Preparation and Characterization of LiNi0. 5Co0. 5O2 and LiNi0

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J. Phys. Chem. C 2007, 111, 11712-11720

Preparation and Characterization of LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 by Molten Salt Synthesis for Li Ion Batteries M. V. Reddy,* G. V. Subba Rao, and B. V. R. Chowdari* Department of Physics, National UniVersity of Singapore, Singapore 117542 ReceiVed: NoVember 19, 2006; In Final Form: June 4, 2007

The layer compounds LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 were prepared by using the molten salt of the eutectic mixture LiNO3-LiCl at temperatures of 750 and 850 °C in air. They were characterized by X-ray diffraction, Rietveld refinement, SEM, EDAX, chemical analysis, surface area, and density techniques. Cathodic behavior was examined at ambient temperature by cyclic voltammetry, charge-discharge cycling, and impedance spectra in cells with Li-metal as the counter electrode at a current rate of 30 mAg-1 with a lower cutoff voltage of 2.5 V up to 120 cycles. The upper cutoff voltages ranged from 4.2 to 4.5 V. The 10th cycle reversible capacities for LiNi0.5Co0.5O2 varied from 131 to 186((3) mAhg-1 depending on the upper cutoff voltage, but capacity fading occurred ranging from 5 to 18% at the end of 50 or 80 cycles. The Al-doped compound showed a 20th cycle reversible capacity of 146 ((3) mAhg-1 (4.3 V cutoff) which was stable up to 120 cycles. Good rate capability was also observed. Thus, LiNi0.5Co0.4Al0.1O2 with only 0.4 mol of cobalt is a cheaper and viable alternative to the presently used cathode, LiCoO2. Impedance spectra on cells cycled up to 60 cycles with 4.3 V cutoff have been analyzed and interpreted.

1. Introduction Commercial lithium ion batteries (LIB) possessing highenergy density use the mixed oxide LiCoO2 as the positive electrode (cathode) material. This compound has a hexagonal layer structure (R-NaFeO2-type). During the charge-discharge process, Li ions undergo deintercalation/ intercalation from the crystal lattice and contribute to the cathodic capacity.1-3 To increase the obtainable cathodic capacity for a given charging voltage and to reduce the Co content, which is an expensive and toxic constituent, substitution at the Co site by the electrochemically active Ni ion has been tried. Studies have shown that LiNixCo1-xO2 forms iso-structural solid solutions for all x.2,4,5 Hence, the cathodic behavior of LiNixCo1-xO2 has been studied by many groups, especially for x g 0.7.2,6-10 The composition with x ) 0.5, viz., LiNi3+0.5Co3+0.5O2, has attracted attention as a prospective cathode, since it can be synthesized in air at high-temperature similar to LiCoO2. A higher cathodic capacity can be obtained for a given charging voltage in this compound since the redox potential of the Ni3+/4+ couple is slightly lower than that of the Co3+/4+ couple which is ∼4 V vs Li metal.2,4-11 For several years, cathodic properties of LiNi0.5Co0.5O2 have been examined.4,5,11-20 However, detailed studies by way of its long-term cyclability have been reported only by two groups: Wang et al.14 reported that cycling of LiNi0.5Co0.5O2 in the voltage range of 3.0-4.4 V at a current density of 0.15 mAcm-2 resulted in continuous capacity fading over 100 charge-discharge cycles, from 165 to 135 mAhg-1, about 18% loss. Recently, Belharouak et al.18 examined the cyclability of LiNi0.5Co0.5O2 in the voltage rang of 2.8-4.3 V and found that the capacity degraded slowly from 150 to 125 mAhg-1, about 16% loss, over 100 cycles at the C/5 rate. Thus, there is a need to understand the reasons for capacity-fading in * Corresponding authors. Tel: (65) 6516-2956. Fax: (65) 6777-6126. E-mail: [email protected]; [email protected].

LiNi0.5Co0.5O2 and, if possible, to suppress it by suitable doping in order that this mixed oxide can be a viable LIB cathode. The compound LiNi0.5Co0.5O2 has been prepared by several groups by the high-temperature reaction in air using raw materials of oxides, hydroxides, carbonates, nitrates, and acetates or through chemical precursors via. Pechini’s method, sol-gel, etc.4,5,11-20 However, these methods involve intimate mixing, repeated heating at high temperatures, >800-850 °C, and burning away of valuable raw materials like carbonates, oxalates, citrates, ethylene glycol, etc. which also produce the green house gas CO2. On the other hand, molten salt synthesis (MSS) is a simple and versatile method to obtain highly crystalline pure, mixed oxides, layered intercalation hosts, and supported catalysts.21 In molten media, reactions are usually controlled by chemical equilibria and proceed much faster than diffusioncontrolled solid-state reactions.21 It is a “one-pot” technique which does not involve mechanical mixing or repeat grinding and heating of constituents and employs salt fluxes to enable the reactants to form the products at suitable high temperatures. Further, the salt flux can be recovered and reused, after filtration and evaporation of water, thus saving on materials and energy. In principle, the MSS method can be adopted from an industrial perspective. In recent years, LIB cathodes like LiMn2O4,22 LiNi1/2Mn3/2O4,23 and LiCoO224-28 have been prepared by the MSS method and studied. The LiCoO2 synthesized using molten salt of KCl26 or KNO327 or LiNO3-LiCl eutectic28 were found to show very good cyclability and stable capacity. Presently we report on the molten salt synthesis and cathodic properties of pure and Al-doped LiNi0.5Co0.5O2. The Al-doped compound shows stable capacity without any noticeable fading up to 120 cycles. Complementary cyclic voltammetry and impedance data are also presented. 2. Experimental Section 2.1. Materials Preparation. The compounds LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 were synthesized by decomposing the

10.1021/jp0676890 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

Synthesis for Li Ion Batteries mixed metal-nitrates in a molten salt of eutectic mixture (0.88LiNO3:0.12LiCl) at high temperature using a procedure similar to that adapted for the synthesis of LiCoO2.28 The metalnitrate-to-eutectic mixture was kept at 1:4 mole ratio. Stoichiometric amounts of high purity Ni-, Co-, and Al-nitrate hexahydrates (Merck or Alfa Aesar (purity 99%)) and the Lisalt mixture (Merck or Fluka, (purity 99%)) were mixed thoroughly, taken in a recrystallized alumina crucible, and heated in a box furnace (Carbolite, U.K.) to 750 or 850 °C in air at the rate of 3 °C/min. The contents were soaked for 8 h at that temperature and cooled to room temperature by furnace shutoff. The obtained product was washed thoroughly with deionized water to remove excess Li salts. The residue was dried in an air oven at 150 °C for 24 h, cooled, ground to fine powder, and stored in a desiccator. The compounds were prepared in 5 g quantities using the above conditions. Characterization Techniques. Powder X-ray diffraction (XRD) patterns were recorded using Bruker AXS D8 diffractometer with Cu KR radiation. Rietveld refinement of XRD data was carried out using TOPAS-R (Version 2.1) software. Morphology of powders and elemental analysis were determined by using scanning electron microscope (SEM; JEOL JSM5600LV) attached with an energy dispersive X-ray analyzer (EDAX). The Brunauer, Emmett, and Teller (BET) surface area and density of the powders were measured by Micromeritics Tristar 3000 (U.S.A.) and AccuPyc 1330 pycnometer, Micromeritics (U.S.A.), respectively. The Li, Co, Ni, and Al contents were determined using inductively coupled plasma (ICP) spectrometer (Thermo Jarrell Ash, IRIS/AP Duo). For electrochemical measurements electrodes were fabricated with the powder of active material, super P carbon black, and binder (Kynar 2801) in the weight ratio 70:15:15 or 80:10:10 using N-methyl pyrrolidone (NMP) as a solvent to the binder. Thick (∼20 µm) composite electrodes on etched Al foil (current collector) were obtained by using the doctor blade technique. The geometrical area of the electrode is 2 cm2, and 2-3 mg/ cm2 active material loading was used for the fabrication of cathodes. Lithium metal foil (Kyokuto metal Co., Japan) was the negative electrode (anode) and 1 M LiPF6 in ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1 V/V) (Merck) was the electrolyte. A polypropylene celgard 2502 membrane was used as the separator. Coin-type test cells (CR2016) were fabricated in an Ar gas filled glove box (MBraun, Germany). More details of cell fabrication have been described elsewhere.9,28 Charge-discharge cycling at a constant current (30 mA/g) and cyclic voltammetry were carried out at ambient temperature (RT ) 25 °C) using the computer controlled Bitrode multiple battery tester (model SCN, Bitrode, U.S.A.) and Macpile II system (Bio-logic, France). The cells were aged for 24 h before the measurements. Computer controlled Solartron Impedance/gain-phase analyzer (model SI 1255) coupled with a battery test unit (model 1470) was used for impedance measurements on cells at RT. The frequency range was from 0.35 MHz to 3 mHz with an ac signal amplitude of 5 mV. Data were analyzed by using Z plot and Z view software (version 2.2, Scribner Associates Inc., U.S.A.) to obtain the Nyquist plots. 3. Results and Discussion 3.1. Structural Aspects. In the presently used one-pot synthesis method, the metal nitrates decompose to the respective oxides above 500-600 °C in the molten salt of the eutectic LiNO3-LiCl, which has a melting point of 280 °C.28 These oxides react with the Li ions to form the desired quasiternary compound at temperatures above 650 °C. Due to the large excess

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11713 of Li, the eutectic composition is little affected. Further, LiNO3 acts as the oxidizing agent to form Ni3+ and Co3+ ions, whereas LiCl is the “mineralizer” and ensures the formation of wellcrystalline compounds. We have not, however, optimized all of the synthesis conditions and presently used only two temperatures (750 and 850 °C), a time of 8 h for soaking the reactant mixture, metal-nitrates-to-eutectic ratio of 1:4, and only 0.1 mole of Al as dopant for the preparation. The pure and Al-doped LiNi0.5Co0.5O2 are black and freeflowing powders. The SEM photographs of LiNi0.5Co0.5O2 prepared at 750 °C showed agglomeration of well-crystalline submicron particles of size 0.5-1 µm as can be seen in Figure 1a. The compound prepared at 850 °C showed well-defined platelets and faceted crystals of size 2-5 µm and a higher degree of crystallinity (Figure 1b). Similar was the case with LiNi0.5Co0.4Al0.1O2 (Figure 1c). Elemental analysis of the compounds for Li, Ni, Co, and Al was carried out by the ICP method on several duplicate samples. The experimental compositions obtained for the nominal LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 to an accuracy of (0.005 are as follows: 750 °C synthesis: Li1.02Ni0.5Co0.47O2; 850 °C synthesis: Li0.98Ni0.5Co0.45O2; 850 °C synthesis: Li0.94Ni0.5Co0.37Al0.06O2. From the results it is clear that the compounds do not contain excess lithium in spite of the large excess of Li salts used as the flux. Also, washing the reaction product with water to remove the salt-flux does not leach out lithium from the crystal lattice of pure and Al-doped LiNi0.5Co0.5O2. Elemental analysis for Ni, Co, and Al was also carried out on select samples by the EDAX method using the SEM. The experimental compositions agreed fairly well with the nominal starting compositions for the pure and Al-doped LiNi0.5Co0.5O2 even though the accuracy here is limited to (12-15%. The ICP and EDAX analyses of pure LiNi0.5Co0.5O2 did not show the presence of Al indicating that Al ions from the crucible material do not get incorporated in to the crystal lattice at the synthesis temperatures of 750-850 °C. Recently, it has been shown that the layered oxide cathode LiNiO2 and oxides with high nickel content, e.g., LiNi0.8Co0.15Al0.05O2 and LiNi0.83Co0.15Al0.02O2, are sensitive to the exposure to moisture and CO2 because of the leaching out of significant amounts of Li ions from the lattice and formation of LiOH/Li2CO3 on the particle surface.29 We examined our compounds, LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2, for their Li leachability: Weighed amounts (0.5 g) of the powders were suspended in 100 mL of deionized water at ambient temperature, and the pH of the solution was monitored for a period of 7 days. We did not find any increase in the pH of the solution above 7.5 ((0.3) indicating that our compounds are stable toward leaching of Li from the lattice. All of the observed peaks in the XRD patterns are indexable on the basis of the R-NaFeO2 structure. Rietveld refinement of the data was carried out assuming the space group R 3hm with R-NaFeO2 type structure with Li+ ions at the 3b site (0,0,1/2), Ni3+ and Co3+ (and Al3+) ions at the 3a sites (0,0,0), and O at the 6c site (0,0,z). The observed and fitted XRD pattern for the compound, LiNi0.5Co0.5O2 prepared at 850 °C is shown in Figure 2a, and Al-doped and pure compound XRD patterns are shown in Figure 2b. The calculated hexagonal a and c lattice parameters, the oxygen positional parameter (z(O); ideal value, 0.25) and the refined R factors for the pure and Al-doped compounds are given in Table 1. The final R factors satisfactorily explain the assumed layer structure. Cation mixing by way of partial occupancy of Li ions at the Ni site in the transition metal layer and vice versa, keeping the total occupancy factor as unity, was tried for the profile fitting but led to larger R values

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Figure 2. (a) Observed (circles) and calculated (line) X-ray diffraction pattern of LiNi0.5Co0.5O2 prepared at 850 °C. The difference pattern and positions of reflections (vertical lines) are also shown. (b) XRD patterns of pure and Al-doped LiNi0.5Co0.5O2 compounds. Miller indices (hkl) are shown in the figures.

Figure 3. Cyclic voltammograms recorded at a scan rate of 58 µV s-1 at ambient temperature. Li metal was the counter and reference electrode. (a) LiNi0.5Co0.5O2 prepared at 750 °C. Cathode composition, 70:15:15; 1-25 cycles in the range of 2.5-4.5 V. (b) Same as in panel a for the 26th cycle in the voltage range 2.5-4.7 V. (c) LiNi0.5Co0.4Al0.1O2 prepared at 850 °C; 2.5-4.4 V. (d) Same as in panel c for the 26th cycle in voltage range 2.5-4.7 V; cathode composition 80:10:10. The numbers in panels a and c refer to cycle numbers. The numbers in panels b and d refer to voltages. Figure 1. SEM photographs of LiNi0.5Co0.5O2 synthesized by molten salt technique at temperature (a) 750 and (b) 850 °C and (c) LiNi0.5Co0.4Al0.1O2 at 850 °C. Bar scale, 10 µm.

in all cases. We conclude that negligible cation mixing in our pure and Al-doped compounds, mainly due to the large excess of Li ions present during the molten salt synthesis. Significant lithium substitution at the Ni/Co site in the transition metal layer is also ruled out by the XRD and Rietveld analysis since no impurity lines due to unreacted NiO or Co3O4 were noted in the XRD patterns. It is known that depending on the method of preparation and the precursor used like hydroxide, oxalic acid, citric acid, glycine, etc., the c lattice parameter of LiNi0.5Co0.5O2 shows slight variations.16 This is due to the different extent of “cation mixing”. The presently observed c lattice parameters,

14.094 Å (750 °C synthesis) and 14.096 Å (850 °C synthesis; Table 1), agree very well with the value of 14.079 Å reported by Belharouak et al.18 (hydroxide method) and 14.105 Å reported by Julien et al.16 (oxalic acid method) for LiNi0.5Co0.5O2. As can be expected, the hexagonal a and c lattice parameters of LiNi0.5Co0.5O2 are an average of those of pure LiNi3+O2 and LiCo3+O2 due to the solid solution formation.4,11 However, contrary to expectations on the basis of ionic radii of the metal ions,9,17 the c parameter for the Al-doped compound is larger than that of the pure compound and it also has the near-ideal value of z(O) (Table 1). Castro-Garcı´a et al.17 also observed an increase in the c lattice parameter in the Al-doped LiNi0.5-yAlyCo0.5O2 (y e 0.3) system and explained it as being

Synthesis for Li Ion Batteries

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TABLE 1: Rietveld Refined R Factors, Oxygen Positional Parameters (z(O)), and Hexagonal Lattice Parameters (a, c) of As-Prepared Oxide Cathodes along with Data on Cycled Electrodes compound; synth. temp.

a, Å

c, Å

c/a

LiNi0.5Co0.5O2; 850°C z(O), 0.2661(1); Rwp, 1.55; GOF, 1.21; R-Bragg, 1.031; cation mixing, 0% LiNi0.5Co0.4 Al0.1O2; 850°C z(O), 0.2555 (1); Rwp, 3.29; GOF, 2.46; R-Bragg, 0.959; cation mixing, 0% LiNi0.5Co0.5O2; 750°C z(O), 0.2632(1); Rwp, 2.0; GOF, 1.90; R-Bragg, 0.662, cation mixing, 0% LiNi0.5Co0.5O2· 850°C : virgin electrode first charge cycle: 3.9V : 4.3 V : 4.4 V : 4.5 V : 4.7 V first discharge cycle : 2.5 V : after 50 cycles; 2.5-4.5V; discharged to 2.5V LiNi0.5Co0.4Al0.1O2; after 120 cycles, 2.5-4.3 V: charged to 4.3 V LiNi0.5Co0.4Al0.1O2; after 50 cycles, 2.5-4.5 V: charged to 4.5 V

2.8422(2)

14.096(4)

4.96

2.8492(2)

14.169(4)

4.97

2.8398(2)

14.094(4)

4.96

2.840(5) 2.801(5) 2.802(5) 2.806(5) 2.807(5) 2.814(5) 2.845(5) 2.845(5) 2.803(5) 2.800(5)

14.10(2) 14.43(2) 14.53(2) 14.55(2) 14.28(2) 13.99(2) 14.14(2) 14.14(2) 14.50(2) 14.39(2)

4.96 5.15 5.19 5.18 5.08 4.97 4.98 4.97 5.17 5.14

due to the involvement of the polarizing effect of Al ion on the metal-ion layers. The presently observed values of the c/a ratio of 4.96-4.97 and well-resolved splitting of the XRD lines assigned to the pairs of Miller indices (006,102) (2θ ∼38°) and (108,110) (2θ ∼65°) in the pure and Al-doped LiNi0.5Co0.5O2 are a very good indication of the layered structure (Figure 2a,b and Table 1). This is also consistent with the observed ratio of relative intensities of the peaks due to (003) and (104) which is >1.5 for the pure and Al-doped LiNi0.5Co0.5O2 (Figure 2a,b). The measured BET surface areas of the powders of the compounds are 1.3-3.0 ((0.1) m2 g-1. These values are typical of the mixed oxides prepared at high temperature. The experimental densities of LiNi0.5Co0.5O2 and Al-doped compounds prepared at 850 °C are 5.055 and 4.807 ((0.002) g cm-3, respectively. These values compare well with the calculated X-ray densities of 4.940 and 4.728 g cm-3. 3.2. Electrochemical Studies. 3.2.1. Cyclic Voltammetry. The cyclic voltammograms (CV) of the cells were recorded with Li metal as the counter and reference electrode, with the upper cutoff voltage up to 4.7 V at RT, at a scan rate of 58 µVs-1 up to 25 cycles. The lower cutoff voltage was 2.5 V. Only selected CV are shown in Figure 3 for clarity. The CV of LiNi0.5Co0.5O2 prepared at 750 °C shows the first-cycle anodic peak (deintercalation of Li ions from the host) at ∼4.2 V (vs Li), whereas the main cathodic (intercalation of Li) peak is broad and occurs at ∼3.45 V. In the second cycle, the anodic peak shifts to a lower voltage (∼3.90 V) and the corresponding cathodic peak shifts to a higher voltage (∼3.6 V). The shifts in the peak voltage are an indication of the “formation” of the electrode in the initial cycles, where by the active material establishes a conducting path for the Li ions and electrons during the current onset with the help of conducting carbon particles, liquid electrolyte, and the Al-current collector. The shifts in the main anodic and cathodic peak voltages are complete by the sixth cycle stabilizing at 3.67 and 3.64 ((0.02) V, respectively. The hysteresis (∆V ) the difference between the sixth anodic and cathodic peak voltages) is 0.03 V indicating very good reversibility of the charge-discharge reaction. Subsequent CV show that ∆V as well as the anodic and cathodic peak areas remain almost constant with the cycle number, up to 25 cycles shown in Figure 3a, indicating good cycling stability. The observation of a single anodic/cathodic peak at 3.67/ 3.64 V in the CV indicates a reversible two-phase reaction

consequent on the deintercalation/ intercalation of Li ions from LiNi0.5Co0.5O2 involving the redox couple, Ni3+/4+, possibly reminiscent of the pure LiNiO2 which shows the hexagonal (H1) to monoclinic (M) phase transition.30,31 Thus, the voltages of the above couples in LiNi0.5Co0.5O2 are lower than those exhibited in pure LiNiO2 and LiCoO2, as has been noted earlier by Ohzuku et al.5,11 Figure 3a also shows small but well-defined anodic peaks at 3.91 and 4.19 V, and the corresponding cathodic peaks at 3.84 and 4.14 V from the sixth cycle onward. Similar peaks were noted during Li deintercalation of LiNi0.5Co0.5O2.19 These persist in all of the subsequent cycles as can be seen in the CV of the 26th cycle shown in Figure 3b recorded up to 4.7 V, after the first 25 cycles of Figure 3a. In addition, Figure 3b shows an additional anodic/cathodic peak in the voltage range, 4.45-4.61 V. However, a more appropriate assignment of these anodic peaks at 3.91, 4.19, and 4.61 V (Figure 3b) will be due to the subtle crystal structure transitions, which are reminiscent of the pure end phases, LiCoO2 and LiNiO2. Indeed, Li1-xCoO2 undergoes reversible structural transitions with increasing x of the type, hexagonal (H1) T monoclinic (M) T hexagonal (H2) T hexagonal (H1-3) T hexagonal (O1).2,3,28 It is also established that Li1-xNiO2 shows reversible structural transitions of the type, hexagonal (H1) T monoclinic (M) T hexagonal (H2) T hexagonal (H3).2,31 Here, (Hi) refer to the hexagonal structure with slightly differing lattice parameters. Although the H1T M occurs in Li1-xCoO2 at 4.2 V (x ) 0.5), the same transition takes place in Li1-xNiO2 at 3.65-3.7 V and is the cause of extensive capacity-fading in the latter when cycled in the range of 2.5-4.2 V.2,31 It is also known that extensive capacity fading occurs in nonoptimized LiCoO2 when cycled in the range of 2.5-4.5 V, attributable to the (H1-3) T (O1) transition occurring at ∼4.5 V vs Li.2,3,28 The H1 T M T H2 transitions in LiCoO2 and LiNiO2 do involve minor but distinct unit cell volume changes which can produce “electrochemical grinding” of the active material in the composite cathode and loss of interparticle connectivity thereby leading to capacity fading on cycling. It is well-known that suppression or elimination of these transitions by suitable methods will significantly improve the cathodic performance.1-3,28 On the basis of the above arguments, we can assign the anodic peaks observed in the CV of LiNi0.5Co0.5O2 at 3.91, 4.19, and 4.61 V and the corresponding cathodic peaks at 3.84, 4.14, and 4.45 V as due to the reversible H1 T M T H2 T H3, respectively, analogous to those exhibited by LiNiO2 (Figure

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Figure 4. Voltage vs capacity profiles of LiNi0.5Co0.5O2 (a and b) 750 °C (c and d) 850 °C and (e,f) 850 °C; LiNi0.5Co0.4Al0.1O2. Voltage range, 2.5-4.3 and 2.5-4.5 V. Electrode composition, 80:10:10. Current rate of 30 mAg-1, ambient temperature.The numbers refer to cycle numbers.

3b). In the CV of the compound prepared at 850 °C, the H1 T M T H2 are barely noticeable, whereas the H2 T H3 transition is clearly observed. On the other hand, in the Al-doped compound, LiNi0.5Co0.4Al0.1O2 prepared at 850 °C, the H1 T M T H2 transitions are completely suppressed, and the H2 T H3 transition is retained, as can be seen from Figure 3c,d. The effect of Al-doping is also clearly noticed in the CV in that the main anodic/cathodic peaks due to Li-extraction/insertion as well as the peaks due to H1-M transition are increased by ∼0.05 V (Figure 3d) as compared to the values in LiNi0.5Co0.5O2 (Figure 3b). In addition, the “electrode formation” is completed only after 10-12 cycles in the doped material. The random distribution of Al ions in the (Ni,Co) layers must be preventing any tendency of structural ordering of the remaining Li ions and (Ni,Co) ions in their respective layers during the extraction/ insertion of Li. 3.2.2. GalVanostatic Cycling. Charge-discharge cycling of the cells with LiNi0.5Co0.5O2 (750 and 850 °C synthesis)and LiNi0.5Co0.4Al0.1O2 (850 °C synthesis) as cathodes were carried out up to 120 cycles at RT at a current density of 30 mAg-1, with the upper cutoff voltages ranging from 4.2 to 4.5 V. The lower cutoff voltage was 2.5 V. The voltage-capacity profiles of the compound, LiNi0.5Co0.5O2 prepared at 750 and 850 °C are shown in Figure 4a-d. For clarity, only select cycles are shown. During the first-charge process, the voltage suddenly increased to ∼3.8 V from the open circuit voltage (OCV ∼ 3.0 V), followed by a plateau till about 25-60 mAhg-1 are reached

and then gradually increased to the upper cutoff voltage. During subsequent cycles, the voltage jumped only to ∼3.6 V and the voltage-plateau presisted. The discharge curves appeared somewhat smooth in all cases. The observed charge and discharge capacities are plotted as a function of the cycle number in Figure 5. Both pure and Al-doped LiNi0.5Co0.5O2 showed irreversible capacity loss (ICL) between the first charge and the first discharge attributable to the “electrode formation” and formation of the solid electrolyte interphase (SEI). The ICL varied from 20 to 36 mAhg-1 for the pure LiNi0.5Co0.5O2, whereas it is 55 mAhg-1 for Al doped compound irrespective of the upper cutoff voltage value. Higher cutoff voltage gave larger charge and discharge capacities as expected. In almost all cases, the “electrode formation” continued up to 5-15 cycles as shown by a gradual increase in the observed capacities. This is in accord with the CV data described earlier and attributable to high degree of crystallinity of the active material particles prepared by the MSS method. After 10-15 cycles, the Coulombic efficiency increased to >98% as shown by the stable charge and discharge capacities for a given cycle number (Figures 4 and 5). The 10th cycle discharge capacities of LiNi0.5Co0.5O2 prepared at 750 and 850 °C ranged from 131 to 186 ((3) mAhg-1 depending on the upper cutoff voltage, 4.2 to 4.5 V. For a given cutoff voltage (4.3 or 4.5 V), there is only a marginal increase if any, in the 10th cycle capacities when the synthesis temperature was increased from 750 to 850 °C (Figure 5a,b). The presently used current rate of 30 mAg-1 corresponds to ∼0.2

Synthesis for Li Ion Batteries

Figure 5. Capacity vs cycle number plots of (a and b) LiNi0.5Co0.5O2 prepared at 750 and 850 °C cycled in various voltage ranges and (c) LiNi0.5Co0.4Al0.1O2, prepared at 850 °C cycled in various voltage ranges. Electrode composition, 80:10:10 at current rate of 30 mAg-1, at ambient temperature. Filled symbols represent the charge capacity and open symbols the discharge capacity.

C assuming a reversible capacity of 146 mAhg-1. The observed capacity values are in good agreement with those reported in the literature on the LiNi0.5Co0.5O2 compounds prepared by a variety of methods.5,11,12,14,16-18,20 For example, our 10th cycle capacities of 146 and 153 mAhg-1 (750 and 850 °C synthesis; Figures 5a) in the voltage range, 2.5-4.3 V agree very well with the 10th cycle value of 147 mAhg-1 reported by Belharouak et al.18 for LiNi0.5Co0.5O2 in the voltage range, 2.8-4.3 V at the C/8 rate. Similarly, our 10th cycle capacities of 163 ((2) mAhg-1 (750 and 850 °C synthesis; Figures 5b) in the voltage range, 2.5-4.4 V agree very well with the 10th cycle value of 165 mAhg-1 reported by Wang et al.14 for LiNi0.5Co0.5O2 in the voltage range, 3.0-4.4 V (current rate,0.15 mAcm-2). Our 10th cycle capacity of 132 mAhg-1 (850 °C synthesis; Figures 5a) in the voltage range, 2.5-4.2 V agrees well with the 10th cycle value of 130 mAhg-1 reported by Sun et al.12 for LiNi0.5Co0.5O2 in the voltage range, 3.0-4.2 V at 0.1C rate. Irrespective of the upper cutoff voltage and synthesis temperature, LiNi0.5Co0.5O2 exhibited capacity-fading from 10th cycle onward: About 10% loss at the 40th cycle, reaching 1118% at the end of 50 or 80 cycles. As mentioned earlier, there are two reports on the long- term cycling, up to 100 cycles, of LiNi0.5Co0.5O2 with upper cutoff voltages of 4.318 and 4.4 V14 which also observed capacity-fading of 15-16% at current rates of C/8 to C/2. Sun et al.12 also observed capacity-fading from 150 to 130 mAhg-1 in the range, 1-10 cycles. We presume that the reversible structural transitions occurring at ∼3.9 and ∼4.2 V in LiNi0.5Co0.5O2 as indicated in the CV curves (Figure 3a,b) are responsible for the observed capacity-fading on cycling. The Al-containing compound LiNi0.5Co0.4Al0.1O2 showed no noticeable capacity fading during 20-120 cycles when cycled in the voltage range of 2.5-4.3 V at a current rate of 30 mAg-1

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11717 (Figure 5c). The 10th cycle reversible capacity of 139 increased to 146 ((3) mAhg-1 at 20th cycle and stabilized. In order to test the rate-capability, the current rate was increased to 60 mAg-1 during the 90-108 cycles. The reversible capacity decreased by ∼10 mAhg-1, but remained stable. On decreasing the current rate to 30 mAg-1 after 108 cycles, the original capacity was recovered and continued to remain stable till the 120th cycle (Figure 5c). The excellent cycling performance of LiNi0.5Co0.4Al0.1O2 in the range of 2.5-4.3 V can be attributed to the complete suppression of the structural reversible phase transitions occurring in the range of 3.8-4.2 V in the compound as reflected in the CV (Figure 3c,d) data. It is pertinent to point out that we have been able to obtain a stable capacity of 146 mAhg-1 up to 120 cycles in a compound which contains only 0.4 mol of cobalt. It is well-known that one of the major aims of positive electrode materials research in the area of LIB is to reduce the Co content due to its high cost and toxicity. The above compound contains 0.5 mol of Ni and 0.1 mol of Al, which are at least ten times cheaper and less toxic than Co, and shows a capacity comparable to that of LiCoO2, viz., 137 mAhg-1 in the voltage range 3.0-4.2 V vs Li metal. Cycling LiNi0.5Co0.4Al0.1O2 to an upper cutoff voltage of 4.4 or 4.5 V yielded higher 10th cycle capacities, but they were found to degrade on long-term cycling. Thus, with 4.4 V cutoff, the 20th cycle capacity of 165 mAhg-1 remained stable till the 45th cycle, after which it degraded to 150 ((3) mAhg-1 at the end of 120 cycles, which corresponds to 9% capacity-fading (Figures 4c and 5c). Similarly, with the 4.5 V cutoff, 7% capacity fading was noted in the range of 10-40 cycles (Figure 5c). The observed capacity fading can possibly be attributed to the nonsuppression of another phase transition that may be occurring in the vicinity of 4.4-4.6 V. It may be possible to improve the performance of LiNi0.5Co0.4Al0.1O2 with a 4.4 V cutoff by optimizing the synthesis conditions like the temperature and time of soaking and metal to salt ratio. The ex situ XRD patterns (figures not shown) of the cycled electrodes of LiNi0.5Co0.5O2 prepared at 850 °C in the charged state (3.9-4.7 V) and discharged state (2.5 V) of several identical cells were recorded after dismantling them and recovering the electrodes (along with the Al foil) in the glovebox. The lattice parameters were calculated by the leastsquare fitting method and are given in Table 1. The a and c parameters of the cycled electrodes in the discharged-state agree well with the bare compound indicating that the crystal structure is stable to cycling. As can be expected, the a parameter decreased and c and c/a increased in the charged-state up to 4.4 V for both LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2. This is due to the electrostatic repulsion of the oxide ion layers caused by the removal of Li ions from Li layer. When charged to 4.5 or 4.7 V, the a parameter shows a slight increase and the c parameters show a decrease in value in comparison to the 4.4 V possibly due to a structural phase transition and also due to significant amounts of the Ni4+/Co4+ ions which have smaller ionic radii present in the compound in the charged-state (Table 1). These observations are in accord with the data reported on LiNi0.5Co0.5O2.11,19 3.2.3. Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) can be used as a technique to understand the electrode kinetics of the cathode and anode metal oxides. In the literature, EIS studies have been carried out on various oxide cathodes such as pure and doped LiCoO2,32-35 LiNiO2,30,32,34 LiNi1-xCoxO2,6,30,36 LiNi1/3Co1/3Mn1/3O2,37 and LiNi1/2Mn1/2O238 and anode materials like graphite,34,39 SnO,40 and CoO.41 Presently, we measured impedance spectra of

11718 J. Phys. Chem. C, Vol. 111, No. 31, 2007

Reddy et al.

Figure 6. Impedance spectra (Z′ vs -Z′′ plots) of (a-c) LiNi0.5Co0.5O2 and (d-f) LiNi0.5Co0.4Al0.1O2 vs Li during 1st and 10th charge-cycles and in the charged state (4.3 V) as a function of cycle number. Voltage range, 2.5-4.3 V at 30 mAg-1, at ambient temperature. The voltages, cycle number, and frequencies at select regions are shown. The symbols represent experimental data. The continuous lines represent fitting with an equivalent circuit to extract the impedance parameters. Geometrical area of the electrode is 2.0 cm2.

Figure 7. Equivalent circuit used for fitting the impedance spectra of Figure 7 consisting Ri and Ri||CPEi combinations. Re, R(sf+ct), and Rb are impedances (resistances) due to electrolyte and cell components, surface film (sf) plus charge transfer (ct) and bulk (b), respectively. CPEi is the respective constant phase element to account for the depressed semicircle in the experimental spectra (dl refers to double layer). Ws is the finite length Warburg (short circuit terminus) element and Cint is intercalation capacitance.

LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 (850 °C synthesis) vs Li at various voltages in the range of 2.5-4.3 V at a current of 30 mAg-1 at RT. The freshly fabricated cells were aged for 24 h, and spectra were recorded during the 1st and 10th cycle. At each voltage, the cells were relaxed for 3 h before measurement. After a select number of cycles, the spectra were recorded in the charged state, 4.3 V. The spectra in the form of Nyquist plots (Z′ vs -Z′′) are shown in Figure 6. They were analyzed by fitting to an equivalent electrical circuit shown in Figure 7. The circuit consists of a resistance (Ri), Ri||CPEi, a Warburg element (Ws), and an intercalation capacitance (Cint). The constant phase element (CPEi) replaces the capacitance of the

electrode due to the observed depressed semicircles in the spectra. The symbols are the experimental data, whereas the continuous lines are the fitted ones in Figure 6. Select frequencies are also shown in the spectra. The proposed circuit and the assignment of Ri|CPEi are in conformity with the recent trends in the interpretation of impedance spectra of electrode materials.6,35,42,43 Here, Re is the combined impedance (resistance) of the electrolyte and cell components. The depressed semicircle in the high-frequency range of 0.3 MHz to 5 kHz is attributed to the surface-film (sf) resistance, whereas that seen in the high-to-medium-frequency range of 5-0.1 kHz is assigned to the charge-transfer (ct) resistance. The semicircle in the low-frequency range of 1000.1 Hz is attributed to the bulk (b) resistance. The semicircle(s) is usually followed by the appearance of straight line Warburgtype region. The respective constant phase elements are CPEsf, CPEdl, and CPEb where dl is the double layer. Many a time, only one semicircle is seen in the high-to-medium-frequency range indicating that effects due to sf and ct are not separable. In this case, a proper assignment will be R(sf+ct) and CPE(sf+dl). The contribution to Rb comes not only from the bulk impedance of the active material (LiNi0.5Co0.5O2) but also from the electrolyte trapped in the pores of the composite electrode and that due to inhomogeneous coating of active material on to the current collector. Accordingly, high values, in the mF-range, of CPEb are encountered in contrast to CPEsf and CPEdl which

Synthesis for Li Ion Batteries

Figure 8. Variation of surface film + charge-transfer resistance (R(sf+ct)) and bulk resistance (Rb) as a function of voltage during the 1st and 10th charge-discharge cycle and cycle number in the charged-state (4.3 V vs Li) of (a-c) LiNi0.5Co0.5O2 and (d-f) LiNi0.5Co0.4Al0.1O2. Filled and open symbols represent charge and discharge cycle respectively. Size of the data points represents the estimated uncertainty in values.

are in the range of µF. Due to changes in the electrode kinetics, these impedance parameters will exhibit a variation as a function of voltage during charge-discharge cycling. The spectra during the 1st and 10th charge cycle at various voltages for pure and Al-doped LiNi0.5Co0.5O2 are shown in Figure 6, panels a and b and d and e, respectively. Spectra during the corresponding discharge cycle are qualitatively similar. Spectra in the charged state (4.3 V) as a function of cycle number are shown in Figure 6, panels c and f, for LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2, respectively. Only a single semicircle in the high-frequency range followed by the Warburg region was observed in fresh cells and also after the respective 1st and 10th discharge cycle, indicating mainly contribution due to R(sf+ct) and CPE(sf+dl). The appearance of a second semicircle is clearly seen in the spectra at V g 3.7 V during any given cycle and more clearly in the fully charged-state indicating that bulk resistance (Rb) falls in the measurable range (Figure 6, panels c and f). The extracted impedance parameters, Ri ,as a function of voltage of the cell are plotted in Figure 8. The lines joining the data points are only a guide to the eye. The size of the data points reflect the uncertainty in the resistance values. The data points during the 10th charge and discharge cycle overlap fairly well showing good reversibility of both the electrodes (Figure 8, panels b and e). Trends in the variation of R(sf+ct) and Rb as a function of voltage are clearly seen in both the compounds during the 1st and 10th cycle. Although the R(sf+ct) decreases in a continuous fashion, by a factor of 2 or 3 upon increasing the voltage from 2.5 to 4.3 V, the Rb decreases by a factor of more than 10 in the voltage range of 3.6-4.0 V and there after remains almost constant. This is indeed expected since the creation of Ni4+ and Co4+ ions in LiNi0.5Co0.5O2 at V > 3.7 V

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11719 due to the removal of Li ions from the lattice will produce enormous decrease of electronic resistivity. In fact, a detailed study of the changes in the electronic resistivity (F), activation energy for conduction (Ea, in the temperature range, 100-300 K), and chemical diffusion coefficient (Li ion mobility, DLi) at 300 K in the Li1-xNi0.5Co0.5O2 system as a function of x by Molenda et al.44 has shown a decrease in F (300 K) by 2 orders of magnitude, from ∼10 to ∼0.1 Ω cm, a change in Ea from 0.2 to 0.07 eV, and an increase in DLi by 3 orders of magnitude, from 3 × 10-11 to 1.5 × 10-8 cm2/s on going from x ) 0 to 0.7. These drastic changes have been interpreted as being due the progressive modification of the electronic band structure of Li1-xNi0.5Co0.5O2 from a semiconductor toward a metallic behavior with increasing x. Analogous semiconductor-to-metal type transitions are encountered as a function of increasing x in Li1-xCoO235 and Li1-xNi0.8Co0.2O2,6 and these transitions are reversible. Variation of R(sf+ct) and Rb as a function of cycle number in the charged-state (4.3 V) gives an insight in to the reasons for the excellent cyclability of the Al-doped compound, LiNi0.5Co0.4Al0.1O2: The value of R(sf+ct) is almost unchanged at 32 ((3) Ω, and Rb increases only from 6 to 20 Ω in the range of 10-58 cycles (Figure 8f). On the other hand, in LiNi0.5Co0.5O2 both R(sf+ct) and Rb increase from 49 to 72 Ω and 19 to 74 Ω, respectively, during 6-60 cycles (Figure 8c). Obviously, this increase is due to the reversible structural transitions which occur in the undoped compound below 4.2 V. Suppression of the above phase transitions in the Al-doped compound, on the other hand, gives rise to small and slowly varying values of R(sf+ct) and Rb as a function of cycle number, and thus contribute to negligible capacity fading up to 120 cycles. The observed CPE(sf+dl) and CPEb values for both the compounds are in the ranges of 20-100 and 30-170 mF, respectively, and showed similar variations as those of R(sf+ct) and Rb as a function of voltage as well as cycle number. The extracted values of Re are 3.5 ((0.5) Ω and Cint varied from 0.1 to 2.0 ((0.05) F in all cases for both pure and Al-doped LiNi0.5Co0.5O2. 4. Summary and Conclusions Layered oxide cathodes, LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 were prepared by a one-pot synthesis method using molten salt eutectic of 0.88LiNO3:0.12 LiCl at temperatures, 750 and 850 °C in air. The mole ratio of starting oxides to salt mixture was kept at 1:4 and a soaking time of 8 h was employed. The compounds were characterized by X-ray diffraction (XRD), SEM-EDAX, chemical analysis, surface area, and density. Rietveld refinement of the XRD data showed that negligible cation mixing for both compounds and this finding can be ascribed mainly due to the presence of large excess of Li during synthesis. Electrochemical properties were studied at ambient temperature in cells with Li metal as the anode at a current density of 30 mAg-1 with upper cutoff voltages in the range of 4.2-4.5 V. The lower cutoff voltage was 2.5 V. Galvanostatic charge-discharge cycling of LiNi0.5Co0.5O2 showed 10th cycle reversible capacities ranging from 131 to 186 ((3) mAhg-1 depending on the synthesis temperature and upper cutoff voltage. Capacity-fading was noted by 5-18% at the end of 50 or 80 cycles in LiNi0.5Co0.5O2 irrespective of the upper cutoff voltage and temperature of synthesis. On the other hand, LiNi0.5Co0.4Al0.1O2 performed well: the 20th cycle reversible capacity of 146 ((3) mAhg-1 with the upper cutoff voltage of 4.3 V was stable up to 120 cycles. Good rate capability was also shown. With a 4.4 V cutoff, the 20th cycle capacity of 164 ((3) mAhg-1 remained stable till the 40th cycle after which it

11720 J. Phys. Chem. C, Vol. 111, No. 31, 2007 degraded by 9% at the end of the 120th cycle. With a 4.5 V cutoff, capacity fading was noted by 7% in the range of 10-40 cycles. Cyclic voltammograms showed indication of two reversible phase transitions in the range of 3.8-4.2 V in LiNi0.5Co0.5O2, and they are found to be suppressed in LiNi0.5Co0.4Al0.1O2. However, another structural transition may exist at 4.54.6 V and could be the reason for the observed capacity-fading when cycled to the 4.4 or 4.5 V cutoff. Impedance spectra of the cells with both the pure and Al-doped LiNi0.5Co0.5O2 have been recorded during the 1st and 10th cycle as well as in the charged-state (4.3 V) after select number of cycles (10-60). The data were fitted to an equivalent circuit, and the relevant parameters were evaluated. Variations in the R(sf+ct) and Rb as a function of voltage and cycle number were interpreted with respect to changes in the electronic resistivity of the active materials and suppression or otherwise of the phase transitions below 4.2 V. The present study shows that LiNi0.5Co0.4Al0.1O2 with only 0.4 mol of cobalt is a cheaper and viable alternative to LiCoO2, the favored cathode for LIB. Scope exists for an improvement in its performance by optimizing the synthesis conditions and Al content. Acknowledgment. Thanks are due to M. Gupta, Mechanical Engg. Dept., NUS, for help with the SEM, to Chen Ping, Physics Dept., NUS, for help with XRD, Ms. Doreen Lai, IMRE, for help with EDAX, and Ms. Leng Lee Eng and Ms. Joanne Soong, Chemistry Dept., NUS, for chemical analysis. K.S. Tan participated in the preliminary synthesis and electrochemical studies. References and Notes (1) Ohzuku, T.; Ueda, A. Solid State Ionics 1994, 69, 201. (2) Nazri, G.-A., Pistoia, G., Eds.; Lithium Batteries: Science and Technology; Kluwer Academic Publishers: New York, 2003. (3) (a) Ohzuku, T.; Ueda, A. J Electrochem. Soc. 1994, 141, 2972. (b) Chen, Z.; Lu, Z.; Dahan, J. R. J. Electrochem. Soc. 2002, 149, A1604. (c) Chen, Z.; Dahn, J. R. Electrochim. Acta 2004, 49, 1079. (4) Delmas, C.; Saadoune, I.; Rougier, A. J. Power Sources 1993, 4344, 595. (5) (a) Ohzuku, T.; Ueda, A.; Nagayama, M.; Iwakoshi, Y.; Komori, H. Electrochim. Acta 1993, 38, 1159. (b) Ueda, A.; Ohzuku, T. J. Electrochem. Soc. 1997, 144, 2780. (6) Nobili, F.; Croce, F.; Scrosati, B.; Marassi, R. Chem. Mater. 2001, 13, 1642. (7) Chebiam, R. V.; Prado, F.; Manthiram, A. Chem. Mater. 2001, 13, 2951. (8) Venkatraman, S.; Manthiram, A. Chem. Mater. 2002, 14, 3907. (9) Madhavi, S.; Subba Rao, G. V.; Chowdari, B. V. R.; Li, S. F. Y. Solid State Ionics 2002, 152, 199. (10) D’Epifanio, A.; Croce, F.; Ronci, F.; Albertini, V. R.; Traversa, E.; Scrosati, B. Chem. Mater. 2004, 16, 3559. (11) Ueda, A.; Ohzuku, T. J. Electrochem. Soc. 1994, 141, 2010. (12) Sun, Y. -K.; Oh, I. - H.; Kim, K. Y. J. Mater. Chem. 1997, 7, 1481. (13) Nakai, I.; Nakagome, T. Electrochem. Solid State Lett. 1998, 1, 259.

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