Effect of Initial Reactants and Reaction Temperature on Molten Salt

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Effect of Initial Reactants and Reaction Temperature on Molten Salt Synthesis of CuCo2O4 and its Sustainable Energy Storage Properties Bobba V.R. Chowdari, Mogalahalli Venkatashamy Reddy, Stefan Adams, and Rajesh Mishra ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00047 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 5, 2016

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ACS Sustainable Chemistry & Engineering

Effect of Initial Reactants and Reaction Temperature on Molten Salt Synthesis of CuCo2O4 and its Sustainable Energy Storage Properties Mogalahalli Venkatashamy Reddy1,2*, Mishra Rajesh2,3,4 , Stefan Adams1**, Bobba Venteshwara Rao Chowdari2 1

Department of Materials Science and Engineering, National University of Singapore,

Singapore 117576 2

3

4

Department of Physics, National University of Singapore, Singapore 117542

St Andrew’s Junior College, 5 Sorby Adams Drive, Singapore 357691 Department of Electrical Engineering, National University of Singapore, Singapore 11

7576.

Corresponding Authors: * [email protected], ** [email protected] ABSTRACT We have prepared CuCo2O4 using 0.5M NaNO3 and 0.5M LiNO3 molten salts at different temperatures (410oC and 610oC) in air. This was later used as an anode material for LIBs. The morphology, structure and electrochemical properties of the products were observed using various techniques such as Scanning Electron Microscopy, X-Ray Diffraction (XRD), Brunauer-Emmett-Teller surface and density method, Cyclic Voltammetry and Galvanostatic cycling tests. The XRD patterns showed a minor CuO phase in addition to major CuCo2O4 phase in most of the reactant and salt combination. CuCo2O4 prepared using copper sulphate and cobalt sulphate at 610oC and copper sulphate and cobalt acetate at 410OC showed the best performance with capacity of 848 mAhg-1 and 882 mAhg-1 and capacity retention of 93% and 94% respectively. 1

ACS Paragon Plus Environment


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KEYWORDS: CuCo2O4, CuO.CuCo2O4, anode, material characterization, electrochemical properties.

INTRODUCTION

The first commercial Li-ion Battery (LIB) was released in 19911. Since then, it has been an advanced and portable source of electricity for both high power appliances and various high energy density applications. Today, LIBs are used in various devices1 ranging from phones and laptops to all-electric cars. These batteries, using graphite as their anode and LiCoO2 as cathode, have revolutionized the world with their ability to recharge. The intercalation-de-intercalation of lithium ions has allowed LIBs to recharge and to be used for far longer periods of time as compared to any other form of battery. However, there are many concerns regarding LIBs' future, most of them relating to the low theoretical capacity of graphite as an anode and the dendrite growth during cycling, which leads to short circuits. Transition metal oxides with binary and ternary oxides that undergo conversion reaction have been prepared using a variety of techniques and their electrochemical properties have been studied2-6. In general, differences in their electrochemical properties were noted depending on preparation methods and reaction conditions like time, reactants, crystal structure, morphology etc. Among all spinel compounds, CoCo2O4 (Co3O4) and other pure and composite oxides are being examined as anode for Li-ion batteries7-12. Co3O4 has shown a reversible capacity in the range of 600-850 mAhg-1, but Co is expensive and toxic. In this project we have replaced part of the Co with low cost and less toxic elements like Cu. CuCo2O4 can be used in other applications such as super capacitors13, Electrocatalyst14-17, oxidation of CO218, Thermoelectric solid oxide fuel cells19, CuO and Co3O4 can be used glucose sensing 20, 21 and photo/electro catalysis22, 23.

Cu, Co-based oxides materials were

2


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prepared by one pot molten salt method (MSM) and we studied the effects of the copper and cobalt salts as well as preparation temperature on the electrochemical properties of the final product. MSM is simple and cost effective as only one pot method of preparation of various materials is used24-32. We have been able to prepare the metal oxide compounds in large quantities using simple MSM technique. This method is simple when compared to other physical deposition techniques.

EXPERIMENTAL

Materials synthesis

CuCo2O4 was prepared using the molten salt method (MSM) at two different temperatures and four different Cu and Co salt combinations.. Sulphates: CuCo2O4 was prepared using 0.5M NaNO3 and 0.5M LiNO3 molten salt using CuSO4.5H2O and CoSO4.7H2O in the ratio 5:5:1:2. All the four compounds were weighed and mixed together in an alumina crucible and kept in the furnace at 410oC for 3hrs in air. The same process was carried out at 610oC. About 40 grams of initial reactants were used. Sulphate and Acetate: The same process as mentioned above was carried out with one change. In this case, the cobalt and copper salts being used were CuSO4.5H2O and Co(CH3COO)2.4H2O in the ratio 1:2. Sulphate and Hydroxide: In this case, the cobalt and copper salts being used were CuSO4.5H2O and Co(OH)2 in the ratio 1:2. Chlorides: In this method, the cobalt and copper salts being used were CuCl2 and CoCl2 in the ratio 1:2. 3



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After 3 hours, the crucible was removed from the furnace. The mixture was then washed with distilled water to remove any excess salts and was then filtered. The powder obtained was then dried overnight in a vacuum oven and stored in a desiccator for later use. The process yielded between 5-8g of final product. Materials Characterization Scanning Electron Microscopy (SEM) (JEOL, Japan), Powder X-ray Diffraction (XRD) (D8 Bruker), the Brunauer-Emmett-Teller (BET) surface area and density method (Micromeritics TriStar, USA), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to determine the physical characterization of our prepared samples and more details on instrumentation are reported in our previous reports33,

34

. Cyclic voltammetry (CV) and

Galvanostatic cycling were used to analyse the electrochemical properties and reaction kinetics of the materials. The lattice parameter values were evaluated using TOPAS software. Electrochemical studies: Composite anodes were prepared as the active material, polyvinylidene fluoride (PVDF) as a binder, Super-P carbon as a conductive additive in the weight ratio of 70:15:15 and lastly N-Methyl-2-pyrrolidone (NMP) as the solvent. To start off, Super-P carbon and CuCo2O4 powder were finely ground in a mortar. Then, the PVDF binder was added to the mixture in the ratio stated above. After this, the powder was transferred into a vial and NMP was added and magnetically stirred overnight to form a uniform slurry. This was then coated onto a copper foil substrate35 to form a 10-15 µm thick layer. After placing the foil at 80oC in an oven overnight, it was passed through a roller-press to press the anode material and the foil substrate together so as to increase the contact area between them. The foil was then punched into discs with a diameter of 1.6 cm (2.0 cm2 geometric area) using an electrode cutter. Discs with uniform coating of the anode material were selected and weighed to determine the total mass of the composite electrode. Finally, 4


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the electrodes were kept in a vacuum oven overnight at 80 °C to let them dry. Weight of active material was 4 to 5 mg. Coin cells were produced using lithium metal, the cathode, and 1M Lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate /dimethyl carbonate (1:1,vol%) as electrolyte. The open-circuit voltage (OCV) was measured after the coin cells were removed from the glove box, then equilibrated for 12 hours35, 36. RESULTS AND DISCUSSION Structure and morphology Scanning electron micrographs (SEM) images of CuCo2O4 prepared using different Cu and Co salts at 410 0C are shown in Fig.1. The images shown in Fig.1 (a), (b), (e) and (f) seem to indicate a flaky structure which means they might have high surface areas as compared to others. Figs. 1(c) and (d) show lumpy particles with a lot of empty space. Lastly, for Figs. 1 (g) and (h), the compound has a crystalline structure with very fine edges and numerous pointed ends protruding out. The obtained samples prepared here are black in colour. SEM images of CuCo2O4 prepared at 610

0

C are shown below in Fig. 2. The

structures shown in Figs. 2 (a), (b), (e) and (f) seem to have similar coral-like structures. For Figs. 2(a) and (b), the particle sizes seem to be uniform which is not the case for the compound in Figs. 2(e) and (f). Similarly, the structures shown in Figs. 2(c), (d), (g) and (h) appear to be alike in their physical features. However, upon a closer look at Figs. 2(d) and (h), the particles in Fig. 2(h) are round and their edges are smoother as compared to the one in Fig. 2(d). All the four samples prepared here are black in colour. We note slight differences in the morphology depending on materials'31 crystals structure, molten salt31, 32 and reaction conditions like preparations temperature10 and time37. Further Insitu microscopy studies are needed to explain above observed surface morphology.

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Figure. 1. SEM micrographs of CuCo2O4 prepared at 410 °C from (a) Cobalt acetate and Copper sulphate, bar scale: 1µm ; (b) Cobalt acetate and Copper sulphate, bar scale: 100nm . (c) Cobalt sulphate and Copper sulphate, bar scale: 1µm; (d) Cobalt sulphate and Copper sulphate; bar scale: 1µm. (e) Cobalt hydroxide and Copper sulphate; bar scale: 1µm. (f) Cobalt hydroxide and Copper sulphate; bar scale: 1µm. (g) Cobalt chloride and Copper chloride; bar scale: 1µm. (h) Cobalt chloride and Copper chloride; bar scale: 1µm.

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Figure 2. SEM micrographs of CuCo2O4 prepared at 610 °C from (a) Cobalt acetate and Copper sulphate; bar scale: 1µm. (b) Cobalt acetate and Copper sulphate; bar scale: 100nm. (c) Cobalt chloride and Copper chloride; bar scale: 1µm. (d) Cobalt chloride and Copper chloride; bar scale: 100nm. (e) Cobalt sulphate and Copper sulphate; bar scale: 1µm. (f) Cobalt sulphate and Copper sulphate; bar scale: 100nm. (g) Cobalt hydroxide and Copper sulphate; bar scale: 1µm. (h) Cobalt hydroxide and Copper sulphate; bar scale: 100nm. The X-ray diffraction patterns of all compounds prepared at 410 and 610°C are shown in Fig. 3. The Rietveld refined patterns of samples prepared at 410OC are shown in Fig. 3iii. The XRD patterns show a mixture of CuCo2O4 and CuO phases. The quantitative phase analysis yielded 20 wt.% CuO with lattice parameters (Å): a=4.700(8), b=3.433(5), c=5.135(7), β (°): 8


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99.54 and 80 wt% CuCo2O4, a (Å)= 8.136. The lattice parameter obtained for CuCo2O4 is close to the one found for a sample prepared at 280°C by Reddy et al10. Irrespective of the molten salt and initial reactants a minor CuO pure phase was noted in our compounds and pure CuCo2O4 phased formed only at 280°C10. Similar CuO.CuCo2O4 phases been reported in literature17. The density and BET surface area values are shown in Table 1. The surface area values are in the range, 0.2 to 6.7 m2g-1 and average pore sizes are 10 to 23 nm. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy studies of select compounds are shown in supporting information file S1 and S2. The fitted XPS binding energy values are given supporting information file Table 1 and all compounds shows Binding values around 933 and 954 eV corresponds to Cu2p3/2, Cu2p1/2 level respectively. Whereas Co element gives a binding energy around 779 and 795 corresponds Co2p3/2 and Co2P1/2 and O1s level show a binding value of 529 eV (Supporting information file Table 1) which corresponds to oxygen bound with metal and 531 eV corresponds to surface oxygen33, 38.

The observed

binding values are close to reported studies on CuO.CuCo2O4 composites17. Raman studies of also are shown supporting information file are shown Fig.S2, the compounds shows a characteristic vibration bands around 270 cm-1, 460-475 cm-1, 460-470, 679 cm-1.

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Fig. 3. (i) X-Ray Diffraction pattern of CuCo2O4 compound prepared using different Cu and Co-salt combinations at 410oC: (a) copper chloride and cobalt chloride (b) copper sulphate and Cobalt hydroxide (c) copper sulphate and cobalt sulphate (d) copper sulphate and Cobalt acetate. (ii) X-Ray Diffraction pattern of CuCo2O4 compound prepared using different Cu and Co-salt combinations at 610oC. (a) Copper chloride and Cobalt chloride (b) Copper sulphate and Cobalt acetate (c) Copper sulphate and Cobalt sulphate (d) Copper sulphate and Cobalt hydroxide. (iii) Rietveld refinement XRD pattern of CuCo2O4 prepared at 410°C. copper chloride and cobalt chloride. # CuCo2O4 phase and * CuO phase. A vertical bar represents (hkl) lines of CuO and CuCo2O4 phases and (hkl) lines are indexed.

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Experimental Surface Area

Pore volume: (cm3/g)

(m2/g)

(Pore Size: nm)

Different Co and Cu salts

density : g/cm3 CuCo2O4 prepared at 410oC

Co-hydroxide and Cu-sulphate

0.12

0.00047(15.29)

5.8153

3.34

0.0131 (15.75)

7.0649

Co- sulphate and Cu-sulphate

2.35

0.0131 (15.75)

6.3318

Co-sulphate and Co-acetate

3.56

Cobalt chloride and copper chloride

0.0131 (15.75)

6.289

CuCo2O4 prepared at 610oC Co- sulphate and Cu-sulphate

4.86

0.0276 (22.73)

6.1811

Co- hydroxide & Cu- sulphate

6.57

0.0226 (13.78)

6.7010

Co-chloride and Cu- chloride

0.35

0.00119 (13.67)

5.9348

Co-sulphate and Cu- acetate

1.36

0.0023 (12.36)

5.8769

Table 1: BET surface area and density values for CuCo2O4 prepared at 410 and 610oC using different salts.

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3.2 Electrochemical Characteristics 3.2.1 Galvanostatic Cycling Galvanostatic cycling studies were carried out in the voltage range 0.005V - 3.0V at a high current rate of 600 mA g-1 and cycling profiles of selected cycles are shown in Fig. 4. Fig.4 (a) shows the 1st, 2nd, 20th, 40th and 80th charge and discharge cycles for CuCo2O4 prepared using cobalt acetate and copper sulphate. During the first cycle, the voltage dropped sharply from the open-circuit voltage (OCV) of 1.6V to 1.1V and the corresponding capacity was around 63 mAh g-1. The discharge corresponds to an intercalation reaction forming LixCuCo2O4, followed by amorphization of the lattice during the conversion reaction. During 1st discharge, nano particles of cobalt metal are embedded into the Li2O matrix. The formation of solid electrolyte interface (SEI) and polymer gel type layer3, 31, 36, 39 also occurs. At the end of the first discharge cycle the capacity is 1352 mAhg-1. For the charge cycle shown in Figure 4a, the charge curve starts with a steep slope until it reaches a voltage of around 1.9V. After this, the slope reduces until the end of the charge cycle where the capacity is 882 mAh g-1. There is a great reduction in capacity as compared to the first discharge cycle. During the charge cycle, the reformation of respective metal oxides occurs. These lead to the above mentioned irreversible capacity loss (ICL) during the 1st cycle. As this only happens in the 1st cycle, the electrochemical reaction is different for the subsequent cycles. For the second cycle, the end of the discharge cycle is marked by a capacity of 897 mAhg-1, with a shorter plateau ending at 480 mAhg-1. For the charge cycle, it does not differ from the first one much in shape and ends with a capacity of 853 mAhg-1 (Table 2). In the subsequent cycles, the capacity reduces and finally, for the 80th cycle, the capacity at the end of discharge cycle is around 811 mAhg-1 and at the end of the charge cycle it is around 800 mAhg-1. However, it is observed that after about 45 cycles the 13



reversible capacity increases and continues all the way until the 80th cycle. This is a very good result and it might continue if the cell is run for a larger number of cycles. The observation can also be seen in Fig. 5(a). The reason for this increase is the development of SEI. Once SEI fully develops and becomes stable, it is beneficial as it allows the unsolvated Li ions to permeate and enable good ion transfer, while being electronically insulating1. The capacity retention of the cell is 94%.

3.0

3.0

(a)

2.0

(b)

2.5

1st Cycle 2nd Cycle 20th Cycle 40th Cycle 80th Cycle

Voltage / V

Voltage / V

2.5

1.5 1.0

2.0 1.5 1.0

0.0

0.0 0

200 400 600 800 1000 1200 1400

0

200

Capacity / mAhg

3.0

600

800 1000 1200

Capacity / mAhg

3.0

(c) 1st Cycle 2nd Cycle 5th Cycle 50th Cycle 100th Cycle

2.0 1.5 1.0

(d)

2.5 Voltage / V

2.5

400

-1

-1

0.5 0.0

1st Cycle 2nd Cycle 5th Cycle 20th Cycle 50th Cycle 100th Cycle

0.5

0.5

Volatge / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1st Cycle 2nd Cycle 15th Cycle 29th Cycle

2.0 1.5 1.0 0.5

0

200

400

600

Capacity / mAhg

0.0

800

0

100

-1

200

300

400

500

600

-1

Capacity / mAhg

Figure 4. Voltage vs. capacity graphs for galvanostatic (600 mAg-1) cycling of CuCo2O4 prepared at 410° C for compound prepared from (a) cobalt acetate and copper sulphate (b) cobalt sulphate and copper sulphate (c) cobalt chloride and copper chloride, (d) cobalt hydroxide and copper sulphate. 14


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Fig. 4(b) shows the 1st, 2nd, 5th, 20th, 50th and 100th charge and discharge cycles for CuCo2O4 prepared using cobalt sulphate and copper sulphate. During the first discharge cycle, the voltage drops sharply from 2.2V to 1.14V and the corresponding capacity is 40 mAh g-1. After this, the voltage stays fairly constant at about 1.1V and then starts sloping downwards. The capacity at the end of this discharge cycle is 1185 mAh g-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.9V. After this, the gradient reduces until the end of the cycle at 3.0V. At the end of the first cycle the capacity is only 766 mAh g-1(Table 2). However, at the end of the second discharge cycle, the capacity is 788 mAh g-1. The plateau for this cycle is much shorter and occurs at around 1.0V. The charge cycle does not differ much from the first one. At the end of the 2nd charge cycle, the capacity is 730 mAh g-1. For the subsequent cycles, the capacity continues to reduce and finally for the 100th cycle, the curve does not have a plateau anymore and the capacity at the end of the discharge cycle is seen to be 653 mAh g-1 and at the end of the charge cycle it is seen to be 640 mAh g-1. The capacity retention of the cell is 88%. For this cell the reversible capacity of the cell again increases after 55 cycles as seen in Fig. 5 (a). Fig. 4(c) shows the 1st, 2nd, 5th, 50th and 100th charge and discharge cycles for CuCo2O4 prepared using cobalt chloride and copper chloride. During the first discharge cycle, the voltage drops sharply from 1.3V to 1.0V and the corresponding capacity is 7 mAh g-1. After this, the voltage continues to drop again until 0.8V after which it stays constant for a short period of time and then starts sloping downwards. The capacity at the end of this discharge cycle is 810 mAh g-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.9V. After this, the gradient reduces until the end of the cycle at 3.0V. At the end of the first cycle the capacity is 281 mAh g-1. However at the end of the second discharge cycle the capacity is only 277 mAh g-1. The plateau for this cycle is much shorter and occurs at around 1.2V. The charge cycle does not differ much from the earlier one. At the 15



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end of the 2nd charge cycle, the capacity is 261 mAh g-1. For the subsequent cycles, the capacity continues to reduce and finally for the 100th cycle, the curve does not have a plateau anymore and the capacity at the end of the discharge cycle and charge cycles are 119 mAh g1

. The retention capacity of the cell is only 46%. Fig.4 (d) shows the 1st, 2nd, 15th and 29th

charge and discharge cycles for CuCo2O4 prepared using cobalt hydroxide and copper sulphate. During the first discharge cycle, the voltage drops sharply from 2.7V to 1.1V and the corresponding capacity is 15 mAh g-1. After this the voltage stays fairly constant at about 1.1V and then starts sloping downwards. The capacity at the end of this discharge cycle is 573 mAh g-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.95V. After this, the steepness reduces until the end of the cycle at 3.0V. At the end of the first cycle the capacity is only 335 mAh g-1 (Table 2). However at the end of the second discharge cycle the capacity is only 337 mAh g-1. The plateau for this cycle is much shorter and occurs at around 1.2V. The charge cycle does not differ much from the earlier one. At the end of the 2nd charge cycle, the capacity is 319 mAh g-1. For the subsequent cycles, the capacity continues to reduce and finally for the 29th cycle, the curve does not have a plateau anymore and the capacity at the end of the discharge cycle is seen to be 292 mAh g-1 and at the end of the charge cycle it is seen to be 287 mAhg-1. The retention capacity of the cell is 90% (Table 2).

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80th cycle

1st cycle

2nd cycle

reversible

reversible

reversible

Retention

Capacity

Capacity

capacity

(2cycle to

(mAhg-1)

(mAhg-1)

(mAhg-1)

nth cycle)

882

853

800 (80 Cycles)

94%

766

730

640

88%

335

319

287 (29 Cycles)

90%

281

261

119

46%

CuCo2O4 prepared at o

Capacity

410 C using different salts

Cobalt acetate and copper sulphate Cobalt sulphate and copper sulphate Cobalt hydroxide and copper sulphate Cobalt chloride and copper chloride

Table 2 Summary of the Galvanostatic cycling studies for CuCo2O4 prepared at 410oC.

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(a)

(b)

Figure 5. Capacity vs. cycle number plots of CuCo2O4 prepared at (a) 410°C, (b) 610°C voltage current density 600 mA g-1, voltage range, 0.005-3.0V vs. Li. Fig.6 shows voltage vs. capacity plots of samples prepared at 610°C and corresponding capacity vs. cycle number plots are shown in Fig.5b corresponding capacity values are summarized in table 3. Fig. 6 (a) shows 1st, 2nd, 5th, 20th, 50th and 100th charge 18


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and discharge cycles for CuCo2O4 prepared using cobalt sulphate and copper sulphate prepared at 610oC. During the first discharge cycle, the voltage drops sharply from 2.5V to 1.0V and the corresponding capacity is 26 mAh g-1. After this the voltage stays fairly constant at about 1.0V and then starts sloping downwards. The capacity at the end of this discharge cycle is 1424 mAhg-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.9V. After this, the gradient decreases until the end of the cycle at 3.0V. At the end of the first cycle the capacity is 848 mAh g-1 (Table 3). However at the end of the second discharge cycle the capacity is 851 mAh g-1. The plateau for this cycle is much shorter and occurs at around 1.3V. The charge cycle does not differ much from the earlier one. At the end of the 2nd charge cycle, the capacity is 823 mAhg-1.

3.0

1.5

Voltage / V

Voltage / V

2.4

1st Cycle 2nd Cycle 5th Cycle 20th Cycle 50the Cycle 100th Cycle

2.0

1.0

1st Cycle 2nd Cycle 5th Cycle 10th Cycle 20th Cycle

2.0 1.6 1.2 0.8

0.5 0.0

(b)

2.8

(a)

2.5

0.4 0

300

600

900

1200

0.0

1500

0

-1

Capacity / mAhg

3.0

3.0

(c) 1stCycle 2nd Cycle 5th Cycle 20th Cycle 50th CYcle 100th Cycle

2.0 1.5 1.0

(d) 1st Cycle 2nd Cycle 5th Cycle 20th Cycle 50th Cycle 100th Cycle

2.0 1.5 1.0 0.5

0.5 0.0

200 400 600 800 1000 1200 Capacity / mAhg-1

2.5 Voltage / V

2.5 Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

200

400 600 800 -1 Capacity / mAhg

0.0

1000

0

200 400 600 800 1000 1200 1400 Capacity / mAhg

19


-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. CuCo2O4 prepared at 610°C cycled in the voltage range, 0.005-3.0V vs. Li, current rate 600 mAg-1. Voltage vs. capacity for certain cycles of the compounds prepared using (a) cobalt sulphate and copper sulphate, (b) cobalt hydroxide and copper sulphate, (c) cobalt acetate and copper sulphate, (d) cobalt chloride and copper chloride. For the further cycles, the capacity continues to reduce and finally for the 100th cycle, the curve does not have a plateau anymore and the capacity at the end of the discharge cycle is seen to be 772 mAh g-1 and at the end of the charge cycle is seen to be 768 mAh g-1. The retention capacity of the cell is 93% (Table 3, Fig. 6a). An interesting observation about this method is that the capacity dips until 70 cycles after which it starts increasing as seen in Fig. 6 (a). This is a very encouraging outcome and it would be beneficial to expand the research further by increasing the number of cycles. Fig. 6(b) shows 1st, 2nd, 5th, 10th and 20th charge and discharge cycles for CuCo2O4 prepared using cobalt hydroxide and copper sulphate. During the first discharge cycle, the voltage drops sharply from 2.6V to 1.1V and the corresponding capacity is 23 mAh g-1. After this the voltage stays fairly constant at about 1.1V and then starts sloping downwards. The capacity at the end of this discharge cycle is 1228 mAh g-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.9V. After this, the steepness reduces until the end of the cycle at 3.0V. At the end of the first cycle the capacity is 745 mAh g-1, and this stays the same at the end of the second discharge cycle. The plateau for this cycle is much shorter and occurs at around 1.3V. The charge cycle does not differ much from the earlier one. At the end of the 2nd charge cycle, the capacity is 720 mAhg-1. In the subsequent cycles, the capacity reduces and finally for the 20th cycle, the capacity at the end of discharge cycle is around 650 mAhg-1 and at the end of the charge cycle is around 645 mAhg-1. Thus, the retention capacity of the cell is 90%. Fig.6 (c) shows the 1st, 2nd, 5th, 20th, 50th and 100th charge and discharge cycles for 20


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CuCo2O4 prepared using cobalt acetate and copper sulphate. During the first discharge cycle, the voltage drops sharply from 2.2V to 1.14V and the corresponding capacity is 54 mAh g-1. After this the voltage stays fairly constant at about 1.1V and then starts sloping downwards. The capacity at the end of this discharge cycle is 1080 mAh g-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.9V. After this, the steepness reduces until the end of the cycle at 3.0V. At the end of the first cycle the capacity is 672 mAh g-1. However at the end of the second discharge cycle the capacity is 697 mAh g-1. The plateau for this cycle is much shorter and occurs at around 1.0V. The charge cycle does not differ much from the earlier one. At the end of the 2nd charge cycle, the capacity is 633 mAh g-1. For the subsequent cycles, the capacity continues to reduce and finally for the 100th cycle, the curve does not have a plateau anymore and the capacity at the end of the discharge cycle is seen to be 460 mAh g-1 and at the end of the charge cycle is seen to be 464 mAhg-1. The retention capacity of the cell is 74%, which is not very high. Fig. 6 (d) shows 1st, 2nd, 5th, 20th, 50th and 100th charge and discharge cycles for CuCo2O4 prepared using cobalt chloride and copper chloride. During the first discharge cycle, the voltage drops sharply from 2.4V to 1.1V and the corresponding capacity is 39 mAh g-1. After this the voltage stays fairly constant at about 1.1V and then starts sloping down. The capacity at the end of this discharge cycle is 1357 mAh g-1. For the first charge cycle, the curve has a very steep upward slope until it reaches a voltage of 1.9V. After this, the steepness reduces until the end of the cycle at 3.0V. At the end of the first cycle the capacity was 776 mAh g-1. However at the end of the second discharge cycle the capacity is 794 mAh g-1. The plateau for this cycle is much shorter and occurs at around 1.1V. The charge cycle does not differ much from the earlier one. At the end of the 2nd charge cycle, the capacity is 772 mAh g-1. For the subsequent cycles, the capacity continues to reduce and finally for the 100th cycle, the curve does not have a plateau anymore and the capacity at the end of the discharge cycle 21



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Page 22 of 33

is seen to be 620 mAh g-1 and at the end of the charge cycle is seen to be 610 mAh g-1. The retention capacity of the cell is 79%, which is not very high. Irreversible capacity between first discharge -charge cycle was noted in all samples prepared at 410 and 610oC which is of the order 250 to 400 mAhg-1. In general, the electrochemical performance of composites of CuCo2O4 prepared at 410 and 610°C showed slight differences in the voltage plateaus and capacity values and fading was noted depending of morphology of the compounds. In the compound prepared at 410°C using Co-sulphate and acetate showed 94 % capacity retention and good cycling stability when compared to CuCo2O4 reported by to previous studies

7-11

and similar improved performance was noted with Co-based mixed oxides12.

1st Cycle CuCo2O4 prepared at Reversible

(mAhg )

(100th )

Retention

Reversible

(2nd cycle

Capacity

& last

(mAhg-1)

cycle)

Capacity

salts -1

Capacity

Reversible

610oC using different Capacity

Last Cycle 2nd Cycle

-1

(mAhg )

Cobalt acetate and 672

633

464

74%

848

823

768

93%

645

90%

745

720

copper sulphate Cobalt sulphate and copper sulphate Cobalt hydroxide and copper sulphate

(20 cycles)

Cobalt chloride and 776

772

copper chloride

22


610

79%

Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: Summary of the capacity values and capacity fading for CuCo2O4 prepared at 610oC. 3.2.2 Cyclic Voltammetry Cyclic voltammetry (CV) allows for the analysis of the redox potentials of the compound. CV is carried out at a scan rate of 0.058 mVs-1 and at a voltage range of 0.005V3.0V vs. Li. Voltammograms of CuCo2O4 prepared at studies for compounds prepared at 410oC (Fig. 7 (a)), during cathodic scan, voltage reduces continuously from 3.0V until it reaches around 1.3V. After this, there is a significant dip in the current until the voltage reaches 0.8V. This is because Li+ ions start to enter the compound and the crystal structure of the compound changes drastically, leading to the reduction of Cu2+ to Cu-metal and Co3+ to Co-metal. As for the anodic scan, a main peak is at 2.05V. This process indicates the reformation of the structure of the compound. For the remaining cycles, the scans are very similar with the cathodic peak occurring at 1.13V and the anodic peak occurring at 2.1V. This indicates that the cell has good reversibility. For Fig.7 (b) the potential reduces continuously from 3.0V until it reaches around 1.6V. After this there are some small dips in current until the voltage reaches 1.0V where there is a huge dip. For the anodic scan, the peak is at 2.05V. For the remaining cycles, the scans are overlap well with the cathodic peak occurring at 0.97V and the anodic peak occurring at 2.12V vs. Li. However, the dip in the cathodic cycle for the subsequent cycles is much smaller as compared to the first. Moreover, as the dips for cycles 2-6 differ, the reversibility may not be that good. A minor peak in the 1st cycle is due to CuO peaks40 .

23



0.003 0.002

2.05125

(a)

0.001

2.05375

(b)

0.001 0.000 -0.001 -0.002

0.24625

1st Cycle 2nd Cycle 3rd Cycle 4th Cycle

0.005

-0.003

Current / A

Current / A

0.000

-0.001

1st Cycle 2nd Cycle 3rd Cycle 4th Cycle 5th Cycle 6th Cycle

0.005

-0.002

-0.004

-0.003 -0.005

1.00375

0.79875

-0.006 0.0

0.5

1.0

1.5

2.0

2.5

-0.004

3.0

0.0

0.5

Voltage / V 0.002

1.5

2.0

2.5

3.0

Voltage / V

2.11125

(c)

1.0

0.002 0.001

(d)

2.0825

-0.002

0.005


-0.004 -0.006

Current / A

0.000

Current / A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

0.000 -0.001

0.005 -0.002 -0.003

1.16

-0.004

-0.008

0.9725

-0.005

-0.010

1.00875 -0.006 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Voltage / V

Voltage / V

Figure 7. Cyclic Voltammogram of CuCo2O4 prepared at 410oC using (a) Cobalt hydroxide and Copper sulphate (up to 4 cycles), (b) Cobalt acetate and Copper sulphate, (c) Cobalt sulphate and Copper sulphate, (d) Cobalt chloride and Copper chloride ( only select cycle only). Voltage range: 0.005-3.0V; scan rate: 0.058mV/sec. For clarity only selected cycles are shown. For Fig.7 (c) during cathodic scan, the voltage reduces continuously from 3.0V until it reaches around 1.4V. After this there is a significant dip in the current until the voltage reaches 0.97V.As for the anodic scan, the peak is at 2.1V. For the remaining cycles, the scans are very similar with the cathodic peak occurring at around 1.1V and the anodic peak occurring at 2.26V. However, the dip in the cathodic cycle for the subsequent cycles is much smaller as compared to the first. Moreover, the dips for cycles 2-6 differ as the voltage is not 24


Page 25 of 33

constant at which the dips occur. For Fig.7(d) the potential reduces continuously from 3.0V until it reaches around 1.4V. After this there is a significant dip in the current until the voltage reaches 1.0V during anodic scan. The anodic scan has a peak of 2.08V.

0.004

0.006

(a)

2.045 0.004

0.002

(b)

2.05375

0.002 0.000

0.000 -0.002


0.005 -0.004 -0.006

Current / A

Current / A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.002 -0.004

0.005


-0.006 -0.008 -0.010

-0.008

-0.012

0.99375

-0.010 0.0

0.5

1.0

-0.014

1.5

2.0

2.5

3.0

0.9975 0.0

Voltage / V

0.5

1.0

1.5

2.0

2.5

3.0

Voltage / V

Figure 8. Cyclic Voltammogram of CuCo2O4 prepared at 610oC using (a) Cobalt acetate and Copper sulphate (b) Cobalt hydroxide and Copper sulphate. Scan rate: 0.058mV/sec. Cyclic voltammetry studies for compounds prepared at 610oC are shown in Fig. 8. Cathodic scans of are shown in Fig. 8(a) the potential reduces continuously from 3.0V until it reaches around 1.44V. After this there are some small dips in current until the voltage reaches 0.99V and then there is a huge dip. Later, for the anodic scan, the peak is at 2.05V. For the remaining cycles, the scans are very similar to the cathodic peak occurring at around 0.84V and the anodic peak occurring at 2.1V. However, the dip in the cathodic cycle for the subsequent cycles is much smaller as compared to the first. Moreover, the dips for cycles 2-6 differ as the voltage as well as the current is not constant. The CVs of Fig.8 (b) the potential reduces continuously from 3.0V until it reaches around 1.3V. After this there is a significant dip in the current until the voltage reaches 0.99V. Later, for the anodic scan, the peak is at 2.05V. For the remaining cycles, the scans are very similar with the cathodic peak occurring 25



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Page 26 of 33

at around 0.94V and the anodic peak occurring at 2.1V. However, the dip in the cathodic cycle for the subsequent cycles is much smaller as compared to the first. Moreover, the dips for Cycles 2-6 differ as the voltage and the current are not constant at the points which the dips occur. Few minor additional peaks in Figs. 8.a,b are due to CuO, since it has a different crystal structure. After first cycle, In all cases main cathodic peak around 1.0V and anodic peak around 2.0V which corresponds to redox behaviours of Cu and Co-ions.

Observed

galvanostatic cycling and cyclic voltammetry studies mechanisms on the above materials are similar to previous reports conversion metal oxides, CuCo2O4, Co3O4, CuO4, 41-43.

CONCLUSIONS

In this project, we studied the effect of molten salts, initial reactants and preparation temperature on CuCo2O4. Materials were characterised using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and the Brunauer-Emmett-Teller (BET) surface and density method. Galvanostatic Cycling and Cyclic Voltammetry were performed to understand the physical and chemical properties, to analyse the reaction mechanics, capacity retention and columbic efficiency. In all the tests, studies and observations, CuCo2O4 prepared at 610 oC from cobalt sulphate and copper sulphate and CuCo2O4 prepared at 410 oC from copper sulphate and cobalt acetate performed the best with good retention capacity. At 410oC, the capacity retention was 94% and at 610oC it was 93%. Moreover, it was also observed that their capacities increased after a certain number of cycles and might produce more fruitful results if tested for higher number of cycles. All our cells were tested at a very high current rate of 600 mA g-1. Due to the catalytic nature of the present hybrid materials with optimized morphology their application is not limited to Li-ion batteries. In the future, we plan to study their performances in air cathodes of Li-air batteries. 26


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: S1: X-ray photoelectron spectra and S2 : Raman spectroscopy studies of CuCo2O4 compounds are shown . Acknowledgements Dr. Reddy and A/Prof. Stefan Adams thank to National Research Foundation (NRF), Prime Minister’s Office, Singapore for support under its Competitive Research Programme (CRP Award NRF-CRP 10-2012-6). Mr. Rajesh would like to thank the Gifted Education Branch of Ministry of Education, Singapore for the opportunity to participate in the Science Research Program (SRP). References 1.

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Effect of Initial Reactants and Reaction Temperature on Molten Salt Synthesis of CuCo2O4 and its Sustainable Energy Storage Properties Mogalahalli Venkatashamy Reddy1,2*, Mishra Rajesh2,3,4 , Stefan Adams1**, Bobba Venteshwara Rao Chowdari2

(a

(b)

(c)

CuO CuCo2O4 composites synthesised from molten salt method studied as anode material for Li-ion batteries

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Effect of Initial Reactants and Reaction Temperature on Molten Salt

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