Effect of Chromium and Niobium Doping on the Morphology and

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The effect of chromium and niobium doping on the morphology and electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4 cathode material Jing Mao, Kehua Dai, Minjie Xuan, Guosheng Shao, Ruimin Qiao, Wanli Yang, Vincent S. Battaglia, and Gao Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00877 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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ACS Applied Materials & Interfaces

The effect of chromium and niobium doping on the morphology and electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4 cathode material Jing Maoa, b, c, Kehua Daid, c, *, Minjie Xuana, b, Guosheng Shaoa, b, Ruimin Qiaoe, Wanli Yange, Vincent S. Battagliac, Gao Liuc,* a

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou

450002, China b

International Joint Research Laboratory for Low-Carbon & Environmental Materials

of Henan Province, Zhengzhou University, Zhengzhou 450002, China c

Energy Storage and Distributed Resource Division, Energy Technologies Area,

Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA d

School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China

e

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720,

USA *

Corresponding Authors. Kehua Dai, Email: [email protected].

Gao Liu, Phone: +1(510) 486-7207, Fax: +1(510) 486-7303, E-mail: [email protected].

1

ACS Paragon Plus Environment


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Abstract Undoped, Cr-doped and Nb-doped LNMO are synthesized via a PVP (polyvinylpyrrolidone)-combustion method by calcinating at 1000 °C for 6h. SEM images show the morphology of LNMO particles is affected by Cr and Nb doping. Crdoping results in sharper edges and corners, smaller particle size while Nb-doping leads to smoother edges and corners, more rounded and larger particles.

The crystal and

electron structure is investigated by XRD and synchrotron-based soft x-ray absorption spectroscopy (sXAS). Cr-doping and light Nb-doping (LiNb0.02Ni0.49Mn1.49O4) improve the rate performance of LNMO. To explore the reason for rate performance improvement, potential intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS) tests are conducted. The Li+ chemical diffusion coefficient at different SOC is calculated and suggests both Cr and light Nb doping fasten Li+ diffusion in LNMO particles. The impedance spectras show both RSEI and Rct are reduced by Cr and light Nb doping. The cycling performance is improved by Cr or Nb doping, and Cr-doping increases both coulombic efficiency and energy efficiency of LNMO at 1C cycling. The LiCr0.1Ni0.45Mn1.45O4 remains 94.1% capacity after 500 cycles at 1C, during the cycling the coulombic efficiency and energy efficiency keeps over 99.7% and 97.5%, respectively.

Keywords: High-voltage spinel; Lithium nickel manganese oxide; Doping; Lithium chemical diffusion coefficient; Cycling performance; Rate performance.

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1. Introduction The LiMn1.5Ni0.5O4 (LNMO) with spinel structure is a promising cathode material candidate for next generation lithium-ion batteries for EV, HEV, energy storage, etc., for its high working voltage of ~4.7 V and capacity of ~130 mAh g−1 1-7. Nevertheless, the commercialization of LNMO material is still limited for the capacity fading due to electrolyte decomposition and concurrent degradative reactions at electrode/electrolyte interfaces

5, 8-10

. One of the most popular way to improve the cycling performance is

the substitution of small amounts of other cations for nickel and/or manganese in the LNMO lattice 11, 12. Besides cycling performance, rate performance of LNMO is also improved by cations doping especially by Cr-doping

13-15

. It is found that Cr-doping increases the

electrical conductivity of LNMO 16. Nb-doping was investigated in LNMO17, LiMn2O4 18

and LiFePO4 19, and also was found to improve the electrical conductivity of LiFePO4

19

. Moreover, rate performance of electrode materials is also dominated by particle

morphology and Li+ diffusion coefficient according to J. Newman’s model 20. However, systematical study of the effect on particle morphology and Li+ diffusion coefficient by Cr or Nb doping is not found in literatures. Although small particles have short Li+ diffusion distance, the high specific surface area is not good for reducing side reactions. Materials composed of single-crystalline big particles mean small specific surface area, low side reactions, low resistance of Li+ through the the solid electrolyte interface and high volume energy density. Their rate performance also can be excellent by increasing Li+ diffusion coefficient through proper synthesis method and doping. 3



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We had synthesized LNMO material with excellent rate and cycling performance via a PVP-combustion method 21, and discussed the complex influence of morphology and particle size 22. It is found that calcination at a high temperature of 1000 °C results in single-crystalline, micro-sized LNMO and the rate performance keeps excellent with the big particles. Though the single-crystalline, micro-sized LNMO already exhibits excellent electrochemical performance, we still want to further improve the performance by combining the PVP-combustion method and doping. We are curious how excellent performance can be achived. Based on the above consideration, in this paper, undoped (LiNi0.5Mn1.5O4), Cr-doped (Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4) and Nb-doped (Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4) LNMO are synthesized via PVP-combustion method and calcined at 1000 °C for 6h. Since the growth rate of particles is fast at 1000 °C, the morphology changes by doping should be more obvious and more easily be studied at such high temperature. There are three doping positions, Ni position (LiMxNi0.5-xMn1.5O4)

16, 23, 24

, Mn position (LiNi0.5MxMn1.5-xO4)

Ni and Mn position (LiMxNi0.5-x/2Mn1.5-x/2O4)

14-16, 28-31

16, 25-27

or

. Among these different

substitution strategies, the LiMxNi0.5-x/2Mn1.5-x/2O4 keeps Ni at +2 valence and Mn at +4 valence, so it is adopted in this paper. The effect of Cr and Nb doping on morphology, structure, electrochemical performance, Li+ chemical diffusion coefficient, coulombic efficiency and energy efficiency are studied.

4


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2. Experimental 2.1. Synthesis procedure The undoped (LiNi0.5Mn1.5O4), Cr-doped (Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4) and Nb-doped (Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4) LNMO were prepared by a polyvinylpyrrolidone (PVP)combustion method. In detail, stoichiometric LiOAc·2H2O, Ni(NO3)2·6H2O, Mn(OAc)2·4H2O, Cr(NO3)3·9H2O or Nb2O5 (nano powder), and PVP (the molar ratio of PVP monomer to total metal ions was 2.0) were dissolved in deionized water and pH = 3 was achieved by adding 1:1 HNO3. The mixture was stirred at 120 °C to obtain dry gel. The dry gel was ignited on a hot plate to induce a combustion process which lasted for several minutes. The resulting precursor was preheated at 450 °C for 3 h and then calcined at 1000 °C for 6 h. The heating rate from 450 °C to 1000 °Cwas 5 °C min−1. After heat treatment, the oven was switched off and the sample was cooled down naturally. 2.2. Morphology and structure characterization The analysis of the phase purity and the structural characterization were made by Xray powder diffraction (XRD) using a Bruker D2 PHASER diffractometer equipped with Cu Kα radiation that was operated over a 2θ range of 10~70ºin a continuous scan mode with a step size of 0.004º. The morphology was examined using a JEOL 7500F scanning electron microscope (SEM). Soft x-ray absorption spectroscopy (sXAS) was performed at Beamline 8.0.1 of the Advanced Light Source (ALS) in LBNL. The undulator and spherical grating monochromator supply a linearly polarized photon beam with resolving power up to 6000. The experimental energy resolution is about 5



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0.15 eV. Experiments were performed at room temperature and with the linear polarization of the incident beam 45ºto sample surfaces. All the spectra have been normalized to the beam flux measured by the upstream gold mesh. 2.3. Electrochemical tests The cathode was prepared by mixing 82 wt.% active material, 10 wt.% acetylene black (AB) and 8 wt.% polyvinylidene fluoride (PVdF) binder in N-methylpyrrolidone (NMP) to form a slurry. The slurry was doctor-bladed onto aluminum foil, dried at 60 °C, and then punched into electrode discs with a diameter of 12.7 mm. The prepared electrodes were dried at 130 °C for 12 h in a vacuum oven and had an active material loading of about 5 mg. The electrochemical cells were fabricated with the LNMO cathode, lithium foil anode, electrolyte (1 mol L−1 LiPF6 in 1:1 EC/DEC), and Celgard 2400 separator in an argon-filled glove box. Electrochemical performances were evaluated using CR2325 coin cells. Galvanostatic charge-discharge tests were performed using Maccor 4000. The potential intermittent titration technique (PITT) tests were conducted using Bio-Logic VMP-3 multichannel electrochemical analyzer. The EIS tests were carried out in two-electrode coin cells using PARSTAT 4000 electrochemical workstation with ±5 mV AC signal and a frequency range from 105 to 0.01 Hz.

3. Results and discussion Fig. 1 exhibits the effect of Cr and Nb doping on the morphology of LNMO. All the samples are composed of micro-sized single-crystalline particles. The undoped LNMO shows mainly truncated octahedra, but its particles size is not uniform enough. The 6


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particles of Nb-0.02 and Nb-0.04 look more rounded and the edges and corners become smoother. Furthermore, the particles grow bigger with Nb content increasing. The morphology change and particle growth were not be discussed in previous reports because these influences of Nb-doping is insignificant at relative low temperature

32

.

Crystal growth becomes fast in the high calcination temperature of 1000 °C, so the impact of doping on morphology in this paper is able to observe. The effect of Crdoping on morphology is contrary to that of Nb-doping. The edges and corners of Cr0.1 and Cr-0.2 particles are sharper than undoped LNMO might due to the strong octahedral field of Cr3+. It also can be seen that the particle size of Cr-doped LNMO becomes smaller and more homogeneous. The particle size distributions are counted from 100 to 200 particles in lower magnification SEM images and shown in Fig. 2 and Table 1. Cr-doping reduces both small and big particles, then decreases the mean particle size. Nb-doping also reduces small particles, but increase big particles so it makes the mean particle size higher than undoped LNMO. Standard deviation indicates uniformity of particle sizes. The lower standard deviation, the more uniform particle sizes. Cr and Nb doping both lessen the standard deviation but the effect of Cr-doping is greater. In summary, at the rather high calcination temperature of 1000 °C, the morphology of LNMO is affected by Cr and Nb doping. Cr-doping results in sharper edges and corners, smaller particle size while Nb-doping leads to smoother edges and corners, more rounded and larger particles. Both Cr and Nb doping narrow down the particle size distribution of LNMO.

7



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Fig. 1. SEM images of the LNMO samples. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4. No doping: LiNi0.5Mn1.5O4.

8


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0.25

0.25

Cr-0.1

Cr-0.2 0.20 Relative Frequency

Relative Frequency

0.20 0.15 0.10 0.05 0.00

1

2

3

4

5

6

0.15 0.10 0.05 0.00

7

1

2

Particle size (m)

3

4

5

0.25

Nb-0.02 0.20 Relative Frequency

Relative Frequency

0.20 0.15 0.10 0.05

1

2

3

4

5

6

0.15 0.10 0.05 0.00

7

1

2

0.25

4

5

6

7

10

Nb-0.04

9

0.20

8 Particle size (m)

Relative Frequency

3



0.15 0.10 0.05 0.00

7

0.25

No doping

0.00

6


7

Standard Deviation Max. value Mean value Min. value

1.0 0.8

6

0.6

5 4

0.4

3 2

Standard Deviation

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


0.2

1

1

2

3

4

5

6

7

0


0.0 Cr-0.2

Cr-0.1

No doping Nb-0.02

Nb-0.04

Fig. 2. Particle size distributions of the LNMO samples and summary of statistics for all the samples (bottom right corner). Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4. Table 1. Statistics for the LNMO samples. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4. Particle size (μm) Standard Sample name deviation Min. value Mean value Max. value Cr-0.2 1.09 2.36 4.05 0.59 Cr-0.1 1.29 2.70 4.93 0.69 No doping 1.00 2.97 5.55 0.97 Nb-0.02 1.64 3.71 5.96 0.90 Nb.0.4 2.08 3.94 6.41 0.81 Fig. 3 shows the XRD patterns of undoped and doped LNMO. All diffraction peaks 9



can be indexed as a cubic spinel structure and 𝐹𝑑3̅𝑚 space group, and all of the peaks are narrow and sharp, indicating good crystallinity. The patterns of Cr-0.1, Cr-0.2 and Nb-0.02 show no any impurity composed of doped element but the sample Nb-0.04 shows a slight LiNbO3 peak at 2θ ≈ 24°

18

. The pattern of undoped LNMO shows a

weak peak related to LizNi1-zO2 at 2θ ≈ 43.7° 16. Nb-doping weakens this peak while the Cr-doped LNMO shows no LizNi1-zO2 impurity for the strong bonding strength of chromium with oxygen is able to suppress Mn3+ content in the LNMO 11. LiNbO3 20

22

24

26

28

Nb-0.04

30

Intensity (arb. unit)

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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Nb-0.02

No doping Cr-0.1

Cr-0.2 20

30

40

50

60

2 (°)

70 43.0 43.5 44.0

2 (°)

Fig. 3. XRD patterns of the LNMO samples. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4.

For studying of the Mn and Ni oxidation states, synchrotron-based soft x-ray sXAS is performed at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. Fig. 4 shows the Mn and Ni L-edge sXAS spectra of the LNMO samples. The spectra consist of well-separated absorption features in two regions, L3 (for Mn 638-646 eV, for Ni 849-856 eV) and L2 (for Mn 649-656 eV, for Ni 867-871 eV), 10


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resulting from the 2p core-hole spin-orbital splitting. The Mn and Ni L3-edge sXAS spectra of all the LNMO samples almost coincide with each other, respectively. The valence of Mn and Ni are mainly +4 and +2 while the Mn3+ peaks are very weak at 642 eV 33. The 642 eV peak for Cr-0.1 seems lower than others and suggesting lower Mn3+ content.

Fig. 4. Mn and Ni L-edge soft X-ray absorption spectra of the LNMO samples. Nb0.02:

LiNb0.02Ni0.49Mn1.49O4.

Nb-0.04:

LiNb0.04Ni0.48Mn1.48O4.

Cr-0.1:

LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4.

Fig. 5a displays the charge and discharge profiles of undoped and doped LNMO at C/5 rate in the 5th cycle. The reason for choosing the 5th cycle to compare is that the coulombic efficiency in the first few cycles was usually low 34-36 and the capacities are unstable due to the severe side reaction at the high potential. The charge and discharge 11



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capacities of undoped LNMO are 133.7 and 127.7 mAh g−1, respectively, and the coulombic efficiency is 95.5%. The Cr-0.1 and Cr-0.2 deliver very close charge capacities to undoped LNMO and a little higher discharge capacities of 129.2 and 130.1 mAh g−1. The coulombic efficiencies of Cr-0.1 and Cr-0.2 are 97.4% and 97.2%, respectively. The higher coulombic efficiencies of Cr-0.1 and Cr-0.2 may be benefited by the strong bonding strength of chromium with oxygen and lower Mn3+ content. The Nb-0.02 delivers a little higher charge and discharge capacities than that of undoped LNMO, and the coulombic efficiency is 95.2% which is closed to that of undoped LNMO. However, although the charge capacity of Nb-0.04 is almost same to that of Nb-0.02, the discharge capacity is as low as 120.4 mAh g−1, and the coulombic efficiency is only 88.9%. The impurity in Nb-0.04 may be the reason of low discharge capacity. Table 2. Electrochemistry performance of the LNMO samples. No doping: LiNi0.5Mn1.5O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Capacity ratio related to

Capacity

Discharge

CE* at

Energy

capacity at C/5

C/5

density at C/5

(mAh g−1)

(%)

(Wh/kg)

1C

5C

10C

15C

No doping

127.7

95.5

586.4

95.8

87.0

81.8

77.0

82.3

Cr-0.1

129.2

97.4

594.6

98.1

94.9

92.6

90.2

94.1

Cr-0.2

130.1

97.2

602.3

98.1

95.0

92.5

90.7

91.9

Nb-0.02

128.8

95.2

589.1

97.6

93.6

89.5

86.4

89.5

Nb-0.04

120.4

88.9

550.7

96.1

91.6

77.8

69.9

92.0

Sample’s name

capacity at C/5 (%)

*

CE: coulombic efficiency

12


retention after 500 cycles at 1C

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5.0 4.8

Voltage (V)

4.6 4.4 4.2 4.0

No doping Nb-0.02 Nb-0.04 Cr-0.1 Cr-0.2

3.8 3.6 3.4

(a) 0

20

40

60

80

100

120

140

-1

Specific capacity (mAh g ) (b)

3+

Cr /Cr 3.6

3.8

4.0

4.2

4+

4.4

4.90V No doping Cr-0.1 Cr-0.2

(c)

No doping Nb-0.02 Nb-0.04

dQ/dV

dQ/dV

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


4.60

4.75 V 4.74 V 4.72 V

4.65

3.6

3.8

4.70

4.0

4.75

4.2

4.80

4.4

3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0

3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0

Voltage (V)

Voltage (V)

Fig. 5. (a) Charge and discharge profiles, (b, c) dQ/dV profiles of the LNMO samples at C/5 rate. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4.

The shape of charge and discharge curves for LNMO is an important index of its electrochemical performance. As shown in Fig. 5a, all the samples show at least three plateaus which suggest 𝐹𝑑3̅𝑚 space group

37-40

. It is well known that the 4.6 V and

4.7 V plateaus are related to Ni2+/Ni3+ and Ni3+/Ni4+ redox, and the 4.0 V plateau is caused by Mn3+/Mn4+ redox 41. Moreover, the Cr-doped LNMO shows a 4.9 V plateau related to Cr3+/Cr4+ redox 23 and the 4.7 V plateau is higher than undoped LNMO. The plateau changes can be clearly recognized in Fig. 5b, the dQ/dV profiles. There are 13



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obvious 4.9 V peaks beside the 4.7 V peaks for Cr-doped LNMO. Compared with undoped LNMO, the ~4.7 V peak of Cr-0.1 in discharge side increases from 4.72 V to 4.74 V, then it shifts to 4.75 V for Cr-0.2, so the energy density is improved along with Cr content increasing (Table 2). The intensity of 4.6 V and 4.7 V peaks decreases with higher Cr content, which originate from more slopping plateau. The 4.0 V peaks of undoped and Cr-doped LNMO do not show too much differences except the intensity of this peak for Cr-0.2 is a little lower. Fig. 5c compares the dQ/dV profiles of undoped and Nb-doped LNMO. The 4.6 V peaks of Nb-doped LNMO are weaker than undoped LNMO but the 4.0 V peaks in discharge side are a little stronger.

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140

1E-9 No doping Nb-0.02 Cr-0.1 Cr-0.2

C/5

1C

1C

110

5C

100

10C

-1

120

DLi+ (cm s )

15C

90 80 No doping Cr-0.1 Cr-0.2 Nb-0.02 Nb-0.04

70 60 50 40

0

5

1E-10

2

Specific capacity (mAh/g)

130

1E-11

(a) 10

15

20

25

(b) 0

30

20

40

5.2

4.2

1E-11

3.6

(c) No doping 20

40

4.2 4.0 3.8

80

100

1E-11

3.6 3.4

60

-1

3.8

1E-10

4.4

2

-1

4.0

+ Potential vs. Li/Li (V)

4.4

4.6

(d) Nb-0.02 0

120

20

-1

1E-9

3.6



1E-11

(e) Cr-0.1

4.2 4.0 3.8

80

100

120

1E-11

3.6 3.4

60

-1

3.8

1E-10

4.4

2

-1

4.0

4.6

(f) Cr-0.2 0

-1

DLi+ (cm s )

2

DLi+ (cm s )

4.2

40

120 1E-9

4.8 1E-10

20

100

Potential DLi+

5.0

4.4

0

80

5.2

Potential DLi+

4.6

3.4

60

Specific capacity (mAh g )

5.2

4.8

40

-1


5.0

DLi+ (cm s )

1E-10

0

1E-9

4.8

4.6

3.4

100

Potential DLi+

5.0

2


4.8

80

5.2

1E-9 Potential DLi+

5.0

60

% SOC

Cycle number

DLi+ (cm s )

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


20

40

60

80

100

120

-1



Fig. 6. (a) Specific capacity of the LNMO samples at different C-rates. (b) Chemical diffusion coefficient of the LNMO samples at different SOC. (c-f) Voltage curves during PITT tests and the calculated diffusion coefficient of the LNMO samples. Nb0.02: LiNb0.02Ni0.49Mn1.49O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4.

The rate performance is studied for it is well known to be very important for many applications. All the cells are charged and discharged at C/5 for 5 cycles, then they are charged at 1 C and discharged at 1 C, 5 C,10 C, and 15 C for 5 cycles each, followed 15



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 16 of 29

by 1C long-term cycling. It can be clearly seen that the Cr-doped LNMO show the best rate performance from Fig. 6a and Table 2. Even at 15C rate, the Cr-doped LNMO still deliver over 90% capacity compared to that of C/5. The rate capability of Nb-0.02 is better than the undoped LNMO but worse than the Cr-doped LNMO. The Nb-0.04 shows the worst rate performance might due to the impurity. In above discussion the Cr and Nb doping affects morphology besides structure, so it needs to be determined that what factors results in the better rate performance. For this question, the lithium chemical diffusion coefficient is determined by potential intermittent titration technique (PITT). In the PITT experiments, a very small potential step size (10 mV) and a low enough cutoff current (C/50) were adopted to ensure the equilibrium states were achieved at every potential step. Then the lithium chemical diffusion coefficient, DLi+, can be calculated from the slope of the linear region in the ln I(t) vs. t plot, as defined in Equation (1) 42, 43

(1) where I is the current in the potential step and L is the diameter of a spherical particle. In this application, the quadratic mean of particle size is used as the L value. The values of DLi+ measured from PITT and the potential plotted with specific capacity are shown in Fig. 6 c-f, and they are compared in Fig. 6b. The lithium diffusion coefficient values vary between 5×10-12 and 9×10-10 cm2 s-1 considerably with Li concentration and three diffusion minima are observed in Fig. 6c and 6d which are corresponding to the three plateau for undoped and Nb-doped LNMO. There are one more minima in the start of discharge in Fig. 6e and Fig. 6f corresponding to Cr3+/Cr4+ redox. Although the DLi+ 16


Page 17 of 29

values within 5% SOC for Cr-0.1 are lower than that for undoped LNMO due to the extra Cr3+/Cr4+ two-phase reaction, the DLi+ values for Cr-0.1 at most SOC are around one time higher than that for undoped LNMO. The DLi+ values for Cr-0.2 at most SOC are lower than that for Cr-0.1. These results suggest that the better rate performance of Cr-0.1 is benefited by both small particle size and higher DLi+ values. The most of DLi+ values for Cr-0.2 are btween those for Cr-0.1 and undoped LNMO, but Cr-0.2 has smaller particle size, so its rate performance are similar with that of Cr-0.1. The DLi+ values for Nb-0.02 at most SOC over 20% are even a little higher than those of Cr-0.1, but the particle size of Nb-0.02 are higher, so its rate performance are higher than that of Cr-0.1 and Cr-0.2 butlower than that of undoped LNMO. Generally speaking, Cr and light Nb doping fasten Li+ diffusion in LNMO particles and benefit rate performance of LNMO. 15

No doping Cr-0.1 Cr-0.2 Nb-0.02

10

-ZIm ()

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


5

0

0

5

10

15

20

25

30

35

40

ZRe ()

Fig. 7. The Nyquist plots of impedance spectra for different LNMO/Li coin cells at 50% SOC after 5 galvanostatic cycles at 0.2 C rate. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4. 17



Fig. 7 shows the Nyqusit plots of impedance spectra for different LNMO/Li coin cells at 50% SOC at the 5th galvanostatic cycle at 0.2 C rate. At first, we measure the Nyqusit plots for a LNMO/Li coin cell from 0% to 100% SOC and found that the highrequency semicircle varies less while the media-frequency semicircle varies largely (The data is not shown here). Thus, in this work, the high-frequency semicircle is ascribed to the resistance of Li+ through the the solid electrolyte interface (RSEI) and the media-frequency semicircle is ascribed to the charge-transfer resistance of Li+ and electrons (Rct). For undoped LNMO, Nb-0.02, Cr-0.1, and Cr-0.2 samples, the RSEI is 20, 18, 15, 15 Ω and the Rct is 12, 6, 5, 4 Ω, respectively. The RSEI decreased slightly with doping. It is clear that the Rct decreased greatly, which can be explained by the improved elctronic conductivity, solid diffusivity of Li+, and more regular morphology as compared to the undoped LNMO particles. The EIS results is consistent well with the rate capability tests, during which Cr-0.2 shows the best rate performance. 140 120

-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

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100 80 60 No doping Cr-0.1 Cr-0.2 Nb-0.02 Nb-0.04

40 20 0

0

100

200

300

400

500

Cycle numbers

Fig. 8. Cycling performance of the LNMO samples at 1C. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4. 18


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The cycling performance of all the samples is displayed in Fig. 8. All the LNMO samples are cycled for 550 times at 1C after rate tests. The capacity retention after 500 cycles is summarized in Table 2. Although undoped LNMO shows good cycling performance before 430 cycles, the Cr-doped LNMO still improves the cycling stability with capacity retention over 90% after 500 cycles. The improvement can be ascribed to no LizNi1-zO2 impurity, the strong bonding strength of chromium with oxygen and higher DLi+44,

45

. The uniformity of particle sizes for Cr-doped LNMO may also

contribute the cycling performance. The cycling performance of Nb-doped LNMO is also better than that of undoped LNMO (Fig. 8, Table 2) although the specific capacity of Nb-doped LNMO is lower. The big particle and smooth surface (mean lower specific surface area) may be the reason of more stable cycling of Nb-doped LNMO than that of undoped LNMO. The undoped LNMO cycles stably with slow capacity fading before 430 cycles, then its capacity decays quickly. To exclude occasionality, the undoped LNMO is tested several times and the results are repeatable. The fast fading after stable cycling may be caused by the continuous side reactions between LNMO and electrolyte. We suppose that the products of side reactions accumulate to a certain degree, the reactions may accelerate and damage electrolyte and anode, then will cause avalanche capacity fading. Coulombic efficiency (CE) of electrode materials is important for good cycling performance of full cells5. Fig. 9a exhibits the CE of all the samples during the 1C rate cycling tests. The CE of all the samples increase from a little low values (the cycling tests are started after rate tests so the “started” CE is not too low) then become stable 19



after around 100 cycles. The undoped LNMO shows stable CE of 99.7% only after around 50 cycles but decreases after about 200 cycles and becomes very noisy after 400 cycles due to the constant side reactions of LNMO and electrolyte. The noisy CE after 400 cycles also explains the quick capacity fading of undoped LNMO. The Cr-0.2 shows the highest CE with stable value around 99.75% and the Cr-0.1 shows stable value around 99.72%. The higher CE suggests Cr-doping reduces the side reaction such as oxidative decomposition of electrolyte and lithium consuming. The Nb-doped LNMO shows lower CE of around 99.55% and the CE of Nb-0.04 are unstable. The reason of the CE decrease for Nb-doping is unclear, may relate to the effect on LNMO structure of Nb-doping. 1.000

0.99

0.998

0.98

0.996

0.97

0.994

0.96

Energy efficiency

Coulombic efficiency

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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0.992 No doping Cr-0.1 Cr-0.2 Nb-0.02 Nb-0.04

0.990 0.988 0.986 0

100

200

300

0.95 0.94 No doping Cr-0.1 Cr-0.2 Nb-0.02 Nb-0.04

0.93 0.92 0.91

(a)

0.90

400

0

100

Cycle numbers

200

300

(b) 400

Cycle numbers

Fig. 9. (a) Coulombic efficiency and (b) energy efficiency of the LNMO samples at different cycles. The rate is 1C. Nb-0.02: LiNb0.02Ni0.49Mn1.49O4. Nb-0.04: LiNb0.04Ni0.48Mn1.48O4. Cr-0.1: LiCr0.1Ni0.45Mn1.45O4. Cr-0.2: LiCr0.2Ni0.4Mn1.4O4.

Energy efficiency (EE) of electrode materials was discussed not much in literatures. Due to the internal resistance, cells’ EE is usually lower than CE. The EE during 1C cycling of all the sample displays in Fig. 9b. The EE of undoped LNMO drops from 97.3% to 95.7%. Before 40 cycles and after 420 cycles, the EE of undoped LNMO is noisy. The decrease of EE suggests the increase of internal resistance. The cells with 20


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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


Cr-doped LNMO show higher EE of about 98% and they drop slower than that of undoped LNMO. The cell with Cr-0.1 has the highest EE of 97.5% at the 450 cycle which indicate slower side reactions. The Nb-doped LNMO shows lower EE. The EE of Nb-0.04 is even below 95%. This suggests higher internal resistance with Nb-doping. Table 3. Comparation of the electrochemical performance for the Cr-doped LNMO in this paper and in literatures. Rate performance Cycling performance Capacity ratio % capacity Coulombic Refs C-rate related to C-rate degradation efficiency capacity at C/5 per cycle This Li Cr0.10Ni0.45Mn1.45O4 15C 90.2% 1C 0.012% 99.7%@1C work LiCr0.2Ni0.4Mn1.4O4 15C 90.7% 1C 0.016% 99.7%@1C 13 LiCr0.2Ni0.4Mn1.4O4 1C 0.2% 14 Li Cr0.10Ni0.45Mn1.45O4 10C 92% 1C 0.037% 15 Li Cr0.10Ni0.45Mn1.45O4 5C 34.8% 1C 0.076% 30 Li Cr0.10Ni0.45Mn1.45O4 10C 72.7% of 1C 1C 0.02% 98.9%@1C −2 0.2mA cm 28 Li Cr0.10Ni0.45Mn1.45O4 0.044% 4. Conclusions The undoped Cr-doped and Nb-doped LNMO are synthesized via a PVP-combustion method and calcined at 1000 °C for 6h. At this rather high calcination temperature, the morphology and structure of LNMO is affected by Cr and Nb doping. The Cr-doping slightly improves the discharge capacity and evidently increases the coulombic efficiency. The Nb-0.02 has a little higher capacity than that of undoped LNMO, and close coulombic efficiency. Cr-doping significantly improves the rate performance of LNMO and light Nb-doping (Nb-0.02) modestly increase the rate performance. Both Cr and light Nb doping fasten Li+ diffusion in LNMO particles. Both Cr and Nb doping improve cycling performance of LNMO. The Cr-0.1 shows the best cycling performance. Cr-doping also increases both coulombic efficiency and energy efficiency 21



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

of LNMO at 1C cycling. In summary, the LiCr0.1Ni0.45Mn1.45O4 shows the best electrochemical performance in this paper. It can be seen from Table 3 that the Cr-doped LNMO also shows the best electrochemical performance compared with previous reports. Acknowledgements This work was supported by the National Natural Science Foundation of China (51204038, U1504521) and the Fundamental Research Funds for the Central Universities of China (N110802002, L1502004). This work was also supported by the Assistant Secretary for Energy Efficiency, Vehicle Technologies Office of the U.S. Department of Energy, under the Advanced Battery Materials Research (BMR) Program and Applied Battery Research (ABR) Program under contract No. DE-AC0205CH11231.

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Table of Contents Graphic

Specific capacity (mAh/g)

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


150

140 120 100 C/5 1C 5C 10C 80

0

5

100

15C

No doping Cr-0.1 Nb-0.02 Cr-0.2 Nb-0.04

60 40

1C

10 15 20 25 30

Cr0.1 Cr0.2 No doping

50 0

0

Nb0.02 Nb0.04

100 200 300 400 500 Cycle numbers

Cycle number

29


Effect of Chromium and Niobium Doping on the Morphology and

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