Synthesis of lithium cobalt oxide by single-step soft hydrothermal method

Synthesis of lithium cobalt oxide by single-step soft hydrothermal method

Journal of Solid State Chemistry 198 (2013) 45–49 Contents lists available at SciVerse ScienceDirect Journal of Solid State Chemistry journal homepa...

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Journal of Solid State Chemistry 198 (2013) 45–49

Contents lists available at SciVerse ScienceDirect

Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Synthesis of lithium cobalt oxide by single-step soft hydrothermal method Kiran Kumar Bokinala a,b,c, M. Pollet b,n, A. Artemenko b, M. Miclau a, I Grozescu a,c a b c

National Institute for R&D in Electrochemistry and Condensed Matter, Timisoara 30024, Romania CNRS, Universite´ de Bordeaux, ICMCB, 87 Avenue du Dr. A. Schweitzer, Pessac F-33608, France Universitatea Politehnica, Timisoara, Romania

a r t i c l e i n f o

abstract

Article history: Received 13 July 2012 Received in revised form 10 September 2012 Accepted 16 September 2012 Available online 25 September 2012

Lithium cobalt double oxide LiCoO2 was synthesized at 220 1C by soft hydrothermal method using Co(OH)2 and LiOH as precursors, LiOH/NaOH as mineralizers and H2O2 as oxidant. The soft hydrothermal synthesis method offers the dual advantage of a much lower synthesis time and a higher purity in comparison with other synthesis methods. The compound was identified by X-ray diffraction and its purity was checked by magnetic and electron magnetic resonance measurements. The grain morphology was studied by Scanning Electron Microscopy and an exponential growth of particle size with synthesis time was observed. & 2012 Elsevier Inc. All rights reserved.

Keywords: Lithium cobalt oxide Soft hydrothermal synthesis Electron magnetic resonance

1. Introduction Lithium cobalt double oxide LiCoO2 is well known for its high performance as cathode part of Li-ion batteries; it offers both a high output voltage and a high specific energy [1–7]. LiCoO2 owes its efficiency to its layered structure which alternates Li and CoO2 layers; in its thermodynamically stable form, it crystallizes with R-3m space group (O3-polytype [8]) with both direct solid state or hydrothermal synthesis methods [9]; other polytypes are also known like O2- [10] and O4-polytypes [11] and for the Li deficient cases, O1- [12], O6- [13], and T2-polytypes [14]. Among the several synthesis methods already proposed to produce LiCoO2, all present at least one of the following weaknesses: high temperature synthesis, multiple steps, long preparation time, long synthesis times, specific care, limited yield and/or presence of impurities in the final product. For standard solid-state synthesis method for instance, the synthesis temperature of the initial mixture of Li2CO3 and Co3O4 reaches 850 1C [15] and Co3O4 impurities are present [16]. In order to reduce the synthesis temperature of lithium cobaltate, a sol–gel method based on an inorganic carboxylic route was successfully developed, but Co3O4 phase was still observed after the thermal treatment at 700 1C [17]. More recently a microwave assisted synthesis with doublecontainment was reported [18]. The hydrothermal synthesis method has already been extensively used to synthesise a wide range of oxide materials [19–22];

n

Corresponding author. E-mail addresses: [email protected] (M. Pollet), [email protected] (M. Miclau). 0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.09.028

it was shown to be advantageous as it requires lower synthesis parameters, it is less expensive and it leads to the good oxygen stoichiometry. The problem of the impurities can be overcome by varying the ratios of precursors: In the specific case of LiCoO2, cobalt impurities can be avoided by increasing the Li to Co ratio. So far, LiCoO2 has been prepared by hydrothermal subcritical as well as in supercritical conditions with 50% concentrations of hydrogen peroxide as an oxidant [23,24]. It is actually challenging to find synthesis conditions which allow to reduce the concentration of hydrogen peroxide and to tune the grain size. In order to optimize the synthesis conditions of LiCoO2 using hydrothermal method, we studied the influence of the precursors’ concentration, the molality of the solution and the synthesis temperature on the resultant phase as well as the influence of the reaction time on the surface morphology and the purity of the samples.

2. Experimental A previous report [25] on the stability diagram of Co–Na–H2O system with NaOH and Co(OH)2 as precursors and NaOH as mineralizer has revealed the systematic presence in the product of secondary phases of precursor and HCoO2 for any synthesis temperature lower than 220 1C whatever the NaOH amount; furthermore, the stability diagram of Na–Co–H2O system indicates an optimal concentration of Co(OH)2 precursor in the solution of 13 mmol/L at 220 1C. These two parameters were used as initial parameters for the present study. Powders of Co(OH)2 and LiOH were selected as precursors. High concentrations of ionic mineralizers (LiOH and NaOH) were used to improve the solubility of Co(OH)2 and 5% hydrogen peroxide as oxidant were

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added. The precursors were first dissolved in water and the 5% H2O2 were added to the solution; this latter was transferred into a 70 mL Teflon-line autoclave with a filling degree of 85%. Samples were heated at 220 1C varying the reaction time from 5 to 80 h and the mineralizers concentration from 1 to 4 m. After cooling to room temperature (RT), the samples were washed and dried at RT. XRD patterns were recorded with a PANalytical X’Pert Pro powder diffractometer in the Bragg–Brentano geometry, using Cu Ka radiation in the range 2y ¼5–801 at RT and analyzed using FULLPROF program [26]. The surface morphology was studied using Scanning Electron Microscopy (Hitachi S4500 field emission microscope). Elemental analysis was carried out by inductively coupled plasma absorption electron spectroscopy (ICP-AES) on a Varian 720-ES. Zero field cooled (ZFC) and field cooled (FC) DCmagnetization data at an applied field of 0.01 T were collected on a superconducting quantum interference device magnetometer (Quantum Design Magnetic Property Measurement System) in the 4–200 K temperature range. EPR measurements were performed with an X-band Bruker spectrometer operating at 9.4 GHz. An Oxford Instruments ESR 9 He cryostat operating in the temperature range 4–300 K was used for the temperature dependence studies of EPR spectra intensities. To perform both quantitative and qualitative analysis the same amount (0.025 g) of powder of LiCoO2 samples was used in the cavity.

and 4 M NaOH, Co(OH)2 hexagonal disappears and it is replaced with CoOOH. The transformation of Co(OH)2 precursor continues with increasing the NaOH molarity and temperature; CoOOH was found to crystallize in the three layered polytype (3R1) and to become the majority phase for 180 1C and 3 M NaOH. Similar results were observed for other cobalt oxyhydroxide samples prepared in oxidative conditions (air or hydrogen peroxide). The same oxidation of Co(OH)2 to (3 R1)-CoOOH was also reported during the synthesis of nano-sized lithium cobalt oxide by sonochemical synthesis [30]. If temperature is increased, Co3O4 forms. The several steps for the reaction can be written as [30] 220 degC 2CoðOHÞ2 þ H2 O2

) LiOH

2HCoO2 þ 2H2 O

ð1Þ

LiOH ) Li þ þ OH

ð2Þ

HCoO2 þ Li þ þOH ) LiCoO2 þ H2 O

ð3Þ

The total reaction is 2201C 2CoðOHÞ2 þ 2LiOH þH2 O2

)

2LiCoO2 þ 4H2 Oð1 þ 2 þ3Þ

5 h,15 h,24 h,60 h

3. Results and discussion Fig. 1 summarizes the result of varying the NaOH: LiOH mineralizer’s ratio in the solution, the temperature (220 1C), the reaction time (24 h), the hydrogen peroxide concentration (5%) and concentration of Co(OH)2 (13 mmol/L) were kept constant. Using only LiOH as mineralizer, LiCoO2 crystallizes only from 0.5 m as a minority phase in a solution containing mostly Co3O4; with the increase of LiOH concentration, LiCoO2 yield increases however with some impurities of CoO for LiOHr2 m; when LiOH Z2 m, the only product visible from XRD is LiCoO2. The inductive coupled plasma (ICP) analysis confirmed the close to stoichiometric ratio Li:Co¼1:1. Our previous work (without hydrogen peroxide, below 150 1C and with a 3 M NaOH solution only) has only revealed a partial evolution of Co(OH)2 precursor (space group: P-3m1; JCPDS 01074-1057) to Co(OH)2 hexagonal (JCPDS 00-001-0357). At 150 1C

NaxCoO2 4m Co3 O4 NaOH (molality)

NaxCoO2 3m

LiCoO2 CoO

2m LiCoO2 1m Co3 O4 CoO

1m

2m

3m

4m

LiOH (molality) Fig. 1. The effect of ionic mineralizers on resultant phases obtaining at 220 1C and after 24 h.

The role of NaOH is actually multiple: First, just like LiOH, it acts as mineralizer; second, NaOH is slightly more oxidative than LiOH and it might help stabilizing higher valence state of cobalt (CoO traces are disappearing on adding NaOH); third, probably in relationship with point 2, it promotes the crystallization of LiCoO2: the crystallinity of LiCoO2 obtained with 1 M LiOH and 1 M NaOH is higher than with 4 M of LiOH only and the FWHM evolution vs NaOH content clearly evidence a narrowing of the diffraction peaks. It is worth noting that for equimolar ratios of LiOH and NaOH, only LiCoO2 forms while NaxCoO2 could also be expected. Several points can be highlighted that explain this result: (i) whatever the conditions used, we were not able to stabilize the stoichiometric compound NaCoO2, meaning that the formed compound always contains a minimum amount of Co4 þ ; as mentioned above, NaOH is more likely to stabilize higher valence state of cobalt however, our phase diagram shows that Co4 þ might be accessible only for strong excess in NaOH; (ii) HCoO2 was shown to be formed at low temperature [ref to the stability diagram of Co–Na–H2O]; in solution it appears as CoO2 which is expected to have a short time life [27] and to fast react with its environment; as lithium has a higher electron affinity than sodium (respectively 0.618 eV and 0.548 eV), one can expect a faster combination with Li þ than with Na þ ; (iii) both compounds are lamellar, however LiCoO2 basically has a more pronounced 3D character (ordered rock salt structure) than NaxCoO2 what can favour its growth. Fig. 2 highlights some typical features in the crystal shapes depending on the mineralizers concentration and the reaction time. For the lowest concentrations of mineralisers and with 24 h of synthesis duration, only desert-rose shape could be observed (Fig. 2a). Keeping constant the overall mineralizer concentration but increasing the LiOH to NaOH ratio results in the growth of small spherical particles (Fig. 2b). Further increasing the reaction time leads to the growth of concave cuboctohedrons (Fig. 2c). Also, the effect of higher mineralizer concentration on the crystal shapes was studied. Some SEM micrographs for the samples synthesized from [Co(OH)2]¼13 mmol/L and equimolar LiOH and NaOH mineralizers amount (2 M) are shown in Fig. 3 for several reaction time.

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Fig. 2. SEM images of LiCoO2 obtained with (a) LiOH—1 M, NaOH—1 M, 24 h; (b) LiOH—1.5 M, NaOH—0.5 M, 24 h; (c) LiOH—1.5 M, NaOH—0.5 M, 48 h.

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Synthesis time Vs Grain size

Grain size (µm)

30 25 20 15 10 5 0 10 20 30 40 50 60 70 80 90 Synthesis time (Hr)

Fig. 3. SEM images of the samples synthesized with (a)  15 h, (b)  24 h, (c)  48 h, (d)  60 h, (e)  80 h reaction times, and (f) growth of particle size with synthesis duration. Synthesis conditions: Co(OH)2– 13 mml/L, LiOH—2 M, NaOH—2 M, H2O2—5% and fill% of autoclave  85%.

Particles size is regularly increasing with the reaction time however their shape is evolving throughout the process. For low reaction time (15 h) only desert-rose shape particles are obtained. With 24 h reaction time a mixture of concave cuboctohedrons and small spheroidal particles are present. With 48 and 60 h reaction time mainly concave cuboctohedrons are visible with few spheroidal particles. With 80 h reaction time the concave cuboctohedron shape looks fully destroyed and mostly classical hexagonal platelets like particles are obtained.. The only available report on synthesis of concave cuboctohedrons mentions the use of nanosize precursors and CoO seeds as well as high concentrations of ionic mineralizing agents [19]. Here, we were able to grow such cuboctohedrons only using standard microsize Co(OH)2, precursor and without CoO seed; furthermore we were not able to detect any presence of this latter at the centre of the particles. This suggests show that an alternative mechanism for the growth of these particles should be considered. In order to analyze the influence of the concentration of Co(OH)2 on the formation of LiCoO2 and to optimize it, the temperature (220 1C), the reaction time (24 h) and the ratio between NaOH:LiOH (2 M:2 M) were kept constant while the

Fig. 4. Dissolution and oxidation of Co(OH)2 at different concentrations. Operating conditions: LiOH—2 M, NaOH—2 M, 5% H2O2, 85% filled autoclave, T¼ 220 1C, t¼ 24 h.

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concentration of Co(OH)2 was varied from 13 mmol/L to 130 mmol/L. The results are shown in Fig. 4. At 130 mmol/L only Co(OH)2 could be identified from XRD. On decreasing the concentration, LiCoO2 slowly appears at ca. 65 mmol/L however with CoO impurities. Only when the initial concentration is lower or equal to 50 mmol/L a pure LiCoO2 compound can be obtained. These results suggest that (i) the solubility limit of Co(OH)2 is close to 50 mmol/L and that, as compared to Co(OH)2 (ii) the seeding of LiCoO2 is energetically favoured (iii) while its growth is energetically unfavoured. Indeed as the Co(OH)2 concentration increases some undissolved traces remain and serve as seeds for the re-crystallization; conversely, for low initial precursor concentration, as the seeding of LiCoO2 compound is much favoured, no trace of Co(OH)2 is visible in the final product. The X-ray diffraction pattern for the product resulting from the synthesis with 26 mmol/L concentration in Co(OH)2 is shown in Fig. 4. The refinement with R-3m space group leads to lattice parameters a¼ b¼2.81547 A˚ and c ¼ 14.0635 A˚ which are in good agreement with previous reports [8,19]. Magnetization data vs. temperature are shown in Fig. 6 for samples prepared using 24 and 60 h reaction time. They evidence a marked decrease in the signal when the synthesis time increases. Pure LiCoO2 compound is only expected to contribute a temperature independent second order Zeeman component while the temperature dependant part of the magnetization is expected to arise from secondary phases and structural defects or impurities [16]. Both data sets could be fitted using the raw data for pure Co3O4 and CoO compounds [16], a temperatureindependent second-order Zeeman contribution and an additional ~ ~ ~ Curie component ðM=HÞ total ¼ ðM=HÞCo3 O4 þ ðM=HÞCoO þ w0 þC=T. In both cases w0 E65  10  6. With 24 h reaction time, the total secondary phase amount is about 4% and it is less than 0.1% after 60 h reaction time. Similarly, C decreases from 0.036 to 0.0009 with reaction time. Assuming the last term of the model function to originate in defects in LiCoO2, such a high difference in the Curie constant can be related to the size and shape of the particles (Fig. 5): For 24 h reaction time the particles size is much lower (and their number is much higher) and a large number of surface defects can be expected resulting in a high Curie constant; conversely, with 60 h reaction time mainly nicely shaped and big concave cuboctohedrons are obtained and the defects amount should be largely reduced supporting the low calculated Curie constant.

EMR studies were performed for the samples prepared with 5, 24 and 60 h synthesis durations. Data were collected in a wide temperature range (4–300 K) in order to study the stability of the charged defects, intrinsic and/or impurity. The detailed analysis of EMR spectra is out of scope of this article and we just present here some results supporting the observations made with macroscopic magnetic measurements. Fig. 7 shows the plots for EMR spectra recorded at 10 K for the samples prepared with 5 and 60 h synthesis durations. It is worth noting, as observed from magnetic measurements, that the intensity of the signal sharply decreases when the reaction time increases. A part of the spectra is similar to the spectra observed with LiCoO2 powder prepared by conventional solid state chemistry [22]. This allows identifying for all the samples resonances arising from Co3O4, from unavoidable Ni3 þ impurity and from Li þ –O  surface centres. As compared to previous work additional resonances are visible in the magnetic field region 800–2400 G up to ca. 80 K. The existence of eight hyperfine lines in this spectrum allows identifying it unambiguously as originating from cobalt ion (59Co) which has 100% natural abundance nuclear spin I¼7/2. Several Co-related defects can however give such resonances. High symmetry Co4 þ (3d5; S ¼1/2 or S¼5/2) related center due for instance to some cation deficiency in the structure have to be discarded: the g-factor value should be close to the free electron one (g ¼2.0023) and the spin–lattice relaxation time is large enough to make it visible at room temperature. Similarly the presence of Co2 þ (3d7 ion with S ¼1/2 LS or S ¼3/2 HS configuration) to compensate for some oxygen-related defects should be discarded: these defects are strongly influencing the crystal field strength and symmetry and lead to the formation of defects with axial symmetry (Co2 þ –V(O) or/and Co2 þ –V(O)  ); such defects were already observed for cobalt ions in the vicinity of the surface of LiCoO2 samples prepared by solid state chemistry [2] and their hallmark (S, g, A) is not matching with present results. The three visible resonances are characterized with g1 ¼5.712(2), 1049A19¼ 70(4) cm  1, g2 ¼4.105(2) and 1049A29 ¼22(2) cm  1, g3 ¼3.518(2) and 1049A39¼45(2) cm  1. For all the reasons listed above, the only possible defect matching both with the crystal symmetry and such parameters has to have an effective spin Seff ¼1 and has to sit in a slightly distorted octahedral environment. The three resonances are then due to two allowed by spin transitions with Dms ¼1 and one forbidden transition with Dms ¼2. Such effective spin could arise from an intermediate spin Co3 þ , or as revealed in the case of LaCoO3 (and consistently with LiCoO2 structure) from

Fig. 5. The XRD pattern of LiCoO2.

K. Kumar Bokinala et al. / Journal of Solid State Chemistry 198 (2013) 45–49

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(iii) the crystallinity of the compound increases as the ratio NaOH/LiOH increases; (iv) the Co(OH)2 concentration threshold is ca. 50 mmol/l; bellow this limit a single phase LiCoO2 grows and above, Co(OH)2 re-crystallizes. In the reaction time window 24–60 h we were able to stabilize CoO-free concave cuboctahedrons particles suggesting that an alternative mechanism for the growth of these particles should be considered. Magnetic measurements (local and macroscopic) highlight the presence of cobalt oxides impurities which amount is decreasing (and nearly vanishes) with reaction time; they also reveal, as already observed, the presence of several kind of close to surface centres.

Acknowledgements

Fig. 6. Magnetization of the samples prepared with 24 (circle) and 60 h synthesis time (square) measured using H¼ 100 Oe (the signal for the 60 h synthesis time sample is multiplied per 10); black lines are the fit to the data.

EMR intensity, arb .un.

3000

T = 10 K LiCoO2

1500

0 60h Ni

* -1500

*

* *

3+

5h

-3000 500

1000 1500 2000 2500 3000 3500 4000 4500 Magnetic field, G

Fig. 7. EMR spectra observed at T ¼10 K for LiCoO2 samples with 5 h and 60 h synthesis durations. Resonances originating from the Co3O4 are marked with asterisks. Left inset: zoom in the region 500–2500 G highlighting resonances ascribed to the cobalt ions in distorted octahedral site of LiCoO2 (spectra are shifted for readability and the intensity of the spectrum of the 60 h reaction time sample is multiplied per 2). Right inset: zoom in the region 2800–3500 G highlighting resonances from unavoidable Ni3 þ impurity (g¼ 2.14) and from Li þ –O  center (g ¼ 2.16).

a low-lying triplet subterm of the 5T2g manifold in the trigonally distorted environment [28,29]. The decrease in the amount of such centres with increasing reaction time from 5 h to 60 h as well as their fast decay with temperature suggest that they are located close to the surface of LiCoO2 particles (just like Li þ –O  centres) which is decreasing as the shape evolves from desertrose to cuboctahedrons (Fig. 5).

4. Conclusions Layered lithium cobalt double oxide was synthesized by soft hydrothermal synthesis method at 220 1C using Co(OH)2 and LiOH as precursors, LiOH/NaOH as mineralizers and 5%H2O2 as oxidant. The effect of ionic mineralizers and the concentration of precursor were studied to optimize the synthesis conditions of LiCoO2: (i) for low mineralizers concentration, only cobalt oxides form; (ii) as the concentration increases, pure LiCoO2 grows;

K.K.B., M.P., M.M. and I.G. acknowledge the financial support by the European project ‘‘SOPRANO’’ under Marie Curie actions (Grant no. PITN-GA-2008–214040). A.A. acknowledges the financial support by the European project ‘‘EPREXINA’’ under Marie Curie actions (Grant no. PIIF-GA-2009-255662).

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