A multiphase LiNi0.33Mn0.54Co0.13O2 cathode ...

Accepted Article Title: A multiphase LiNi0.33Mn0.54Co0.13O2 cathode material with very good capacity retention for Li-ion batteries

Authors: Prasant Kumar Nayak; Judith Grinblat; Mikhael Levi; Ortal Haik; Elena Levi; Sangryun Kim; Jang Wook Choi; Doron Aurbach

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To be cited as: ChemElectroChem 10.1002/celc.201500339 Link to VoR: http://dx.doi.org/10.1002/celc.201500339

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ChemElectroChem

10.1002/celc.201500339

A multiphase LiNi0.33Mn0.54Co0.13O2 cathode material with very good capacity retention for Li-ion batteries Prasant Kumar Nayak,[a] Judith Grinblat,[a] Mikhael Levi,[a] Ortal Haik,[a] Elena Levi,[a] Sangryun Kim,[b] Jang Wook Choi,[b] Doron Aurbach*[ a] [a]

Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel 5290002

[b]

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea

Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea

Abstract An integrated layered-spinel LiNi0.33Mn0.54Co0.13O2 was synthesized by self-combustion reaction (SCR), characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. It was studied as a cathode material for Li-ion batteries and its electrochemical performance was compared with that of the layered cathode materialLiNi0.33Mn0.33Co0.33O2being operated at a wide potential window. The Rietveld analysis of LiNi0.33Mn0.54Co0.13O2 indicated the presence of monoclinic Li[Li1/3Mn2/3]O2 (31%) and rhombohedral (LiNixMnyCozO2) (62 %) phases as the major components, and spinel (LiNi0.5Mn1.5O4) (7 %) as a minor component, which is well supported by TEM and electron diffraction. A discharge specific capacity of about 170 mAh g-1 is obtained in the potential range of 2.3-4.9 V vs. Li at low rate (C/10) with excellent capacity retention upon cycling. On the other hand, LiNi0.33Mn0.33Co0.33O2 (NMC111) synthesized by SCR exhibits an initial discharge capacity of about 208 mAh g-1 in the potential range of 2.3-4.9 V, which decreases to a value of 130 mAh g-1 after only 50 cycles. In turn, the multiphase structure of LiNi0.33Mn0.54Co0.13O2 seems to stabilize the behavior of this cathode material even when polarized to high potentials. LiNi0.33Mn0.54Co0.13O2 shows superior retention of average discharge voltage upon cycling as compared to that of LiNi0.33Mn0.33Co0.33O2 when cycled in a wide potential range. Overall,

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LiNi0.33Mn0.54Co0.13O2 can be considered as a promising low cobalt content cathode material for Li ion batteries. Key words: Li-ion batteries, layered-spinel integrated cathodes, Li[NiMnCo]O2, layered-tospinel transformation, HRTEM *[email protected] 1. Introduction Li-ion batteries are widely used as high energy density electrochemical power sources for a variety of portable electronic devices. In recent years, advanced Li ion batteries are implemented in more challenging applications such as electric vehicles. High capacity, high voltage and long cycle-life are essentially desirable for cathode materials to be applicable for those challenging Li-ion batteries.[1-5] However, the most widely used cathode material, LiCoO2 can provide only 140 mAh g-1. Despite its successful commercialization with decades of history, the use of this cathode material still raises some concerns about the cost, abundance of cobalt, and potential safety limitations at elevated temperatures, besides the limited specific capacity.[6] On the other hand, layered LiNi1/3Mn1/3Co1/3O2 is a promising cathode material, which can provide a specific capacity of 160 mAh g-1 in the potential range of 2.5-4.3 V with a very good cyclability and good safety features.[7-22] Its Co content is also only 1/3 of that present in LiCoO2. It is iso-structural to layered LiCoO2 and also integrates the features of LiCoO2, LiNiO2 and LiMnO2. LiNi1/3Mn1/3Co1/3O2 can also deliver higher specific capacity near 200 mAh g-1 when cycled to higher voltages (e.g. 4.6 V) which enables the use of the Co3+/Co4+ redox reaction.[23,24] Layered oxide systems in the form of LiNi1-y-zCoyAlzO2 (y=0.1-0.15, z=0.05) (NCA) also exhibit high specific capacity of about 200 mAh g-1 in the potential range of 3.0-4.5 V vs. Li.[25] 2

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However, there is a severe capacity fading of LiNi1/3Mn1/3Co1/3O2 observed with extensive cycling in this wide potential range as compared to normal potential range of cycling (2.5-4.3 V). Recently, we had systematically investigated the effect of upper potential limit on the electrochemical performance of LiNi1/3Mn1/3Co1/3O2, where a large capacity fading was observed when cycled to potential higher than 4.3 V.[24] Hence, the layered cathode materials are usually cycled in a limited potential range, below 4.3 V. This low value of upper potential of cathodes limits the energy density of Li-ion batteries. Recently, there is a great interest in Li- and Mn-rich integrated cathode materials which are usually cycled to potentials higher than 4.6 V in order to extract their maximum capacities.[2639]

These cathode materials can exhibit specific capacities ≥ 250 mAh g-1 in a wide potential

range of 2.0-4.8 V. However, these materials also undergo capacity fading as well as voltage decay with extensive cycling due to structural layered-to-spinel transformation accompanying the activation of Li2MnO3 on cycling to potential ≥ 4.5 V.[30-32] Also, a high irreversible capacity is usually observed in the 1st cycle, upon activation of the Li2MnO3 component in these Li and Mn rich cathode materials. On the other hand, LiNi0.5Mn1.5O4 spinel cathode materials are usually cycled to a potential ≥ 4.9 V because of the redox activity of Ni2+/Ni4+ at about 4.7 V.[40 44]

It is known that the cubic-close-packed oxygen arrays in both layered and spinel Li-metal-

oxide structures are compatible with each other.[45,46] In recent years, Li- and Mn-rich cathode materials that were explored, included also composites of layered and spinel compounds, which are usually cycled in a wide potential range (2.0-5.0 V) with good electrochemical stability.[45-51] Lee et al. studied layered-spinel composites with different proportion of layered to spinel components

and

reported

a

specific

capacity

xLi[Li0.2Mn0.6Ni0.17Co0.03]O2.(1-x)Li[Mn1.5Ni0.425Co0.075]O4

of

about

200

mAh

g-1

for

(x=0.5 and 0.75).[46] Long et al. 3

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introduced a spinel component to suppress the voltage fade of high capacity Li and Mn-rich layered-layered xLi2MnO3.(1-x)LiMO2 (M=Mn, Ni, Co) electrodes in Li cells.[48] Kim et al. also tried to improve the electrochemical performance and cycling stability by embedding a spinel component into the layered structure.[49] We also recently reported the electrochemical performance of a Co-free layered-spinel composite Li[Ni1/3Mn2/3]O2, which exhibited a specific capacity of about 220 mAh g-1 at C/10 rate in the potential range of 2.4-4.9 V Vs. Li/Li+.[51] All of these investigations coherently imply that cathode materials with multi-phase compositions could be advantageous for their electrochemical performance. In this study, we synthesized an integrated layered-spinel material LiNi0.33Mn0.54Co0.13O2 by SCR, characterized and studied its electrochemical performance as a cathode material for Liion batteries, over a wide potential range of 2.3-4.9 V, in order to extract its maximal capacity. The material with composition of LiNi0.33Mn0.54Co0.13O2 can be considered to be very close to Li[Ni1/3Mn2/3]O2, where some Mn cations are substituted by Co while the amount of Ni remains the same. The substitution of Mn by Co results in the decrease of the amount of Li2MnO3 from 77 % in Li[Ni1/3Mn2/3]O2 to 31 % in LiNi0.33Mn0.54Co0.13O2 and simultaneously increases the amount of rhombohedral LiMnNiCoO2 (62 %) and spinel LiNi0.5Mn1.5O4 (7 %), thus making the whole active mass more electrochemically active than Li[Ni1/3Mn2/3]O2. The presence of spinel component in the pristine active mass may suppress the fatal layered-to-spinel phase transition and help in the stabilization of the active mass as reported previously [49] as it is known that the rhombohedral LiMnNiCoO2 undergoes capacity fading when cycled to potential ≥ 4.5 V.[23,24] In order to clarify the role of the given stoichiometry, the layered compound LiNi0.33Mn0.33Co0.33O2 (NMC 111) was included in the present study for a comparison purpose. An obvious advantage of LiNi0.33Mn0.54Co0.13O2 over the NMC 111 reference cathode material is the low content of Co

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(40 %). By simply varying the content of Mn and Co, a very interesting layered-spinel integrated cathode with excellent electrochemical performance has been obtained and explored in the present study. The tools for characterization of the material included X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and electrochemical impedance spectroscopy (EIS) in conjunction with standard electrochemical techniques. The present study is a continuation of our on-going research aiming at mapping the behavior of vario-stoichiometric high-voltage and high-capacity lithiated transition metal oxides cathode materials. 2. Results and discussion The elemental analysis of these materials (synthesized by SCR) using the inductive coupled plasma technique (ICP) confirmed the designed composition, i.e., Li1.03Ni0.32Mn0.52Co0.13O2 and Li1.02Ni0.32Mn0.33Co0.33O2 for LiNi0.33Mn0.54Co0.13O2 and LiNi0.33Mn0.33Co0.33O2, respectively. 2.1. Structure and morphology Fig. 1 shows the XRD pattern and the results of a Rietveld analysis for the product of the synthesis of our target Mn rich cathode material by SCR. The phase compositions, structural parameters and R-factors (quality of fitting) are presented in the Table 1. The details of the Rietveld analysis for the materials LiNi0.33Mn0.54Co0.13O2 and LiNi0.33Mn0.33Co0.33O2 are given in Tables 1S and 2S, respectively (see supporting information). The synthesized material LiNi0.33Mn0.54Co0.13O2 was found to be composed of three phases: LiMnNiCoO2 (62 %), Li2MnO3 (31 %) and LiMn1.5Ni0.5O4 (7 %), whereas LiNi0.33Mn0.33Co0.33O2 was completely a single phase (rhombohedral) material.

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Fig. 1. Rietveld analysis of (a) LiNi0.33Mn0.54Co0.13O2 and (b) LiNi0.33Mn0.33Co0.33O2 synthesized by SCR and annealed at 900 ◦C. The calculated patterns are shown by a solid black curve; red dots show the observed intensities. The difference between the observed and calculated intensities is presented by a blue curve. The short vertical bars indicate the position of Bragg reflections: a - from upper to bottom: monoclinic Li2MnO3, cubic spinel LiNi0.5Mn1.5O4 and rhombohedral Li(Mn,Ni,Co)O2. 6

ChemElectroChem

Table

1.

10.1002/celc.201500339

The

results

of

the

Rietveld

analysis

of

LiNi0.33Mn0.54Co0.13O2

and

LiNi0.33Mn0.33Co0.33O2.

Sample


Phase

Li2MnO3

Li(Ni,Mn,Co)O2 LiNi0.5Mn1.5O4 LiNi0.33Mn0.33Co0.33O2 LiNi0.33Mn0.33Co0.33O2

Space group

Lattice parameters

a = 4.946 Å C 2/m b = 8.559 Å c = 5.036 Å  = 109.220 R -3m

*a = 2.884 Å *c = 14.296 Å

F d-3m a = 8.190 Å R -3m

*a = 2.862 Å *c = 14.238 Å

Phase content (%)

31

Fitting quality Rb (%)

2

12.86 3.21

62

7.49

7

10.21

100

6.89

2.56

*hexagonal setting

SEM images of our target material LiNi0.33Mn0.54Co0.13O2 and the reference material LiMn0.33Ni0.33Co0.33O2 are shown in Fig. 2. Submicron size particles of 200-500 nm are observed for both samples. TEM provided supporting information regarding the structure of these cathode materials. Based on our experience from previous studies,[30-32] the results were interpreted on the basis of the two-phase system model consisting of structurally integrated layered monoclinic Li2MnO3 (C2/m) and rhombohedral LiMO2 ( R 3 m ) components (Fig. 3). In addition, the cubic spinel LiNi0.5Mn1.5O4 is present as a minor component (Fig. 4). The lattice parameters for the monoclinic phase were a=4.9411Å, b=8.5289 Å, c=5.0252 Å, β=109.1° and the lattice parameters for the rhombohedral phase were a=2.849 Å, c=14.19 Å. The HRTEM images clearly show the presence of atomic planes corresponding to the monoclinic phase (Fig. 3c). The CBED taken from a 4 nm area marked by white arrow in Fig. 4a shows the reflections that can be 7

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indexed as rhombohedral phase (Fig. 4b) and CBED pattern from Fig. 4c can be uniquely indexed to spinel phase (Fig. 4d). The rhombohedral crystal structure of LiNi0.33Mn0.33Co0.33O2 is clearly observed by the CBED and HRTEM in our previous study.[24]

(a)

(b)

Fig. 2 SEM images of (a) LiNi0.33Mn0.54Co0.13O2 and (b) LiNi0.33Mn0.33Co0.33O2.

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Fig. 3 (a) Typical bright field image of LiNi0.33Mn0.54Co0.13O2nano-particles; (b) SAED taken from image (a) showing reflections characteristic of the monoclinic phase; (c) HRTEM image showing characteristic atomic planes of the monoclinic phase; (d) FFT analysis of the area marked by white square in (c) displaying sets of the reflections that where indexed as the monoclinic phase.

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Fig. 4 (a), and (c) images of LiNi0.33Mn0.54Co0.13O2 nano-particles; (b) CBED taken from a 4 nm area marked by white arrow in (a) showing the reflections that were indexed as rhombohedral LiNi0.33Mn0.33Co0.33O2, (d) CBED taken from a 4 nm area marked by white arrow in (c) showed the reflections that is uniquely indexed to spinel LiNi0.5Mn1.5O4.

2.2. Electrochemical performance Galvanostatic

charge-discharge

cycling

of


and

LiNi0.33Mn0.33Co0.33O2 cathodes was carried out at 20 mA g-1 (around C/10) in the voltage range of 2.3-4.9 V in standard electrolyte solutions. Since the Li counter electrodes in the cells were in

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large excess, all the data in these figures reflect solely the behavior of the cathodes. Typical charge-discharge voltage profiles and the differential capacity plots in the 1st cycle are presented in Fig. 5. The potential increases suddenly from open circuit potential (ocp) to 4.0 V and then gradually increases as a plateau corresponding to the de-intercalation of Li+ ions accompanied by Ni2+/Ni4+ redox couple in the case of LiNi0.33Mn0.54Co0.13O2. A plateau starts from 3.8 V during charge in the case of LiNi0.33Mn0.33Co0.33O2. The first cycle charge and discharge specific capacities are 180 and 158 mAh g-1 for LiNi0.33Mn0.54Co0.13O2; 244 and 208 mAh g-1 for LiNi0.33Mn0.33Co0.33O2 cathodes, respectively. Thus, initially high specific capacities are obtained (a) 1st cycle

Potential / V vs. Li/Li

+

4.8 4.4 4.0 3.6 3.2 2.8


2.4


-20 0

20 40 60 80 100 120 140 160 180 200 220 240 260

Specific capacity / mAh g

-1

600

(b) LiNi0.33Mn0.54Co0.13O2

-1

(dQ/dV) / mAh g V

-1

800

400


200 0 -200 -400 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0


+

Fig. 5 (a) 1st galvanostatic charge-discharge profiles and (b) differential capacity plots of (i) LiNi0.33Mn0.54Co0.13O2and (ii) LiNi0.33Mn0.33Co0.33O2 cathodes at 20 mAh g-1 (about C/10 rate) in the potential range of 2.3-4.9 V in EC-DMC 1:1/1M LPF6 solutions. 11

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for LiNi0.33Mn0.33Co0.33O2 as compared to LiNi0.33Mn0.54Co0.13O2 cathodes when operating in a wide potential window. The coulombic efficiencies are found to be 87.8 and 85.2 % in the 1st cycle for LiNi0.33Mn0.54Co0.13O2 and LiNi0.33Mn0.33Co0.33O2, respectively in this wide potential range. The differential capacity plots (Fig. 5b) clearly show the peaks corresponding to the plateaus in the charge-discharge curves, e.g., major peaks upon charge at 3.75 V and 4.03 V for LiNi0.33Mn0.33Co0.33O2 and at 4.0 V and 4.75 V for LiNi0.33Mn0.54Co0.13O2 cathodes, respectively and a major peak upon discharge around 3.7 V for both electrodes (Fig. 5a). Typical

charge-discharge

voltage

profiles

of


and

LiNi0.33Mn0.33Co0.33O2 electrodes in different cycles are shown in Fig. 6. Upon cycling, there are severe decreases in the specific capacity as well as in the average discharge voltage of LiNi0.33Mn0.33Co0.33O2 electrodes (Fig. 6b). Simultaneously, there is an increase in the average charge

voltage

of

these

electrodes,

indicating

that

the

reversibility

of

the

intercalation/deintercalation processes of Li+ ions is impaired with cycling. In turn, the capacity retention of LiNi0.33Mn0.54Co0.13O2 electrodes is reasonably good. There is a gradual decrease in the average discharge voltage upon cycling of these electrodes, but their average charge voltage is stable. Hence, making one change with these 5 element cathode materials: increasing the amount of Mn on the account of Co, leads to the formation of a multi-phase structure (in contrast to the single phase structure of LiNi0.33Mn0.33Co0.33O2). This multi-phase structure enables intercalation/deintercalation of Li+ ions along a wide potential domain, keeping the overall capacity stable, although there are obvious structural changes occurring upon cycling, well reflected by the gradual change observed in the discharge voltage (Fig. 6a). The decrease in the discharge voltage can be ascribed to the activation of Li2MnO3 at potentials higher than 4.5 V, 12

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which is usually observed in case of Li- and Mn-rich cathode materials.[31,32] Both layered phases (Li2MnO3 and LiMnNiCoO2) are known to undergo layered-to-spinel transformation upon cycling. The layered LiMO2 transforms to spinel through migration of transition metals ions to the Li sites without breaking down the lattice whereas the transition from Li 2MnO3 to spinel involves the removal of Li+ and O2- with breakdown of the parent lattice.[30] Thus, the presence of spinel phase in the pristine LiNi0.33Mn0.54Co0.13O2 may help in suppression of layered-to-


+

4.8

(a) LiNi0.33Mn0.54Co0.13O2

4.4

3rd 25th 50th 100th

4.0 3.6 3.2 2.8 2.4 0

20

40

60

80

100 120 140 160 180 200


(b) LiNi0.33Mn0.33Co0.33O2


+

4.8

-1

3rd 25th 50th

4.4 4.0 3.6 3.2 2.8 2.4 0

40

80

120

160


200

240

-1

Fig. 6 Galvanostatic charge-discharge cycles of (a) LiNi0.33Mn0.54Co0.13O2 and (b) LiNi0.33Mn0.33Co0.33O2 cathodes at 20 mAh g-1 (about C/10 rate) in the potential range of 2.3-4.9 V in EC-DMC 1:1/1M LPF6 solutions. 13

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spinel phase transition and stabilization of active mass.[49] The plots of specific capacity and average voltage upon cycling are shown in Figs. 7 and 8. The discharge specific capacity of 160 mAh g-1 increases to a value of 175 mAh g-1 in about 8 cycles and a steady capacity is then observed even after 100 cycles in case of LiNi0.33Mn0.54Co0.13O2 cathodes (Fig. 7 (i)). The initial increase in the discharge capacity can be due to the presence of Li2MnO3, which takes a few cycles in order to get activated.[52]

Potential range: 2.3-4.9 V

Specific capacity / mAh g-1

240

(i) LiNi0.33Mn0.54Co0.13O2 (ii) LiNi0.33Mn0.33Co0.33O2

200

160

120

80

0

20

40

60

80

100

Cycle number Fig. 7 Plot of specific capacity vs. cycle number from galvanostatic charge-discharge cycling of (i) LiNi0.33Mn0.54Co0.13O2 and (ii) LiNi0.33Mn0.33Co0.33O2 cathodes at 20 mAh g-1 (about C/10 rate) in the potential range of 2.3-4.9 V in EC-DMC 1:1/1M LPF6 solutions.

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Potential range: 2.3-4.9 V

Avg. Potential / V vs. Li/Li

+

4.4

4.2

4.0

(i) LiNi0.33Mn0.54Co0.13O2 (charge) (i) LiNi0.33Mn0.54Co0.13O2 (discharge)

3.8

(ii) LiNi0.33Mn0.33Co0.33O2 (charge) (ii) LiNi0.33Mn0.33Co0.33O2 (discharge)

3.6

3.4

3.2

0

20

40

60

80

100

Cycle number

Fig. 8 Plot of average potential vs. cycle number from galvanostatic charge-discharge cycling of (i) LiNi0.33Mn0.54Co0.13O2 and (ii) LiNi0.33Mn0.33Co0.33O2 at 20 mAh g-1 (about C/10 rate) in the potential range of 2.3-4.9 V in EC-DMC 1:1/1M LPF6 solutions. On the other hand, the initial specific capacity of 208 mAh g-1 for LiNi0.33Mn0.33Co0.33O2 cathodes rapidly decreases to a value of 130 mAh g-1 after only 50 cycles when cycled in the potential range of 2.3-4.9 V, showing its inferior electrochemical stability in this wide potential range. From our previous studies, it is known that as the upper potential of cycling of LiNi0.33Mn0.33Co0.33O2 cathodes gets higher (≥4.3 V), the capacity fading upon cycling becomes more severe.[24] The average discharge voltage of LiNi0.33Mn0.54Co0.13O2 cathodes decreases from 3.62 V to a value of 3.32 V, thus retaining about 91.7 % after 100 cycles (Fig. 8). This behavior, i.e., the decrease in the discharge voltage in case of Li-and Mn-rich cathodes is usually ascribed to the layered-to-spinel transformation that results from the activation of monoclinic Li2MnO3 15

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upon cycling to potential higher than 4.5 V. Also, a decrease in the average charge voltage was detected in the initial cycles and subsequently a steady voltage was maintained over 100 cycles upon charging the LiNi0.33Mn0.54Co0.13O2 cathodes (Fig. 8). It is clear however that the change in average voltage upon cycling is much more pronounced with LiNi0.33Mn0.33Co0.33O2 electrodes when cycled in the wide potential range.

220

LiNi0.33Mn0.54Co0.13O2 C/10



-1

200 180

C/10

C/5

160

C/2

140

C

120

2C

100

4C 80 60 40

0

5

10

15

20

25

30

35

40

Cycle number

Fig. 9 Rate capability tests of LiNi0.33Mn0.54Co0.13O2 cathodes at different current values (C rates) in the potential range of 2.3-4.9 V in EC-DMC 1:1/1M LPF6 solutions. In order to complete the presentation of the electrochemical behavior of these electrodes, the rate capability tests were performed at different current densities (C rates), as shown in Fig. 9. On increasing the rate from C/10 to 4C, the specific capacity of LiNi0.33Mn0.54Co0.13O2

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electrodes decreases from 180 mAh g-1 to 80 mAh g-1, thus retaining about 47 % capacity. LiNi0.33Mn0.33Co0.33O2 exhibits a high specific capacity close to 200 mAh g-1 at C/10 rate. However, on increasing the rate, its specific capacity decreases and about 70 mAh g-1 is obtained at 4C rate, thus retaining about 35 % capacity. The high rate capability of LiNi0.33Mn0.54Co0.13O2 can be ascribed to the presence of spinel component in the active mass, which is known to facilitate the diffusion of Li+ ions. After cycling at high rates, the specific capacity returns to the high values measured initially at low rates. 2.3. Electrochemical impedance spectroscopic studies The

electrochemical

impedance

spectra

of


and

LiNi0.33Mn0.33Co0.33O2 electrodes were recorded at various equilibrium potentials with an amplitude of 5 mV in the frequency range of 100 kHz-0.01 Hz. The cells were subjected to 10 galvanostatic charge-discharge cycles for activation and stabilization before the impedance measurements. The Nyquist plots recorded at various potentials (3.8-4.6 V) during charging are shown in Fig. 10 for qualitative comparison. While these impedance spectra obviously represent the transport properties and phenomena related to these systems, it is impossible to exactly and precisely assign the EIS features to the various relevant time constants, due to the composite structure of the electrodes. The Nyquist plots consist of two distinguishable semicircles followed by a nearly linear z’’ vs. z’ response at low frequency region. The high-to-medium frequency semicircles are usually assigned to the time constants related to the surface films that cover all lithiated transition metal oxides in standard electrolyte solutions.[31,32,34] These time constants include resistance due to Li+ ions migration through the surface films (Rf), coupled with film capacitance (Cf). The 2nd semicircle present in the medium-to-low frequency domain can be assigned to interfacial charge-transfer resistance (Rct) coupled with an interfacial capacitance 17

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(Cdl). The low frequency straight lines are usually assigned to diffusion of Li+ ions into the electrode active mass, resembling a Warburg-type impedance (W). The Rf was found to be nearly invariant with potential, for both types of electrodes explored in the present study. For LiNi0.33Mn0.54Co0.13O2, the Rct first decreases from 3.8 V to 4.0 V and starts increasing thereafter

-500

(a)

Z'' / a.u.

-400 -300

3.8 V 4.0 V 4.2 V 4.4 V 4.6 V

-200 -100 0 0

100

200

300

Z' / 

400

500

600

-500

Z'' / a.u.

-400

(b)

-300

3.8 V 4.0 V 4.2 V 4.4 V 4.6 V

-200 -100 0 0

100

200

300

Z' / 

400

500

600

Fig. 10 Nyquist plots of (a) LiNi0.33Mn0.54Co0.13O2 and (b) LiNi0.33Mn0.33Co0.33O2 electrodes with an amplitude of 5 mV, measured at equilibrium potentials in the potential range of 3.8-4.6 V in EC-DMC 1:1/1M LPF6 solutions.

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upon further charging. A similar trend in Rct is observed for LiNi0.33Mn0.33Co0.33O2 electrodes. Up to 4.4 V, the impedance of the latter electrodes is lower. This could be the origin of the higher specific capacity of LiNi0.33Mn0.33Co0.33O2 as compared to that of LiNi0.33Mn0.54Co0.13O2. However, at high voltages (see the spectrum measured at 4.6 V), the impedance of LiNi0.33Mn0.33Co0.33O2 cathodes is dominated by high Rct values, which dominate over the low frequency features assigned to diffusion related time constants. The higher Rct observed at potentials above 4.4 V relates to the poor cycling performance of layered LiNi0.33Mn0.33Co0.33O2 when cycled in the wide potential range. This high value of Rct measured at high voltages with LiNi0.33Mn0.33Co0.33O2 electrodes is also reflected in the charge-discharge curves by high ohmic drop, as exhibited in the voltage profiles presented in Fig. 6b. On the other hand, the kinetics of LiNi0.33Mn0.54Co0.13O2 is controlled by the diffusion of Li+ ions, reflected by features at the low frequency domain in the EIS even at potential higher than 4.4 V. Thus, EIS shows a clear difference between the electrode kinetics of these cathode materials at higher potential (> 4.4 V) when cycled in a wide potential range of 2.3-4.9 V. 2.4. Raman spectra of cycled electrodes Fig. 11 compares the Raman spectra of pristine and cycled (after 100 cycles) LiNi0.33Mn0.54Co0.13O2 electrodes. The Raman band at 426 cm-1 associated with the monoclinic Li2MnO3 component which is seen in the spectrum of pristine electrodes (Fig. 11 (i)) does not exist in the spectra of cycled electrodes. The other two significant Raman peaks which appear around 485 and 591 cm-1 in the spectra of the pristine electrodes are blue shifted to higher values in the spectra of cycled electrodes (to 497 and 624 cm-1, Fig. 11 (ii)). The Raman band around 620 cm-1 is a characteristic peak of the spinel phase.[40] Thus, the appearance of a Raman band at 624 cm-1 can be ascribed to a spinel-type cation ordering upon cycling, as reported previously for 19

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all the integrated Li- and Mn-rich cathode materials.[31,32] This layered-to-spinel structural transformation is once again associated with the shifted average discharge voltage of Li- and Mn-rich cathodes to lower values upon cycling to potentials higher than 4.5 V.

624

(i) pristine (ii) cycled (iii) cycled

591

614

Intensity / a.u.

485 426

(i)

485

497

(iii) (ii)

200

300

400

500

600

700

800

900

-1

Raman shift / cm

Fig. 11 Raman spectra of (i) pristine; and ((ii), (iii)) different regions of cycled electrodes of LiNi0.33Mn0.54Co0.13O2. 3. Conclusions Our present study elucidated the electrochemical performance of a multi-phase LiNi0.33Mn0.54Co0.13O2 composition, containing initially Li2MnO3, LiNiMnCoO2 and spinel LiNi0.5Mn1.5O4, synthesized by SCR. The reference compound for this study was the single phase LiNi0.33Mn0.33Co0.33O2 (synthesized by SCR as well), operating in the same wide potential 20

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window, with an upper limit of 4.9 V. The structural studies by XRD and TEM confirmed the presence of monoclinic Li[Li1/3Mn2/3]O2 and rhombohedral (LiNi0.33Mn0.33Co0.33O2) phases as the major components, and spinel (LiNi0.5Mn1.5O4) as a minor one. A high discharge capacity of about 170 mAh g-1 was obtained in the potential range of 2.3-4.9 V vs. Li at low rates (C/10) with excellent capacity retention upon cycling for 100 cycles. On the other hand, LiNi0.33Mn0.33Co0.33O2 cathodes exhibited initially a specific discharge capacity close to 210 mAh g-1, but dropped to a value of 130 mAh g-1 after only 50 cycles. The excellent cyclability of LiNi0.33Mn0.54Co0.13O2 can be ascribed to the suppression of layered-to-spinel phase transformation due to presence of a spinel component LiNi0.5Mn1.5O4 in the pristine active mass. Thus, LiNi0.33Mn0.54Co0.13O2 can be considered as a promising cathode material with enhanced cyclability for Li-ion batteries. The rate capability of this cathode material is good, with a specific capacity > 80 mAh g-1 at 4C rate, with no capacity fading due to the cycling at high rates. Hence, the high Mn concentration in this cathode material (compared to the reference compound) leads to the formation of multi-phase structure in the synthesis and stabilization of the capacity upon cycling in a wide potential range. This Mn rich material exhibit the wellknown and documented gradual decrease of the average voltage upon cycling, due to the well explored layered-to-spinel

transformation (reflected

clearly by Raman spectroscopic

measurements). However, due to the stable specific capacity, the voltage decrease upon cycling can be well predicted, so it may be possible to draw a clear functional relationship between cycle number and average discharge voltage. In such a case, the voltage change can be compensated electronically by the computerized battery management systems (BMS) anyway used in advanced battery systems. Finding cathode compositions that ensure fully stable capacity and

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well predicted average discharge voltage decrease, may promote acceptance of these high capacity/high voltage cathodes for automotive applications.

Experimental Section Analytical grade chemicals: Mn(NO3)2 (Fluka), Ni(NO3)2, Co(NO3)2, LiNO3, sucrose, Poly(vinylidene fluoride) (PVDF), 1-methyl-2-pyrrolidinone (NMP) (Aldrich) were used as received. Doubly-distilled (DD) water was used to dissolve the metal nitrates and sucrose. Synthesis and Structural Characterization The layered-spinel integrated cathode LiNi0.33Mn0.54Co0.13O2 was synthesized by a SCR using the precursors of LiNO3, Ni(NO3)2, Mn(NO3)2 and Co(NO3)2, which act as the oxidants, and sucrose acting as the fuel as reported previously. In a typical synthesis, the precursors were taken in the stoichiometric ratio of Co(NO3)2:Ni(NO3)2:Mn(NO3)2:LiNO3=1:2.538:4.154:7.692. Excess LiNO3 (10 % by wt.) was added in order to compensate for the Li loss during high temperature annealing. The metal nitrates (1.455 g of Co(NO3)2, 3.691 g of Ni(NO3)2, 5.213 g of Mn(NO3)2, 2.652 g of LiNO3) were dissolved in 80 ml DD water. Then sucrose (with metal nitrates to sucrose ratio of 1:2) was added to this solution with continuous stirring for about 6 h. The water was evaporated slowly by heating this mixture to produce a syrupy mass, which on further heating at 300 °C led to the self-ignition of the reactants to give the amorphous compound. The powder material was ground finely and annealed at 450 ºC for 2 h in air. Again the product was ground to form a fine powder that was annealed in two steps, at 700 ºC for 1 h and then at 900 ºC for 15 h in air, resulting in the well-crystallized material. LiNi0.33Mn0.33Co0.33O2 was synthesized by SCR following the same procedure with the precursors in the appropriate stoichiometric ratio as reported previously.[24]

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The elemental analysis of the synthesized materials was carried out using the inductive coupled plasma technique (ICP-AES, spectrometer Ultima-2 from Jobin Yvon Horiba). The Xray diffraction (XRD) studies were performed with a Bruker Inc. (Germany) AXS D8 ADVANCE diffractometer (reflection - geometry, Cu K radiation, receiving slit 0.2 mm, High-Resolution Energy-Dispersive Detector). The data was analysed by the Rietveld structure refinement program, FULLPROF.[53] The structural data for the modelling were taken from previously reported articles for spinel LiMn1.5Ni0.5O4,[54] for monoclinic and rhombohedral layered phases.[46] The Thompson-Cox-Hastings pseudo-Voigt function was used for the peakshape approximation. The background was fitted manually by linear interpolation. The morphology of the product was investigated by a scanning electron microscope Magellan XHR 400L FE-SEM-FEI. The characterization of the cathode materials by HRTEM was carried out with a JEOL-JEM 2100 electron microscope with LaB6 emitter operating at 200 kV. Samples for the TEM studies were prepared by dispersing and sonicating the powdered samples in ethanol and adding a few drops of the resulting suspension to a TEM copper grid. Transmission electron microscopic (TEM) studies were performed in order to provide information about the morphology and structure of the synthesized LiNi0.33Mn0.54Co0.13O2 material. These studies were performed using the bright field (BF)/dark field (DF) imaging technique, conventional selectedarea electron diffraction (SAED) with 300 nm aperture and convergent-beam electron diffraction (CBED) using a 4nm convergent beam. Fast Fourier transform analysis (FFT) was applied for analysis of the high resolution images. The structural assignment of the particles was based on analyzing the SAED, the CBED or the FFT patterns. The information embedded in the reflections in these diffraction patterns (or FFT) in most cases allows the unambiguous determination of the exact structure of the compound. Micro-Raman spectroscopy studies of

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pristine and cycled electrodes were performed using a micro-Raman spectrometer from Renishaw in Via (UK), equipped with a 514 nm laser, a CCD camera, and an optical Leica microscope. A 50x objective lens to focus the incident beam and an 1800 lines/mm grating were used. Electrodes Preparation The electrodes for electrochemical studies were prepared by making a slurry of 80 wt % active material, 10 wt % of conductive super P carbon, and 10 wt % PVDF binder in N-methyl2-pyrrolidinone (NMP) as the solvent. The slurry was coated by using a doctor-blade onto Al foil current collectors, dried at 80 ◦C for 12 h in an oven. The coated Al foils were then pressed uniformly and then cut into circular electrodes of 14 mm diameter. The electrodes were finally dried at 110 ◦C for 12 h under vacuum. Electrochemical Tests The electrochemical performance of these composite cathode materials was tested using coin-type cells 2032 (NRC, Canada) assembled in an argon-filled glove box (made by MBraun). Li metal foils were used as the counter and reference electrodes. Typical loading of the active mass was 4-5 mg/cm2. A commercial battery electrolyte solution LP 30 (Merck) consisting of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) (1:1 w/w) was used. A porous polypropylene based membrane (Celgard) was used as the separator. The cells were stored for 12 h at room temperature to ensure the complete impregnation of the electrodes and the separators with the electrolyte solution. Galvanostatic charge-discharge cycling was carried out in the potential range of 2.3-4.9 V vs. Li/Li+ using a computerized multi-channel battery testing instruments from Arbin Inc. Electrochemical impedance spectra (EIS) were recorded at various 24

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equilibrium potentials during charging using a Solartron model SI 1287 electrochemical interface and a 1255 HF Frequency Response Analyzer, with an amplitude of 5 mV around equilibrium in the frequency range of 100 kHz-0.01 Hz. The electrochemical measurements were performed at 30 ºC in thermostats. Acknowledgements Partial support for this work was obtained by the ISF–Israel Science Foundation, in the framework of the INREP project. References [1] M. S. Whittingham, Chem. Rev. 2004, 104, 4271-4301. [2] M. Armand, J. M. Tarascon, Nature 2008, 451, 652-657. [3] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 2011, 4, 3243-3262. [4] A. Manthiram, J. Phys. Chem. Lett. 2011, 2, 176-184. [5] J. B. Goodenough, K. -S. Park, J. Am. Chem. Soc. 2013, 135, 1167-1176. [6] K. Mizushima, P. C. Jones, P. J. Wiseman, J. B. Goodenough, Mater. Res. Bull. 1980, 15, 783-799. [7] T. Ohzuku, Y. Makimura, Chem. Lett. 2001, 7, 642-643. [8] K. M. Shaju, S. Rao, B. V. R. Chowdari, Electrochim. Acta 2002, 48, 145-151. [9] S. Patoux, M. M. Doeff, Electrochem. Commun. 2004, 6, 767-772. [10] D. C. Li, T. Muta, L. Q. Zhang, M. Yoshi, H. Noguchi, J. Power Sources 2004, 132, 150-155. [11] B. Lin, Z. Wen, J. Han, X. Wu, Solid State Ionics 2008, 179, 1750-1753. 25

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