Single-Phase Lithiation and Delithiation of Simferite Compounds Li

Oct 17, 2014 - Brookhaven National Laboratory, Upton, New York 11973, United States. § Northeastern Center for Chemical Energy Storage, Department of...
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Single-Phase Lithiation and Delithiation of Simferite Compounds Li(Mg,Mn,Fe)PO4 Fredrick Omenya,† Joel K. Miller,† Jin Fang,† Bohua Wen,† Ruibo Zhang,† Qi Wang,†,‡ Natasha A. Chernova,† and M. Stanley Whittingham*,†,§ †

Chemistry and Materials, Binghamton University, Binghamton, New York 13902-6000, United States Brookhaven National Laboratory, Upton, New York 11973, United States § Northeastern Center for Chemical Energy Storage, Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States ‡

S Supporting Information *

ABSTRACT: Understanding the phase transformation behavior of electrode materials for lithium ion batteries is critical in determining the electrode kinetics and battery performance. Here, we demonstrate the lithiation/delithiation mechanism and electrochemical behavior of the simferite compound, LiMg0.5Fe0.3Mn0.2PO4. In contrast to the equilibrium two-phase nature of LiFePO4, LiMg0.5Fe0.3Mn0.2PO4 undergoes a one-phase reaction mechanism as confirmed by ex situ X-ray diffraction at different states of delithiation and electrochemical measurements. The equilibrium voltage measurement by galvanostatic intermittent titration technique shows a continuous change in voltage at Mn3+/ Mn2+ redox couple with addition of Mg2+ in LiMn0.4Fe0.6PO4 olivine structure. There is, however, no significant change in the Fe3+/Fe2+ redox potential.

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INTRODUCTION There has been a great deal of interest in understanding the reaction mechanism of the olivine structure since it was first reported as a potential cathode by Goodenough’s group.1 Partially delithiated LiFePO4 of particle sizes above 100 nm demonstrate a narrow solid solution of lithium-rich triphylite and lithium-poor heterosite at room temperature.2 Reducing the particle dimension or introducing defects can increase the extent of the solid solution range.3−8 A disordered single phase is formed over the whole lithium compositional range at elevated temperatures.9 The lithiation and delithiation mechanism of olivine compound is considered to be either a twophase reaction or a single-phase reaction.1,10 These two possibilities have been under intense investigation in recent years. Nonetheless, there is a general consensus that at equilibrium partially charged LiFePO4 shows the two-phase coexistence either in the same particle or in different particles.10−12 In the understanding of the reaction mechanism of olivines, Yamada et al.13 studied binary olivines LiMnyFe1−yPO4. They demonstrated the existence of a single-phase reaction at the Fe3+/Fe2+ redox potential. On the other hand, Molenda et al.14 reported the existence of a single-phase reaction in LixFe0.55Mn0.45PO4 for the entire Li composition. Contrary to these findings, Bramnik et al.15 reported the existence of two two-phase reaction regions for both Fe3+/Fe2+ and Mn3+/Mn2+ © XXXX American Chemical Society

redox couples in Li1−xMn0.6Fe0.4PO4. The two regions are separated by a narrow single phase located between the Fe3+/ Fe2+ and Mn3+/Mn2+ redox reaction regions. In addition to the binary olivines, Kang’s group16 has studied multicomponent olivine systems and reported the existence of a single-phase reaction in LixMn1/3Fe1/3Co1/3PO4 throughout the 0 ≤ x ≤ 1 range. Ostensibly contrary to the above result, Nam et al.17 reported the existence of a two-phase mechanism and intermediate phases in the lithiation and delithiation of Li1−xMn1/4Fe1/4Co1/4Ni1/4PO4. The existence of a singlephase reaction pathway is kinetically favorable as it allows the systems to overcome the nucleation and growth limitations.10 Therefore, clear understanding of lithiation/delithiation mechanism of mixed transition-metal olivines is important in understanding and development of cathode materials for lithium ion batteries. Naturally occurring mineral simferite, Li 0.5 Mg 0.5 Fe0.3Mn0.2PO4, has been reported to exist as a single phase.18 In this compound, Fe, Mg, and Mn occupy the Fe site with both the Fe and Mn being in trivalent state. The composition is charge-compensated by the existence of Li vacancies in the structure. If the Li0.5Mg0.5Fe0.3Mn0.2PO4 composition exists, it Received: July 31, 2014 Revised: October 16, 2014

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in a 1:1 volume ratio of ethylene carbonate and dimethyl carbonate (DMC) was used as electrolyte. The cells were tested using a VMP2 multichannel potentiostat (Biologic). The galvanostatic charge and discharge experiments were performed at a current density of 1.25 μA/ cm−2 corresponding to 0.002 (1 C corresponds to 160 mA·h/g) over voltage ranges of 2.0−4.3 or 2.2−4.2 V. For ex situ experiments, the electrochemical tests were done at 0.05 C and stopped at different states of charge/discharge before disassembling the cells in a helium atmosphere. The electrodes were washed with DMC and dried under vacuum before extracting the samples from the current collector. In the galvanostatic intermittent titration technique (GITT), the cells were charged at a current density of 30−35 μA/cm2 corresponding to C/20 for 2 h followed by open circuit relaxation for 24 h. This was repeated until a cutoff voltage of 4.3 V was reached followed by a discharge to 2 V. In the potentiostatic intermittent titration technique (PITT) measurements, a 5 mV step was applied after initial galvanostatic charge−discharge, and the current cutoff was set at a current density corresponding to C/200.

would be of interest to determine if its lithiation/delithiation mechanism is equilibrium single-phase or equilibrium twophase reaction for the intermediate states. Herein, we investigate the effect of Mg substitution in LiFe0.6Mn0.4PO4 in terms of the reaction pathway and shift in potential of the transition-metal ions.

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EXPERIMENTAL SECTION

The LiFePO4 and LiMgy(Fe0.6Mn0.4)1−yPO4, y = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, were synthesized by both the solid-state (SS) and the hydrothermal (HT) methods. In solid-state synthesis, required amounts of lithium carbonate (Li2CO3), iron(II) oxalate dihydrate (FeC2O4·2H2O), magnesium oxalate dihydrate (MgC2O4·2H2O), manganese carbonate (MnCO 3), and ammonium dihydrogen phosphate (NH4H2PO4) were wet ball-milled via planetary ballmilling machine for 10 h. The mixtures of the precursors were dried and pelletized. The pellets were then preheated at 350 °C for 8 h followed by manual grinding with pastel and mortar and repelletizing before heating at 600 °C for 12 h at a heating rate of 5 °C/min under hydrogen/helium atmosphere. Hydrothermal synthesis was done by using iron(II) sulfate heptahydrate (FeSO4·7H2O), manganese(II) sulfate monohydrate (MnSO4·H2O), magnesium sulfate heptahydrate (MgSO4·7H2O), lithium hydroxide monohydrate (LiOH·H2O), phosphoric acid (H3PO4), and L-ascorbic acid as starting materials. The theoretical yield of each reaction was 0.01 mol. With the exception of lithium hydroxide, for which a threefold excess was used, the molar ratio of reactants used matched the element ratio in the chemical formula of the desired product. L-Ascorbic acid (1.4 g/L) was used as a reducing agent. The 80 mL reaction solution was sealed in the autoclave and the reaction done at 200 °C for 48 h. The phase composition and the crystal structure of the synthesized samples were determined by powder X-ray diffraction (XRD) using a Scintag XDS2000 θ−θ diffractometer equipped with a Ge(Li) solidstate detector and Cu Kα sealed tube (λ = 1.54178 Å). The data were collected in the range of 2θ = 10°−70° with a step size of 0.02° while spinning the sample to minimize preferred orientation. Highresolution synchrotron powder XRD data were collected using beamline 17-BM at the Advanced Photon Source (APS), Argonne National Laboratory, with an average wavelength of 0.72808 Å and National Synchrotron Light Source, beamline X7B, wavelength 0.3196 Å. Neutron powder diffraction (NPD) data were collected using the BT-1 32 detector neutron powder diffractometer at the NCNR, NBSR. A Cu(311) monochromator with a 90° takeoff angle, λ = 1.5403(2) Å, and in-pile collimation of 60 min of arc were used. Data were collected over the range of 3°−168° 2θ with a step size of 0.05°. The sample was sealed in a vanadium container of length 50 mm and diameters either 6 or 9.2 mm inside a dry He-filled glovebox. A closed-cycle He refrigerator was used for temperature control. The data for the structural refinement was taken at room temperature. For the magnetic ordering studies, the data was taken at 4 K and at a temperature above the Neel temperature TN (either 60, 70, 90, or 100 K, depending on sample composition). Also, the intensity of the 101 peak was measured as a function of temperature to determine the Neel temperature (TN). XRD and NPD data Rietveld refinement was performed using the GSAS/EXPGUI package.19,20 The morphology and particle sizes were obtained by high-resolution scanning electron microscopy (SEM), ZeissSupra-55 field emission scanning electron microscope, operating at 10 kV. The Superconducting Quantum Interference Device magnetometer (Quantum Design MPMS XL-5) was used to measure the dc magnetic susceptibility (χ) of the samples from 350 to 2 K in a 1000 Oe magnetic field. For the electrochemical characterization, 80% of the active material was mixed with 12% of carbon black and 8% polyvinylidene fluoride using 1-methyl-2-pyrrolidinone as the solvent. The prepared slurry was casted on carbon-coated aluminum current collector. The dried electrodes, of area 1.2 cm2, with 3−8 mg of active material, depending on the experiment requirement, were assembled in 2032- or 2325-type coin cells in a He-filled glovebox. Pure lithium foil was used as the counter electrode and Celgard 2400 as the separator. One molar LiPF6

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RESULTS AND DISCUSSION The electrochemical performance and reaction mechanism of olivine−LiFePO4 depends on particle size and the morphology.3,21 Therefore, we examined the difference in particle sizes and morphology between hydrothermal and solid-state samples using SEM. The particle size and morphology of the various synthesized materials in this study were found to depend majorly on the synthesis method and not on the compositions of the compounds (Figure 1). The hydrothermal particles have

Figure 1. SEM micrographs of (a) hydrothermally and (b) solid-state synthesized LiMg0.5Fe0.3Mn0.2PO4.

large hexagonal-like cross sections and are about 1 μm in length. On the other hand, the solid-state samples are spherical with particle sizes around 50 nm. The phase purity of the as-synthesized samples, LiMgy(Fe0.6Mn0.4)1−yPO4, 0 ≤ y ≤ 0.5, prepared by solid-state method and hydrothermal synthesis was determined by XRD analysis. All samples show single olivine phase, which can be indexed with the Pnma space group, with the diffraction patterns shown in Figure 2. The magnified diffraction patterns in Figure 2 (top) reveal a continuous shift in the diffraction peak positions with the increase of Mg in the LiMgy(Fe0.6Mn0.4)1−yPO4, confirming formation of solid solution. The solubility of Mg in LiFe0.6Mn0.4PO4 was further confirmed by the systematic increase or decrease in the normalized peak intensities as exemplified by the magnified section. The 200 and 210 peaks show a decrease in the normalized peak intensity, while the 101 shows an increase with the increase in the concentration of Mg in the structure. The a and b lattice parameters and unit cell volume decrease linearly with increase of Mg2+ in the structure while the c parameter shows very insignificant changes not affecting the overall trend of the unit cell volume. Hydrothermal LiMgy(Fe0.6Mn0.4)1−yPO4, 0 ≤ y ≤ 0.5, also shows single olivine phase with lattice parameters comparable to those of the solid state of corresponding B

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Figure 2. XRD patterns of the LiMgy(Fe0.6Mn0.4)1−yPO4 series. The top panel shows the magnified section of the diffraction pattern in the rectangular box.

compositions. However, the a unit cell parameter is slightly larger as compared to the SS as shown in Figure 3 and Table S1. Such increase may indicate sample compositions different from the target values or presence of structural disorder. Therefore, we determined selected sample compositions by the ICP method and refined site occupancy from both X-ray and neutron diffraction data. The ICP results shown in Table 1 for the representative hydrothermal samples reveal Li deficiency and transition-metal excess; the ICP results for the solid-state samples are consistent with the targeted compositions. The XRD data refinement for the hydrothermal samples shows modest improvement in the goodness-of-fit values when some transition metal is allowed to occupy the Li site, which is compensated by lithium deficiency in the structure. The occupancy of the lithium site by Fe was shown to be 4%−3% by the NPD data refinement. For example, the refinement of hydrothermal LiMg0.5Fe0.3Mn0.2PO4 had a χ2 of around 1.843 assuming full lithium site occupancy by Li ions. When the Li occupancy was allowed to refine freely, the occupancy dropped to around 0.82 but with insignificant change in χ2. This is consistent with ICP results; therefore, we constrained the Li concentration to the ICP values. The χ2 dropped to below 1.8 when about 4% of Fe was allowed to occupy the Li site. Attempt to force 4% Mn or Mg instead of Fe leads to an increase in χ2 to about 1.845 and 1.826 for the latter. In NPD, Fe and Mn have different scattering factors, 9.45 and −3.73, respectively; this allows us to differentiate between the two transition-metal ions which would otherwise have similar scattering factors in XRD. Contrary to the hydrothermal samples, solid-state samples do not show such mixing. An attempt to introduce transition-metal ions at the lithium site leads to an increase in the Rp and χ2. The occupancy of the lithium site by a heavy metal was found to be below 1%. We investigated magnetic properties with the primary goal to corroborate the results of structural refinements, based on our previous findings that structurally ordered LiFePO4 undergoes antiferromagnetic ordering, while TM-excess samples show ferrimagnetism.22 Also of interest were the effect of dilution with nonmagnetic Mg2+ ions on the magnetic ordering, and

Figure 3. Linear decrease in lattice parameters and unit cell volume as a function of Mg2+ content in solid state (SS) and hydrothermal (HT) LiMgy(Fe0.6Mn0.4)1−yPO4.

Table 1. ICP Results for the Concentration of Various Elements in the Hydrothermally Synthesized Simferite Series y in LiMgy(Fe0.6Mn0.4)1−yPO4

Li

Mn

Fe

Mg

P

0.0 0.2 0.5

0.93 0.90 0.92

0.42 0.34 0.22

0.63 0.52 0.32

0 0.18 0.5

1.0 1.0 1.0

possible presence of magnetic impurities, which can be detected beyond the limit of XRD. The temperature-dependent susceptibility data is shown in Figure 4. All the samples undergo magnetic ordering at temperatures below that of LiFePO4, even when 0.5 Mg is present at the Fe site. For the LiFe0.6Mn0.4PO4 samples, TN is about 44 K, as expected from linear interpolation between LiFePO4 (TN = 50 K) and LiMnPO4 (TN = 35 K).23−25 No additional magnetic ordering transitions were observed over the studied temperature range, which further confirms pure olivine phases. The Neel temperature (TN) of the solid-state simferite series were observed to linearly decrease with the increase of Mg in the structure; this further corroborates the formation of solid C

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ature (Figure 5) of the solid-state samples linearly decrease with an increase of Mg in the olivine structure due to the weakening

Figure 5. Linear dependence of TN on Mg concentration in LiMgy(Fe0.6Mn0.4)1−yPO4 prepared by hydrothermal (HT) and solidstate (SS) methods. The transition temperatures were determined by neutron diffraction and magnetization measurement.

of magnetic interactions between Mn2+ and Fe2+ by the nonmagnetic Mg2+. The Curie constant also decreases with Mg content. For antiferromagnetically ordered samples below the Neel temperature, we would expect only a decrease in susceptibility; however, in SS Mg-substituted olivine samples we observe a decrease in susceptibility below the transition temperature followed by an increase. The behavior becomes more pronounced above 20% Mg concentration. It is interesting to note that, even at high Mg2+ concentration, 50 mol %, in the structure, there is still considerable magnetic interaction. In contrast to the solid-state samples, Mg-substituted HT samples do not show obvious antiferromagnetic transition in the magnetization measurement as shown in Figure 4b, as no decrease in the susceptibility is observed below the transition temperature. However, neutron powder diffraction measurements at 4 K reveal antiferromagnetic ordering, similar to that of LiFePO4 (see Supporting Information). Since, in the case of LiFePO4, magnetic unit cell is the same as atomic, no additional reflections are observed below TN. Instead, some peaks’ intensities increase below TN, and it becomes impossible to perform accurate Rietveld refinement without taking magnetic ordering into account. The refinements incorporating the magnetic structure are presented in Figures S2 and S3, and the resulting parameters are presented in Table S2. The Neel temperatures obtained from the 101 peak intensities measured as a function of temperature are presented in Figure 5. Magnetic susceptibility and neutron diffraction data together indicate that the magnetic moments in the TM site are antiferromagnetically ordered as in LiFePO4, while the Fe excess at the Li site causes an increase of the magnetic susceptibility below TN. It corroborates our conclusion from the structural studies as well as our previous analysis of Feexcess material.22 The crystallographic changes of Li1−xMg0.5Fe0.3Mn0.2PO4 at different Li content were investigated by ex situ XRD upon delithiation. The electrodes were electrochemically delithiated using a current density of 60 μA/cm2 (0.05 C) to different

Figure 4. Temperature dependence of the magnetic susceptibility of LiMgy(Fe0.6Mn0.4)1−yPO4, y = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, for samples synthesized by different methods: (a) solid-state and (b) hydrothermal methods. The inset shows inverse molar susceptibility corrected for the temperature-independent contribution and their fit to the Curie− Weiss law.

solution already observed by XRD. All the samples of the LiMgy(Fe0.6Mn0.4)1−yPO4 series exhibit paramagnetic behavior in the high-temperature region. The thermal evolution of 1/(χ − χ0) at temperatures between 150 and 350 K obeys the Curie−Weiss law (Figure 4). The Curie constant (C) and Curie−Weiss temperature (θ) of the LiMgy(Fe0.6Mn0.4)1−yPO4, y = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, were determined from the temperature dependence of the magnetic susceptibility using the Curie−Weiss law (Table 2). The absolute value of the Curie−Weiss temperature (Figure S1) and the Neel temperTable 2. Magnetic Parameters of LiMgy(Fe0.6Mn0.4)1−yPO4, y = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, Synthesized by Solid-State Methoda y

TN, K

0 0.1 0.2 0.3 0.4 0.5

44.0 39.5 34.1 31.0 23.7 17.0

χ0, emu/mol 5.8 5.3 1.0 −1.7 −3.5 1.4

× × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4

θ, K

C, emu·K/mol

μexp eff , μB

μtheor eff , μB

−90.2 −80.5 −62.9 −60.0 −51.7 −45.9

4.187 3.706 2.987 2.756 2.306 1.969

5.78 5.74 5.46 5.61 5.54 5.61

5.43 5.43 5.43 5.43 5.43 5.43

TN is determined as an inflection point of the M(T) dependence; μexp eff theor 1/2 is determined using μexp eff = [8C/(1 − y)] ; in calculations of μeff , the 2+ magnetic moment of Fe is assumed to be 5.09 μB, as in LiFePO4, since it is difficult to account for its orbital contribution; the Mg2+ is not magnetically active; the magnetic moment of Mn2+ is assumed as spin-only 5.92 μB. a

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states as shown in Figure 6, with the value of x showing the amount of Li removed from the structure. The diffraction

Figure 6. Ex situ XRD profiles for electrochemically delithiated Li1−xMg0.5Fe0.3Mn0.4PO4 synthesized by solid state. The plots on the right correspond to the magnified section in the rectangular boxes.

Figure 7. Variation of lattice parameters as a function of Li concentration x in Li1−xMg0.5Fe0.3Mn0.4PO4.

patterns of the charged samples show two major features: (1) the absence of additional phases at all states of charge and (2) continuous shift in the diffraction peak positions during charge. To illustrate the shift in peak position, a line is drawn in Figure 6 showing the peak position of the pristine material as the reference point and all other peaks are observed to continuously shift with the amount of Li removed. In the magnified regions “a” and “b”, it is evident that the different peaks shift in different directions. For example, the peaks in region a, between 38° and 39° 2θ, are well-resolved in the pristine material but merge into one single peak at 48% state of charge. The peak between 39.5° and 41° 2θ splits into two peaks at higher states of charge. The absence of a second phase and the shift in the diffraction peak positions are clear indications of a single-phase reaction during the delithiation process of the LiMg0.5Fe0.3Mn0.2PO4. Otherwise, a two-phase reaction mechanism would be characterized by the emergence and growth of the second phase during the charge/discharge process at the expense of the initial phase. The diffraction profiles for the delithiated samples were refined as a single phase with orthorhombic Pnma space group. The lattice parameters of the partially delithiated samples show continuous changes. The delithiation of the LiMg 0.5 Fe0.3Mn0.4PO4 leads to a decrease in a and b and the unit cell volume whereas the c lattice parameter increases (Figure 7). The continuous changes in the lattice parameters confirm the solid-solution reaction mechanism for the simferite compound. The volume change between LiMg0.5Fe0.3Mn0.4PO4 and Li0.5Mg0.5Fe0.3Mn0.4PO4 of below 3% is observed. The small change in the volume reduces the lattice strain during lithiation/delithiation processes. The change in the olivine reaction can therefore be attributed to the Mg substitution in the structure, which decreases the lattice mismatch between the lithiated and delithiated end-member phases. The smaller the mismatch, the easier it is for the system to undergo single-phase reaction due to the suppression of the phase separation. Additionally, the influence of Mg substitution on the reaction mechanism was further evaluated by potentiostatic intermittent titration technique method. The PITT voltage step and the corresponding current relaxation curves of LiMg 0.5 Fe0.3Mn0.2PO4 were compared to the well-known two-phase LiFePO4. Upon application of the 5 mV voltage step, the current increases to a maximum followed by current relaxation

to C/200 (Figure 8). The nature of the current relaxation time curves differs depending on the transformation of the material. Considering LiFePO4, a faster current relaxation with a

Figure 8. PITT curves for (a) LiFePO4 and (b) LiMg0.5Fe0.3Mn0.2PO4. The voltage curve is shown in blue and the corresponding current curve is red. E

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diffusion-type limited transformation occurs only close to the end-member phases. This region is associated with the solid solution. Outside the narrow solid solution region, a two-phase reaction is observed to dominate its reaction characterized by a bell-shaped curve, which rises to a maximum before falling off to the cutoff current with time. Such a feature is attributed to the biphasic phase transition resulting from the nucleation and growth limited transformation and is consistent with an earlier report.3 The Mg-substituted sample, however, is characterized by fast current relaxation throughout the process; this current relaxation time curve characterizes mostly a diffusion-limited process. The current decay as a function of time for Li1−xMg0.5Fe0.3Mn0.4PO4 can be divided into two parts depending on the rate of current relaxation. At the beginning of delithiation and at the transition between the Fe2+/Fe3+ and Mn2+/Mn3+ redox potential, the current relaxation is much faster and, as such, I(t) is composed of a higher density of unresolved individual currents. Outside these regions, the current relaxation still exhibits a diffusion-type behavior but with slightly slower current relaxation rates, and as a result the individual rates are resolved. Galvanostatic charge−discharge technique, where a constant current is applied to an electrochemical cell, suffers from polarization resulting from kinetic limitations, leading to deviation from the equilibrium conditions. At constant current charge−discharge, SS materials show good cycling performance at moderate current densities. On the other hand, HT materials exhibit poor electrochemical performance at much lower current densities. This can be attributed to the large particle size and excess of transition-metal cations at the lithium site. To avoid the kinetic limitations, we adopted a GITT technique to determine the quasi-equilibrium redox potentials at different Li content for both LiFe0.6Mn0.4PO4 and LiMg0.5Fe0.3Mn0.2PO4. In this technique, a small current, C/20, is applied for a short period before relaxing the cell to an equilibrium state. In a multicomponent olivine, LiMgy(Fe0.6Mn0.4)1−yPO4, Mg is electrochemically inactive, while Mn and Fe are oxidized/ reduced at different potentials. The quasi-OCV profiles of the Fe3+/Fe2+ redox couple in both samples exhibits sloping voltage profiles as shown in Figure 9. However, at the Mn3+/Mn2+ redox potential, the two samples show different behavior: in LiFe0.6Mn0.4PO4, the Mn redox couple exhibits a flat voltage profile, a characteristic of a two-phase reaction, while the OCV at the Mn3+/Mn2+ redox potential in LiMg0.5Fe0.3Mn0.2PO4 exhibits a sloping voltage profile. Such a voltage slope characterizes a solid-solution reaction, which is consistent with our XRD results discussed above. The existence of solid solution in LiMnyFe1−yPO4 was speculated by Malik et al.26 to result from the strong attraction between Mn2+−Li+, which is disturbed by the Fe3+−Li+ repulsion due to the random coexistence of Mn2+ and Fe3+ in the olivine structure in the partially lithiated/delithiated state, thereby decreasing the driving force for the phase separation. However, at the Mn3+/ Mn2+ redox couple, there are no such competing interactions since all Fe exists as 3+. In LiMg0.5Fe0.3Mn0.2PO4, the existence of electrochemically inactive Mg2+ randomly distributed in the structure with Mn3+ and Fe3+ during charge can be used to explain the existence of single-phase reaction at both Fe and Mn redox potentials. The OCV measurements of LiFe 0.6 Mn 0.4 PO 4 and LiMg0.5Fe0.3Mn0.2PO4 show nearly equal equilibrium voltage at the Fe3+/Fe2+ redox couple at around 3.5 V as shown in Figure 9; this is slightly higher than the OCV of Fe3+/Fe2+

Figure 9. Charge GITT curve for LiFe 0.6 Mn 0.4 PO 4 and LiMg0.5Fe0.3Mn0.2PO4. The dotted line is for the clarity of the voltage shift. The bottom graph shows the quasi-equilibrium voltage plots for the two samples.

redox couple in LiFePO4. The latter has a flat OCV at around 3.45 V. Unlike Mn substitution into LiMnyFe1−yPO4, where the Fe3+/Fe2+ voltage continuously shifts to higher values with the increase of Mn in the structure, the Fe3+/Fe2+ redox potential does not show significant dependence on the concentration of Mg2+. In the 4 V region where Mn is oxidized/reduced, the Mn3+/Mn2+ redox couple in LiFe0.6Mn0.4PO4 shows a flat equilibrium voltage. The voltage is slightly lower than that of LiMnPO4; this is consistent with that shown in a previous publication.27 In the simferite case, the observation is a little different; the OCV of the Fe3+/Fe2+ remains unchanged while the Mn3+/Mn2+ redox couple shifts slightly to higher voltages as compared to that of LiFe0.6Mn0.4PO4. Considering the changes in the electronegativity of Mg2+-substituted LiMgy(Fe0.6Mn0.4)1−yPO4, the substitution of more electronegative Mg2+ ion is expected to lower the transition metal ions’ Fe3+/Fe2+ or Mn3+/Mn2+ redox potentials. Mg2+ (72 pm) is smaller than both Fe2+ (78 pm) and Mn2+ (83 pm), and such substitution of a smaller cation for larger ones is expected to shorten the length and increase the covalence of Fe−O and Mn−O bonds. Stronger covalent bonding would decrease the M3+/M2+redox couple. On the other hand, using first-principle calculations, Malik et al.26 explained the voltage shift observed in binary olivine phosphate in terms of relative energy of the intermediate compounds. Using the latter principle to explain the change in the voltages of the Fe3+/Fe2+and Mn3+/Mn2+ couples in the simferite system, we would expect an increase in voltage at both Fe3+/Fe2+and Mn3+/Mn2+ redox couples in the simferite, LiMg0.5Fe0.3Mn0.2PO4, since Mg2+ is electrochemically inactive and does not take part in the reaction. A combination of the two effects, stronger covalent bonding resulting in a decrease in voltage and the energy of the intermediates increase, which ideally should result in higher voltage, may cancel the effect of each other, thereby resulting in unchanged voltage in the Fe3+/Fe2+ redox couple. F

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CONCLUSION The effect of magnesium substitution in mixed olivine, LiFe0.6Mn0.4Po4, was investigated by studying the lithiation/ delithiation mechanism of LiMgy(Fe0.6Mn0.4)1−yPO4 (y = 0, 0.1, 0.2, 0.3, 0.4, and 0.5). While a two-phase reaction mechanism of LiFePO4 under equilibrium thermodynamic conditions is observed, introducing Mg into the olivine modifies the olivine structure by reducing the lattice misfit between the endmember phases. The combination of smaller lattice mismatch and interactive forces introduced by the different cations, Mn, Fe, and the electrochemically inactive Mg2+ favors a singlephase reaction during the lithiation/delithiation process. The one-phase mechanism is more kinetically appealing since no nucleation and growth is required and therefore has faster rate kinetics. In addition, Mg substitution raises the voltage at the Mn3+/Mn2+ redox couple as well as lowering the polarization at the same redox couple. The latter suggests an improvement in the reaction kinetics while the former is understood in terms of the relative energies of the intermediates.

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ASSOCIATED CONTENT

S Supporting Information *

Curie−Weiss constant as a function of Mg content, lattice parameters from X-ray and neutron diffraction data, magnetic neutron diffraction data, and refinements for Mg contents 0 and 0.5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported as part of the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DESC0001294. Use of the Advanced Photon Source at Argonne National Laboratory and the National Synchrotron Light Source at Brookhaven National Laboratory is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract Nos. DE-AC02-06CH11357 and DEAC02-98CH10886, respectively. We also acknowledge the support of the National Institute of Standards and Technology U.S. Department of Commerce, in providing the neutron research facilities used in this work. We thank Dr. Hui Wu of NIST for her help with neutron data collection and analysis.

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REFERENCES

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dx.doi.org/10.1021/cm502832b | Chem. Mater. XXXX, XXX, XXX−XXX