Modulating the Electrochemical Performances of Layered Cathode

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Modulating the Electrochemical Performances of Layered Cathode Materials for Sodium-Ion Batteries through Tuning Coulombic Repulsion between Negatively Charged TMO2 Slabs Zheng-Yao Li, Huibo Wang, Wenyun Yang, Jinbo Yang, Li-Rong Zheng, Dongfeng Chen, Kai Sun, Songbai Han, and Xiangfeng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15590 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Applied Materials & Interfaces

Modulating the Electrochemical Performances of Layered Cathode Materials for Sodium-Ion Batteries through Tuning Coulombic Repulsion between Negatively Charged TMO2 Slabs Zheng-Yao Lia,, Huibo Wangb, Wenyun Yangc, Jinbo Yangc, Lirong Zhengd, Dongfeng Chena, Kai Suna, Songbai Hana*, Xiangfeng Liub* a)

Neutron Scattering Laboratory, Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, P. R. China *E-mail: [email protected]; [email protected]

b)

College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China. *E-mail: [email protected] c)

d)

School of physics, Peking University, Beijing 100871, P. R. China.

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China.

Keywords: Layered oxides, Coulombic repulsion, neutron diffraction, cathode materials, sodium-ion batteries

ABSTRACT: Exploiting advanced layered transition metal oxide cathode materials is of great importance to rechargeable sodium batteries. Layered oxides are composed of negatively charged TMO2 slabs (TM = transition metal) separated by Na+ diffusion layers. Herein, we propose a novel insight, for the first time, to control the electrochemical properties by tuning Coulombic repulsion between negatively charged TMO2 slabs. Coulombic repulsion can finely tailor the d-spacing of Na-ion layers and material structural stability, which can be achieved by employing Na+ 1

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cations to serve as effective shielding layers between TMO2 layers. A series of O3NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 1.0, 0.9, 0.8, and 0.7) have been prepared and Na0.7Mn1/3Fe1/3Cu1/6Mg1/6O2 shows the largest Coulombic repulsion between TMO2 layers, the largest space for Na-ion diffusion, the best structural stability and also the longest Na-O chemical bond with weaker Coulombic attraction, thus leading to the best electrochemical performance. Meanwhile, the thermal stability depends on the Na concentration in pristine materials. Ex-situ X-ray absorption (XAS) analysis indicates that Mn, Fe and Cu-ion are all electrochemical active components during insertion and extraction of sodium-ion. This study enables some new insights to promote the development of advanced layered NaxTMO2 materials for rechargeable sodium batteries in future.

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1 INTRODUCTION The increasing demand for energy and electrical energy-storage makes sodium-ion batteries (SIBs) the promising electric power sources in future. Compared to lithium-ion batteries (LIBs), Cu foil can be replaced by Al foil as current collectors for both anode and cathode in SIBs and Na is also much richer and more widespread in the Earth’s crust than Li, thus resulting in a largely reduced cost1-7. Electrode materials are the supporters for energy-storage and conversion along with the insertion and de-insertion of the Li/Na-ion in both LIBs and SIBs2, 3, 8. The structural and physical/chemical properties of electrode materials can severely determine the final properties of rechargeable batteries such as reversible specific capacity, rate capability and lifetime. Thus, electrode materials are significantly important for rechargeable batteries. Layered transition metal oxides, NaxTMO2 (TM = transition metal), is one of promising groups of current cathode materials, due to the high reversible capacity, volumetric density and easy preparation in air9. Intercalation reaction is very typical that can be limited by solid-state diffusion of Li/Na-ion and solid-state diffusion is one of significant rate-limiting steps in rechargeable batteries10. Layered oxide materials are presence of open 2-deminational spaces that serve as hosts to accommodate guest ions. Thus structures that permit rapid Li/Na-ion motion are significantly favorable to design electrode materials with high-rate performance for rechargeable batteries10-12. In 2006, Ceder and co-workers firstly accomplished the idea that the increased spacing of Li-ion motion layers can achieve cathode with high-rate performance for LIBs12. Computational results by Ceder further proved that the activation energy barrier of Li motion remarkably decreases by expanding d-spacing of Li motion layers11,12. Zhou also successfully introduced an advanced Sn-doped Li-rich cathode material with enhanced rate performance by enlarging the spacing of Li-ion diffusion layers for LIBs13. Our group firstly synthesized the P2-NaxTMO2 cathodes with high-rate performance for SIBs and simultaneously revealed the effect of Co on P2-NaxTMO2 cathodes for SIBs on the basis of tunable d-spacing of Na-ion layers14,15. However, the increased d-spacing 3



of Li/Na-ion layers in reported results mainly caused by foreign-ion substitutions in the TMO2 sheets13-15. Electrostatic interaction is very universal and has been successfully applied in the nano-fields, such as the self-assembly process of isolated nano-monomer to complex nanostructures and also nanosheets synthesis by exfoliating layered materials10,16,17. Within the group of NaxTMO2 materials, the NaxTMO2 has been divided into two main groups including P-types with the prismatic coordination and O-type with octahedral coordination18. However, one of the critical features of layered NaxTMO2 is the alternative stacking sequences of interslabs (Na-ion layers) and slabs (TMO2 layers) along c-axis in both types, where two TMO2 sheets are separated by one Na-ion layer to form sandwich configuration19,20. In-depth understanding of NaxTMO2 can provide some new insights for material design and exploitation. The TMO2 sheets are not only negatively charged, but also can accommodate oppositely charged guest cations between TMO2 sheets10. Therefore, the Coulombic repulsive effect causes negatively charged TMO2 sheets to be far away from each other according to electrostatic interaction aspect, which can expand the interplanar distance between TMO2 sheets. This implies the d-spacing of Na-ion layers can be truly adjusted by tuning the Coulombic repulsion between negatively charged TMO2 sheets. The Coulombic repulsion can be effectively varied by Na+ cations resided at the spaces between TMO2 sheets, because Na+ cations can serve as shielding layers to alleviate such repulsion interaction21,22, in terms of the Na+ concentration in Na-ion layers. The Na+ cations in Na-ion layers hold the TMO2 sheets together and can mitigate the repulsive interaction between TMO2 sheets along c-axis9,10,23. Therefore, the more Na+ cations between negatively charged TMO2 sheets, the lower Coulombic repulsion by the effective screening of Na+ cations, due to the reduced effective charge numbers between negative and positive charges. On the other hand, in fact, during insertion/extraction of Li/Na-ion, the interlayer space along c-axis and lattice parameter c change gradually, in part owing to the varied Coulombic repulsion between TMO2 sheets9. These indicate the rationality and reliability of controlling the d-spacing of Na-ion layers by skillfully tuning Coulombic interaction using the 4


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screening of Na+. Herein, for the first time, we propose that the electrochemical performances of NaxTMO2 for SIBs can be effectively controlled by smartly tuning the Coulombic repulsion between negatively charged TMO2 sheets, on the basis of the above analysis and previous studies. This study offers some new insights into designing layered oxides cathodes with high performance for SIBs.

2 EXPERIMENTAL SECTION 2.1 Material Synthesis: The NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 1.0, 0.9, 0.8, 0.7) cathode material were synthesized by sol-gel approach using citric acid as chelating agent and metal nitrate as raw materials. The Citric acid and Ethylene glycol (EG) were dissolved into de-ionized water to obtain a clear solution. Then stoichiometric ratio of Manganese nitrate, Iron nitrate, Copper nitrate, Magnesium nitrate and sodium nitrate (5% sodium excess) were dissolved into de-ionized water to get a metal solution. The above two solutions were mixed and maintained at 80 °C for 5h and then dried at 150 °C overnight. The obtained dried gels were ground and calcined at 500 °C for 5 h, then calcined at 900 °C for 12 h in air. All the above chemicals were purchased from China National Medicines Corp. Ltd and were used without any further purification. 2.2 Material Characterizations: Powder X-ray diffraction (XRD) patterns were collected by the Persee instrument using Cu (Kα) radiation in steps of 0.01° and 2θ range of 10-70°. Neutron power diffraction data was collected by High Intensity Power Diffractometer (HIPD) of China Advanced Research Reactor (CARR) in China Institute of Atomic Energy. The wavelength is 1.4812 Å and the scanning step is 0.1°. Fullprof software was applied to refine the unit cell parameters based on Rietveld method. X-ray absorption spectra (XAS) were collected on the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). Scanning electron microscopy (SEM) measurements were performed on a Hitachi S-4800 apparatus (2 kV). X-ray photoelectron spectroscopy (XPS) analysis was tested on an ESCALAB MK II X-ray photoelectron spectrometer and Mg was the exciting source. Differential scanning calorimetry (DSC) analysis were carried out by using a DSC 200 PC 5



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(NETZSCH, Germany) at a temperature scan rate of 2℃ min-1 in flow N2 atmosphere. R2025 coin-type cells were fully charged to 4.2 V at 0.05C and opened in an Ar-filled glovebox. After washed by PC electrolyte carefully and removed from the surface of the electrode, the active cathode materials (3-5 mg) were collected to accomplish DSC experiments. Chemical compositions of the samples were analyzed by inductively coupled plasma/Atomic emission spectrometry (ICP-AES) (Prodigy 7, Leeman). 2.3 Electrochemical Measurements: The electrochemical performance was carried out by coin cells (R2025) composed of metal sodium plate as counter electrode and 1.0 M NaClO4 in propylene carbonate (PC)/ethylenecarbonate (EC) mixed solvent (volume 1:1) as electrolyte as well as glass fiber membrane GF/D (Whatman) as separator. The active materials were mixed with Super P carbon and poly(vinylidene fluoride) (PVDF) (mass ratio is 75:15:10) by N-methylpyrrolidinone (NMP) to form uniform slurry. The slurry was uniformly pasted on an Al foil then dried at 120°C in an vacuum oven for 12h. The sodium-ion batteries were prepared in Ar-filled glovebox. Galvanostatic charge-discharge tests were performed in the voltage range of 1.5-4.2V versus Na+/Na by an automatic galvanost at (NEWARE) at various

current

densities.

Electrochemical

Impedance

Spectroscopy

(EIS)

measurements were performed on an electrochemical workstation (PGSTAT302N, Autolab).

3 RESULTS and DISCUSSION 3.1 Material Design It is well believed that structure and composition determine the properties in materials science24,25, as shown in Figure 1a-d. The core idea here is that TMO2 sheets are negatively charged and strong Coulombic repulsion exist between adjacent TMO2 sheets in oxides. Such repulsive effect can lead to the distance change of interlayer distance according to the electrostatic interaction aspect. Thus the interlayer distance can be precisely controlled by employing cations to serve as shielding layers, such as Li+/Na+ cations10. To adjust the d-spacing of Na-ion layers and balance the Coulombic 6


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repulsion between negatively charged TMO2 sheets, it should keep the chemical compositions in TMO2 sheets constant to guarantee the same Coulombic repulsion and identical TMO2 layers. The rapid development of LIBs using Ni/Co as raw materials has largely increased the costs of Ni/Co26,27. In contrast, Mn and Fe are abundant and low prices, thus ideally suitable for large-scale application of rechargeable batteries in future24,28-30. Moreover, copper is less toxic and less expensive than Ni/Co. Previous studies have found the electrochemical activation of Cu2+/Cu3+ and also the excellent air-stability of Cu-based NaxTMO2 for SIBs31,32. Therefore, for the first time, we designed Co/Ni-free Na-controlled NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 1.0, 0.9, 0.8, 0.7) cathodes and demonstrated the impact of the controlled Coulombic repulsion by the screening of Na+ ions on the structural and electrochemical performances. Our interest is to adjust the d-spacing of Na-ion layers by tuning the Coulombic repulsive effect and to further understand the relationship between controlled electrochemical performances of and the resultant microstructures.

Figure 1. The properties depend on material structure type and composition. (a) tetrahedron of relationship; (b) P2-type layered oxides; (c) P3-type layered oxides; (d) 7



O3-type layered oxides.

3.2 Structural Characterization by XRD and NPD The NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 1.0, 0.9, 0.8, 0.7, denoted as MFCM-1, MFCM-2, MFCM-3, MFCM-4, respectively) samples were prepared by sol-gel approach. The obtained precursors were sintered at 500℃ for 5h and 900℃ for 12h in air, respectively. Figure 2a shows the X-ray diffraction (XRD) patterns of the as-prepared materials. All the diffraction peaks of each pattern can be indexed to the O3-type type of α-NaFeO2 structure with space group R3m29,33-37. Some labeled peaks by asterisks are indicative of impurity CuO, which has also occurred in P2-type materials32, indicating the limited solubility of Cu. The structural type is significantly dependent upon the preparation temperature and Na content9,38. However, it is interesting that all the samples are O3-types here, even for the MFCM-4 sample (Na = 0.7), which is different from previous reports where higher temperature and lower Na content are prone to be P2-type structures9,24,38. The chemical compositions of as-prepared materials are analyzed by ICP-AES, which are close to the designed materials, as shown in Table S1. The (003) diffraction peak shifts to lower angle, as shown in Figure 2b, indicating the expanded d-spacing from MFCM-1 to MFCM-4. The d-spacing of (003) peak is related to the distance of layer-to-layer between adjacent TMO2 sheets39,40. Zhou and co-workers have demonstrated that the Sn-doped Li-rich cathode material shows a lower 2-theta of (003) peak, but an increased d-spacing in comparison with Sn-free material13. The enhanced rate performance of Sn-doped cathode has been proved to derive from the accelerated lithium-ion motion by larger d-spacing of (003) peak13. X-ray diffraction (XRD) technique is a versatile method to probe crystal structure, but it is hard to distinguish light elements because of the existence of heavy elements. However, neutron interact with nucleus and is more sensitive to light elements such as Li, Na, O and adjacent elements such as Mn, Fe, Cu in elementary periodic table, due to significant differences of cross sections between different elements. Therefore, neutron power diffraction (NPD) technique is a more powerful tool to analyze the fine material structure compared to XRD. The 8


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pristine patterns of NPD are collected at room temperature and shown in Figure 2c. Figure 2d exhibits the shift of (003) peak of neutron diffraction toward low angle with reducing Na+ concentration and is well agreement with the results of XRD in Figure 2b.

Figure 2. Observed patterns of as-prepared materials. (a) XRD results; (b) The shift of (003) peak by XRD; (c) NPD results; (d) The shift of (003) peak by NPD.

To inspect the precise evolution of crystallographic parameters, the pristine data of NPD are refined based on single model of O3-type with space group R-3m regardless of the impurity CuO, as shown in Figure 3a-d. The relative crystallographic parameters are presented in Table S2-S5. Figure 4 and Table 1 show the evolution of refined crystallographic parameters. The lattice parameter a and lattice volume V shrink while lattice parameter c increases linearly from MFCM-1 to MFCM-4. The increase of parameter c derives from the increased Coulombic repulsion along c-axis between negatively charged TMO2 slabs by reducing Na+ cations concentration. Thus, it can be clearly inferred that the smallest Coulombic repulsion between TMO2 sheets 9



is MFCM-1, followed by MFCM-2 and MFCM-3, MFCM-4 is the largest one according to the Na+ concentration in Na-ion layers. The d-spacing of (003) peak in Table 2 (in the following section) increases from MFCM-1 to MFCM-4 and this is consistent with the shift of (003) peak in Figure 2b and Figure 2d. The increased Coulombic repulsion can account for such change of d-spacing of (003) peak. The decreased lattice volume is responsible for the increased material density d from MFCM-1 to MFCM-4, as shown in Table 1. Higher material density implies higher volumetric energy density, which is dramatically significant for rechargeable batteries9.

Figure 3. The refined NPD results. (a) MFCM-1; (b) MFCM-2; (c) MFCM-3; (d) MFCM-4.

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Figure 4. The evolutions of lattice parameters of as-prepared materials Table 1. Refined crystallographic parameters of as-prepared materials by NPD. Samples

a/Å

c/Å

V / Å3

d /g cm-3

MFCM-1

2.9965

16.0747

125.004

4.229

Factors / % Rp = 4.17 Rwp =6.21 Rp = 3.71

MFCM-2

2.9886

16.1184

124.678

4.568 Rwp =5.42 Rp = 4.40

MFCM-3

2.9824

16.1540

124.581

5.03 Rwp =6.62 Rp = 4.35

MFCM-4

2.9686

16.2607

124.100

5.653 Rwp = 6.18

3.3 Structural Reconstruction and Analysis The relationship between structure and properties is the foundation and various structures show different properties in materials science24,25, as shown in Figure 1a-d. However, for a given material, fine modulation in microstructure can significantly alter the final properties, in particular the rechargeable batteries, because guest ions must insert/extract the electrode materials and structural stability can affect the lifetime of batteries. Ceder and co-workers have proved that Li-ion transportation largely relates to the interplay spacing between oxygen layers since Li-ion motion is 11



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constrained to a 2D space in layered oxides, thus very small reduction of such d-spacing can lead to remarkable increase of activation energy barrier11,12. Therefore, to further understand the structural evolution, the crystal structures of MFCM-1 and MFCM-4 as examples are reconstructed based on the refined NPD results. The local environmental analysis including d-spacing of (003) peak, thickness of TMO2 sheets and Na-ion diffusion layers, chemical bond lengths, chemical bond angles are presented in Figure 5a-b and Table 2-3.

Figure 5. The refined crystal structures and individual alignment composed of NaO6 octahedrons or TMO6 octahedrons. (a) MFCM-1; (b) MFCM-4; (c) Alignment of NaO6 octahedrons; (d) Alignment of TMO6 octahedrons. Table 2. The evolutions of d-spacing of (003) peak, TMO2 sheets and Na-ion diffusion layers in materials. MFCM-1

MFCM-2

MFCM-3

MFCM-4

d(003) / Å

5.3582

5.3728

5.3847

5.4202

TMO2 / Å

2.1245

2.1242

2.1231

2.1225

Na-ion layer / Å

3.2337

3.2486

3.2616

3.2977

12


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The d-spacing of (003) peak shares a harmonic trend with the d-spacing of Na-ion layers in Table 2. Thus the d-spacing of (003) peak seems to be equivalent to the d-spacing of Na-ion layers. However, from the viewpoint of material structure, further analysis of the crystal structure in Figure 5a-b demonstrates that the d-spacing of (003) peak involves two parts including one TMO2 sheet and one Na-ion layer, thus specific size of each part is much more reliable than the inference only by the evolution of d-spacing of (003) peak. It is significantly urgent and rational that an equation should be proposed to quantitatively calculate the thickness of TMO2 sheets and Na-ion layers. Herein, two equations can be proposed, for the first time, on the basis of the crystal structure in Figure 5a-b:

d (003) = d ( slab ) + d (int erslab )

(1)

d ( slab ) = 2d (003) − (1 − 2 Z OX )c

(2)

Where d(003), d(slab) and d(interslab) represent d-spacing of (003) peak, thickness of single TMO2 sheet and d-spacing of single Na-ion layer, respectively. ZOX is coordinate of oxygen in structure, as shown in Table S2-S5, c is the lattice parameter, as shown in Table 1. The calculated results are presented in Table 2 and Figure S1. The d-spacing of (003) peak and the thickness of Na-ion layers (interslabs) both increase while the thickness of TMO2 sheets (slabs)reduces from MFCM-1 to MFCM-4. The increase of d-spacing of Na-ion layer mainly derives from the increased Coulombic repulsion between the negatively charged TMO2 sheets and also the shrinkage the thickness of TMO2 sheets9,10,14. The results suggest that the evolution of Coulombic repulsion significantly depends on the Na concentration, but the d-spacing of Na-ion layers dramatically relates to the Coulombic repulsion between TMO2 layers. Zhou and co-workers have also suggested that structural difference determines the Na diffusion kinetics25. In general, guest Li/Na-ion diffusion kinetics is rate-limiting step in solid-state materials for rechargeable batteries10,28. The d-spacing increase of guest-ion diffusion layer can effectively reduce activation energy barrier11,12, resulting in the accelerated Li/Na-ion motion and improved rate performance of electrode materials based on previous experimental and computational reports11-15. In addition, 13



in typical layered oxide cathode materials, the modified interlayer can also be beneficial to transfer charges, which is favorable for insertion and extraction of guest ions10. Material structural stability severely determines the cycling performance of electrode for rechargeable batteries and local environments of crystal structures including chemical bond lengths and chemical bond angles of interatoms are responsible for the material stability. Table 3 shows the corresponding evolutions of chemical bond lengths and bond angle in MFCM-1 and MFCM-4. Interatomic distances of Na-Na, TM-TM, TM-O and O-O in TMO6 octahedrons all reduce while Na-TM, Na-O all increase from MFCM-1 to MFCM-4. The interatomic distances of Na-Na, TM-TM and O-O equal the lattice parameter a, as shown by the three blue parallelograms in Figure 5a-b, and they share the same values23,41. In general, electrostatic repulsion not only exist in the adjacent layers, but also exist between Na+-Na+ and TM-TM cations in individual Na-ion layer and TMO2 layer10,23, such as strong electrostatic interactions between Na+-Na+ and also the Co4+/3+ charge ordering in NaxCoO2 material9,20,42. As shown in Figure 5a-b, the reduced distance of Na+-Na+ relates to the decreased electrostatic repulsion between Na+-Na+ in Na-ion layers19,20, because of the reduced Na concentration in constrained 2D space from MFCM-1 to MFCM-4. This indicates the impact of Na content on repulsion interaction in Na-ion layers. Previous studies have proved that the compressed chemical bond lengths such as TM-O and O-O means the improved bonding energy43, which is beneficial to stabilize the crystal structure and enhance the cycling performance of cathode for SIBs. In addition, Na-O length increases from MFCM-1 to MFCM-4 in Table 3. The larger Na-O length implies the weaker electrostatic attraction interaction between Na+ and O2-, which is also favorable to promote the insertion/extraction of Na-ion and improve the rate capability for SIBs. The shrinkages of O-O lengths in TMO6 octahedrons lead to the volume contraction of TMO6 octahedrons from MFCM-1 to MFCM-4, which can cause the reduced thickness of TMO2 sheets. The shrinkage of TMO2 sheets is also responsible for the expansion of Na-ion layers, except for the strong Coulombic repulsion by controlling Na+ cations. Figure 5c-d demonstrates the 14


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evolutions of chemical bond angles in the single Na-ion diffusion layer composed of NaO6 octahedrons and individual TMO2 sheet composed of TMO6 octahedrons, respectively. The bond angles of Na-O-Na and TM-O-TM both decrease from MFCM-1 to MFCM-4 and such reduction might originate from the shrinkages of TM-TM and TM-O chemical bond lengths. Thus the main factor that affects Na-ion diffusion kinetics here is the d-spacing of Na-ion layers, because of the same chemical compositions of TMO2 sheets in all materials, which is significantly different from the previous reports13-15,44.

Table 3. The evolutions of chemical bond lengths and bond angles in samples. Bonding length

MFCM-1

MFCM-2

MFCM-3

MFCM-4

Na-Na / Å

2.9966

2.9886

2.9785

2.9686

Na-TM / Å

3.1892

3.1928

3.1947

3.2066

Na-O / Å

2.3680

2.3697

2.3701

2.3774

TM-O / Å

2.0301

2.0261

2.0209

2.0165

TM-TM / Å

2.9966

2.9886

2.9785

2.9686

O1-O3 in TMO6 / Å

2.9966

2.9886

2.9785

2.9686

O1-O4 in TMO6 / Å

2.7398

2.7380

2.7320

2.7301

O3-O4 in TMO6 / Å

2.7398

2.7380

2.7320

2.7301

O3-O6 in TMO6 / Å

2.9966

2.9886

2.9785

2.9686

O1-O2 in TMO6 / Å

4.0604

4.0523

4.0418

4.0331

O1-O2 in NaO6 / Å

4.7360

4.7394

4.7402

4.7548

TM-O3-TM / °

95.125°

95.039°

94.986°

94.793°

Na-O3-Na / °

78.503°

78.185°

77.921°

77.267°

In terms of the above detailed analysis of each crystal structure, it can be concluded that skillfully tuning Coulombic interaction between negative charged TMO2 slabs by the screening of Na+ can adjust the d-spacing of Na-ion layer. The d-spacing of Na-ion layer expands while Na-O length extends with weaker electrostatic attraction from MFCM-1 to MFCM-4, which is of particular importance to promote Na-ion 15



motion and improve the rate capability of SIBs. The reduction of chemical bond lengths such as TM-O and O-O can stabilize the material structure, which can effectively enhance cycling stability of SIBs. Therefore, on the basis of above material structure analysis, it can be clearly predicted that MFCM-4 will show the best electrochemical performance, followed by MFCM-3, then by MFCM-2 and MFCM-1 is inferior, which will be evidenced in following discussion.

3.4 Oxidation State Analysis and Morphological Characterization Figure S2 shows the oxidation states of metal ions of the samples by X-ray photoelectron spectroscopy (XPS). The peak of 642.26eV and peak of 653.82eV assigned into Mn 2P3/2 and Mn 2P1/2 demonstrate the existence of Mn4+oxidation state in the oxides. The existence of Fe3+ can be manifested by the Fe 2P3/2peak and Fe 2P1/2 peak at around 710.94eV and 724.08eV. The Cu 2P3/2 peak and Cu 2P1/2 located at 933.42eV and 953.29eV state that the oxidation state of Cu is +2 in the oxides. Thus the average oxidation states of Mn, Fe and Cu in NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 0.7, 0.8, 0.9, 1.0) are +4, +3 and + 2, respectively. Previous studies have demonstrated that Mn, Fe and Cu are all electrochemical active components upon inserting/extracting Na-ion24,29-31,36,45. Figure S3 shows morphological analysis by SEM images. All the oxide materials exhibit plate-like morphology and the particles are around several micrometers size, but the thickness of particles is about 500-600nm.

3.5 Electrochemical Characterization

16


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Figure 6. (a) The initial charge/discharge profiles of as-prepared cathode materials; (b) comparison of Coulombic efficiency in the voltage range of 1.5-4.2V at 10mA g-1.

The sodium half cells were made by NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 0.7, 0.8, 0.9, 1.0) cathode materials and were used to evaluate the relationship between material microstructures and electrochemical properties. Figure 6a shows the initial charge/discharge profiles of as-prepared cathodes in the range of 1.5-4.2V at 10mA g-1. In general, step-like profiles are typical of layered NaxTMO2 cathodes, especially the P2-type cathodes, which are related to a series of phase transformations and Na+/vacancy ordering and coupled charge ordering5,9,19,20. However, smooth charge and discharge profiles of NaxMn1/3Fe1/3Cu1/6Mg1/6O2 here demonstrate the little effect of phase transformations in O3-type25. Hu and co-workers have evidenced that large difference in ionic radii of TM-ions, indicative of around 15%, and rational ratio are beneficial for generating ordered arrangement in TMO2 layers, thus the authors have employed Cr3+ and Ti4+ to synthesize the cation-disordered cathode material, resulting in

smooth

slopes

of

charge/discharge

curves42.

However,

in

cases

of

NaxMn1/3Fe1/3Cu1/6Mg1/6O2, the largest difference of ionic size between Cu2+ and Mn4+ is about 27% (Mn4+ = 0.53Å, Fe3+= 0.645 Å, Cu2+= 0.73Å, Mg2+= 0.69Å ), and the TM ratio of Mn: Fe: Cu: Mg is 2:2:1:1, thus NaxMn1/3Fe1/3Cu1/6Mg1/6O materials seem to be ordered structures. However, it is interesting that the results here are inconsistent with the smooth charge/discharge profiles and the previous report42. On one hand, foreign-ion substitution can result in a more disordered structure in TMO2 layers9. On the other hand, both Cu-ion and Mg-ion can hinder phase transformation and smooth the profiles upon inserting and extracting of Na-ion32,47. The initial charge/discharge specific capacity is 164/103, 162/109, 142/123 and 136/135mAh g-1 of MFCM-1, MFCM-2, MFCM-3 and MFCM-4, respectively. The average discharge voltage is around 3.1V for all the materials, close to that of the NaFe0.45Co0.5Mg0.05O2 cathode48, which may be associated with the Fe-ion and Cu-ion with high redox potential31,48, indicating little effect of Na concentration on average discharge voltage. The Coulombic efficiency increases obviously from MFCM-1 to MFCM-4, as shown 17



Page 18 of 31

in Figure 6b, indicating the impact of Na concentration on the initial efficiency. Hu and

co-workers

have

argued

that

the

low

Coulombic

efficiency

of

P2-Na0.66[Li0.22Ti0.78]O2 can be attributed to the reduction of electrolyte49. The initial Coulombic efficiency is also of great importance to evaluate rechargeable batteries, duo to the loss of electrochemical active Na upon charging and discharging. The corresponding Na content per formula unit of insertion/extraction are calculated according to the reversible charge/discharge capacity in Figure 6 and shown in Table S6. The results in Table S6 can be concluded as follows. The higher Na concentration in pristine NaxMn1/3Fe1/3Cu1/6Mg1/6O2 (x = 1.0, 0.9, 0.8, 0.7) materials: 1) the more Na removal upon charging, such as 0.66 Na for MFCM-1 compared to 0.51 Na for MFCM-4, but the lower Na insertion upon discharging, such as 0.41 Na for MFCM-1 compared to 0.50 Na for MFCM-4. 2) the higher immovable Na in structure after charging, such as 0.34 Na in structure for MFCM-1 compared to 0.19 Na in structure for MFCM-4, which means not all the Na can move upon charging and discharging. 3) the lower Na availability indicated by the ratio of effective insertion content to total content in pristine, such as 41.00% for MFCM-1 compared to 71.43% for MFCM-4, which means lower available active Na concentration and lower reversible capacity. Thus it clearly demonstrates that the evolution of Na insertion/extraction during charge and discharge severely depends on the initial Na content in pristine materials. These phenomena may be related to the available vacancies42 in pristine NaxMn1/3Fe1/3Cu1/6Mg1/6O2 materials, where the lower Na concentration, the higher available vacancy. Typically, O3-type always shows higher Na content than P2-type in pristine materials, which means P2-type has much more available vacancies than O3-type9,24. This is consistent with much higher reversible discharge capacities of P2-tpyes than O3-types24. The MFCM-4 shows the maximum of available vacancy and exhibits the largest discharge capacity compared to others, especially the MFCM-1. To the best of our knowledge, it is the first time that the above results and phenomena are reported and it can provide new insights to design advanced NaxTMO2 cathodes for SIBs in future. It is interesting that the charge/discharge curves become less smooth with decreasing Na content in pristine materials, indicating the influence 18


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of Na content on curve shapes. Results of material structure analysis have predicted the evolution of rate capabilities. MFCM-4 will be the finest, followed by MFCM-3, then by MFCM-2, and MFCM-1 will be the inferior. Figure 7, Table 4 and Figure S4 show the comparison of rate capabilities in the range of 1.5-4.2V at various current densities (1C = 200mAh g-1). The batteries made by the cathodes are firstly cycled three times at low current density of 10mA g-1 to activate the as-prepared cathodes. However, the Coulombic efficiency of all the cathodes during the activation process is improved in the following cycles. The experimental results are well agreement with the theoretical prediction by material structure analysis. The rate capability of MFCM-4 is superior to that of MFCM-3, followed by MFCM-2 and the last is MFCM-1. For example, MFCM-4 even can be charged and discharged at very high rates of 8C and 10C in comparison with MFCM-1. Bruce, Rojo and co-workers have proved that enhanced rate performance may be relevant to the larger Na disorder caused by Mg doping in the TMO2 sheets50. Guo and co-workers also found that the enhanced electrochemical performance derived from the synergetic contributions of multi-metal ions, where different metal-ion exhibits various effects48. Also multiple TM-ions in TMO2 layers can affect the Na ordering and kinetics in P2-type and higher ordering may be detrimental to the Na-ion diffusion kinetics51. However, the chemical compositions (TMO2 = Mn1/3Fe1/3Cu1/6Mg1/6O2 here) are identical to all the cathode materials in transition metal layers. Therefore, we believe the improved rate capability mainly derives from the expanded d-spacing of Na-ion layers achieved by smartly tuning the Coulombic repulsion between TMO2 sheets and also the weaker electrostatic attraction interaction between Na+ and O2- resulted from the extended Na-O chemical bond from MFCM-1 to MFCM-4. The effect of expanded d-spacing on Li/Na-ion diffusion has been detailed studied through experimental and computational results11-15. It also should be noted that the rate capabilities in this work are not very superior in comparison with some reported results, because of the electrochemical inactive Mg and the relatively lower electrochemical activation of Cu32,47,52. However, the significance is that such ideas and new sights in this work can be beneficial for 19



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further improving electrochemical performance of NaxTMO2 cathodes in future.

Figure 7. The rate capability comparison of as-prepared cathode materials at various current densities: a) MFCM-1, b) MFCM-2, c) MFCM-3, d) MFCM-4.

Table 4. Rate capability comparison of cathodes at various current densities Rate 20

40

100

200

400

1000

1600

2000

MFCM-1 (mAh g-1)

88

64

43

27

12

3

1

1

MFCM-2 (mAh g-1)

101

83

64

51

36

10

3

2

MFCM-3 (mAh g-1)

114

107

95

77

60

31

15

7

MFCM-4 (mAh g-1)

126

121

106

91

73

47

32

23

(mA g-1)

Material structural stability significantly affects the reversible behaviors and prolonged lifetime of rechargeable batteries. Figure 8a-b shows the cycling performances and capacity retentions of cathode materials in the range of 1.5-4.2V at 20mA g-1. The discharge specific capacities at 1st/100th cycle of MFCM-1, MFCM-2, MFCM-3 and MFCM-4 are about 88/65, 101/80, 114/93 and 126/108 mAh g-1, 20


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respectively, corresponding to capacity retentions of around 73.8%, 79.2%, 81.5% and 85.7%. The results indicate the improved capacity retention from MFCM-1 to MFCM-4. The cycling performance results are well consistent with theoretical prediction based on material structure analysis. The contractions of TM-O and O-O chemical bonds in TMO6 octahedrons imply the high bonding energy, thus can effectively strengthen the structural stability, which is favorable for improvement of the cycling performance43. This largely differs from the reports about substitutions by electrochemical inactive ions in TMO2 layers21 37 44, but there are identical TMO2 layers here. The electrochemical performance results clearly evidence that the successful materials structural design of NaxTMO2 materials with expanded d-spacing of Na-ion layers and the stable structures, which is achieved by tuning the Coulombic repulsion between negative charged TMO2 sheets through the screening of opposite Na+ in Na-ion layers. The report in this work is significantly different from previous studies13-15,44, because of utilization-free of foreign-ion dopants or substitutions, but careful and skillful consideration of the layered features in NaxTMO2 and Coulombic interaction between negatively charged TMO2 layers separated by oppositely charged Na+ cations in Na-ion layers.

Figure 8. Comparison of a) cycling stabilities and b) capacity retentions of as-prepared cathode materials at 20mA g-1

3.6 Thermal Stability Analysis and Diffusion coefficient of Na-ion 21



The thermal stabilities of the electrochemically desodiated MFCM-1 an MFCM-4 cathodes as examples were analyzed by DSC technique. The temperature of exothermic peak of MFCM-1 is about 239.2℃, which is a little higher than that of 233.4℃ of MFCM-4, as shown in Figure 9a. Thus, the MFCM-1 shows a higher thermal stability than MFCM-4. The more thermal stability of electrode, the higher safety of battery. Sun and co-workers have investigated the increased Ni composition is detrimental to thermal stability in layered cathodes, but Mn composition mainly contributes to the increased thermal stability53,54. The results reveal that Na concentration can affect the thermal stability of such cathodes, suggesting higher Na concentration in pristine material leads to higher thermal stability, which is in favor of improving the safety of rechargeable sodium batteries. The higher thermal stability of MFCM-1 might be relevant to the higher immovable Na in structure during charge compared to MFCM-4, as shown in Table S6.

Figure 9. a) Comparison of DSC results of the electrochemically desodiated cathodes, b) the EIS analysis of cathodes.

The diffusion coefficients of sodium-ion in MFCM-1 and MFCM-4 are analyzed by Electrochemical Impedance Spectroscopy (EIS) measurements. Figure 9b shows the Nyquist plots of MFCM-1 and MFCM-4 cathode materials. The Nyquist plots consist of a small semicircle at high frequencies and a linear part at low frequencies. The magnified plots in Figure 9b show the ohmic resistances (Rs) such as electrolyte. The semicircle at high frequencies along Z’ axis represents the electrochemical reaction 22


Page 22 of 31

Page 23 of 31 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


resistance

(Rct) and

the

linear

part at

low

frequencies

represents

the

diffusion-controlled Warburg impedance (Zw)55. The corresponding slop plots between Zánd frequency ω-1/2 are shown in Figure S5. Rs (MFCM-4) is about 7.90Ω, which is a little lower than 8.22Ω of MFCM-1 cathode. Rct (MFCM-4) is about 200Ω , which is much lower than 459Ω of MFCM-1 cathode. These results indicate the reduced resistance of MFCM-4 compared to MFCM-1 cathode. The diffusion coefficients of sodium-ion can be calculated by equations (3) and (4)55:

=

(3)

Z´ = + + σ /

(4)

Where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of transfer electron, F is Faraday constant, and C is the concentration of sodium ion, σ is the plot slop between Zánd frequency ω-1/2. The calculated diffusion coefficient of sodium-ion in MFCM-4 is about 3.30×10-14cm2/s, which is higher than 2.74×10-14cm2/s in MFCM-1. The expanded diffusion layers are responsible for the enhanced motion of sodium-ion. This is agreement with the improved rate capability of MFCM-4.

3.7 The Ex-situ XAS Analysis X-ray absorption spectroscopy (XAS) experiments were used to investigate the charge compensation mechanisms in MFCM-1 and MFCM-4 cathodes, as shown in Figure 10a-f. The Mn k-edge absorption exhibits little shift when firstly charged to 4.2V (C4.2V), but shifts to lower energy region close to Mn2O3 when discharged to 1.5V (D1.5V). This indicates Mn stays in +4 oxidation state during first charge, but can be activated during first discharge, thus Mn-ion can participate in charge compensation after first discharge45,56. The Fe K-edge absorption shifts to higher energy direction when charged to 4.2V and return back to initial region compared to Fe2O3 and fresh electrode when discharged to 1.5V. When charged to 4.2V, the shift of Cu K-edge absorption toward the higher energy position is obviously observed and 23



return back to initial energy region compared to the fresh electrode. The observed shifts of Mn, Fe and Cu suggest that the charge compensation is achieved by Mn4+/Mn3+, Fe4+/3+ and Cu3+/Cu2+ redox reaction during insertion and extraction of sodium-ion. The results are well agreement with previous studies31,32,36,45,48. Figure S6 compares the K-edge results of Mn, Fe and Cu in fresh electrodes. The main peak positions of each elements in MFCM-1 and MFCM-4 materials are very close to each other, suggesting the same oxidation state, which are consistent with the results by XPS analysis.

Figure 10. Normalized ex situ XANES spectra at a) Mn K-edge of MFCM-1 and b) Mn K-edge of MFCM-4 and c) Fe K-edge of MFCM-1 and d) Fe K-edge of MFCM-4 24


Page 24 of 31

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and e) Cu K-edge of MFCM-1 and f) Cu K-edge of MFCM-4 collected during first charge and discharge.

4 CONCLUSIONS In summary, comprehensively and smartly considering the layered features and Coulombic repulsion between negatively charged TMO2 layers in NaxTMO2 oxides, for the first time, we propose a new insight into modulating the electrochemical properties by skillfully tuning Coulombic repulsion between TMO2 layers to precisely adjust d-spacing of Na-ion layers and material stabilities. This idea is successfully achieved by employing positively charged Na ions served as effective shielding layers in interslabs to tailor the Coulombic repulsion between TMO2 layers. Also we found that the evolution of Coulombic repulsion is related to the Na concentration in pristine materials. Systematically investigated by material structures via NPD and Rietveld refinements, electrochemical measurements, thermal stability analysis and X-ray absorption (XAS) technique, the results reveal that the successful design and modulation of material structure by tuning the Coulombic repulsion between TMO2 slabs. The theoretical predictions and experimental results both demonstrate that MFCM-4 shows the largest Coulombic repulsion between TMO2 layers, the largest space for Na-ion diffusion, the most stability of crystal structure and also the longest Na-O chemical bond with weaker Coulombic attractive effect, thus leading to the best electrochemical performances including rate capability and cycling stability compared to others. Meanwhile, we also found that, for the first time, reversible specific capacity, Coulombic efficiency, the content evolutions of Na insertion/extraction upon charging and discharging as well as thermal stability all dramatically depend on the Na concentration in pristine materials. Lower Na concentration in pristine materials results in larger Coulombic repulsion between TMO2 sheets, higher reversible discharge capacity, higher Coulombic efficiency, higher available Na per unit upon charging and discharging, but lower immovable Na in structures after charging. However, lower Na content is unfavorable to thermal stability. Ex-situ XAS results indicate that Mn, Fe and Cu-ion are all electrochemical active components and 25



Page 26 of 31

the charge compensation is accomplished by Mn4+/Mn3+, Fe4+/Fe3+ and Cu3+/Cu2+ redox reactions. The results and findings in this work can provide some new insights to design layered oxides as advanced cathodes for SIBs in future.

5 ASSOCIATED CONTENT Supporting Information The chemical composition analysis of as-prepared by ICP-AES, refined crystal sites of MFCM-1, MFCM-2, MFCM-3 and MFCM-4 by the Rietveld method, the thickness evolutions of TMO2 slabs and Na-ion diffusion layers for as-prepared materials, the oxidation states of metal ions of the as-prepared materials by X-ray photoelectron spectroscopy (XPS), The SEM images of as-prepared materials,

the

evolution of Na concentration during first charge and discharge of as-prepared materials, the comparison of rate capability of as-prepared cathodes at various current densities, the corresponding slop plots between Zánd frequency ω-1/2 of MFCM-1 cathode and MFCM-4 cathode, the comparisons of Mn, Fe and Cu K-edge absorption of fresh cathodes in MFCM-1 and MFCM-4.

Notes The authors declare no competing financial interest.

6 Acknowledgments This work was supported by National Natural Science Foundation of China (Grant 11575192), the State Key Project of Fundamental Research (Grants 2014CB931900) of Ministry of Science and Technology of the People's Republic of China, the Scientific Instrument Developing Project of the Chinese Academy of Sciences (ZDKYYQ20170001), and “Hundred Talents Project” of the Chinese Academy of Sciences.

7 REFERENCES 26


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Cathodes with

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O3-P3 Phase

Transition for

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Modulating the Electrochemical Performances of Layered Cathode

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