Improved Cycling Stability and Fast ChargeâDischarge Performance

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Improved Cycling Stability and Fast Charge-Discharge Performance of Cobalt-Free Li-Rich Oxides by Magnesium-Doping Ting-Feng Yi, Yan-Mei Li, Shuang-Yuan Yang, Yanrong Zhu, and Ying Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11724 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

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ACS Applied Materials & Interfaces

Improved Cycling Stability and Fast Charge-Discharge Performance of Cobalt-Free Li-Rich Oxides by Magnesium-Doping Ting-Feng Yi, a* Yan-Mei Li, a Shuang-Yuan Yang, a Yan-Rong Zhu a, Ying Xie b* a

School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan,

Anhui 243002, PR China b

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of

Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China

∗

Corresponding author:

E-mail: [email protected] (Dr. Ting-Feng Yi) E-mail: [email protected] (Dr. Ying Xie).

1

ACS Paragon Plus Environment


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ABSTRACT

Layered

Li-rich,

Li1.17Ni0.25-xMn0.58MgxO2

cobalt-free

and

(0≤x≤0.05),

manganese-based

was

successfully

cathode synthesized

material, by

a

co-precipitation method. All prepared samples have typical Li-rich layered structure, and Mg has been doped in the Li1.17Ni0.25Mn0.58O2 material successfully and homogeneously. The morphology and the grain size of all material are not changed by Mg doping. All materials have a estimated size of about 200 nm with a narrow particle size distribution. The electrochemical property results show that Li1.17Ni0.25-xMn0.58MgxO2 (x=0.01 and 0.02) electrodes exhibit higher rate capability than pristine one. Li1.17Ni0.25-xMn0.58MgxO2 (x= 0.02) indicates the largest reversible capacity of 148.3 mAh g−1 and best cycling stability (capacity retention of 95.1% ) after 100 cycles at 2 C charge-discharge rate. Li1.17Ni0.25-xMn0.58MgxO2 (x= 0.02) also shows the largest discharge capacity of 149.2 mAh g−1 discharged at 1 C rate at elevated temperature (55 °C) after 50 cycles. The improved electrochemical performances may be attributed to the decreased polarization, reduced charge transfer resistance, enhanced the reversibility of Li+ ion insertion/extraction and increased lithium ion diffusion coefficient. This promising result gives a new understanding for designing the structure and improving the electrochemical performance of Li-rich cathode materials for the next-generation lithium-ion battery with good high rate cycling performance. KEYWORDS

Li-ion battery, Cobalt-free Li-rich cathodes, Magnesium doping,

Cycling performance, Fast charge-discharge property, Density functional theory 2


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1. INTRODUCTION With the global warming and energy crisis, low-emission electric vehicles (EVs) have attracted considerable interest and Li-ion batteries (LIBs) also have been considered as one of the most prospective candidates as the power sources.1,2 Unfortunately, low capacity of the cathode materials leads to a low energy density or power density of conventional LIBs, for instance LiCoO2 (~150 mAh g-1) or LiMn2O4 (~130 mAh g-1).3,4 One effective way of increasing the energy density or power density of Li-ion batteries is to develop the new cathode material with high the specific capacity. Recently, Li-rich cathode materials, solid solutions between Li2MnO3 and LiMO2 (M= Co, Ni and Mn) have been researched extensively due to the high capacities over 250 mAh g-1 cycled above 4.5 V.5,6 In xLi2MnO3·(1-x)LiMO2 structures, the Li2MnO3 component is able to provide extra lithium ions when charged above 4.5 V, and then increases the operational voltage and capacity.7,8 However, the low rate capability and poor cycling stability of Li-rich materials hinder their commercial applications, such as hybrid electric vehicles (HEVs) and electric vehicles (EVs).9 Accordingly, many efforts, such as smart microstructure design,10-12 surface modification13-16 and cation doping,17-22 have been explored to enhance the electrochemical property of Li-rich materials. As we know, magnesium element is abundant and less expensive than the cobalt, so Mg-doped Li-rich materials are expected to be positive electrode materials of Li-ion battery with low cost. Mg substitution not only lowers the polarization but also improves the overall insertion kinetics of some cathodes by increasing the electronic conductivity. Mg-doping has 3



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already

been

used

to

enhance

the

electrochemical

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property

of

Li-rich

manganese-based positive electrode materials. The electrochemical performance of Mg-doped cobaltiferous Li rich material by Li site or transition metal (TM) site doping were reported by some researchers, and most Mg-doped cobaltiferous electrodes exhibited good rate capacity and cycling stability at room temperature,23-28 but the electrochemical performance at elevated temperature was not reported. In addition, all samples mentioned above contains toxic and expensive cobalt metal, which impede their practical application. As we know, the cost of cathode materials occupies about 35% of the total cost of batteries.29 Hence, developing positive-electrode material using abundant elements with high performance is one of the key challenges for LIBs, and cobalt-free, Mn-based Li-rich positive electrode materials are one of the most attractive choices.30,31 Wang et al. reported the electrochemical property of Co-free and Mg-doped Li rich material (Li[Li0.2Ni0.2−xMn0.6−xMg2x]O2) by TM site doping. However, this doped material showed poor capacity at high rate,32 and the cycling performance at high rate or elevated temperature was not given. Yu et al. reported the electrochemical

properties

of

Mg-doped

and

Co-free

Li

rich

material

(Li1.4Mg0.1[Mn0.75Ni0.25]O2+δ) by Li site and TM site doping, and it showed excellent cycling stability and rate capacity.33 Unfortunately, the charge rate is only 0.2 C when at high discharge rate, and the electrochemical performance at elevated temperature was also not given. As we know, fast charge-discharge property (charge and discharged at high rate) has been considered as one of the most significant properties 4


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of LIBs for uprated applications, such as PHEVs, HEVs and EVs. Hence, it is advisable to study the electrochemical behavior of fast charge-discharge performance. A great challenge for the high-voltage electrode material is instability of currently used LiFP6-based electrolytes with carbonate solvent at elevated temperature.34 However, such electrolytes usually suffer from chemical degradation at elevated temperature, and then lead to the generation of HF, which can accelerate Li, Mn and Ni dissolution from Li rich electrode. Various reaction products, such as MnF2, NiF2, LiF, and polymerized organic species, can deposit the surface of Li rich electrode, and the directly affects to a battery safety. In addition, the crystal structure of nickel-containing layered cathode material changes from layer to spinel and rocksalt, and then O2 can be released at elevated temperature.35 The oxygen evolution results in thermal track due to reaction between the cathode and electrolyte, and then arouse fire or explosion of the cell. Hence, the evaluation of cycling satiability at elevated temperature of the Mg-doped Li rich cathode is important. Though the electrochemical performance at room temperature of Mg-doped Li rich materials mentioned above have been reported, the fast charge-discharge property and the cycling stability at elevated temperature are almost not reported. Mn-based Li-rich positive electrode materials were prepared by solid-state reaction,36 co-precipitation synthesis,37 hydrothermal method,38 sol–gel method,39 etc. However, the product prepared by solid-state method often suffer from irregular morphology, uncontrollable particle growth and agglomeration. Though other synthesis methods, such as hydrothermal or sol-gel synthesis could conquer the 5



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shortcomings of solid-state synthesis, they also have intricate synthesis routes and expensive cost. Among all the synthesis methods, the co-precipitation has been considered as one of the traditional and cheap methods to prepare the ultimate Li-rich materials. With these consideration mentioned above, we have developed a co-precipitation method to synthesize Mg-doped Li-rich material Li1.17Ni0.25Mn0.58O2. Using co-precipitation method, an ideal Mg-doped Li1.17Ni0.25Mn0.58O2 material with large discharge capacity, large capacity at high charge-discharge rate, well fast charge-discharge property and excellent cycling stability at elevated temperature can be easily synthesized. Also, the mechanism of electrochemical performance improvement is explored. These research results shed new light on how to improve the electrochemical performance of Mn-based Li-rich material by easy and reproducible Mg-doping.

2. EXPERIMENTAL The Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) were synthesized via co-precipitation method, as illustrated in Figure 1, and the more detail is given in the Supporting Information (SI). The structure of the materials were researched by XRD (X-ray diffraction) tests examined on a Rigaku instrument with Cu Kα radiation. Particle size, morphology and microstructure were performed using HRTEM (Tecnai G2 F20 S-TWIN) and SEM (SU8000). CR2025 coin-type half cells were used to perform the all electrochemical characterizations, and the more detail on the preparation of CR2025 coin cells is given in the SI. The galvanostatic charge and 6


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discharge tests were examined on LAND CT2001A (Wuhan Lanhe) between 2.0 and 4.8 V (vs. Li0/Li+) at different charge-discharge rates (where 1 C corresponds to 200 mA g-1). Electrochemical workstation (CHI 1000C) was used to carry out CV test (cyclic voltammetry) in 2.0-4.8 V with a scaning rate of 0.2 mV s-1. EIS (Electrochemical impedance spectroscopy) study was conducted by using a electrochemical working station (Princeton P4000) over a frequency between 10-2 Hz and 105 Hz at a potentiostatic signal amplitude of 5 mV. NaOH, NH4OH Dissolve

Water bath

Centrifuging, Drying, Ball-milling

Annealing

Mn2+

Ni2+

Presintering

SO42-

Li1.17Ni0.25Mn0.58O2

Figure 1. Schematic illustration of Li1.17Ni0.25Mn0.58O2

3. RESULTS AND DISCUSSION Figure 2a shows the XRD patterns of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05). Well-defined and sharp peaks can be easily found in all XRD patterns, indicating that all synthesized samples are universally well crystallized. The main peaks are indexed to the α-NaFeO2 phase with layered symmetry R-3m space group except for a number of peaks located at 20-25° which could be indexed to Li2MnO3 phase with C2/m 7



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space group. These results show that there is an ordered occupancy of Li, Mn, Ni atoms in the transition metal layers.39-41 In the XRD patterns, the distinct splitting (006)/(102) and (108)/(110) peaks reveal that each material have a representative layered structure.42 The I(003)/I(104) intensity ratio is usually applied to indicate the degree of positive ion mixing in the Li-layers.43 If the value of I(003)/I(104) is greater than 1.2, the positive ion mixing is considered as low and the layered structure is usually fine. The I(003)/I(104) intensity ratio of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) samples are 1.92, 1.93, 1.87 and 2.01, respectively. Obviously, all intensity ratio values exceed 1.2, indicating that the layered structure has a low positive

ion

mixing.44

Li1.17Ni0.25-xMn0.58MgxO2

can

be

denotes

as

0.41Li2MnO3·0.59LiNi0.51-2xMn0.49O2 (x=0, 0.01, 0.02, and 0.05). The related atomic arrangement and the structure of 0.41Li2MnO3·0.59LiNi0.51-2xMn0.49O2 cathode material are shown in Figure 2b. In the Li2MnO3 (C2/m), the Li ions occupy 2b, 2c and 4h site, and Mn ions occupy 4g site . In the LiMO2 (R3-m), the Li ions occupy 3a site, and transition metal ions (Mn and Ni) occupy 3b site. The bivalent nickel ions (Ni2+) easily occupy lithium sites in the layered LiMxNi1-xO2 (M=Mn, Co) cathode because the there is a very small difference of ionic radius between Li+ (0.76 Å) and Ni2+ (0.69 Å), resulting in the cation disorder.45 The ion radius of Mg2+(0.72 Å) is close to the that of Li+ ion and Ni2+ (0.69 Å) ion, but the ion radius of Mg2+ ion is less than that of Li+ ion, and greater than that of Ni2+. Hence, most probably, most inert alien Mg2+ ions preferentially occupy the Li+ site (3a site) in Li1.17Ni0.25-xMn0.58MgxO2 (x≤0.02), and few Mg2+ ions passively occupy the Ni2+ site (3b site). These results are 8


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confirmed by the XRD Rietveld refinement profiles of all cathode materials (Figure S1 and Table S1 of SI). It can be seen that Mg2+ prefers to occupy the 3a (Li+) site in the Li1.17Ni0.51-2xMn0.49O2 (x=0.01, 0.02) phase, and Mg2+ passively occupy 3b site in the Li1.17Ni0.51-2xMn0.49O2 (x=0.05) phase. We can infer that the introduction of inactive magnesium into the lithium (3a) site can stabilize the crystal structure, and restrain the migration of nickel ions into lithium sites, and improve the cycling performance of the Mg-doped Li rich material. Furthermore, no impurity peaks are detected for the doped ones. These results indicate that magnesium inserts into crystal lattice and the crystal structure does not change because of a small number of Mg-doping.

(108/110) (113)

(107)

(105)

(101) (006/102)

(104)

(b)

(003)

(a)

Intensity

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


(iv)

(iii) (ii)

(i) 10

20

30

40

50

60

70

80

2 Theta / degree

Figure 2 (a) XRD patterns of Li1.17Ni0.25-xMn0.58MgxO2 (i) x=0, (ii) x=0.01, (iii) x=0.02, (iv) x= 0.05 and (b) the

related atomic arrangement and the structure of

Li1.17Ni0.25-xMn0.58MgxO2 cathode material To determine where the manganese is located, DFT+U method implemented in the VASP code was applied. The plane-wave energy cutoff is set to 600 eV, and the sampling over Brillouin zone was handled by a (4×2×4) Monkhorst-Pack mesh. 9



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Spin-polarization was also considered. This set of parameters can guarantee a good convergence, and the optimization process will be repeated till the energy change of the system was less than 1.0×10-6 eV and the force on each atom less than 0.003 eV Å-1. The calculations in Table S2 of SI clearly showed that when Mg is located in the 4h site of Li2MO3 phase, the system is the most stable, while the anti-site configuration is 0.176 eV higher in energy, indicating that Mg species is difficult to substitute the transition metal ions in Li2MO3 phase. However, in the LiMO2 phase, the situation is different. The anti-site configuration and the one where Mg is located at 3a site do have rather similar energies, and the difference is only about 3 meV. Therefore, Mg species can be found either in the Li (3a) positions or in the Ni (3b) sites of LiMO2 phase, which is consistent with experimental observations. Figure 3 shows the SEM and TEM images of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) samples. All materials have a estimated size of about 200 nm with a narrow particle size distribution. It can not find evident differences in the particle size and morphology with the Mg doping. The small particle size and uniform size distribution can shorten the diffusion lengths of lithium ions and supply a big interface area between the active materials and electrolyte, and are conducive to the migration of lithium ions. It can be also seen that the lattice fringes of the body part are distinctly visible. The primary lattice fringe distance equal to 0.419 nm, which corresponds to the interplanar distance of the (020) plane for the Li-rich sample (Figure 3e).46 In order to prove the Mg-doped cathode materials homogeneously, EDS mapping of Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) powder is given in Figure 4. The 10


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presence of Mn, Mg, Ni and O elements are found. It can be found that not only transition metal elements, such as Mn and Ni, are similarly distributed on the surface of the grain but also Mg element is equably distributed in Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) cathode. It can be concluded that Mg has been doped into the Li1.17Ni0.25Mn0.58O2 material successfully and homogeneously. (a)

(b)

(c)

(d)

(e)

(f)

11



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Figure 3 SEM images of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) (a) x =0, (b) x=0.01, (c) x=0.02, (d) x=0.05 and (e, f) TEM image of Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) (a)

(b)

(c)

(d)

(e)

Figure 4. Elemental mapping images of Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) (a) SEM image, (b) Mn element, (c) Ni element, (d) Mg element and (e) O element. The first 3 cycles cyclic voltammograms (CV) of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) samples were given in Figure 5a-d. All samples present two oxidation and 12


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reduction peaks over a voltage range form 2.0 to 4.8 V. For all samples, two oxidation peaks appear at 4.0 and 4.79 V in the first anodic process: the former is attributed to the oxidation of transition metals (Ni2+ to Ni4+); the latter could correspond to a removal of the redundant Li from the TM layer along with coinstantaneous O2 evolution. In the following cathodic course, only one peak is founded at 3.8 V, which is related to the reduction of tetravalent nickel ions (Ni4+) to bivalent nickel ion (Ni2+). In the subsequent cycling, it can be found that the strongest peak at around 4.79 V vanishes, and occurs a new wide peak at about 4.0 V, corresponding to a reduction of Mn3+/Mn4+ and Ni2+/Ni4+ couples. From the previous reports, we can conclude that the non-reversibility can be connected with the removal of Li+ ion from Li2MnO3 structure and O2 evolutions.6,47 Based on the CV curves for the first 3 cycles, a steady current is arrived at the second cycle, and the 2nd and the 3rd cycle curves are almost overlapped. These results further verify the good cycling performance of the prepared samples. The polarization degree of the Li-rich electrode can be reflected by the potential differences between cathodic and anodic peaks. From CV curves, it can be calculated that the potential difference of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) samples are 310, 280, 290 and 400 mV, respectively. Obviously, suitable amount Mg doping enhances electrochemical kinetics and reduce the polarization of Li1.17Ni0.25Mn0.58O2 material.

13



0.8

(a)

0.9

1st 2nd 3rd

(b)

1st 2nd 3rd

0.6

Current / mA

Current / mA

0.6 0.3 0.0 -0.3

0.4 0.2 0.0 -0.2

-0.6

-0.4

2.0

2.4

2.8

3.2

3.6

4.0

4.4

4.8

2.0

2.4

2.8

3.2

0.6

(c)

1st 2nd 3rd

0.3

(d)

4.0

4.4

4.8

4.0

4.4

4.8

1st 2nd 3rd

0.2

Current / mA

Current / mA

0.4

3.6

Potential / V

Potential / V

0.2 0.0

0.1 0.0

-0.2

-0.1 -0.4 2.0

2.4

2.8

3.2

3.6

4.0

4.4

-0.2 2.0

4.8

2.4

2.8

3.2

3.6

Potential / V

Potential / V 10000 (e)

(i) (ii) (iii) (iv)

8000

3600

(f) (i)

3000

1000 800

(i) (ii) (iii) (iv)

Zre/ohm

6000

- Z'' / Ω

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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600

4000

2400 (iv)

1800

400

2000

0

1200

200 0 0

0

2000

4000

(ii) 200

400

6000

600

800

8000

600

1000

10000

1.5

(iii) 2.0

2.5

3.0

3.5

4.0

−0.5

Z' / Ω

ω

(g)

Figure 5. Cyclic voltammograms of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) (a) x =0, (b) x=0.01, (c) x=0.02, (d) x=0.05; (e) Nyquist plots (Inset is the enlarged Niquist plots), 14


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(f) curve chart of real part of the impedance (Zre) plotted against ω-1/2 at low-frequency region for Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) (i) x =0, (ii) x=0.01, (iii) x=0.02, (iv) x=0.05, and (g) equivalent circuit. To explore the effect of magnesium doping on the electrochemical property of Li1.17Ni0.25-xMn0.58MgxO2, EIS are measured. Figure 5e shows the EIS curves of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) electrodes measured before charging. The insets are the enlarged Nyquist plots of all cathode materials and the selected equivalent circuit applied to fit the EIS is shown in Figure 5g. The fitting curves are given in Figure S3 of SI. The plots of all samples comprise a straight line in the low-frequency region and a semicircle in the high-frequency region. Moreover, the straight line represents a semi-infinite Warburg diffusion process and the semicircle is related to the charge-transfer process. Rs is solution resistance, and W points to the semi-infinite Warburg diffusion impedance in the bulk, respectively. The high-frequency area diminutive semicircle is relevant to the Rf and CPE1, and Rf and CPE1 (Constant Phase Angle Element) reflects the lithium-ion migration resistance across the surface films and film capacitance.

48,49

The middle-frequency area is related to charge

transfer resistance (Rct) and interfacial capacitance (CPE2). The fitted Rct values of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) samples are 376.2, 259.6, 87.5, and 151.4 Ω, respectively. Apparently, the Mg doping decreases charge transfer resistance of Li1.17Ni0.25Mn0.58O2 electrode, indicating that the Mg doping reduces the charge transfer resistance between the cathode and electrolyte interface, and then increases the conductivity of the Li-rich electrode. The lithium ion diffusion is vital to 15



accelerate the Li-ion intercalation / deintercalation during cycling. The Li+ ion diffusion coefficients (DLi) are calculated based on the above EIS data. The DLi can be obtained with the data in the low-frequency area using the equations as follows: 50

Z re = Rct + Rs + σω DLi =

−

1 2

(1)

R 2T 2 2 A2 n 4 F 4CLi2 σ 2

(2)

The nomenclature for these equations is given in the SI. The Warburg factor is connected with the real part of the impedance (Zre), and it can be gotten from the slope of the lines in Figure 5f. The DLi values of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) estimated to be 7.88×10-16, 9.29×10-16, 2.10×10-15 and 5.48×10-16 cm2 s-1, respectively. Obviously, the Li-ion diffusion is improved because of the Mg doping. Obviously, Li1.17Ni0.25-xMn0.58MgxO2 (x= 0.02) may have the highest electrochemical activity during cycling due to the largest DLi value and smallest Rct value among all samples. 4.8 (a)

4.8 (b)

(iii)

(iv)

4.2

(ii) (i)

(iii)

4.2

3.6

(iii) (ii)

3.0

(iii) (ii)

3.0

0

60

(i)

(iv)

(iv)

(ii)

3.6

(i)

2.4

(i)

(iv)

Voltage / V

Voltage / V

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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2.4 120

180

240

300

360

0

60

120

−1

Specific capacity / mAh g

180

240

300

−1

Specific capacity / mAh g

Figure 6. (a) The first charge and discharge curves and (b) the second charge-discharge curves of the Li1.17Ni0.25-xMn0.58MgxO2(0≤x≤0.05)/Li cells (i) x =0, (ii) x=0.01, (iii) x=0.02, (iv) x=0.05. 16


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The first and second charge and discharge curves of the Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) are shown in Figure 6a and b. In the initial charge process below 4.4 V, the charge mechanism might be described as follows (Figure7a): Charge xLi 2 MnO 3 ⋅ (1 − x )LiMO 2  → xLi 2 MnO3 ⋅ (1 − x )MO 2 + (1 − x )Li Below 4.4V

(3)

In this electrochemical reaction below 4.4 V, lithium ions extract from the LiMO2 component associated with the Ni2+ ion to Ni4+ ion oxidation. However, lithium ions is difficult to diffuse into the Mn layer, and then lead to a electrochemical inactivity of Li2MnO3. In the initial charge profiles, it can be found a long potential plateau accompanied by the oxygen release at about 4.5 V for all samples, corresponding to a high irreversible oxygen loss and electrochemical activation of the Li2MnO3 component as Li2O from the layered lattice. The charge mechanism over 4.5 V is described follows (Figure 7b): Charge xLi 2 MnO 3 ⋅ (1-x )MO 2  → xMnO 2 ⋅ (1-x )MO 2 +xLi 2 O above 4.4V

(4)

During the discharge process, lithium ions are back into the rock salt structure accompanied by the Ni4+ to Ni2+ and Mn4+ to Mn3+ ion reduction. The total charge electrochemical reaction equation is as follows: Charge xLi 2 MnO 3 ⋅ (1 − x )LiMO 2  →(1 − x )Li+xMnO 2 ⋅ (1-x )MO 2 +xLi 2 O

(5)

The discharge mechanism is described as follows(Figure 7c,d): Discharge xMnO 2 ⋅ (1-x )MO 2 +Li   → xLiMnO 2 ⋅ (1-x )LiMO 2

17


(6)


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

(a)

Charge below 4.4 V

(b) Charge above 4.4 V

(c)

Discharge

(d) Discharge

Figure 7. Structural change schematic of the Li2MnO3·LiMO2 material during the initial cycle. (a) charge reaction below 4.4 V, (b) charge reaction above 4.4 V, (c) and (d) discharge reaction. (i) x =0, (ii) x=0.01, (iii) x=0.02, (iv) x=0.05. 18


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However, the 2nd charge curve and the initial one are different. The plateau over 4.5 V disappears in the subsequent charge process because the irreversible lithium oxide (Li2O) is taken from the layered Li2MnO3 structure. The initial discharge (charge) capacities of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) are 193.1 (308.9), 206.4 (329.5), 201 (228) and 120.9 (154.9) mAh g-1, and the corresponding coulombic efficiencies are 62.5%, 62.6%, 88.2% and 78.1%, respectively. Obviously, the Mg-doped materials have higher discharge capacities and coulombic efficiencies than those of pristine one. In the subsequent cycles, a improved coulombic efficiency for all Li-rich electrodes can be found because of the activation process. The differential capacity curves for the 1st and 2nd cycle are shown in Figure S2 of SI. The differences between charge and discharge plateau of the Li-rich electrode can be reflected by the potential differences between cathodic and anodic peaks. It is easy to find that a small amount of doping of Mg can reduce the polarization of Li1.17Ni0.25Mn0.58O2 electrode during cycling. Figure 8a shows the rate performance of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.01, 0.02, and 0.05) electrodes at different charge-discharge rates from 0.2 to 5 C rate. Among all samples, the Li1.17Ni0.25-xMn0.58MgxO2 (x=0.01 and 0.02) electrodes show a higher discharge capacities at various current densities (for the sample x=0.01, 206.4, 175.4, 154.2, 137.4 and 104.4 mAh g-1 at 0.2, 0.5, 1, 2, and 5 C rate, respectively), whereas the pristine Li1.17Ni0.25Mn0.58O2 electrode provides much smaller specific capacities at each rate (193.1, 157.4, 139.3, 127.3 and 101.7 mAh g-1 at 0.2, 0.5, 1, 2 and 5 C rate, respectively). In addition, the capacity retentions of the all samples are near 100% of 19



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Page 20 of 34

original value, when the charge-discharge rate was returned to 0.2 C. This result reveals

that

all

samples

exhibit

excellent

restorability.

However,

Li1.17Ni0.25-xMn0.58MgxO2 (x=0.01 and 0.02) electrodes also show higher capacity than the undoped one. The such excellent cycling performance and rate property of Li1.17Ni0.25-xMn0.58MgxO2 (x=0.01 and 0.02) is superior to the reported data of Mg-doped

Co-free

Li

rich

material

in

the

literature

(for

the

sample

Li1.2Ni0.19Mn0.59Mg0.02O2 charged at 0.1 C, 170 mAh g-1 at 0.5 C and 30 mAh g-1 at 5 C).32 Li1.4Mg0.1[Mn0.75Ni0.25]O2+δ material reported by Yu et al. showed excellent rate performance (230, 220, 170 mAh g-1 at 0.5, 1 and 5 C discharge rate, respectively), but unfortunately the charge rate is only 0.2 C at each discharge rate.

33

It is well

known that the electrode material can charge much more electric quantity at low charge rate than at high charge rate, and then the battery charged at low rate can supply high discharge capacity. However, in this work, the charge rate is equal to the discharge rate. Li1.17Ni0.25-xMn0.58MgxO2 (x=0.05) material exhibits the lowest discharge capacity among all electrodes at all rates, indicating that it is important to control the amount of Mg doping so as not decrease the capacity. To obtain more insight into the effect of the Mg doping, the EIS test results of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.02) samples after 1 and 60 cycles are given in Figure 8b,c, and the fitting curves are given in Figure S4 and Figure S5 of SI. After the first cycle, the high-frequency area semicircle becomes smaller. Furthermore, the low-frequency area straight line turns to be an clear arc-like profile representing a finite Nernst diffusion process on the surface of electrode, and a short straight line 20


Page 21 of 34

appears in the low-frequency area after 60 cycles (Figure 8c). It is therefore reasonable to infer that an obvious SEI (solid electrolyte interface) film is formed after the cycling. The equivalent circuit of Figure 8b, c and f is given in Figure 5e. (a)

(i) (ii) (iii) (iv)

0.2 C

200

0.5 C 2C

5C 0.2 C

100

1000

500

50 0

(i) (ii)

1500

1C

150

(b)

2000

-Z'' / Ω

Discharge Capacity / mAhg

-1

250

0

10

20

30

40

50

0

60

0

500

1000

(c)

(d) -1

Discharge Capacity / mAh g

360

-Z'' / Ω

2000

160

480

(i) (ii)

240

120

0

1500

Z' / Ω

Cycle number

0

300

600

900

120 (i) (ii)

80

(iii) (iv)

40

1200 0

-Z' / Ω

25

50

75

100

Cycle number 250 (e)

(f)

3000 200 2400 150

- Z'' / Ω

Discharge Capacity / mAh g-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


100

0

0

10

(i) (ii) (iii) (iv)

1200

(i) (ii) (iii) (iv)

50

1800

600

20

30

40

0

50

0

800

1600

2400

Z' / Ω

Cycle number

21


3200

4000

4800


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 22 of 34

Figure 8. (a) rate capacity of Li1.17Ni0.25-xMn0.58MgxO2(0≤x≤0.05) materials at different rats between 0.2 and 5 C (i) x =0, (ii) x=0.01, (iii) x=0.02, (iv) x=0.05; (b) and (c) Nyquist plots of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) materials after 1 and 60 cycles,

respectively

(i)

x=0,

(ii)

x=0.02;

Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) samples at

(d)

cycling

performance

of

2 C charge-discharge rate; (e)

cycling performance of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) at 1 C rate and 55 °C (i) x =0, (ii) x=0.01, (iii) x=0.02, (iv) x=0.05; and (f) Nyquist plots of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) cycled at 1 C rate and 55 °C after 50 cycles (i) x =0, (ii) x=0.01, (iii) x=0.02, (iv) x=0.05. According to the fitting results, the values of Rf of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.02) are 108.6 and 71.2 Ω, respectively while Rct values are 1306 and 1109 Ω. Based on the SEI film model principle, relationships among Rf, ρ (SEI film conductivity), l (thickness of SEI film), and A(surface area of the electrode) are as follows:51

l=

Rf A

ρ

(7)

If the change of electrode area and SEI film conductivity are not considered during the cycling process, it is likely that the film thickness is decreased because the Mg doping. Hence, it can be concluded that Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) has more conductive than that of pristine one during cycling. High Rf value also means more lithium was absorbed on the surface of pristine Li1.17Ni0.25Mn0.58O2 electrode during cycling, and then decrease the reversible capacity. In addition, the small Rct value of Li1.17Ni0.23Mn0.58Mg0.02O2 indicates a higher conductivity compared with the pristine 22


Page 23 of 34

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one.

The low Rf and

Rct reveal good electrode/electrolyte interface in

Li1.17Ni0.23Mn0.58Mg0.02O2 electrodes.52 This is why the Li1.17Ni0.23Mn0.58Mg0.02O2 indicates the better cycling performance and rate capacity than pristine one (Figure 8a and d). Fast charge-discharge performance has been considered as one of the most significant electrochemical performance of lithium-ion battery for uprated applications, such as PHEVs, HEVs and EVs. The cycling performance of Li1.17Ni0.25-xMn0.58MgxO2 (0≤x≤0.05) at 2 C charge-discharge rates are presented in Figure 8d. Clearly, the Li1.17Ni0.23Mn0.58Mg0.02O2 electrode obtains the largest initial discharge capacity of 155.9 mAh g-1, while the Li1.17Ni0.25-xMn0.58MgxO2 samples (x=0, 0.01 and 0.05) only reach 116.6, 142 and 62.4 mAh g-1. The discharge capacities of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.02) are 107.8 and 148.3 mAh g-1 after 100 cycles, respectively. The corresponding capacity retentions of both electrodes are 92.4% and 95.1%, respectively, revealing that the appropriate Mg doping can enhance the fast charge-discharge performance and cycling stability. Evidently, the wonderful rate-capability of Li1.17Ni0.25-xMn0.58MgxO2 (x=0, 0.02) originates from the decreased polarization on one hand, but most primarily from the immensely enhanced electronic conductivity and lithium ion migration ability. The Li1.17Ni0.25-xMn0.58MgxO2 (x=0.05) shows the poorest electrochemical property, and the possible cause is that the over doping jeopardizes the stability of the crystal structure. The cycling property of Li1.17Ni0.25-xMn0.58MgxO2(0≤x≤0.05) discharged at 1 C rate (charge rate is 0.2 C) at elevated temperature (55 °C) is given in Figure 8e, and then the EIS was tested after 50 cycles (Figure 8f). The discharge capacity of the 23



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

pristine Li1.17Ni0.25Mn0.58O2 decays from 174.7 mAh g-1 at initial cycle to 128.3 mAh g-1 at the 50th, which is only 73.4% of the initial capacity. By contrast, the initial discharge capacity of Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) is 214.1 mAh g−1 and it decreases to 149.2 mAh g−1 at 50 cycles, which maintains a capacity retention of 69.7%. Though the Li1.17Ni0.23Mn0.58Mg0.02O2 has a slightly lower capacity retention than the undoped one, the doped one exhibit larger discharge capacity than the pristine cathode over the whole cycles. From Figure 8f, it can be found that the plot of all samples comprises a semicircle in the high frequency region, an arc-like profile and a straight line, which is similar to Figure 8c, and the fitting curves are given in Figure S6 of SI. This obviously indicates that an SEI film can be also formed. According to the fitted results (Table S3), Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) has the lowest Rf and Rct values among all samples, reflecting the best conductivity and lowest electrochemical polarization. It can be inferred that the capacity fading can be suppressed with the Mg substitution. This is the reason why Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) has the best high rate cycling performance and highest discharge capacity at elevated temperature.

4. CONCLUSIONS The layered Li-rich, cobalt-free and manganese-based positive electrode material, Li1.17Ni0.25Mn0.58O2, was successfully synthesized by a co-precipitation method and further modified with Mg doping. The morphology and the grain size of all material are not changed by Mg doping, and materials have a estimated size of about 200 nm with a narrow particle size distribution. Li1.17Ni0.25-xMn0.58MgxO2 24


Page 24 of 34

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(x=0.02) shows a significant increase in cycling stability, fast charge-discharge performance, rate capability and discharge capacity compared with the undoped one even at elevated temperature. The improved electrochemical performances of Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) is attributed to the decreased electrode polarization and charge transfer resistance, and then improve the reversibility of Li+ ion intercalation-deintercalation and lithium ion diffusion coefficient, leading to its correspondingly larger rate capacity. The fast charge-discharge performance, high cycling stability, excellent high-rate cycling property at elevated temperatures and low cost make the Li1.17Ni0.25-xMn0.58MgxO2 (x=0.02) be a positive electrode material with great promise for practical high-power lithium-ion battery, and the same strategy used in this work can be conducive to developing and exploring other desired Li-rich cathode materials with high cycling stability and excellent rate capacity. ■ ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Material

preparation

of

Li1.17Ni0.25-xMn0.58MgxO2,

battery

preparation,

nomenclature for equations (1) and (2), Rietveld refinement profiles, Rietveld refinement results of XRD data, elemental mapping images, differential capacity vs voltage plots, fitting curves of EIS, fitted results of EIS, and total energies for different arrangements of Manganese in the lattice. (PDF)

■AUTHOR INFORMATION 25



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

Corresponding Author *E-mail: [email protected] (Dr. Ting-Feng Yi); [email protected] (Dr. Ying Xie). Tel.: +86 -555 2311807. Fax: +86 555 2311552

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported by Anhui Provincial Natural Science Foundation (no. 1508085MB25), the National Natural Science Foundation of China (nos. 51274002 and 51404002), Anhui Provincial Science Fund for Excellent Young Scholars (no. gxyqZD2016066), and the Program for Innovative Research Team in Anhui University of Technology (no. TD201202).

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