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A high-capacity and long cycle life aqueous rechargeable lithium-ion battery with FePO4 anode Yuesheng Wang, Shize Yang, Ya You, Zimin Feng, Wen Zhu, Vincent Gariepy, Jiexiang Xia, Basile Commarieu, Ali Darwiche, Abdelbast Guerfi, and Karim Zaghib ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18058 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

A high-capacity and long cycle life aqueous rechargeable lithium-ion battery with FePO4 anode Yuesheng Wang1, Shi-Ze Yang2, Ya You3, Zimin Feng1,Wen Zhu1, Vincent Gariépy1, Jiexiang Xia4, Basile Commarieu1, Ali Darwiche1, Abdelbast Guerfi1, Karim Zaghib1* 1. Center of Excellence in Transportation Electrification and Energy Storage, Hydro-Québec, 1806 boulevard Lionel-Boulet, Varennes, QC, Canada J3X1S1 2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA 3. Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United State. 4. Jiangsu University, School of Chemistry and Chemical Engineering, 301 Xuefu Road, Zhenjiang, P R China Correspondence to Karim Zaghib (Email: [email protected]) Key words: aqueous lithium-ion battery; FePO4 and FePO4·2H2O anode; stable lowcurrent cycling; in-situ XRD in aqueous battery; electrochemical window

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Abstract Aqueous lithium-ion batteries are emerging as strong candidates for a great variety of energy storage applications due to their low-cost, high-rate capability and high safety. Exciting progress has been made in the search for anode materials with high capacity, low toxicity and high conductivity; yet most of the anode materials, due to their low equilibrium voltages, facilitate hydrogen evolution. Here, we show the application of olivine FePO4 and amorphous FePO4·2H2O as anode materials for aqueous lithium-ion batteries. Their capacities reached 163 mAh/g and 82 mAh/g at a current rate of 0.2 C, respectively. The full cell with amorphous FePO4·2H2O anode maintained 92% capacity after 500 cycles at a current rate of 0.2C. The acidic aqueous electrolyte in the full cells prevented cathodic oxygen evolution, while the higher equilibrium voltage of FePO4 avoided hydrogen evolution as well, making them highly stable. A combination of in-situ X-ray diffraction analyses and computational studies revealed that olivine FePO4 still has the bi-phase reaction in aqueous electrolyte and that the intercalation pathways in FePO4.2H2O form a 2-D mesh. The low cost, high safety and the outstanding electrochemical performance make the full cells with olivine or amorphous hydrated FePO4 anodes commercially viable configurations for aqueous lithium-ion batteries.


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Introduction Aqueous lithium-ion batteries may resolve the safety issues associated with lithium-ion batteries that currently use flammable organic electrolytes15

. Presently, a few cathode materials have been evaluated, such as spinel-

type LiMn2O4 and LiNi0.5Mn1.5O2; layered-LiCoO2 and LiNi1/3Co1/3Mn1/3O2; olivine-LiFePO4 and LiMnPO4 and CuHCF1, 6-15. While improvements are always hoped for, they are not the barrier for further improvements. On the anode side, the situation seems less optimistic. Ever since the aqueous LiMn2O4/VO2 system was reported1, various anode materials were proposed like activated carbon, LiV3O8, V2O5, H2V3O8, TiO2, Mo6S8, LiTi2(PO4)3, Li4Ti5O12, organic Ppy and polyimide7,8,16-31. Based on earlier studies, however, the majority of these anode materials showed significant capacity fading during Li+ intercalation and de-intercalation. The search for a suitable anode material for aqueous lithium-ion batteries is not trivial due to the harsh operating conditions. In particular, a material suitable for anode applications must not react with water and be (almost) insoluble in water. The rate of dissolution scales mainly with surface area and hence materials with high specific surface area are usually less viable in aqueous environments. For example, the vanadium oxides VO2, LiV2O5 and LiV3O8



are typically synthesized at low temperature with a relatively high surface area, and consequently they are not optimum electrode materials in aqueous lithium-ion batteries. Furthermore, hydrogen evolution reactions must be prevented over a modest range of voltages. Although for an anode to be useful, it must function at a voltage as low as possible; nevertheless, too low a working voltage will also accommodate hydrogen evolution and raise stability issues. For example, since the lithium-ion intercalation potential in LiTi2(PO4)3 is 2.45 V vs. Li/Li+, it is theoretically not stable in pH=7 aqueous solutions at room temperature (should be at least 2.63 V vs. Li/Li+ to be stable). Sometimes stability in aqueous solutions is attained by adjusting the electrolyte pH. For example, at pH=13, the minimum stable equilibrium voltage is 2.27 V vs. Li/Li+, hence LiTi2(PO4)3 is stable under this circumstance. But electrolytes with too high pH values lead to O2 evolution on the cathode side. Thus, an inappropriate choice of electrode materials cannot always be rectified by adjusting pH. Finally, the electrode material must have high ionic and electric conductivity, be structurally stable, non-toxic and economical for mass production. These are commonly desired features for all electrode materials; yet in reality one usually has to be content with a compromise among them. For example, vanadium oxides are well-studied but they exhibit modest toxicity to the environment. With active


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carbon anodes, one can cycle the aqueous lithium-ion batteries with tens of thousands of cycles. Although the stability seems amazing, it suffers from the extremely low capacity and by far the highest cost among all candidate anode materials. Organic electrode materials are less expensive, but have very low electric conductivity and low volumetric energy density. There are two common approaches to boost the electrochemical performance of cells. One is to reduce the particle size of the electrode materials, for example, to tens of nanometers. In this way, the diffusion pathways are shortened and higher rates are achieved. For example, nano-sized LiTi2(PO4)3 can reach a rate of 20C32. But reducing the particle size inevitably increases the surface area and promotes more (undesired) surface side effects. The other approach is to enhance performance by greatly increasing the concentration of electrolyte to saturation. By doing this, a stable solid-electrolyte interface can be formed to protect the electrodes during cycling. However, this also shifts the stability voltage window7, 8. Thus, in order to benefit from nano-sizing, the electrode materials must not have significant side reactions with water; and to benefit from highly concentrated electrolytes, the electrode materials must have suitable voltages. These contradicting requirements make the search for a suitable anode material for aqueous lithium-ion batteries a challenging task. The on-going ACS Paragon Plus Environment


pursuit of high capacity, low cost and stable anode materials represent the key issues for the commercialization of aqueous lithium-ion batteries. Here, we report on olivine FePO4 and amorphous FePO4·2H2O as anode materials for aqueous lithium-ion batteries. Although FePO4 has been a hot topic for lithium-ion batteries, given its voltage of 3.4 V vs. Li+/Li, it has historically mostly been used as a cathode material33-35. Nevertheless, by properly adjusting the electrolyte pH and combining with an electrode with even higher working voltage, olivine FePO4 and amorphous FePO4·2H2O can be suitable anodes for aqueous lithium-ion batteries. The capacity of olivine FePO4 reached 163 mAh/g at 0.2C rate, twice as large as that of FePO4·2H2O. However, the full cell with amorphous FePO4·2H2O anode outperformed its olivine counterpart in cycle life by maintaining 92% capacity after 500 cycles at the same 0.2 C rate, thus providing an alternative solution to the long-standing technical issue that aqueous batteries cannot have long cycle life at low current. The operating voltage ranges fall between the voltages of oxygen and hydrogen evolution, thus suggesting high sustainable cycle life.

Results Structure and morphology


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Commercially available amorphous FePO4·2H2O costs about 300 $/Ton, significantly lower than other mainstream anode materials like activated carbon and vanadium-based oxides. An X-ray diffraction (XRD) study was performed on the purchased amorphous FePO4·2H2O (see Fig. 1a) and it clearly indicates an amorphous peak was centered at 35o (Co-Kα) while no sharp peak was seen from 10-80o. To further confirm the results, scanning transmission electron microscopy (STEM) study was performed and no crystalline structure was observed (left inset Fig. 1a). There are continuous hollow rings in the SEAD pattern (left inset Fig. 1a), confirming the amorphous structure. The typical particle size ranges from 30 to 100 nm (see Fig. S1 in Supporting Information). The water content of FePO4·2H2O is about 20% by weight, determined by thermogravimetric analysis (TGA) (shown in right inset Fig. 1a). The BET area of FePO4·2H2O is 29.5 m2/g (Shown in Fig. S2). Fig. 1b shows the XRD and scanning electron microscopy (SEM) results of olivine FePO4. All of the diffraction peaks are in good agreement with the PDF 01-074-9600 and is isostructural with olivine FePO4. It indicates that the sample has high purity and the particle size is around 1 µm. Lithium storage performance in aqueous system



The electrochemical properties of amorphous FePO4·2H2O as an anode material was tested in a three-electrode system using Ag/AgCl as the reference electrode and Pt as the counter electrode with aqueous electrolyte of 15 M LiTFSI in water in a voltage range of -0.8 V to 0.2 V at 0.2C. It delivers a capacity of 82 mAh/g, which corresponds to ~ 0.5 Li insertion per formula unit. The pH of the electrolyte is 5.0. The average Li storage potential is around -0.08 V vs. Ag/AgCl (3.03 V vs. Li+/Li), which falls between the O2 and H2 evolution potentials (2.74 V and 3.98 V vs. Li+/Li at pH = 5 at this concentration of salt. Details to follow in the next section). It is possible for amorphous FePO4·2H2O to be crystalized when Li ions are inserted. This will be discussed in the next section. The cyclic voltammetry (CV) of amorphous FePO4·2H2O electrode features a pair of redox peaks around -0.08 V which is attributed to the reversible reactions of the Fe3+/Fe2+ redox couples corresponding to lithium insertion/extraction into/from amorphous FePO4·2H2O. This result is consistent with the electrochemical profile in Fig. 2b. Aqueous Li-ion full cells are fabricated as three-electrode systems (TES1 and TES2) and two R2302 coin cells (CS1 and CS2). TES1 uses olivine FePO4 as the counter electrode and TES2 uses amorphous FePO4·2H2O instead; this is similar for CS1 and CS2. Both TES1 and TES2 use LiMn2O4


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as the working electrodes and Ag/AgCl as the reference electrodes; all electrodes are immersed in a 15M LiTFSI/H2O electrolyte. The full cell, working electrode and counter electrode profiles are obtained simultaneously by in-situ monitoring; thus it greatly facilitate the analysis and understanding of the electrochemical properties of each and every part of the full cell. For TES2, the operating voltage of the working electrode (LiMn2O4) and counter electrode (FePO4·2H2O) are 0.85 V and -0.08 V, respectively. This three-electrode system was charged and discharged between 0 and 1.9 V. It delivers a reversible capacity of 82 mAh/g at a current of 0.2 C. For TES1, the operating voltage of the working electrode (LiMn2O4) and counter electrode (olivine FePO4) are 0.85 V and 0.3 V, respectively. The lithium storage capacity of olivine FePO4 is 163 mAh/g, which doubles that of amorphous FePO4·2H2O. Likewise, CS2 operates in the same voltage range and exhibits an operating voltage of 1.1V, comparable to TES2. Fig. 3a shows that this coin cell delivered a reversible capacity of 82 mAh/g at a current rate of 0.2C, and that it is even capable of retaining 52% of the capacity at a high current rate of 20C. Upon cycling at 0.2C for 500 cycles, the FePO4·2H2O electrode exhibits a capacity retention of 92% as shown in Fig. 3c. The initial coulombic efficiency is 91% and after initial cycles it reaches 99.2%. Compared with amorphous FePO4·2H2O, CS1 with an



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olivine FePO4 anode exhibited a reversible capacity of 163 mAh/g at 0.2C. At 20C, it retained 70% of the capacity delivered at 0.2C, perceivably showing better kinetics than amorphous FePO4·2H2O. The long-term cycle life of CS1 (shown in Fig. 3f) at a low current rate of 0.2C is over 300 cycles with capacity retention of 71%. The initial coulombic efficiency is 92% and after initial cycles it reaches 99.9%. Table 1. Electrochemical properties of full cell aqueous lithium-ion batteries.

Cathode

LiFePO4/C LiMn2O4 LiMn2O4 LiMn2O4 LiCoO2 LiMn0.05Ni0.05Fe0.9PO4 LiMn2O4 LiMn2O4 LiMn2O4

Anode

Electrolyte

VO2 VO2 TiP2O7 LixV2O5-Ppy Ppy LiTi2(PO4)3 Mo6S8 FePO4 FePO42H2O

LiNO3 LiNO3 LiNO3 (5M) LiNO3 (5M) Li2SO4 Li2SO4 LiTFSI (21M) LiTFSI (15M) LiTFSI (15M)

Capacity retention (cycle) 94% (50) 83% (42) 85% (10) 82% (60) 63% (120) 53% (50) 78% (100) 70% (300) 92% (500)

Current density (mA/g) C/3 0.2C 0.2C 0.2C 0.1C 0.2C 0.15C 0.2C 0.2C

Anode Capacity (mAh/g) 106 120 80 50 104 87 105 163 82

Ref. 36 37 26 38 39 40 7 This work This work

A summary of currently available anode materials for aqueous lithium-ion batteries capable of low rate is presented in Table 1. We observe from this table that crystalline electrode materials do not usually have high stability at low current rate. This observation is again exemplified in the two materials reported here. To the best of our knowledge, olivine FePO4 has the highest capacity and amorphous FePO4·2H2O has the best cycle life at low current. The olivine FePO4 is preferred when more capacity or higher power are required; and the amorphous FePO4·2H2O should be preferred when cycle


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life is emphasized. This amorphous FePO4.2H2O shows better cycling performance than olivine FePO4, likely due to the fact that they have much smaller particle size, which makes them a lot more resistant to the stresses and strains during lithium intercalation/deintercalation.

Discussion The stability of electrode materials in aqueous solutions remains a big challenge1,2,41. The stability and safety for anodes are achieved if the possibility of hydrogen evolution resulting from lithium-involving reactions is excluded because lithium ions are the most active species inside LixFePO4 or LixFePO4·2H2O. Following the method of Dahn1, the equilibrium reaction in H2O is: Liint + H2O ⇔ Li+ + OH- + 0.5H2 Here, Liint represents the intercalated lithium, whose chemical potential is expressed as: uint=u0-eV

(1),

where u0 is the chemical potential of lithium in its standard condition (lithium metal); V is the equilibrium voltage between the anode material and



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lithium metal and e is the charge of an electron (taken as 1 if energy is in eV and voltage in volt). If hydrogen gas forms tiny bubbles at the surface of the anode, the chemical potential of H2 is then considered the same as that at standard condition. And further assuming ideal solution, the change of chemical potentials due to concentration changes is calculated via addition of the logarithms of the new concentrations. Given the Gibbs energy change of the reaction Li + H2O → Li+ + OH- + 0.5H2 is -2.23 eV per lithium atom at standard condition and the equilibrium constant of H+ and OH- is 10-14, the final expression of V that maintains the above equilibrium is: V = 3.0565 - 0.0591 pH - 0.02568 ln[Li+]

(2),

where [Li+] is the concentration of lithium ions. In order to avoid hydrogen evolution, the anode material must have an equilibrium redox potential that is higher than V (vs. lithium metal). As seen from the expression of V, higher concentrations of Li+ reduce the minimum equilibrium redox potential of the anode, and this explains why “water-in-salt” typed electrolytes increase the voltage windows of the cells. Hence, the olivine FePO4 and amorphous FePO4·2H2O materials are stable in the pH range


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between 0 and 14 with any concentration of Li+, as their equilibrium redox potentials are 3.40 V and 3.03 V, respectively. However, it should be noted that the equilibrium potential of LixFePO4·2H2O (x close to 0.5) is around 2.69 V. In this case, the pH should not be too low. In the coin cells, the pH is 5, which helps utilize the full capacity of FePO4·2H2O. It is also worth noting that a lot of traditional cathode materials had to work in an alkaline environment because of the limitations from the chemical stability of the anodes. For example, LiFePO4 cathodes usually work at pH 7~14, LiCoO2 over pH 9, LiNi1/3Co1/3Mn1/3O2 over pH 11, LiMn2O4 over pH 8 (Kim et al.5). If put in an acidic environment, the same cathode materials should work more stably since the cathodic oxygen evolution is suppressed, if these materials do not react with acid and the anodic hydrogen evolution is not facilitated as a result. To further confirm the viability of these materials in Li-ion batteries, advanced aberration-corrected electron microscopy was used to study the structure at atomic scale. The high-angle annular dark field (HAADF) images of lithiated FePO4·2H2O are shown in Fig. 4b and Supplementary Fig. S5 with the corresponding FFT inserted. We could indeed observe some nano-crystalline domains which are not seen in the pristine amorphous FePO4·2H2O anode. Electron energy loss spectroscopy (EELS) mapping (Fig. S6) show the homogenous distributions of Li, Fe, and



P. To identify the nano-crystals formed, the lithiated FePO4·2H2O at the end of discharge anode was measured by XRD, 7 discernible peaks matched the calculated spectrum of Li0.5FePO4·2H2O (space group: P21/n) suggesting that the phase could be Li0.5FePO4·2H2O. The XRD results clearly revealed that the crystal structure did transform from amorphous to nano-crystalline during the lithium insertion. The charge transfer of this material is then studied with STEM-EELS, (as shown in Fig. 4c). The Fe L-edge in amorphous FePO4·2H2O is quite similar to the reference spectrum of Fe2O3 (EELS Atlas of Digital Micrograph). A chemical shift of about 1 eV was observed with the cycled material. The L3/L2 ratio is 4.73±0.23 for the initial amorphous material and 3.58±0.15 for the cycled material (shown in Fig. 4b&4c). The chemical shift and lowering of L3/L2 ratio are clear evidence of reduction of Fe atoms due to Li insertion. In-situ XRD have been widely applied in non-aqueous lithium ion batteries, but seldom in the studies of aqueous batteries because (1) previously available aqueous batteries are usually unstable at low current rate and thus the charge and discharge are too fast for XRD scans; (2) the commonly adopted “window materials” for XRD, beryllium and aluminum, react with water and consequently setting up an XRD experiment for aqueous batteries requires alternative procedures. In this work, we employed in-situ XRD to study


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structure evolution during the cycling of the aqueous batteries. Kapton film was used as the X-ray window with current collector and titanium mesh underneath. The evolution of XRD spectra during cycling are plotted in Fig. 5a and b. The olivine FePO4 anode undergoes a reversible bi-phase transformation (FP ↔ LFP), which is consistent with previous studies using organic electrolytes42, thus confirms lithium intercalation/de-intercalation. The LiMn2O4 cathode also appears to be reversible and stable with a continuous contraction/expansion of the lattice during charge/discharge. An ab initio molecular dynamics (AIMD) calculation was performed to reveal the lithium diffusion mechanism in the nano-FePO4·2H2O crystals. The lithium trajectories showed interesting diffusion patterns. Fig. 6 illustrates the diffusion channels in a 23131 supercell showing the pathways of lithium atoms during the AIMD simulation, where the movement of the FePO4·2H2O matrix is not shown for clarity. The intercalation paths form a two-dimensional mesh in the plane of c = 0.5. The Fe-octahedra and Ptetrahedra flank the main path that parallels the diagonal of a unit cell in the a-b plane and strips the crossing paths in its conjugate direction, leaving a parallelogram void in the middle. This unique pattern is created by the special arrangement of the Fe-octahedra and P-tetrahedra. In fact, the closest P-atom pairs are found in the middle parallelogram void, when viewed in the



c-direction within the c = 0.5 plane, where the O-atoms are at the shared vertices between the P-tetrahedra and Fe-octahedra. A nudged-elastic-band calculation shows activation energy of 0.65 eV for lithium hopping in this mesh. 2 Li atoms were put in simulation, and the strong correlation between them caused the artificial detachment of the diffusion pathway in the upperleft corner of Fig. 6c. But as we can see in its 3 symmetric locations, the diffusion paths indeed traverse that area.

Conclusion In summary, olivine FePO4 and amorphous FePO4·2H2O are successfully applied as the anode material for aqueous lithium-ion batteries. Olivine FePO4 shows a highly reversible capacity of 163 mAh/g at 0.2C, with an average operating voltage of 3.40 V in the three-electrode system and capacity retention of 70% at 20C rate. Amorphous FePO4·2H2O shows a reversible capacity of 82 mAh/g at 0.2 C rate with an average operating voltage of 3.03V in the three-electrode system. A full cell with amorphous FePO4·2H2O anode and LiMn2O4 cathode exhibits excellent cycle life with 92% retention after 500 cycles at 0.2 C rate, which is more stable than olivine FePO4. Comparing the two materials, olivine FePO4 delivers higher capacity and higher rate, while amorphous FePO4·2H2O has longer cycle life at lower rate. Through the in-situ XRD, STEM and DFT characterisation,


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olivine FePO4 in aqueous electrolytes has bi-phase reaction, the same mechanism as in non-aqueous electrolyte. And the we can just noticed that amorphous FePO4·2H2O morphs to nano crystals and its lithium diffusion pathways differ significantly from those in the olivine FePO4. Both materials possess the advantages of low cost, non-toxicity, and safety. We believe these findings demonstrate novel and possibly commercially viable anode materials for aqueous lithium-ion batteries. Experimental Materials synthesis Commercial amorphous FePO4·2H2O (>99%) was purchased from Sigma. LiMn2O4 was purchased from Umico. Olivine FePO4 and LiTFSI are from Hydro Quebec. Powder diffraction patterns were obtained from SmartLab XRay Diffractometer (Rigaku) with Co-Kα radiation. Thermogravimetric and differential scanning calorimetry analyses of the commercials FePO4·2H2O were performed under Ar atmosphere using a TGA Analyzer (TGA 550). STEM experiments: The electrode materials were first removed from the nickel-net electrode and grinded into powder form. The powders were then dispersed onto holey carbon TEM grids using ethanol. The as-prepared STEM samples were ACS Paragon Plus Environment


baked at 150 oC for 8 hours. Imaging was performed with an aberrationcorrected scanning transmission electron microscope (Nion UltraSTEM200) operated at 200 kV using the high angle annular dark field (HAADF, inner angle 80 mrad, outer angle 300 mrad) imaging mode with a beam current of about 24 pA. The probe convergence semi-angle was set to 30 mrad. The EELS inner collection semi angle was set to 48 mrad. Electrochemistry Composite electrodes were fabricated with amorphous FePO4·2H2O or olivine FePO4, super Carbon and polytetrafluoroethylene (PTFE) binder in a mass ratio of 75:17:8 or 80:12:8, which were pressed onto a titanium mesh. Two sets of 3-electrode systems were made. One system consists of Ag/AgCl immersed in saturated KCl, Pt and FePO4·2H2O as the reference, counter and working electrodes, respectively. This 3-electrode system (The mass of the working electrode is 6 mg and the area is 0.5 cm2) was used to measure the voltage window in an aqueous solution. The other one contains Ag/AgCl immersed in saturated KCl, FePO4·2H2O or olivine FePO4, and LiMn2O4 as the reference, counter and working electrodes, respectively. This 3-electrode system was used to simultaneously monitor the electrochemical performances of olivine FePO4 or FePO4·2H2O, and LiMn2O4 electrodes. The electrolyte in both 3-electrode systems is 15M LiTFSI dissolved in ACS Paragon Plus Environment

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deionized water. The coin-type (CR2032) cells were assembled in air with a LiMn2O4 cathode (the ratio between LiMn2O4 (8 mg/cm2) and FePO4·2H2O (12 mg/cm2) or olivine FePO4 (11.6 mg/cm2) is 1.1 or 1.8. The area of anode (FePO4·2H2O or olivine FePO4) is 1.12 cm2), and glass fiber separator. The charge and discharge measurements were carried out on an EC-Lab battery test system (UK) in the voltage ranges of 0 ~ 1.2 V, 0.0 ~ 1.9 V, -0.8 ~ 0.2 V and -1.2V ~ 1.2 V with the three-electrode systems at room temperature. The capacity of all the batteries and three electrode system was calculated based on the weight of FePO4 or FePO4·2H2O in the electrode. Ab initio molecular dynamics calculations Our calculations were carried out with VASP43 using the Perdew-BurkeErnzerhof exchange-correlation functional44. The chosen supercell contains 98 atoms of which 2 were lithium atoms. The energy cut-off was 300eV. The k-points were chosen at a density of 27.14 k-points per Å-3 in the Brillouin Zone. The target temperature was 1200 K. This temperature is above the decomposition temperature of FePO4.2H2O in air, but since we chose a relatively confined supercell, the structure of the material is well maintained during the entire AIMD simulation and the elevated target temperature acted merely as a means of acceleration. A total of 68 points were simulated, and the results indeed depicted the intercalation pathways. ACS Paragon Plus Environment


The hopping activation energy is obtained with the nudged elastic band method45. Hubbard U correction for Fe3+ (U=4.9) is adopted for the calculation of the activation energy46.

Author contributions K. Z. and Y. W. designed this work; Y. W. carried out the synthesis and electrochemical experiments; Y. W., V. G. and W. Z. performed XRD and in- situ XRD measurements; J. X. performed the BET test. S.-Z. Y. measured and analysed STEM and EELS results; B. C. perform the TGA measurements; Z. F. performed the first principles calculations, Y. W. and Z. F. wrote the paper; all the authors participated in analysis of the experimental data and discussions of the results as well as preparing the paper. Acknowledgements: The electron microscopy at ORNL (S.-Z. Y.) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Supporting Information: Additional figures indicating the details of characterisation ACS Paragon Plus Environment

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Figure1. Structures of FePO42H2O and FePO4. (a) Powder X-ray diffraction patterns of FePO42H2O. Left inset: TEM image and SEAD image of the sample. Right inset: TGA curve in Argon, heat rate 10 °C/min. (b) Powder X-ray diffraction patterns of olivine FePO4. Inset: SEM image (Scalar bar: 1µm).



Figure 2. Lithium storage performance of FePO42H2O and FePO4 electrode in threeelectrode system. (a) Cyclic voltammogram curves of the FePO42H2O in three electrode at 0.2 −1

mV s . (b) Galvanostatic charge/discharge curves of FePO42H2O in the three-electrode system in the voltage range of -0.7-0.2 V at 0.5 C rate. (c) Profile of working electrode (LiMn2O4) and counter electrode (FePO42H2O). (d) Profile of working electrode (LiMn2O4) and counter electrode (FePO4). The capacities calculated based on the weight of anode materials.


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Figure 3. The electrochemical performances of LiMn2O4 | FePO42H2O and LiMn2O4 | FePO4 full cell in aqueous electrolytes. LiMn2O4 | FePO42H2O full cell: (a) Charge/discharge curves in the voltage range of 0 and 1.9V. (b) Rate performance. (c) Cycling performance at 0.2C rate. LiMn2O4 | FePO4 full cell: (d) Charge/discharge curves in the voltage range of 0 and 1.9V. (e) Rate performance. (f) Cycling performance at 0.2C rate. The capacities calculated based on the weight of anode materials.



Figure 4. Structure evolution and charge transfer. (a) XRD patterns of lithiated FePO42H2O anode at the charging voltage of 1.9V in full cell. (b) STEM image of lithiated Li0.5FePO42H2O at the charging voltage of 1.9V in full cell. (c) Comparison of EELS of Fe in FePO42H2O and Li0.5FePO42H2O at the charging voltage of 1.9V in full cell.


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Figure 5. Structure evolution during electrochemical lithiation and de-lithiation. In-situ XRD patterns of (a) olivine FePO4 and (b) LiMn2O4.



Figure 6. Lithium diffusion pathways. The ergodic image of lithium positions during the AIMD simulations of lithium movement in P63/n FePO42H2O. The red balls represent oxygen atoms, pink balls hydrogen atoms, Fe-octahedra are in yellow and P-tetrehedra in blue. The movement of the FePO42H2O matrix is not shown. (a) Lithium pathways viewed from a-axis shows 2dimensional Li diffusion. (b) A perspective view. (c) Li diffusion pattern viewed from c-axis without the FePO42H2O matrix present.


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