Prelithiation Activates Fe2(MoO4)3 Cathode for Rechargeable Hybrid Mg2+/Li+ Batteries Nan Wang, Hancheng Yuan, Yanna Nuli, Jun Yang, and Jiulin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10705 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

Prelithiation Activates Fe2(MoO4)3 Cathode for Rechargeable Hybrid Mg2+/Li+ Batteries Nan Wang, Hancheng Yuan, Yanna NuLi,* Jun Yang and Jiulin Wang School of Chemistry and Chemical Engineering, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. E-mail: [email protected] KEYWORDS: prelithiation, cathode, Fe2(MoO4)3, hybrid Mg2+/Li+ batteries, Mg batteries

ABSTRACT. The development of rechargeable Mg-based batteries with high energy density is restricted by the high-voltage cathodes and the parasitic side reactions between battery components and electrolytes operating at relatively high potentials. Here, we develop a hybrid Mg2+/Li+ cell using monoclinic or orthorhombic Fe2(MoO4)3 cathode, a Mg anode and a simple (PhMgCl)2-AlCl3+LiCl electrolyte. Hastelloy-C alloy is proposed as a current collector of highvoltage cathode for the hybrid Mg2+/Li+ battery within a Swagelok-type cell. The application of Hastelloy-C alloy current collector breaks the crucial bottleneck of incompatibility between currently available current collectors and electrolytes. The hybrid cell features a low voltage polarization between the discharge and charge profiles, which is in favor of practical application. On the other hand, since all Li+ ions are supplied by the electrolyte in a hybrid Mg2+/Li+ battery, a high Li+ concentration is required to operate at high capacities for the hybrid battery. We

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further prove first-hand evidence about the compensation of Li+ ions by simple soaking the cathode in the hybrid electrolyte. The preliatition of Li+ ions into monoclinic Fe2(MoO4)3 significantly enhances the cycling stability and reversibility.

1. INTRODUCTION Renewable energy sources have attracted ever-increasing attention due to the exhaustion of fossil fuels and environmental concerns. Due to the intermittent nature of the renewable energy, the utilization of these energy sources depends on the availability of large-scale energy storage systems. In the past decades, great efforts have been made to develop new electrochemical energy-storage devices with high power and energy density, which is triggered by the safety and sustainability of using Li metal with high volumetric energy density (2062 mAh cm-3) as an anode for the rechargeable batteries.1 Owing to wide distribution in the Earth’s crust, chemical stability, low formula-weight, reasonable potential (-2.4 V vs. SHE), two-electron redox and dendrite-free features of Mg anode, rechargeable Mg batteries are preferable for large-scale electrochemical energy-storage systems with high volumetric energy density (3832 mAh cm-3), low cost (more than 24 times cheaper than Li), and high safety.2,3 The intrinsic strong coulombic interactions between the host materials and bivalent Mg2+ ions, however, cause large polarization and sluggish Mg2+ diffusion, and consequently, rapid capacity decay and/or low Mg2+ intercalation levels.4,5 To circumvent the issues related to the intercalation process but still take advantage of the high safety and high capacity of Mg deposition-dissolution, hybrid Mg2+/Li+ batteries have recently been constructed combining an Li+-intercalation electrode and an Mg anode, along with an electrolyte containing both Mg2+ and Li+.6 In the proposed hybrid Mg2+/Li+ batteries, Li+ ion intercalation-deintercalation preferentially takes place in the cathode due to its


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superior mobility in solid structures compared to the bivalent Mg2+ ions, while the reversible Mg deposition-dissolution proceeds firstly at the Mg anode owing to its higher redox potential compared to Li (-2.37 V and -3.04 V vs. SHE for Mg and Li, respectively). The active Li+ ions for cathode and Mg2+ ions for anode are provided by the hybrid Mg2+/Li+ electrolyte. These batteries combine the advantages of Li+ intercalation cathode (high voltage and fast kinetics) and Mg metal (low cost and safety), outperforming all the previously reported rechargeable magnesium batteries in terms of cycling stability, rate capability, and specific capacity. To overcome the challenges associated with Mg electrolyte, such as the limited anodic stability, parasitic corrosion of currently available metal current collectors and side reactions at the electrolyte-electrode interface, low voltage materials have been successively proposed, including Mo6S8,7 MoS2,8 TiS2,9,10 TiO2,11,12 Li4Ti5O1213,14 and MoO215 intercalation materials, FeSx (x=1 or 2)16 and S17 conversion materials, etc. By now, however, the voltage is still lower than the currently state-of-the-art intercalation transition metal oxide cathodes, even though the anodic stability of the reported hybrid Mg2+/Li+ electrolytes can be maximized to around 3.2 V (vs. Mg) on the molybdenum current collector in the magnesium electrolytes containing chloride.18 Preliminary attempt on high voltage material LiFePO4 (LFP) manifests very low coulombic efficiencies owing to the limited anodic stability of electrolytes, poor control over the electrolytecathode interface and ineffective design of the cell architecture.19 Wu et al. applied a solid membrane (LISICON) to separate a LiFePO4 cathode in an aqueous lithium salt solution and a Mg anode in a Grignard reagent-based organic electrolyte, avoiding the parasitic corrosive reactions and realizing a high operating potential.20 As a result, the cell exhibited 2.1 V average discharge voltage with good cycling life and stable discharge platform, but along with inferior coulombic efficiency and large voltage polarization. With the development of high stable


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electrolytes (3.4 V vs. Mg) based on inorganic magnesium aluminium chloride complex,21-23 high-voltage hybrid Mg2+/Li+ batteries using [Mg2Cl2(DME)4][AlCl4]2+LiTFSI electrolyte and LFP or LiMn2O4 (LMO) cathode with the molybdenum metal current collector were proposed,24 but still along with issue since the redox potentials of LFP and LMO are below and close to the decomposition of the electrolyte, respectively. In order to further enhance the anodic stability of the electrolytes, bis(trifluoromethylsulfonyl)imide (TFSI-) derived and chloride-free hybrid electrolytes 0.5 mol L-1 Mg(TFSI)2-LiTFSI/diglyme and 0.25 mol L-1 Mg(CB11H12)2LiTFSI/diglyme, anodically stable up to 3.8 V (vs. Mg) on the aluminium current collector, were used for the practical utilization of high voltage lithium-ion intercalation cathodes (LiMn2O4, LiCoO2 and LiNi1/3Mn1/3Co1/3O2).25 Nevertheless, the electrolytes have lower reversibility of Mg deposition-dissolution and larger over-potential for both Mg deposition and dissolution processes. Zhang et al. assembled a high voltage battery using LFP cathode with flexible pyrolytic graphite film current collector, 0.5 mol L-1 (PhMgCl)2-AlCl3+0.4 mol L-1 LiCl electrolyte and a pouch cell configuration (instead of a coin cell), and further demonstrated the capability of the cell to operate at -40 °C.26 The hybrid battery with the electrolytes containing tetrahydrofuran (THF) significantly outperforms the lithium ion batteries with carbonate based electrolytes at -20 and -40 °C due to the lower melting points and viscosity of THF-based solvents. The development of high voltage hybrid Mg2+/Li+ batteries is faced up with the main roadblock to identify an applicable electrolyte, where Mg could be compatible with all battery components whilst being deposited reversibly. All phenyl electrolyte 0.4 mol L-1 (PhMgCl)2-AlCl3 (called as APC) can be easily prepared and displays 100% Mg deposition-dissolution efficiency, relatively high anodic stability (approximately 3.3 V vs.


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Mg on the Pt electrode) and 2-5 ×10-3 S cm-1 ionic conductivity at room temperature, which is similar to that of standard lithium electrolytes.27 Despite the optimal properties, the electrolyte is drastically corrosive to common current collectors of rechargeable batteries, which reduces anodic electrochemical stability of the electrolytes on non-noble metal below 2.0 V (vs. Mg).28,29 Molybdenum and tungsten with a high anodic stability (>2.8 V) could form passive surface layers, which were found to be applicable for the highly corrosive APC electrolyte.30 Excellent corrosion resistances were observed on Inconel and Hastelloy with high anodic stability (3.3 and 3.8 V, respectively) in magnesium

bis(hexamethyldisilazide)

((HMDS)2Mg)-based

non-nucleophilic

electrolytes.31-33 Until now, it remains a major challenge for rechargeable Mg batteries to identify an electronically conducting, electrochemically inert and affordable current collector. Herein, we propose a reliable and facile strategy for the development of a hybrid Mg2+/Li+ battery with easily prepared APC+LiCl hybrid Mg2+/Li+ electrolyte by applying Hastelloy-C alloy as a current collector within a Swagelok-type cell configuration, which can not only be easily processed, but also preserves the anodic stability of the APC electrolyte exceeding 2.35 V (vs. Mg). Fe2(MoO4)3, which is one of the most promising cathode material for rechargeable lithium and sodium batteries in view of the non-toxic and inexpensive iron, was chosen as the cathode material.34,35 Furthermore, we provide first-hand evidence of the compensation of Li+ ions through a chemical prelithiation process by simply soaking the electrode in the hybrid electrolyte. 2. MATERIALS AND METHODS 2. 1 Preparation of Fe2(MoO4)3


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Monoclinic and orthorhombic Fe2(MoO4)3 micro-sized materials with three-dimensional architectures were prepared by a template-free hydrothermal process.36 Ammonium molybdatetetra hydrate ((NH4)6Mo7O24·4H2O, Alfa Aesar, analysis, 99%) and Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, Alfa Aesar, ACS reagent, 98%) without further purification were dissolved in distilled water, respectively. Then, (NH4)6Mo7O24 solution was added into the Fe(NO3)3 solution at room temperature under magnetic stirring in order to form a homogeneous solution. The pH values were adjusted to 1.65 and 3 by HNO3 (1.3 mol L-1) and dilute ammonia solution, respectively. Hydrothermal treatment of the particulate dispersion at 140 oC for 12h led to pale green and brown precipitations, respectively. The precipitations were washed and calcinated at 600 °C to form yellowish-green and brownish-yellow products, respectively. 2.2 Preparation of electrolyte APC+LiCl/THF electrolytes with different LiCl amounts (0, 0.5, 1, 1.5 mol L-1) were obtained by dissolving PhMgCl (Sigma-Aldrich, 95%), AlCl3 (Aldrich, 99.99%) and LiCl (Alfa Aesar, ultra dry, 99.9%) with the predetermined amount in tetrahydrofuran (THF, Aladdin, further dried using a 3 Å molecular sieve) for at least 24h under stirring in an argon-filled glove box (Mbraun, Unilab, Germany). 2.3 Characterization The conductivities of the electrolytes were conducted on a DDB-303A conductivity meter (INESA INSTRUMENT). X-ray powder diffraction (XRD) measurements were carried out on a Rigaku diffractometer D/MAX-2200/PC with Cu Kα radiation (λ = 0.15418 nm). Structure characterization was measured by using scanning electron microscopy (SEM) on a JEOL field-emission microscope (JSM-7401F) and transmission electron microscopy (TEM) on


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a JEOL high-resolution electron microscope (JEM-2010). Brunauer-Emmett-Teller (BET) surface areas were measured by using a Micromeritics ASAP 2010 surface area analyzer. Raman spectra were conducted at room temperature using a DXR Raman microscope (Thermo Scientific) with the excitation source of a HeNe laser. Fourier transform infrared spectroscopy (FTIR) of the solutions was carried out by a Spectrum 100 FT-IR spectrometer (Perkin Elmer, Inc., USA). X-ray Photoelectron Spectroscopy (XPS) analysis was performed using an AXIS Ultra DLD (Kratos, Inc.; Japan) with monochromatic Al Kα (1486.6 eV) radiation and the C 1s line (284.8 eV) as reference. After discharging-charging to different states and soaking in the electrolytes for 24h, the Fe2(MoO4)3 electrodes were washed with THF solvent to remove soluble residue in an argon-filled glove box and then transferred out of the glove box without exposure to the atmosphere. 2.4 Electrochemical Measurements. Cyclic voltammograms (CVs) were measured inside an argon-filled glove box using a threeelectrode glass cell by an electrochemical instrument of CHI604A Electrochemical Workstation (Shanghai, China). The working electrode was Pt, stainless steel (SS, type 316), Hastelloy-C alloy (type 276, Ni 57%, Cr 14.5%~16.5%, Mo 15.0%~ 17.0%, W 3.0%-4.5%, Fe 4.0%-7.0%, Si 0.08%, Mn 1.0%, C 0.01%, Co ≤ 25%, V ≤ 0.35%, P ≤ 0.025%, S ≤ 0.01%, Goodfellow) or Mo, while magnesium ribbons (Sigma-Aldrich, 99.5%, 0.15 mm thickness) as counter and reference electrodes. 80 wt % Fe2(MoO4)3, 10 wt % Super-P carbon powder (Timcal) and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidinone (NMP) were mixed and coated onto 1 cm2 Hastelloy-C alloy sheets (Weidi Metals Co., Ltd., 100 µm thickness). The coated sheets were dried at 80 oC under vacuum for at least 10h to form electrode disks after volatilization of the


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solvent. Electrochemical performance was examined via two-electrode Swagelok-type cells made from corrosion-resistant polypropylene (PP) swagelok, assembling with polished Mg counter electrode, Entek PE membrane separator with 37% porosity and 20 µm thickness, and APC+LiCl/THF electrolyte. The Swagelok-type cells were assembled in an argon-filled glove box. Galvanostatic discharge-charge measurements were conducted on a Land battery measurement system (Wuhan, China) at ambient temperature. There was 4h rest time for the cells before the beginning of the galvanostatic discharge-charge measurements. 3. RESULTS AND DISCUSSION Fig. 1a shows Cyclic voltammograms (CVs) of the electrochemical Mg depositiondissolution from APC+LiCl/THF electrolytes on Pt electrode with different LiCl concentrations (0, 0.25, 0.5, 1, 1.5 mol L-1) at 50 mV s-1. The increasing cathodic currents from about -0.3 V (vs. Mg/Mg2+) correspond to Mg deposition. The anodic peaks at around 1 V are related to the electrochemical dissolution of deposited Mg. In the following positive scan, a minimal anodic/oxidation currents are observed below the potential of 2.8 V (vs. Mg/Mg2+). APC electrolyte exhibits an anodic stability of 2.8 V (vs. Mg/Mg2+) on noble Pt and the stability increases slightly after the addition of the Li salt, as shown in inset of Fig. 1a. The addition of the suitable amount of LiCl also enhances the magnesium deposition-dissolution properties with improved currents. LiCl shows remarkable solubility, and clear and transparent solutions can be obtained at the concentration lower than 1.0 mol L-1. Photo of the solutions in Fig. 1b exhibits that attempts with higher concentrations than 1.5 mol L-1 are unsuccessful. The ionic conductivities of the electrolytes with the LiCl concentrations of 0, 0.25, 0.5, 1.0, and 1.5 mol L-1 are in the range of 2.3-2.7 mS cm-1, and APC+0.5 mol L-1 LiCl/THF electrolyte exhibits the highest conductivity (Fig. 1c). The improved magnesium deposition-dissolution properties of the


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electrolytes with addition of LiCl are attributed to the higher ionic conductivities, the significantly reduced interfacial resistance and/or improved interfacial compatibility between the electrolyte and Mg metal. As shown in Fig. 1d, APC+0.5 mol L-1LiCl/THF electrolyte exhibits an anodic stability about 3.0 V (vs. Mg/Mg2+) on Pt electrode, but only 2.2 V (vs. Mg/Mg2+) stability on stainless steel (SS), which is a key component of current collector and traditional coin-cell casing. However, the electrolyte has an enlarged anodic stability over 2.65 V (vs. Mg/Mg2+) on Hastelloy-C alloy and Mo, implying higher corrosion-resistances to the electrolyte than that on SS. Furthermore, the CVs upon cycling exhibits the magnesium depositiondissolution processes are more stable on Hastelloy-C alloy than that on Mo, as shown in Fig. S1 in the Electronic Supplementary Information (ESI), which is probably due to better interfacial compatibility between the metal and the electrolyte.


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Figure 1. CVs of Mg electrochemical deposition-dissolution from APC+LiCl/THF electrolytes with different LiCl concentrations (0, 0.25, 0.5, 1.0, 1.5 mol L-1) at 50 mV s-1 on Pt electrode (a). The photo of the electrolytes with different LiCl concentrations, showing the solubility (b). The ionic conductivity of APC+LiCl/THF electrolytes with different LiCl concentrations (0, 0.25, 0.5, 1.0, 1.5 mol L-1) (c). CVs of Mg electrochemical deposition-dissolution on Pt, stainless steel (SS), Hastelloy-C alloy and molybdenum (Mo) electrode from APC+0.5 mol L-1 LiCl/THF electrolyte at 50 mV s-1 (d).

Prototype Swagelok-type two-electrode cell, schematic drawing shown in Fig. 2a, was designed by using Hastelloy-C alloy, which is more easily processed than Mo metal, as the cathode current collector. The use of Hastelloy-C alloy spring ensures cells with electrochemical stability and appropriate sealing. Fig. 2b shows CVs of the electrochemical Mg depositiondissolution on Hastelloy-C alloy electrode from APC+0.5 mol L-1 LiCl/THF electrolyte at 50 mV s-1 using Swagelok-type cell. There is no obvious increase of anodic/oxidation current until the potential of 2.7 V (vs. Mg). Fig. 2c depicts the galvanostatic Mg deposition-dissolution profiles. There is 1h discharge process for Mg deposition and 2.7 V charge voltage limit for Mg dissolution. After about a dozen cycles, over 100% coulombic efficiency (dissolution time/deposition time) appears and the charge voltage decreases and stabilizes at about 2.5 V, indicating the corrosion of Hastelloy-C alloy in the electrolyte. This phenomenon demonstrates that there is probably tiny corrosion in the cell system which is accumulated during cycling and begins to display after dozens of cycles. Fig. 2d and the inset show the galvanostatic Mg deposition-dissolution profiles with 2.4 V charge voltage limit and the corresponding Mg deposition-dissolution efficiencies for 100 cycles. The deposition-dissolution curves are normal profiles as expected when the upper limit of charge voltage is 2.4 V. The first Mg deposition-


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dissolution efficiency is 89.9% and the value increases to over 95% after ten cycles and stabilizes at nearly 100% upon subsequent cycles. High Mg deposition-dissolution efficiency and alleviated over potentials (±0.05V) upon cycling demonstrate that Hastelloy-C alloy is a promising candidate as a cathode current collector for Mg (hybrid) batteries with APC+LiCl/THF electrolyte when the upper limit of charge voltage for the battery system is lower than 2.4 V.


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Figure 2. Schematic drawing of Swagelok-type two-electrode cell (a). CVs of APC+0.5 mol L-1 LiCl/THF electrolyte at 50 mV s-1 in the two-electrode Swagelok-type cell, using Mg as the counter electrode and Hastelloy-C alloy as the working electrode (b). Mg deposition-dissolution curves of APC+0.5 mol L-1 LiCl/THF electrolyte in the Swagelok-type two-electrode cell, the upper limit of charge voltage being 2.7 V (c). Mg deposition-dissolution cycling efficiency of APC+0.5 mol L-1 LiCl/THF electrolyte in the Swagelok-type two-electrode cell, the upper limit of charge voltage being 2.4 V (d), the inset is the corresponding cycling curves of Mg deposition-dissolution.

Monoclinic and orthorhombic Fe2(MoO4)3 materials have been prepared via template-free hydrothermal treatment at 140 oC for 12h of particulate dispersions made of Fe(NO3)3 and (NH4)6Mo7O24 at the pH values of 1-1.65 and 3, respectively.36 The pH value of the solution is important for the formation of Fe2(MoO4)3 with different crystal structures and morphologies. Herein, monoclinic and orthorhombic Fe2(MoO4)3 micro-sized particles with higher crystallinity were fabricated via the hydrothermal treatment at 140 oC for 12h followed by 600 oC calcination of the dispersions obtained at the pH values of 1.65 and 3, respectively. The as-prepared monoclinic and orthorhombic Fe2(MoO4)3 materials were pale green and brown, respectively. The X-ray diffraction (XRD) patterns and scanning and transmission electron microscope (SEM and TEM) images are presented in Fig. 3. XRD diffraction peaks in Fig. 3a and Fig. 3b can be attributed to monoclinic Fe2(MoO4)3 with the cell parameters: a=15.706, b=9.236, c=18.249 (JCPDS No. 83-1701, space group: P21/a(14)) and orthorhombic Fe2(MoO4)3 with cell parameters: a=9.361, b=12.859, c=9.220 (JCPDS No.33-0661, space group: Pnca(60)), respectively. The crystallinity of the materials decreases without the 600 oC calcination.


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Monoclinic Fe2(MoO4)3 is composed of microspheres with rough surfaces and diameters of about 20-25 µm (Fig. 3c), each of which is composed of densely packed nano-flakes with thicknesses of 60-110 nm growing from the center in an radial way (inset of Fig. 3c). Orthorhombic Fe2(MoO4)3 is flower-like aggregates with diameters of 40-50 µm (Fig. 3d), each of which is composed of flakes with thicknesses of 400-700 nm (inset of Fig. 3d). TEM images in Figs. 3e and 3f show the broken flakes of the monoclinic and orthorhombic Fe2(MoO4)3 obtained by an ultrasonic treatment for approximately five minutes, respectively. The selected area electron diffraction (SAED) patterns of Fe2(MoO4)3 in Fig. 3e and 3f are taken along < 114 > and < 011 > zone axis, indicating that the flakes are monoclinic and orthorhombic crystals, respectively. The Brunauer-EmmettTeller (BET) surface area of monoclinic and orthorhombic Fe2(MoO4)3 is 14.1215 and 1.5531 m2 g-1, respectively. The monoclinic Fe2(MoO4)3 particles show a higher BET surface area due to smaller microspheres although they are constructed by more densely packed building units.


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Figure 3. X-ray diffraction pattern of monoclinic Fe2(MoO4)3 and the sample without calcination along with the standard pattern of JCPDS-ICDD 83-1701 (a). X-ray diffraction pattern of orthorhombic Fe2(MoO4)3 and the sample without calcination along with the standard pattern of JCPDS-ICDD 33-0661 (b). Low and high resolution (inset) SEM (c, d) and TEM (e, f) images of the monoclinic and orthorhombic Fe2(MoO4)3, respectively. The inset of (e, f) is the corresponding SAED (selected area electron diffraction).

Fig. 4a shows the first three galvanostatic discharge-charge curves of monoclinic Fe2(MoO4)3/Li Swagelok-type two-electrode cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte between 1.8 V and 3.5 V at 0.05 C (1 C = 90.6 mAh g-1). During the first discharge, monoclinic Fe2(MoO4)3 has a well-defined platform at approximately 3.01 V. The first charge platform is observed at a slightly higher potential of 3.02 V. In the following two cycles, the discharge and charge platforms move hardly, manifesting the Fe2(MoO4)3 features a low voltage polarization between the discharge and charge profiles, which is good for practical application. CV results in Fig. S2a further demonstrate the low voltage polarization and good reversibility. The 1st, 2nd and 3th discharge capacities are about 84.1 (corresponding to about 1.86 Li+), 78.8 and 77.4 mAh g-1 with coulombic efficiency of 93.1%, 95.7% and 93.7%, respectively. The discharge and charge capacities at 0.05 C as a function of cycle number are shown in inset of Fig. 4a. The charge capacity matches well with discharge capacity during cycles. At the 50th cycle, the discharge and charge capacity are 32.1 mAh g-1, 29.4 mAh g-1, respectively. We further investigated the discharge-charge performance of Fe2(MoO4)3/Mg cell with (PhMgCl)2AlCl3+LiCl/THF hybrid electrolytes and the effects of LiCl concentration on the performance.


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Fig. 4b shows discharge capacities as a function of cycle number of monoclinic Fe2(MoO4)3/Mg Swagelok-type two-electrode cell and inset exhibits the 20th discharge-charge curve, using APC+LiCl/THF electrolytes with different LiCl amounts (0, 0.5, 1.0, 1.5 mol L-1) between 1.0 V and 2.35 V at 0.05 C. For pure APC electrolyte without the addition of LiCl, monoclinic Fe2(MoO4)3 only delivers 7.8 mAh g-1 initial discharge capacity, indicating that Mg2+ could barely intercalate into the Fe2(MoO4)3 structure. This phenomenon is different from the traditional Mo6S8 cathode where Li+ and Mg2+ co-intercalation reaction occurres,7 but similar to LiFePO4 cathode where only the Li+ intercalation can take place at the cathode due to the strong electrostatic interactions between PO43– anions and Mg2+ ions.19 Herein, the strong electrostatic interactions mainly occurs between (MoO4)2– anions and Mg2+ ions. When the LiCl amount increases in the electrolyte, the discharge capacity of the cells gradually enhances. The cells with APC+1.0 mol L-1 LiCl/THF electrolyte and APC+1.5 mol L-1 LiCl/THF electrolyte exhibit higher discharge capacities, while the latter has the best cycling stability. Since all Li+ ions are supplied by the electrolyte, a high Li+ concentration is preferable for the hybrid battery. Obviously, a better electrochemical performance is obtained from the hybrid electrolyte with a higher LiCl concentration. This demonstrates that Li+ ions in the electrolytes are the main intercalation species and a higher LiCl concentration is able to provide sufficient Li+ ions for the electrochemical reaction and intercalation kinetics of cathode. Considering the solubility of the solutions, APC+1.5 mol L-1LiCl/THF electrolyte was chosen for further study. Fig. 4c shows the first three discharge-charge curves of monoclinic Fe2(MoO4)3/Mg Swagelok-type cells with APC+1.5 mol L-1 LiCl/THF between 1.0 V and 2.35 V at 0.05 C. The curves during the first cycle display a very flat discharge and charge plateau at 2.21 V and 2.27 V (vs. Mg), respectively, ascribing to the two-phase reaction of lithium insertion with a low voltage gap of 0.06 V. The


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higher voltage polarization in Mg cell than that in Li cell (Figs. 4c and 4a, Figs. S2b and S2a) is likely related to the lower ionic conductivity of the mixed salt electrolyte than purelithium electrolyte (2.57 and 11.2 mS cm-1, respectively). The discharge and charge plateaus hardly exhibit change in two subsequent cycles. The 1st, 2nd and 3th discharge capacities are about 74.1 (corresponding to about 1.64 Li+), 72.1 and 71.5 mAh g-1, which are lower than those of monoclinic Fe2(MoO4)3/Li cell with lithium electrolyte in Fig. 4a. However, the 1st, 2nd and 3th charge capacities are about 102.5, 95.8 and 91.0 mAh g-1, larger than the corresponding discharge capacities, and that of monoclinic Fe2(MoO4)3/Li cell in Fig. 4a. The exceeding capacities may be resulted from some side reactions, for example, the corrosion of the current collector. Galvanostatic discharge-charge experiments were conducted, further improving the charge voltage limits to 2.4 V and 2.5 V for monoclinic Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte, and the results are shown in Fig. 4d. In a typical charge profile at 2.5 V charge voltage limit, the voltage increases closely to 2.5 V, and then decreases and maintains closely to 2.4 V, indicating the corrosion phenomenon of the current collector appears when the charge voltage is higher than 2.4 V. The corrosion-resistances of the Swagelok-type cell at 2.3 V, 2.35 V, 2.4 V and 2.5 V in APC+1.5 mol L-1 LiCl/THF electrolyte were further investigated by chronoamperometry. As shown in Fig. S3, high current densities are observed in the curves at 2.4 V and 2.5 V. However, a much lower current density of about 0.5 µA cm-2 appears in the profiles at 2.3 V and 2.35 V, confirming stability of the Swagelok-type cell in the electrolyte at 2.35 V. Therefore, the exceeding charge capacities than discharge capacities in Fig. 4c (the limit of charge voltage is at 2.35 V) are not mainly resulted from the corrosion of the current collector but other process, such as chemical prelithiation during the rest time of the cell, which will be discussed in detail as below. The lower discharge capacity in the hybrid electrolyte (Fig. 4c) than


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that in the Li electrolyte (Fig. 4a) may be due to some Li+ ions introduced during the chemical prelithiation process, and occupying some space of the interstitial sites which are electrochemically active for Li+-storage. It has been reported that Li+ ions can be inserted chemically into the Fe2(MnO4)3 framework in tetrahedral interstitial sites formed at the edges of two FeO6 octahedra to obtain orthorhombic Li2Fe2(MoO4)3 (Pbcn).37 As shown in inset of Fig, 4c, the monoclinic Fe2(MoO4)3/Mg cell with hybrid electrolyte has a discharge capacity of 42.2 mAh g-1 at the 50th cycle, indicating a better cycling performance than that of the monoclinic Fe2(MoO4)3/Li cell with pure lithium electrolyte. The fresh hybrid electrolyte was further added in the monoclinic Fe2(MoO4)3/Mg cell after cycles to speculate the reason of the capacity decay. As shown in the Fig. S4, the capacities increase slightly after adding fresh electrolyte during the initial several cycles, and then decrease quickly. This means the main reason of the capacity degradation should be related to the change of the cell system upon cycling, such as the separation of the active material from the current collector, although the electrolyte loss also plays a role.


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Figure 4. Discharge-charge profiles of monoclinic Fe2(MoO4)3/Li Swagelok-type two-electrode cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte between 1.8 V and 3.5 V at 0.05 C (a). Cycling performance of monoclinic Fe2(MoO4)3/Mg Swagelok-type cell using APC+LiCl/THF electrolytes with different LiCl amounts (0, 0.5, 1.0, 1.5 mol L-1) between 1.0 V and 2.35 V at 0.05 C, the inset corresponding to the 20th discharge-charge profiles (b). Discharge-charge profiles of monoclinic Fe2(MoO4)3/Mg Swagelok-type cells with APC+1.5 mol L-1 LiCl/THF electrolyte at 0.05 C between 1.0 V and 2.35 V (c), the inset is the corresponding cycling performance. Typical charge profiles of monoclinic Fe2(MoO4)3/Mg Swagelok-type cells with


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APC+1.5 mol L-1LiCl/THF electrolyte at 0.05 C to different upper potential limits of 2.3 V, 2.35 V, 2.4 V and 2.5 V (d).

Fig. 5a shows the first three galvanostatic discharge-charge curves of orthorhombic Fe2(MoO4)3/Li two-electrode Swagelok-type cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte between 1.8 V and 3.5 V at 0.05 C. Orthorhombic Fe2(MoO4)3 also has a well-defined plateau at about 3.0 V with low voltage polarization. The behaviour is the nature of two-phase reaction of lithium intercalation into orthorhombic Fe2(MoO4)3.35 The 1st, 2nd and 3th discharge capacities are about 88.4 (corresponding to about 1.95 Li+), 89.2 and 89.1 mAh g-1 with coulombic efficiency of 98.0%, 99.6% and 99.4%, respectively. The discharge and charge capacities at 0.05 C as a function of cycle number are shown in the inset of Fig. 5a. At the 50th cycle, the discharge and charge capacity are 64.7 and 61.3 mAh g-1, respectively. The capacities are higher and the cycling performance is better than that of monoclinic Fe2(MoO4)3, indicating that the orthorhombic Fe2(MoO4)3 is more suitable for the electrochemical insertion and extraction of Li+ ions. Fig. 5b shows the first three discharge-charge curves of orthorhombic Fe2(MoO4)3/Mg Swagelok-type cell with APC+1.5 mol L-1 LiCl/THF electrolyte between 1.0 V and 2.35 V at 0.05 C. The curves during the first cycle also display a flat discharge and charge plateau at 2.2 V and 2.25 V (vs. Mg), respectively, which is similar to that of monoclinic Fe2(MoO4)3/Mg cell. The 1st, 2nd and 3th discharge capacities are about 86.9, 80.1 and 74.4 mAh g-1, which are lower than those of orthorhombic Fe2(MoO4)3/Li cell with lithium electrolyte in Fig. 5a. However, the 1st, 2nd and 3th charge capacities are about 92.7, 86.3 and 78.6 mAh g-1, larger than the corresponding discharge capacities. The chemical prelithiation in the hybrid Mg2+/Li+ electrolyte also occurs in orthorhombic Fe2(MoO4)3 electrode with lower amounts than those of monoclinic


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Fe2(MoO4)3 electrode. However, the cycling capacities in the inset of Fig. 5b indicate a worse cycling stability up to 50 cycles than that of orthorhombic Fe2(MoO4)3/Li with pure lithium electrolyte. After the 50th cycle, 42.8 mAh g-1 discharge capacity with 49.3% cycling retention is achieved, compared with 64.7 mAh g-1 discharge capacity with 73.1% cycling retention for orthorhombic Fe2(MoO4)3/Li cell with pure lithium electrolyte. As the charge-discharge rate increases to 0.1 C (Fig. 5c), the discharge capacities for both of monoclinic Fe2(MoO4)3 and orthorhombic Fe2(MoO4)3 of the hybrid cells are slightly higher than that of Li cells. Monoclinic Fe2(MoO4)3 still maintains larger capacities in hybrid electrolyte at 0.2 C and 0.3 C. However, the capacities of orthorhombic Fe2(MoO4)3 decease obviously at high rates. As shown in Fig. 5d, for orthorhombic Fe2(MoO4)3, the voltage polarizations in the charge-discharge processes of Fig. 5c are obviously larger than that of monoclinic Fe2(MoO4)3, especially at high rates. The results in Fig. 4 and Fig. 5 manifest that orthorhombic Fe2(MoO4)3 in the hybrid Mg2+/Li+ electrolyte has larger capacities at 0.05 C, but monoclinic Fe2(MoO4)3 exhibits a better cycling stability and higher rate capability. It has been reported that the Li+ ions in the chemical prelithiation process can retain in the lattice, improving the electrical conductivity and in favour of the Li+ intercalation and de-intercalation in following charge and discharge process.38,39 The Li+ ions occupying the interstitial site of monoclinic Fe2(MoO4)3 lattice may stabilize the structure and decrease the electrostatic interaction between Li+ ions and monoclinic Fe2(MoO4)3 layers in interlayer during the discharge, and thus improve the electrochemical performance. However, the chemical prelithium process in the orthorhombic Fe2(MoO4)3 probably causes a larger volume change and make the structure unable to return to the pristine structure, which results in the decrease of cycling performance and rate capability.


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Figure 5. Discharge-charge profiles of orthorhombic Fe2(MoO4)3/Li Swagelok-type twoelectrode cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte between 1.8 V and 3.5 V at 0.05 C (a). Discharge-charge profiles of orthorhombic Fe2(MoO4)3/Mg Swagelok-type cell with APC+1.5 mol L-1LiCl/THF electrolyte between 1.0 V and 2.35 V at 0.05 C, the inset corresponding to the cycling performance (b). The rate cycling performance and the coulombic efficiency of monoclinic and orthorhombic Fe2(MoO4)3/Mg Swagelok-type cells with APC+1.5 mol L-1 LiCl/THF electrolyte between 1.0 V and 2.35 V and Fe2(MoO4)3/Li Swagelok-type cells with LiPF6/EC+DMC electrolyte between 1.8 V and 3.5 V (c). The voltage polarizations in the charge-discharge processes at different rates (d).


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Figure 6. Ex situ XRD patterns of Fe2(MoO4)3 electrodes at different discharge and charge states at a rate of 0.05 C and pristine Fe2(MoO4)3 electrode soaking into the electrolyte for 24h. Monoclinic Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte (a). Monoclinic Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte (b). Orthorhombic Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte (c). Orthorhombic Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte (d). The Li2Fe2(MoO4)3 phases are marked in red and dash lines.

Ex situ XRD patterns were collected to investigate the phase transitions at different dischargecharge states during the first cycle for monoclinic and orthorhombic Fe2(MoO4)3/Mg cells with APC+1.5 mol L-1 LiCl/THF electrolyte and monoclinic and orthorhombic Fe2(MoO4)3/Li cells


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with 1.0 mol L-1 LiPF6/EC+DMC electrolyte. For monoclinic Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte, all peaks of the pristine and fully lithiated Fe2(MoO4)3 (after discharging to 1.0 V) can be well attributed to monoclinic Fe2(MoO4)3 (JCPDS card No. 83-1701) and Li2Fe2(MoO4)3 (JCPDS card No. 84-1001) with the different structures, respectively. When soaking the electrode in electrolytes, the intensities of the major peaks for monoclinic Fe2(MoO4)3 maintain in the Li+ electrolyte, but decrease in the hybrid electrolyte, along with some Li2Fe2(MoO4)3 phase. This means that the chemical lithium process occurs in monoclinic Fe2(MoO4)3 when soaking the electrode in the hybrid electrolyte. The obvious colour change of the electrode in the hybrid electrolyte from yellowish gray to dark brown also indicates the occurrence of chemical lithiation reaction.37 During the discharge process, the intensities of the major peaks at (311), (214), (202), (122) and (420) related to monoclinic Fe2(MoO4)3 decrease gradually and disappear finally when the 1.0 V discharge voltage limit is reached. At the same time, a new Li2Fe2(MoO4)3 phase gradually forms as observed with the appearing and increasing intensity of (111), (102), (021), (112), (310), (212), (130), (312), (231) and (402) peaks marked by the dash lines. No peak shifts appear in both Fe2(MoO4)3 and Li2Fe2(MoO4)3 phases during the discharge process. In the charge process, the peak evolution is observed in the opposite way against the discharge process, and all the diffraction peaks return back to the same positions as that of the soaked electrode, suggesting the recovery of the structure lattice during one discharge-charge cycle. The co-existence of both Fe2(MoO4)3 and Li2Fe2(MoO4)3 phases with changing ratio confirms a typical two-phase reaction in the material during the discharge and charge process. The good reversibility indicates that Li+ ion insertion dominates the cathode reaction upon the cycling in the hybrid cell. Moreover, the monoclinic Fe2(MoO4)3 phase can be recovered after 50 cycles, suggesting a structural reversibility. The


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similar phenomena is also observed in monoclinic Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte. Figs. 6c and 6d show the ex situ XRD pattern of orthorhombic Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte and orthorhombic Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte. Li2Fe2(MoO4)3 phase also appear when soaking orthorhombic Fe2(MoO4)3 electrode in the hybrid electrolyte, suggesting the chemical lithium process occurs in the electrode as well. However, the orthorhombic Fe2(MoO4)3 phase cannot be fully recovered after 20 cycles in pure lithium electrolyte, and cannot even be recovered after the initial cycle in the hybrid electrolyte, indicating an incomplete structural reversibility and explaining the unsatisfying rate capability. It has been reported the chemical lithiation method was used to insert Li+ ions in V2O5.40,41 This chemical lithiation process can give nearly the same results as the electrochemical lithiation and reach the full lithiation capacity. V2O5 powder dispersed in fresh distilled hexane was reduced by an n-butyllithium (BuLi) solution under argon atmosphere to obtain LixV2O5 during 48h for x1.40 For V2O5 nanoribbon, the Li intercalation was done by immersing the nanoribbons into BuLi reagent for the desired time in inert atmosphere, and the lithium was dispersed across the entire nanoribbon after 2h chemical lithiation.41 In this work, the chemical lithiation process occurs when the Fe2(MoO4)3 electrode contacts and soaks in the hybrid electrolyte.


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Figure 7. Raman spectra of APC+1.5 mol L-1LiCl/THF electrolyte and APC+1.5 mol L-1 LiCl/THF electrolyte after soaking monoclinic Fe2(MoO4)3 electrode for 24h (a). FTIR spectra of pristine monoclinic Fe2(MoO4)3 electrode and the electrodes after soaking in APC, APC+1.5 mol L-1 LiCl/THF, and 1.0 mol L-1 LiPF6/EC+DMC electrolytes for 24h, and FTIR spectra of the monoclinic electrodes after discharging to cut-off potentials at 0.05 C for Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte and Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte (b).

In order to verify the chemical lithiation process, the APC+1.5 mol L-1 LiCl/THF electrolytes before and after soaking monoclinic Fe2(MoO4)3 electrode were measured by Raman spectroscopy and the results are shown in Fig. 7a. For the APC+1.5 mol L-1 LiCl/THF electrolyte, the peaks at 914, 1031 and 1072 cm-1 are related to THF. The peaks at 180 and 348 cm-1 are attributed to AlCl4-, while peak at 212 cm-1 corresponds to MgCl2·4THF. Peaks at 297, 624, 680, 997 cm-1 are all related to PhAlCl3-/PhAlCl2·THF, with that at 658 cm-1 referring to Ph3AlCl/Ph3Al·THF.42 The equilibrium species presenting in the hybrid electrolyte are AlCl4-, PhAlCl3-,


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Ph3AlCl- and MgCl2. The spectrum for the electrolyte after soaking the monoclinic electrode shows appreciable changes. One striking change is the emergence of three new peaks, at 241, 270 and 667 cm-1. The former one is attributed to Mg2Cl3+·6THF, and the latter ones are related with Ph2AlCl2-/Ph2AlCl·THF. Moreover, the peak intensities at 914, 1031 and 1072 cm-1 related to THF increase. At the same time, the peak at 348 cm-1 referring to AlCl4- almost disappears. The equilibrium species presenting in the electrolyte after soaking the electrode are AlCl4-, PhAlCl3-, Ph2AlCl2-, Ph3AlCl- and Mg2Cl3+. Those suggest the hybrid electrolyte appears change after soaking the electrode in the electrolyte. The chemical lithiation process of Fe2(MoO4)3 soaking in (PhMgCl)2-AlCl3+1.5 mol L-1 LiCl/THF electrolyte can be preliminarily speculated as follows: 4PhMgCl + 2AlCl4-·THF + 2Li+ + 4PhAlCl2·THF + Fe2(MoO4)3 + 8THF → 2(Mg2Cl3+·6THF) + 2Ph2AlCl·THF + 4PhAlCl3- + Li2Fe2(MoO4)3. The structural evolution for the soaked electrodes in electrolytes before cycling and the electrodes after discharging to cut-off potentials was further probed by FTIR spectroscopy. As shown in Fig. 7b, for pristine monoclinic electrode the peak at 960 cm-1 can be attributed to stretching vibration of Mo-O bonds in the MoO4.43 After soaking monoclinic Fe2(MoO4)3 electrode in APC/THF and 1.0 mol L-1 LiPF6/EC+DMC electrolytes under open-circuit condition for 24h, no changes are observed. However, the FTIR spectrum changes after soaking the electrode in APC+1.5 mol L-1 LiCl/THF electrolyte. The peak at 960 cm-1 disappears and new peak at 930 cm-1 appears, being same as the electrodes after discharging to cut-off potentials in the monoclinic Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1 LiCl/THF electrolyte and monoclinic Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte. This means there is a chemical insertion of Li+ ions when soaking the monoclinic electrode in the APC+1.5 mol L-1 LiCl/THF electrolyte. However, the process does not occur in 1.0 mol L-1 LiPF6/EC+DMC and APC/THF


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electrolytes. Mg2+ ions in the hybrid electrolyte play a key role and accelerate the chemical intercalation of Li+ ions in Fe2(MoO4)3. That is to say, Mg2+ ions contribute to the chemical intercalation of Li+ in the materials. Mg2+ ions also participate in this reaction or just ‘catalyse’ the reaction is still not clear at this stage. The intercalation process is probably dominated by a highly synergetic interaction between Mg2+and Li+. Further studies are needed to determine the exact mechanism involved.

Figure 8. XPS spectra of pristine monoclinic Fe2(MoO4)3 electrode (a, d) and monoclinic Fe2(MoO4)3 electrodes in Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte after discharging to 1.8 V (b, e), and soaked in the APC+1.5 mol L-1 LiCl/THF electrolyte for 24h (c, f).

XPS measurements were further carried out to identify the oxidation states of Fe and Li in the pristine monoclinic Fe2(MoO4)3 electrode and the electrodes after soaking in the hybrid


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electrolyte and discharging to cut-off potential in Li electrolyte. Fig. 8a, 8b and 8c shows the Fe 3p spectra of pristine electrode and the electrodes discharging to 1.8 V in the Li electrolyte and soaking in the hybrid electrolyte for 24h, respectively. It could be noted that the characteristic peak of Fe 3p at about 56.8 eV exists in three Fe2(MoO4)3 electrodes. After discharging to 1.8 V in the Li electrolyte and soaking in the hybrid electrolyte, a new Li 1s peak at 55.7 eV exists in both spectra. It indicates that chemical Li+ intercalation and electrochemical Li+ intercalation exhibit similar phenomenon. In Fig. 8d, the spectrum of Fe 2p3/2 for pristine electrode can be split into two peaks at approximately 711.8 eV, 713.0 eV, corresponding to Fe3+. After discharging to 1.8 V in the Li electrolyte or soaking in the hybrid electrolyte in Fig. 8e and 8f, the peak at about 711.8 eV shifts to a lower energy direction to 711.3 eV, which is related to Fe2+.44 It means that the valences of Fe in the electrodes discharging to 1.8 V in the Li electrolyte and soaking in the hybrid electrolyte are +3 and +2, respectively, suggesting the chemical intercalation reaction happens for the electrode soaking in the hybrid electrolyte. XPS and FTIR measurements were also conducted to orthorombic Fe2(MoO4)3 electrodes on the same conditions. XPS spectra in Fig. S5 exhibit same results with those in monoclinic Fe2(MoO4)3, demonstrating common reaction process occurs in two electrodes. FTIR results of orthorombic Fe2(MoO4)3 electrodes shown in Fig.S6 are nearly similar to those of monoclinic Fe2(MoO4)3 electrodes on the same conditions. For orthorombic Fe2(MoO4)3 electrode after soaking in APC+1.5 mol L-1LiCl/THF, the peak at 960 cm-1 merely weakens except that the new peak at 930 cm-1 appears. This probably means the chemical intercalation of Li+ ions in orthorombic Fe2(MoO4)3 is not so obvious compared to monoclinic Fe2(MoO4)3, which is demonstrated by the lower coulombic efficiencies in the orthorombic Fe2(MoO4)3 than monoclinic Fe2(MoO4)3 (Figs. 5b and 4c).


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In a hybrid Mg2+/Li+ battery, all the Li+ ions come from the cathode materials or electrolytes. Owing to the irreversible reaction during the charge process, a certain amount of Li+ ions cannot go back to the cathode materials or electrolytes during the discharging process, which decreases the energy density and specific energy of the battery. When the cathode materials have no Li+ ions, the hybrid battery, in which electrolytes supply all Li+ ions, needs an adequate Li+ concentration to operate at high capacities. Therefore, the hybrid Mg2+/Li+ batteries with optimal performance should have minimum requirements on the electrolyte amount. Developing new electrolytes, such as the solvent-in-salt type electrolytes, could reduce the volume of electrolytes.7 Chemical prelithiation with negligible negative effects on their chemical and structural stability before cell fabrication process enables to effectively compensate the lithium loss. In this work, the chemical prelithiation process is achieved through simply soaking the electrode in the hybrid Mg2+/Li+ electrolyte. In general, the prelithiation materials should exhibit high volumetric and specific capacities as lithium-ion donors. On the other hand, stability is vital factor for a good prelithiation material.38 Monoclinic Fe2(MoO4)3 consists of MoO4 tetrahedra and FeO6 octahedra sharing oxygen atoms interconnected through corner. During insertion process, Li+ ions fill the defined positions that allows the change from the pristine monoclinic phase to the lithiated orthorhombic phase. The host lattice goes through a concerted rotation of rigid polyhedral subunits actuated by strong interactions with the Li+ ions, resulting in an ordered lithium arrangement.45 Meanwhile, Li+ ions can be extracted from Li2Fe2(MoO4)3 returns back to the original monoclinic framework phase Fe(MoO4)3 without the change in lattice parameter.37 Orthorhombic Fe2(MoO4)3 is composed of FeO6 octahedra sharing all six corners with MoO4 tetrahedra and MoO4 tetrahedra sharing all four corners with FeO6 octahedra. The intercalated Li ions capture simultaneously all four sites through a “discrete-occupation” path to form


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Li2Fe2(MoO4)3 phase apart from the original Fe2(MoO4)3 phase.35 The results in this work prove monoclinic Fe2(MoO4)3 to be a more suitable prelithiation material than orthorhombic Fe2(MoO4)3.

4. CONCLUSION The hybrid Mg2+/Li+ batteries can combine the merits of Mg and Li electrochemistry and exhibit outstanding cycling stability as well as rate performance. However, the electrolyte should provide sufficient Li+ ions for the cathode electrochemical reaction and insertion kinetics, particularly when there are no Li+ ions in the cathode materials. In this work, we study the performance of a hybrid Mg2+/Li+ battery using monoclinic or orthorhombic Fe2(MoO4)3 cathode and APC+LiCl/THF electrolyte in a Swagelok-type cell with Hastelloy-C alloy as the cathode current collector. There is a chemical prelithiation process when soaking the Fe2(MoO4)3 electrode in the hybrid electrolyte, and monoclinic Fe2(MoO4)3 is a more suitable prelithiation material than orthorhombic Fe2(MoO4)3. The chemical prelithiation provides Li+ ions during the following charge and maintains the capacity during the cycles, thus enhancing the cycling stability and reversibility. Our results demonstrate the chemical prelithiation of Li+ ions into cathode to be a promising strategy for supplying sufficient Li+ ions in future hybrid magnesiumlithium batteries.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. CVs of Mg electrochemical deposition-dissolution on Hastelloy-C alloy and Mo electrodes at 50 mV s-1 from APC+1.5 mol L-1 LiCl/THF electrolyte. CVs of monoclinic Fe2(MoO4)3/Li Swagelok-type cells with LiPF6/EC+DMC electrolyte between 2.5 V and 3.2 V and monoclinic Fe2(MoO4)3/Mg Swagelok-type cells with APC+1.5 mol L-1 LiCl/THF electrolyte between 1.0 V and 2.4 V. Chronoamperograms of Swagelok-type cells at different potentials in APC+1.5 mol L-1 LiCl /THF electrolyte. The cycling performance of monoclinic Fe2(MoO4)3/Mg Swagelok-type cells with APC+1.5 mol L-1 LiCl/THF electrolyte at 0.05 C between 1.0 V and 2.35 V after adding the fresh electrolyte. XPS spectra of pristine orthorhombic Fe2(MoO4)3 electrode and orthorhombic Fe2(MoO4)3 electrodes in Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte after discharging to 1.8 V, and soaked in the APC+1.5 mol L-1 LiCl/THF electrolyte for 24h. FTIR spectra of pristine orthorhombic Fe2(MoO4)3 electrode and the electrodes after soaking in APC, APC+1.5 mol L-1LiCl/THF, and 1.0 mol L-1 LiPF6/EC+DMC electrolytes for 24 h, and FTIR spectra of the orthorhombic electrodes after discharging to cut-off potentials at 0.05 C for Fe2(MoO4)3/Mg cell with APC+1.5 mol L-1LiCl/THF electrolyte and Fe2(MoO4)3/Li cell with 1.0 mol L-1 LiPF6/EC+DMC electrolyte.

Corresponding Author E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Financial support from the National Natural Science Foundation of China (No. 21273147, 21573146) and the Shanghai Municipal Science and Technology Commission (Project No. 11JC1405700) is gratefully acknowledged.

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Prelithiation Activates Fe2(MoO4)3 Cathode for Rechargeable Hybrid Mg2+/Li+ Batteries

A hybrid Mg2+/Li+ battery using monoclinic or orthorhombic Fe2(MoO4)3 cathode with Hastelloy-C alloy current collector, Mg anode and (PhMgCl)2-AlCl3+LiCl electrolyte delivers a low voltage polarization. The preliatition of Li+ ions into monoclinic Fe2(MoO4)3 by soaking the electrode in the hybrid electrolyte enhances the cycling stability and reversibility.


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Prelithiation Activates Fe2(MoO4)3 Cathode for ... - ACS Publications

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