Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 36:71–81, 2006 Copyright # 2006 Taylor & Francis Group, LLC ISSN: 0094-5714 print/1532-2440 online DOI: 10.1080/15533170500474130

Electrochemical Characterization of Nanocrystalline LiMxCo1-xO2 (M 5 Mg, Ca) Prepared by a Solid-State Thermal Method R. Sathiyamoorthi, R. Chandrasekaran, P. Santhosh, K. Saminathan, R. Gangadharan, and T. Vasudevan Department of Industrial Chemistry, Alagappa University, Karaikudi, India

used cathode material in the majority of commercially available lithium-ion batteries due to its ease of synthesis, high reversibility and long cycle life.[1] The structure of LiCoO2 belongs to the rhombohedral space group R3¯m (layered a-NaFeO2-type structure) with lithium atoms in the 3a positions, transition-metal atoms in the 3b positions, and oxygen atoms in the 6c positions (with respect to hexagonal axes).[2] Attempts to improve the cyclability beyond 4.25 V by doping have been reported.[3,4] The high cost and toxicity Co, however, makes of LiCoO2 inhibits its use as a commercial cathode. In order to reduce the cost and to improve the cell voltage and specific energy, other transition metals such as Cr,[5] Mn,[6] Fe,[7 – 9] Ni[9 – 11] and Rh[12] are used to partially substitute Co in LiCoO2. Non transition metals such as Al, Ca and Mg, which have fixed oxidation states, may also be used to particularly substitute Co. Ceder et al.[13] have predicted, from first principles and also shown by experiment, that substitution, of Al increases the potential as well as the performance of LiCoO2. Consequent to this theoretical prediction several studies of the substitution of Co by Al have been reported.[13 – 21] Since Mg is light in weight and cheap, it has been considered as a substituent for Co. By contrast, there have been very few investigations of Mg substitution.[22] It is believed that doping with M-additives could stabilize the layered framework, which extends the cyclability and enhances the capacity of the electrochemical Li/ LiCo1-xMxO2.[23] In order to overcome the cycling problems at high voltages, recently, Cho et al.[24 – 26] has demonstrated that by coating LiCoO2 with ceramic materials like Al2O3 and ZrO2, show stable capacities even when cycled up to 4.5 V and thus was also confirmed by Chen and Dahn.[27] However, there is no report on the prolonged cycling of doped LiCoO2 at high voltages (4.5 V) involving metal ions such as Mg and Ca. Hence, a study was made to dope LiCoO2 with Mg and Ca respectively by solid-state thermal reaction method. In this communication, the physical characterization and

Mg-doped LiCoO2 and Ca-doped LiCoO2 are prepared by a solid state thermal method and are used as cathode active materials for lithium ion batteries. The synthesized electrodes are characterized by X-ray diffraction and SEM studies. LiCoO2 prepared by solid state reaction method, shows a capacity decay over 4.3 V but LiCoO2 doped with Mg and Ca provide an improvement in charge – discharge cycling performance. This effect may be attributed to the fact that Mg21 and Ca21 ions have the same size as the Li1 ion and they preferably locate at the inter-slab space. Therefore, they give a pathway for the intercalation – deintercalation of Li1 during the charge and the discharge studies and prevent distortion of the structure. The products possess good morphology, nano level particle size. The excellent electrochemical performance of the LiMxCo1-xO2 cathode active material is attributed to the novel preparation and is significant to further studies.

Keywords

nanocrystalline, lithium cells, charge-discharge studies, magnesium, SEM, XRD

INTRODUCTION Lithium-ion batteries are emerging as a major source of power in view of their varied applications ranging from cell phones to electric vehicles as also in the medical field. The system usually involves the use of lithiated transition metal oxides, namely, LiCoO2, LiNiO2 and LiMn2O4 cathode material and also as lithium source. However, among these materials, lithium cobalt oxides (LiCoO2) is the most widely Received 20 July 2005; accepted 28 October 2005. Financial assistance received from the Department of Science and Technology (DST), Government of India, is gratefully acknowledged. Address correspondence to T. Vasudevan, Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003, India. E-mail: [email protected].

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FIG. 3. TG/DTA curves of precursors of LiCaxCo1-xO2.

FIG. 1. Systematic preparation procedure for Mg-doped or Ca-doped LiCoO2 by solid-state thermal reaction method.

electrochemical performance of the synthesized materials in lithium rechargeable cells are attempted. EXPERIMENTAL Doped LiCoO2 of the type LiMxCo1-xO2 with Mg or Ca has been prepared by mixing stoichiometric amounts of LiNO3, Co(NO3)2, with Mg(NO3)2 or Ca(NO3)2 and urea as a fuel self heat generating and heated slowly in a muffle furnace to

FIG. 2. TG/DTA curves of precursors of LiMgxCo12xO2.

1508C for 2 hours, and then cooled the materials. The above said materials were ground and mixed well before finally annealing to 8008C for 10 hours. The synthesized mixture was cooled slowly in the furnace and was ground well to particle size. The product was characterized physically and electrochemically. The above procedure is presented in Figure 1. The phase purity of the synthesized powder was analyzed by X-ray (XRD, JDX 8030) with Cu Ka radiation. The scan range was 10 –808 with a scan step of 0.028 and a scan speed of 108 per minute. Thermogravimetric analysis (TGA) was helped to determine the weight loss up to 6008C. The surface of the synthesized powders was examined by a scanning electron microscope (SEM)(JEOL, JSM –840 A). Electrochemical measurements were carried out at room

FIG. 4. XRD patterns of LiCoO2 (8008C).

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temperature (288C) with cells that include carbon as the anode, a positive electrode as cathode with a mixture composed of 85% weight percent (w/o) of active material, 10% (w/o) acetylene black and 5% (w/o) polyvinylidene difluoride (PvDF) in N-methyl-2-pyrrolidone (NMP) as a binder. The electrolyte was a 1 M LiBF4 in EC-DMC in 1 : 1 volumetric ratio. A microporous polypropylene film was used as a separator. When a lithium ion is inserted into the active material, the electron must move simultaneously. The charge-discharge studies were galvanostatically done with a battery cycler (WonA Tech., WBC 100) at a specific current of 0.1 mAh/g. The voltage ranges of charge and discharge were from 4.5 to 3.0 V.

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RESULTS AND DISCUSSION Structural Studies TG/DTA Curves The solid precursor was the intermediate product obtained on the way to synthesizing LiMgxCo1-xO2 and LiCaxCo1-xO2 by the solid-state thermal reaction method. The thermal behaviour of the gel precursor has been found[28] to determine the process condition, the crystallinity of the powder, the particle, size, the lattice constant, and the specific surface area. Accordingly, the thermal behaviour was analyzed by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) (Figures 2 and 3).

FIG. 5. XRD patterns of LiMgxCo1-xO2 at different compositions (8008C).

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The TG/DTA curves for the LiMgxCo1-xO2 and LiCaxCo1-xO2 precursors obtained from lithium nitrate, cobalt nitrate, with magnesium nitrate or calcium nitrate with urea as a fuel are presented in Figure 2 and 3. A weight loss of 20% occurs initially until 1508C, and an additional decrease of 10% is observed at 4008C and water present in the salts. The initial decrease is due to the thermal decomposition of solvent, nitrate and urea.[29] The secondary weight loss is due to the thermal decomposition between carbon and oxygen of the urea.[29] There is no weight loss above 6008C. It is safer to keep the solid precursor must be heated above 5008C. The above experiments indicate that for minimum

temperatures in between 6008 to 8008C because 70% of the weight was lost below 5008C. It is possible that loss of weight LT-products of LiMgxCo1-xO2 and LiCaxCo1-xO2 (low temperature) and HT- LiMgxCo1-xO2 and LiCaxCo1-xO2, can be prepared according to the calcination temperatures. There varieties show are differences in electrochemical characteristics. It is observed that Liþ intercalation-deintercalation of LT-LiMgxCo1-xO2 and LiCaxCo1-xO2 occurs at lower voltages than that of HT- LiMgxCo1-xO2 and LiCaxCo1-xO2 type compounds. The primary calcinations temperature condition is for LT varieties in between 4008C to 5008C for 10 hours, and the secondary calcinations are in between 6008C to 8008C

FIG. 6. XRD patterns of LiCaxCo12xO2 at different compositions (8008C).

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for 10 hours. However in this paper only HT-LiMgxCo1-xO2 and LiCaxCo1-xO2 which were synthesized at 8008C and studies in detail. X-ray Diffraction Figures 4, 5(a – d) and 6(a –d) show the powder X-ray diffraction patterns of bare LiCoO2, Mg and Ca doped lithium cobaltates at 8008C with different compositions. The XRD patterns of LiMgxCo1-xO2 and LiCaxCo1-xO2 samples are dominated by a strong Bragg peak located at ca. 2u ¼ 198 and Bragg peaks with medium intensity at 368 and 448, respectively. Considering the intensity position of the Bragg peaks, it is possible to index either to a rhombohedral unit cell (R3¯m) of a layered structure (or) because the c/a ratio equals 4.90, to a face centered cubic unit cell (Fd3¯ m) of a spinel

FIG. 7. Variations of lattice parameters with Mg content.

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structure.[30,31] LiM0.1Co0.9O2, LiM0.2Co0.8O2, LiM0.3Co0.7O2 and LiM0.5Co0.5O2 where, M ¼ Mg, Ca grow with the tendency of a layered a-NaFeO2-type structure. Their XRD patterns appear to be similar to those reported for LiCoO2.[32] In the diagrams of LiMgxCo1-xO2 and LiCaxCo1-xO2 powders, we detect the presence of a small amount of a second phase. The rhombohedral R3¯m structure is based on alternative ordering of Li in the octahedral sites between (Co1-xMxO2) infinite slabs formed by edge-sharing octahedral. Hexogonal cell parameters of the oxides prepared by different doping elements with different metal-metal compositions that were calculated by least-squares refinement are given in Figure 7. It is seen distinctly that substitution of Mg for Co in LiCoO2 results in decrease in ‘a’ and increase ‘c’ parameters. This phenomenon is due to shrinking of the inter-atomic distance within the CoO2 layer, which results from the substitution of Co3þ by the smaller Mg2þ ion. The LiMgxCo1-xO2 compounds grown by solid-state thermal

FIG. 8. SEM photograph of different magnifications of LiMgxCo1-xO2 (8008C) (x ¼ 0.2).

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reaction method exhibits XRD patterns with well defined (006,102) and (009,107) doublets. The c/a ratio higher than the critical value 4.721 and the clear splitting of the (006) and (102) as well as (009) and (107) diffraction lines indicate that, as far as XRD patterns are concerned, an ordered distribution of lithium and transition-metal ions exists in the structure. On the other hand, the XRD diagram of LiCaxCo1-xO2 materials at 8008C (Figure 6(a–d)) exhibits peaks due to impurities. Calcium up to a composition of x ¼ 0.2 forms a solid solution and the XRD patterns are also indexed to a hexagonal system with a R3¯m space group, as seen in Figure 6(a – d). For compositions beyond x ¼ 0.2, it is a mixture of CaO and the layered compound. The formation of a solid solution is more pronounced with the hexagonal lattice parameters “a” and “c” (not shown) of the R3¯m space group. It is observed that both the parameters increase with the substitution of Ca2þ, which is a significantly larger ion than Co3þ.

FIG. 10. Cyclic voltammograms of bare LiCoO2 electrode (8008C) at scan rate of a 50 mV/s with 1 M LiBF4 in EC:DMC as the electrolyte.

Scanning Electron Microscope Representative scanning electron micrographs of LiMgxCo1-xO2 and LiCaxCo1-xO2 samples are presented in Figures 8 and 9. The particles are found to be crystalline with well-defined facets that have a wide range of distribution (100 – 200 nm). Further, the particles exhibit a uniform distribution and are smaller than the parent LiCoO2.[33] From the all above SEM photographs, the proportions of x ¼ 0.2 is the best in the sense of particle size, which is smaller than other compositions of the both Mg-doped and Ca-doped LiCoO2.

FIG. 9. SEM photograph of different magnifications of LiCaxCo1-xO2 (8008C) (x ¼ 0.2).

FIG. 11. Cyclic voltammograms of LiMgxCo1-xO2 electrode (8008C) (x ¼ 0.2) at scan rate of a 50 mV/s with 1 M LiBF4 in EC:DMC as the electrolyte.

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Electrochemical Studies Cyclic Voltammograms and Charge-Discharge Studies The substituted materials have been used as positive electrode in lithium-ion batteries in numerous laboratories. From a practical point of view, several improvements must be achieved vs. LiCoO2 with the following order of priority: .

. .

.

Increase of the stability of the lithium deintercalated phase to improve the cell life, decrease of the fading, decrease of the irreversible capacity for a good capacity balance with the carbon negative electrode, increase of the reversible capacity.

All these points must be optimized simultaneously, keeping in mind that the cost of the material is one of the most important parameters. Among all the substituting cations, which have been proposed, so far, cobalt seems to play a key role in obtaining a strictly layered structure, which leads to a decrease of the irreversible capacity and to an increase of the reversible one. Moreover, it slightly improves the material stability in the oxidised state.[34 – 36] However, substituting cations that increase the 3D character of the structure (like Fe) must be avoided since they lead to a poor cyclability. As stated by Ohzuku, aluminium substitution limits the amount of lithium that can be deintercalated and therefore prevents lithium over deintercalation.[37] Moreover, aluminium strongly stabilises the deintercalated material, leading to a safer battery.[37,38] Recently, simultaneous substitution of magnesium and titanium for nickel allowed FMC to propose a very safe electrode.[39] In order to assess electrochemical activity, cyclic voltammograms of LiMgxCo1-xO2 and LiCaxCo1-xO2 were recorded.

FIG. 12. Cyclic voltammograms of LiCaxCo1-xO2 electrode (8008C) (x ¼ 0.2) at scan rate of a 50 mV/s with 1 M LiBF4 in EC:DMC as the electrolyte.

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The voltammogram of LiCoO2 (Figure 10) shows an anodic current peak at 4.14 V and a cathodic peak at 3.72 V. There are also two weak cathodic peaks at 4.0 and 4.13 V, which may be due to an order-disorder transition of Li-ions in the CoO2 framework, as reported by Dokko et al.[40] The cyclic voltammograms of LiMg0.2Co0.8O2 and LiCa0.2Co0.8O2 (Figures 11 and 12) are similar to that of LiCoO2 (Figure 10). The weak cathodic peaks, however, are absent in the Mg and Ca substituted LiCoO2. These data qualitatively suggest that the compounds prepared by solid-state thermal reaction method are electrochemically active. The possibility to use bare – LiCoO2, Mg-doped and Cadoped LiCoO2 as cathode materials in Lithium-ion batteries were tested in constant current experiment (Figure 13(a – c)) respectively. In the case of LiCaxCo1-xO2, the discharge

FIG. 13. Charge-discharge curves: (a) LiCoO2(10.2 mg) (b) LiMg0.2Co0.8O2 (8008C) (11.4 mg); and (c) LiCa0.2Co1-xO2 (8008C) (11.8 mg) ((O) Charging; (†) discharging; current ¼ 0.mAh/g).

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curves are steep; the steepness increases at higher values of x. It is seen that the efficiency of discharge-charge capacities is more than 80% in LiCoO2, LiCa0.2Co0.8O2, but is only about 60% in LiCa0.5Co0.5O2. After charging the Mg doped samples to 4.5 V, the cells were allowed to equilibrate for 20 hours and the open-circuit voltage (OCV) was measured (Figure 14). Changes in discharge capacity with cycling were measured in the voltage range of 3.0 –4.5 V. The cycling performance of bare-LiCoO2, Mg-doped LiCoO2 and Ca-doped LiCoO2 calcined at 8008C of several compositions in the voltage range of 3.0–4.5 V are presented in Figure 15(a–c), respectively. The first discharge capacity of LiCoO2 is as high as 127 mAh g21, and stabilizes at 100 mAh g21 after a few cycles. The average capacity is
96 mAhg21, which is comparable with values reported in the literature.[14,18] In the present study of LiCa0.2Co0.8O2, the first discharge capacity is as high as 106 mAh g21 and the capacity on the 10th cycle is 60 mAh g21 (Figure 15c). Thus, there is a 40% decrease of capacity by the end of 10 cycles. Nearly, all other composition of Ca-doped LiCoO2, the charge retentions is found to be less than that of LiCa0.2Co0.8O2 composition. It is thus found that LiCaxCo1-xO2 with x ¼ 0.2 is a better than variants with x . 0.2, in terms of both discharge and capacity retention. In the present study, it is found that the average discharge capacity is 80 mAh g21, which is higher than that found by other workers.[14,18] Cycle life data of LiMg0.2Co0.8O2 (8008C) is also given in Figure 15(b). The first discharge capacity of the electrode is as high as 132 mAh g21, but at the 10th cycle is only 86 mAh g21. The average discharge capacity of LiMg0.2Co0.8O2 is 98 mAh g21. Here also, the other compositions have low discharge capacities than the LiMg0.2Co0.8O2 compound. Electrochemical Impedance Measurements Electrochemical impedance spectroscopy studies reveal several important parameters of batteries and battery electrodes.[41] These parameters are seminal: (i) to derive information on the optimum utilization of the storage batteries, (ii) to find the modes of cell failure, and (iii) to determine the state of

FIG. 14. Open-circuit voltage of LiMgxCo1-xO2 (8008C).

FIG. 15. Cycle life data of (a) LiCoO2; (b) LiMg0.2Co0.8O2 at 8008C; and (c) LiCa0.2Co0.8O2 at 8008C.

charge (SOC). In the present study, this technique is used to evaluate the electrodes of different compositions and to find a correlation between the AC impedance and DC charge – discharge cycling data. Nyquists plots of impedance of LiMgxCo1-xO2 and LiCaxCo1-xO2 at different compositions are shown in Figures 16, 16a and 17, 17a, respectively. In general, the impedance spectrum is characterized by a pair of semicircles at high and mediumfrequency ranges, and occasionally, a linear spike at low-frequency range. The lowfrequency spike, which is prominent only at high SOC values, is due to diffusion-controlled Warburg impedance. In as much as the spike does not appear in all impedance spectra, and hence this part of the spectrum is ignored, and the data corresponding to the semicircles part are subjected to nonlinear least squares (NLLS) fitting.[41] The equivalent

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FIG. 16. Electrochemical impedance spectrum of LiMgxCo1-xO2 electrode (8008C) (x ¼ 0.2). (a) At high frequency end.

circuit used for this purpose is shown in Figure 18. Various circuit elements shown in Figure 18 are explained as follows: the element RV represents the ohmic resistance, which is due to the electrolyte, current collectors, electrical leads, etc. As the two capacitive semicircles are depressed, a constant phase element (CPE) is taken in place of capacitance. Accordingly, R1 and Q1 are resistance and CPE, respectively, corresponding to the high-frequency semicircle; R2 and Q2 are resistance and CPE, respectively, corresponding to the low-frequency semicircle. The CPE arises due to the microscopic material properties. The interfaces between the electrode/ electrolyte are not smooth and uniform, as the electrodes are

made using fine particles of the active materials. The intercalation and deintercalation of lithium process are uniform throughout the surface of the electrodes. Reaction resistance and capacitance may differ with the electrode position, nonuniform thickness of the electrode materials, etc. The admittance representation of CPE is given as:[41] Q ¼ Q0 ð jvÞn

ð1Þ

where Q0 is an adjustable parameter and v ¼ 2pf, f being the AC frequency. For n ¼ 0, CPE represents a resistance R (55Q21 5Q0); for n ¼ 0.5, a 0 ); for n ¼ 1, a capacitance C (5 Warburg impedance; and for n ¼ 21, an inductance L

FIG. 17. Electrochemical impedance spectrum of LiCaxCo1-xO2 Electrode (8008C) (x ¼ 0.2). (a) At high frequency end.

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FIG. 18. Equivalent circuit used for the NLLS fitting of the experimental impedance data. See the text for definition of symbols.

(55Q21 0 ). The spectrum consists of two semicircles of different sizes, the high frequency semicircle being smaller than the low-frequency one. It has been found that the impedance plot obtained by using a cell with only a fully charged carbon electrode is similar to the larger semicircle at low frequencies. Based on these observations, of the low-frequency larger semicircle was attributed to the charge-transfer reaction at the carbon electrode, and of the high-frequency smaller semicircle to the charge-transfer reaction at corresponding Mg- and Cadoped LiCoO2 electrode (Figures 16, 16a, 17 and 17a). In a similar single electrode study with LiNiO2, LiCoO2, and LiMn2O4 electrodes in nonaqueous media, Aurbach et al.[42] have reported a pair of semicircles of unequal size. The values of these two parameters are minimal at x ¼ 0.2, and they increase with an increase of Mg and Ca concentrations respectively. The minimal values of R1 and R2 for the sample with x ¼ 0.2 support the maximum discharge capacity obtained. With an increase of x beyond 0.2, there is an increase of R1 and R2 consistent with a decrease in the discharge capacity. These results support that the LiMgxCo1-xO2 and LiCaxCo1-xO2 prepared from solid-state thermal reaction route is electrochemically active as a positive electrode material of Li-ion cells. CONCLUSIONS The positive electrode materials LiMgxCo1-xO2 and LiCaxCo1-xO2 have been synthesized using a solid-state thermal reaction method at 8008C. X-ray diffraction studies suggested that the samples with x ¼ 0.1, 0.2, 0.3 and 0.5 have a layer structure. Specific capacity of the sample Mgand Ca-doped lithium cobaltates with x ¼ 0.2 is found to be 132 and 106 mAh g21 respectively, which is higher than the capacity value of LiCoO2 synthesized by the same method and also the value reported for it in the literature. Impedance spectroscopy shows that the charge-transfer resistance is minimum for the composition x ¼ 0.2, thereby supporting the high and stable capacity obtained for this sample. REFERENCES 1. Julien, C.; Gastro-Garcia, S. Lithiated cobaltates for lithium-ion batteries: Structure, morphology and electrochemistry of oxides grown by solid-state reaction, wet chemistry, and film deposition. J. Power Sources 2001, 97 – 98, 290– 293.

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