Understanding the Improved Electrochemical Performances of Fe

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J. Phys. Chem. C 2009, 113, 15073–15079

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Understanding the Improved Electrochemical Performances of Fe-Substituted 5 V Spinel Cathode LiMn1.5Ni0.5O4 Jun Liu and Arumugam Manthiram* Electrochemical Energy Laboratory and Materials Science and Engineering Program, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: May 7, 2009; ReVised Manuscript ReceiVed: July 6, 2009

Partial substitution of Fe for Ni alone or both for Ni and Mn in the 5 V spinel cathode LiMn1.5Ni0.5O4 has been investigated by characterizing the samples by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), charge-discharge measurements in lithium cells, electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and X-ray photoelectron spectroscopy (XPS). The Fe-substituted samples exhibit remarkably superior cycling performances and higher rate capabilities than the pristine LiMn1.5Ni0.5O4. The greatly enhanced electrochemical performances by the Fe substitution are attributed to the (i) stabilization of the structure with cation-disorder in the 16d octahedral sites of the spinel lattice, (ii) suppression of the formation of a thick solid-electrolyte interfacial (SEI) layer due to the Fe-enrichment and Ni-deficiency on the surface, (iii) production of Mn3+ and the consequent enhancement in electronic conductivity, and (iv) much reduced polarization loss arising from both fast charge transfer kinetics and lithium ion diffusion kinetics in the bulk. Introduction With the advancement in portable electronic devices and intense interest in plug-in hybrid electric vehicles, there is great demand to increase the energy and power capabilities of lithium ion batteries. In this regard, the 5 V spinel cathode LiMn1.5Ni0.5O4 has drawn much attention due to its high operating voltage (∼4.8 V) and the high intrinsic rate capability offered by the 3-dimensional lithium ion diffusion in the spinel lattice.1,2 Moreover, the difficulties encountered with the dissolution of manganese3,4 and Jahn-Teller distortion5,6 in the 4 V LiMn2O4 cathode are suppressed in LiMn1.5Ni0.5O4 as it does not contain Mn3+ due the presence of all Mn as Mn4+ and Ni as Ni2+. However, LiMn1.5Ni0.5O4 is encountered with the formation of the LixNi1-xO impurity phase, and its electrochemical performance is influenced by the large lattice parameter difference among the three cubic phases formed during cycling.7,8 To improve the electrochemical performances, various synthesis routes2,9-12 including the sol-gel method, the molten salt method, the coprecipitation method, and the resorcinolformaldehyde assisted solution based method10 have been pursued. Also, partial substitutions of other cations such as Li, Al, Mg, Ti, Cr, Fe, Co, Cu, Zn, and Mo for Mn and Ni in LiMn1.5Ni0.5O4 have also been pursued.8,13-20 Recent studies8,21-23 have shown that the cationic substitutions help to eliminate the formation of the LixNi1-xO impurity phase and stabilize the spinel structure with a disordering of the Mn4+ and Ni2+ ions in the 16d octahedral sites (space group Fd3jm) instead of the cation-ordered structure formed by the pristine LiMn1.5Ni0.5O4 (space group P4332), resulting in a smaller lattice parameter difference among the three cubic phases formed during cycling and improved electrochemical performances. Among the various cationic substitutions, we showed previously that the substitution of Co is particularly effective in improving the electrochemical performances.8 Fey et al.17 * To whom correspondence should be addressed. Phone: 512-471-1791. Fax: 512-471-7681. E-mail: [email protected].

investigated the substitution of Fe for Ni and found that LiMn1.5Ni0.4Fe0.1O4 exhibits better electrochemical performance than LiMn1.5Ni0.5O4 although the synthesis methods were not optimized. We present here by adopting a coprecipitation method the synthesis of LiMn1.5Ni0.5O4, LiMn1.5Ni0.42Fe0.08O4, LiMn1.42Ni0.42Fe0.16O4, and LiMn1.5Ni0.34Fe0.16O4 and their characterization by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), electrochemical charge-discharge measurements, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The characterization data are used to develop an in-depth understanding of the roles played by Fe on enhancing the electrochemical performances. Experimental Section The LiMn1.5Ni0.5O4, LiMn1.5Ni0.42Fe0.08O4, LiMn1.42Ni0.42Fe0.16O4, and LiMn1.5Ni0.34Fe0.16O4 samples were synthesized by a hydroxide precursor method as described previously.8,20 The procedure involves the precipitation of the hydroxide precursors first from a solution containing the required quantities of manganese, nickel, and iron acetates by adding KOH, followed by firing the ovendried hydroxide precursors with a required amount of LiOH · H2O at 900 °C in oxygen for 12 h with a heating/cooling rate of 1 deg/ min. XRD patterns were recorded with a Phillips X-ray diffractometer with Cu KR radiation between 10° and 90° at a scan rate of 0.01 deg/s. FTIR spectra were recorded with KBr pellets with a Perkin-Elmer IR spectrophotometer. The characteristic vibrational bands of the metal-oxygen bonds between 700 and 400 cm-1 were used to examine the ordering of cations in the 16d sites of the spinel lattice. Particle size was measured by diffraction laser spectroscopy (DLS) particle size analyzer (90 plus/BI-Mas, Brookhaven Instruments Corporation). The average radius of the particles was ∼1.0 µm for all the samples investigated. XPS data were collected at room temperature with a Kratos Analytical Spectrometer and monochromatic Al KR (1486.6 eV) X-ray source to assess the chemical state and

10.1021/jp904276t CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

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Figure 1. XRD patterns of the pristine LiMn1.5Ni0.5O4 and the Fesubstituted samples.

concentration of the transition metal ions both on the surface and in the bulk of samples. Multiplex spectra of various photoemission lines were collected at medium resolution using an analyzer pass energy of 40 at 0.1 eV step and an integration interval of 1 s/eV. Sputtering was performed with an argon ion beam gun operating at 2 kV with a spot size of 1 × 1 mm2. All spectra were calibrated with the C 1s photoemission peak at 285.0 eV to correct for the charging effect. Electrochemical performances were evaluated with CR2032 coin cells between 5.0 and 3.5 V. The coin cells were fabricated with the spinel oxide cathodes, metallic lithium anode, 1 M LiPF6 in 1:1 diethyl carbonate/ethylene carbonate electrolyte, and Celgard polypropylene separator. The cathodes were prepared by mixing 75 wt % active material with 20 wt % conductive carbon and 5 wt % polytetrafluoroethylene (PTFE) binder, rolling the mixture into thin sheets, and cutting into circular electrodes of 0.64 cm2 area. The electrodes typically had an active material content of 6-8 mg. EIS data were collected at 50% state of charge (SOC) with an ac amplitude of 10 mV in the frequency range of 10 kHz to 50 mHz by a Solartron 1260 impedance analyzer. Chronoamperometry was carried out on the fully charged electrodes with the Autolab PG 302 potentiostat and stepping the potential from the open circuit potential (OCP) of the fully charged state to 3.6 V. Li foil served as both counter and reference electrodes in these measurements. Results and Discussion Structural Characterization. XRD patterns of the LiMn1.5Ni0.5O4, LiMn1.5Ni0.42Fe0.08O4, LiMn1.42Ni0.42Fe0.16O4, and LiMn1.5Ni0.34Fe0.16O4 samples are compared in Figure 1. The weak reflections observed at around 37.6°, 45.7°, and 63.5° due to the LixNi1-xO impurity phase in the pristine LiMn1.5Ni0.5O4 are absent in the Fe-substituted samples, indicating the effectiveness of Fe in eliminating the impurity phase. The ordering of the cations in the 16d octahedral sites of the (Li)8a[Mn1.5Ni0.5]16dO4based spinel cathodes is known to influence significantly the electrochemical performances, especially the rate capability.23 FTIR spectroscopy has been proved to be an effective technique in differentiating the ordered versus disordered structures of the LiMn1.5Ni0.5O4-based cathodes.24,25 As seen in Figure 2, The characteristic bands (at around 465 and 430 cm-1) corresponding to the cation-ordered structure appear only in the FTIR spectra of the pristine LiMn1.5Ni0.5O4 and are absent in the spectra of

Liu and Manthiram

Figure 2. FTIR spectra of the pristine LiMn1.5Ni0.5O4 and the Fesubstituted samples.

Figure 3. Cyclability of the pristine LiMn1.5Ni0.5O4 and the Fesubstituted samples.

the Fe-substituted samples, indicating that the Fe substitution stabilizes the cation disordered structure. Electrochemical Performances. Capacity and Capacity Retention. Figure 3 compares the cycling performances of the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples. While the pristine LiMn1.5Ni0.5O4 sample delivers a capacity of ∼130 mAh/g with a capacity retention of 92% in 50 cycles, the Fesubstituted LiMn1.5Ni0.42Fe0.08O4, LiMn1.42Ni0.42Fe0.16O4, and LiMn1.5Ni0.34Fe0.16O4 deliver a capacity of respectively 136, 131, and 127 mAh/g with a remarkable capacity retention of 100%, 99%, and 100% in 100 cycles. The substantially improved cycling performance of the Fe-substituted samples could be partly due to the stabilization of the cation-disordered structure and the much reduced lattice parameter differences among the three cubic phases formed during cycling.8,26 The LiMn1.5Ni0.42Fe0.08O4 composition offers a combination of high capacity and excellent cyclability among the compositions investigated. Previous Mossbauer spectroscopic study has shown that the valence of Fe in LiMn1.5Fe0.5O4 is +3, and it varies between +3 and +4 during the charge-discharge process.27 Our XPS results also reveal that Fe is present in the +3 state (see later). Figure 4 compares the discharge profiles of LiMn1.5Ni0.5O4, LiMn1.5Ni0.42Fe0.08O4, LiMn1.42Ni0.42Fe0.16O4, and LiMn1.5Ni0.34Fe0.16O4. While the capacity in the 5 V region is due to the reduction of Ni4+ to Ni2+ and Fe4+ to Fe3+, that in the 4 V region is due to the reduction of Mn4+ to Mn3+ (Table 1). It is interesting to note that while the

Fe-Substituted 5 V Spinel Cathode LiMn1.5Ni0.5O4

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Figure 4. Discharge curves of the pristine LiMn1.5Ni0.5O4 and the Fesubstituted samples. The filled circles represent the separation points between the 4 and 5 V regions.

TABLE 1: Comparison of Discharge Capacities and Mn3+ Contents in the Pristine and Fe-Substituted LiMn1.5Ni0.5O4 discharge capacity (mAh/g) sample

5 V region

4 V region

amount of Mn3+

LiMn1.5Ni0.5O4 LiMn1.5Ni0.42Fe0.08O4 LiMn1.42Ni0.42Fe0.16O4 LiMn1.5Ni0.34Fe0.16O4

117.8 121.4 118.5 105.0

12.5 14.9 13.1 21.3

0.08 0.10 0.09 0.14

capacity in the 4 V region of LiMn1.42Ni0.42Fe0.16O4 is similar to that of pristine LiMn1.5Ni0.5O4, the capacity values in the 4 V regions of both LiMn1.5Ni0.42Fe0.08O4 and LiMn1.5Ni0.34Fe0.16O4 are larger than that of LiMn1.5Ni0.5O4. This is because the substitution of Fe3+ for Ni2+ reduces the corresponding amount of Mn4+ to Mn3+ in both LiMn1.5Ni0.42Fe0.08O4 and LiMn1.5Ni0.34Fe0.16O4, resulting in an increase in the capacity of the 4 V region. On the other hand, the substitution of equal amounts of Fe3+ for Ni2+ and Mn4+ keeps the oxidation state of Mn unchanged, resulting in no significant change in capacity in the 4 V region on going from LiMn1.5Ni0.5O4 to LiMn1.42Ni0.42Fe0.16O4. Rate Capability. The rate capabilities of the pristine and Fesubstituted samples were assessed after 5 charge-discharge cycles. Figure 5 compares the discharge profiles of pristine and Fe-substituted samples at various C rates. The Fe-substituted samples exhibit much higher rate capability than LiMn1.5Ni0.5O4. For instance, while the pristine LiMn1.5Ni0.5O4 offers ∼30 mAh/g at 10 C rate, LiMn1.5Ni0.42Fe0.08O4 delivers a remarkable capacity of 106 mAh/g at 10 C rate. In order to illustrate the differences in the rate capabilities between the pristine and Fesubstituted samples in a better manner, the discharge capacity values at various C rates are normalized to the discharge capacity value at C/6 rate and plotted in Figure 6. Clearly, the rate capability increases in the order LiMn1.5Ni0.5O4 < LiMn1.42Ni0.42Fe0.16O4 < LiMn1.5Ni0.42Fe0.08O4 < LiMn1.5Ni0.34Fe0.16O4. The results indicate that the substitutions of Fe for Ni alone or for both Ni and Fe improve the rate capability drastically, but the substitution of Fe for Ni alone is more effective in enhancing the rate capability compared to the substitution of Fe for both Ni and Mn. Factors Controlling the Rate Capability. Rate capability of cathodes is influenced by the lithium ion insertion/extraction kinetics. Insertion/extraction of lithium ions into/from the cathode materials involves (i) lithium ion diffusion through the surface solid-electrolyte (SEI) layer, (ii) charge transfer reaction, and (iii) lithium ion diffusion in the bulk of the material, which

imposes respectively ohmic polarization, activation polarization, and diffusion polarization on the electrode. The process with the slowest kinetics leads to the largest polarization and becomes the rate determining step. The sections below focus on understanding the factors controlling the rate capabilities in the samples investigated. Polarization Study. Figure 7 compares the dQ/dV curves of LiMn1.5Ni0.5O4 (exhibiting the worst rate capability) and LiMn1.5Ni0.34Fe0.16O4 (exhibiting the best rate capability). The much smaller difference in potential between the anodic and cathodic peaks in LiMn1.5Ni0.34Fe0.16O4 compared to that in LiMn1.5Ni0.5O4 suggests faster lithium insertion/extraction kinetics in the former.14 Additionally, small redox peaks appear above 4.9 V only in the case of LiMn1.5Ni0.34Fe0.16O4, which have been attributed to the redox reaction of Fe3+/4+,27,28 confirming the electrochemical activity of Fe in the Fe-substituted samples. To quantify the polarization behaviors of the pristine and Fesubstituted samples, the total polarization resistance RP was extracted from the voltage (V) vs. mass current (I) curve (not shown here) in the domain where the voltage bears a linear relationship with current.29 The RP vs. DOD curves of the pristine and Fe-substituted samples are shown in Figure 8. As seen, the RP value increases with increasing DOD, which indicates that the kinetics of the discharge reaction becomes more and more unfavorable during the lithium insertion process. Also, the RP value decreases in the order LiMn1.5Ni0.5O4 > LiMn1.42Ni0.42Fe0.16O4 > LiMn1.5Ni0.42Fe0.08O4 > LiMn1.5Ni0.34Fe0.16O4, which is exactly the reverse order of the rate capability seen in Figure 6. As we pointed out previously,29 RP consists of three components: ohmic resistance RΩ, activation resistance (also called charge transfer resistance) Rct, and diffusion resistance Rd. The relative contribution of these various components to the RP values of the different samples investigated here can be evaluated from the electrochemical impedance spectroscopy and lithium ion diffusion kinetics studies presented below. Electrochemical Impedance Spectroscopy (EIS). To gain a further understanding of the polarization behaviors, electrochemical impedance spectroscopic (EIS) studies were carried out on both the pristine and Fe-substituted samples. Before the EIS measurements, all the samples were charged to 50% SOC (state of charge) after 5 charge-discharge cycles at C/6 rate to obtain an identical status. The EIS spectra of the pristine and Fe-substituted samples are compared in Figure 9. A possible equivalent circuit for the 5 V spinel cathode has been proposed in our previous study.29 The first semicircle (at the highfrequency region) is ascribed to lithium ion diffusion through the surface layer (SEI layer in this study), the second semicircle (at the medium-to-low frequency region) is assigned to charge transfer reaction, the intercept of the first semicircle with the ZRe axis at very high frequency refers to RΩ, and the slope (at the low-frequency region) is attributed to lithium ion diffusion in the bulk material.30 As seen in Figure 9, RΩ is negligible for all four samples, which is probably due to the high content of conductive carbon (20 wt %), while the values of the surface resistance (Rs) and charge transfer resistance (Rct) of the pristine and Fe-substituted samples differ significantly as seen in Table 2. It should be noted that the Rs and Rct values include the resistances of the surface film and charge transfer from both the cathode and Li foil. The contribution of Li foil to both Rs and Rct can be assumed identical for all the samples since Li foil undergoes the same electrochemical process before the EIS measurement in all cases. Thus, the Rs and Rct values obtained provide a relative measure of the

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Figure 5. Discharge profiles of the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples at various C rates.

Figure 6. Normalized discharge capacity values of the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples at various C rates.

resistances of the various cathodes, and they can be used to compare the surface resistance and charge transfer resistance of the different cathode samples. Clearly, Rs decreases in the order LiMn1.5Ni0.5O4 > LiMn1.5Ni0.42Fe0.08O4 > LiMn1.42Ni0.42Fe0.16O4 ≈ LiMn1.5Ni0.34Fe0.16O4, which is in the reverse order of the amount of Fe in the material. The results suggest that the partial substitution of Fe may help to form thinner SEI layers. Although the lithium ion diffusion process in the SEI layer can cause polarization, this step cannot be the controlling factor of rate capability due to the extremely short lithium diffusion length through the SEI layer.29 Rct decreases in the order LiMn1.5Ni0.5O4 > LiMn1.42Ni0.42Fe0.16O4 > LiMn1.5Ni0.42Fe0.08O4 > LiMn1.5Ni0.34Fe0.16O4, which is in the reverse order of the rate capability. The results suggest that charge transfer reaction could be one factor controlling the rate capability. Li Ion Diffusion in the Bulk Material. Although the order in Rct agrees well with the reverse order in rate capability, this does not mean that rate capability is related to only Rct. Since

Figure 7. dQ/dV vs. voltage curves of LiMn1.5Ni0.5O4 and LiMn1.5Ni0.34Fe0.16O4.

solid state diffusion is always a slow step at room temperature, lithium ion diffusion in the bulk materials could also play an important role in deciding the rate capability. The lithium ion diffusion coefficient in the bulk of LiMn1.5Ni0.5O4 has been determined by Kim et al.20 using GITT measurement. However, Deiss31,32 claimed that lithium ion diffusion coefficients measured by GITT and PITT are spurious due to the neglect of the finite kinetics of the charge transfer reaction in the theories of both GITT and PITT. To avoid the influence of charge transfer kinetics on the measurement of lithium ion diffusion coefficient, chronoamperometry with a large potential step (from the OCP of the fully charged state to 3.6 V) was applied in this work. In this case, charge transfer kinetics is too fast to be a rate determining step as the rate of the charge transfer reaction increases exponentially with overpotential.33 Figure 10a shows the chronoamperometry of pristine and Fe-substituted samples. Current responses in

Fe-Substituted 5 V Spinel Cathode LiMn1.5Ni0.5O4

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Figure 8. Relationship between the total polarization resistance (RP) and depth of discharge (DOD) for the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples.

Figure 10. (a) Chronoamperometry and (b) relationship between ln (-I) and t for the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples. The cathodic current is defined negative here.

Figure 9. EIS spectra of the pristine LiMn1.5Ni0.5O4 and the Fesubstituted samples.

Figure 10a can only be ascribed to lithium ion diffusion in the bulk of the spinel sample. Although SEM images (not shown here) indicated octahedral shape particles for all the samples investigated here, we assume spherical particles for simplicity in the calculation. Diffusion current I for spherical particles can be expressed by the following equation in the long-time domain (t . r2/Dπ2),34

I)-

2nFADC 0 exp[-(Dπ 2 /r 2)t] r

(1)

where n is the number of electrons involved in the charge transfer reaction, F is Faraday constant, A is the real surface area of the electrode, D is the average diffusion coefficient of lithium ions, C0 is the concentration of lithium ions in the fully discharged spinels, r is the diffusion length of

lithium ion in the spherical particle (∼1.0 µm as indicated in the Experimental Section), and t is the diffusion time. Equation 1 can be rewritten as

(

ln(-I) ) ln

)

2nFADC 0 - (Dπ 2 /r 2)t r

which indicates a linear relationship between ln I and t, and D can be obtained from the slope (-[Dπ2/r2]). Figure 10b shows the relationship between ln(-I) and t in the time domain of 2800-3600 s. The good linear relationship confirms that this large-potential-step discharge process is controlled by lithium ion diffusion in the bulk spinel. The values of the slopes (-[Dπ2/r2]) and the average diffusion coefficient (D) are listed in Table 2. The D values measured in this work range from 4.92 × 10-13 to 7.48 × 10-13 cm2 s-1, which are close to that (2.8 × 10-13 cm2 s-1) measured in the 4 V spinel LiMn2O4 by Deiss.31,32 Actually, Dπ2/r2 is the reciprocal of the critical diffusion time tc (tc ) r2/Dπ2), defined as the time needed for a species diffusing from the center to the boundary of a spherical particle, and so it can be used to quantitatively compare the diffusion

TABLE 2: EIS Data and Lithium Ion Diffusion Kinetic Parameters of the Pristine and Fe-Substituted LiMn1.5Ni0.5O4 Rs (ohm g) Rct (ohm g) -(Dπ2/r2) (10-4 s-1) D (10-13 cm2 s-1)

(2)

LiMn1.5Ni0.5O4

LiMn1.5Ni0.42Fe0.08O4

LiMn1.42Ni0.42Fe0.16O4

LiMn1.5Ni0.34Fe0.16O4

0.095 0.156 -4.85 4.92

0.087 0.100 -6.35 6.44

0.070 0.115 -6.28 6.37

0.066 0.890 -7.36 7.48

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Figure 11. XPS of the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples before and after 5 min of sputtering.

TABLE 3: Comparison of the Surface and Bulk Concentrations of Mn, Ni, and Fe in the Pristine and Fe-Substituted LiMn1.5Ni0.5O4 LiMn1.5Ni0.5O4

LiMn1.5Ni0.42Fe0.08O4

LiMn1.42Ni0.42Fe0.16O4

LiMn1.5Ni0.34Fe0.16O4

quantity

Mn

Ni

Mn

Ni

Fe

Mn

Ni

Fe

Mn

Ni

Fe

nominal % surface % bulk %

75.0 77.8 75.2

25.0 22.2 24.8

75.0 73.5 74.9

21.0 16.4 20.9

4.0 10.1 4.2

71.0 68.7 71.0

21.0 16.2 21.0

8.0 15.1 8.0

75.0 72.8 71.1

17.0 10.5 20.9

8.0 16.7 8.0

rates of spherical particles with different D and/or r values. For the pristine and Fe-substituted samples, both D and Dπ2/r2 increase in the order LiMn1.5Ni0.5O4 < LiMn1.42Ni0.42Fe0.16O4 < LiMn1.5Ni0.42Fe0.08O4 < LiMn1.5Ni0.34Fe0.16O4, which is the same as the order found with the rate capability. The results indicate that lithium ion diffusion in the bulk spinel is the other important factor controlling the rate capability. Electrons also migrate accompanying the lithium ion diffusion in the bulk material to maintain local charge neutrality. Extensive work carried out by Amatucci et al.35,36 demonstrated electronic conductivity (i.e., rate of electron migration) has great influence

on the rate capability of 5 V spinels. Therefore, it can be speculated that the rate of electron migration may affect the results of the D measurement. Amatucci et al.35,36 also proved that high electronic conductivity results from the Mn3+ ions in lattice, and the larger the amount of Mn3+, the better the electronic conductivity. The amount of Mn3+ can be calculated from the discharge capacity values observed in the 4 V region and the results are given in Table 1. Table 1 shows that the amount of Mn3+ increases in the order LiMn1.5Ni0.5O4 < LiMn1.42Ni0.42Fe0.16O4 < LiMn1.5Ni0.42Fe0.08O4 < LiMn1.5Ni0.34Fe0.16O4. Interestingly, the D value increases following a similar order.

Fe-Substituted 5 V Spinel Cathode LiMn1.5Ni0.5O4 The results imply that the rate of electron migration is one factor (but not necessarily the only factor) controlling the D values. Kim et al.20 also mentioned that the changes in the lithium ion diffusion coefficients of cation-substituted 5 V spinels may be caused by both the changes in the phase transition and in metal-oxygen bonding. The higher lithium ion diffusion coefficient in the Fe-substituted samples might be due to (i) the increased rate of electron migration induced by a larger amount of Mn3+, (ii) suppression of the phase transition (from ordered structure to disordered structure) by the partial substitution by Fe, and (iii) changes in the metal-oxygen bonding in the host structure. A stronger Fe-O bond (390.4 ( 17.2 KJ/ mol) compared to the Ni-O bond (382.0 ( 16.7 KJ/mol) would result in a stronger total metal-oxygen bonding in the host lattice, resulting in weaker Li-O bonding and consequently easier lithium ion diffusion in the lattice.37 X-ray Photoelectron Spectroscopic (XPS) Study. Normally, both surface and bulk properties such as chemical state and composition can have great influence on the electrochemical performances. However, not much attention has been focused to compare the surface and bulk properties of the 5 V spinels cathodes. XPS combined with sputtering could be used to compare the surface and bulk properties. Figure 11 compares the XPS spectra of Mn, Ni, and Fe in the pristine and Fe-substituted samples before and after 5 min of sputtering. The Fe 2p3/2 peaks appear at ∼711.8 eV in all the Fe-substituted samples, indicating the presence of Fe as Fe3+ in these samples,38 which is in good agreement with the study of Ohzuku et al.27 The sputtering depth in this work was estimated to be a few tens of nanometers by using Si as the sputtering reference sample. There is no obvious peak shift before and after sputtering for all the investigated elements, indicating that the chemical states of the surface elements are similar to those of the bulk elements. However, the relative peak intensity changes after sputtering, suggesting a difference in the relative concentrations of the investigated elements on the surface and in the bulk. The relative concentrations of each transition metal ion before and after sputtering are given in Table 3. Interestingly, while the relative concentrations of the elements in the bulk (after sputtering) are very close to the nominal values, those on the surface are different from the nominal concentrations. Specifically, a Fe-enrichment and Ni-deficiency on the surface of the Fe-substituted samples were observed. We believe the Feenrichment on the surface may be due to the formation of a passive Fe2O3 layer on the surface that can alleviate electrolyte decomposition at high voltage and prevent the formation of thick SEI layer. This might be another reason for the better cyclability of the Fesubstituted samples compared to that of the pristine LiMn1.5Ni0.5O4. Additionally, the less-developed insulating SEI layer on the Fesubstituted sample compared to that with the pristine sample leads to lower surface resistance and faster charge transfer kinetics as seen earlier with the EIS data. Conclusion The partial substitution of Fe for Ni alone or for both for Ni and Mn in the 5 V spinel cathode LiMn1.5Ni0.5O4 has been found to significantly improve the cycling performance and rate capability. Especially, the LiMn1.5Ni0.42Fe0.08O4 sample with a substitution of Fe for Ni alone delivers a capacity of 136 mAh/g at C/6 rate with a capacity retention of 100% in 100 cycles and a remarkably high capacity of 106 mAh/g at 10 C rate. Through systematic investigation, it is found that the superior electrochemical performance of the Fe-substituted samples is due the stabilization of a cationdisordered structure, surface enrichment by Fe, suppression of the formation of thick SEI layers, and much lower polarization

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