O3 Biphasic Na0.67Mn0.55Ni0

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Article 0.67

0.55

0.25

0.2-x

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Li-substituted Co-free Layered P2/O3 Biphasic Na Mn Ni Ti LiO as High-rate Capability Cathode Materials for Sodium Ion Batteries Zheng-Yao Li, Jicheng Zhang, Rui Gao, Heng Zhang, Lirong Zheng, Zhongbo Hu, and Xiangfeng Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11983 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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The Journal of Physical Chemistry

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Li-substituted Co-free Layered P2/O3 Biphasic Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 as 5 6

High-rate Capability Cathode Materials for Sodium Ion Batteries 7 8 9

Zheng-Yao Li,1 Jicheng Zhang, 1 Rui Gao,1 Heng Zhang,1 Lirong Zheng,2 Zhongbo Hu1 and 10 1

Xiangfeng Liu1* 12 13 15

14 1

College of Materials Science and Opto-Electronic Technology, University of Chinese Academy

16 17 18

of Sciences, Beijing 100049, P. R. China 19 21

20 2

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

2 24

23

Sciences, Beijing 100049, China 25 27

26

ABSTRACT: The effects of Li substitution for Ti on the structure and 28 30

29

electrochemical performances of Co-free Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 (x = 0, 0.1, 0.2) 31 32

layered cathode materials for sodium ion batteries (SIBs) have been comprehensively 3 35

34

investigated. X-ray diffraction (XRD) and Rietveld refinement results demonstrate 36 38

37

that Li mainly occupies TM(TM=transition metal) sites in the crystal structure to 39 40

maintain the P2-structure majority and a small amount of Li enter Na sites to generate 41 43

42

some O3-phase. The discharge voltage, reversible capacity, rate capability, cycling 4 46

45

performance and Coulombic efficiency all have been improved by Li substitution, 47 48

which can be largely attributed to the integration of P2 and O3. Li substitution also 49 51

50

raises the average discharge voltage from 2.6V to 3.1V. Na0.67Mn0.55Ni0.25Li0.2O2 (L02) 52 54

53

56

5

can deliver an initial capacity of about 158mAh g-1 at 0.05C (12mA g-1) in comparison with Li-free sample (147mAh g-1). Even at the high rates of 480 (2C), 57

60

59

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1200 (5C) and 1920mA g-1 (8C), Na0.67Mn0.55Ni0.25Li0.2O2 (L02) can also display ca.

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ACS Paragon Plus Environment


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93, 65 and 38mAh g-1 discharge capacities, respectively. The rate capability is higher 6

5

4

than what’s reported in the previous Li-substituted cathode materials. In addition, 7 9

8

Li-substitution in transition metal sites generates more defects to maintain the charge 10 12

1

neutrality, which enhances the electronic conductivity and also has a positive effect on 13 14

Na-ion diffusion coefficient. The electronic conductivity and Na-ion diffusion 15 17

16

coefficient have been enhanced by 122% and 29%, respectively, with the substitution 18 20

19

of Li for Ti. Our results also show that the oxidation peaks become sharper with 21 2

increasing 23

Li

content,

which

indicate

the

feasibility

of

Na-ion

25

24

intercalation/de-intercalation in the integrated P2/O3 phase. This study also offers 26 28

27

some new insights into designing high performance cathode materials for sodium ion 29 30

batteries. 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

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INTRODUCTION 5

4

Due to the insufficient lithium resources and high-cost of lithium ion batteries 6 8

7

(LIBs) for the large-scale applications in electrical vehicles and portable devices, 9 10

specific attention has been paid on an alternative of energy-storage system. 1 13

12

Considering the high abundance, low cost and non-toxicity of sodium element, 14 16

15

sodium ion batteries (SIBs) has attracted great interest as promising candidates to 18

17

meet grid-scale energy storage demands in future1-4. The layered transition metal 19 21

20

oxides (NaxTMO2), which have been classified into two groups of P2 and O3 type by 2 23

Delmas5, has drawn much attention as electrode materials for SIBs. 25

24 26

Generally, the layered sodium-based oxides possess various structures including O3, 27 29

28

P2 and P3-type, which are largely related to the preparation temperature and the ratio 30 31

of sodium to transition metals6,7. Previous studies have shown that the type of the 34

3

32

structures has an important effect on the electrochemical property8-12. For examples, 35 37

36

Komaba and co-workers have found that P2- and O3-type Nax[Fe1/2Mn1/2]O2 can 38 40

39

exhibit different behavior and the reversible specific capacity of P2-type is better than 42

41

that of the O3-type cathode8. And the stability of P2 structure is higher than that of the 43 45

4

O3 phase9-12. Therefore, a series of layered-structure materials have been explored as 46 48

47

cathodes for SIBs13-34. Meanwhile, the advanced progress has also been achieved for 50

49

O3-structure materials35-38. For examples, the binary O3-NaFe0.50Co0.5O2 cathode35 51 53

52

can deliver over 100mAh g-1 capacity even at the very high rate of 30C. The tri-phase 54 56

5

O3-NaNi1/3Co1/3Fe1/3O2 cathode36 can display initial capacity of around 165mAh g-1 at 58

57

C/20 and about 80mAh g-1 even at very high rate of 30C. Ceder has reported a 59 60

quaternary O3-NaMn0.25Fe0.25Co0.25Ni0.25O2cathode38, which can show about 180mAh 3



1 2 3

g-1 capacity with great cyclic performance and further investigated the structure 6

5

4

evolution of O3-P3-O3’-O3” by in-situ X-ray diffraction (XRD). 7 9

8

Recently, multiple-phase composite has drawn great attention due to the excellent 10 1

performance in comparison with the single-structure material39-42. For instance, Zhou 13

12 14

and Wei have prepared a layered P3/P2 biphasic cathode material with improved 15 17

16

electrochemical performance and the cathode can deliver ca. 53mAh g-1 reversible 18 19

capacity at 651mA g-1 with good cycling property39. Johnson and co-workers have 21

20 2

reported the advanced layered P2/O3 intergrowth Na1-xLixNi0.5Mn0.5O2 cathode with 23 25

24

high rate performance40. Zhou has also reported a high-energy cathode of layered P2 26 28

27

and O3-type Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ composite, which can deliver 69mAh 30

29

g-1 at 5C (500mA g-1) and about 75% capacity retention after 150 cycles at 0.5C41. 31 3

32

The results make the multiple-phase composites promising candidates as high 34 36

35

performance electrodes for sodium ion batteries. 37 38

Considering the above ideas and the environmental friendliness as well as low cost 39 41

40

and toxicity, the Co-free cathode material is a better choice. Herein, we synthesized 42 4

43

Co-free layered P2 and O3 biphasic composite Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 (x = 0.1 45 46

and 0.2) by Li-substitution for Ti and investigated the electrochemical performance as 47 49

48

rechargeable sodium-ion batteries. Some new insights into the effects of Li 50 52

51

incorporation on the structure and electrochemical performances have been proposed. 53 54

Different from what’s observed in previous studies where the TM sites are fully 5 57

56

occupied by TM elements and is Li-free40,41, Li element here is designed to mainly 58 60

59

enter the transition metal (TM) sites. X-ray diffraction (XRD) and Rietveld

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refinement results prove that Li mainly occupies TM sites in the crystal structure to 5 6

maintain the P2-structure majority and a small amount of Li enter Na sites to form 7 9

8

some O3-phase. The relative content of the P2 and O3 phases has also been calculated 10 12

1

based on the two-phase model (Na-based P2 and O3 phase) refinements. The lattice 13 14

parameters a and c both decrease with increasing Li substitution for Ti, which is 15 17

16

different from what’s observed in the previous study41. The discharge voltage, 18 20

19

reversible capacity, rate capability, cycling performance and Coulombic efficiency 21 2

have all been improved with Li substitution, which can be largely attributed to the 23 25

24

integration of P2 and O3 induced by Li substitution for TM. In addition, 26 28

27

Li-substitution in transition metal sites generates more defects to maintain the charge 29 30

neutrality and enhances the electronic conductivity which also benefits to the 31 3

32

improvement of the rate capability. In addition, the oxidation peaks become sharper 34 36

35

with 37

increasing

Li

content,

which

further

indicate

that

the

38

intercalation/de-intercalation of Na-ion is more feasible with the substitution of Li. 39 41

40

This is also in well agreement with the enhanced electronic/ion conductivity and the 42 4

43

rate capability of Li-substituted samples. Last but not least, we also find the average 45 46

discharge voltage has been enhanced from 2.6 to 3.1V with the substitution of Li, 47 49

48

which is favorable to the increase of the energy density of SIBs. 50 51 52 53 54

EXPERIMENTAL SECTION 5 57

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Synthesis of Cathode Materials 58 60

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Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 (x = 0, 0.1, 0.2) cathode materials were synthesized

5



1 2 4

3

via a simple sol-gel route. They are designed as L0, L01 and L02, respectively. The 5 6

Citric acid and Ethylene glycol (EG) were dissolved into de-ionized water to obtain a 7 9

8

clear solution. Then stoichiometric ratio of Manganese acetate, Nickel acetate, 10 12

1

Titanium dioxide, Lithium acetate (5% lithium excess) and Sodium acetate (5% 13 14

sodium excess) were added into the above solution, respectively. The mixed solution 15 17

16

was maintained at 80 °C then dried at 150 °C overnight to obtain dried porous gel. 18 20

19

The dried gel was ground and calcined at 500 °C for 5 h, then calcined at 900 °C for 21 2

12 h in air. All the above chemicals were purchased from China National Medicines 23 25

24

Corp. Ltd and were used without any further purification, except the Titanium dioxide, 26 28

27

which was purchased from Aladdin Corp. Ltd. 29 30

Material Characterization and Analysis 31 3

32

Powder X-ray diffraction (XRD) patterns were tested by the Persee instrument 34 36

35

using Cu (Kα) radiation in steps of 0.01° and 2θ range of 10-70°. The unit cell 37 38

parameters were refined using Fullprof software based on Rietveld method. Scanning 39 41

40

electron microscopy (SEM) measurements were carried out on a Hitachi S-4800 42 4

43

apparatus (2 kV).X-ray photoelectron spectroscopy (XPS) analysis was measured on 45 46

an ESCALAB MK II X-ray photoelectron spectrometer using Mg as the exciting 47 49

48

source. The electrical resistivity measurements were made on disc-shaped pellets by 50 52

51

the two-point dc method at room temperature. Then the electric conductivity is 53 54

calculated by the value of electrical resistivity. X-ray-absorption fine structure (XAFS) 5 57

56

spectra were collected on the 1W1B beamline of the Beijing Synchrotron Radiation 58 60

59

Facility (BSRF Beijing, China).

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Electrochemical Measurements 5 6

Electrochemical studies were performed using R2025 coin cells. The metal Na was 7 9

8

used as the counter electrode, 1.0 M NaClO4 in propylene carbonate (PC) and the 10 12

1

glass fiber GF/D (Whatman) was used as the electrolyte and separator. 75 wt % 13 14

as-prepared active materials and 15 wt % super P conducted carbon and 10 wt % 15 17

16

poly(vinylidene fluoride) (PVDF) were mixed in N-methylpyrrolidinone (NMP) 18 20

19

solvent to produce a uniform slurry. Then the slurry was pasted uniformly onto an Al 21 2

foil and dried overnight at 120°C in a vacuum oven. The batteries were assembled in 23 25

24

Ar-filled gas glove-box. Galvanostatic charge-discharge tests were carried out in the 26 27

voltage range of 1.5-4.2V versus Na+/Na using an automatic galvanostat (NEWARE) 29

28 30

at different current density and room temperature. The cyclic voltammogram (CV) 31 3

32

and Potentiostatic Intermittent Titration Technique (PITT) were measured by the 34 36

35

electrochemical workstation (VMP3, Bio-Logic). 37 38

RESULTS AND DISSCUSSION 39 41

40

Structural characterization 42 4

43

The power X-ray diffraction (XRD) patterns of the as-synthesized P2/O3 biphasic 45 46

Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 (x = 0, 0.1, 0.2) materials are shown in Figure 1(a). 47 49

48

Almost all the diffraction peaks in the spectrums can be indexed in the hexagonal 50 52

51

layered P2-type structure with the space group of P63/mmc. However, with the 53 54

increase of Li content (x), extra peaks labeled by asterisks in XRD patterns of L01 5 57

56

and L02 in Figure 1(a) appear and the intensities also strengthen, which can be 58 60

59

assigned to the O3-structure phase in R-3m space group. The XRD results of the

7



1 2 3

P2/O3 biphasic composites are in good agreement with the recent report by Zhou41. In 5

4 6

comparison with Li-free pure P2-structure L0, the Li-substitution in transition metal 7 9

8

sites can induce the formation of O3 phase and the O3-phase component raises with 10 12

1

increasing Li content, considering the enhanced intensities of XRD diffraction peaks 14

13

for the P2/O3 biphasic composite39. The pristine XRD data also were further refined 15 17

16

on the basis of double-phase model associated with the Na-based P2 and O3 phase, 18 19

according to reported results40-44. Figure (b), (c) and (d) show the refinement plots of 21

20 2

L0, L01, and L02, respectively. As shown in Figure (b), (c) and (d), the calculated 23 25

24

plots fit the observed data very well and the refinement results of L01, L02 and L03 26 28

27

are summarized in Table S1, S2 and S3, respectively. The results demonstrate that the 29 30

relative mass content of P2 and O3 phase is 94.05% and 5.95% for L01 and 85.13% 31 3

32

and 14.87% for L02, respectively, which also further indicate that the content of O3 34 36

35

increases with the increase of Li substitution. 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 4

43

Figure 1. The observed XRD patterns and refinement results of as-prepared materials: (a) XRD 45 46

patterns, (b) L0, (c) L01, (d) L02. (e) The shift of (002) peak, (f) The linear relationship of lattice 47 49

48

parameters. 50 51 52 54

53

In spite of some Li-substituted materials have been reported40-42,44 the differences 5 57

56

should be carefully taken into account. According to the chemical formula, the 58 60

59

transition metal sites are fully occupied by Mn, Ni and/or Co from Johnson’ report40,44,

9



1 2 3

Zhou’ work41 and Komaba’s study42, thus the lithium may inhabit the Na sites in 5

4 6

crystal structure. In contrast, for the P2/O3 biphasic composite in this work, the 7 9

8

refined XRD results demonstrate the Li-ion is designed to mainly enter the transition 10 12

1

metal sites and just a little amount of Li-ion occupies the Na sites. This is consistent 14

13

with the previous studies40,43,44. Meng and co-workers have proved that, for the 15 17

16

Na0.80[Li0.12Ni0.22Mn0.66]O2material, most of the Li-ion occupied the transition metal 18 20

19

(TM) sites by neutron diffraction and Li nuclear magnetic resonance (NMR) 2

21

spectroscopy43. In addition, the (002) peaks shift to higher angle, as shown in Figure 23 25

24

1(e), indicating the decreased lattice parameters with the substitution of Li. The 26 28

27

refined results of XRD data further show that both the lattice the parameters, a and c, 29 30

decrease with increasing the Li content, as shown in Figure 1(f) and the Table 1. This 31 3

32

is consistent with the shift of (002) peak. However, the ionic size gap between Li+ 34 35

(0.76 Å) and Na+ (1.06 Å) is about 0.30 Å, which is almost twice larger than that of 38

37

36

0.16 Å between Li+ (0.76 Å) and Ti4+ (0.60Å). Thus if all the Li-ion enter the 39 41

40

transition metal sites, in general, the lattice parameters should increase, but the 42 4

43

experimental observation shows the completely different results and coincides with 45 46

the large difference of ionic size gap. On the basis of the above analysis, the main 47 49

48

P2-phase revealed by XRD patterns and refined results in Figure 1(a)-(d) and previous 50 52

51 53

studies40,43,44, we believe the Li-ion occupies Na and TM sites in crystal structure, but 54

the Li-ion mainly enter the transition metal sites to maintain the P2-majority structure 5 57

56

and also some Li-ion occupy the Na sites to generate a small amount of O3-phase in 58 60

59

the P2-majority. It should be noted that the parameters, a and c, of O3-phase also

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decrease with increasing Li content as shown in Table 1, which is similar to the case 5 6

of P2-phase indicating the substitution of Li also takes places in O3-phase. In addition, 7 9

8

the previous studies have demonstrated that the multiple phases co-exist at a atomic 10 1

scale40,41. 13

12 14

Table 1 Crystallographic lattice parameters refined by Rietveld method of as-prepared materials. 15 16

L01 18

17

Samples 19

Space group a /Å c /Å v /Å3 24

23

2

21

20

L0 P63/mmc 2.8924(2) 11.1824(9) 81.02(1)

L02

P2 O3 P2 O3 P63/mmc R-3m P63/mmc R-3m 2.8816(6) 2.9003(7) 2.8803(2) 2.8598(5) 11.1532(7) 14.3761(5) 11.1415(7) 14.3134(6) 80.204(7) 104.727(8) 80.045(8) 101.378(7)

26

25

28

Rp% Rwp% 29

Mass % 30

27

7.02 9.59

7.18 10.10 94.05

9.22 11.80 5.95

85.13

14.87

31 32 34

3

The particle size and morphology are tested by SEM as shown in Figure 2(a)-(c). 35 36

The particle surface is very smooth, but particle sizes are not uniform. Almost all the 37 39

38

particles of the three samples exhibit plate-like shape and the average thickness of 40 42

41

plate is about 500 nm. 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

11



1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38

Figure 2. The SEM images of as-prepared materials (a) L0, (b) L01, (c) L02. 39 40 41 42 4

43

The X-ray photoelectron spectroscopy (XPS) was carried out to investigate the 45 46

oxidation states of transition metals of the as-prepared materials, as shown in Figure 47 49

48

3(a) and (b).The Mn 2P3/2 main peak is around 642.19eV, which is close to the Mn4+ 50 52

51

in MnO2, indicating the Mn-ion is in the +4 state. Considering the possibility of lower 54

53

oxidation state of Mn-ion at higher temperature and similar size of Li-ion and Mn3+ 5 57

56

by Komaba42, the X-ray absorption spectroscopy technique was used to probe the 58 60

59

oxidation state of Mn-ion. The Mn K-edge results of the L02 power material further

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demonstrate that the Mn4+ exists in the oxide, as shown in Figure S1. The Ni 2P3/2 6

5

4

main peak is around at 854.5eV, which demonstrates the existence of Ni2+. The results 7 9

8

are in consistent with the previous studies16,20,21,45,46. The XPS results show the 10 1

existence of Mn4+ indicating the electrochemical inertness owing to the Mn4+ is hard 13

12 14

to be oxidized to higher oxidation state during sodiation and desodiation process. 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 53

52

Figure 3. The XPS spectra of as-prepared power materials (a) Mn, (b) Ni. 54 5 56 57 58

The first three galvanostatic charge/discharge curves of the as-synthesized L0, L01, 59 60

L02 cathodes in the range of 1.5 - 4.2V at 12mA g-1 (0.05C, 1C = 240mAh g-1) are 13



1 2 4

3

shown in Figure 4(a), (b) and (c), respectively. The initial discharge capacities 6

5

increase with the Li content (x), which is 147, 150 and 158mAh g-1 for L0, L01, L02, 7 9

8

respectively. The L0 cathode shows a little lower capacity decay than that of the L02 10 12

1

and L01 cathodes during the subsequent two cycles. In comparison with the pristine 14

13

Na2/3[Ni1/3Mn2/3]O2 cathode, based on the Ni4+/Ni2+ redox reaction, the discharge 15 17

16

capacity is lower than 173mAh g-1, which might be associated with the reduced Ni 18 19

content here47-49. As shown in Figure4, the three samples here all show much 21

20 2

smoother charge/discharge curves than that of the pristine Na2/3[Ni1/3Mn2/3]O2 cathode, 23 25

24

which displays a few obvious plateaus below 3.9V in charge/discharge profiles47-49. 26 28

27

However, among the three cathodes, the multiple voltage steps in charge/discharge 29 30

profiles gradually become a little clearer with the increasing the Li content (x), which 31 3

32

indicates that Li-ion cannot effectively smooth the charge and discharge profiles of 34 36

35

such materials but is better than the pristine Na2/3[Ni1/3Mn2/3]O2 cathode. Additionally, 38

37

as for the reported Na2/3Ni1/3Mn2/3-xTixO2 (0≤x≤2/3) materials50 with great cyclic 39 41

40

stability and rate capability, the charge and discharge profiles become much smoother 42 4

43

with increasing the Ti content from 1/6 to 1/3 and 2/3. However, the further 45 46

observation indicates that the obvious plateaus appear in the charge profiles of the 47 49

48

P2-Na2/3Ni1/3Mn1/2Ti1/6O2 with a lower Ti content. Considering the shape evolution of 50 52

51

profiles and the decreased discharge capacity of Na2/3Ni1/3Mn2/3-xTixO2 (0≤x≤2/3) 53 54

with increasing the Ti content and the electrochemical behaviors of L0 material in this 5 57

56

work, it is believed that the intermediate Ti content such as 0.2 can effectively smooth 58 60

59

the curves and shows no obvious impact on the capacity. In addition, for L02 cathode,

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the potential plateaus around 4.1V is very obvious and the plateau has been ascribed 5 6

to the P2-O2 phase transformation at highly charged state via the generation of 7 9

8

stacking faults by in situ XRD study47. However, a surprisingly high capacity of 10 1

200mAh g-1 for the P2-Na5/6[Li1/4Mn3/4]O2 material introduced by Komaba and 14

13

12

co-workers42 is beyond the theoretical value depending on the Mn4+/Mn3+ reaction 15 17

16

and a similar result of long potential plateau around 4.1V also occurs, thus the authors 18 20

19

proposes the charge compensation partially arises from the oxygen removal in the 21 2

crystal structure. The Li-ion behavior in the Na-based transition metal oxide has also 23 25

24

been discussed in previous literatures28-30,40,43 and Johnson40 has declared that there is 26 28

27

little impact on the electrochemical performance. Meng and co-workers have reported 29 30

that Li ions occupy the transition metal sites and can hinder the phase transformation 31 3

32

even charged to 4.4V for the Na0.80[Li0.12Ni0.22Mn0.66]O2 cathode43 and found the 34 36

35

reversible behavior of Li-ion motion. In addition, Amine, Sun and co-workers found 37 38

that 39

Li-ion

cannot

suppress

the

O3-P’3

phase

transition

in

41

40

O3-Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 cathode material37 upon charge and discharge 42 4

43

process. Therefore, further work need to deeply understand this experimental 45 46

observation. 47 48 49 50 51 52 53 54 5 56 57 58 59 60

15



1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50

Figure 4. The first three galvanostatic charge/discharge curves of as-prepared cathodes in the 51 53

52

voltage range of 1.5-4.2V versus Na+/Na at 12mA g-1(a) L0, (b) L01, (c) L02. 54 5 56 57 58

The average discharge operating voltage is 2.6V for L0, which is lower ca. 0.5V 59 60

than that of the Li containing L01 and L02 (about 3.1V), demonstrating Li 16


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substitution can improve the overage discharge operating voltage. Interestingly, the 6

5

result of L0 is very different from the reported P2-Na2/3Ni1/3Mn1/2Ti1/6O2 cathode50, 7 9

8

which possesses a much higher average working voltage ca. 3.7V. The results here 10 12

1

also show huge difference, when compared to the Li-substituted P2-type 14

13

Na0.80[Li0.12Ni0.22Mn0.66]O2 cathode in the range of 2.0-4.4V reported by Meng43 and 15 17

16

Na0.85Li0.17Ni0.21Mn0.64O2 in the 2.0-4.2V by reported Johnson44, where the materials 18 20

19

perform very smooth charge/discharge curves via a solid solution mechanism. The 21 2

discharge voltages of 3.1V for the Li-substituted L01 and L02 cathodes are very close 23 25

24

to 3.2V of the Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ cathode reported by Zhou41, but the 26 28

27

charge/discharge curves here largely differ from the P2-O3 composite cathode, which 29 30

show much smoother curves. 31 3

32

The CV plots of the first 3 cycles are shown in Figure 5(a)-(c). Multiple redox 34 36

35

peaks can be observed in the whole voltage range of 1.5-4.2 V, which is in well 38

37

agreement with the previous reports21,25,26,39,41. However, the results are a little 39 41

40

different 42

from

the

reported

P2-O3

composite

by

Zhou41

and

4

43

P2-Na0.5[Ni0.23Fe0.13Mn0.63]O2 electrode25 and more complex CV plots are obtained in 46

45

this work compared to the above materials. The Mn4+ is electrochemical inert above 47 49

48

2.0V during charge and discharge processes according to previous studies15,16,43,47-49. 50 52

51

Thus considering the results of XPS and the electrochemical active components of Mn 53 54

and Ni here, a series of oxidation/reduction peaks (3 couples for L0, 4 couples for L01, 5 57

56

5 couples for L02) between 3.0 V and 3.9V can not only be ascribed to the Ni4+/Ni3+ 58 60

59

and Ni3+/Ni2+ pairs with a double-electron process25, but also the Na+/vacancy

17



1 2 3

ordering process26,51. However, the Mn-ion valence here is +4 state on the basis of 5

4 6

XPS analysis, indicating the electrochemical inert. On the basis of the reported 7 9

8

studies15,16,43,47-49,52,53 and the obvious charge/discharge plateaus below 2.0V in this 10 1

work, the Mn4+ can be activated after the initial Na-ion insertion process, thus a 14

13

12

couple of oxidation reduction peaks below about 2.0V may come from the Mn4+/Mn3+ 15 17

16

redox pair16,25,41. Other visible weak peaks between around 2.0-3.0V are probably 18 19

associated with the Na+/vacancy ordering structure26,51. In contrast, the Li containing 2

21

20

materials L01 and L02 show more peaks of the Na+/vacancy ordering structure from 23 25

24

the CV plots. In comparison with the L0 cathode, there are extra peaks for the 26 28

27

Li-contained samples: one couple for L01 and two couples for L02 as shown in Table 29 30

S4. The position of each couple (denoted as 1 to 6) does not exhibit a significant shift, 31 3

32

as shown in Table S4. This may be associated with the more Na+/vacancy ordering by 34 35

the Li+ than Ti4+. In fact, P2-structure Na0.67MnO2 is vacancy-predominated material 38

37

36

where exists strong interaction of Na-vacancy ordering process in the structure54. The 39 41

40

results prove that the Li+ substitution cannot effectively hinder the ordering 42 4

43

transformation during the Na+ insertion/de-insertion process. The CV results are 45 46

consistent with the characteristics of above charge and discharge profiles. In addition, 47 49

48

with increasing Li content the oxidation peaks become sharper indicating the 50 52

51

intercalation/de-intercalation of Na-ion becomes more feasible with the substitution of 53 54

Li, which is also in well agreement with the enhanced rate capability of Li-substituted 5 57

56

samples. 58 59 60

18


Page 18 of 34

Page 19 of 34


1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 53

52

Figure 5. The first three CV plots of as-prepared cathodes in the voltage range of 1.5-4.2V versus 54 56

5

Na+/Na at 0.1mV/s (a) L0, (b) L01, (c) L02. 57 58

To further evaluate the electrochemical properties of the as-prepared cathodes, rate 59 60

capabilities were tested in the range of 1.5-4.2V at various current densities and the 19



Page 20 of 34

1 2 4

3

charge and discharge curves at various rate, as shown in Figure 6(a)-(d). The Li-free 5 6

L0 cathode can deliver discharge capacities of ca. 122, 114, 101, 80, 60, 15, 2mAh g 7

-1

9

8

at various current densities of 24, 48, 120, 240 480, 1200 and 1920mA g-1, 10 1

corresponding to 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 8C(1C=240mA g-1), respectively. 13

12 14

However, for the Li-containing cathodes, the rate capabilities are largely improved, 15 17

16

indicating the Li-substitution can enhance the rate performance of such kind of 18 19

material. The L01 can display about 130, 121,110, 99, 80, 42, 16mAh g-1 at 24, 48, 2

21

20

120, 240 480, 1200 and 1920mA g-1, which correspond to 0.1C, 0.2C, 0.5C, 1C, 2C, 23 25

24

5C and 8C, respectively. The L02 cathode performs the best rate capacities, which are 26 27

about 142,129, 120, 109mAh g-1 at the current density of the 24(0.1C), 48(0.2C), 30

29

28

120(0.5C), 240mA g-1 (1C), respectively. Even at very high rate of 480 (2C), 1200 31 3

32

(5C) and 1920mA g-1 (8C), L02 can also deliver the discharge capacities of ca. 93, 65 34 35

and 38mA h g-1, respectively, showing the outstanding rate capability. The discharge 38

37

36

capacity 65mAh g-1 at 5C (1200mA g-1) is almost equal to 69 mAh g-1 at 5C (500mA 39 41

40

g-1) of the excellent Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ cathode reported by Zhou41 and 42 4

43

another excellent cathode P2-Na0.5[Ni0.23Fe0.13Mn0.63]O2 by Passerini and Hassoun25. 45 46

However, by carefully comparing the values of current densities, an important thing 47 49

48

should be carefully noted that the current density of 5C in this work is 1200mA g-1, 50 52

51

54

53

which is more than twice larger than the 500 mA g-1 (5C) of the reported Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ cathode by Zhou41. In contrast, the current density of 5 57

56

500mA g-1 (5C) of Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ cathode is almost equal to the 58 60

59

480mA g-1 (2C) in this work, indicating the largely improved rate capabilities of

20


Page 21 of 34


1 2 3

Li-contained cathodes than the pure “Na sites” doping, especially the L02 cathode. 6

5

4

The results indicate that the rate capability of L01 and L02 is much higher than what’s 7 9

8

reported in the previous Li-substituted cathode materials41,43. The great rate 10 12

1

performance can be attributed to the P2/O3 biphasic composite materials, according to 14

13

the recent studies40,41. The results of enhanced electrochemical performance induced 15 17

16

by Li-ion are consistent with Johnson’s work40, where the possible mechanism has 18 20

19

been thoroughly discussed. In addition, Johnson 21

40

et al has declared the amount of

2

Li-ion insertion and de-insertion should be very low in comparison with the 23 25

24

sodium-ion because of the lack of lithium salt in electrolyte. Therefore, the 26 27

contribution of Li-ion itself on the capacity can be neglected.40 29

28 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 59

58

Figure 6. (a) The rate capabilities of the cathodes at various current densities. The rate curves of 60

cathodes in the voltage range of 1.5-4.2V versus Na+/Na at various density current: (b) L0, (c) L01, 21



1 2 4

3

(d) L02. 5 6 7 9

8

To further evaluate the long-term cycling performance, the half-cells are measured 10 1

for 50 cycles at 24mA g-1 (0.1C) and 240mA g-1 (1C) as shown in Figure 7(a),(b). The 13

12 14

L0 cathode offers 76% capacity retention after 50 desodiation and sodiation processes 15 17

16

at 0.1C, in comparison with the 84% and 83% for the L02 and L01, respectively. The 18 20

19

Coulombic efficiency of L0 is over 95%, which is a little lower than those of the 21 2

Li-substituted L01 and L02 cathodes (above 97%). When at 1C, the L0 cathode offers 23 25

24

84% capacity retention after 50 desodiation and sodiation processes at 0.1C, when 26 28

27

compared to the 87% and 85% for the L02 and L01, respectively. The results 29 30

demonstrate that Li substitution can improve both the cyclic performance and 31 3

32

Coulombic efficiency. 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60

22


Page 22 of 34

Page 23 of 34


1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 37

36

Figure 7. The cycling performance of the as-prepared cathodes in the voltage range of 1.5-4.2V 38 40

39 41

versus Na+/Na at 24 mA g-1 (a) and 240 mA g-1 (b), respectively. 42 43 45

4

The electronic conductivity and diffusion coefficient of Na-ion are tested to further 46 48

47

assess the effect of Li-ion on the cathode materials as shown in Table 2. Firstly, the 49 50

power materials were prepared to disc-shaped with diameter of 7mm by pressure of 51 53

52

1.2 ton. Secondly, the power electrical resistances were measured by the two-point dc 54 56

5

method. Finally, the power electrical conductivities were calculated by the values of 57 58

electrical resistances. In comparison with the Li-free L0 powder material, the 59 60

Li-substituted samples show the improved electric conductivity, which might be due 23



1 2 4

3

to the more defects to be produced to keep the charge neutrality by the lower valence 6

5

of Li+ than the Ti4+. The electronic conductivity of L02 has been enhanced by 122% 7 9

8

in compared to L0. The defect by substitution can largely improve the electronic 10 1

conductivity of cathode material55. The results prove that Li-substitution can improve 14

13

12

the electronic conductivity and this is in well agreement with previous study33. In 15 17

16

addition, the electrical conductivities here are very close to the reported results by 18 19

Amine, Sun and co-workers37, if considering the difference of the used measurement 21

20 2

technique. The diffusion coefficients of Na-ion were measured by Potentiostatic 23 25

24

Intermittent Titration Technique (PITT). The cells were discharged to 3.1V at 12mA 26 27

g-1 and titration time is 5 hour. The PITT experiment technique is based on one 29

28 30

dimensional transport according to Fick's second law and detail analysis can be seen 31 3

32

in previous literatures56-58. The diffusion coefficients of Na-ion can be calculated 34 36

35

according to the equation: D  38

37

d ln( I ) 4 L2 exp 2 , where L is the characteristic diffusion dt 

39 40

length, D is the diffusion coefficient. The slope 41 42

d ln( I ) is calculated by the linear dt

4

43

regions of the ln (I) vs. t plots, which can be obtained by the relationship of transition 45 46

current (I) depend on time (t) plots, as show in Figure 8(a)-(c). The measured 47 49

48

diffusion coefficients are around 10-14 cm2 s-1, which are well consistent with the 50 52

51 53

previous reports16,59,60. In compared to Li-free sample the diffusion coefficients of 54

Li-substituted samples increase a little (29% for L02), as shown in Figure 8(d) and 5 57

56

Table 2. The results can indirectly indicate that the Li-ion may occupy both Na and 58 60

59

TM sites. Otherwise, according to the larger ionic size of Li+ ion (0.76Å) than Ti4+

24


Page 24 of 34

Page 25 of 34


1 2 4

3

(0.60Å), the lattice distance should be increased and the Na-ion removal should be 5 6

accelerated during charge/discharge process. 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34

Figure 8. The relationship of transition current (I) depend on titration time (t) at 3.1V.(a) L0, 35 37

36

(b)L01, (c)L02, (d) the comparison of diffusion coefficient. 38 40

39

Table 2 The powder electric conductivity and diffusion coefficients of cathode materials. 41 42

Electrical

Electrical

Diffusion

resistance/

conductivity/

coefficient/

Ω·cm

S cm-1

cm2 s-1

L0

5.28× 105

1.89 × 10-6

7.56 × 10-14

L01

2.88× 105

3.47 × 10-6

8.28 × 10-14

L02

2.37× 105

4.21 × 10-6

9.81 × 10-14

43 45

4

Specimen 46 47 48 49 51

50

54

53

52

57

56

5

58 59 60

CONCLUSIONS 25



1 2 4

3

In summary, the advanced Li-substituted Co-free P2-Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 (x 5 6

= 0, 0.1, 0.2) cathode materials for SIBs have been synthesized by a simple sol-gel 7 9

8

method. The results prove that the Li-ion occupy both Li and TM sites, but mainly 10 12

1

enter the transition metal sites to maintain the main phase of P2-structure and some 13 14

Li-ion occupy the Na sites to generate partial O3-structure in P2-majority phase. The 15 17

16

rate capabilities of the composites are largely improved with increasing Li content as 18 20

19

well as the increase of O3-phase content. In particularly, L02 delivers ca.158mAh 2

21

g-1discharge capacity at 0.05C (12mA g-1) and show improved discharge operating 23 25

24

voltage 3.1V in contrast to the Li-free L0 sample (ca.147mAh g-1 and 2.6V). At 26 27

1200mA g-1 (5C), the capacity of L02 is largely increased from 15 to 65mAhg-1, 29

28 30

indicating the large enhancement of rate capability. The Coulombic efficiency of Li 31 3

32

substituted samples has been improved to over 97% in comparison with that of over 34 36

35

95% for L0. After 50 cycles, the cathodes show enhanced cycling performance and 38

37

the capacity retention of L02 and L01 are ca. 83% and 84% at 0.1C (24mA g-1), which 39 41

40

are higher than that of 76% for L0 sample. The results prove that Li can improve the 42 4

43

rate and cycling performance of such materials and lift the average discharge voltage. 45 46

Li-substitution in transition metal site can generate more defects to balance the charge 47 49

48

neutrality and improve both the electronic conductivity and the diffusion coefficient 50 52

51

of Na-ion. More single Li-substitution cannot effectively smooth the profiles of 54

53

charge and discharge and can deteriorate the Na+/vacancy ordering process during the 5 57

56

sodiation and de-sodiation processes. Thus, the Li-containing sodium-based insertion 58 60

59

oxides should be carefully taken into account to further improve the electrochemical

26


Page 26 of 34

Page 27 of 34


1 2 4

3

performance for advanced SIBs. 6

5

ASSOCIATED CONTENT 7 9

8

Supporting Information Available: The refined crystal sites and atom occupancies 10 12

1

of L0, L01, L02 by Rietveld method, the Mn K-edge analysis results of L02 power 13 14

material by way of the X-ray absorption spectroscopy technique and the comparison 15 17

16

of oxidation and reduction peaks of the three cathodes. This Supporting Information is 18 20

19

available free of charge via the Internet at http://pubs.acs.org. 21 2 23 25

24

AUTHOR INFORMATION 26 28

27

Corresponding Author 29 30

*E-mail:[email protected]; Tel +86 10 8825 6840 (X.L.). 31 3

32

Notes 34 36

35

The authors declare no competing financial interest. 37 38 39 41

40

ACKNOWLEDGEMENTS 42 4

43

This work was supported by National Natural Science Foundation of China (Grant 45 46

11575192), the State Key Project of Fundamental Research (Grant 2012CB932504 47 49

48

and 2014CB931900) of Ministry of Science and Technology of the People's Republic 50 52

51

of China, and “Hundred Talents Project” of the Chinese Academy of Sciences. 53 54

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O3 Biphasic Na0.67Mn0.55Ni0

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