1. INTRODUCTION :

Several techniques have been devised to improve the capacity and efficiency of the Li-

Ion batteries, among which is the use of 3D transition metal oxides. Amongst the 3D transition

metal oxides cobalt oxides (Co3O4, CoO) have shown the highest capacity and the best cyclic

performance compared to nickel oxides and iron oxides. However efforts have been made to

replace Co3O4 partially by environment friendly and less expensive metal oxides to lower the

toxicity and reduce the cost. For this reason ternary cobalt based metal oxide particles like

ZnCo2O4, which are larger than Co3O4 particles, have been used as an anode material of Li-Ion

batteries. For improving the electrochemical performance one dimensional nanostructured

materials have been developed because of their high surface to volume ratio and excellent

electronic transport property. ZnCo2O4 1D nanostructure have not been obtained until now

because of its spinel structure.

The main topic of research carried out by the author is synthesis of ZnCo2O4 nanowires

from a microemulsion of ZnCo2 (C2O4)3. The reversible capacity of ZnCo2O4 nanowires is much

higher than ZnCo2O4 nanoparticles. The ZnCo2O4 nanowires are obtained from ZnCo2 (C2O4)3

under annelating conditions. Here ZnCo2O4 acts as a sacrificial template for the synthesis of

ZnCo2O4. The as- synthesized porus ZnCo2O4 nanowires were applied as anode materials of Li-

Ion batteries, which showed superior capacities and cycling performance.

(a) Experimental section:

Chemical reagents:

a. CTAB (Cetyltrimethylammonium bromide)

b. Cyclohexane

c. n-Pentanol

d. 1 M H2C2O4 aqueous solution

e. Mixture of 0.05M Zn(NO3) and 0.1 M Co(NO3)2

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(b) Preparation of sacrificial template:

One gram of (a) was dissolved in a mixture of 35ml of (b) and 1.5 mL of (c) and stirred

for 30 minutes to form a microemulsion. Then, 2mL of reagent (d) was added to the

microemulsion and the whole mixture was again stirred for an addition one hour. Finally 1.25

mL of an aqueous solution of reagent (e) was added to the above microemulsion and stirred for 2

hours at room temperature. The precipitates (pink) were obtained by centrifugation and dried in

air at 80oC. The precipitates were heated at 500-700oC for 3 hours to obtain the final product

(dark grey)

(c) Sample characterization:

The final product was characterized by using X-ray diffraction (XRD) with

monochromized Cu Kα radiations (γ= 1.54178 Angstrom units).

The morphology and structure of the samples were examined by field emission scanning

electron microscopy (FESEM), transmission electron microscopy (TEM), and high resolution

transmission electron microscopy (HRTEM) with an energy-dispersive X-ray spectrometer

(EDX).

The BET surface area and pore volume were tested using a Beckman Coulter Omnisorp

100cx.

2. IMPORTANT FEATURES OF THE PAPER :

1. The paper mainly concentrates on production of 1D nanowires via a sacrificial template

using microemulsion based technique.

2. A microemulsion based method was developed for synthesizing 1D nanostructures with

well controlled dimensions.

3. The porous one dimensional nanostructures and large surface area were found

responsible for superior performance.

4. The surface of the nanowires was found to be smooth and no isolated nanoparticles were

detected.

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5. The reversible capacities of ZnCo2O4 nanowires were found to be much higher than that

of ZnCo2O4 nanoparticles.

6. The initial Coulombic efficiency for porous ZnCo2O4 nanowires was found to be 82%

which was due to the formation of a Solid electrolyte interphase film and some

undecomposed Li2O phase.

7. The porous ZnCo2O4 nanowires synthesized at 500oC showed larger capacity and better

cycling performance than prepared at 700oC because of higher porosity and larger surface

area.

3. LITERATURE SURVEY :

It is known that rechargeable Li-Ion batteries are the key components for portable

equipments.

The major hurdle in such batteries is to overcome their disintegrity over many discharge

and recharge cycles. So the electrodes made of nanoparticles of transition metal oxides of Ni, Co

or Fe was developed. The advantages of metal oxide based Li ion cells over commercially used

LiCoO2/C cells are that metal oxides have twice the capacity of Carbon per unit mass and three

times its density and six times capacity of carbon per unit volume. [1]

Spinel LiMn2O4 is a promising candidate to replace layered Ni or Co oxide materials as -

cathode in lithium ion batteries because of its intrinsic low-cost, environmental friendliness, high

abundance, and better safety. [2]

The timing of the reaction controls the product’s dimensions. Shorter reaction times at

optimal “cube” conditions reduce the nanocube edge lengths to ~70 nm; longer reaction times

lead to nanocubes with edges of up to ~175 nm, in a well-controlled manner. [3]

There are potential advantages and disadvantages associated with the development of

nano electrodes for Lithium batteries.

Advantages include:

Better accommodation of the strain of lithium insertion/removal, improving cycle life

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New reactions not possible with bulk materials

Higher electrode/electrolyte contact area leading to higher charge/discharge rates.

Short path lengths for electronic transport (permitting operation with low electronic

conductivity or at higher power).

Short path lengths for Li+ transport (permitting operation with low Li+ conductivity or

higher power).

Disadvantages include:

An increase in undesirable electrode/electrolyte reactions due to high surface area,

leading to self-discharge, poor cycling and calendar life.

Inferior packing of particles leading to lower volumetric energy densities unless special

compaction methods are developed

Potentially more complex synthesis. [4]

Nanostructure material can improve the electrochemical properties of Li-Ion batteries

compared to their bulk counterpart but to maximize this potential, organizing nanomaterial on an

attractive template is needed. [5]

1D nanostructures have attracted considerable attention because of their potential use in a

wide range of advanced applications in the past decade. Studies have shown that the key to

fabricating a 1D nanostructure can be focused on the way in which atoms or other building

blocks are rationally assembled into structures with nanometer-sized diameters but much higher

lengths. 1D nanostructures have attracted considerable attention because of their potential use in

a wide range of advanced applications in the past decade. Studies have shown that the key to

fabricating a 1D nanostructure can be focused on the way in which atoms or other building

blocks are rationally assembled into structures with nanometer-sized diameters but much higher

lengths. [6]

Silicon is an attractive anode material for lithium batteries because it has a low discharge

potential and the highest known theoretical charge capacity (4,200 mAh g). Although this is

more than ten times higher than existing graphite anodes and much larger than various nitride

and oxide materials silicon anodes have limited applications because silicon’s volume changes

by 400% upon insertion and extraction of lithium, which results in pulverization and capacity

fading. [7]

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It has been found that 3D transition metal oxides such as nickel oxide, cobalt oxide, and

iron oxide exhibit reversible capacities about three times larger than those of graphite at a

relative low potential. Among them, cobalt oxides (Co3O4 and CoO) have shown the highest

capacity (700 mAh g–1) and best cycle performance (93.4 % of initial capacity was retained after

100 cycles), compared with nickel oxide (NiO) and iron oxides (Fe2O3 and Fe3O4). [8]

Co3O4 nanotubes were developed via thermal decomposition of a sacrificial template

which was used as anode material for Li-Ion batteries and also as gas sensors.Co3O4 nanotubes

were found to be much better than Co3O4 nanorods or nanoparticles. The improved performance

has aroused from the effects of hollow inside and porous surface character of the tubes. [9]

Nanotubes have been considered as one of the most promising structures for Li-Ion

batteries due to higher surface to volume ratios than other one dimensional nanostructures such

as nanowires. But the initial Coulombic efficiency for porous Co3O4 nanotubes was only 70%

which was the same for earlier performed research. The TEM image and the XRD pattern of the

porous ZnCo3O4 nanotube indicated that morphology and structure have been basically

maintained after first charge and discharge process. [8,10]

It was reported that nano sized metal oxides exhibited good electrochemical performance

on account of larger specific area and higher reaction activity for Li-Ion insertion. For transition

metal oxides multi-electron reaction is dominant in the electrochemical reaction processes,

leading to higher electrochemical property. [11]

The main cause for low initial Coulombic efficiency i.e. large initial irreversible capacity,

were solid electrolyte interphase (SEI) film formation and incomplete decomposition of Li2O

during first discharge process. Co3O4 specimen exhibited similar properties in terms of capacity

and cycle life even when there was a change in synthetic time. [12]

Ferrous oxides being environment friendly were combined with Cobalt to form cobalt

ferrite which showed superior capacities. But the major drawback of the ferrite for anode

materials of Li-Ion batteries is that, there is dramatically decrease in the reversible capacities. [13]

Co3O4 produces a cubic inverse spinel structure. [14] Spinel-type ZnCo2O4 has long been

used as pigments or dying material and also in use as a catalyst for some reactions. Spinel

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compounds have a general formula MY2O4, in which M is a divalent metal and Y is a trivalent

one. Spinels (involve ZnCo2O4) containing transition metal ions can act as efficient catalysts in a

number of heterogeneous chemical processes. [15]

ZnCo2O4 was also used as an anode material and demonstrated high energy density. The

initial discharge capacity is high but irreversible capacity is low due to the formation of solid

electrolyte interface (SEI) and the irreversible reaction of lithium with oxygen atoms in the

active material. [16]

Poizot et.al; have proposed that mechanism of lithium storage in the cobalt oxide

materials is due to the reversible oxidation and reduction of cobalt oxides.

ZnCo2O4 is an attractive material for evaluation as anode for Lithium Ion batteries. It is a

normal spinel with the bivalent Zn-ions occupying the tetrahedral sites in the cubic spinel

structure and the trivalent Co-ions occupying the octahedral sites. Both Zn and Co are

electrochemically active metals with respect to Li-metal. Traditional spinel-formation reactions

usually occur at planar interfaces, or in a powder form at high sintering temperatures (>1,000

◦C). [17]

When nano-size particles of ZnCo2O4 are electrochemically discharged with Li-metal, the

crystal structure destruction occurs followed by the formation of Zn- and Co-metal nano-

particles and Li2O, as per the Equation 1. Upon deep discharge, Zn can reversibly react with Li

to form Li-Zn alloy (1:1) at potentials below 1.0 V to contribute to the anodic capacity, as shown

in Equation 2. In addition, at potentials above 1.0 V during charge-reaction, both Zn- and Co-

metal nano-particles can reversibly react with Li2O by the displacement reaction forming the

respective nano-size metal oxides (Equations. 3 and 4). In the most favourable case, the bivalent

Co2+ ions in CoO can further react reversibly with Li2O to form some trivalent Co3+ ions to give

Co3O4 as in Equation 5.

ZnCo2O4 + 8Li++8e–→Zn+ 2Co + 4 Li2O (1)

Zn + Li++e–↔ LiZn (2)

Zn + Li2O↔ZnO+ 2Li++2e– (3)

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2Co + 2Li2O↔2CoO+ 4Li++4e– (4)

2CoO+ 2/3Li2O↔2/3Co3O4 + 4/3Li+ + 4/3e– (5)

ZnCo2O4 gives high and stable capacities only under deep discharge-charge conditions

that enabled the formation and decomposition of the beneficial polymeric gel-type layer. During

the first-discharge reaction, the crystal structure of the starting material was destroyed followed

by the reduction of metal ions to metal nanoparticles. [18]

4. CRITICAL ANALYSIS

4.1 ORIGINALITY:

The work done by the author/ authors is the modification and extension to the other

researches carried out in the field. The ZnCo2O4 nanowire synthesis was already carried out by

the other researchers and the same work has been modified by synthesizing the material at

different annealing temperatures. The references which mention this work are 10 th, 11th, 22nd,

25th, and 26th.

Assumptions:

1. The large surface area and unique porous 1 D nanostructure may be responsible for good

performance and higher capacity.

2. The higher surface area at 500 oC is due to smaller diameters and larger quantities of

nanocrystals and nanopores.

3. ZnCo2O4 nanowires give superior performance in comparison to ZnCo2O4 nanoparticles

4.2 TECHNICAL CORRECTNESS:

1. The assumptions made are technically sound and reasonable. The microemulsion

technique used is one of the common techniques employed for nanowire synthesis as the

advantage of the method is biocompatibility and biodegradability. [18]

2. The author says that reversible capacity of ZnCo2O4 nanowires is much higher than that

of ZnCo2O4 nanoparticles but there are no facts supporting the statement as in 11 th

reference it is mentioned that the reversible capacities of ZnCo2O4 nanoparticles were

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98% over 60 cycles but the paper mentions that initial Coulombic efficiency was 82% in

case of ZnCo2O4 nanowires.

Some of the other discrepancies observed were as follows;

a. The effect of temperature is discussed but variation of the product property with respect

to time is not discussed.

b. The thermal conductivity of the material and its variation with temperature is not

discussed.

c. There are some typographical errors existing in the paper like Coulombic efficiency

being cited as Colombic efficiency.

d. Why the synthesis of ZnCO2O4 spinel structure is difficult is not mentioned in any of the

references cited.

e. The chemical name of ZnCO2O4 and name of the substrate used has not been mentioned

anywhere in the paper.

f. The initial Coulombic efficiency is mentioned but the average efficiency after 20 cycles

is not mentioned.

g. Whether the precipitates formed after microemulsion were aged and then annealed or

whether they were directly annealed after preparation is not clearly understood.

h. The apparatus in which the products obtained were prepared or stored is not mentioned.

i. Results with samples prepared with different aging times as well as at different CTAB

concentrations were not compared.

4.3 CLARITY:

The paper presented is an extension to the other researches that were carried out in the

same field. The work was already carried out as per the references 10 th, 11th, 22nd, 23rd, 25th, 26th.

The paper is prepared taken into consideration that the reader is very well aware of the basic

aspect relative to the subject. For a beginner the paper is very tedious to understand and interpret

its contents.

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4.4 BIBLIOGRAPHY:

1. The references are not fully cited and unnecessary semicolons are used for separating two

names.

2. As per the 1st reference which is cited, the author mentions that it gives information about

efforts made for the improvement of cyclic performance, Coulombic efficiency and

capacity. But such things are not at all mentioned in the 1 st reference but the given

information is found in 6th reference.

3. At some places information is taken from the references but the references are not cited

as in; the low Coulombic efficiency was due to the formation of solid electrolyte

interphase film and incomplete decomposition of Li2O during the discharge process is

taken from 11th reference but was not cited.

4. Some of the references are not cited in a proper manner as in; reference 8 th the year is

cited incorrectly (cited 2009 instead of 2007)., reference 7 th the page number is cited

incorrectly.

5. The author states that reversible capacities of ZnCo2O4 nanowires are much higher than

that of ZnCo2O4 nanoparticles as per cited in references 10th and 11th. But both the

references describe the nanoparticles of ZnCo2O4 and no comparison between the

nanoparticles and nanowires is being made

6. 15th reference does not clarify any significant details about the study of research relative

to the given paper.

4.5 TITLE/ABSTRACT:

1. The abstract contains the same content as that of conclusions.

2. The title “Porous ZnCO2O4 nanowires synthesis via Sacrificial Templates: High

Performance Anode materials of Li-Ion Batteries” it mentions use of templates but the

work contains the use of only a single template. The title is somewhat inappropriate as it

does not clearly define the process. The title suitable would be “ZnCo2O4 nanowire

synthesis by annealing ZnCo2 (C2O4)3 template; used as anode in Li-Ion batteries”.

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4.6 ILLUSTRATIONS AND TABLES:

There are too many figures or illustrations depicted in the paper. The illustrations are

quite clear but one of the figures given in the abstract is not at all the part of the paper.

.

Figure 1: XRD pattern of ZnCo2O4 at 500 oC

The figure given below is a XRD pattern of the product prepared by calcinations of

ZnCo2 (C2O4)3 nanowires at 500oC. The x-axis of the graph does not signify the units used

whether the 2Ө is in degrees or radians.

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Figure 2: (b) EDX pattern (c) SEM image, (d, e) TEM images, (f) HRTEM image, of ZnCo2O4

nanowires

For the above figures the author has wrongly mentioned figure (b) and (c) to be SEM

images but b is an EDX pattern of ZnCo2O4 at 500oC.

Figure 3: Cyclic voltammogram curves of electrodes made from ZnCo2O4

In the above figure the x-axis title is defined and it is mentioned that these curves are

voltammogram curves of the electrodes made from ZnCo2O4 nanowires at a scan rate of 0.5 mVs-

1.

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Figure 4: Discharge capacity versus cycle number for electrodes made from ZnCo2O4 nanowires

synthesized at 500oC and 700oC

Figure 4 compares the discharge capacity of versus cycle number for porous ZnCo 2O4

nanowires synthesized at 500 oC and 700 oC. The reversible capacity of nanowires synthesized at

500 oC was maintained at 1197.9 mA h/g while that synthesized at 700 oC exhibits a value of

957mAh/g. This was due to the larger surface area of the nanowires obtained at 500 oC than at

700 oC. If a clear observation is made, it is seen that there is a slight increase in the capacity after

5-6 cycles and then the capacity remains fairly constant. This slight increase in the value is not

clarified.

4.7 ALTERNATIVE INTERPRETATION:

The research concludes with the comparison of nanowires with nanoparticles but does not

compare nanowires with nanotubes. Also the comparison made is not on a firm base as no

references cited clearly mention the facts that are stated. Some of the research papers mentioned

in the references state that nanotube is more efficient than nanowire. The author could have

carried out further research on synthesis of nanotubes under different annealing conditions and

testing its Coulombic efficiency, cycling performance and capacity.

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5. CONCLUSIONS:

On basis of the critical analysis performed on the given research paper and on the

basis of my understanding I conclude that the author/authors tried to synthesize the

product by microemulsion technique by modification according to the need. But the work

could not clearly explain whether the comparision made was between ZnCo2O4

nanowires synthesized at different annealing temperatures or between ZnCo2O4

nanoparticles or nanowires or various other metal oxides with ZnCo2O4.

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366-375, 2005. 5. Nam K. T., Kim D. W., Yoo P. J., Chiang C Y., Meethong N. L., Hammond P T.,

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Huang X H., Tu J P., Adv. Mater., 19, 4505-4509, 200711. Wang G., Gao X. P., Shen P W., J. Power Sources, 192, 719-723, 2009.12. Kang Y M., Kim K T., Kim J H., Kim H S., Lee P S., Lee J Y., Liu H K., Dou S X , J.

Power Sources, 133, 252-259, 200413. Chu Y Q., Fu Z W., Qin Q Z., Electrochim. Acta., 49, 4915-4921, 2004.14. Wei X H., Chen D H., Tang W J., Mater. Chem. Phys., 103, 54-58, 200715. Ai C C., Yin M C., Wang C W., Sun J T., J. Mater. Sci., 39, 1077-1079, 200416. Fan H J., Knez M., Scholz R., Nielsch K., Pippel E., Hesse D., Zacharias M., Gosele U.,

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