Ti-Doped Tunnel-Type Na4Mn9O18 Nanoparticles as Novel Anode

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Energy, Environmental, and Catalysis Applications

Ti-doped Tunnel-type Na4Mn9O18 Nanoparticles as a Novel Anode Material for High-performance Supercapacitors Peiyuan Ji, Chengshuang Zhang, Jing Wan, Meili Zhou, Yi Xi, Hengyu Guo, Chenguo Hu, Xiao Gu, Chuanshen Wang, and Wendong Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08350 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Materials & Interfaces

Ti-doped Tunnel-type Na4Mn9O18 Nanoparticles as a Novel Anode Material for High-performance Supercapacitors Peiyuan Ji, #a Chengshuang Zhang, #a Jing Wan, a Meili Zhou,b Yi Xi, a,c Hengyu Guo,a Chenguo Hu,a Xiao Gu,*a Chuanshen Wang,a Wendong Xue*b a

Department of Applied Physics, State Key Laboratory of Power Transmission Equipment & System Security and

New Technology, Chongqing University, Chongqing 400044, China

b

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing,100083

China

c

Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083, China

#

These authors contributed equally to this work

Key words: Ti doped Na4Mn9O18, nano-particles, tunnel structure, flexible super capacitors, density functional theory (DFT), ball milling.



Corresponding authors. Tel: +86 23 65678362, Fax: +86 23 65678362, E-mail: [email protected] (Y Xi) Tel: +86 23 65678362, Fax: +86 23 65678362, E-mail: [email protected] (X Gu) Tel: +86 10 62332666, Fax:+86 10 62332666, E-mail: [email protected] (W Xue) 1

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Abstract The nanomaterial with the tunnel structure is extremely attractive to be used for the electrode materials in the electrochemical energy storage devices in. Tunnel structured Ti doped Na4Mn9O18 nano-particles TNMO-NPs) was synthesized by a facile and high-production method of the solid state reaction with a high-energy ball milling process. As the electrode material in the supercapacitor cell, the as-synthesized TNMO-NPs exhibit the high specific capacity of 284.93 mAh g-1 (0.57 mAh cm-2/1025.75 F g-1). A superior rate capability with the decay of 36% is achieved by increasing scan rates from 2 to 25 mV s-1. To further explore the storage mechanism of Ti doped Na4Mn9O18 materials, the density functional theory (DFT) calculations were used to calculate the activation energy for the ion immigration in the electrode, and the results show that the minimum ions diffusion barrier energy is 0.272 eV, indicating that the sodium ions could insert into the system easily. Through the scan-rate dependence cyclic voltammetry analysis, the capacity value indicates a mixed charge storage of capacitive behavior and Na+ intercalation progress. A maximum energy density of 77.81 Wh kg-1 at the power density of 125 W kg-1 is achieved, and maintains a high energy density of 54.79 Wh kg-1 even at an ultrahigh power density of 3750 W kg-1. The TNMO-NPs super capacitors show excellent flexibility at various bent (0-180°) states. The capacitive performance of the TNMO-NPs makes it promising cathode materials for flexible super capacitors with high specific capacities and high energy densities. 1. Introduction In the past decades, great effort has been devoted to renewable energy due to the declining supply of fossil fuels and environmental issues1-5. The sustainable energy source requires an advanced energy storage system such as LIBs (Li-ion batteries) and super capacitors (SCs) to store energy6-8. In particular, the flexible energy-storage devices have been integrated in complex networks for the internet of things as 2


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wearable system, bendable mobile phones and medical device9-13. SCs has been considered as an efficient energy storage thanks to high power density, fast charge-discharge rate, ultra-long cycle life and environment-friendly14-16. However, the low energy density is a crucial issue in SCs

17

. It is well known that pseudo

capacitor materials exhibit relatively high energy density but poor cycling stability18, 19

. Therefore, there is a need to explore the better super capacitor materials with high

rate capacity, high energy density, excellent stability and structural flexibility. The metal oxides are extremely attractive for energy storage in the electrode materials owing to the reversible oxidation/reduction reactions with change in oxidation states of the primary metal on the electrode surface, typically Mn in Mn xOy systems presenting various oxidation states (Mn2+, Mn3+, Mn4+)20-23. RuO2 and Co3O4 show high specific capacity but suffered from toxicity and high cost19, 24. As we known, the electrochemical performance of SCs is tied to the morphology and the crystalline structure of the electrode materials

25-27

. Since the capacitor stores

charges in the first few nanometers near the surface of the electrodes, and at the same time, the bulk reactions time (t) of ions diffusion depends on the on the diffusion length (L) (t≈L2/D), decreasing the particle size can increase active material usage 28, 29

. Based on high specific surface area, nanostructured material can enhance the

interfacial exchange and shorten the effective electrolyte ionic pathway30, 31. Many efforts have been made to break this limit on the specific area as well as to retain the benefits of tunnel structure2, 25-27, 29-32. Therefore, increasing the specific area of the electrode materials may be the effective approach to improve the electrochemical performance

15

. In particular, the need for flexible working environment should be

fulfilled with the electrode materials. Recently, tunnel structured Na ion (Na+) transition-metal materials have been widely studied because of their nontoxic, low cost and high earth-abundant32-37. Nevertheless, the low specific area of origin micro-structured material makes it not good for super capacitor materials, and thus the electrode materials with nanoscale 3



structures is a way to improve the rate capacity, Na+ storage kinetics and cycling stability. Furthermore, the detailed energy store mechanism of TNMO has not been reported yet and thus it is very necessary to explore the energy storage process by the theoretical calculations. In this work, Ti doped Na4Mn9O18 nano-particles (TNMO-NPs) is firstly synthesized by a facile and high-production method of solid state reaction followed by ball milling. The as prepared TNMO-NPs electrode exhibits the highest specific capacity of 284.93 mAh g-1 (0.57 mAh cm-2/1025.75 F g-1), and a superior rate capability with the decay of 36% is achieved by increasing scan rates from 2 to 25 mV s-1. In a typical three electrode test system, as the result, a maximum energy density of 77.81 Wh kg-1 at the power density of 125 W kg-1 is obtained, and maintains a high energy density of 54.79 Wh kg-1 even at an ultrahigh power density of 3750 W kg-1, which is much higher than the results reported in literature. The TNMO-NPs super capacitors also show excellent flexibility at various bent (0-180°) states. Furthermore, the detailed energy store mechanism of TNMO has not been reported yet and thus it is very necessary to explore the energy storage process by the theoretical calculations. Density functional theory (DFT) calculations and scan-rate dependence cyclic voltammetry were employed to study the charge storage mechanism and ion-transport process in the electrode. 2. Experimental Unless otherwise specified, the raw material or reagents used in this work are used directly without further refinement. 2.1 Material synthesis We use a simple solid state reaction to obtain the tunnel-structured Ti doped Na4Mn9O18 (TNMO) 38. Firstly, Mn2O3, Na2CO3 and TiO2 with the molar atomic ratio 1: 1.02: 1.03 were mixed together in an agate mortar, after ground carefully, the mixture was pressed into circle slice using the tablet machine (YLJ-15) under pressure 4


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of 20 MPa. Thirdly, the circle slice was directly heated at 900 ℃ for 12 h with a heating rate of 5 ℃ min-1 in a ceramic crucible. TNMO-NPs were obtained by using a planetary high-energy ball mill (DECO-PMB-V-0.41) with 6 h and 12 h milling time. The detailed process is illustrated in supporting information, Figure S1. 2.2 Electrode preparation For a current collector, the base part must be capable of conducting electricity, stable in acidic, neutral and alkaline environments and easy to cut into any shapes we want. In this work, carbon fiber fabric (CFF) was used as the base part to provide a stable plant for active materials. The CFF was immerse into the hydrochloric acid and subsequently put in ethanol and deionized water under sonication progressively before used. After that, the CFF was cut into pieces of 1 × 2 cm2 and dried in the oven. The carbon black/TNMO-NPs/CFF (C/TNMO-NPs/CFF) electrodes were used to measure the electrochemical performance of active materials. We mixing 80 wt% active materials, 15 wt% carbon black (C) and 5 wt% Polytetrafluoroethylene (PTFE) together in a suitable amount of absolute ethyl alcohol. The mixtures were then pressed onto the cleaned CFF of 1 × 2 cm2 and dried at 60 ℃ in an electric oven overnight, the final mass loading on CFF was 2 mg cm-2. To further study the cycling stability of three materials, the electrode was charge-discharge for 3000 times to ensure those materials in a stable condition. C/CFF electrodes and TNMO/CFF electrodes were fabricated by the similar method without TNMO or carbon black respectively. The solid-state flexible super capacitors were assembled as follows. Two pieces of C/TNMO-NPs/CFF electrodes (2 × 5 cm2) after immersing the PVA-LiCl gel were assembled together with a separator (Whatman 8 μm filter paper) sandwiched in between. 2.3 Structure characterization For the nano materials, the morphology and element analyzing are important. The morphologies and microstructure of as-made samples were characterized by a Field-emission scanning electron microscope (FESEM, JEOL JSM-7800F). The 5



crystal structure of the products was characterized by X-ray diffraction (XRD) on a PANalytical X’Pert Powder with Cu Kα radiation. The elemental composition was analyzed by energy-dispersive X-ray spectroscope (EDX) attached to the JEOL JSM-7800F microscope and X-ray photoelectron spectrometer (XPS) analysis on an ESCA Lab MKII using Mg Ka as the exciting source. The specific area was determined by the multipoint Brunauer-Emmett-Teller (BET) method with a Quadrasorb 2MP system. The pore size distribution was also measured from the desorption isotherm by Barrett–Joyner–Halenda (BJH) method. 2.4 Electrochemical measurements Electrochemical measurements were performed by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) with a three-electrode system on an electrochemical workstation (CHI660D) in 2 M aqueous Na2SO4 electrolyte, where platinum was used as the counter electrode and Ag/AgCl served as reference electrode. Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range from 0.1 Hz to 100 KHz at open circuit potential. The specific capacity, energy density and power density are calculated according to the formulas shown in supporting information 37. 3 Results and discussion

6


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Figure 1. (a) SEM image, (b) XRD analysis and (c) XPS spectra of the typical TNMO. The core-level XPS spectra of (d) Mn2p and (e) Ti2p. (f) Crystal structure of TNMO.

Figure 1a shows the SEM image of the typical TNMO. The synthesized TNMO has a rod-like morphology within a diameter about 0.5 μm to 3 μm in length. The X-ray diffraction pattern (XRD) of the product is shown in Figure 1b, indicating that the as prepared TNMO is isostructural with Na4Mn9O18 (space group Pbam, PDF#32-1129). There is a small amount of other characteristic reflections most likely due to the Ti doped 38. XPS spectra of TNMO microcrystals is displayed in Figure 1c, confirming the presence of stable Na, Mn, Ti, and O cations. Figure 1d and 1e show the core-level XPS spectra of Ti 2p and Mn 2p., The typical Ti bonding energy at 458.4/464.0 eV and Mn at 641.6/653.3 eV are observed and the spin energy separation between the peaks are 5.6 and 11.7 eV respectively, proving the existence of Ti4+ and Mn3+ states

39, 40

. Ti atoms are inactive and remain at Ti4+ state

26

. This

result demonstrates Ti had been successfully doped into the Na4Mn9O18 crystal structure, which replace the Mn4+ in edge. Figure 1f depicts the projection of TNMO crystal structure along c axis. This framework has a three-dimension tunnel structure composed of S-shaped tunnel and hexagonal tunnel point-sharing or edge-sharing MnO6/TiO6 polyhedral. The Na ions are in channels along the c axis.

Figure 2. The energy-dispersive spectrum (EDS) and EDS-mapping images of the elements of TNMO. The energy-dispersive spectrum (EDS) of TNMO shows the elemental composition of Na, Mn, Ti and O in Figure 2a, which is consistent with the results from XPS. Furthermore, EDS-mapping images of the elements clearly verify the homogeneous distribution within TNMO microrods (Figure 2b). 7



To evaluate the effect of the TNMO and carbon black on the electrochemical properties of the electrode, TNMO-NPs/CFF, C/CFF and C/TNMO-NPs/CFF electrodes were fabricated and tested by CV at 20 mV s-1, and GCD at the same current density. Compared with the CV curves of carbon black and the TNMO-NPs, C/TNMO-NPs/CFF exhibits larger enclosed area, while the response current from the carbon black is very low, as shown in Figure S2a. Therefore, the TNMO-NPs is the major contributor to the overall capacity of the developed super capacitor. The carbon black allows for the fast electronic transports and a shortened diffusion path for the ions. This is also confirmed by the GCD curves as shown in Figure S2b. The C/TNMO-NPs/CFF electrode was further measured in different operating potential windows from 0.5 to 1 V. CV curves were tested at a constant scan rate of 20 mV s-1 in Figure 3a. The results show that the potential window can extend up to 1 V without the polarization phenomenon, demonstrating high reversible capacity of the super capacitor. In Figure 3b, GCD curves tested at a constant current density of 5 mA follow the same path up to at the potential of 1 V, indicating excellent charge storage properties.

Figure 3. (a) CV curves at 20 mV/s and (b) GCD curves at 5 mA of C/TNMO-NPs/CFF electrodes within different potential window from 0.5 to 1 V. (c) The XRD patterns of TNMO with different milling time. Comparison of the TNMO with different milling time: (d) CV curves at the scan rate of 5 mV/s, (e) specific capacitance, (f) Nyquist plots. 8


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Given that decreasing the particle size helps to increase active materials usage. The TNMO-NPs were prepared by high-energy ball-milling the TNMO for 6 h TNMO-NPs M6) and 12 h TNMO-NPs M12) respectively. The comparison of the XRD patterns of TNMO with different treatments are shown in Figure 3c. It indicates that all the diffraction peaks for the ball-milled products are in good agreement with the pristine TNMO without impure phase. There is an enlarged width of the peaks that should be due to the decreased particle size by the ball-milling process 41. As a result, ball-milling process did not change phase structure of the TNMO except with the decreased particle size. Figures S3a-c show the FE-SEM images of the pristine TNMO, TNMO-NPs M6 and TNMO-NPs M12, respectively at the same magnification. It is obvious that the size of the particles becomes smaller as the milling time became longer. In Figure S3b, TNMO-NPs M6 shows inhomogeneous size distribution and the length of the rod-like particles is within a range from several tens of nanometers to a few micrometers, while TNMO-NPs M12 has a nanometric morphology with a little agglomeration (Figure S3c). BET surface areas were calculated to be 44, 88, and 87 m2 g-1 for pristine TNMO, TNMO-NPs M6 and TNMO-NPs M12, respectively (Figure S4a-c). The product milled for 6 h shows the highest specific surface area, which means a high electrochemical surface accessibility and is in favor of improving the Na+ flux across the electrode-electrolyte interface

42

. Moreover, Figure S4d-f

show that the average pore size of the samples is in the nano porous region. The TNMO-NPs M6 shows a pore size distribution within the range of 2-5 nm, which was identified as the possible reasons for the improved power capability and energy density

28

. Therefore, the reduction of the particle size and the appropriate nano

porous structure of TNMO expose the larger surface areas to the electrolyte and shorten the Na+ travel distance for the charge transport. When NMTO approaches to nanoscale dimensions, both surface behavior and diffusion process would be enhanced sharply

41

. Therefore, the TNMO-NPs M6 with unique tunnel structure 9



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considered as better energy storage. In Figure 3d, CV curves of pristine TNMO, TNMO-NPs M6 and TNMO-NPs M12 electrodes were examined in the applied potential difference range of 0-1V at the scan rate of 5 mV/s indicating that the capacity can be attributed to both surface and bulk reactions. As observed, TNMO-NPs M6 exhibits markedly larger enclosed area of CV curve than others, indicating a significantly improved capacity. This can be due to larger surface areas and more appropriate pore size distribution achieved by the appropriate ball milling process. The area capacity (CA) can be calculated using Eq. S1 from the CV curves to be 0.32, 0.53 and 0.46 mAh cm-2 (1.16, 1.9 and 1.67 F cm-2) as shown in Figure 3e. The electrochemical performance of various products was further studied by EIS measurements (Figure 3f). The straight line in the low frequency region represents the Warburg resistance relevant to the electrolyte diffusion process, and TNMO-NPs M6 shows a near vertical line, revealing fast ion diffusion in the electrolyte and the ideal capacitive behavior in the charge-discharge progress. Furthermore, the depressed semicircle in the high- and medium-frequency regions shows the charge transfer resistance (Rct) at the interface between electrode and electrolyte

43

. The Rct were

calculated to be approximately 0.7, 1 and 2 Ω, indicating that TNMO-NPs M6 can provide favorable ion transport pathway and enable the fast faradaic reaction 16.

10


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Figure 4. Electrochemical performance of TNMO-NPs M6. (a) CV curves at various scan rates. (b) GCD curves at various current density. (c) The specific capacitance at different scan rates. (d) XRD patterns of electrode materials charged and discharged. (e) The calculated barrier energy and diffusion coordinate. (f) Possible diffusion paths for sodium ions.

The detailed electrochemical performance of TNMO-NPs M6 was measured by CV and GCD in the potential window of 0-1 V, as shown in Figure 4a and b. The CV curves from 2 to 30 mV s-1 show nearly reversible properties with discernable redox peaks, suggesting the combination of surface properties and Na+ intercalation mechanisms

44

. There are noticeable peak shifts to a low potential in cathodic scans

and to a high potential in anodic scans with increasing scan rate. On the one hand, at low scan rates, the redox reactions can take place over the entire active surface area. However, at faster rates, the diffusion of Na ions will be limited and a higher over potential is required to transport the ions for redox exchange

45

. Therefore, the peak

separation increases with the scan rates. The TNMO-NPs M6 shows the highest specific capacities (CS)/area specific capacities (CA) calculated by Eq. S1 and Eq. S2 of 284.93 mAh g-1 (0.57 mAh cm-2) at 2 mV s-1, which is much higher than the results in literature. A superior rate capability with the decay of 36% is achieved by increasing scan rates from 2 to 25 mV s-1, as shown in Figure 4c. To understand the structure evolution of TNMO-NPs M6 during Na+ extraction and insertion, XRD patterns of electrode materials with charging and discharging were conducted (Figure 4d). The electrodes were charged/discharged within the potential range of 0 - 1 V at the current of 1 mA. There is no impurity phase formation upon Na+ extraction/insertion from/into TNMO-NPs M6. It is found that most of the peaks represent slight shift to higher degrees when charged to 1 V and back to the pristine position when discharged to 0 V. The magnified views of the peaks are given in the inset of Figure 4d. According to the Bragg equation of 2dsinθ=nλ, the shift of the peaks to larger angles means that the lattice constants and planar inter-spacing (d) become smaller

46

. This demonstrates that the Na+ is extracted from the structure 11



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through an extraction/insertion mechanism during charging. Furthermore, the phase evolution can be reversibly restored to the pristine structure upon discharged, indicating a high structure stability in which the main channel structure of the TNMO-NPs M6 do not break after the entire charge/discharge process 38, 47. To further investigate the properties and diffusion process of Na+ in TNMO-NPs, we used the first-principles studies based on the density function theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP)

48

. The generalized

gradient approximation (GGA) with the Perdew-Burke-Ernzerhof was employed for the exchange-correlation functional. The wave functions were expanded in a plane wave basis truncated at the plane wave energy of 500 eV, and a 3 x 1 x 9 Monkhorst-Pack grid was used for ƙ-space sampling in the calculation. The geometry optimizations were converged to a force cutoff 0.01 eV/Å and energy cutoff10−6 eV. For the computation of the diffusion process of sodium in TNMO-NPs system, the nudged elastic band (NEB) method was used to find the minimum energy paths (MEP) 49

. TNMO has a space group Pbam structure, with lattice parameters a = 9.215 Å, b

= 26.490 Å, c = 2.874 Å. To get the TNMO structure, we used 6 Ti atoms to substitute Mn atoms, all the configurations have been relaxed, and the most stable crystal structure was showed in Figure 1f. It is well known that，the ions diffusion barrier energy is a significant factor for super capacitors, the lower ions diffusion barrier energy indicates the electrons and ions are easily transferred from inner system to surface of materials. To give a comprehensive understanding, we have chosen three possible diffusion paths for Na+, as shown in Figure 4f. The calculated barrier energy and diffusion coordinate are showed in Figure 4e. The barrier energies of path1, path2 and path3 are 0.272 eV，0.371 eV and 0.873 eV, respectively. From which we can find that path1 and path2 have lower ion diffusion energy, demonstrating the sodium ions could insert into the system easily. The possible electrochemical process in TNMO-NPs is shown as follows, 12


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𝑁𝑎0.66 𝑀𝑛0.66 𝑇𝑖0.34 𝑂2 ↔ 𝑁𝑎𝑥 𝑀𝑛0.66 𝑇𝑖0.34 𝑂2 + (0.66 − 𝑥)𝑁𝑎+ + 𝑥𝑒 −

(1)

Figure 5. (a) Determination of b-value correspond to the peak currents of OX.1 using power law. (b) Contribution ratios between the two different charge storage processes.

To investigate the charge transfer kinetics of the TNMO-NPs M6 electrode, we further calculated the plot of log(i) versus log(v) from 0.2 to 10 mV s-1 corresponding to the anodic peak currents of OX.1 (Figure 5a) based on the power law 41, 50: 𝐼 = 𝑎𝑣 𝑏

(2)

Where I is the peak current in CV curve, v is the sweep rate and a, b are appropriate values, the b-value is determined from the slope of the plot. It is noticeable that slope b = 1 indicating the total contribution of capacitive behavior via surface or near-surface reversible redox reactions and absorption/desorption of electrolyte ions, while slope b = 0.5 representing the diffusion-limited process caused by Na+ intercalation to overall charge storage. The b-value for the anodic peaks reaches to 0.88, indicating the mixed charge storage of capacitive behavior associated with the surface of TNMO-NPs M6 nanostructures and intercalation progress of Na+ in the bulk TNMO-NPs M6 lattice. The total current response depends on the above two parts of surface-controlled capacitive effects ( I s  k1v ) and diffusion-controlled bulk 1/ 2 intercalation ( I d  k2v ), which can be quantified according to 25:

𝐼 = 𝑘1 𝑣 + 𝑘2 𝑣 1/2 Or

13


(3)


𝐼 𝑣 1/2

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= 𝑘1 𝑣 1/2 + 𝑘2

(4)

k1 and k2 can be calculated by plotting I/v1/2 versus v1/2.Thus, the contributions from capacitive charge and diffusion-limited Na+ intercalation during the CV cycling are extracted quantitatively (Figure 5b and Table S1). The charge contribution analysis obviously shows that capacitive storage is the significant contribution to the total capacity, which is mainly attributed to the decreased particle size, similar to the micro/nano-structures of some transition metal oxides reported previously

51

. The

surface-controlled reactions present excellent scan rate stability as shown in Figure 4c.

Figure 6. (a) Ragone plot (specific energy densities vs specific power densities). (b) CV curves under different bending degrees. (c) Cycling performance of TNMO with different milling time at a current density of 20 mA (d) The photographs of the flexible demonstration under different bending degrees (0°, 90°, 150°, 180°).

Specific energy densities and specific power densities are evaluated from GCD 14


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curves following Eq. S3 and Eq. S4, and the Ragone plot as shown in Figure 6a. A maximum energy density of 77.81 Wh kg-1 at the power density of 125 W kg-1 is obtained, and still maintains a high energy density of 54.79 Wh kg-1 even at an ultrahigh power density of 3750 W kg-1Compared with other reports, the energy density is much higher 37, 52-56. Meanwhile, the cycling stability is an important parameter for super capacitors. As depicted in Figure 6c that TNMO-NPs exhibits much higher and more stable duration than pristine TNMO. TNMO-NPs M6 and TNMO-NPs M12 retains 109.4% and 109.1% of its initial capacity, respectively, compared with 82.2% for the origin TNMO after 7000 cycles, for the reason that as the cycling time goes by, the wettability of the materials becomes better, more active sites would be exposed to the electrolyte, furthermore, the nanoparticles decomposed from the materials were adsorbed on the active carbon instead of fell off the carbon cloth, so that the contact area between the material and the electrolyte was raised. As for practical applications, flexible SCs hold great promises for flexible and wearable electronics. Thus, CV measurements of TNMO-NPs M6 at the scan rate of 20 mV s-1 were tested at various bent (0-180°) states (Figure 6c and d). The shape and area of the CV curves under different angles are almost identical, demonstrating its excellent mechanical stability and flexibility. Figure S5 shows the CV curves of full solid-state super capacitors at different scan rates ranging from 5 to 50 mV/s, and the GCD curves at different current ranging from 1 to 30 mA in the potential window of 0 to 1 V. The specific capacity of solid-state super capacitors decreases with an increase in current density as shown in Figure S5c, and the highest specific capacity is 180.28 mAh/cm2 at 1 mA.

15



Figure 7. Energy-storage mechanism of C/TNMO-NPs/CFF electrode during the charge/discharge.

The excellent electrochemical performance of TNMO-NPs originates from the unique tunnel structure, and the high specific surface area that are beneficial to high-efficient energy storage (Figure 7). The improved charge storage properties of our super capacitors electrode is attributed to the following factors: (1) the minor portion of carbon black allowed for the fast electronic conductivity. The milling time decreased the particle size providing the sufficient electrochemically active sites for the reversible redox reactions and absorption/desorption of electrolyte ions. (2) Three-dimensional tunnel structure and better ion transport due to the short diffusion pathway can provide facile access to the repeated Na+ intercalation and storage. (3) The domination of the total capacity by fast capacitive storage via surface or near-surface and the Ti-substitution are in favor of the excellent cycle performance. 4 Conclusion In summary, the nanostructured TNMO was prepared through a facile and high-production method of solid state reaction followed by a high energy ball milling. The sample milled for 6 h maximizes the electrochemical performance of TNMO and shows the highest specific capacity of 284.93 mAh g-1 (0.57 mAh cm-2/1025.75 F g-1). 16


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A superior rate capability with the decay of 36% is achieved by increasing the scan rates from 2 to 25 mV s-1. TNMO-NPs M6 maintains 109.4% of the initial capacity over 7000 cycles, which is much higher than pristine TNMO. The TNMO-NPs M6 super capacitors also show excellent flexibility at various bent (0-180°) states. Density functional theory (DFT) calculations show that the minimum ions diffusion barrier energy is 0.272 eV, which demonstrates the sodium ions could insert into the system easily. By scan-rate dependence cyclic voltammetry analysis, the capacity value indicates mix charge storage of capacitive behavior and Na+ intercalation process. The electrochemical performance of the TNMO-NPs M6 makes it promising cathode materials for flexible super capacitors with high specific capacities and high energy densities. Acknowledgments This work is supported by the NSFC (51772036, 51572040), the Fundamental Research Funds for the Central Universities (2019CDXZWL001), National Key Research and Development Programs - Intergovernmental International Cooperation in Science and Technology Innovation Project (Grant No.: 2016YFE0111500). Supporting Information Additional details including capacitive currents and intercalation currents as a function of the scan rates, the equations of the specific capacitance, power and energy density, schematic illustration of the synthesis of TNMO-NPs and the fabrication process of C/TNMO-NPs/CFF electrodes, comparison of the C, TNMO-NPs and C/TNMO-NPs/CFF electrodes, N2 physisorption isotherm and pore size distribution of different materials, CV curves and GCD curves of the full solid-state supercapacitors.

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Ti-Doped Tunnel-Type Na4Mn9O18 Nanoparticles as Novel Anode

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