Improved Electrochemical Investigation of Combustion Synthesized Cd-doped LiCoO 2 Powders

The LiCo1-xCdxO2 (x = 0 to 0.1) cathode materials were synthesized by the combustion method (annealed at 800 °C for 12 h). Structural analyses of the synthesized materials were carried out using XRD. All the samples had the R-3m structure. LiCo0.90Cd0.1O2 sample contained CdO phase as an impurity. Fourier transform infrared (FTIR) studies were carried out to understand the nature of the metal-ligand bond. The surface morphology and particle agglomeration was investigated using scanning electron microscope. The room temperature electrical conductivities of the sample increased with Cd content. For LiCo1-xCdxO2 (x = 0, 0.01, 0.03, 0.05 and 0.1), the first discharge capacity increased with increase in Cd content. Among these samples, LiCo0.95Cd0.05O2 had good cycling performance (discharge capacity 195 mAh g at n = 30).


Introduction
Advances in electrode active material play a critical role on the development of lithiumion batteries.Among the several cathode materials, lithium transition metal oxides have shown attractive electrochemical performance in the applications involving lithium battery devices and they are utilized as positive electrodes due to their high redox potential and specific capacity during the lithium intercalation/deintercalation processes [1].LiCoO 2 is the most preferred cathode active material as it is widely used in commercial lithium-ion batteries and was first reported as cathode active material by Misushima et al. [2].It is well known that the practical capacity of LiCoO 2 is limited to around 140 mAhg -1 i.e., nearly half of its theoretical capacity (273 mAhg -1 ).This material has an ordered rock salt structure (α-NaFeO 2 structure) and belongs to R-3m space group.The high cost and toxicity of cobalt, however, makes the use of LiCoO 2 undesirable.In order to reduce the cost and to improve the cell voltage and specific capacity, attempts have been made to partially substitute cobalt with either transition metals or non-transition metals [3][4][5].Recent attempts to improve the performance of LiCoO 2 by means of doping or coating have shown promising results for obtaining higher capacity with enhanced stability of the LiCoO 2 electrodes.Such modifications led to either improved capacity retention characteristics by suppressing the surface reactions of LiCoO 2 particles or higher achievable specific capacity by increasing the upper limit of the intercalation voltage through the formation of stronger M-O bonds and to stabilize the layered structure in its fully delithiated state [6].Intensive investigations have been carried out on doped LiCo 1- y M y O 2 oxides (M = Ti, Cr, Mn, Ni, Fe, Cu, Fe, Bi, Zn, Mo) [7][8][9][10][11][12][13][14], which show interesting structural and electrochemical properties.Doping with transition metals such as cadmium has gained increasing interest for the following reasons: (i) the low cost and low toxicity, (ii) the fact that cadmium substitution for transition metal oxides will increase the capacity, (iii) the cadmium doping stabilizes the layered structure and extends the cyclability and enhances the capability of the electrochemical cells.To the best of our knowledge, no study has been made on the Cd-doped LiCoO 2 cathode material so far.Thus, in the present work, we report on the synthesis of LiCo 1-x Cd x O 2 powder by combustion technique.The influence of Cd on the material properties are discussed and correlated to the electrochemical properties of these cathode materials.

Experimental
For the synthesis of LiCo 1-x Cd x O 2 (x = 0, 0.01, 0.03, 0.05 and 0.1) powders, a combustion route was adopted by the following procedure shown in Fig. 1.Calculated amount of 'water soluble' starch was dissolved in 20 ml of hot distilled water.To the hot starch solution was added stoichiometric amount of lithium hydroxide monohydrate (LiOH.H 2 O), cobalt (II) nitrate hexahydrate (Co(NO 3 ) 2 .6H 2 O) and cadmium acetate Cd(CH 3 COO) 2 .2H 2 O solutions that were (metal ions ratio Li:Cd:Co: is 1:x:1-x ) mixed with starch solution.The obtained solution was evaporated at 80 °C for 6 hours with constant stirring.Continuous evaporation lead to the formation of a dark pink coloured polymeric resin.The polymeric resin was further dried at 120 °C for 12 hours to remove the excess water, which lead to the formation of a solid mass.Subsequently, these intermediates were heat treated at 800 °C for 12 hours.Unlike glycine, a molar correlation in terms of a chemical equation may not be possible in the case of starch.Being a polysaccharide (-C 6 H 10 O 5 -) n starch does not have a definite n value [15].
The crystalline structure was determined by X-ray diffraction using X'pert PRO (PANalytical) diffractometer with CuKα radiation (λ = 0.15405 nm) and employing scanning rate of 5° min -1 over the 2θ range of 15-75 °.The particle size and morphology was examined using scanning electron microscope (SEM) Hitachi S-3000H model.
Infrared absorption spectra were recorded at room temperature using a Fourier transform interferometer (model Bruker IFS113v) in the range of 100-700 cm -1 .For SEM studies the samples are coated with Au using JFC-1100 model instrument.The electrical conductivity was measured by a two probe method from room temperature to 150 °C uniformly on circular pellets of diameter of ~10 mm and thickness of ~2 mm.The electrochemical performance was studied by assembling 2016 coin-type cells.Typical cathode was prepared from a slurry of the synthesized LiCo 1-x Cd x O 2 powder (80 %), acetylene black (10 %) and (polyvinylidene fluride) PVDF binder (10 %) in n-methyl pyrrolidinone (NMP) solvent.The slurry was coated on an aluminium foil (current collector) and was dried at 110 °C in an oven for 12 hours.It was then pressed under a pressure of 4 tonne inch -2 , for 1 min.Finally, circular discs of 18 mm diameter were cut and used as cathode.The cells were assembled using Li metal as anode and LiPF 6 in EC: DMC (1:1 vol.%) as electrolyte within an argon filled glove box.Galvanostatic discharge studies were carried out at a current rate of 0.1 C.

Results and Discussion
Fig. 2 shows the XRD patterns of LiCo 1-x Cd x O 2 powders annealed at 800 ºC for 12 h.The presence of high intensity reflections are assignable to a highly crystalline and phase pure layered material and the peak signatures could be indexed to α-NaFeO 2 layered structure with R3m space group (JCPDS card no.: 75-0532).Further, the XRD pattern of LiCo 0.90 Cd 0.1 O 2 exhibits impurity peaks (marked with *) around diffraction angles 32.91º may be attributed to traces of CdO.It is apparent that such impurity peaks are not observed with LiCo 1-x Cd x O 2 (x = 0, 0.01, 0.03, and 0.05) samples.The appearance of doublets at planes 0 0 6/1 0 2 and 1 0 8/1 1 0 indicate the stabilization of two-dimensional structure and an ordered distribution of lithium and cobalt ions in the inner lattice [16].
The lattice constants 'a' and 'c' were calculated for (003) and (104) planes using the formula given by ( ) where (h k l) are the Miller indices of the plane concerned and a and c are the lattice constants are tabulated (Table 1).The lattice parameter a indicates the metal-metal intralayer distance while c represents the interlayer spacing [17].Lattice substitution by elements with larger ionic radii often leads to expansion of the unit cell parameters.Thus, considering the large difference in ionic radii of Cd 2+ (∼0.103 nm) compared Co 3+ (∼ 0.063 nm), the observed increase in a and c values in LiCo 1-x Cd x O 2 powders can be explained.These results indicates that addition of Cd stabilizes the layered structure of LiCoO 2 and increases the interlayer spacing with respect to that for pristine LiCoO 2 which would facilitate Li + ion intercalation-deintercalation.Further evidence of a good cation ordering is shown by a high c/a ratio value, which is greater than the critical value 4.899 (√24) [18] other than (x=0.1)sample.The intensity ratio value I (0 0 3) /I( 1 0 4) increases with increasing Cd molar concentration.However it decreases to 0.94 for LiCo 0.90 Cd 0.1 O 2 sample, less than the critical value of 1.2 [19], suggesting cation mixing at higher doping level.FTIR features can be divided into two parts, the high wave-number region of strong absorption corresponding to the broad rock-salt band broken into several distinct components at ca. 600-400 cm -1 .An isolated strong band centered at ca. 240-250 cm -1 is observed (infrared active modes 2A 2u +2E u ) in the low wave number region.Fig. 3 shows the FTIR spectra of LiCoO 2 and LiCo 0.90 Cd 0.1 O 2 samples.These results indicate the formation of compressed modes of CoO 6 and LiO 6 elongated octahedra.Upon Cd substitution, the vibrational mode of LiO 6 units is observed in the far-infrared region which remains quite stable.The shift has been observed toward the low-frequency side for solid solution with Cd (x=0.1)doping.These results show that, by doping, the local environment of lithium ions surrounded by oxygen anions is not affected and the CoO 2 layer covalency has increased slightly.Further, the broadening of the infrared peaks can be observed as an increase in CoO 6 distortion due to the addition of Cd in CoO 2 layers [20].

(c) (e)
Fig. 4 shows the SEM photographs of LiCo 1-x Cd x O 2 powders annealed at 800 ºC for 12 h.The particles are found to be well-defined facets that have a wide range of distribution ranging from 100 to 200 nm.The pristine LiCoO 2 sample contains bundle of agglomerated particles and relatively small particles on the agglomerates.Figs. 4 (b) and (c) show agglomerated particles with less voids as compared to pristine sample.Uniformly distributed particles with good electrical contact between the particles were observed in Figs. 4 (d) and (e).
The electronic conductivity is a very important property of a cathode material.For a better charge transfer process during lithium intercalation-deintercalation in lithium-ion cell conductivity plays a significant role [21,22].Fig. 5 shows the electrical conductivity of LiCo 1-x Cd x O 2 system measured from room temperature to 150 °C.The room temperature conductivity of pristine LiCoO 2 is found to be 4.52 10 -5 Scm -1 .All the Cddoped compounds show higher room temperature conductivity with respect to pristine LiCoO 2 .The conductivity increases with increasing amount of Cd per mole of LiCoO 2 .A conductivity value of 3.61 10 -3 Scm -1 is observed for Cd (x=0.1) which is about two orders of magnitude higher than pristine LiCoO 2 .Electrochemical characterization was carried out by galvanostatic discharge cycling studies between 3.0 and 4.5 at the current density 0.1 mAg -1 employing 0.1 C rate.Fig. 6 shows the first discharge curves of LiCo 1-x Cd x O 2 (x = 0, 0.01, 0.03, 0.05 and 0.1) powders.It is observed that Cd-doping increases the discharge capacity from 186 to 197mAg -1 .The first and 30th discharge capacities were measured for all the materials and thes are shown in Table 2.The increase of discharge capacity can be ascribed to the occupancy of Cd 2+ ions in the inter-slab space along with Co 3+ ions which facilitates the transit of Li + ions above 4.2 V.The relatively larger ionic radius of Cd 2+ (∼1.03 Å) than that of the Co 3+ ion (0.63 Å) allow free movement of Li + ions due to pillaring effect [23,24] and also prevent the vacancy ordering.This fact is corroborated from the marginal increase observed in the 'c' lattice (Table 1).As the charge-discharge proceeds the dopant ions located on the electrode surface form a solid solution with Co 4+ ions.This solid solution acts as an intermediate layer and minimizes the dissolution of Co 4+ ions into the electrolyte.This phase formation stabilizes the structure and enhances the cycle stability [23].Nevertheless, the morphology and the uniformly distributed particles without voids of the Cd-doped LiCoO 2 also supplement the high discharge capacity as it obviously provides shorter diffusion path lengths for lithium-ion transport.Doping of Cd 2+ in LiCoO 2 enhances the conductivity of LiCoO 2 as well as its structural stability as it creates more Co 4+ ions due to charge compensation.Subsequently, Li + vacancies are generated to Discharge capacity (mAh/g) Potential (V) Vs Li/Li + balance the covalence greatly increase the conductivity of LiCoO 2 and thereby enhance the electrochemical performance [25].The capacity fading rate f c , is defined as where C 1 and C n are the specific discharging capacities of first and nth cycle, and n is the number cycles.The capacity fading rate are 1.03, 0.66, 0.6, 0.33 and 0.73 mAhg -1 per cycle for LiCo 1-x Cd x O 2 (x = 0, 0.01, 0.03, 0.05 and 0.1) powders respectively.The capacity fading rate decreases first with increase in Cd content up to x=0.05 of Cd and then increases with Cd content x=0.1.LiCo 0.90 Cd 0.1 O 2 compound shows the decrease in reversible capacity due to cation mixing as revealed in XRD analysis The above results clearly show that the LiCo 0.95 Cd 0.05 O 2 , show much better capacity retention in comparison to pristine LiCoO 2 .

Conclusion
LiCo 1-x Cd x O 2 has been successfully synthesized by starch assisted combustion route method.XRD patterns suggest that Cd-doping favours the formation of highly layered structure with marginally expanded 'c' lattice thereby providing more space for lithiumion intercalation/deintercalation.Microscopy studies reveal that the particle size increases with the increase of dopant content In addition, the doping enhances the conductivity thereby improving the reversibility of the electrode reaction.LiCo 0.95 Cd 0.05 O 2 deliver an average discharge capacity of around 195 mAhg -1 suggesting that Cd-doping enhances the electrochemical performance of LiCoO 2 .

Fig. 2 .
Fig. 2. X-ray diffraction patterns of LiCo 1-x Cd x O 2 powders prepared by starch assisted combustion route method.

Table 1 .
The lattice parameters of LiCo 1-x Cd x O 2 samples.

Table 2 .
Cathode discharge and irreversible capacities of LiCo 1-x Cd x O 2 sample.