Enhanced Sodium Ion Storage in Interlayer Expanded Multiwall

Aug 1, 2018 - We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNT) as an active anode material in sodium ion battery by ...
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Enhanced Sodium Ion Storage in Interlayer Expanded Multiwall Carbon Nanotubes Ajay Piriya V. S., Manoharan Muruganathan, Kamaraj M., Hiroshi Mizuta, and Ramaprabhu Sundara Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02275 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Enhanced Sodium Ion Storage in Interlayer Expanded Multiwall Carbon Nanotubes Ajay Piriya Vijaya Kumar Saroja †,‡, Manoharan Muruganathan*,§, Kamaraj Muthusamy‡, Hiroshi Mizuta§,∥, and Ramaprabhu Sundara † †

Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, ‡Department of Metallurgical and Materials Engineering; Indian Institute of Technology Madras, Chennai 600036, India, §School of Materials science, Japan advanced Institute of science and Technology, Asahidai 1-1, Nomishi, Ishikawa 923-1292, Japan, 4Hitachi Cambridge Laboratory, ∥Hitachi Europe Ltd., J. J. Thomson Avenue, CB3 0HE Cambridge, United Kingdom ABSTRACT We report an effective approach of utilizing multiwalled carbon nanotubes (MWCNT) as an active anode material in sodium ion battery by expanding the interlayer distance in few outer layers of multiwalled carbon nanotubes (MWCNT). The performance enhancement was investigated using Density functional tight binding (DFTB) molecular dynamics simulation. It is found that sodium atom forms a stable bonding with the partially expanded MWCNT (PECNT) with the binding energy of -1.50 eV based on density functional theory calculation with van der Waals correction, where sodium atom is caged between the two carbon hexagons in the two consecutive MWCNTs. Wavefunction and charge density analyses shows that this binding is physisorption in nature. This larger exothermic nature of binding energy favors the stable bonding between PECNT and sodium atom thereby it helps to enhance the electrochemical performance. In the experimental works, partially opening of MWCNT with the expanded interlayer has been designed by the well-known Hummer’s method. It has been found that introduction of functional groups causes partial opening of the outer few layers of MWCNT, with the inner core remaining undisturbed. The enhanced performance is due to expanded interlayer of carbon nanotubes, which provide sufficient active sites for the sodium ions to adsorb as well as to intercalate into the carbon structure. PECNT shows a high specific capacity of 510 mAh g-1 at a current density of 20 mA g-1 which is about 2.3 times the specific capacity obtained for pristine MWCNT at the same current density. This specific capacity is higher when compared to other carbon-based materials. The PECNT also shows a satisfactory cyclic stability at a current density of 200 mA g-1 for 100 cycles. Based on our experimental and theoretical results, an alternative perspective for the storage of sodium ions in MWCNT is proposed. 1 ACS Paragon Plus Environment

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Keywords: carbon nanotubes, sodium ion battery, binding energy, expanded interlayer, intercalation, partially expanded. Large scale energy storage technology plays a significant role in the integration of renewable energy resources with the grid power supply. Lithium ion battery (LIB) technology is considered as a key device among all energy storage devices due to its high energy density and long cycle life1,2. Despite these advantages, lithium ion battery has failed to meet the needs of large scale energy storage system due to the uneven distribution of lithium resources and consequent problems in importing the lithium resources leading to an increase in cost3. As sodium ion belongs to the alkaline family, the similar physical and chemical properties of sodium to lithium (redox potential -2.7 V for Na/Na+ vs standard hydrogen electrode(SHE) and -3.02 V for Li/Li+ vs SHE) draws significant attention and can be one of the most promising alternatives to lithium based storage technology4. The high abundancy of sodium in the earth’s crust makes sodium ion batteries appealing to be a cost-effective storage technology for large scale applications5. Though sodium metal has high theoretical specific capacity of 1166 mAh g-1, sodium metal batteries which works on the mechanism of sodium metal stripping and plating faces the problem of dendrite formation. Though many research groups are working to minimize the dendrite formation, the commercialization of sodium metal batteries is difficult due to safety concern6,7. So, sodium ion battery is considered to be a promising alternative battery technology. However, it remains a great challenge for the sodium ion battery (NIB) to achieve comparable performance to LIB due to its larger ionic radius (1.07 Å for Na+ and 0.76 Å for Li+) and slow reaction kinetics8. In addition, the larger size of the sodium ions affects the mass transport properties, electrochemical storage and causes huge volume expansion. These negative effects restrict the adaptability of the current satisfactory LIB electrode materials to NIB technology. For instance, graphite is an intercalation-based anode material used in commercial LIB and it delivers a high reversible specific capacity of 372 mAhg-1 in the operating voltage of 0.1 to 0.2 V vs Li/Li+. Unfortunately, the specific capacity of graphite for the insertion of the sodium ions is limited due to the thermodynamic consideration wherein sodium ions form plating on the surface of the anode before being intercalated into graphite. The sodium intercalated graphite forms NaC 64 9,10 compound rather than NaC 6, as in lithium it forms stage-I intercalated compound LiC 6 and therefore the reversible specific capacity obtained for graphite in sodium ion battery is limited to 2 ACS Paragon Plus Environment

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30 mAhg-1

9,11

. This is in accordance with the theoretical results that sodium intercalation /

deintercalation is possible in the carbon structures only with a minimum interlayer spacing of 0.37 nm10. Different strategies have been adopted to increase the sodium ion storage in graphite. A technique was employed wherein diglyme based solvent molecules were co intercalated into the graphite structure to achieve an increased interlayer spacing and it exhibited a stable reversible capacity of 100 mAhg-1 with a potential plateau of 0.6 V12. Though reversible capacity can be obtained by this method, the primary obstacle lies in the huge volume expansion resulting from the process of intercalation and deintercalation, which limits its commercialization. Therefore, it is necessary to find a suitable host material to accommodate and transport Na+ ions. So far, various materials like carbonaceous materials13, metal and metal oxide-based systems12– 17are explored as anode in sodium ion battery18. The conversion and alloying based materials like metals and metal oxides exhibit huge volume expansion which leads to rapid capacity fading19,20. Among these materials, carbon-based electrode materials are considered as dominant electrode materials in the energy storage technology due to its good electrical conductivity with better electrochemical properties and cost effectiveness. To overcome the various challenges faced by NIB technology, many other carbon-based materials like hard carbon21, soft carbon22, porous carbon23, hollow carbon nanowires24 and expanded graphite25 have been used as an anode material in sodium ion battery in Table S1 . Unfortunately, low coulombic efficiency and poor cyclic stability remains an obstacle for the use of carbon-based materials. Yang et al. reported expanded graphite as sodium ion battery anode by the process of partial oxidation and reduction of graphite and it delivers a specific capacity of 284 mAh g-1 at 20 mA g-1. Here the rate capability of electrode is not satisfactory due to the low electrical conductivity of the expanded graphite25. Yun et.al. have investigated reduced graphene oxide as an anode for sodium ion battery which delivers a specific capacity of about 174 mAh g-1 at 40 mA g-1. Though better rate capability is obtained for reduced graphene oxide, the specific capacity obtained is very low26. Nanostructured one-dimensional materials like nanowires and nanotubes provides new opportunities to improve the properties of energy storage materials for various batteries due to its good electrical conductivity and structural stability. Yuliang et.al. employed one dimensional hollow carbon nanowire as an anode in sodium ion battery and the specific capacity obtained is 251 mAh g-1 at 50 mA g-1 which offers better specific capacity than two-dimensional carbon-based materials. The enhanced specific capacity is due to increased interlayer spacing of 0.37 nm which is also proved by theoretical studies. Though 3 ACS Paragon Plus Environment

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high specific capacity can be obtained the synthesis technique has been carried out at a very high temperature which increase the production cost of materials27. One dimensional carbon nanostructures like multiwalled carbon nanotubes (MWCNT) have good electrical conductivity with an interlayer spacing of 0.34 nm. MWCNT are considered as an excellent active material for lithium ion storage because it can store lithium ions similar to graphite28. Moreover, due to its higher aspect ratio it can deliver higher capacity than graphite. Due to its higher electrical conductivity MWCNT have been used as a conductive support as well as a buffering matrix for controlling the volume expansion of alloy type metal and metal oxides29–31. Though MWCNT have good electrical conductivity, the storage of sodium ions is limited due to its interlayer spacing. Multiwalled carbon nanotubes have been used as anode in sodium ion battery where the specific capacity obtained is around 100 mAh g-1at 30 mA g-1 32 . It was concluded from this work that MWCNT are unfavorable for the insertion of sodium ions due to its insufficient d-spacing and low active sites. This limited capacity hinders the use of MWCNT as a high capacity anode material in sodium ion battery. In this aspect, we have designed to expand only a few outer layers of MWCNT with the inner core of MWCNT remaining undisturbed using Hummer’s method. Hummer’s method offers better advantage compared to other techniques in unzipping the outer few layers. This helps to obtain partial unzipping of only few outer layers of MWCNT without causing any damage to the inner core of the MWCNT. But in other methods like use of strong acids and other techniques, complete opening of MWCNT to form graphene nanoribbon is obtained. Also, by other methods there is a possibility to damage the inner morphology of MWCNT thereby it affects the electrical conductivity of the sample. With the modified structure the interlayer spacing of outer few layers get expanded to 0.8 nm due to the intercalation of functional groups. The interlayer expanded outer few layers offer more sites for the insertion of sodium ions as well as more active sodium ion adsorption sites and the undisturbed inner core offers good electrical conductivity which helps to attain both better rate capability and high specific capacity. A comparison with MWCNT employed as anode material is conducted. Further investigations on enhancement in capacity, is studied using DFTB molecular dynamics based theoretical calculation. Also, the binding energy of sodium ions with PECNT is determined using DFT calculations where the larger exothermic nature of binding energy of sodium atom favors the formation of stable bonding with PECNT and thus delivers high specific capacity. 4 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION The crystalline structure and the phase purity were obtained using X ray diffraction technique (XRD). Figure 1a shows the XRD pattern of MWCNT and PECNT. The intense peak observed at 26.2° in MWCNT corresponds to C (002) hexagonal plane of graphite with an interlayer spacing of 0.34 nm. The intense peak is due to the long-range order and high crystallinity of MWCNT. The signals at 43° and 54° corresponds to (100) and (002) planes respectively. The introduction of functional groups into the graphitic carbon materials causes an increase in the interlayer spacing from 0.34 nm to a higher value depending on the quantity of functional groups attached. In PECNT, along with the graphitic peak C(002) centered at 25.8°, another peak is observed at 9.23° corresponding to C (002) plane33. The shift in C (002) to lower angle strongly suggests that the introduction of oxygen functional groups is into the outer layers of the carbon nanotubes while with the inner core of the carbon nanotubes remains undisturbed. The intense peak centered at 25.8° with broadening in peak from 20 to 35° confirms the unraveled outer layers of the MWCNT. Thus, it again confirms that during the oxidation process only few outer layers get oxidized leaving the inner layers intact. Further, the opening of outer layers of MWCNT can be quantified from the ratio of the intensity of the peak centered at 9.23° to the sum of the intensities of the peaks centered at 25.8° and 9.23° and the corresponding ratio is determined to be 0.2534. This signifies partial opening of the multiwalled carbon nanotubes. The molecular vibrational characteristics were determined by Raman spectroscopy technique. The Raman spectra of MWCNT and PECNT are shown in Figure 1b. Three major peaks corresponding to D band, G band and 2D and are observed at 1341 cm-1, 1558 cm-1and 2689 cm-1 for MWCNT, and 1333 cm-1, 1563 cm-1 and 2685 cm-1 for PECNT respectively. The G band is due to the sp2 bond stretching of E 2g mode, the D band is due to the structural imperfections present in the carbon basal plane and at the edge sites35 and the 2D band is used to define the quality of the graphene. The I D /I G ratio defines the degree of disorder present in the carbon nanostructures and this ratio for PECNT increases to 0.982 from 0.65 as in the case of MWCNT. A low intensity D band is observed in MWCNT, which arises mainly due the defects created during the process of acid purification to remove the catalyst. The introduction of functional groups in PECNT causes decrease in the average size of sp2 domain after the oxidation process and thus leads to increase in the I D /I G ratio. Moreover, an upshift in the G band is observed in PECNT which is ascribed to the 5 ACS Paragon Plus Environment

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shortening of C-C bond due to the electron transfer from MWCNT to the functional groups and is an indication of intercalation of functional groups in the PECNT35. Thermogravimetric analysis was carried out to determine the thermal stability of the material as well as to confirm the presence of functional groups. As shown in Figure 1c, MWCNT shows significant weight loss above 700° C, which is due to the decomposition of multiwalled carbon nanotubes. In case of PECNT, an initial weight loss around 100° C is due to the removal of water molecules and upon increase in the temperature, weight loss occurs between 150° and 250⸰°C. This is due to the formation of CO and CO 2 gas which are released as the result of easily removable functional groups attached on the surface of PECNT. Weight loss between 250° C and 650° C indicates the removal of more stable oxygen containing functional groups36 and the decomposition of PECNT occurs beyond 650° C. From the TGA curve, it can be inferred that around 15wt% of functional groups are present in PECNT. Moreover, the thermal stability of PECNT is reduced when compared to MWCNT as the introduction of oxygen functional groups weakens the van der Waals forces between the layers and also disrupts the hexagonal carbon basal plane, accelerating the process of weight loss. To evaluate the presence of different functional groups, FTIR measurement was carried out as shown in Figure 1b. During the process of oxidation using H 2 SO 4 and KMnO 4, the carboxylic, epoxy and hydroxyl functional groups are attached on the surface of MWCNT. This process creates defect and stretching of C-C bond leading to opening of the few outer layers of MWCNT. The presence of broad and intense peak as observed in PECNT (Figure 1b.) from 3200 cm-1 to 3500 cm-1 corresponds to -OH stretching. A weak -OH stretching peak is also observed in MWCNT, because of the attachment of few functional groups during the purification process of MWCNT. The PECNT shows signals at 1049 cm-1 and 1408 cm-1 corresponding to carboxyl and epoxy functional groups respectively, which confirms the presence of -COOH groups. Also, an intense peak present at 1690 cm-1 corresponds to carboxylic C=O group. The peaks arising at 2920 cm-1 and 2845 cm-1 corresponds to the symmetric and asymmetric vibrations of -CH x group. These functional groups are not prominent in case of MWCNT which confirms the structure of the MWCNT is not attached with any functional groups37.

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Figure 1. (a) XRD patterns of MWCNT, PECNT. (b) FTIR spectra of MWCNT, PECNT. (c) TGA curves of MWCNT, PECNT. (d) Raman spectra of MWCNT, PECNT. The nitrogen adsorption and desorption isotherm are used to determine the specific surface area and porosity of the MWCNT and PECNT (Figure S1). The porosity and specific surface area are interpreted using Brunauer-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller (BET) method respectively. The BET isotherm for MWCNT and PECNT are done at atmospheric pressure and temperature and follows type IV isotherm. This indicates the presence of relatively large pores in the sample. The BET surface area of PECNT (124 m2/g) is much higher than that of MWCNT (60 m2/g). The surface area enhancement also serves as an evidence for the successful opening of the outer layers of MWCNT. As seen from Figure S1 the adsorption of nitrogen in PECNT is higher when compared to MWCNT even at low pressure. This is due to the fact that opening of carbon nanotubes by the oxygen functional groups induces defects and pores on the surface of carbon nanotubes which helps in adsorption. The desorption pore volume as a function of pore width is calculated using BJH method and the distribution of pore size is shown in Figure S1. where the

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maximum pore diameter is centred at 2.4 nm indicating the mesoporous structure. This facilitates faster diffusion of sodium ions into PECNT when compared to MWCNT. The morphological structure of MWCNT and PECNT was investigated using transmission electron microscopy technique. The typical TEM images of MWCNT and PECNT are shown in Figure 2. Morphology of MWCNT shows a smooth outer surface with a curved and entangled structure and has an average outer diameter in the range of 60 nm and length in the range of micrometres. After the process of oxidation, it is clearly observed that MWCNT undergoes a morphology transformation. The surface of PECNT looks like a distorted graphene nanoribbon covered over the surface of the carbon nanotubes. It is clearly observed from Figure 2d that only a few outer layers of the carbon nanotubes are unzipped successfully along the longitudinal direction of the multiwalled carbon nanotubes yielding shorter graphene fragments. The length as well as inner core of the carbon nanotubes still could be identified, this thereby helps in maintaining the conductivity of the carbon nanotubes. This is in good agreement with the XRD data as shown in Figure 1a. This unique feature thus helps to achieve additional sodium ion storage sites as well as aids in achieving good rate capability.

Figure 2. TEM images of (a) & (b) MWCNT, (c) & (d) PECNT 8 ACS Paragon Plus Environment

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XPS technique was carried out to understand the nature of functional groups attached to PECNT. Figure S2. Represents the xps spectrum of PECNT. The survey spectrum shows only the presence of carbon and oxygen and there are no additional metal impurities present in the sample. After deconvolution and fitting five peaks are centered at 284.2 eV, 285.1 eV, 286 eV, 287.1 eV and 288.7 eV corresponding to C-C sp2 carbon, sp3 carbon, C-OH, C-O-C and O-C=O respectively. This indicates the attachment of functional groups on the surface of PECNT. To understand the electrochemical property of MWCNT and PECNT, these materials were used as electrodes in halfcell experiments. Figure S3 shows the initial three CV curves of MWCNT and PECNT that are recorded at scan rate of 0.1 mV s-1 in the potential range of 0.01 V to 2.8 V vs Na/Na+. Two major reduction peaks are observed at 0.42 V and at 0.01 V in PECNT (Figure S2 a.) and one reduction peak at 0.01 V in CNT (Figure S3 b) in the first cycle. The reduction peak in the first cycle is due to the decomposition of the electrolyte to form a solid electrolyte interface on the surface of the electrode. The reduction peak at 0.42 V is due to the irreversible reaction of functional groups present on the surface of PECNT with electrolyte. There is no obvious reduction peak at 0.42 V in MWCNT which confirms the absence of functional groups on the surface of MWCNT. The disappearance of the irreversible cathodic peak in the next cycles confirms the formation of stable solid electrolyte interface (SEI) layer. Also, reduction peak observed at 0.01 V is due to the insertion of Na+ ions into the carbon structure as well as the adsorption of Na+ ions on the active surface of the electrode38. The cathodic peak current at 0.01 V is observed to be higher in PECNT when compared to MWCNT which is ascribed to the enhanced Na+ ion storage. The enhanced sodium ion storage in PECNT is mainly contributed from the expanded interlayers by the functional groups which provides more active sites for the insertion of Na+ ions into the carbon structure. Additionally, the presence of rectangular shaped curves indicates that the storage mechanism is also contributed by the capacitive behaviour i.e. physical adsorption of sodium ions. To further investigate the electrochemical behavior, galvanostatic charge discharge studies were carried out in the potential range of 0.01 V to 2.8 V at different current densities. Figure S4. shows the charge discharge profile cycled at 20 mA g-1. In the initial cycle, MWCNT shows discharge capacity of 597 mAh g-1 and a charge capacity of 179 mAh g-1 with a coulombic efficiency of 29.9 %. PECNT shows an initial discharge capacity of 1095 mAh g-1 and a charge capacity of 342 mAh g-1 with a coulombic efficiency of 31.2 %. The first cycle coulombic efficiency depends on the irreversible formation of SEI layer, which in turn depends on the surface area of the active material. 9 ACS Paragon Plus Environment

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Since the surface area of the PECNT is high compared to hard carbon, the first cycle coulombic efficiency is low. In further cycles the coulombic efficiency is close to 100%, which confirms the formation of a stable SEI layer in the initial cycle25. The decomposition of propylene carbonate in the presence of PVDF binder leads to the formation of thicker SEI layer. This results in low initial columbic efficiency. The initial columbic efficiency may be improved by using other type of electrolyte solvents like ethylene carbonate: diethyl carbonate or ethylene carbonate: propylene carbonate. In addition, the use of additives in the electrolyte can also improve the coulombic efficiency. The charge discharge studies were carried out for different current densities and the profile shows a sloping and a plateau region for both PECNT and MWCNT as shown in Figure 3a and 3b.As reported by previous studies on sodium storage in hard carbon and soft carbon the Na+ storage in the high voltage region corresponds to the insertion of sodium ions into graphene layers and at low voltage corresponds to the insertion of Na+ into pores and defects39. This signifies that the capacity contributed from the high voltage region (> 0.7 V) is due to the insertion of sodium ions into the graphitic layers in PECNT and in the low voltage region (< 0.7 V) is due to the adsorption of sodium ions into the pores and defects. From Figure 3a and 3b, the capacity contribution from the sloping region up to 0.7 V is 150 mAh g-1 in case of PECNT and 50 mAh g1

in case of CNT at a current density of 20 mA g-1. This further confirms that intercalation of

sodium ions is facilitated in case of PECNT rather than CNT. The synergistic advantage of enlarged d spacing in PECNT facilitates the transport and accommodation of more Na+ ions into the graphitic layers as well as the partial opening of PECNT into few layer graphene nanoribbons helps to adsorb more sodium ions which in turn helps to achieve enhanced specific capacity. The complete opening of MWCNT into graphene nanoribbons helps to adsorb more of sodium ions. But simultaneously it causes the problem of restacking during the extended charging and discharging cycles40. The synergistic role of inner core and outer graphene nanoribbons helps to achieve simultaneously good specific capacity and electrical conductivity due to the inner core.

The rate capability measurements of MWCNT and PECNT were performed at different current densities from 20 mA g-1 to 500 mA g-1 as shown in Figure 3c. When cycled at different current densities, better rate capability with technologically significant specific capacities of 510 mAh g1,

249 mAh g-1, 165 mAh g-1, 116 mAh g-1 and 67 mAh g-1 at a current density of 20 mA g-1, 50

mA g-1, 100 mA g-1, 200 mA g-1 and 500 mA g-1 respectively are obtained for PECNT. MWCNT 10 ACS Paragon Plus Environment

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shows a specific capacity of 222 mAh g-1,105 mAh g-1, 66 mAh g-1, 26mAh g-1and 1 mAh g-1at a current density of 20 mA g-1, 50 mA g-1, 100 mA g-1, 200 mA g-1 and 500 mA g-1 respectively. PECNT shows a specific capacity which is about 2.3 times the specific capacity obtained for MWCNT. This enhancement in capacity is contributed mainly by the increased interlayer spacing which facilitates transport and accommodation of Na+ ions, the high specific surface area offered by the opening of carbon nanotubes facilitates faster Na+ transport even at higher current density. When cycled back to 50 mA g-1, PECNT shows a capacity retention of more than 80 % and thus demonstrates the stability of PECNT over wide charge discharge conditions. PECNT exhibits better rate capability when compared to hard carbon in the previous reports12,21,41. The long term cyclic stability was carried out at a low current density of 50 mA g-1 for 10 cycles and then at a high current density of 200 mA g-1 for 100 cycles as shown in Figure 3d. PECNT also exhibits a satisfactory cycling stability at 200 mA g-1 with a specific capacity of 120 mAh g-1 after 100 cycles, while MWCNT shows 40 mAh g-1.

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Figure 3 (a) Charge/discharge curves of PECNT at different current densities. (b) Charge/discharge curves of MWCNT at different current densities (c) Rate capability of PECNT, MWCNT. (d) cyclic stability of PECNT and MWCNT To further understand the reaction kinetics, the cyclic voltammetry was carried out at different scan rates from 0.1 mV s-1to 1 mV s-1 and the graph is shown in Figure S5 a. The maximum anodic peak currents as shown in Figure S5 a and S5 b is plotted as a function of square root of the scan rate. Their linear relationship shown in Figure S5 b indicates the diffusion-controlled reaction where the Randles- Sevcik equation is applicable. 𝐼𝐼𝑝𝑝 = (2.69 × 105 )𝑛𝑛3/2 𝐴𝐴𝐷𝐷1/2 𝐶𝐶𝑣𝑣 1/2

− (1)

where I p is the peak current; n is the number of electrons transferred; A is the surface area of the electrode; D is the diffusion coefficient; C is the concentration of Na+ ions in the electrode material; and v is the scan rate. The linear relationship between peak current and square root of the scan rate indicates that the process is a diffusion-controlled process (Figure S4 c and S4 d). Also, the plot of logarithmic scan rate versus logarithmic peak current as shown in Figure S6 indicates the linear relationship. The linear fitting of MWCNT gives a slope of 0.9 which indicates that the process is surface capacitive process. The slope of PECNT gives a value of 0.6 indicating the diffusioncontrolled process. Also, to evaluate the capacity contribution from diffusion-controlled reaction and surface capacitive process, CV curves at different scan rates like 0.1 mV s-1,0.2 mV s-1, 0.4 mV s-1,0.6 mV s-1, 0.8 mV s-1 and 1 mV s-1 are done as shown in Figure S5 a) and b). According to Dunn42, the proportion of sodium ion capacity contribution can be quantified by separating the current response at a certain potential into capacitive or surface-controlled reaction (proportional to scan rate v) and diffusion-controlled reaction (k 2 v1/2): 𝑖𝑖(𝑉𝑉) = 𝑘𝑘1 + 𝑘𝑘2 𝑣𝑣 1/2

For analytical purposes, the equation is rearranged to: 1

⋯ (2)

1

𝑖𝑖(𝑉𝑉)/𝑣𝑣 2 = 𝑘𝑘1 𝑣𝑣 2 + 𝑘𝑘2 ⋯ (3)

By determining the constants k 1 and k 2 the fraction of the current from surface controlled and diffusion controlled reactions can be determined43. The quantification indicates that for PECNT at 0.1 mV s-1 the diffusion-controlled reaction contributes to about 63% and surface-controlled 12 ACS Paragon Plus Environment

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reactions contributes to about 37%. (Figure S7. c)). This indicates that majority of the capacity originated from the intercalation of sodium ions. But in the case of MWCNT at 0.1 mV s-1 the majority of the capacity originated from the surface-controlled process (Figure S7. d)). This again confirms the ease of intercalation of sodium ions into PECNT rather than MWCNT. In order to understand the interaction between sodium atom and MWCNT, density functional tight binding (DFTB) molecular dynamics simulation is done at 300 K. At first 1261 atoms double-walled carbon nanotube (DWCNT) is constructed using inner CNT (5,5) and outer CNT (10,10) as shown in Supporting Movie SM1, where the distance between two CNTs is 3.39 Å. This atomistic structure is smaller compared to the experimental structure. As DFTB calculation are more accurate and computationally heavier than classical molecular dynamics results, we have adopted these structures in order to study the microscopic details of the Na atom interaction with PECNT. Moreover, the realistic movement of sodium atom inside PECNT is given by molecular dynamics calculation, which takes care the temperature of the environment. At first, Sodium atom is kept at the edge of DWCNT and Langevin molecular dynamics simulation is carried out at 300 K. Even though sodium interacts well with carbon atoms in CNT, it is not able to enter the gap between outer and inner CNTs (Movie SM1). This is consistent with the experimental results of lower specific capacity of MWCNT. To understand the sodium atom interaction with PECNT, we created partially opened DWCNT with carboxylic functional groups and hydrogen atom termination as shown in Figure. 4. We carried out DFTB Langevin molecular dynamics simulation at 300 K by keeping sodium atom inside partially opened DWCNT (Movies SM2-SM3). Radial distribution function is plotted between the sodium and the carbon atoms (Figure S8). The distribution function shows a pronounced peak at 2.77 Å, indicating that this is the dominant nearest-neighbor distance between the sodium and the carbon atoms at room temperature. The pronounced peak indicates the stable binding characteristics of sodium atom in the PECNT. Furthermore, molecular trajectories of sodium atom do not show any rapid movement of sodium atom (Movies SM2-SM3). Sodium is caged between the hexagonal rings in the outer and inner DWCNTs (Movie SM4), which leads to stable binding of the sodium atom to PECNT. To investigate the stability of sodium atom binding to PECNT, the binding energy is calculated based on density functional theory (DFT) simulation. In the binding energy calculation, we have to account the van der Waals interactions properly, which are underestimated by local and semilocal exchange-correlation functionals. In our calculation, the long-range van der Waals interaction 13 ACS Paragon Plus Environment

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is included through the semi-empirical corrections based on Grimme D244. Geometrically optimized atomistic structure of PECNT and sodium complex is given in Figure 4. The binding energy of the sodium atom is calculated based on the following equation: 𝐸𝐸𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 𝐸𝐸𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀+𝑁𝑁𝑁𝑁 − (𝐸𝐸𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 + 𝐸𝐸𝑁𝑁𝑁𝑁 )

− (4).

Here, E MWCNT+Na , E (MWCNT) , and E Na are the total energies of PECNT and sodium complex, PECNT, and Sodium atom, respectively. The calculated binding energy is -1.50 eV. Based on the equation (4), this larger negative value of binding energy indicates a more favorable exothermic reaction occurring between the PECNT and the sodium atom. Even though binding energy is much higher than thermal energy, they don’t form chemical bonding. This binding energy is larger than Sodium-pristine Graphene and Sodium-pristine MWCNT (5,5) complexes (Figure S9), which are calculated to be -1.17 and -1.22 eV, respectively. In general, this is consistent with our experimental results. In order to further analysis, the bonding nature of sodium atom with MWCNT, density of states (DOS), wavefunction and electron difference density analysis are carried out (Figure 5). Projected DOS of sodium atom does not any show strong state around the fermi-energy, the peak at - 0.11 eV is the next closer state (inset of Figure 5a). This state shows that there is very weak overlap of wavefunction between sodium atom and MWCNT (Figure 5b). Furthermore, electron difference density of entire PECNT-sodium complex indicates no appreciable charge mixing between sodium atom and MWCNT (Figure 5c). These analyses clearly manifest that sodium atom is not chemically bonded to PECNT, which makes it suitable for sodium ion battery anode.

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Figure 4 Geometrically optimized atomistic structure of partially unzipped MWCNTSodium complex (a) front view (b) side view. Grey, red, light-blue, and purple balls are carbon, oxygen, hydrogen and sodium atoms, respectively.

Figure 5 (a) Total DOS plot of PECNT-Sodium complex. Inset: Projected DOS plot Sodium atom. Around the fermi-level, Na atom has a state at - 0.11 eV. Fermi-level is aligned to 0 eV in these plots. (b) Wavefunction plot of PECNT-sodium complex at - 0.11 eV. Isovalue: 0.005. Colour of the wavefuction indicates the phase of the wavefunction. (c) Electron difference density plot of PECNT-Sodium complex CONCLUSION In summary, expansion of a few outer layers in multiwall carbon nanotubes by functional groups has been successfully obtained by Hummer’s method, and confirmed by XRD, FTIR and TGA analyses. The partial opening of a few outer layers in MWCNT facilitates the intercalation of sodium ions into the carbon structure and also provides sufficient active sites for the adsorption of sodium ions due to its increased surface area. The inner core of PECNT remains undisturbed, which helps to maintain the electrical conductivity and is evident from the better rate capability. van der Waals interaction corrected DFT calculations showed exothermic binding energy of -1.50 eV in sodium-PECNT complex compared to -1.17 and -1.22 eV in sodium-pristine Graphene and sodium-pristine MWCNT (5, 5) complexes, respectively. This larger binding energy confirms the enhancement in specific capacity of PECNT compared to pristine MWCNT. Energy levels, wavefunction and charge density analyses shows that this binding is physisorption in nature. The larger negative value of binding energy favors the bonding between PECNT and sodium atom, which helps to enhance the electrochemical performance. The proof of concept reported here will 15 ACS Paragon Plus Environment

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be beneficial for the design and manufacture of a rechargeable sodium ion battery with PECNT as a promising anode candidate for sodium ion battery. Method of Preparation of PECNT MWCNT was synthesized by catalytic chemical vapor deposition technique, where misch metal and nickel based AB 3 alloy were used as the catalyst for the growth of MWCNT. The growth of MWCNT was carried out at 700°C in an argon atmosphere using acetylene as the carbon precursor. As obtained MWCNT was further air oxidized at 450°C and acid treated with HNO 3 to remove amorphous carbon and metal particles respectively45 PECNT was prepared by Hummer’s method46. Briefly, the calculated quantity of MWCNT in conc. H 2 SO 4 for 5 min at ice temperature. NaNO 3 was added very slowly to the above mixture, followed by the addition of KMnO 4 . The ratio of MWCNT: KMnO 4 was maintained in a certain ratio in order to obtain the controlled oxidization of the outer few layers leaving the inner core of MWCNT intact. Warm water (50°C) was added very slowly to the reaction mixture, followed by the addition of hydrogen peroxide. The addition of hydrogen peroxide removes the unreacted compounds formed in the process. Also, to completely remove the byproducts of the reaction the sample is immersed in the DI water and decanted repeatedly till the he pH of the solution become neutral. Then the solution is filtered and washed several times with DI water to make sure the removal of all impurities present. The presence of functional groups causes partial opening of MWCNT. Material Characterization Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku Smartlab X-ray diffractometer with Cu Kα radiation (λ= 1.5418 A°). All measurements were taken in the range of 10° to 90° with the step size of 0.02°. Raman spectra were recorded using WiTec Alpha Raman Spectrometer in the range of 500 to 3000 cm-1. Laser source used was Nd: YAG (532 nm). Thermogravimetric analysis was carried out in SDTQ600 analyzer from TA instruments in air atmosphere to 1000° C at a heating rate of 20°C /min with a flow rate of 160ml/min. The surface area and pore size analysis were done using Micromeritics ASAP 2020 instrument using BrunauerEmmett-Teller (BET) theory. In order to study the morphology of the samples high resolution transmission electron microscope (HRTEM) images were recorded in Technai G20 instrument by drop casting sample solution in ethanol on holey carbon coated 200 mesh copper grids. A Perkin Elmer FTIR spectrometer was used to record FTIR spectrum of samples. 16 ACS Paragon Plus Environment

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Electrochemical characterization The electrode was fabricated by a slurry coating process. The slurry was prepared by mixing 75% of active material and 10% of acetylene black with 15% of binder polyvinylidene difluoride in Nmethyl-2-pyrrolidone. The working electrodes were prepared by coating the slurry on the copper foil, which were then dried at 80° C overnight in a vacuum oven. The dried electrodes were cut in the form of a circular disc of 12mm. The mass loading of the active material is 1 mg/ cm2. 2032coin cells were assembled in an argon filled glove box where the oxygen and moisture levels were maintained below 0.1ppm using sodium metal as the counter electrode and reference electrode and glass fiber membrane (Whatman GF/C) as the separator. The electrolyte used was 1M NaClO 4 dissolved in propylene carbonate (PC). The cells were galvanostatically cycled at various current densities between 0.01 V and 2.8 V on Biologic BCS 810 battery cycler. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on Biologic SP 300. EIS measurements were conducted by applying an AC signal of amplitude 5 mV in the frequency range of 10 mHz to 100 kHz. The cyclic voltammetry was carried out in the potential range of 0.01 V to 2.8 V at the scan rate of 0.5 mV/s for 5 cycles. Computational method In this work, all DFT calculations are performed using the density functional theory (DFT) code Quantumwise ATK 47,48,which is based on a linear combination of numerical atomic orbitals and norm-conserving Troullier−Martins pseudopotentials. The Perdew−Burke−Ernzerhof exchange correlation functional, which is derived within the generalized gradient approximation (GGA) is used with semi-empirical corrections based on Grimme D2 for van der Waals interaction. Double zeta plus polarized basis sets are used and Calculations are carried out with the supercells shown in corresponding figures. In order to closely resemble with the experimental works, all the edge atoms of MWCNT are terminated with hydrogen atoms and carboxylic groups. To avoid any spurious interactions with neighbour super cells, a minimum vacuum distance of 10 Å was kept between adjacent MWCNT in all the electronic calculations. DFTB calculations are done using Slater-Koster parameters 49,50 implemented in Quantumwise ATK. ASSOCIATED CONTENT Supporting Information Available 17 ACS Paragon Plus Environment

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Figures showing Nitrogen adsorption and desorption isotherm and the distribution of pore size for MWCNT and PECNT, xps spectrum of PECNT and deconvoluted peaks of C 1s and O 1s, cyclic voltammogram measured at 0.1 mV s-1 of PECNT and MWCNT. First cycle charge discharge curves of MWCNT and PECNT, cv curves of PECNT and MWCNT at different scan rates, plot of logarithmic anodic peak current versus scan rate for MWCNT and PECNT, Bar diagram showing the percentage of capacity contribution calculated from cyclic voltammetry curve, Radial distribution between sodium and carbon atoms in PECNT complex, Initial atomistic structure of Na atom on graphene and its geometrically optimized final atomistic structure. Table showing the comparison of performances of various carbon-based materials with PECNT. AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID: M. Manoharan: 0000-0001-5421-5160 Notes The authors declare no competing financial interest ACKNOWLEDGEMENT Authors thank Indian Institute of Technology Madras, India, RCI, Hyderabad for supporting this work. For FTIR measurements, the authors acknowledge SAIF, IIT Madras.

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