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Electrochemical kinetics and cycle stability improvement with Nb doping for lithium-rich layered oxides MUHAMMAD ZUBAIR, Guangyin Li, Boya Wang, Lin Wang, and Haijun Yu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01534 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Energy Materials

Electrochemical Kinetics and Cycle Stability Improvement with Nb Doping for Lithium-Rich Layered Oxides Muhammad Zubair†, Guangyin Li†, Boya Wang, Lin Wang, Haijun Yu* College of Materials Science & Engineering, Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology, Beijing 100124, China

ABSTRACT: Lithium-rich layered oxide materials are extremely important for improving the energy density of lithium-ion batteries.

However, the electrochemical kinetics and cycle stability of these materials

are still not good enough for further industrial application.

Here, the effects of Nb doping on the

crystalline structure, surface chemistry, cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2 are studied.

Results show that Nb doping can significantly promote the

cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2.

After 200 cycles, the

discharge capacity retention increases from 59.2% (pristine material) to 78.8% (1% Nb element doping) with obvious enhanced rate performance.

Via X-ray diffraction, scanning electron

microscope, energy dispersive spectroscopy, X-ray photoelectron spectroscopy and galvanostatic intermittent titration analysis, it is confirmed that Nb element has been successfully doped into the bulk of Li1.13Mn0.52Ni0.26Co0.10O2.

Doping sites of Nb element in structure and influence on

lithium migration has also been studied by the Rietveld refinement and first principle theoretical calculation methods.

Doped Nb element efficiently changes the lattice parameters, inhibits the

resistance rise, accelerates lithium ion diffusion, and finally promotes the cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2

KEYWORDS: cycle stability, electrochemical kinetics, Lithium-rich layered oxides, Lithium ion batteries, Nb doping.

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1. INTRODUCTION With decades of rapid development, lithium-ion batteries (LIBs) have achieved great success in the power supply of portable electronics and electric vehicles (EVs).1-3

However, lack of high

performance cathode materials has become the bottleneck of the development of advanced LIBs. At present, commercialized cathode materials such as layered LiCoO2,4 LiNi1/3Mn1/3Co1/3O2,5 LiNi0.8Co0.15Al0.05O2,6 olive LiFePO4,7 spinel LiMn2O4,8 can provide a limited reversible capacity (120-200 mAh g-1).

In order to overcome this challenge, it is important to develop cathode

materials with higher energy densities.

Lithium-rich layered oxides (LLOs) with the general

formula xLi2MnO3·(1-x)LiMO2 (M=Mn, Co, Ni, etc.) appear to be the most promising candidates because of their higher reversible capacity (up to 300 mAh g-1) and lower costs.9 However, the practical application of these materials is hindered by inherent disadvantages, including: (1) the low initial Coulombic efficiency; (2) the irreversible structure transition and voltage decay during cycling; (3) the low electronic/ionic conductivity and the poor rate performance.10,11 To address these problems, various strategies have been proposed, such as optimizing element composition and distribution,12-15 surface coating,16-18 bulk doping19-23 and particle size adjustment.24

In particular, bulk doping has been employed to alter the electronic structure and

stabilize the oxygen close-packed structure of the LLOs.

Doping of fixed-valence-state

elements, such as Al3+,25 Mg2+ ,26 Ru5+ ,27 Sn4+ ,28 Cr3+,29 could effectively suppress the migration of transition metal (TM) during cycling, mitigate the capacity and voltage decay.

Nonmetallic

element doping (B, P) also demonstrates a superior cycle performance through enhanced oxygen stability.30-31

Besides cycling stability, bulk doping also has a significant effect on the lithium

diffusion kinetics in the cathode materials.

For example, doped alkali metal ions (K+, Na+) with

a large ionic radius have been shown to accelerate the lithium diffusion kinetics of LLOs materials by extending the Li ion diffusion channel.32-33

Therefore, bulk doping could prevent the structure

transformations and improve the conductivity of electrons and lithium ions in the cathode material. The dissociation energy of Nb-O (753 kJ mol-1) is higher than that of Mn-O (402 kJ mol-1), which contributes to the stability of the layered structure.34

Meanwhile, compared with Mn4+ (0.53 Å),


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the doped Nb element is expected to expand the lattice parameter due to its large radius (0.69 Å). As far as we know, the systematic study of Nb doping on the cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2 (or expressed as 0.3Li2MnO30.7LiMn0.42Ni0.42Co0.16O2) cathode materials is seldom reported, and the specific effects and mechanisms are still unclear. In this work, Nb doped Li1.13Mn0.52Ni0.26Co0.10O2 were prepared by employing the Nb2O5 coated Mn0.59Ni0.29Co0.12CO3 precursor and subsequent

high temperature sintering with Li2CO3, and the

effects of Nb doping on the structure, surface chemistry, cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2 are studied thoroughly.

The mechanisms of the enhanced

cycle stability and electrochemical kinetics are also studied. 2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The precursor Mn0.59Ni0.29Co0.12CO3 was synthesized by the carbonate co-precipitation method. An aqueous solution containing appropriate proportions of NiSO46H2O, CoSO47H2O and MnSO45H2O with the total metal ions concentration of 2.0 mol dm-3 was pumped into a continuously stirring tank reactor (CSTR, Volume of 2 dm-3).

Meanwhile, 2.0 mol dm-3 Na2CO3

solution and desired amount of 0.2 mol dm-3 NH4OH solution were separately fed into the CSTR, which were used as the precipitating agent and coordinating agent for the transition metals ions, respectively.

The co-precipitation temperature was controlled at 55 °C, and the pH was

constrained via a PH control device.

The precursor powders were filtered, washed, and dried at

90 oC in a vacuum oven. To obtain Nb doped Li1.13Mn0.52Ni0.26Co0.10O2, Nb2O5 was first coated on the surface of Mn0.59Ni0.29Co0.12CO3 precursor.

Specifically, Nb2O5 was dispersed in ethanol at room

temperature and stirred for 30 min, and then the Mn0.59Ni0.29Co0.12CO3 precursor was poured into the solution with continuous stirring for 30 min. The powder was collected after the solvent was evaporated.

The powder was dried in a vacuum oven at 90 oC for 12 hours to obtain the Nb2O5

coated Mn0.59Ni0.29Co0.12CO3 precursor. Then, the required amount of prepared precursor powder and Li2CO3 were accurately weighed and thoroughly mixed with a three-dimensional mixer, and a



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5 wt% excess of Li2CO3 was used to compensate for lithium loss during high-temperature sintering. The mixed powders were loaded into alumina crucibles and sintered in the air to obtain the final product.

The following steps were used during the sintering process: step 1, heating

from room temperature to 500 °C and hold for 5 h to promote complete removal of CO2 from the carbonate precursor; step 2, heating from 500 to 850 °C and hold for 12 h; and step 3, naturally cooled down to room temperature.

The molar ratios of Nb/(Ni+Co+Mn) were 0, 0.005, 0.01,

0.02, 0.04, labeled as Nb-0, Nb-0.005, Nb-0.01, Nb-0.02, Nb-0.04, respectively. 2.2. Material Characterization. The synthesized samples was identified by powder X-ray diffraction (XRD, Cu Kα radiation, Bruker D8 Advance) in the range of 2θ from 10° to 130° in a steps size of 0.01° and a counting time of 0.5 s.

The morphology of the samples was investigated by scanning electron microscopy

(Sirion 2000, FEI) in conjunction with energy-dispersive spectroscopy (EDS, Horiba, EX-250) energy-dispersive spectroscopy (EDS, Horiba, EX-250).

The surface chemistry was analyzed by

X-ray photoelectron spectroscopy (XPS) using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV).

The C 1s hydrocarbon peak at 284.80 eV was used as the final adjustment criterion for

the energy scale in the spectrum. The prepared sample was characterized by BET surface area. 2.3. Electrochemical Measurements The electrochemical performances of the synthesized materials were assessed in 2032 coin-type cell.

The cell contains a cathode and a metallic lithium foil anode separated by a porous

polypropylene film (Celgard 2400).

The cathode slurry was prepared by homogeneously mixing

of the active material (80 wt%), carbon black (15 wt%) and polyvinylidenefluoride (5 wt%) in N-methyl-2pyrrolidine (NMP) solvent.

The slurry was coated on an aluminum foil, and then

dried at 90 °C for 4 h to remove the NMP solvent.

The electrode was punched into a 12 mm

diameter round disc, then pressed with a roller and dried in a vacuum oven at 120 °C for 12 hours. The geometric surface area of the electrodes exposed to the electrolyte solution was 1.13 cm2. The electrode loading of active material is about 2 mg·cm-2.

The cells were assembled in an

argon-filled glove box with H2O and O2 concentrations below 0.01 ppm.

The electrolyte was 1

M LiPF6 dissolved in ethylene carbonate (EC) / dimethyl carbonate (DEC) (1:1 by volume).


The

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volume of electrolyte used in the cells is 100 μL with the pipetting device.

All electrochemical

performance was tested on the LAND CT2001C (Wuhan, China) battery program-control test system with a designed charge/discharge current density at room temperature (25 °C). Galvanostatic intermittent titration (GITT) was tested using a Solartron Analytical instrument. Electrochemical impedance spectroscopy (EIS) was studied using a CHI 660 E (Shanghai, China) electrochemical workstation from 106 to 10-2 Hz with amplitude of 5 mV. 2.4. Calculations Details The total energy calculation is implemented in the Vienna ab initio package (VASP) with pseudopotential

established

Perdew-Burke-Ernzerh (PBE).

by

the

projector-augmented

wave

(PAW)

and

the

A 450 eV cutoff for plane waves is used to test exact values.

The Brillouin zone was used with a 3×3×5 Γ-centered k-mesh for 2 a×1 b×1 c supercell (Containing eight Li2MnO3 molecular units).

All structures were relaxed until the self-consistent

force was less than 10-2 eVÅ-1 and the energy between two consecutive steps was less than 10-5 eV.

In the GGA+U schemes, U eff (U-J) is fixed at 4.9 eV and 1.5 eV for the Fe-3d state and

Co-3d. 3. RESULTS AND DISCUSSION 3.1. Crystal Structure Analysis Figure 1a shows the XRD patterns of pristine and Nb doped Li1.13Mn0.52Ni0.26Co0.10O2 samples in the range of 10°-130°.

All diffraction patterns are consistent with the hexagonal α-NaFeO2

structure (R-3m group space) except for the weak reflections between 20°-23°, which are caused by the formation of Li2MnO3-like structure in LLOs.35

The significant splitting of (018)/(110)

peaks indicates that a highly ordered layered structure is obtained (Figure 1d). The enlarged view angle ranges of 17°-20°, 43°-46° and 63°-66°are shown in Figure 1b, Figure 1c and Figure 1d, respectively.

The peaks around 18.7° and 44.6° gradually move toward lower angles, indicating

that a significant amount of Nb atoms may doped into the bulk of the LLOs.

Rietveld refinement

is based on the supposition that the Li+/Nb5+ occupy 3b (0, 0, 0), transition metal ions/Nb5+ occupy 3a (0, 0, 0.5), while the oxygen anions occupy 6c (0, 0, Zox).


Results shows that the lattice


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parameters (a, c, V) increase slightly after Nb doping. This may be related to the larger ionic radius of Nb5+ (0.64 Å) than the ionic radius of Co3+ (0.54 Å) and Mn4+ (0.53 Å). Nb5+ prefers to occupy the 3b site (Table 1).

Particularly,

When the Nb doping amount is smaller (for

Nb-0.005 and Nb-0.01 samples), all of Nb5+ occupies the 3b site.

As doping amount increases,

most of Nb5+ occupies 3b site, but part of Nb5+ occupies 3a site (for Nb-0.02 and Nb-0.04 samples).

The changes in lattice parameters and the introduction of Nb5+ in slabs may introduce

a strong distortion field, electronic structure transformation, and then stimulate the electrochemical performance of these materials seriously.36

Note that as the amount of doped Nb

increases, some tiny residual peaks appear (marked by rhombus), which can be indexed as Li3NbO4 phase.

Figure 1. (a) XRD patterns of pristine and Nb doped Li1.13Mn0.52Ni0.26Co0.10O2; The enlarged view in the 2θ range of (b) 17°-20°, (c) 43°-46° and (d) 63°-66°; (e) XRD patterns of Nb-0.04 and the standard patterns for each phase; (f) Rietveld refinement of Nb-0.01 using Topas software.


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Table 1 The refined lattice parameters based on LiNiO2 model with R-3m space group

Sample

Nb-0

a (b), Å

V, Å3

c, Å

Zox

*Occ

*Occ

(Nb5+ in

(Nb5+

3b)

in 3a)

Rwp (%)

2.85761(5)

14.2354(6)

100.672(6)

0.2531(1)

0

0

5.29

2.85975(3)

14.2446(4)

100.888(3)

0.2560(1)

0.0050(4)

0.0000(4)

4.76

2.86061(3)

14.2477(4)

100.970(3)

0.2568(1)

0.0100(4)

0.0000(4)

4.40

2.86165(4)

14.2538(5)

101.087(5)

0.2599(1)

0.0162(5)

0.0037(5)

4.73

2.86305(5)

14.2645(6)

101.262(6)

0.2604(1)

0.0102(6)

0.0297(6)

5.72

Nb-0.0 05 Nb-0.0 1 Nb-0.0 2 Nb-0.0 4 *Constraints: total occupancy of 3a sites (Li layer), Li+3a+Nb5+3a = 1, total occupancy of 3b sites (TM layer), TMn+3b+Li+3b+Nb5+3b = 1, Nb5+ occupancy in 3a and 3b sites, Nb5+3a+Nb5+3b=Nb doping amount.

Figure 2. The schematic structure diagram of Nb doped into (a) TM layer and (b) lithium layer, respectively; (c) the lithium ion migration barrier energy; the diagram of lithium ion migration path when Nb is doped into (d) TM layer and (e) lithium layer, respectively.

To better understand the effects of Nb doping, a first-principle density functional theory (DFT) was used.

Figure 2a,b shows the schematic structure diagram of Nb doped into the TM layer and



lithium layer, respectively.

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The formation energy of O vacancy of the bare material is 2.23 eV.

After Nb doping, a stable Nb-O6 octahedron is formed, and the formation energy is increased. When Nb is doped, the formation energy of the O vacancy in the TM layer is 2.63 eV, and the formation energy in the lithium layer is 3.66 eV.

In addition, the barrier energy of lithium ion

migration is also calculated, and the results and schematic diagram of the lithium ion migration path are shown in Figure 2c-e.

The barrier energy of the bare material is 0.72 eV.

When Nb is

doped, the barrier energy in the TM layer is 1.35 eV, and the barrier energy in the lithium layer is 0.44 eV.

Therefore, Nb preferably occupies the 3b site (lithium layer), and Nb doped into the

lithium layer can form a stable structure and promote lithium ion migration, which is consistent with the XRD analysis. 3.2. Morphology and Element Distribution Analysis


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Figure 3. (a-j) SEM images of pristine and Nb-doped Li1.13Mn0.52Ni0.26Co0.10O2: (a, b) Nb-0, (c, d) Nb-0.005, (e, f) Nb-0.01, (g, h) Nb-0.02, (i, j) Nb-0.04,; (k-o) EDS mappings of Nb-0.01, (p) SEM image of the cross-section of a single particle of Nb-0.01, and the inset graph (p’) is the EDS mapping of Nb element.

SEM images of pristine and Nb doped Li1.13Mn0.52Ni0.26Co0.10O2 samples are shown in Figure 3a-j. The morphology of Li1.13Mn0.52Ni0.26Co0.10O2 particles is closely spherical.

The micrometer

sized Li1.13Mn0.52Ni0.26Co0.10O2 particles have a broad particle size distribution, and are assembled from a number of primary particles.

As shown in high resolution images, some mesopore can be

clearly observed on the surface of the particles, which may be due to CO2 evolution during high temperature calcination.37 doped samples by SEM.

There is no obvious difference observed between pristine and Nb Moreover, the EDS maps of the selected particles of Nb-0.01 are shown



in Figure 3k-o.

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As shown in Figure 3k-o, the Nb element is uniformly distributed on the surface

of the particles of Nb-0.01, as well as other components (Mn, Co, Ni) of the material. Furthermore, Figure 3p is the SEM image of the cross-section of a single particle Nb-0.01, and the inset Figure 3p’ is the EDS map of the Nb element, which is used to characterize the elements distribution in the cross-section.

As shown in Figure 3p’, the Nb element can be detected in the

cross-sectional surface, and the element distribution of Nb is uniform.

Therefore, Nb element

can be successfully transported to the internal bulk of Nb-0.01 particle and doped into the crystal lattice after high temperature sintering. In addition, the measurement of BET surface area shows slight decrease after Nb doping. The surface area of Nb-0 is 8.56 m2g-1, while the surface area of Nb-0 is 5.23 m2g-1. The decreased surface area may be beneficial for the interface stability. 3.3. Surface Chemistry Analysis XPS was used to study the surface element composition and chemical state of Nb-0 and Nb-0.01 samples, as shown in Figure 4.

The results were analyzed by XPSPEAK software.

As shown

in Figure 4a, the peak of Nb3d5/2 is located at 206.3 eV, and the peak of Nb3d3/2 is located at 209.1 eV, indicating that the chemical valence of Nb in Nb-0.01 sample is +5. Ni2p3/2 shifts to low binding energy after Nb doping. related to an increase in the Ni2+ ratio.

The peak shifts to low energy may be

After Nb doping, the proportion of Ni2+ increased from

64% (Nb-0) to 81% (Nb-0.01) (Figure 4b). more than Nb-0 sample.

Meanwhile, the peak of

Therefore, Ni2+ appears on the surface of Nb-0.01

The strong O1s peak of Nb-0 sample at 529.2 eV corresponds to lattice

oxygen, and the small broad peak at higher energy level (531.3 eV) belongs to weakly adsorbed surface oxygen species (LiOH/Li2CO3).38

The proportion of lattice oxygen increases from 51%

to 54%, while the proportion of adsorbed oxygen decreases from 48% to 45%. peak of lattice oxygen shifts to a higher binding energy after Nb doping.

In addition, the

The bond dissociation

energy of Nb-O (Hf (Nb-O)=753 kJ mol-1) is stronger than that of Mn-O (Hf (Mn-O)=402 kJ mol-1), which can improve the stability of the crystal structure.34 Several similar situations have been studied.

For example, Sn4+ doping39 can stabilize the lattice oxygen of Li1.17Ni0.25Mn0.58O2

due to the strong Y-O bond, Y3+ doping40 has a positive stabilizing effect on the overall structure of Li1.2Mn0.6Ni0.2O2.

Therefore, Nb doping is an effective method for stabilizing TM-O6

octahedron and inhibiting the migration of transition ions to the lithium layer.


In combination

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with the C 1s spectrum in Figure 4d, the carbon peak at 289.3 eV (Nb-0.01) of Li2CO3 shows a value lower than that of Nb-0 sample, indicating that the amount of Li2CO3 on the surface of Nb-0.01 is less than that of Nb-0 sample actually.

As it is commonly known, these surface

impurities (LiOH/Li2CO3) may have a profound effect on the electrochemical performance of active material since the surface property is very important for the electrochemical reaction.

In

addition, the less surface impurities may be associated to the changed surface property of Nb-0.01 since these surface impurities are related with the spontaneous reactions between the surface of material and CO2 in the atmosphere.

Figure 4. XPS spectrums of (a) Nb3d, (b) Ni2p, (c) O1s, and (d) C1s taken from the surface of Nb-0 and Nb-0.01 samples.

3.4. Electrochemical Performance 3.4.1. Initial Charge/Discharge Analysis Figure 5a shows the initial charge/discharge curves of the samples with various Nb doping amount at the current density of 20 mA g-1 between 2-4.8 V vs. Li/Li+.

All samples represent the

characteristic curve of the sloped and plateau regions of the LLOs during initial charging, as well



as the sloped discharge curve.

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The slope region (< 4.5 V) is related to the lithium ion extraction

process with the oxidation of Ni2+ and Co3+, the long plateau region (>4.5 V) corresponds to the lithium ion extraction process, accompanied with Li2MnO3-sturcture activation (Manganese and Oxygen element activation).41

It can be clearly seen that there is a large difference in the

charge/discharge curve between pristine and Nb doped samples.

For the initial charge curve of

the Nb doped sample, a slope below 4.5 V moves toward a lower voltage (indicated by arrows in Figure 5a).

The reduced polarization may be related to the expanded lithium ion channel caused

by Nb doping.

According to previous studies, the main kinetics of lithium in this process is

related to the extraction of lithium ions from the LiMn0.42Ni0.42Co0.16O2 and Li2MnO3 component.41

Therefore, it can be concluded that the kinetics of lithium ion extraction becomes

easier after Nb doping. The discharge capacity of Nb-0 sample is 256 mA h g-1, and a Coulombic efficiency of 84.8%. After Nb doping, as expected, the discharge capacity increases firstly, and then decreases (Figure 5b).

For Nb-0.005, Nb-0.01, Nb-0.02, Nb-0.04, they were 277 mAh g-1, 289 mAh g-1, 286 mAh

g-1 and 265 mAh g-1, and Coulombic efficiency 85.2%, 83.5%, 83.6%, 81.7%, respectively. Therefore, an appropriate Nb doping amount is very important for the performance of Li1.13Mn0.52Ni0.26Co0.10O2, which may be related to the expanded lattice parameters and the newly formed Li3NbO4 phase.

When the amount of Nb is small, the positive effect of Nb doping on

lithium ion diffusion cannot be fully manifested;42

when it is large, an “extra” Li3NbO4 phase

with a cationic disordered rock salt-type structure is formed, and which may affect lithium migration and kinetic process of charge transfer at the electrode/electrolyte interface, resulting in reduced discharge capacity.43


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Figure 5. (a) Initial charge/discharge curves and (b) specific charge/discharge capacity & Coulombic efficiency of pristine and Nb-doped Li1.13Mn0.52Ni0.26Co0.10O2 samples.

3.4.2. Cycle Stability Analysis



Figure 6. (a) The galvanostatic cycling and (b) capacity retention ratio of pristine and Nb doped Li1.13Mn0.52Ni0.26Co0.10O2 at the current density of 200 mA g-1 between 2.0-4.6 V after the 1st activation at the current density of 20 mA g-1 between 2.0-4.8 V; (c-g) The charge discharge curves from galvanostatic cycle performance: (c) Nb-0, (d) Nb-0.005, (e) Nb-0.01, (f) Nb-0.02, (g) Nb-0.04; (h) plots of the discharge mean potential versus cycle number.

The inset of (h) is the plot of E vs cycle number, where E = Ed-Ed1st, Ed is the

discharge mean voltage.

In order to understand the effect of Nb doping on the cycle stability of Li1.13Mn0.52Ni0.26Co0.10O2, the galvanostatic charge/discharge cycle test was carried out at the current density of 200 mA g-1


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between 2-4.6 V vs. Li/Li+ at room temperature (25 oC).

Figure 6 shows the evolution of cycle

capacity, voltage curve and mean voltage of pristine and Nb doped samples during 200 charge discharge cycles.

At higher cut-off voltage (4.6V), the discharge capacity and capacity retention

ratio of each sample continue to decrease as the cycle progresses.

Specifically, after 200 cycles,

the discharge capacity and capacity retention ratio for Nb-0.01 are 172 mAh g-1 and 78.8%, while Nb-0 sample are 117 mAh g-1 and 59.2% (Figure 6a,b). Meanwhile, Nb doping can slow down the voltage decay, as shown in Figure (6c-g). In order to clearly show the positive effect of Nb doping on voltage decay, the graph of the mean discharge voltage vs. cycle number is shown in Figure 6h. The discharge voltage of Nb-0.01 decreases from 3.71 V (1st) to 3.39 V (200th) (E = 0.32 V), while for Nb-0 sample, the discharge voltage decreases from 3.66 V to 3.18 V after 200 cycles (E = 0.48 V).

In addition, the inset in Figure 6h shows the graph between E vs. cycle number.

E reflects electrochemical polarization.

Compared with the pristine sample (Nb-0), Nb doped

samples exhibit a slow rising trend and a small polarization, indicating that Nb doping can reduce electrode polarization and improve cycle stability. The degradation in electrochemical performance is related to the structural changes of the electrode material during the cycle, and the bulk doping affects the stability of the structure.

It

has been reported that the electrolyte decomposition at high potentials and continuous structural changes of LLOs from layered structure to spinel structure and then to rock-salt are considered to be the main cause of the voltage decay.44,45

In particular, this inherent structural change usually

occurs from the surface of the particle to the internal bulk during cycling.

Nb doping improves

the surface and internal bulk phase stability of LLOs, and thereby reducing cycle capacity and voltage decay.

Therefore, Nb doping is an effective method to improve the cycle stability of

Li1.13Mn0.52Ni0.26Co0.10O2.



Page 16 of 28

Figure 7. Nyquist plots of (a) Nb-0 and (b) Nb-0.01 after the 1st, 100th and 200th cycles at the current density of 200 mA g-1 between 2.0-4.6 V; (c) A typical Nyquist and fitting plots using the inset equivalent circuit. The impedance spectra were collected at a charged state of 4.3 V.

Electrochemical impedance spectroscopy (EIS) probes the evolution of cell impedance during 200 cycling times.

Figure 7 shows the impedance spectra of Nb-0 and Nb-0.01 samples at the 1st,

100th and 200th cycle.

The high-frequency small semicircle is generated by the surface film

impedance (Rs), the high to low frequency larger semicircle is generated by the charge-transfer impedance (Rct), and the low-frequency line is generated by the lithium ion diffusion impedance (W).

The high frequency intercept with the real axis arises from the electrolyte and cell

component resistance (Re).

The fitting results of the impedance spectra are listed in Table 2.

Table 2 Fitting results of the impedance spectra using the equivalent circuit inset in Figure 7c (unit: ohm) Nb-0

Nb-0.01

cycles

Re

Rs

Rct

Re

Rs

Rct

1st

4.3

9.7

76.4

4.1

7.8

55.6

100th

6.5

13.6

212.5

5.9

9.6

89.4

200th

8.4

23.4

407.2

7.6

17.1

206.9

As shown in Table 2, Re and Rs are the minimum resistances that increase slightly with the


Page 17 of 28 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


extension of the cycle, which indicates that the resistances of electrolyte and cell component, the surface film impedance has little effect on the cycle stability of Li1.13Mn0.52Ni0.26Co0.10O2. However, Rct is severely affected by Nb doping.

With an extended cycle, the Rct of the Nb-0

sample increased significantly, but the Rct of Nb-0.01 did not (Table 2).46

Previous studies have

shown that significant changes in Rct are associated with unstable microstructure changes.46 Therefore, Nb doping can enhance the structural stability and promote enhancement of electrochemical cycling, which is consistent with XRD and cycle stability analysis. 3.4.3. Electrochemical Kinetics Analysis

Figure 8. (a, b) Charge/discharge curves of (a) Nb-0 and (b) Nb-0.01 samples at the current densities of 20 mA g-1, 40 mA g-1100 mA g-1 and 200 mA g-1 between 2-4.6 V vs. Li/Li+; (c-f) OCV curves and GITT of Nb-0 (c, e) and Nb-0.01 (d, f) samples during the first, second charges/discharges process at 20 mA g-1 with a time interval of 3600 s; (g, h) lithium ion diffusion coefficients calculated from the GITT curves as a function of the cell voltage (OCV) during the charge/discharge process.



Page 18 of 28

Rate performance testing was used to study the electrochemical kinetics, as shown in Figure 8 a,b. Firstly, Nb-0 and Nb-0.01 cells were charged and discharged at a current density of 20 mA g-1 and the potential range was 2-4.8 V to activate them.

The cells then charged and discharged at the

current densities of 20 mA g-1, 40 mA g-1, 100 mA g-1, 200 mA g-1 with a potential range of 2-4.6 V.

As expected, the discharge capacity decreased as the current density increases, but Nb-0.01

showed a higher rate capacity than that of Nb-0 sample.

Specifically, the discharge capacities of

Nb-0.01 are 257 mAh g-1, 245 mAh g-1, 231 mAh g-1, 212 mAh g-1 at 20 mA g-1, 40 mA g-1, 100 mA g-1, 200 mA g-1, while those of Nb-0 sample are 248 mAh g-1, 230 mAh g-1, 206 mAh g-1, 185 mAh g-1, respectively. GITT was used to study the lithium ion diffusion in electrode materials.

Cells were charged and

discharged at a constant current for intervals of 3600 seconds, and the current density is 20 mA g-1. Subsequently, the cells were held at an open-circuit voltage (OCV) for 3600 seconds to relax the voltage to a steady state value.

The charge/discharge procedure were tested in the potential

range of 2-4.8 V vs. Li/Li+. As shown in Figure 8c-f, the OCV curve is significantly affected by Nb doping.

Each procedure

of the charge/discharge segment is repeated several times during charging/discharging. The more these procedures are repeated, the larger the charge/discharge capacity.

For the Nb-0 sample, the

procedure is repeated 20 times and 15 times during the 1st and 2nd charge cycles, 14 times and 13 times during the 1st and 2nd discharge cycles; for the Nb-0.01 sample, the procedure is repeated 24 times and 18 times during the 1st and 2nd charge cycles, and is repeated 19 times and 17 times during the 1st and 2nd discharge cycles, respectively.

Obviously, the charge and discharge

capacity of Nb-0.01 is larger than that of Nb-0 sample. In addition, the polarization of the over-potential can be alleviated after Nb doping.

The high

over-potential is related to the kinetic control steps and concentration polarization during the lithium ion extraction/insertion reaction.

Previous studies have shown that the lithium ion

extraction process of Li2MnO3 component and activated LiMnO2 component is kinetically limited compared with that of LiMn0.42Ni0.42Co0.16O2 component, and the insertion process of lithium ion


Page 19 of 28 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


into activated MnO2 component is kinetically limited compared with that into Mn0.42Ni0.42Co0.16O2 component.47

Compared to the Nb-0 sample, the over-potential is mitigated after Nb doping

(Nb-0.01), especially during the stage of charging plateau, implying that Nb doping promotes the kinetic process of Li1.13Mn0.52Ni0.26Co0.10O2. Based on GITT technique, the lithium ion diffusion coefficient of Li1.13Mn0.52Ni0.26Co0.10O2 can be determined by Fick’s second law of diffusion and calculated by the following eq (1):47 2

D Li

4  m V   E    B M  S   M B S   E 

2

( = L2 / D Li ) (1)

Where mB and MB are the molecular weight and mass of the cathode material, VM is the molar volume of the Li1.13Mn0.52Ni0.26Co0.10O2 material, which is derived from the crystallographic data of 19.84 cm3 mol-1.

S is the active surface area of the electrode, which is 8.56 m2g-1 (Nb-0) and

5.23 m2g-1 (Nb-0.01) deduced from the BET testing, and L is the thickness of the electrode. Figure 8g,h shows variation in the lithium ion diffusion coefficient (DLi+) as a function of OCV in the 1st, 2nd charge/discharge processes. show similar trends of DLi+.

As shown in Figure 8g,h, Nb-0 and Nb-0.01 samples

Specifically, when the OCV is higher than 4 V, the DLi+ (Nb-0) of

the charging process is about 10-13 cm2 s-1, and then increases a little as the OCV increases to about 4.5 V, which is related to the plateau charging process; when the OCV is lower than 4.4 V, the DLi+ (Nb-0) of the discharge process is about 10-13 cm2 s-1, then decreases slightly as the OCV decreases to about 3.5 V, and then drops sharply to 10-15 cm2 s-1.

In contrast, DLi+ (Nb-0.01)

showed the same trend as that of Nb-0 sample, but the value of DLi+ (Nb-0.01) is higher than that of Nb-0 sample, indicating that Nb doping promotes the enhancement of electrochemical kinetics.

4. CONCLUSIONS Effects of Nb doping on the structure, surface chemistry, cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2 are studied in this work.

Nb doping significantly promotes

the cycle stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2.

After 200 cycles,

the discharge capacity of the pristine Li1.13Mn0.52Ni0.26Co0.10O2 is 117 mAh g-1, and the capacity



Page 20 of 28

retention ratio is 59.2% at the current density of 200 mA g-1, while it increases to 172 mAh g-1 and 78.8% after Nb doping.

Moreover, Nb doped samples exhibit improved rate performance.

Via

the XRD, SEM, EDS, XPS and GITT analysis, it is confirmed that Nb element has been successfully doped into the bulk of Li1.13Mn0.52Ni0.26Co0.10O2.

Nb5+ prefers to occupy the 3b site.

Doped Nb element effectively changes the lattice parameters, suppresses resistance rise (Rct), accelerates lithium ion diffusion (DLi+), and finally promotes cyclic stability and electrochemical kinetics of Li1.13Mn0.52Ni0.26Co0.10O2.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

†Muhammad

Zubair and

Guangyin Li contribute equally to this work.

Notes The authors declare no competing financial interest. Muhammad Zubair and Guangyin Li contribute equally to this work.

ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Grants 51622202, U1507107, 21503009 and 21603009), the National Key R&D Program of China (Grant No. 2016YFA0202500), Beijing Natural Science Foundation (B) (KZ201610005003), Guangdong Science and technology project (2016B010114001) and the Funding Projects for “Thousand Youth Talents Plan”, China Postdoctoral Science Foundation funded project (2017M610730) and Postdoctoral Science Foundation of Chaoyang Distinct, Beijing, China(2017ZZ-01-16).


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Table of Contents/Abstract Graphic

Nb element has been successfully doped into the bulk of Li1.13Mn0.52Ni0.26Co0.10O2. As a promising cathode material, Nb doped Li1.13Mn0.52Ni0.26Co0.10O2 sample exhibits good electrochemical kinetics and cycle stability, which provides effective methods for improving the electrochemical performance of lithium-rich layered oxides or similar high-energy materials with better electrochemical kinetics and cycle stability.


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Electrochemical Kinetics and Cycle Stability ... - ACS Publications

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