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Mn-based cathode with synergetic layered-tunnel hybrid structures and their enhanced electrochemical performance in sodium ion batteries Zhen-Guo Wu, Jun-Tao Li, Yan-Jun Zhong, Xiao-Dong Guo, Ling Huang, Ben-He Zhong, Daniel Adjei Agyeman, Jin-Myoung Lim, Duho Kim, Maenghyo Cho, and Yong-Mook Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 9, 2017

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ACS Applied Materials & Interfaces

Mn-based cathode with synergetic layered-tunnel hybrid structures and their enhanced electrochemical performance in sodium ion batteries Zhen-Guo Wu †, Jun-Tao Li*,‡, , Yan-Jun Zhong†, Xiao-Dong Guo*,†, Ling Huang§, Ben-He Zhong†, Daniel-Adjei Agyeman#, Jin-Myoung Lim£, Du-ho Kim£, Maeng-hyo Cho£, and YongMook Kang*,# *

†

School of Chemical Engineering, Sichuan University, Chengdu, 610065 (PR China);

‡

College of Energy, Xiamen University, Xiamen,361005 (PR China)

§

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen,361005 (PR China)

#

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul, 04620, (Republic of Korea);

£

Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-gu, Seoul, (Republic of Korea) KEYWORDS: sodium ion battery, Manganese based cathode, layered-tunnel hybrids, synergistic approach, ultrahigh rate capability.

ABSTRACT:A synergistic approach for advanced cathode materials is proposed. Sodium manganese oxide with a layeredtunnel hybrid structure was designed, synthesized and subsequently investigated. The layered-tunnel hybrid structure provides fast Na ion diffusivity and high structural stability thanks to tunnel phase, enabling high rate capability and greatly improved cycling stability compared to pure P2 layered phase, while retaining the high specific capacity of P2 layered phase. The hybrid structure provided a decent discharge capacity of 133.4 mAh g-1 even at 8 C, which exceeds the reported best rate capability for Mn based cathodes. It also displayed an impressive cycling stability, maintaining 83.3 mAh g-1 after 700 cycles at 10 C. Theoretical calculation and PITT demonstrated that this hybrid structure helps to enhance Na ion diffusivity during charge/discharge, resultantly attaining the unprecendented electrochemical performance.

1.INTRODUCTION Sodium ion batteries (SIBs) provide many advantages, such as cost effectiveness, extensive sodium (source material) reserves, and similar chemical characteristics to lithium ion batteries (LIBs). It is widely regarded as one of the most suit1-9 able batteries for large scale energy storage systems. Among various cathode materials, Mn based P2 layered materials are the most widely utilized ones because of their low cost, low toxicity, high capacity, and relatively high volt10,11 age. However, Jahn-Teller distortion related to the pres12 ence of Mn(III) and sodium ion diffusion limited to one dimensional pathway inside its layered structure significantly 13 deteriorates its cycling stability and rate capability. Several strategies have been suggested to address these serious issues, including partial substitution of other metal ions (Fe, 14 Co, Ni, Mg, etc.) for Mn and modification of the layered 15 structure. However, serious challenges still remain to realize Mn based layered cathodes with acceptable cycling stability and rate capability. Recently, heterostructured materials combining layered and spinel structures have attracted significant attention as the promising cathode candidates for LIBs. Because the spinel structure includes three dimensional (3D) fast Li ion diffusion pathways that contribute to improved cycling stability 16-19 as well as rate capability, layered-spinel hybrid structure has turned out to be an effective strategy to improve both electrochemical performance and stability of cathode materials. As to SIBs, some pioneer work about cathodes with heterostructure also showed interesting results and demonstrated the synergy effect of various layered structures. Though enhanced cycling stability and rate capabilities had

been obtained with dual layered structures (P2/O3, P3/P2, 20,21 etc.), the rate performances are still hampered by the narrow two-dimensional ion diffusion path. Considering that the crystal structures of layered cathode materials for LIBs are analogous to those for SIBs, the design strategy based on hybrid or composite structure seems to also help get over the limitation of single crystal structure for SIBs in terms of not only electrochemical performance but also stability. However, the significant ionic radius difference between Na and Li ions can negate the merits of hybrid structure and so we need to investigate whether hybrid structures can also work for SIBs. Spinel and subsequent hybrid structures have been 22 reported to be difficult to realize. Fortunately, another sodium manganese oxide with tunnel structure (Na0.44MnO2, Pbam) can substitute for that with spinel structure because it can be easily synthesized and has larger diffusion channels 13 for sodium ion transport. Some pioneering studies have already demonstrated that the Na0.44MnO2 with tunnel structure delivers excellent cycling stability and superior rate capability even if its theoretical capacity is limited below about -1 23-25 121.0 mAh g . In addition, it has been observed that both P2-type layered structure and tunnel structure can be simultaneously stabilized when the ratio of sodium ions to man13 ganese ions is reduced. Based on the above consideration, a novel class of design strategy based on hybrid structure was proposed and examined theoretically and experimentally to find out more advanced cathode materials for SIBs. The cathode composites with a characteristic hybrid structure composed of layered and tunnel structures were synthesized through a simple coprecipitation method. Herein, we first investigated the

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synergistic effect of hybrid structure on the electrochemical performance of P2 type Mn-based layered materials for SIBs. The effect of metal ion doping on the final structure was carefully studied and optimized as well. As a result, it was successfully demonstrated that its synergistic effect could overcome the electrochemical limitation of each single structure. First-principles calculations based on density functional theory validated that the corresponding hybrid structure assembled with layered and tunnel structures is superior to pure layered structure in every aspect. The optimized Na0.6MnO2 composite incorporating layered and tunnel structures provides the best rate capability for Mn based -1 layered cathodes reported to date: maintaining 133.4 mAh g -1 at 8.0 C which corresponds to 1600 mA g . Significantly enhanced cycleability was also achieved, maintaining 135.4 mAh -1 g after 100 cycles at 1.0 C (Its initial discharge capacity was -1 -1 193.6 mAh g at 1.0 C) and 83.3 mAh g even after 700 cycles at 10 C.

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New Materials Co., Ltd., Jiangsu, China). The cells were galvanostatically charged and discharged on a battery test system (LAND-2001A, Land Electronic Co., Ltd., Wuhan, China) with cut off voltage 1.5–4.3 V (vs. Na/Na+) at 30 °C. Potentiostatic intermittent titration technique (PITT) experiments were conducted on a CHI660D electrochemical working station. After charging to 4.3 V, the as-prepared materials were subjected to successive potential steps in the initial discharge process to provide chronoamperometric curves (I-t). The potential difference for each step was ΔE=50 mV, and the time span was 3600 s. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (Zennium IM6) at 100 kHz–10 mHz with an alternating current amplitude 5 mV. 2.3 First-Principles Calculations The characteristics of ionic diffusion inside tunnel and pure layered NaxMnO2 was investigated by spin-polarized density functional theory (DFT) calculations using Vienna ab initio simulation package (VASP) with the projector augmented-wave (PAW) method. For the exchange-correlation functional, the Perdew-Burke-Ernzerhof (PBE) of generalized-gradient approximation (GGA) functional was used, which was extended by the Hubbard U correction to consider the strong correlation of Mn 3d band. The U value of 4.64 eV for Mn ion was chosen from the previous studies for the calculation of layered structure cathode materials. For more accurate Na+, the Na pseudopotentials in all calculations were used with 3s one electron and 2p six electrons. Through the convergence test within 2.0 meV per formula unit (f.u.), A plane-wave energy cut-off of 500 eV and k-point mesh of 4 × 4 × 2 were utilized. The migration barriers of both structures were calculated using the climbing-image nudged elastic band (ci-NEB). The atomic models for the tunnel (Space group: Pbna) and layered (Space group: P63/mmc) structures were developed by 8 f.u. and 18 f.u., respectively. Finally, both ions and cells were fully-relaxed for all thermodynamic calculation

Thus, the corresponding hybrid structure could provide a new class of composite cathode materials that are able to break the limitations of single layered structures. The superior performance from the hybrid layered-tunnel structured Mn based cathode makes it a promising candidate for SIBs, which will be applicable to large-scale energy storage systems. 2.EXPERIMENTAL METHODS 2.1Preparation of NaxMnO2 NaxMnO2 materials were synthesized by coprecipitation. Analytical reagent grade NaCH3COO.3H2O and Mn(CH3COO)2.4H2O with different molar ratios corresponding to 0.05 mol of the target products were dissolved in 100 ml of deionized water. 12.61 g of H2C2O4 .2H2O dissolved in 100 ml of deionized water was then added. Water was evaporated at 80°C to produce a milky white precursor. After drying at 80°C overnight, the precursor was pressed into pellets and annealed at 450°C for 6 h with subsequent heat treatment at 800°C for 15 h. Finally, the pellet was naturally cooled to room temperature and stored in an Ar atmosphere glove box.

3. RESULTS Figure 1a shows that P2 layered structure is composed of a sheet of edge sharing MnO6 octahedra with Na ions accommodated at two prismatic sites: Na1 and Na2 (Table S1), which shares face or edge with MnO6 octahedra, respective26 ly. Sodium ions could migrate from one prismatic site to adjacent sites through open square bottlenecks surrounded by four oxide ions and a relative small repulsive interaction could be expected. And the Na+ diffusion is also influenced by the sodium/vacancy chemical compositions. Moreover, the Na+ diffusion mechanism could be altered by the unavoidable P2-O2 phase transition. Figure 1b shows the tunnel structure where the framework incorporates four MnO6 octahedral sites and one MnO5 square pyramidal site. In detail, 4+ the Mn1, Mn3 and Mn4 sites are occupied by Mn (Table S2), 3+ while the Mn2 and Mn5 are occupied by Mn . The double tunnel structure is assembled with double and triple chains of MnO6 octahedra as well as single chains of MnO5 square pyramids by either edge or corner sharing. Na1 sites are located in the smaller tunnels while Na2 and Na3 are situated in the large S-shaped tunnels. The small tunnels are fully occupied and the large S-shaped tunnels are only half occupied. Na ions in the large tunnels are highly mobile, enabling

2.2Characterization And Electrochemical Measurements The morphology and structure of the as-prepared samples were characterized by field emission scanning electron microscopy (SEM, HITACHI S-4800), transmission electron microscopy (TEM, JEM 2100), and powder X-ray diffraction (XRD, Philips X’pert Pro Super X-ray diffract meter, Cu Kα radiation) analysis. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI QUANTUM 2000 instrument.Electrodes of sodium half cells were made by spreading a mixture of 75 wt % active material, 20 wt % acetylene black and 5 wt % PVDF onto aluminum foil current collectors. The mass loading of the active materials was ~2.5 mg cm-2. The as-prepared electrodes were dried at 80°C in a vacuum oven for 12 h. Electrochemical properties of the electrodes were monitored by assembling them into coin cells (type CR2025) in an argon-filled glove box with water and oxygen content less than 20 ppm. Metallic Na was used as the counter electrode and glass fiber (GF/A, Whatman) as the separator. The electrolyte was NaClO4 (1 mol L-1) and a mixture of PC/EC at volume ratio 1:1 (purchased from Fosai

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ACS Applied Materials & Interfaces ered-tunnel material are slightly smaller than that of pure P2 layered Na0.7MnO2. This contraction could imply that the P2 3+ phase in layered-tunnel Na0.6MnO2 contains less Mn ions 4+ (the ionic radius of Mn is 0.530 Å, whereas ionic radius of 3+ Mn is 0.645 Å) than pure P2 layered structure, which reduces the Jahn-Teller distortion and ensures better structural 27 stability. The XRD pattern of pure tunnel-structured Na0.44MnO2 was also obtained for comparison (Table S4), showing slightly smaller structure parameters (a = 9.110 Å, c =2.829 Å). Thus, the tunnel structure in layered-tunnel Na0.6MnO2 may provide larger tunnels for faster Na ion diffusion and facilitate the accommodation for structural strain during Na ion insertion/extraction, providing superior rate capability and cycling stability. XRD results confirmed the coexistence of P2-layered and tunnel structures in the Na0.6MnO2. The layered--tunnel Na0.6MnO2 also displays reasonably modified structure parameters compared with pure layered Na0.7MnO2, which are definitely beneficial for its electrochemical performances. The ICP test indicate that the composition of Na0.6MnO2 hybrid cathode is Na0.602MnO2, which is consistent with the results.

high Na ion diffusivity. The large double tunnel structure can also help tolerate structural stress during charge/discharge, 23 which can offer good cycling stability. Figure 1c and 1d shows XRD patterns of Na0.6MnO2 and Na0.7MnO2, analyzed by Rietveld refinement using PDXL software to identify the detailed structure. Though some weak peaks ascribed to impure phase could be observed, most of the diffraction lines in Na0.6MnO2 could be related to the hexagonal lattice of layered structure or tunnel structure (Figure S1). A reasonable refinement could be completed based on P2 layered structure (S.G. P63/mmc) and tunnel structure (S.G. Pbam) finally demonstrating that the Na0.6MnO2 possesses the hybrid structure made up of layered and tunnel structures. The XRD pattern of Na0.7MnO2 indicates that the diffraction peaks may be ascribed to the hexagonal lattice with space group P63/mmc, which is isostructural with P2 structure. And a relative ideal XRD Rietveld could also be obtained with P63/mmc structure. Moreover, the refinement of XRD pattern with Cmcm (P’2) (Figure S2) (the crystal structure parameters are listed in table S3) was also carried out. No peak splitting typical of orthorhombic distortion was observed. And the R-values for Cmcm space group are higher than that of P63/mmc. According to the above results, the hexagonal mode could be more suitable.

Table1 Refined parameters of hybrid Na0.6MnO2 and pure P2 Na0.7MnO2.

Space group

layered-tunnel

Layered-tunnel

Layered

Na0.6MnO2

Na0.7MnO2

Pbam

P63/mmc

P63/mmc

Lattice parameters

a=9.125

a=2.878

a =2.879

(Å)

b=26.42

b=2.878

b=2.879

c=2.973

c=11.24

c=11.20

34.64

65.36

/

Mass ratio(%) Rwp(%)

4.76

6.00

Rp(%)

3.06

3.57

Figure 1 Schematic crystal structures of (a) P2 layer and (b) tunnel structures; Rietveld refined XRD patterns of hybrid layered- tunnel (c) Na0.6MnO2 and (d) pure P2 layered Na0.7MnO2 Table 1 shows the refined parameters from XRD analyses for the layered-tunnel structure and pure layered P2 structure. In the hybrid layered-tunnel structure, the ratio of P2type layered Na0.7MnO2 and tunnel Na0.44MnO2 is 65.4% : 34.6%. The lattice parameter of P2-type structure, c=11.25 Å, in layered-tunnel Na0.6MnO2 is larger than that (c=11.20 Å ) of pure P2 layered Na0.7MnO2, which indicates that the layeredtunnel structure has enlarged interlayer spacing, providing more space for Na ion migration. This feature could activate more Na ions and accelerate Na ion diffusion in the hybrid 20 structure. In contrast, a and b lattice parameters for lay-

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gies (Figure S3), whose lattice fringes correspond to 0.245 nm ((010) plane). Basing on this observation, the layered-tunnel Na0.6MnO2 basically looks like a composite made up of separate layered structured particles and tunnel structured particles probably due to the big differences between these two crystal structures. Interestingly, though the two structures possess distinctive oxygen arrangement patterns, some particles possess both layered and tunnel structures could also be observed in the as-prepared sample (Figure S4), which could be linked with the compatible close-packed oxygen arrays along with some special axis in layered/tunnel structures. This phenomenon could also be evidenced by the lattice parameter analysis, which proved that tunnel structure is partly merged into P2-type layered structure to make a whole hybrid particle. The formation mechanism of the hybrid structure was preliminarily investigated by detecting the structure change along with the calcination process (Figure S5). The results evidenced that after pre-calcination, the precursor with layered structure could be formed. When the temperature rose to 800℃，both layered and tunnel structures appeared. What’s more, the diffraction peaks of layered structure became sharp along with the calcination process, while the peaks belong to tunnel structure turned to be weak. So, in this experiment, the layered and tunnel structures was 29 formed at the same time. According to Zheng’s study, the layered structure component can be formed from the thermal reaction between precursor and available Na source. Since the Na source is insufficient, part tunnel structure with less Na content was obtained. The detailed formation mechanism need a further investigation. Figure 3a shows the charge/discharge curves of hybrid layered-tunnel Na0.6MnO2 at 1st, 2nd, 10th, 20th, 30th and 50th cycle. In the initial charge-discharge process at 1C, the -1 charge capacity is 104.6 mAh g , which is much lower than -1 that of the corresponded discharge capacity (188.8 mAh g ). And the low initial charge capacity could be attributed to the low sodium content in the pristine sample. The charge/discharge curves of layered-tunnel Na0.6MnO2 have multiple voltage plateaus, which were attributed to P2 layered as well as tunnel structure. As the cycle number increases, the increment of overpotential can be clearly observed during charge and discharge. It could be found that layered-tunnel Na0.6MnO2 displayed more complex voltage plateaus compared with pure layered Na0.7MnO2 and tunnel Na0.44MnO2 (Figure S6). Part of the voltage plateaus could be assigned to layered structure. And the other plateaus belong to tunnel structure. When the current density increased to 1.0 C, the layered-tunnel Na0.6MnO2 hybrid displayed much less polarization in comparison with layered and tunnel structures. And the voltage plateaus could still be retained with increased current density. Moreover, the discharge plateau around 2.3 V in layered-tunnel Na0.6MnO2 is more reversible than that of layered Na0.7MnO2, which indicates less 3+ Jahn-Teller effect with Mn . The layered-tunnel Na0.6MnO2 showed significantly enhanced cycling stability in comparison with pure layered Na0.7MnO2 as shown in Figure 3b. -1 Herein, high reversible discharge capacity of 193.6 mAh g was obtained with layered-tunnel Na0.6MnO2, and 135.4 mAh -1 g (about 70% of the initial capacity) was maintained even after 100 cycles. On the other hand, pure layered Na0.7MnO2

Figure 2. Scanning electron microscope (SEM) images comparing morphologies of (a, b) layered-tunnel Na0.6MnO2 composite and (g, h) pure layered Na0.7MnO2. High resolution transmission electron microscope (HTEM) images and corresponding selected area electron diffraction patterns (SAED -insets) of (c, d) rod-like and (e, f) plate-like particles obtained in layered-tunnel Na0.6MnO2. Figure 2 compares the morphologies of hybrid layeredtunnel Na0.6MnO2 and P2 layered Na0.7MnO2. Two different morphologies of particles could be observed in layeredtunnel Na0.6MnO2 (Figure 2a). Plate-like particles with welldefined layered structure and rod-like particles can be simultaneously detected in the magnified images (Figure 2b). HRTEM and SAED analyses were conducted to obtain the structural information about the rod and plate-like particles. As shown in the HRTEM image (Figure 2d), the lattice fringe of the rod-like particle is ~0.452 nm, corresponding to the interplanar distance of (200) plane in the tunnel structure. Its SAED pattern provides a characteristic spot pattern of tunnel structure with an orthorhomic lattice (space group: Pbam). Figure 2f demonstrates that the plate-like particle has hexagonal lattice attributed to P2 structure. Its SAED pattern along [001] direction is also evident of its layered structure. Figure 2g and 2h indicate that pure P2 layered Na0.7MnO2 particles have well-defined plate-like morphologies, which is 28 consistent with previous reports, whereas pure tunnel Na0.44MnO2 with space group Pbam has rod-like morpholo-

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ACS Applied Materials & Interfaces -1

b

4.0 1st 2nd 10th 20th 30th 50th

3.5 3.0 2.5

300

Discharge capacity / mAh g

+

Voltage / V (vs.Na/Na )

4.5

-1

a

2.0

1C 1.5 40

80

120

160

200

Specific capactiy / mAh g

240

-1

150 100 50

1C 0

d

0.20

0.10 0.05 0.00 -0.05

1st 2nd 3rd

-0.10

20

40

60

80

100

Cycle number 0.25

Layered Na0.7MnO2

Current / mA g-1

Current / mA g-1

0.15

Layered Na0.7MnO2

200

0 0

c

Layered-tunnel Na0.6MnO2

250

Layered-tunnel Na0.6MnO2

0.15 0.10 0.05 0.00 -0.05 -0.10

1st 2nd 3rd

-0.15 -0.20

1.5

2.0

2.5

3.0

3.5

4.0

f

0.5 C

1.0 C

2.0 C

150

4.0 C 8C

100

Layered-tunnel Na0.6MnO2 Layered Na0.7MnO2

50

2.5

3.0

3.5

Voltage/V(vs.Na/Na+)

4.0

4.5

+

0.1 C

2.0

4.5

Voltage / V (vs.Na/Na )

Specific capacity/mAh g-1

250 200

1.5

4.5

Voltage / V(vs.Na/Na+)

e

0

1 C=200 mA g-1

4.0 3.5

0.1 C 0.5 C 1.0 C 2.0 C 4.0 C 8.0 C

3.0 2.5 2.0 1.5

0

g

5

10

15

Cycle number

20

25

0

40

80

120

160

200

Specific capacity / mAh g

240

280

-1

200 100 160

60 Charge Discharge Efficiency

80

40

40

20

10 C

Layered-tunnel Na0.6MnO2 0 0

100

200

300

400

Cycle number

500

Efficiency/%

80 120

600

4+

cathodic current peaks could be assigned to Mn /Mn re30-32 dox reaction, resulting from high Mn content. The redox process that occurred in the higher voltage region (3.3–4.3 V) was ascribed to two-phase oxidation reaction associated with 33,34 the formation of O2 phase at low sodium content. The other small cathodic/anodic current peaks corresponded to 13,14 the rearrangement and migration of Na ion vacancy. And some other broad peaks could be attributed to the singlephase evolution accompanied with Na+ insertion and extraction During consecutive cycling sweeps, the intensity of current peaks slowly decreased, suggesting that the redox reaction here is partially irreversible. The anodic peak related to 3+ 4+ Mn /Mn redox shifted to higher potentials, indicating the increased polarization probably resulting from irreversible 2+ structural changes (e.g. Mn dissolution due to dispropor3+ 35 tional Mn ). The layered-tunnel Na0.6MnO2 showed more complex electrochemical behaviors (Figure 3d). In addition to the characteristic cathodic/anodic peaks of pure P2 layered Na0.7MnO2, several pairs of symmetrical redox peaks could be also observed, which are assigned to the complex biphasic transition mechanism during the charge-discharge of tunnel structure (Figure S9), and confirms the coexistence 25,36,37 of layered structure and tunnel structure. The multiple peaks indicate the biphasic transitions, which caused by the extraction of Na3, Na2 and Na1 with different electrostatic repulsion. The sodium ions in Na3 sites with the strongest repulsion are first deintercalated, and then the Na2 and Na1 sites are extracted sequentially. The overlapping curves during cycling prove the high reversibility of layered-tunnel Na0.6MnO2 during charge/discharge and its CV curves averagely showed lower polarization compared to pure layered Na0.7MnO2. Figure 3e compares the rate capabilities of layered-tunnel Na0.6MnO2 and pure layered Na0.7MnO2. Table S5 summarizes the average capacities of each sample at various C rates, indicating that layered-tunnel Na0.6MnO2 showed highly improved kinetic properties compared with pure layered Na0.7MnO2 and its rate capability was best even among the reported values with various Mn-based layered materials. From Figure 3e, 1st discharge capacity of layeredtunnel Na0.6MnO2 is lower than pure P2 layered Na0.7MnO2, probably due to low theoretical capacity of tunnel structure, but its charge/discharge curve at 8.0 C demonstrates that this hybrid composite has much less polarization even com38 pared with most of previous reports. The small polarization followed by an outstanding rate capability may be relevant to fast Na ion diffusion in the layered-tunnel Na0.6MnO2. As mentioned above, the rate capability of layered-tunnel Na0.6MnO2 is superior to all previously reported performanc15,20,26,28,39-45,46-49 es for Mn based cathodes (Table 2) The superior electrochemical performances of layered-tunnel Na0.6MnO2 looks like coming from the synergistic effect of hybrid layered-tunnel structure. Galvanostatic discharge/charge profiles of layered-tunnel Na0.6MnO2 at various rates demonstrates that the increase of overpotential is not large even at 8.0 C (Figure 3f). Figure 3g shows the longterm cycling performance of Na0.6MnO2 at 10 C. Layeredtunnel Na0.6MnO2 displays not only high initial capacity -1 (114.4 mAh g ) but also an unprecedentedly great cyclic re-1 tention coming around 72% (83.3 mAh g ) after 700 cycles, again proving the excellent structure stability of this hybrid composite.

Electrochemical behaviors of layered-tunnel Na0.6MnO2 and pure P2 layered Na0.7MnO2 were investigated by cyclic voltammetry (CV) measurements between 1.5 and 4.3 V at a -1 scan rate of 0.2 mV s . For P2 layered Na0.7MnO2 (Figure 3c), couple of symmetric cathodic and anodic peaks could be observed, which have not been clearly identified, and indicate the complicated process of Na ion insertion and extraction. In the lower potential region (2.0–2.5 V), the anodic and

Specific 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

3+

had much lower capacity coming up to 166.6 mAh g at 1st -1 cycle, and only 63.5 mAh g (about 38% of the initial capacity) was retained after 100 cycles. Although the tunnel Na0.44MnO2 showed excellent cycleability, the capacity is insufficient (Figure S7). As shown in Figure S8, the layeredtunnel hybrid cathode possesses the highest energy density among the three Mn-based cathodes. The layered Na0.7MnO2 displayed higher initial energy density than that of tunnel Na0.44MnO2. But the cycling performance is very insufficient. And the results also reveal that further effort should be carried out to enhance the cycle performance of layered-tunnel Na0.6MnO2.

0 700

Figure 3(a) Galvanostatic discharge/charge profiles of layered-tunnel Na0.6MnO2 at 1C, (b) Cyclic performances of layered Na0.7MnO2 and layered-tunnel Na0.6MnO2, Cycle voltammetry of (c) layered Na0.7MnO2 and (d) layered-tunnel Na0.6MnO2, (e) Rate capabilities of layered Na0.7MnO2 and layered-tunnel Na0.6MnO2, (f) Galvanostatic discharge/charge profiles of layered-tunnel Na0.6MnO2 at various rates, (g) Extended cyclic performance of layered-tunnel Na0.6MnO2 at 10 C

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Table 2 Electrochemical performance comparison between the previously reported NaxMnyM1-yO2 (y>0.6) cathodes and layeredtunnel Na0.6MnO2 composite cathode. Cathode materials

Cycling stability

Rate capability

-1

~130 mAh g after 100 cycles Na0.5[Ni0.23Fe0.13Mn0.63]O2

-1

Ref. -1

60mAh g (715 mA g )

39

-1

(100 mA g ) -1

~125 mAh g after 50 cycles Na0.67[Mn0.65Ni0.15Co0.2]O2

-1

-1

40

-1

-1

20

75 mAh g (400 mA g )

-1

(20 mA g ) P2/O3 composite Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ

-1

~125 mAh g after 150 cycles

69 mAh g (500 mA g )

-1

(50 mA g ) -1

~120 mAh g after 100 cycles P2-NaxMg0.11Mn0.89O2

-1

-1

41

-1

-1

42

-1

28

63 mAh g (610 mA g )

-1

(12 mA g ) -1

118 mAh g after 30 cycles Na0.66Ni0.26Zn0.07Mn0.67O2

79 mAh g (768 mA g )

-1

(12 mA g ) -1

105 mAh g after 100 cycles Na0.67Mn0.65Ni0.2Co0.15O2

-1

93mAh g (1200 mA g )

-1

(120 mA g ) -1

120 mAh g after 100 cycles Na0.45Ni0.22Co0.11Mn0.66O2

-1

-1

43

-1

44

76 mAh g (610 mA g )

-1

(12mA g ) -1

110 mAh g after 50 cycles Na0.8[Li0.12Ni0.22Mn0.66]O2

-1

71 mAh g (590 mA g )

-1

(12mA g ) -1

101 mAh g after 50 cycles Na0.67Mn0.80Ni0.10Mg0.10O2

-1

-1

66mAh g (1200 mA g )

45

-1

(240 mA g ) -1

135.4 mAh g after 100 cycles Na0.6MnO2

-1

-1

133.4 mAh g (1600 mA g )

-1

This work

(200 mA g )

The voltage profile and dQ-dV curves are depicted in Figure S10. Smooth charge-discharge curves could be detected, which could be related with the high current density. The electrochemical polarization increased with different cycles, which may indicate structure changes. The dQ-dV curves showed that the reduction current peaks became weak and implied low crystallinity. Moreover, though the peak intensity reduced, the current peaks assigned to layered structure (~2.2V) and tunnel structure (~2.8V) could still be retained and infer the synergy effect throughout the cycling test. The

fast kinetics and a lower proportion for the layered structure with slower kinetics. This would result in less polarization, which is consistent with the charge-discharge curves at different C rates. The deteriorated performance mainly due to the lattice change of layered structure, which could be negotiated by reduce the corresponded current at high C-rate.

enhanced performance could be associated with the dis tribution of the effective charge-discharge current onto the two structures assessed according to their kinetics and ratio 50 in the electrode. This would mean a higher proportion of the current would be utilized for the tunnel structure with

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ACS Applied Materials & Interfaces Na0.7MnO2with the same loading mass and thickness (L≈ 6.5μm, FigureS12). Figure S13 shows the chronoamperometry and ln(I)-t plots of layered-tunnel Na0.6MnO2 and pure lay+ ered Na0.7MnO2. Figure 4b displays the variation of DNa , which was obtained from dln (I)/dt slopes in Figure S13, as a + function of step potential. For pure layered Na0.7MnO2, DNa gradually decreases with Na ion insertion, which could be attributed to the narrowing of Na ion diffusion pathway. An + instant increment of DNa between 2.7 and 2.4 V was corre13 + lated with the phase transition from OP4 to P2. DNa again started to decrease between 2.4 and 1.5 V due to the repulsive force between Na ions. +

Meanwhile, layered-tunnel Na0.6MnO2 had higher DNa than pure layered Na0.7MnO2 throughout the whole discharge process in great accordance with the rate perfor+ mance. DNa of layered-tunnel Na0.6MnO2 showed a similar tendency to pure layered Na0.7MnO2 above 2.4 V. Herein, the + increase of DNa between 2.2 and 1.5V was interestingly con+ sistent with the variation of DNa (Figure S14) for tunnel+ structured Na0.44MnO2, indicating that DNa of layeredtunnel Na0.6MnO2 is affected by those of layered structure or tunnel structure and this region is dominated by Na ion diffusivity of tunnel structure. Through first-principles calculation based on density functional theory, we theoretically demonstrated Na ion diffusivity behaviors of tunnel NaxMnO2 and pure layered NaxMnO2, and fundamentally elucidated the origins and mechanism for the improved diffusivity of layered-tunnel hybrid composite. Figure 5a indicates that the calculated migration barriers for Na ion as a function of Na content (see Figure S15 for more details) are comparable to the experimental results in Figure 4b.

Figure4. (a) Nyquist plots of layered-tunnel Na0.6MnO2 and layered Na0.7MnO2 electrodes after 100 cycles at 1C (inset: + equivalent circuit); (b) ln(DNa ) variation as a function of step potential during the initial discharge of layered-tunnel Na0.6MnO2 and layered Na0.7MnO2. Figure 4a compares the Nyquist plots of layered-tunnel Na0.6MnO2 and pure P2-layered Na0.7MnO2 electrodes, which were obtained after 100 cycles at 1.0 C. Rs in the equivalent circuit (Figure 4a inset) represents the ohmic resistance, and the charge transfer resistance was expressed by Rct,. W1 corresponds to the Warbug impedance, which is correlated with 42,51 Na ion diffusion in bulk structure. Herein, Rct of layeredtunnel Na0.6MnO2 looks much smaller than that of pure layered Na0.7MnO2 in great agreement with lower polarization in both charge/discharge curves and CVs, implying its improved Na ion kinetics followed by excellent rate capability. The lower Rct of layered-tunnel Na0.6MnO2 seems to result from tunnel structure in the layered-tunnel hybrid composite. The b lattice parameter determining the size of Na ion diffusion channel in tunnel structure is significantly larger than the c lattice parameter, which correlates with Na ion diffusion path in layered structure. Hence, the tunnel structure may contribute mainly to promoting fast Na ion diffusion and reducing charge transfer resistance (Rct) (Figure 36 S11).

The migration barriers for both structures are similar below approximately x =0.45 in NaxMnO2. In this region, even if there is a little difference of the calculated migration barriers between tunnel and layered structures, the Na ion diffusion length of tunnel NaxMnO2 (1.330 Å) is significantly shorter than that of the layered NaxMnO2 (3.002 Å), and the distance between oxygens of the tunnel structure (3.668 Å) is approximately 10% larger than that of the layered structure (3.384 Å) as shown in the inset figures of Figure 5a, which + can produce higher DNa in tunnel NaxMnO2. The superiority of tunnel structure begins to significantly reduce its migration barrier around ~0.45 mol of Na ion in comparison with the layered structure. To be summarized, the lower migration barrier of Na ion in tunnel Na0.6MnO2 looks closely related to the shorter migration path and the longer distance between oxygens finally increasing the diffusion coefficient + of tunnel NaxMnO2 (Figure S15). Thus, the change of DNa above 2.4 V in layered-tunnel NaxMnO2 is influenced by both + layered structure and tunnel structure, whereas DNa between 2.2 and 1.5 V is governed by the structural merits of tunnel structure such as shorter diffusion length and larger space compared to the layered structure. Figure 5b proves that the electrochemical superiority of layered-tunnel Na0.6MnO2 compared to pure layered Na0.7MnO2 primarily originates from the voltage region between 2.2 and 1.5 V in good agreement with Na ion diffusivity and migration barrier. At 0.5 C, the decay of reversible capacity between 2.2 and 1.5V looks like the main factor for the poor cycling perfor mance of pure layered Na0.7MnO2, and its degradation was

To further understand the difference between layeredtunnel Na0.6MnO2 and pure layered Na0.7MnO2. PITT (Potentiostatic intermittent titration technique) method was employed to obtain Na ion diffusion coefficients during the initial discharge. Assuming that the diffusion process may be 52 described by Fick’s second law, diffusion coefficients could be calculated by the following equation; 2 where I (A) is step current, L (μm) is diffusion distance, which is approximately the cathode thickness, and t (s) is step time. To minimize experimental error, we utilized the electrodes of layered-tunnel Na0.6MnO2 and pure layered-

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Figure 6. Schematic diagram for the synergistic effect of layered-tunnel Na0.6MnO2. Figure 6 shows the proposed mechanism for the synergistic effect in layered-tunnel Na0.6MnO2. From previous 25,36 studies , Na ion insertion into tunnel Na0.44MnO2 would be normally completed around 2.2 V. So, when layeredtunnel Na0.6MnO2 was charged and discharged between 4.3 and 2.2 V, both layered structure and tunnel structure in Na0.6MnO2 would take Na ion directly from the electrolyte Because of relatively higher content of layered structure, the + discharge curves and DNa variation in this range looks mainly dominated by the features of layered structure, and thus + DNa is quite slow. When the voltage reaches 2.2 V, the sodiation into tunnel structure is almost completed extending its 24 lattice parameter. As a result, larger Na ion diffusion chan+ nels and higher DNa can be secured when the layered structure gets Na ion from both the electrolyte and tunnel struc12 + ture below 2.2V. times higher DNa of tunnel structure than that of pure layered structure normally renders the sodiation into tunnel structure to be finished shortly. Then, the layered structure seems to have more Na ions through this tunnel structure with higher Na ion diffusivity finally showing higher kinetics and lower polarization. The synergistic effect of tunnel and layered structure between 2.2 and 1.5 V could 4+ 3+ help to retain the reversibility of the Mn /Mn redox and 3+ reduce the Jahn-Teller effect linked to Mn . 4. CONCLUSION In summary, a new class of sodium manganese oxide composite (Na0.6MnO2) with layered and tunnel heterostructures was prepared by a one-pot co-precipitation method. The hybrid material integrated the advantage of P2 layered structure with high capacity and those of tunnel structure with excellent cycling stability and superior rate performance. The coexistence and interaction of layered and tunnel structures also look like producing some advantageous structural changes, finally leading to great electrochemical enhancement. A high initial discharge capacity of 193.6 mAh -1 g was secured at 1.0 C, and also its cyclic stability was superb -1 with 135.4 mAh g retained after 100 cycles at 1.0 C and 83.3 -1 mAh g retained after 700 cycles at 10 C. This hybrid material exhibited the best rate performance among all reported Mn based cathodes for SIBs. The synergistic effect of tunnel and layered structure in Na0.6MnO2 was demonstrated by the improved Na ion diffusivity from PITT. The significance of tunnel structure inside this composite was validated by First principles calculation. Finally, the experimental results and theoretical calculation commonly indicated that the tunnel structure can enhance Na ion diffusivity and the reversibility 4+ 3+ of Mn /Mn during the redox process, which provides superior cycling stability and rate performances. Therefore, this novel layered-tunnel Na0.6MnO2 can be a promising candi-

Figure 5. (a) Na ion migration barriers with Na contents in tunnel NaxMnO2 and layered NaxMnO2; insets: the distance between oxygen and Na migration (left: tunnel structure, right: layered structure). (b) Charge-discharge curves of layered-tunnel Na0.6MnO2 and layered Na0.7MnO2 at 0.5 C; (c) 2nd and 10th discharge curves of layered-tunnel Na0.6MnO2 and layered Na0.7MnO2 at 1.0 C. accelerated when the charge/discharge rate was increased as shown in Figure 5c and Figure S16. Therefore, the reason why the layered-tunnel Na0.6MnO2 possess better reversibility is mostly attributed to the positive contribution of tunnel structure especially between 2.2 and 1.5 V. The oxidation state of Mn at the end of discharge were measured (Figure S17). And no obvious change could be detected at the end of discharge in comparison with the pristine one, which is consistent with the charge-discharge curve and may indicate an enhanced structure stability with the synergy effect.

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date cathode for large-scale energy storage application of SIBs. This study also seems to extend the potential cathode material for SIBs to mixed structures or composites away from the dependence on single-phase cathode material, finally providing a new approach to design cathode material for advanced SIBs.

[2] Wang, P.-F.; You, Y.; Yin, Y.-X.; Wang, Y.-S.; Wan, L.J.; Gu, L.; Guo, Y.-G., Suppressing the P2–O2 Phase Transition of Na0.67Mn0.67Ni0.33O2 by Magnesium Substitution for Improved Sodium-Ion Batteries. Angew. Chem., Int. Ed. Engl. 2016, 55 (26), 7445-7449. [3].Hong, S. Y.; Kim, Y.; Park, Y.; Choi, A.; Choi, N.-S.; Lee, K. T. Charge Carriers in Rechargeable Batteries: Na Ions vs. Li Ions. Energy Environ. Sci. 2013, 6, 2067-2081.

ASSOCIATED CONTENT Supporting Information

[4].Islam, M. S.; Fisher, C. A. J. Lithium and Sodium Battery Cathode Materials: Computational Insights into Voltage, Diffusion and Nanostructural Properties. Chem. Soc. Rev. 2013, 43, 185-204.

The atom sites in P2-Na0.7MnO2, Na0.44MnO2, P’2-Na0.7MnO2; XRD refinement results of tunnel Na0.44MnO2 with Pbna and Na0.7MnO2 with Cmcm, XRD patterns of samples at different calcination stages; SEM images, CV curves, cycling performance at 1 C and EIS spectra of tunnel Na0.44MnO2; TEM images, charge-discharge curves and corresponded dQ-dV curves during the long-term cycling test of layered-tunnel Na0.6MnO2; the charge-discharge curves comparison, energy density comparison and rate capability comparison of layered-tunnel Na0.6MnO2, tunnel Na0.44MnO2, layered Na0.7MnO2; Cross-sectional SEM images, Chronoamperometry plots and the relationship between ln(I) and t in the PITT test of Na0.6MnO2,Na0.7MnO2 and Na0.44MnO2; Migration energy barriers of Na ions for tunnel NaxMnO2 and layered NaxMnO2; high resolution Mn2p spectrum before and after cycling.

[5].Kim, S.-W.; Seo, D.-H.; Ma, X.; Ceder, G.; Kang, K. Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 710-721. [6].Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angew. Chem., Int. Ed. 2015, 54, 3431-3448. [7].Palacín, M. R. Recent Advances in Rechargeable Battery Materials: AChemist’s Perspective. Chem. Soc. Rev. 2009, 38, 2565-2575.

AUTHOR INFORMATION [8].Palomares, V.; Casas-Cabanas, M.; Castillo-Martínez, E.; Han, M. H.; Rojo, T. Update on Na-based Battery Materials. A Growing Research Path. Energy Environ. Sci. 2013, 6, 2312-2337.

Corresponding Author * E-mail: [email protected] (Y.-M. Kang), [email protected] (J.-T. Li)，[email protected] (Z.-G. Wu) and [email protected] (X.-D. Guo)

[9].Han, M. H.; Gonzalo, E.; Singh, G.; Rojo, T. A Comprehensive Review of Sodium Layered Oxide: Powerful Cathode for Na-ion Battery. Energy Environ. Sci. 2014, 8, 81-102.

Author Contributions Funding Sources Notes

[10].Chagas, L. G.; Buchholz, D.; Vaalma, C.; Wu, L.; Passerini, S. P-type Na x Ni 0.22 Co 0.11 Mn 0.66 O 2 Materials: Linking Synthesis with Structure and Electrochemical Performance. J. Mater. Chem. A 2014, 2, 20263-20270.

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A2A1A15055227), National Natural Science Foundation of China (21373008), the National Key Research and Development of China (2016YFB0100202), China Postdoctoral Science Foundation (2014M562322), the Science and Technology Pillar Program Of Sichuan University (2014GZ0077), and the Development of Advanced Electrode and Electrolytes for LIB (AutoCRC Project 1-111).

[11].Xiang, X.; Zhang, K.; Chen, J. Recent Advances and Prospects of Cathode Materials for Sodium-Ion Batteries. Adv. Mater. 2015, 27,5343-5364. [12].Lu, Z.; Beaulieu, L. Y.; Donaberger, R. A.; Thomas, C. L.; Dahn, J. R. Synthesis, Structure, and Electrochemical Behavior of Li[Ni[subx]Li[sub1/3−2x/3]Mn[sub 2/3−x/3]]O[sub 2]. J. Electrochem. Soc. 2002, 149, A778-A791. [13].Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev.2014, 114, 11636-11682. [14].Billaud, J.; Singh, G.; Armstrong, A. R.; Gonzalo, E.; Roddatis, V.; Armand, M.; Rojo, T.; Bruce, P. G. Na0.67Mn1−xMgxO2 (0 ≤ x ≤ 0.2): A High Capacity Cathode for Sodium-ion Batteries. Energy Environ. Sci. 2014, 7, 1387-1391.

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Mn-Based Cathode with Synergetic Layered ... - ACS Publications

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