Electrochemical Performance of a Layered-Spinel Integrated Li [Ni1

Article pubs.acs.org/cm

Electrochemical Performance of a Layered-Spinel Integrated Li[Ni1/3Mn2/3]O2 as a High Capacity Cathode Material for Li-Ion Batteries Prasant Kumar Nayak,† Judith Grinblat,† Mikhael D. Levi,† Ortal Haik,† Elena Levi,† Michael Talianker,‡ Boris Markovsky,† Yang-Kook Sun,§ and Doron Aurbach*,† †

Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel § Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea ‡

S Supporting Information *

ABSTRACT: Li[Ni1/3Mn2/3]O2 was synthesized by a self-combustion reaction (SCR), characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy, and studied as a cathode material for Li-ion batteries at 30 °C and 45 °C. The structural studies by XRD and TEM confirmed monoclinic Li[Li1/3Mn2/3]O2 phase as the major component, and rhombohedral (LiNiO2), spinel (LiNi0.5Mn1.5O4), and rock salt Li0.2Mn0.2Ni0.5O as minor components. The content of the spinel phase increases upon cycling due to the layered-to-spinel phase transition occurring at high potentials. A high discharge capacity of about 220 mAh g−1 is obtained at low rate (C/10) with good capacity retention upon cycling. However, LiNi0.5Mn1.5O4 synthesized by SCR exhibits a discharge capacity of about 190 mAh g−1 in the potential range of 2.4−4.9 V, which decreases to a value of 150 mAh g−1 after 100 cycles. Because of the presence of the spinel component, Li[Ni1/3Mn2/3]O2 cathode material exhibits part of its capacity at potentials around 4.7 V. Thus, it can be considered as an interesting high-capacity and high-voltage cathode material for high-energy-density Li-ion batteries. Also, the Li[Ni1/3Mn2/3]O2 electrodes exhibit better electrochemical stability than spinel LiNi0.5Mn1.5O4 electrodes when cycled at 45 °C. materials.11−22 The first step has to involve polarization to potentials >4.5 V vs Li, in order to activate the Li2MnO3 phase. These composite cathode materials undergo capacity fading as well as voltage decay upon cycling, due to structural transformation of the layered material to a spinel phase.23−25 In fact, the spinel structure can be considered as the most stable one for Li−Mn−O materials.26 However, the operation of spinel materials such as LiNi0.5Mn1.5O4 over a wide potential range causes irreversible, detrimental structural changes that lead to an inevitable loss of capacity. Li- and Mn-rich cathode materials that initially contain Li2MnO3 must operate over a wide potential range, 2.5−4.8 V, in order to extract their high capacity. However, since these materials undergo the layeredto-spinel transition, this means an obvious increase in activity around 3 V on account of activity at higher potentials upon cycling. This results in a gradual lowering of the average voltage upon cycling, as more spinel phase is formed. Li2MnO3 can be represented as Li[Li1/3Mn2/3]O2, where 1/3 of the total Mn-ions in the transition metal layer is replaced by

1. INTRODUCTION Rechargeable Li-ion batteries are considered as high-energy density electrochemical power sources that can be suitable to propel electric vehicles. Their energy density can be increased by using high-capacity electrode materials.1−5 The layered Liand Mn-rich oxide Li2MnO3 can be considered as a cathode material, because of its high specific capacity that can reach practical values up to 250 mAh g−1 after activation by polarization to high potentials (>4.7 V vs Li).6−9 This activation involves remarkable structural changes in which delithiation and oxygen evolution are involved. The final specific capacity that can be obtained depends on the particle size, morphology, etc., which in turn depends on the synthetic method and also on the annealing temperature.10 Although nanoparticles of Li2MnO3 were found to exhibit high capacity, there is severe capacity fading due to structural instability of activated Li2MnO3 with cycling.6−9 The electrochemical activation of this material by extraction of Li-ions and oxygen can be described as follows: Li 2MnO3 → MnO2 + 2Li+ + 1/2O2 ↑+ 2e−

(1)

Received: February 2, 2015 Revised: March 9, 2015

Many lithiated transition-metal oxides containing initially Li 2 MnO 3 have been studied as high-capacity cathode © XXXX American Chemical Society

A

DOI: 10.1021/acs.chemmater.5b00405 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

powder that was annealed in two steps, at 700 °C for 1 h and then at 900 °C for 15 h in air, resulting in the well-crystallized material. LiNi0.5Mn1.5O4 was synthesized by following the same procedure with the precursors in the appropriate stoichiometric ratio. The elemental chemical analysis of the materials was carried out using the inductive coupled plasma technique (ICP-AES, spectrometer Ultima-2 from JobinYvon Horiba). The X-ray diffraction (XRD) studies were performed with a Bruker Inc. (Germany) AXS D8 ADVANCE diffractometer (reflection θ−θ geometry, Cu Kα radiation, receiving slit 0.2 mm, high-resolution energy-dispersive detector). Diffraction data for the Rietveld refinement were collected in the angular range of 10° < 2θ < 140°, step size 0.02°, step time 1.7 s/step for spinel LiMn1.5Ni0.5O4 and 6 s/step for the integrated material Li[Ni1/3Mn2/3]O2. The data was analyzed by the Rietveld structure refinement program, FULLPROF.34 The structural data for the modeling were taken from previously reported articles for spinel LiNi0.5Mn1.5O4,35 for monoclinic and rhombohedral layered phases,33 and for the rock-salt material.36 The Thompson−Cox−Hastings pseudo-Voigt function was used for the peak-shape approximation. The background was fitted manually by linear interpolation. The morphology of the product was investigated by a scanning electron microscope Magellan XHR 400L FE-SEM-FEI. The characterization of the cathode materials by HRTEM was carried out with a JEOL-JEM 2100 electron microscope with LaB6 emitter operating at 200 kV. Samples for the TEM studies were prepared by dispersing and sonicating the powdered samples in ethanol and adding a few drops of the resulting suspension to a TEM copper grid. Micro-Raman spectroscopy studies of pristine and cycled electrodes were performed using a micro-Raman spectrometer from Renishaw in Via (U.K.), equipped with a 514 nm laser, a CCD camera, and an optical Leica microscope. A 50× objective lens to focus the incident beam and an 1800 lines/mm grating were used. 2.2. Electrode Preparation. The electrodes for electrochemical studies were prepared by making a slurry of 80 wt % active material, 10 wt % of conductive super P carbon, and 10 wt % PVDF binder in Nmethyl-2-pyrrolidinone (NMP) as the solvent. The slurry was coated by using a doctor-blade onto Al foil current collectors, dried at 80 °C for 12 h in an oven. The coated Al foils were then pressed uniformly and then cut into circular electrodes of 14 mm diameter. The electrodes were finally dried at 110 °C for 12 h under vacuum. 2.3. Electrochemical Tests. The electrochemical performance of these composite cathode materials was tested using coin-type cells 2032 (NRC, Canada) assembled in an argon-filled glovebox (MBraun). Li metal foils were used as the counter and reference electrodes. Typical loading of the active mass was 4−5 mg/cm2. A commercial battery electrolyte solution LP 30 (Merck) consisting of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) (1:1 w/w) was used. A porous polypropylene-based membrane (Celgard) was used as the separator. The cells were stored for 12 h at room temperature to ensure the complete impregnation of the electrodes and the separators with the electrolyte solution. Galvanostatic charge− discharge cycling was carried out in the potential range of 2.4−4.9 V vs Li/Li+ using a computerized multichannel battery testing instrument from Arbin, Inc. Electrochemical impedance spectra (EIS) were recorded at various equilibrium potentials during charging by using a Solartron model SI 1287 electrochemical interface and a 1255 HF frequency response analyzer, with an amplitude of 5 mV around equilibrium in the frequency range of 100 kHz to 0.01 Hz. The impedance data were further treated by a nonlinear least-squares (NLLS) fitting procedure. The electrochemical measurements were performed at 30 and 45 °C in thermostats.

Li+. There are various combinations of Mn, Ni, and Co in layered metal oxides, such as LiNi 0 .5 Mn 0 .5 O 2 , LiNi0.4Mn0.4Co0.2O2, and LiNi1/3Mn1/3Co1/3O2, which have been studied as cathode materials for Li-ion batteries.27,28 Lu et al. reported the electrochemical performance of layered Li[NixLi(1/3−2x/3)Mn(2/3−2x/3)]O2 (x = 1/3, 5/12, and 1/2) by substituting Li+ and Mn4+ by Ni2+.29 They reported a maximum specific capacity of about 200 mAh g−1 for this compound with x = 1/3 in the potential range of 2.0−4.6 V at 30 °C.29 They also mentioned that Ni2+ can coexist with Mn4+ in the transition-metal layer of layered materials. However, there are few reports on the structure and electrochemical performance of Li[Ni1/3Mn2/3]O2, which is formed when the excess Li-ions present in the transition-metal layer of Li[Li1/3Mn2/3]O2 are replaced by Ni2+-ions. Recently, Zhao et al. reported the electrochemical performance of LixNi1/3Mn2/3O2 (x > 2/3) with O3 structure, which exhibited a discharge capacity of 230 mAh g−1 in the voltage range of 2.5−4.7 V at 50 °C.30 It is also well-known that the first delithiation of Li- and Mn-rich cathode materials such as Li1.2Ni0.2Mn0.6O2 that contain Li[Li1/3Mn2/3]O2 involves migration of Ni2+ and Mn-cations to the octahedral sites in the Li layer.11 It is worth mentioning that the Li- and Mn-rich cathode materials that were explored in recent years also included integrated composites of layered and spinel compounds.31−33 These layered-spinel composite cathode materials are cycled in a wide potential range of 2.0− 5.0 V with very good cycling stability.32,33 Thus, cathode materials with multiphase composition may be advantageous for their electrochemical performance. In this study, Li[Ni1/3Mn2/3]O2 and LiNi0.5Mn1.5O4 were synthesized by a self-combustion reaction (SCR) and were characterized and studied as cathode materials for Li-ion batteries, over a wide potential range, in order to extract the maximal capacity from these materials. The latter compound was included in the present study as a high voltage Co-free reference cathode material. Using cobalt-free cathode materials has advantages in terms of toxicity and price. The tools for this study included high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical impedance spectroscopy (EIS) in conjunction with standard electrochemical techniques.

2. EXPERIMENTAL SECTION Analytical grade chemicals: Mn(NO3)2 (Fluka), Ni(NO3)2, LiNO3, sucrose, poly(vinylidene fluoride) (PVDF), and 1-methyl-2-pyrrolidinone (NMP) (Aldrich) were used as received. Doubly distilled (DD) water was used to dissolve the metal nitrates and sucrose. 2.1. Synthesis and Structural Characterization. The integrated cathode Li[Ni1/3Mn2/3]O2 was synthesized by a SCR using the precursors of LiNO3, Ni(NO3)2, and Mn(NO3)2, which act as the oxidants, and sucrose acting as the fuel. In the SCR method, nanoparticles are formed initially, and the particle size can be tuned by varying the annealing temperature and the duration of the calcination step. In a typical synthesis, the precursors were taken in the stoichiometric ratio of Ni(NO3)2/Mn(NO3)2/LiNO3 = 1:2:3.3. Excess LiNO3 (10% by wt) was added in order to compensate for the Li loss during high temperature annealing. The metal nitrates (2.326 g of Ni(NO3)2, 4.016 g of Mn(NO3)2, and 1.810 g of LiNO3) were dissolved in 80 mL of DD water. Then sucrose (with metal nitrates to sucrose molar ratio of 1:2) was added to this solution with continuous stirring for about 6 h. The water was evaporated slowly by heating this mixture to produce a syrupy mass, which on further heating at 300 °C led to the self-ignition of the reactants to give the amorphous compound. The powder material was ground finely and annealed at 450 °C for 2 h in air. Again the product was ground to form a fine

3. RESULTS AND DISCUSSION The self-combustion reactions of transition-metal (Mn, Co, and Ni) nitrates and lithium nitrate with sucrose result in amorphous nanoparticles of lithiated metal oxides.37,38 Highly crystalline particles are obtained by the calcination of the amorphous products at elevated temperatures; for example, in the present study, the temperature was 900 °C. Also the B


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Figure 1. Rietveld plots for (a) Li[Ni1/3Mn2/3]O2 and (b) spinel LiNi0.5Mn1.5O4 materials annealed at 900 °C. The calculated pattern is shown by a solid curve; red dots show the observed intensities. The difference between the observed and calculated intensities is presented by a blue curve. The short vertical bars indicate the position of Bragg reflections for the respective phases listed in Table 1.

Table 1. Results of the Rietveld Analysis for the Materials under Study fitting quality sample LiNi1/3Mn2/3O2

LiNi0.5Mn1.5O4

space group

lattice parameters

phase content (%)

Rb (%)

χ2

Li2MnO3

C2/m

77

7.58

5

Li0.2Ni0.5Mn0.2O LiNi0.5Mn1.5O4 LiNiO2

Fm3m ̅ Fd3m ̅ R3̅m

2 4 17

13.21 9.26 7.46

LiNi0.5Mn1.5O4 Li0.3Ni0.5Mn0.2O

Fd3̅m Fm3m ̅

a = 4.939 Å b = 8.547 Å c = 5.039 Å β = 109.24° a = 8.288 Å a = 8.169 Å a = 2.897 Å c = 14.350 Å a = 8.176 Å a = 8.276 Å

99 1

4.37 38.97

phase

2

layered compound based on monoclinic Li2MnO3 and a Ni-rich rhombohedral phase marked as LiNiO2 (Figure 1a). Additionally, spinel (LiNi0.5Mn1.5O4) and rock-salt phase isostructural to LixNi1−xO are present in minor proportions. It should be mentioned that almost all the phases allow for a Ni−Mn substitution, which is difficult to quantify by X-ray analysis. As a result, it is impossible to correlate the phase analysis to the ICP data. The amount and the composition (Li/vacancy ratio) of the rock-salt phase are greatly affected by the synthesis temperature and the general composition of the material. In fact, synthesis of Li[Ni1/3Mn2/3]O2 at 700 °C, instead of 900 °C, results in the increase of the rock-salt component up to 27% (see Supporting Information). The phase composition obtained for the Li[Ni1/3Mn2/3]O2 material is in a good agreement with the existence of the three-phase regions in the phase diagram presented by McCalla et al.36,41 Interestingly, such phase separation can be predicted based on the tolerance factors calculated in a recent work for different LiNi1−yMyO2 systems (M = Li, Mn, Co, Fe, Cr, Al, Mg).42 The large difference in the sizes of Li+ (0.76 Å) or Ni2+ (0.69 Å) and Mn4+ (0.53 Å) located in the same cationic layer of the oxygen close-packing results in high lattice strains and decomposition of the solid solutions formed at high temperature in the Li− Ni−Mn−O system. It is important to note that the multiphase composition and structure of such cathode materials may be advantageous for their electrochemical performance.

particles are coated uniformly with a layer of carbon produced from sucrose decomposition during the synthesis, as reported previously.22 The carbon coating increases the electronic conductivity of the active mass and also mitigates to some extent surface reactions of the oxide materials with the electrolyte solutions. It is well-known that all the lithiated transition metal oxides are reactive with several components in the standard electrolyte solutions based on LiPF6 and alkyl carbonate solvents.39,40 Their surface reactions lead to precipitation of surface films, which may lead to high impedance.18,22,25,40 The ICP analysis of the synthesized material indicated a composition of Li1.05Ni0.31Mn0.64O2, which matches well to the target composition Li[Ni1/3Mn2/3]O2. 3.1. Structure and Morphology. Figure 1a,b shows the Rietveld profiles for the products of the synthesis by SCR. The phase compositions, structural parameters, and R-factors (quality of fitting) are presented in Table 1. As can be seen, the synthesis of LiNi0.5Mn1.5O4 by SCR results in an almost pure spinel phase (Figure 1b). The amount of impurity (rocksalt phase) does not exceed 1%. (In spite of the minor quantity, this cubic phase can be identified by small separate diffraction peaks at 37.6 and 43.7 degrees. Account of this phase in the fitting results in the decrease of the global χ2 parameter from 3.3 to 2.0.) In contrast, the Li[Ni1/3Mn2/3]O2 material crystallizes as a mixture of four different compounds. The material is mainly composed of two components: a Mn-rich C


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Figure 2. SEM images of (a) Li[Ni1/3Mn2/3]O2 and (b) LiNi0.5Mn1.5O4 synthesized by SCR and annealed at 900 °C.

Figure 3. (a) Typical bright field image of the microstructure of the pristineLi[Ni1/3Mn2/3]O2; (b) HRTEM image showing typical lattice fringes belonging to the monoclinic structure (inset) and a clearly visible 4 nm carbon coating; (c) FFT analysis from the area marked by white square the in panel b displaying sets of reflection indexed as the monoclinic phase; (d−g) CBED diffractions from the marked areas in panel a, showing the presence of the monoclinic, rhombohedral, cubic spinel, and the cubic rock salt phases, respectively.

SEM images of these materials are shown in Figure 2. Submicron size particles of 200−500 nm are observed for both samples. Transmission electron microscopic (TEM) studies were performed in order to provide information about the morphology and structure of the synthesized Li[Ni1/3Mn2/3]O2. These studies were performed using the bright field (BF)/dark field (DF) imaging technique, conventional selected-area electron diffraction (SAED) with 300 nm aperture, and convergent-beam electron diffraction (CBED) using a 4 nm convergent beam. Fast Fourier transform analysis (FFT) was applied for analysis of the high resolution images. The structural assignment of the particles was based on analyzing the SAED, the CBED, or the FFT patterns. The information embedded in the reflections in these diffraction patterns (or FFT) in most cases allows the unambiguous determination of the exact structure of the compound. Figures 3 and 4 present the results of TEM analysis of the pristine material. The structural information obtained using TEM methods is in good agreement with the results of the XRD analysis, which showed that the structure of the pristine material can be interpreted on the basis of a four-phase system model, consisting mainly of the

structurally integrated layered monoclinic Li2MnO3 (C2/m) phase. The material also contains three additional minor phases: rhombohedral LiNiO2 (R3̅m), cubic spinel LiNi0.5Mn1.5O4 (Fd3̅m), and cubic rock salt Li0.2Ni0.5Mn0.2O (Fm3̅m). The calculations were based on data from ICDD 2010: PDF# 01−084−1634, a = 4.9371 Å, b = 8.52 Å, c = 5.03 Å, β = 109.46° for the monoclinic phase; PDF#00−009−0063, a = 2.878 Å, c = 14.190 Å for the rhombohedral phase; PDF# 01−080−2185, a = 8.2770 Å for the cubic spinel phase; and for the cubic rock salt we used a cubic (Fm3̅m) model with a = 8.288 Å. Figure 3a shows a typical bright field image of a few agglomerated nanoparticles of Li[Ni1/3Mn2/3]O2. Probing the local structure of nanoparticles using the CBED technique indicates the presence of four structurally compatible phases: monoclinic Li2MnO3 (C2/m) as the major phase and three additional minor phases: rhombohedral LiNiO2 (R3̅m), cubic spinel LiNi 0.5 Mn 1.5 O 4 (Fd3̅ m ), and cubic rock salt Li0.2Ni0.5Mn0.2O (Fm3m ̅ ). Examples of CBED patterns (Figure 3d−g), obtained from the areas marked by white arrows on the nanoparticle (Figure 3a), show sets of reflections that were uniquely indexed on the basis of the monoclinic unit cell D


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coating (thickness ≈ 4 nm), which is most probably carbon. It is a known fact that the use of sucrose in the synthesis facilitates the formation of a thin surface layer of carbon.22,43 Figure 4b shows the SAED pattern taken from the particle shown in Figure 4a. The pattern displays the streaks associated with monoclinic reflections. The presence of these streaks indicates that the particles are apparently very thin nanometer sized plates of the monoclinic component. This is clearly seen on the corresponding dark field image presented in Figure 4d, which was taken from the reflections associated with the streak (Figure 4b). HRTEM images of these thin monoclinic plates in Figure 4c show the lattice fringes corresponding to the (021) family of planes (d021 = 3.17 Å). The TEM studies of the single component material, the spinel LiNi0.5Mn1.5O4, are presented in Figure 5. The nanobeam electron diffraction (NBED) pattern (Figure 5a, taken from a 4 nm area) clearly indicates the presence of the spinel phase. The lattice fringes corresponding to the (111) plane of a spinel phase are shown in the HRTEM image (Figure 5b). The Raman spectra of the pristine Li[Ni1/3Mn2/3]O2 and LiNi0.5Mn1.5O4 are shown in Figure 6. The Raman bands observed at 368, 423, 479, and 600 cm−1 are closely associated with the monoclinic Li2MnO3 component.44 The Raman bands of the LiNi0.5Mn1.5O4 material around 361, 483, and 620 cm−1 are indicative of the spinel phase.45 The peaks at about 361 and 483 cm−1 can be assigned to the Ni2+−O stretching mode and the peak at about 620 cm−1 to the symmetric Mn−O stretching vibration of MnO6 octahedra.45 3.2. Electrochemical Cycling Performance. We examined the voltage profiles by galvanostatic cycling of Li[Ni1/3Mn2/3]O2 electrodes in different potential ranges (at least three cycles in each potential range). Typical voltage profiles and their derivatives (dQ/dE vs E) are shown in Figure 7a,b. Low specific capacity values, around 30−50 mAh g−1 with sloping voltage profiles, were obtained when the upper potential limit was below 4.5 V (Figure 7a) as the major monoclinic phase Li2MnO3 remains electrochemically inactive in this potential range. Increasing the upper potential limit to 4.6 and 4.9 V provided specific capacity of about 115 and 180 mAh g−1, respectively, during initial cycling, and the voltage

Figure 4. (a) TEM bright field image; (b) selected area electron diffraction (SAED) of Li[Ni1/3Mn2/3]O2 taken from particles in panel a, displaying the streaks associated with the reflections of the thin plates of the monoclinic microstructure; (c) HRTEM image taken from the particle in panel b displaying the lattice fringes and the arrangements of the thin monoclinic plates; (d) TEM dark field image displaying the thin plates of the monoclinic structure.

(Figure 3d), rhombohedral (Figure 3e), spinel (Figure 3f), and rock-salt phase (Figure 3g). HRTEM imaging was helpful in visualizing and identifying the monoclinic phase as the major component in this material. Figure 3b shows the HRTEM image of a nanoparticle revealing well-resolved crystal planes of the monoclinic Li2MnO3 phase. The FFT (Figure 3c), taken from the area marked by white square in Figure 3b, was indexed in terms of the monoclinic unit cell of Li2MnO3. The filtered image of this area (inset in Figure 3b) clearly shows the d(020), and the d(−111) family of atomic planes in the monoclinic Li2MnO3. The image also displays a thin uniform amorphous

Figure 5. HRTEM measurements of LiNi0.5Mn1.5O4 synthesized by SCR and annealed at 900 °C, (a) NBED image taken with a 4 nm probe from the marked areas, showing the presence of spinel phase; (b) a typical image of a particle displaying the lattice fringes corresponding to the spinel phase. E


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includes also a layered-to-spinel phase transition that increases the content of the spinel phase beyond its concentration in the pristine material. As a result of this activation process of Li[Li1/3Mn2/3]O2 at potential ≥4.5 V, the voltage profile of Li[Ni1/3Mn2/3]O2 deviates when cycled to potentials higher than 4.5 V from those cycled up to 4.5 V (Figure 7a). The electrochemical activity at the low potentials (around 3 V) should be related mostly to the Mn4+/Mn3+ redox couple.25,33 Further cycling of these electrodes (Li[Ni1/3Mn2/3]O2 and LiNi0.5Mn1.5O4) were carried out in the potential range 2.4−4.9 V, in order to extract their maximal capacity, as presented in Figures 8−10 (galvanostatic cycling in two electrode cells at 30

Figure 6. Raman spectra of pristine (i) Li[Ni1/3Mn2/3]O2 and (ii) LiNi0.5Mn1.5O4, synthesized by SCR and annealed at 900 °C.

Figure 8. (a) Voltage profiles obtained during galvanostatic charge− discharge cycling of Li[Ni1/3Mn2/3]O2 electrodes at 20 mA g−1 (C/10) rate and (b) differential capacity plots related to the voltage profiles of second, 30th, 50th, and 80th cycles at 30 °C.

°C). Since the Li counter electrodes in the cells were in large excess, all the data in these figures reflect solely the behavior of the cathodes. Cycling these electrodes at higher temperatures, for example, 45 °C, showed very similar results and trends. Typical charge−discharge voltage profiles of Li[Ni1/3Mn2/3]O2 electrodes in different cycles are shown in Figure 8a and their derivative dQ/dE vs E curves are presented in Figure 8b. Plateaus in the voltage profiles that correspond to phase transition processes appear in the latter curves as sharp peaks. Upon cycling, we observe a slight increase in the capacity related to the LixNi0.5Mn1.5O4 component at the high voltage domain and flattening of the voltage profile around 3 V (an indication of a redox activity of a LixMn2O4 spinel phase).49−51 It is well-known that Li- and Mn-rich cathodes containing Li[Li1/3Mn2/3]O2 undergo a voltage decay during prolonged cycling.22,25 Similarly, we observed a decrease in the average voltage of Li[Ni1/3Mn2/3]O2 as it contains Li[Li1/3Mn2/3]O2 as a major phase. The average voltage during charge and discharge are plotted in Figure 9. The average charge voltage decreases from 4.31 to 3.85 V after 100 cycles, whereas the average discharge voltage decreases from 3.46 to 3.24 V, thus retaining about 93.6% discharge voltage after 100 cycles. Here we can

Figure 7. (a) Voltage profiles obtained during galvanostatic charge− discharge cycles of Li[Ni1/3Mn2/3]O2 electrodes at 10 mA g−1 in different potential ranges (indicated); (b) differential capacity plots related to the voltage profiles in panel a at 30 °C.

profiles show plateaus at the high potentials. On the basis of previous studies,11−16,22,25 we can assign these plateaus to the activation of the Li2MnO3 phase at potential >4.5 V and the redox activity of the Ni4+/Ni2+ of the spinel phase. The two sets of sharp peaks appearing in the derivative curves (Figure 7b) at potential >4.6 V can be considered as an electrochemical signature of the LiNi0.5Mn1.5O4 spinel phase.46−49 On the basis of previous studies,23−25 we suggest that the activation process F


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Figure 11. Plots of specific capacity vs cycle numbers from galvanostatic charge−discharge cyclability tests of (i) Li[Ni1/3Mn2/3]O2 and (ii) LiNi0.5Mn1.5O4 electrodes at 20 mA g−1 (C/10 rate) for 100 cycles at 30 °C.

Figure 9. Plots of average voltage (both charge and discharge) vs cycle numbers from galvanostatic charge−discharge cyclability tests of Li[Ni1/3Mn2/3]O2 electrodes measured at 20 mA g−1 (C/10 rate) at 30 °C.

30 °C at C/10 rate during 100 cycles. The specific capacity of the Li[Ni1/3Mn2/3]O2 cathodes increases during initial cycling from 80 to 220 mAh g−1 and thereafter remains stable. An initial Coulombic efficiency of 93% was found for Li[Ni1/3Mn2/3]O2, which increased during the first 10−15 cycles and then was stabilized at about 99%. On the basis of previous studies of Li2MnO3 electrodes, we can explain the behavior of these electrodes as a completion of activation of the Li2MnO3 component as cycling proceeds. However, the electrodes studied here exhibit more attractive and interesting electrochemical behavior than Li2MnO3. The results of the present work seem to indicate that the multiphase structure has a stabilization effect on the active mass. The specific capacity of the LiNi0.5Mn1.5O4 electrodes reach a maximal capacity of 190 mAh g−1 in the first five cycles, which then decays gradually to 150 mAh g−1, thus retaining about 79% capacity, after 100 cycles. This behavior is in fact expected. LixMn2O4 and LixNi0.5Mn1.5O4 (0 < x < 2) spinel cathode materials have two redox activity domains at high (4.1 and 4.8 V, respectively) and low voltages (around 3 V for both spinel materials), each of them related to intercalation/deintercalation of one Li-ion per M2O4 unit. It is well-known that operating these materials over the entire activity domains involves structural distortions (the Jahn−Teller effect) that interfere badly with their stability upon cycling.49−51 Consequently, these spinel materials are operated only at their high voltage domains, which is associated with a reversible capacity that cannot exceed 145 mAh g−1.45−48 Hence, the capacity fading for the LiNi0.5Mn1.5O4 electrodes shown in Figure 11 is well understood. In turn, the increase in the capacity of the Li[Ni1/3Mn2/3]O2 electrodes upon cycling and its stabilization around 220 mAh g−1, as seen in Figure 11, is interesting. Since the LiNi0.5Mn1.5O4 containing about 1% rock salt as impurity undergoes capacity fading during prolonged cycling, rock salt is not effective for stabilizing the capacity. Also, layered LiNiO2 and LiNi1/3Mn1/3Ni1/3O2 (isostructural to LiNiO2) are usually cycled in the potential range of 2.5−4.3 V.52−54 When cycled to higher potential beyond 4.3 V, these undergo capacity fading.53,54 Thus, the layered LiNiO2 cannot be effective in stabilization of LiNi0.5Mn1.5O4. Therefore, we can conclude that from the other three phases, only Li[Li1/3Mn2/3]O2 (which is usually cycled to potential higher than 4.6 V in order to be activated) can stabilize the spinel

observe the difference in the average charge, and discharge voltage (ΔE = 0.85 V) in the initial cycle decreases to a value of ΔE = 0.61 V in the 100th cycle, indicating that there is an increase in the reversibility of intercalation/deintercalation of Li+-ions into the active mass of Li[Ni1/3Mn2/3]O2 with an increase in cycling. Figure 10a,b presents typical voltage profiles and their derivative curves for the reference LiNi0.5Mn1.5O4 spinel electrodes. Their main expected redox activities around 4.7 V (Ni4+/Ni3+/Ni2+) and 3 V (Mn4+/Mn3+) are clearly indicated (plateaus in Figure 10a and sets of peaks in Figure 10b). Figure 11 compares the typical cycling performance of Li[Ni1/3Mn2/3]O2/Li and LiNi0.5Mn1.5O4/Li cells measured at

Figure 10. (a) Voltage profiles measured during galvanostatic charge− discharge cycles (first, fourth, 30th, 50th, and 80th cycles) and (b) the relevant differential capacity plots for LiNi0.5Mn1.5O4 electrodes at 20 mA g−1 (C/10) rate at 30 °C. G


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Chemistry of Materials LiNi0.5Mn1.5O4 phase during prolonged cycling in the potential range of 2.4−4.9 V. It means that the integrated structure, in which the spinel and the layered phases are interconnected at the nanometer level, stabilizes the spinel structure, despite operation of the cathode material over a wide voltage domain. We cannot yet offer a full explanation. However, this article aims at addressing the phenomenon as clearly as possible. The galvanostatic charge−discharge curves of Li[Ni1/3Mn2/3]O2/Li and LiNi0.5Mn1.5O4/Li cells recorded at 45 °C are shown in Figure 12. A long plateau is observed

Figure 13. Galvanostatic charge−discharge cyclability test of (i) Li[Ni1/3Mn2/3]O2 and (ii) LiNi0.5Mn1.5O4 at 20 mA g−1 (C/10) rate for 80 cycles at 45 °C.

and also at elevated temperature of 45 °C as compared to spinel LiNi0.5Mn1.5O4. In order to complete the presentation of the electrochemical behavior of these Li[Ni1/3Mn2/3]O2 electrodes, Figure 14

Figure 12. Voltage profiles measured during galvanostatic charge− discharge cycles (first, fourth, 30th, 50th, and 80th cycles) of (a) Li[Ni1/3Mn2/3]O2 and (b) LiNi0.5Mn1.5O4 electrodes at 20 mA g−1 (C/ 10) rate at 45 °C. Figure 14. Rate capability tests of Li[Ni1/3Mn2/3]O2 electrodes at different current values (C rates) at temperatures of 30 and 45 °C.

during charge in the first cycle, which corresponds to the activation of Li2MnO3 and then around 4.7 V corresponding to the deintercalation of Li+-ions from LiNi0.5Mn1.5O4 as observed in case of cycling at 30 °C (Figure 12a). The initial charge and discharge capacities are found to be 170 and 140 mAh g−1, respectively. Although a lower specific capacity of about 140 mAh g−1 is obtained initially, the reversible capacity of about 220 mAh g−1 is obtained only after 10 cycles. This indicates that the electrode kinetics becomes faster with an increase in temperature, and it is activated much faster when cycled at 45 °C as compared to that cycled at 30 °C. The variation of specific capacity of Li[Ni1/3Mn2/3]O2/Li and LiNi0.5Mn1.5O4 /Li cells during prolonged cycling (for 80 cycles) at 45 °C are shown in Figure 13. Initially, there is an increase in discharge capacity for 20 cycles, and then a stable capacity of 220 mAh g−1 is obtained up to 50 cycles, which then decreased to a value of 190 mAh g−1, thus retaining about 86% capacity after 80 cycles. However, the specific capacity of LiNi0.5Mn1.5O4 decreased from 190 mAh g−1 to 140 mAh g−1, thus retaining about 74% capacity. It is interesting to note that Li[Ni1/3Mn2/3]O2 exhibited excellent cycling stability at 30 °C

presents the rate capability tests at 30 and 45 °C. It is interesting to note that higher specific capacities are measured at 45 °C than at 30 °C as the activation processes that provide a stable capacity of around 220 mAh g−1 require less than 10 cycles at this higher temperature. This cathode material cannot be considered as a very fast one. On increasing the rate from C/ 10 to 1C and 2C at 30 °C, the specific capacity decreases from 220 to 120 and 80 mAh g−1, respectively. After cycling at high rates, the specific capacity returns to its high specific capacity values when cycled at low rates. The electrochemical impedance spectra of Li[Ni1/3Mn2/3]O2 and LiNi0.5Mn1.5O4 electrodes were recorded for various equilibrium potentials at both 30 and 45 °C with an amplitude of 5 mV in the frequency range of 100 kHz to 0.01 Hz. The cells were subjected to 25 galvanostatic charge−discharge cycles for activating and stabilizing the electrodes before the impedance measurements. The Nyquist plots recorded at various potentials during charging at 30 °C are shown in Figure H


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Chemistry of Materials 15. The Nyquist plots include high-to-medium frequency semicircles, which are usually assigned to the time constants

Figure 16. Plot of (a) Rf and (b) Rct calculated by fitting the impedance spectra to the equivalent circuit analog (inset of Figure 15) vs potential for (i) Li[Ni1/3Mn2/3]O2 and (ii) LiNi0.5Mn1.5O4 electrodes at 30 °C.

tively. Hence these EIS measurements reflect the usual, expected response of all kinds of lithiated transition-metaloxide electrodes, as already discussed in detail.18,22,25 They also reflect the faster kinetics of the LiNi0.5Mn1.5O4 spinel electrodes, as expected (spinel compounds with 3D solidstate diffusion are known to be very fast electrode materials).45−48 Figure 17 compares the Raman spectra of pristine and cycled electrodes (after 100 cycles) of Li[Ni1/3Mn2/3]O2. The Raman

Figure 15. Electrochemical impedance spectra presented as Nyquist plots of (a) Li[Ni1/3Mn2/3]O2 and (b) LiNi0.5Mn1.5O4 electrodes recorded at different potentials during charging with an amplitude of 5 mV at 30 °C.

related to the surface films that cover all lithiated transition metal oxides in standard electrolyte solutions.18,22,25 These time constants include resistance due to Li+-ion migration through the surface films (Rf), coupled with film capacitance (Cf). The Nyquist plots of these electrodes also contain medium-to-low frequency semicircles that can be assigned to interfacial chargetransfer resistance (Rct) coupled with an interfacial capacitance (Cdl). Additionally, low frequency straight lines are also observed corresponding to the solid-state diffusion of Li+-ions into the electrode active mass, resembling Warburg-type impedance (W). The impedance data were analyzed by NLLS fitting procedure to the appropriate equivalent circuit analog shown in the inset of Figure 15a. This fitting allows us to extract the values of Rf and Rct, which are plotted as a function of the potential (Figure 16). The Rf values were found to be nearly invariant with potential, for both types of electrodes explored in the present study, and are lower for the LiNi0.5Mn1.5O4 electrodes. The Rct values calculated are nearly potential invariant and similar for both types of electrodes in the potential range of 4.0−4.6 V, and are higher beyond this potential range. The general behavior of these values calculated from the impedance spectra of the Li[Ni1/3Mn2/3]O2 electrodes at 45 °C resembles that at 30 °C. Both Rf and Rct values calculated from the data related to the higher temperature are lower, as expected for activation controlled processes: Li-ion migration through surface films and interfacial charge transfer, respec-

Figure 17. Raman spectra of (i) pristine and (ii) cycled electrodes (100 cycles) of Li[Ni1/3Mn2/3]O2.

band at 423 cm−1 associated with the monoclinic Li2MnO3 component, which is seen in the spectrum of pristine electrodes (Figure 6) does not exist in the spectra of cycled electrodes. The other two significant Raman peaks that appear around 479 and 600 cm−1 in the spectra of the pristine electrodes are blueshifted to higher values in cycled electrodes (to 486 and 621 I


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Chemistry of Materials cm−1), respectively (Figure 17ii), and their integral intensities decrease substantially in the spectra of cycled electrodes. The Raman band at 620 cm−1 is a characteristic peak of the spinel phase (Figure 6ii). Thus, the appearance of a Raman band at 621 cm−1 can be ascribed to a spinel-type cation ordering upon cycling, as reported previously for the integrated Li- and Mnrich cathodes.22,25 The results of HRTEM studies of the cycled Li[Ni1/3Mn2/3]O2 electrode are presented in Figures 18 and 19. Figure 18a

Figure 19. (a) TEM bright field image of Li[Ni1/3Mn2/3]O2 electrode (after 100 cycles); (b) SAED taken from the particle in panel a; (c) TEM dark field image taken with the (044) sp reflection displaying the spatial locations of the spinel phase on the surface of the monoclinic structure; (d) HRTEM image showing typical lattice fringes belonging to the (044) family of planes of the spinel phase.

structure. The DF image in Figure 19c was taken using the reflection (044) that is unique to the spinel phase (Figure 19b). Hence the bright spots seen in this DF image (Figure 19c) show the spatial distribution of the spinel phase that is formed on the surface of the composite particles during the cycling process. The HRTEM image in Figure 19d strengthens this conclusion even more, as clear small polycrystalline areas were discerned with lattice-fringes d(220) = 2.9 Å of the spinel (044) family of planes. In agreement with Raman and electrochemical studies, the HRTEM observations show that the cubic spinel is a major component of the cycled material due to the process of massive layered-to-spinel transformation, which takes place in the course of charging. It is also important to note that the streaks in the SAED patterns that are easily seen with the pristine material are not observed in the cycled active mass (Figures 18b and 19b). As pointed above, the streaks (Figure 4b and 4d) are indicative of the presence of thin plates of the monoclinic component. In addition, taking into account the HRTEM images that show mostly well-resolved lattices belonging to the spinel phase (Figure 19d), we can conclude that upon cycling, the fine plates of the monoclinic phase in the pristine material are mostly destroyed as a result of the layeredto-spinel transformation. However, it is interesting and important to emphasize that the monoclinic phase still remains after prolonged cycling, despite the high upper potential applied (4.9 V) at which the monoclinic phase is expected to fully disappear, as found with integrated layered Li- and Mnrich high-capacity cathode materials.22,25 The presence of this monoclinic phase may play an important role in stabilizing the capacity measured with these materials when they are cycled in a wide potential domain; research into the precise mechanism is being carried out.

Figure 18. (a) Typical bright field image of the microstructure of Li[Ni1/3Mn2/3]O2 electrodes after 100 cycles; (b) SAED taken from the particle in panel a; (c,d) CBED diffractions taken from the marked areas, showing the presence of the monoclinic and the spinel phase.

shows a typical bright-field image of a few agglomerated nanoparticles scraped from an electrode that was cycled 100 times. The SAED pattern in Figure 18b reveals the presence of two phases, monoclinic (m) and spinel (sp). There are three sets of reflections in this pattern: two sets belong to the spinel phase, and the weak set of reflections was ascribed to the monoclinic phase, which still remains after cycling (Figure 18b). Using CBED technique, we were also able to show, on the nanoscale, the presence of the two distinct structures, monoclinic and spinel phases. The CBED patterns in Figure 18c,d (obtained from the areas marked by white arrows in Figure 18a) show sets of reflections that were uniquely indexed, one on the basis of the monoclinic unit cell (Figure 18c) and the other, shown in Figure 18d, for the spinel phase. The massive transformation of the layered-to-spinel phase upon cycling is also supported by the TEM-HRTEM image, as presented in Figure 19. The SAED in Figure 19b that was taken from the particle imaged in Figure 19a shows reflection that is unambiguously indexed as the spinel phase. The important fact is that there are no additional reflections in this pattern so we can deduce that either there are no other reflecting phases in the analyzed particle or their amount is below the detectable limit needed for a diffraction pattern. The TEM BF image in Figure 19a and, mainly, the DF image in Figure 19c show the presence of small grains that were found to have a spinel J


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4. CONCLUSIONS Synthesis of the cathode material with the stoichiometry Li[Ni1/3Mn2/3]O2 by SCR, followed by annealing at 900 °C, provides a multiphase active mass containing a monoclinic Li2MnO3 phase as the major component, together with LiNiO2, LiMn1.5Ni0.5O4, and Li0.2Mn0.2Ni0.5O as minor phases. The electrochemical response of the cycled Li[Ni1/3Mn2/3]O2 electrodes shows a clear signature of the LiNi0.5Mn1.5O4 component, being active around 4.7−4.8 V. The Li[Ni1/3Mn2/3]O2 cathodes studied here could deliver a discharge capacity of about 220 mAh g−1 at C/10 rate at 30 °C with high electrochemical cycling stability, in the voltage domain of 2.4− 4.9 V. These electrodes demonstrate much better performance than the reference LiNi0.5Mn1.5O4 electrodes, working in the same potential domain, in terms of high capacity and stability. The polarization of the integrated cathode material Li[Ni1/3Mn2/3]O2 to the high voltages drives a very significant layered-to-spinel phase transition upon cycling, which was clearly analyzed by Raman spectroscopy, TEM, electron diffraction, and electrochemistry. It seems that the multiphase structure of Li[Ni1/3Mn2/3]O2 helps to stabilize the spinel phase in prolonged cycling. The structural analysis of cycled electrodes reveals that some monoclinic phase remains in the integrated structure (apart from the spinel phase), despite the high potentials applied. We suggest that the steady presence of the monoclinic phase in the integrated Li[Ni1/3Mn2/3]O2 structure may contribute to the stability of this cathode material despite its cycling over a wide potential domain. More work is needed to optimize its properties, to confirm stabilization mechanisms, and to test full Li-ion cells in order to demonstrate impressive long-term stability.

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ASSOCIATED CONTENT

* Supporting Information S

XRD Rietveld refinement results of Li[Ni1/3Mn2/3]O2 synthesized by SCR and annealed at 700 °C. 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 Partial support for this work was obtained from the Israel Science Foundation, ISF, in the framework of the INREP project.

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REFERENCES

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Electrochemical Performance of a Layered-Spinel Integrated Li [Ni1

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