Relationships between Structure, Composition, and Electrochemical

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C: Energy Conversion and Storage; Energy and Charge Transport

Relationships between Structure, Composition, and Electrochemical Properties in LiNixMn2-xO4 ( = 4.43, 3.61, 3.06, 2.86 and 2.60) Spinel Cathodes for Lithium Ion Batteries Dongsheng Lu, Jianglong Li, Jia He, Ruirui Zhao, and Yue-Peng Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11085 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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The Journal of Physical Chemistry

Relationships between Structure, Composition, and Electrochemical Properties in LiNixMn2-xO4 ( 2 x x = 4.43, 3.61, 3.06, 2.86 and 2.60) Spinel Cathodes for Lithium Ion Batteries Dongsheng Lu,*,† Jianglong Li,† Jia He,† Ruirui Zhao,† and Yuepeng Cai†,‡ † Institute

of chemistry and environment, South China Normal University, Guangzhou,

510006, P.R. China; ‡ Guangzhou

Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou,

510006, P.R. China

1

ACS Paragon Plus Environment

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ABSTRACT: A series of off-stoichiometric LiNixMn2-xO4 (x = 0.37, 0.43, 0.49, 0.52 and 0.56) spinels are prepared by adjusting Mn/Ni molar ratio, and are used to investigate the correlations between Mn3+ content, structural ordering degree, oxygen vacancies, impurities and electrochemical properties in these spinels through inductive coupled plasma atomic emission

spectroscopy,

scanning

electron

microscopy,

Fourier

transform

infrared

spectroscopy, X-ray photoelectron spectroscopy, Rietveld refinement of the X-ray diffraction data, galvanostatic charge/discharge test and first principles computation. Results show that the relationships between these factors in the off-stoichiometric LiNixMn2-xO4 spinels are obviously different from that in common oxygen-deficient LiNi0.5Mn1.5O4-ơ spinel due to their different Mn3+ formation mechanisms. Specifically, structural ordering degree and oxygen vacancy concentration almost remain constant when Mn3+ content varies obviously, which is attributed to that prolonged annealing (600℃, 12 h) combined with slow cooling (1℃/min ) steps during LiNixMn2-xO4 preparation can order the distribution of Ni2+ and Mn4+ ions in spinel structure, and compensate the oxygen loss due to calcining at 800 ℃. Electrochemical properties (capacity, first Coulombic efficiency and rate capability) are significantly improved with an increase in Mn3+ content because the increase of Mn3+ can reduce rock-salt impurity, improve electronic conductivity, and Li+ diffusion in LiNixMn2-xO4 structure.

2


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■ INTRODUCTION As a promising cathode material for Li-ion batteries, spinel LiNi0.5Mn1.5O4 (LNMO) has attracted much attention in the past decade. 1-7 Due to the redox reactions of Ni2+/Ni4+, this compound inserts Li+ reversibly at a high potential of ~ 4.7 V (vs. Li/Li+), making its energy density (650 Wh/Kg) 20% and 30% higher than that of conventional LiCoO2 and LiFePO4 materials, respectively.1 LNMO is generally considered to have ordered and disordered cubic crystal structures. In the ordered structure (space group P4332), Ni and Mn atoms occupy the 4a and 12d Wyckoff positions, respectively, while in the disordered structure (space group Fd3m), both types of the atoms are randomly distributed in the octahedral 16d sites. 8 Almost all the related studies have shown that the disordered LNMO delivers better rate capability and cyclic stability than the ordered LNMO.8-11 One proposed reason for this result is the higher electronic conductivity of the disordered LNMO, which is based on an early work by Amatucci et al. 12 In that work, the electronic conductivities of a series of LNMO powders with different degrees of structural ordering were measured by an AC impedance method. The results show that the highest conductivity of the disordered LNMO is 2.5 orders of magnitude higher than that of the ordered LNMO, which is ascribed to the presence of a little Mn3+ in the disordered LNMO. So far, however, few studies have been related to the correlation between the degree of Ni/Mn ordering and the electronic conductivity of LNMO, the reason why the disordered LNMO has higher electronic conductivity still remains in question. Another reason is thought to be the faster Li+ diffusion in the disordered LNMO,

13

while the theoretical

calculation indicates that the barriers for Li+ motion are similar in both types of structure. 14 The other reason is that compared to the order LNMO, which is has a two-step phase 3



transitions between three cubic phases, the disordered LNMO undergoes a much smaller strain during charging and discharging, originating from the one-step phase transition between two cubic phases. 15 Further studied suggest that the better electrochemical properties of the disordered LNMO are closely related to the oxygen vacancies in the structure.16-18 In spite of several methods for preparing LNMO, all methods require a calcination process at high temperatures ranging from 700-1000℃, which inevitably causes varying degrees of oxygen loss (oxygen vacancies) and produces oxygen oxygen-deficient LNMO (LiNi0.5Mn1.5O4-δ). In addition, accompanied by the oxygen loss, part of Mn4+ ions are reduced to Mn3+ due to the charge neutrality. Several theoretical calculations show that oxygen vacancies induce disorderly distribution of Ni and Mn atoms in the LNMO.17-18 However, direct evidence of oxygen vacancies is difficult to obtain because of the limited tool available to measure with accuracy the content O2- in a solid. So the content of Mn3+ is often used to indirectly characterize the concentration of the oxygen vacancies in the disordered LNMO. For the disorder LNMO, Mn3+ is essential, and its electrochemical properties are very sensitive to the amount of Mn3+.10,15,19 Nevertheless, another suggestion is that the oxygen loss and the generation of Mn3+ are not correlated. Under high-temperature calcination condition, the obtained LNMO is found to deviate from the theoretical stoichiometry (Li: Ni: Mn = 1:0.5:1.5, molar ratio), which contains an excess of Mn. The Mn excess leads to the formation of some Mn3+, and is compensated by the formation of a Ni-rich secondary phase with a rock-salt structure, not by the creation of oxygen vacancies.11 Thus, the mechanism by which the disordered LNMO has better electrochemical properties needs to be further clarified. 4


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In this work, a series of Ni-rich and Mn-rich LNMO samples were prepared for the first time by changing the Mn/Ni molar ratio from 4.43 to 2.60. In order to eliminate the influence of oxygen vacancies in the structure, all samples were cooled to 600℃at a very low rate (1℃/min) and then annealed for 12h after 15 hours of calcination at 800 ℃. We systematically studied the relationships between Mn3+ content, structural ordering degree, impurity and electrochemical properties in the LNMO materials, and obtained some new results.

■ EXPERIMNTAL AND COMPUTATIONAL METHODS Materials Preparation. Spherical precursor NixMn2-x(CO3)2 (x = 0.368, 0.434, 0.492, 0.518, 0.556) compounds were prepared by a co-precipitation method. An aqueous solution of NiSO4·6H2O and MnSO4·H2O with 2 mol/L total concentration of Ni2+ and Mn2+ was pumped into a continuously stirred flask. The molar ratio of Mn2+ to Ni2+ in the mixed solution was 3.15, 2.68, 2.40, 2.10 and 1.80 for the five precursors, respectively. At the same time, Na2CO3 solution (aq.) of 1 mol/L and desired amount of NH3·H2O solution (aq.) were also separately pumped into the flask. The molar ratio of Na2CO3 to Ni2+ and Mn2+ ions was fixed at 1. The PH value of the reaction mixture was controlled at 7.8, and the reaction temperature was 25 ℃. The precipitates were filtered and washed using distillated water and then dried at 50 °C in a vacuum oven for 8 h. The obtained precursors were heated at 600 °C for 8 h, and the resulting powders were thoroughly mixed with appropriate amount of LiOH·H2O (molar ratio of Li / (Ni + Mn) = 1/2). The mixtures were calcined at 800 ℃ for 15 h in air, cooled to 600℃at 1℃/min, and then annealed at this temperature for 12 h, followed 5



by cooling to 400℃at 1℃/min and eventually cooled naturally to room temperature. The obtained LNMO samples were correspondingly labeled as MN4.43, MN3.61, MN3.06, MN2.86 and MN2.60. Material Characterization. The chemical compositions of the samples were analyzed by inductive coupled plasma atomic emission spectroscopy (ICP-AES Perkin-Elmer Plasma 3200). The morphologies of the obtained LNMO powders were observed using scanning electron microscopy (SEM, Ultra 55, Carl Zeiss). Fourier transform infrared spectroscopy (FT-IR) spectra were acquired with a Perkin-Elmer Spectrum Two instrument with 2 cm-1 resolution using KBr pellets containing 0.5 wt % LNMO. X-ray photoelectron spectroscopy (XPS) was performed by an Axis Ultra DLD (Kratos, U.K.) instrument, and the spectrum was collected with a monochromatic Al Kα source (1486.7 eV) under ultrahigh vacuum condition. Powder X-ray diffraction (XRD, D8 advance, Bruker) measurements using Cu Kα radiation were employed to identify the crystalline phase of the LNMO samples with a step size of 0.03° from 10° to 80°. The diffraction patterns were analyzed by the Rietveld refinement program GASA/EXPGUI. Electrochemical Analysis. The LNMO electrode consisted of 80: 12: 8 wt% LNMO, super P carbon black, and polyvinylidene fluoride (PVDF), which were mixed with N-methyl-2pyrrolidone (NMP) and coated onto an Al foil via the doctor-blade method. The coin cell was comprised of an LNMO cathode and a lithium metal anode separated by a porous polypropylene film (Celgard 2400). The mass loading of LNMO in the cathode is about 5mg. The electrolyte used in the test cell was composed of 1M LiPF6 salt in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) solution (3:7, by volume). Galvanostatic 6


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charge/discharge tests were carried out in the voltage range of 3.5-4.9 V with a Land-CT2001A battery test system at room temperature. The LNMO electrodes were extracted from the long-term cycled cells and rinsed with anhydrous dimethyl carbonate (DMC) three times in an argon-filled glovebox, followed by vacuum drying overnight at room temperature. XRD and SEM were used respectively to characterize the structure and morphology of the mixed powders scraped from the Al foil of the dry LNMO electrodes. Computational Methods. Spin-polarized calculations were performed using the software CASTEP based the density functional theory (DFT). Generalized gradient approximation (GGA) in the parameterization of Perdew, Burke, and Ernzerhof (PBE) pseudopotential was used to describe the exchange-correlation potential. The Hubbard-type U correction was introduced to describe the effect of localized d electrons of transition metal ions, and the effective U value given to Mn ions is 5 eV, to Ni ions is 6 eV. The primitive cell consisting of 36 atoms was used through all calculations. The plane-wave cutoff was set to 300 eV, and the reciprocal space k-point mesh was 4×4×4. Geometry optimizations were performed by using a conjugate gradient minimization until all the forces acting on ions were less than 10-6 eV per atom.

■ RESULTS AND DISCUSSION Surface Morphology. The morphologies of the five LNMO samples characterized by SEM are shown in Figure 1. As can be seen, all the samples show almost identical spherical morphologies, indicating that the variation in Mn/Ni molar ratio from 4.43 to 2.60 has little effect on the morphology of the obtained LNMO. These secondary spherical particles are about 2 μm in diameter and are composed of smaller submicron primary particles with 7



octahedral shape. In addition to the spherical particles, a small amount of micro-size (about 1 μm) octahedral particles can also be seen in all the samples.

Figure. 1. SEM images of the obtained five LNMO samples: (A) MN 4.43, (B) MN 3.61, (C) MN 3.06, (D) MN 2.86, (E)MN 2.60

Structural Analysis. Figure.2a-f shows XRD patterns of the five LNMO samples and Rietveld refinement results of these XRD data. Obviously, all samples have the typical profiles of the spinel phase (JCPDS No. 80-2162). With the increase of Mn/Ni molar ratio, the lattice parameters show an increasing trend from 8.16224 to 8.16545 Å (Table 1), which may 8


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be related to an increase of Mn3+amount due to the charge neutrality. It is believed that the replacement of Mn4+ (ionic radius 0.053 nm) by the larger Mn3+ (0.065 nm) leads to the expansion of the unit cell and a larger lattice parameter. 11,19

50

60

65

70

75

80

20 25 30 35

40 45 50

25

30

35

40

50

55

60

65

70

75

80

(311) (222)

10

15

20

25

30

35

2(degree)

50

55

60

(533) (622)

(440) (531) 65

70

75

80

(400)



(222) Intensity (A.U.)

(533) (622)

(440) (531)

(333)

(400) (531)

(311)

45

(311)

b (f)

Observed Calculated difference Std.phase

(222)

40

2(degree)

(e) MN2.60

(111)

75 80

Observed Calculated Difference Std.phase

(111)

Intensity(arb.units)

(533) (622)

(440) (531)

(531) 45

(333)

(400)

(311) (222)

20

55 60 65 70

(d) MN2.86

Observed Calculated difference std.phase

(111)


15

(333)

(311) (222) 10 15

2(degree)

(c) MN3.06

10

(400)

(111) Intensity(arb.units)

(533) (622)

(333) 55

(533) (622)

45

2degree

(440)

40

(440) (531)

35

(333)

30

(400)

25

Observed Calculated Difference Std.phase

(531)

20

(531)

(222) 15

(440) (531)

(400)

(311)

(111) Intensity(arb.units) 10

(b) MN3.61

Observed Calculated difference Std.phase

(531)

(a) MN4.43


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




2.60



2.86



3.06

   

3.61 4.43

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

36

2(degree)

38

40

42

2 degree

44

62

64 66

2 degree

Figure 2. (a-e) Rietveld refinement profiles of XRD data for the five LNMO samples. (f) Enlarged region to show the LixNi1-xO impurity phases marked by asterisks. 9


The Journal of Physical Chemistry 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

Page 10 of 27

Table 1. Structural parameters obtained from XRD Rietveld refinement of the five LNMO samples based on a space group of Fd3m. sample

lattice constant a/ Å

atom

MN 4.43

8.16545

MN 3.61

8.16507

MN 3.06

8.16483

MN 2.86

8.16261

MN 2.60

8.16224

Li Mn Ni O Li Mn Ni O Li Mn Ni O Li Mn Ni O Li Mn Ni O

Wyckoff position

x

8a 16d 16d 32e 8a 16d 16d 32e 8a 16d 16d 32e 8a 16d 16d 32e 8a 16d 16d 32e

0.125 0.5 0.5 0.26017 0.125 0.5 0.5 0.26118 0.125 0.5 0.5 0.25715 0.125 0.5 0.5 0.25876 0.125 0.5 0.5 0.25876

y

z

0.125 0.5 0.5 0.26017 0.125 0.5 0.5 0.26118 0.125 0.5 0.5 0.25715 0.125 0.5 0.5 0.25876 0.125 0.5 0.5 0.25876

0.125 0.5 0.5 0.26017 0.125 0.5 0.5 0.26118 0.125 0.5 0.5 0.25715 0.125 0.5 0.5 0.25876 0.125 0.5 0.5 0.25876

Site occupancy

Rp /%

Rwp /%

Weigh percent of impurity/ wt.%

1 0.75 0.25 0.907 1 0.75 0.25 0.896 1 0.75 0.25 0.893 1 0.75 0.25 0.891 1 0.75 0.25 0.889

3.57

4.73

0

1.9259

1.9695

4.81

6.27

0

1.9115

1.9618

4.17

5.71

0.3

1.9090

1.9595

4.03

5.44

3.2

1.8911

1.9566

4.16

5.45

5.6

1.8909

1.9563

10


Li-O /Å

Mn/Ni-O /Å

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In the case of MN 2.60 sample, there are visible weak peaks at 2θ= 37.5°, 43.6° and 63.5° which could be assigned to a rock-salt impurity phase (Li1-xNixO, (LixMn0.66Ni0.34)y O, and/or Ni6MnO8), it is a common concomitant impurity in the LNMO.11,19 The impurity peaks become less apparent for sample MN 3.06 and almost disappear for samples MN 3.61 and MN 4.43. As can be seen from Table 1, the contents of the impurities in the five samples estimated from the XRD data are 5.6 wt.% for MN 2.60, 3.2 wt.% for MN 2.86, 0.3 wt.% for MN 3.06, 0 wt.% for MN 3.61 and 0 wt.% for MN 4.43. In the literatures, for those annealed LNMO spinel samples, several superlattice peaks (2θ= 15.4°, 39.8°, 45.8° and 57.6°) assigned to the cubic P4332 symmetry can be often observed from their XRD patterns, indicating Ni/Mn long-range ordering in the spinel lattice.9,19 However, no superlattice peak appears in Figure 2, which shows that all samples have the cubic spinel structure with a space group of Fd3m instead of P4332. To further identify structures of the five LNMO samples, we performed Rietveld refinements. The refinements were performed using the Fd3m space group, the sites in which the atoms were assumed to be located were as follows: Li atoms in 8a sites, Ni and Mn atoms in 16d sites, and O atoms in 32e sites. The refinement profiles for all the samples show that all of the peaks fit well to the space group of Fd3m, as seen in Figure 2a-e, and the refined results are listed in Table 1. Average Mn/Ni-O and Li-O bond lengths are found to gradually increase with increasing the Mn/Ni molar ratios in the LNMO samples, which is consistent with the variation tendencies of the lattice constants. It is worth noting that the oxygen occupancies are close to 0.9 for all the LNMO samples, indicating that oxygen vacancies are present in these samples and their concentrations 11



vary little with the Mn/Ni molar ratios in the range of 4.43 to 2.60. Figure 3 compares the FTIR spectra of the five LNMO samples. The bands at 590 and 621 cm-1 were assigned to Ni-O and Mn-O vibrations, respectively, and their intensity ratio has been often used as a qualitative measure for Mn/Ni ordering degree in the LNMO spinel.4,20,21 As can be seen, the two peak intensity ratios of the five LNMO samples are close to 1, indicating that the ordering degree of two transition metal ions in LNMO structure is not significantly related to their molar ratios. The above results are contradictory to some conclusions in the literature. 8,10,16,18

MN2.60 MN2.86

Absorbance(a.u.)

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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MN3.06 MN3.61 MN4.43

621

700

650

590

MN4.43 MN3.61 MN3.06 MN2.86 MN2.60

600

550

500

450

-1

Wavenumber(cm )

Figure 3. FT-IR spectra of the five LNMO samples.

12


400

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6

6

measured by XPS measured by ICP

5 5

Mn/Ni molar ratio

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


4.43

4

3.61 3

3.06

3

3

2.86

2.60 2

2

1

0

Mn 4.43

Mn 3.61

Mn 3.06

Mn 2.86

Mn 2.60

Figure 4. Comparison of Mn/Ni molar ratios of the five LNMO samples measured by XPS and ICP. Compositions Analysis. Mn/Ni molar ratios of the five LNMO samples measured by XPS and ICP were compared in Figure. 4. For the manganese-rich samples (MN4.43, MN3.61 and MN3.06), the values measured by XPS are obviously higher than those obtained by ICP. However, the case of the nickel-rich samples (MN2.86 and MN2.60) is just the opposite. These results indicate that Mn and Ni in the LNMO samples are not uniformly distributed. Specifically, the Ni content on the surface of the nickel-rich samples is higher than that of the substrate due to the rock-salt impurity phase detected by XRD. In the case of the manganese-rich LNMO, the surface Mn concentration is higher, indicating that there is excessive manganese on the surface of these samples. XPS was used to further analyze the chemical states of Mn and Ni on the surface of these LNMO samples (Figure 5). As can be seen, all these samples show almost the same Mn 2p and Ni 2p XPS spectra, indicating that the chemical states of Mn and Ni on their surfaces are not 13



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significantly different. Ni 2p3/2 peaks locates at 854.8 eV and 2p1/2 peaks are at 872.1 eV, Mn 2p3/2 and 2p1/2 peaks appear at 642.3 and 654.3 eV, respectively. Difference of binding energies between two spin splitting peaks for Mn 2p is 12.0 eV, and that for Ni 2p is 17.3 eV. Based on the above results, it can be inferred that there are Ni2+ and Mn4+ on the surface of these samples.22 However, the existence of Mn3+ and Ni3+ cannot be determined from these XPS spectra, which may be related to their low content on the surfaces of LNMO samples.

Ni2p

Mn2p MN4.43

2P3/2

MN4.43

2P3/2

2P1/2

2P1/2

MN3.61

MN3.61

MN3.06

MN3.06



12.0eV

642.3eV



MN2.86

17.3eV

MN2.60

854.8eV

654.3eV

640

644

648

652

MN2.86

MN2.60

872.1eV

656

660850

860

870

880

890

Figure 5. Mn 2p and Ni 2p XPS spectra of the five LNMO samples.

14


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Electrochemical Properties. Figure 6 shows the first three charge-discharge curves of the five LNMO samples. All samples have a voltage plateau around 4.7 V, which corresponds to the Ni2+/Ni4+ redox couple. A relatively small plateau can be found for MN4.43, MN3.61 and MN3.06 in the region around 4.0 V, which is attributed to the Mn3+/Mn4+ redox couple.10,15 The size of the small plateau is commonly used to evaluate Mn3+ content in LNMO spinel.10,19 As can be seen from Figure 6, with the decrease of Mn/Ni molar ratio, the length of the small plateau gradually shortens until almost disappears, indicating that the content of Mn3+ decreases gradually from MN4.43 to MN2.60 (Table 2). Generally, the charging voltage profile (around 4.7 V) of LNMO can be divided into two regimes: first half was associated with the Ni2+→Ni3+ transition and the latter was attributed to the Ni3+→Ni4+ transition. The first Ni2+→Ni3+ plateau voltage can be used to characterize Mn/Ni ordering in the LNMO spinel. 10 It can be seen from Table 2 that the plateau voltage is about 4.72 V for all LNMO samples, showing that these samples have disordered spinel structure and the ordering degree of transition metal ions in these samples is almost equal. Obviously, this result is very consistent with the above XRD refinement and FTIR results.

In addition, it can also be found that the first

Coulombic efficiency of these samples increases with the increase in Mn/Ni molar ratio (Table 2), indicating that the nickel-rich rock-salt impurity on the surface of the LNMO spinel can accelerate the oxidation of electrolyte.

15



5.0

5.0

3 Ni

4.8 4.6

Ni

4+

Ni

3+

Ni

3+

Ni

2

3

1

4+

2

1

4.8 4.6

2+

4.4

Mn Mn

4.2

Voltage /V, vsLi/Li

Voltage /V, vsLi/Li

+

+

Ni

2+

4+

3+

Mn

4+

Mn

4.0

3+

3.8

4.4 4.2 4.0 3.8

MN4.43 3.6

Mn/Ni=3.61

3.6

1 3.4 0

20

40

60

80

100

2

1

3 3.4

120

140

160

0

20

Discharge specific capacity (mAh/g)

60

80

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120

140

160

5.0

3

1

2

3 4.8

4.6

4.6

2

1

Voltage /V, vsLi/Li

+

4.8

+

Voltage /V, vsLi/Li

40

3 2


5.0

4.4 4.2 4.0 3.8

4.4 4.2 4.0 3.8

MN2.86

MN3.06

3.6

3.6

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2 3 1

3 3.4

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3

+

4.6 4.4 4.2 4.0 3.8

MN2.60 1 3 2

3.4 0

20

40

60

80

100

120

40

60

80

100

120

140

160

Figure 6. The first three charge-discharge curves of the five LNMO samples measured at 0.2 C between 3.5 and 4.9 V.

1

2

4.8

3.6

20


5.0

Voltage /V,vsLi/Li

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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140

160


16


Page 17 of 27

Table 2. Summary of the first three charge-discharge property of the five LNMO samples. Amount of Mn3+ per formula unit of LiNixMn2-xO4*

The 3rd discharge capacity/mAh•g

Potential of Ni2+→Ni3+ plateau/V

First Coulombic efficiency/%

MN4.43

0.11

122.4

4.719

81.8

MN3.61

0.090

119.4

4.724

77.7

MN3.06

0.039

121.0

4.725

76.8

MN2.86

0.029

117.4

4.715

72.8

MN2.60

0.020

110.7

4.725

72.2

Sample

*The amount of Mn3+ was calculated by (the 3rd discharge capacity in 4-V region)/(overall discharge capacity). The 4-V region is the voltage range of 4.40-3.60 V.

140

0.5C

0.2C

130 120


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


1C

0.2C 2C

110

5C

100 90

10C

80 70 60 50

MN4.43 MN3.61 MN3.06 MN2.86 MN2.60

40 30 20 10 0 0

5

10

15

20

25

30

35

40

Cycle number

Figure 7. Rate capability of the five LNMO samples measured at different C-rates between 3.5-4.9 V.

17



-3.2 -7400

Femi energy Free energy

-7200 -2.8

-7000

-2.6

-200

-0.2 0.0

Free energy/ev

-3.0

Femi energy/ev

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 18 of 27

Mn 4.43

Mn 3.61

Mn 3.06

Mn 2.86

Mn 2.60

0

Figure 8. Comparison of Fermi level and free energy of the LNMO five samples. Rate capability of the five LNMO samples is shown in Figure7. The rate capability of these samples from excellent to inferior in sequence is MN4.43, MN3.61, MN3.06, MN2.86 and MN2.60, which is good consistent with the order of their Mn3+ content from high to low. This indicates that Mn3+ content in LNMO spinel is an important factor affecting its rate capability. It is well known that the rate capability depends on the electronic conductivity of LNMO and the Li+ diffusion rate in LNMO structure. Kunduraci et al 12 claimed that electronic conductivity of LNMO spinel with Mn3+ can be 2-3 orders of magnitude higher than that of Mn3+ free LNMO spinel, which is attributed to the lower activation energy of electron hopping in the former structure. As can be seen from Figure 8, the Fermi level of the five LNMO samples gradually decreases according to the order of MN4.43, MN3.61, MN3.06, MN2.86 and MN2.60, indicating that the increase in Mn3+ content can enhance the electronic conductivity of LNMO spinel. 18


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Actually, this result is consistent with the result of Kunduraci et al. It can be also seen from Figure 8 that free energy of the five LNMO spinels decreases with the decrease of Mn3+ content in their structure, showing that the LNMO sample with low Mn3+ content has a more stable structure, which is good agreement with the above results of XRD refinement in Table 1. Although Mn3+ can reduce the structural stability of LNMO to a certain extent, the expansion of the unit cell due to Mn3+ facilitates Li+ diffusion in LNMO structure. From the above results, Mn3+ can greatly enhance the electrochemical properties (capacity, first Coulombic efficiency and rate capability) of LNMO, which is consistent with the results reported in some literatures.

8-12

An increase in Mn3+ content is often

believed to trigger the transition from the ordered (space group P4332) to the disordered (space group Fd3m) spinel.8,17 A small amount of Mn3+ has been found even in the near perfectly ordered LNMO with the P4332 space group, indicating that LNMO remains ordered at low Mn3+ content but disorders when Mn3+ increases to a critical level. In our work, the ordering degree of two transition metal ions in LNMO structure is almost independent on the Mn3+ content, probably because Mn3+ content in these LNMO samples is higher than the critical value. Unfortunately, the exact Mn3+ critical value is still unknown. Because of the better electrochemical properties, the disordered LNMO has attracted more attention. Despite of many preparation methods for LNMO, all of these methods require a calcination step at high temperatures ranging from 800 to 1000 ℃, which inevitably leads to oxygen evolution and the consequence of formation of oxygen 19



Page 20 of 27

vacancies in oxygen-deficient LiNi0.5Mn1.5O4-ơ spinel.16 To satisfy the charge balance, some of Mn4+ ions in the spinel are reduced to Mn3+, and so Mn3+ content increases with the increase of oxygen vacancies. The result of first principles simulations shows that the increase of oxygen vacancies causes a decrease in the ordering degree of Mn/Ni site occupation in LNMO structure.17 Obviously, concentration of oxygen vacancies, Mn3+ content and ordering degree of Mn/Ni are closely correlated under this situation. As the oxygen loss due to calcining at high temperatures is reversible, annealing at 600-700℃ or slow cooling are often used to tune the Mn3+ content of the oxygen-deficient LNMO.16,23-25 In this work, the Mn3+ content of LNMO samples was tuned through a change in Mn/Ni molar ratio, and it was found that the concentration of oxygen vacancies, like the ordering degree of Mn/Ni, remains almost constant for all LNMO samples with different Mn3+ content. This result suggests that the formation mechanism of Mn3+ in off-stoichiometric LNMO (the Mn/Ni molar ratio deviates from 3.) is different from that of Mn3+ in oxygen-deficient LiNi0.5Mn1.5O4-ơ spinel.16

Since the ionic charge of Ni

species is less positive than that of Mn species, Mn3+ in the LiNixMn2-xO4 (Mn/Ni molar ratio >3, x < 0.5) spinel increases with an increase in Mn/Ni molar ratio to maintain electrical neutrality. Nevertheless, when Mn/Ni molar ratio is near or less than 3, a Ni-rich rock-salt impurity is easily segregated from the LiNixMn2-xO4 spinel, as the spinel can no longer accommodate excess Ni2+ in the structure. In addition, almost equal oxygen vacancy concentration in all LNMO samples shows that most oxygen vacancies in spinel structure may be eliminated by combining prolonged annealing with slow cooling steps

20


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during the sample preparation, so electrochemical properties of the LNMO spinel depends largely on amount of the Ni-rich rock-salt impurity.

■ CONCLUSIONS Due to different formation mechanism of Mn3+ in off-stoichiometric LNMO and common oxygen-deficient LNMO, the relationships of Mn3+ content, structural ordering degree, oxygen vacancies, impurities and electrochemical properties in the former are not the same as those in the latter. In the case of off-stoichiometric LNMO, both structural ordering degree and oxygen vacancies hardly change with Mn3+ content, which may be related to prolonged annealing combined with slow cooling steps during the sample preparation. Electrochemical properties are significantly improved with an increase in Mn3+ content because the increase of Mn3+ can reduce the Ni-rich rock-salt impurity, improve electronic conductivity, and facilitates Li+ diffusion in LNMO structure. However, for oxygen-deficient LNMO, the relationships of Mn3+ content, structural ordering degree, oxygen vacancies, impurities and electrochemical properties are closely correlated with each other.

21



■ AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

ORCID Dongsheng Lu: 0000-0001-5334-8957 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The authors acknowledge financial support by the National Natural Science Foundation (No.21571069), the Scientific and Technological Plan of Guangdong Province (No. 2017A040405048) and Applied Science and Technology Planning Project of Guangdong Province, Guangzhou, China (No.2015B010135009 and 2017B09091700 2).

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TOC Graphic

27


Relationships between Structure, Composition, and Electrochemical

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