Surface-segregated, high-voltage spinel lithium-ion battery cathode material LiNi0.5Mn1.5O4 cathodes by aluminium doping with improved high-rate cyclability

Journal of Alloys and Compounds 703 (2017) 289e297

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Surface-segregated, high-voltage spinel lithium-ion battery cathode material LiNi0.5Mn1.5O4 cathodes by aluminium doping with improved high-rate cyclability Ying Luo a, b, c, Taolin Lu a, b, Yixiao Zhang b, c, Liqin Yan b, c, Samuel S. Mao d, Jingying Xie a, c, d, * a

Department of Applied Chemistry, Harbin Institute of Technology, Harbin, 150001, China Shanghai Power & Energy Storage Battery System Engineering Tech. Co. Ltd., Shanghai, 200241, China Shanghai Engineering Center for Power and Energy Storage Systems, Shanghai, 200245, China d Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 September 2016 Received in revised form 18 January 2017 Accepted 22 January 2017 Available online 25 January 2017

A series of Al-doped Li1þxNi0.5AlxMn1.5-xO4 cathode materials for lithium ion batteries are successfully synthesized via a co-precipitation method. Raman spectroscopy studies reveal that the cation-disordered tends to increase with the content of Al. The data of X-ray photoelectron spectroscopy (XPS) depth profiles reveal that the inert Al3þ ions segregates preferentially to the surface during the synthesis process. The results of the electrochemical tests suggest that the substitution of a small amount of Al has the ability to improve the rate capability of LiNi0.5Mn1.5O4 spinel with conventional electrolytes. Especially, the Al-doped (x ¼ 0.06) sample delivers a long cycle-life at high rate (20 C). The enhanced performance is attributed to the formation of Al-enriched surface, providing a more stable interface with the electrolyte at high voltage (~4.7 V), along with the stabilization of the spinel structure with a disordering of the cations and improved Li-ion diffusion, based on results of the potentiostatic intermittent titration technique. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lithium ion battery LiNi0.5Mn1.5O4 cathodes Surface segregation High-rate cyclability

1. Introduction Developing lithium ion battery with high energy density is one of the key challenges for the electric vehicle (EV). In generally, there are two ways to improve the energy density of lithium ion battery. One is developing high capacity materials, including cathodes [1] and anodes [2e8]; another is increasing the voltage of lithium ion battery through adopting a high potential cathode [9,10] or low potential anode. The spinel LiNi0.5Mn1.5O4 is considered to be a promising cathode material for lithium ion battery owing to its high working voltage at 4.7 V, making its energy density (650 Wh kg1) 30% higher than that of conventional LiMn2O4 and LiFePO4 cathodes [11e13]. So far, the synthesis routes available for LiNi0.5Mn1.5O4 can be divided into solid-state method [14] and wet chemical method, such as sol - gel [15], co-precipitation [16],

* Corresponding author. Department of Applied Chemistry, Harbin Institute of Technology, Harbin, 150001, China. E-mail address: [email protected] (J. Xie). http://dx.doi.org/10.1016/j.jallcom.2017.01.248 0925-8388/© 2017 Elsevier B.V. All rights reserved.

microwave assist method [17], and hydrothermal synthesis [18,19]. In terms of electrochemical performance, LiNi0.5Mn1.5O4 provides good, reversible cyclic performance and rate capability [20]. However, due to the working voltage over stability voltage window of electrolyte, there are undesirable side reactions at electrolyte/electrode interface, especially in the charged state where Ni2þ is oxidized to Ni4þ, resulting in significant capacity fading [21,22]. In order to improve the electrochemical performance, one of the commonly used methods is to coat the surface with various compounds: Al2O3, MgF2, LaF3, Li4SiO4, RuO2, Li7La3Zr2O12, Li1.3Al0.3Ti1.7(PO4)3, Fe2O3, or carbon nanotube [23e31]. Most of the time, the role of surface coating layer is to prevent the electrode material from direct contacting with electrolyte, suppress the side reactions occurred at the electrolyte/ electrode interface, and enhance the cycling stability and safety of the electrodes. Since most of the coating layers are not good conductors, surface coating with fast ionic conductors or by simple techniques with low cost methods should be developed. In addition, a uniform surface coating around the whole LiNi0.5Mn1.5O4 particle is also obviously difficult to achieve. Another useful

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modification method for LiNi0.5Mn1.5O4 is to partially substitute Ni and/or Mn with elements such as Co, Ti, Cu, Fe, Cr, or Ru [32e37]. The improved cyclic performance after substitution is related to the enhancement of structural stability, which leads to a smaller change in the lattice parameter during the charge-discharge process. Meanwhile, Manthiram et al. [38,39] observed that some of doped-cation such as Cr, Fe, Ga, Al could segregate preferentially to the surface of particle, which suppresses the formation of the SEI layer and improves the cycle life at elevated temperatures. In previous research, doping with aluminium has been an effective way to improve the electrochemical properties of LiNi0.5Mn1.5O4, due to the increased electrical conductivity with Al doping [40,41]. Substitution of Mn site by Al in the spinel structure was first reported for LiMn2O4, which improved the capacity retention and cyclic performance at elevated temperature [42,43]. Chen et al. [44] reported that the Mn-substituted sample Li1.05Ni0.5Mn1.45Al0.05O4 presented excellent cyclic performance and rate capability. In this paper, we synthesize a series of Li1þxNi0.5AlxMn1.5-xO4 cathode materials via a co-precipitation method, and Al doping offers a more stable electrode/electrolyte interface. The content of Al is controlled from 0 to 0.09 and the content of Li is altered to keep charge neutrality. We show here that the Al-doped spinels exhibit remarkable high-rate cyclability with conventional electrolytes. The systematic characterization and comparison of the physical and electrochemical properties of the pristine and Al-doped samples are presented to understand the origin of the superior performance. 2. Experimental 2.1. Materials synthesis For the preparation of Li1þxNi0.5AlxMn1.5-xO4 spinel samples, the raw materials, including NiSO4$6H2O (99%, AR), MnSO4$5H2O (99%, AR), Na2CO3 (99%, AR), Li2CO3 (99%, AR) and nano-Al2O3 (99%, AR, 20 nm), were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Polyvinylidene fluoride (PVDF) was purchased from Solvay Ltd. Super P (SP) was purchased from Timcal Ltd. N-methyl2-pyrrolidone (NMP) was purchased from Mitsubishi Chemical Corporation. The Li1þxNi0.5AlxMn1.5-xO4 (0  x  0.09) samples were synthesized by a co-precipitation method. An aqueous solution (1 mol L1, 0.3 L) of a stoichiometric amount of NiSO4$6H2O, and MnSO4$5H2O was pumped into a continuously stirred tank reactor (CSTR, 1 L). At the same time, an aqueous solution of Na2CO3 (1 mol L1, 0.3 L) was also pumped into the reactor as a precipitation agent. The pH and temperature of the mixture in the reactor were maintained at 8 and 60  C, respectively. After vigorous stirring for 15 h, the nickel manganese carbonates were obtained. The powder was filtered and washed using distillated water, and then the particles were dried at 100  C in an oven for 24 h in air. The particles thus obtained were calcined at 600  C in air for 5 h, and subsequently cooled down to room temperature. The intermediate products were mixed with a stoichiometric amount of Li2CO3 and Al2O3. After that, the mixtures were first calcined at 500  C for 5 h, subsequently calcined at 900  C for 20 h, and finally annealed at 700  C for 15 h in air. 2.2. Characterization The surface morphology of the samples was observed using scanning electron microscopy (SEM, HITACHI S-4800) with an energy dispersive spectrometer (EDS) for element analysis and transmission electron microscopy (TEM, JEOL JEM-2100F) operated at an accelerating voltage of 200 kV. The crystalline structure

characterization was performed using an x-ray diffractometer (XRD, D8 X) with Cu Ka as the irradiation wavelength. The scanning range was from 10 to 80 . The lattice constants and the phase contents were calculated from the XRD data by Rietveld method using Jade 5.0 software. Raman spectra were obtained using a Renishaw in Via Reflex system with a 780 nm excitation wavelength. The contents of Ni, Mn, Al elements for the Li1þxNi0.5AlxMn1.5-xO4 samples were analyzed with inductively coupled plasma (ICP, JY2000ULTRACE). X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250 Xi system (Thermo Scientific) with Al Ka radiation (hn ¼ 1486.6 eV) to assess the chemical state and concentration of the metal ions both on the surface and in the bulk of samples. The pass energy was fixed at 30.0 eV to ensure sufficient resolution and sensitivity. Sputtering was performed with an argon ion beam gun operating at 3 kV with a spot size of 1  1 mm2. All spectra were calibrated using the C1s photoemission peak at 284.6 eV to correct for the charging effect. Xray absorption spectroscopy (XAS) was carried out using the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF, China). XAS spectra on electrodes were recorded in transmission mode for both Mn and Ni K-edges. The Demeter software package based on IFEFFIT was used to process the spectra. The active material Li1þxNi0.5AlxMn1.5-xO4, polyvinylidene fluoride as a binder, and Super P as conductive carbon were mixed together with a weight ratio of 80:10:10. This mixture was then blended with NMP (ca. 20 wt%) to form a paste-like material. After 8 h of an ultrasonic dispersion, the paste-like material was coated uniformly on aluminum foil and dried at 80  C for 4 h in a blower oven followed by 24 h at 60  C in a vacuum oven. After that, the cathode was formed by pressing the coated aluminum foil using a roller. The mass loading of the composite electrodes for all samples is ~3 mg cm2, and the final electrode thickness is 15 mm, not including the thickness of the Al foil support. CR2016-type coin cells were assembled in an Ar-filled glove box using the aforementioned coated aluminum foil as the cathode, and a lithium wafer as the anode, Celgard 2400 (polypropylene) as the separator and a mixed solution of 1.2 M LiPF6 inethylene carbonate (EC) and ethylmethyl carbonate EMC (3:7, v/v) as the electrolyte solution. The cells were galvanostatically cycled on a multi-channel battery cycler (LAND, CT2001A, Wuhan) at room temperature in a voltage window from 3.5 to 4.9 V at a various C-rates (1 C ¼ 147 mA g1). The performances at high temperature were tested at 55  C by laying the cells in an oven. The cyclic voltammetry (CV) of Li1þxNi0.5AlxMn1.5-xO4/Li cells were measured on a Solarton SI1260 electrochemical workstation at a scan rate of 0.1 mV s1 from 3.5 to 4.9 V. In the PITT measurements, a potential step of 10 mV was applied after the electrode had reached equilibrium as evidenced by a residual current of less than 2 mA. The voltage window was set from 4.6 to 4.8 V. Electrochemical impedance spectroscopic (EIS) data were collected with an ac amplitude of 5 mV in the frequency range of 100 kHz to 0.01 Hz by a Solartron SI1260 electrochemical workstation. The parameter values of the equivalent circuits for the impedance spectra were determined by computer simulation using Zsimpwin electrochemical impedance software. 3. Results and discussion The morphologies of the Li1þxNi0.5AlxMn1.5-xO4 samples are shown in Fig. 1. All of the samples consist of irregular polyhedral particles. The morphologies suggest that the samples are wellcrystallized spinels. A gradual reduction in size is observed as the content of Al increases. From the results of the EDS mapping (see Fig. S1 ~ S4), we can see that the distribution of the element Al, Ni, and Mn is uniform in the Li1þxNi0.5AlxMn1.5-xO4 samples. The

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Fig. 1. SEM (a, c, e, g) and TEM (b, d, f, h) images of the Li1þxNi0.5AlxMn1.5-xO4 samples: (a, b) x ¼ 0; (c, d) x ¼ 0.03; (e, f) x ¼ 0.06; (g, h) x ¼ 0.09.

contents of Ni, Mn, and Al of the Li1þxNi0.5AlxMn1.5-xO4 samples are shown in Table 1. The Mn/Al ratios of all samples are consistent with the value of experimental design. A slight lower Ni content is observed, which may be associated with the higher solubility product of NiCO3, resulting in more difficult precipitation compared with MnCO3. Fig. 2a shows the XRD patterns of the Li1þxNi0.5AlxMn1.5-xO4

powders. All of the samples exhibit a pure (x ¼ 0) or a main phase of the typical cubic spinel diffraction pattern (JCPDS No. 80-2162). The lattice parameters of the Li1þxNi0.5AlxMn1.5-xO4 samples are shown in Table 1. It can be seen that the lattice parameter of Li1þxNi0.5AlxMn1.5-xO4 increases with the content of Al. This is attributed to the increase of larger Mn3þ ions from the latter result of CV. A weak reflection observed at 2q ¼ 43.6 in Al-doped samples are

Table 1 The contents of Ni, Mn, Al and the lattice parameter of the Li1þxNi0.5AlxMn1.5-xO4 samples. Samples

Mn/ppm

Al/ppm

Ni/ppm

Mn/at%: Al/at%: Ni/at%

lattice parameter (Å)

x¼0 x ¼ 0.03 x ¼ 0.06 X ¼ 0.09

44.342 42.210 42.540 43.180

0 0.425 0.912 1.336

15.965 14.870 15.350 15.830

1.495: 1.466: 1.442: 1.414:

8.1659 8.1698 8.1752 8.1787

0: 0.486 0.030: 0.483 0.063: 0.487 0.089: 0.485

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Fig. 2. XRD patterns (a) and Raman spectra (b) of the Li1þxNi0.5AlxMn1.5-xO4 powders.

from the impurity phase, which is a cation-rich rock-salt phase. Zhong et al. [45] first suggested the rock-salt phase is LixNi1-xO and proposed that it was unlikely that the rock-salt phase contains Mn, by comparison of lattice parameters of NiO and MnO. Cabana et al. [46] found that the rock-salt phase contained Mn with a composition of (LixMn0.66Ni0.34)yO by transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). Because of the close coherent scattering lengths of Ni and Mn, it is difficult to distinguish between the Ni and Mn atoms using XRD. Depending on the ordering of Ni in the lattice, LiNi0.5Mn1.5O4 has two different space groups including a disordered Fd3m structure and an ordered P4332 structure [47]. To distinguish clearly the P4332 structure from Fd3m structure, Raman spectra of the Li1þxNi0.5AlxMn1.5-xO4 composites are shown in Fig. 2b. The 399 and 490 cm1 bands are associated with the Eg and F(2) 2g vibration modes of the Ni2þeO bond. The acromion at 588 cm1 is associated with the symmetric Mn4þeO stretching vibration (F(1) 2g ). The band at 632 cm1 can be unequivocally assigned to the symmetrical MnO6 octahedral stretching vibration (A1g). For the pristine, some new bands at 218 and 240 cm1 are detected and the bands at 399 and 490 cm1 become strong. Meanwhile, the F(1) 2g band splits into two components, suggesting that the structure is in accordance with space group P4332. For the Al-doped (x ¼ 0.03) sample, the splitting 1 of the F(1) are still 2g band and the bands around 218 and 240 cm observed, which is considered to be clear evidence of the ordered structure (P4332). With the increasing content of Al, the intensities of all these bands become gradually lower and the bands around 399, 240 and 218 cm1 almost disappear. These results suggest that the introduction of Al increases the content of the disordered phase

in the spinel materials. According to a study by Strobel et al. [48], the charge difference between Mn and other metal (M) atoms is the main driving force for such an octahedral cation disordering in Li2Mn3MO8. Therefore, the disordering degree of Ni/Mn ions is increased with the content of Al. It is known that the disordered spinel structure exhibits better electrochemical performances due to higher electronic conductivity and lithium diffusion coefficient, compared with the ordered spinel structure [47]. Fig. 3 compares the elemental depth profiles of the Li1þxNi0.5AlxMn1.5-xO4 recorded with XPS. XPS spectra of the Li1þxNi0.5AlxMn1.5-xO4 samples with different sputtering times are shown in Fig. S5 ~ S8. As seen, the relative concentrations of the elements Al, Ni, and Mn in the bulk (after sputtering for 200 s) are very close to the nominal values, while Al - enrichment and Ni deficiency on the surface of the Al-doped samples are observed, indicating a self-surface segregation of Al during the synthesis process. The pristine and Al-doped (x ¼ 0.06) samples are also analyzed using X-ray absorption near edge spectra (XANES) regions of the XAS spectrum to study the influence of Al doping on the oxidation state of Mn and Ni. The XANES spectra of both pristine and Aldoped (x ¼ 0.06) samples at the Mn and Ni K-edges are represented in Fig. 4a and b. The position of the X-ray absorption edge can be correlated to the oxidation state of the absorbing atom. In principle, with an increasing valence state of the absorbing atom, the position of the absorption K-edge should systematically shift to higher energy and vice versa [49,50]. However, since the outer porbits are more sensitive to chemical changes, the difference in the position of the 1s/4p peak is more pronounced [51]. Therefore, as shown in Fig. 4a, by comparing the XANES region of the Mn K-edge with the reference material MnO2, it may be inferred that the average valence state of Mn in the pristine is close to þ4. However, the position of the 1s/4p peak at the Mn K-edge of the Al-doped (x ¼ 0.06) sample shifts to lower energy, suggesting that the average valence state of Mn is lower than that of the pristine. It can be inferred that a higher content of Mn3þ ions is observed for the Al-doped (x ¼ 0.06) sample. In contrast, there is no obvious change in the position of the 1s/4p peaks of the Ni K-edge (see Fig. 4b), suggesting a similar oxidation state for the pristine and Al-doped (x ¼ 0.06) samples. The CV curves of the Li1þxNi0.5AlxMn1.5-xO4 samples are shown in Fig. 5. There are two main regions of electrochemical activity at ~4.7 V and ~4.0 V. The activity near 4.7 V is assigned to the Ni2þ/ Ni3þ and Ni3þ/Ni4þ redox couples, whereas the activity near 4.0 V is due to the Mn3þ/Mn4þ redox couple. It is interesting to notice that the discrepancy between these peaks has a clear relationship to the Al content. When x  0.06, the two redox peaks near 4.7 V from the Ni2þ/Ni3þ and Ni3þ/Ni4þ redox couples are clearly observed. However, there is only one redox peak observed near 4.7 V for the samples with x  0.03. Gu et al. [52] indicated that the voltage difference near 4.7 V from the Ni2þ/Ni3þ and Ni3þ/Ni4þ redox couples in the space group Fd3m is greater than that in the space group P4332. Therefore, this further demonstrates that the disordering degree of Ni/Mn ions is increased with the content of Al in the spinel. Furthermore, as shown in the inset of Fig. 5d, the intensity of the peak around 4.0 V is gradually increased, indicating that the content of Mn3þ ions also increases with the content of Al. Amatucci et al. [53] had proved that high electronic conductivity results from the Mn3þ ions in lattice, and the larger the amount of Mn3þ, the better the electronic conductivity. The cyclic performances of the Li1þxNi0.5AlxMn1.5-xO4 samples at 1 C and 55  C are shown in Fig. 6a. The pristine delivers the largest initial discharge capacity of 136 mAh g1 with the capacity retention of 92% after 100 cycles. In contrast, the initial discharge capacity is decreased due to the introduction of Al, which may be

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Fig. 3. The elemental depth profiles of the Li1þxNi0.5AlxMn1.5-xO4 samples.

Fig. 4. XANES of the Mn (a) and Ni (b) K-edges for the pristine and Al-doped (x ¼ 0.06) samples, as well as for the reference material MnO2 and NiO, respectively.

ascribed to the formation of a rock-salt impurity phase. However, it is clearly observed that Al doping can improve the cyclic performance of LiNi0.5Mn1.5O4 spinel. The Al-doped (x ¼ 0.06) electrode exhibits improved capacity retention of 96.2% after 100 cycles, which compensates for its lower initial capacity of 125 mAh g1. Although Al doping increases the content of Mn3þ, the enrichment of Al on the surface minimizes the contact of Mn3þ with the electrolyte and thereby suppresses the Mn dissolution, resulting in an improved cycle performance at elevated temperatures (55  C). The rate capabilities of these spinels are evaluated to further investigate the roles of Al doping. Fig. 6b shows the normalized discharge capacity (0.5 C-rate capacity ¼ 100%) of the samples at a current density range from 0.5 C to 20 C (all samples are charged at 0.5 C). The pristine sample exhibits a slow decrease in capacity at low current density. However, a difference in the rate capability is observed at high current density (5 C). At 20 C, the Al-doped (x ¼ 0.06) sample delivers the largest normalized capacity with 87% of the 0.5 C values. In addition, for the Al-doped (x ¼ 0.09) sample, the normalized capacity is decreased, which may be arose from the blocking of migration pathway of Liþ ions in octahedral site [54]. The long-term cyclic performance tests are investigated at 10 C and 20 C rates (all samples are charged at 0.5 C) and the results are shown in Fig. 7a and b. As shown in Fig. 7a, the capacity of the pristine spinel obviously fades with increasing the cycle number. However, Al-doped samples show a better cycling performance at 10 C. The capacity retention of the Al-doped (x ¼ 0.06, 0.09) electrodes are as high as 100% even after 100 cycles. Even at 20 C, the capacity retention of the Al-doped (x ¼ 0.06) electrode is still 84% after 600 cycles, with an initial discharge capacity of 110 mAh g1, while a strong capacity fading of the pristine is apparent after 200 cycles (see Fig. 7b). The improved cyclic performance can be partially attributed to the reinforced crystal structure by the incorporation of Al in the structure. Furthermore, the discharge curves of the pristine and Al-doped (x ¼ 0.06) electrodes at

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Fig. 5. CVs of the Li1þxNi0.5AlxMn1.5-xO4 samples: (a) x ¼ 0; (b) x ¼ 0.03; (c) x ¼ 0.06; (d) x ¼ 0.09; Inset in Fig. 5d is the enlarged CVs between 3.6 and 4.3 V.

different cycle numbers are shown in Fig. 8a and b (both of the samples are charged at 0.5 C and discharged at 20 C). The discharge voltage platform of the Al-doped (x ¼ 0.06) electrode is increased compared with that of the pristine, suggesting that polarization is effectively suppressed by Al doping. The potentiostatic intermittent titration technique (PITT) was performed to identify the apparent chemical diffusion coefficients of the pristine and Al-doped (x ¼ 0.06) samples. The diffusion coefficient can be extracted from the current response assuming a standard cottrell solution for the lithium flux at the surface [55]. The lithium diffusion coefficient, DLi, is determined from the following equation:

IðtÞ ¼

Fig. 6. (a) The cyclic performance of Li1þxNi0.5AlxMn1.5-xO4/Li cells in the range of 3.5e4.9 V at 1 C and 55  C; (b) Normalized capacity values of Li1þxNi0.5AlxMn1.5-xO4/Li cells in the range of 3.5e4.9 V at 0.5e20 C.

 2  2nFADLi C* p DLi L2 exp t ; t[ 2 2 DLi L 4L

The slope of the linear region in the plot of lnI (t) versus t, where I is the current and L is the thickness of the electrode, gives DLi [56]. The example of the current-time transition on the deintercalation process (4.8e4.81 V vs. Li/Liþ) and its plot of ln[I(t)] vs. t for the pristine sample are illustrated in Fig. S9. An approximately linear section is observed over a certain time period in the plot of ln[I(t)] vs. t (Fig. S9b). From the slope of the straight line in the plot of ln[I(t)] vs. t, the chemical diffusion coefficient of the pristine sample at 4.8 V is calculated. Using the same procedure, the chemical diffusion coefficients at various potentials from 4.6 to 4.8 V are obtained. Fig. 9 shows the variation of chemical diffusion coefficients in the pristine and Al-doped (x ¼ 0.06) samples during the deintercalation process. DLi changes with the content of Liþ. The minimum of DLi appears between 4.65 and 4.75 V. Interestingly, the DLi of Al-doped (x ¼ 0.06) sample is one order of magnitude higher than that of the pristine sample. Moreover, it has been previously shown that the existence of Mn3þ improves the electrical conductivity of a disordered Fd3m spinel [47]. Thus, the higher chemical diffusion coefficient and

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Fig. 7. The cyclic performance of Li1þxNi0.5AlxMn1.5-xO4/Li cells at 10 C (a) and Li1þxNi0.5AlxMn1.5-xO4/Li cells (x ¼ 0, 0.06) at 20 C (b).

electronic conductivity lead to the improved high rate capability of Al-doped (x ¼ 0.06) sample. To further understand the underlining mechanisms responsible for the improved high rate cyclability of the Al-doped samples, Fig. 10 compares the EIS spectra of the pristine and Al-doped (x ¼ 0.06) samples after 200 cycles at 20 C. Inset in Fig. 10 is the corresponding equivalent circuit, where RS is the solution resistance, RSEI reflects the surface resistance, and Rct is the charge transfer resistance. Base on the fitting results, both of RSEI and Rct of the Al-doped (x ¼ 0.06) sample are found to be much lower than that of the pristine. These results may suggest that the Alenriched surface of Al-doped (x ¼ 0.06) sample reduces the total resistance by minimizing the side reactions with the electrolyte and suppressing the formation of thick solid-electrolyte interfacial (SEI) layer, which may be another reason for the better cyclability of the Al-doped (x ¼ 0.06) sample compared to that of the pristine.

295

Fig. 8. The discharge curves of the pristine (a) and Al-doped (x ¼ 0.06) (b) samples at different cycle numbers at 20 C.

the content of Al. The cyclic performance at evaluated temperatures (55  C) and the rate capability at 10 C are significantly improved by Al doping without an obvious reduction in capacity at the optimized Al concentration of 0.06  x  0.09. At 20 C, the discharge capacity of the Al-doped (x ¼ 0.06) electrode is 87% of the 0.5 C values, and the capacity retention is 84% after 600 cycles. Through systematic investigation, it is found that the good rate capability of the Al-doped (x ¼ 0.06) sample is attributed to the stabilization of a cation disordered structure, enhanced lithium diffusivity, and the

4. Conclusions Surface characteristics in the high-voltage LiMn1.5Ni0.5O4-based spinels greatly influence both the cyclability and rate capability. Aldoped Li1þxNi0.5AlxMn1.5-xO4 cathode material with good electrochemical properties was prepared via a co-precipitation method. With the introduction of Al, the spinel samples contain small amounts of cation-rich rock-salt impurity, changing the space group from P4332 to Fd3m. The amounts of Mn3þ ions increase with

Fig. 9. Apparent chemical diffusion coefficient of the pristine and Al-doped (x ¼ 0.06) samples calculated from PITT measurement.

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Fig. 10. Electrochemical impedance spectra of the pristine and Al-doped (x ¼ 0.06) samples after 200 cycles at 20 C in the frequency range from 100 kHz to 0.01 Hz at room temperature. The inset shows the equivalent circuits for the EIS measurements.

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