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Anchoring Iodine to N-Doped Hollow Carbon Fold-Hemisphere: Toward Fast and Stable Cathode for Rechargeable Lithium-Iodine Batteries Kaidi Li, Bo Lin, Qiufeng Li, Huifeng Wang, Sen Zhang, and Chao Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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ACS Applied Materials & Interfaces

Anchoring Iodine to N-Doped Hollow Carbon FoldHemisphere: Toward Fast and Stable Cathode for Rechargeable Lithium-Iodine Batteries Kaidi Li,a Bo Lin,a Qiufeng Li,b Huifeng Wang,b Sen Zhang,b,* Chao Denga,* a

Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education; Key

Laboratory of Photochemical Biomaterials and Energy Storage Materials, Heilongjiang Province; Harbin Normal University, Harbin, 150025, Heilongjiang, China. b

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education; College

of Material Science and Chemical Engineering, Harbin Engineering University, Harbin, 150001, Heilongjiang, China.

Corresponding Author

E-Mail: [email protected] (C. Deng)

[email protected] (S. Zhang)

1 Environment ACS Paragon Plus


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Abstract: Rechargeable Lithium-iodine batteries with abundant raw materials and low cost are promising electrochemical energy storage systems. Herein, we demonstrate that anchoring iodine to N-doped hollow carbon fold-hemisphere (N-FHS) is highly efficient to overcome slow kinetics and low stability of iodine cathode in lithium-iodine batteries. For the first time, significant effects of carbon framework architecture on the lithium storage performance of iodine cathode are studied in detail. Notably, the fold-hemisphere (N-FHS) is more effective than the similar architectures, such as hollow sphere (N-S) or hemisphere (N-HS), in modifying slow ion transport capability and fast structure deterioration. The superior property of iodine@N-FHS is associated with its highly porous structure and strong interconnection to iodine. The iodine deterioration mechanism in lithium-iodine battery is analyzed, and the deterioration processes of iodine in different carbon frameworks during cycling are investigated. This work opens a new avenue to solve the key problems in lithium-iodine batteries, allowing it an important candidate for energy storage.

Keywords: electrochemistry • energy storage • lithium-iodine battery • electrochemical property • folded hollow sphere


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1 Introduction The need for long-range electric vehicles and large-scale grid storage integrated with intermittent energy sources has prompted the development of advanced rechargeable batteries.1,2 Lithium ion and sodium ion batteries have been widely studied as renewable energy storage systems.3,4 However, the limited specific energy restricts their application in ever-growing energy demands. Therefore, it is necessary to develop new rechargeable battery systems that could solve the problems of various energy applications.5,6 Among the current available candidates, rechargeable lithium-iodine batteries with metal lithium as anode and iodine as cathode are promising one.7,8 As an inorganic material, the price of iodine is much lower than those of the rare metals of cobalt or lithium in metal based electrode materials, which makes lithium-iodine battery a good alternative to realize green and sustainable energy storage.

7-16

However, the development of rechargeable lithium-iodine batteries is severely restricted in the past decades due to the low thermal stability and poor kinetics. The inferior stability and low conductivity of iodine lead to the fast deteriorated capacity, inferior kinetics and low columbic efficiency of rechargeable lithium-iodine battery. Therefore, great efforts have been carried out to improve the performance of lithium-iodine batteries to realize its full potentials. In the past decade, the fundamental electrochemistry of rechargeable lithium-iodine batteries has been intensively investigated. Aiming at delimiting the iodine dissolution in electrolyte, liu et al. prepared an all-solid-state lithium iodine battery.12 Although holding simple battery configuration, the inferior kinetics and poor rate capability originating from the solid electrolyte make it far from satisfactory. On the contrary, Zhao et al. fabricated an aqueous lithium-iodine battery with aqueous iodine cathode to seek fast kinetics.13 The aqueous electrode achieves good cyclic performance and high energy density, however, the complex configuration



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severely restricts its practical application and large-scale fabrication. Taken the advantages of both simple configuration of all-solid-state system and fast kinetics of aqueous electrode, a rechargeable lithium-iodine battery with simple cell structure is constructed.7,14 It is composed of a lithium metal anode, an iodine-C composite cathode and an organic aqueous electrolyte. The cathode electrode is made of iodine/C composite, which is constructed through loading active iodine into the carbon framework. The simple structure as well as the improved kinetics enables it a promising energy storage system. Recently, a flexible iodine/carbon cloth electrode is developed by zhao et al., which promotes its development in practical applications.14 Although great improvements have been achieved, some important issues are still open for the rechargeable lithium-iodine battery now. One of the key issues is how to construct high performance iodine/C cathode. In the iodine/C cathode, the electrochemical redox reaction is constrained inside the carbon framework. The carbon network greatly influences the lithium storage performance for the iodine electrode and lithium-iodine battery. Therefore, it is important to controllable construct highly-efficient carbon framework and anchor iodine to achieve highperformance iodine/C composites. Among the architectures of functional materials, the three-dimensional hollow spheres show superiority in energy storage applications.17-21 Its highly porous architecture facilitates uniform distribution of electrolyte and allows highly-efficient mass transport. Both promote the application of hollow spherical architecture in diverse electrochemical systems. However, one fatal drawback severely restricts its development. The large inner cavity of hollow sphere leads to the low stacking density, which greatly affects its packing density and leads to low volume energy density. To overcome this drawback, one effective strategy is cutting the hollow sphere into halves, which achieves a closer stacking capability and improved volume energy density.22-


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24

The continuous electron pathway will be formed among the stacking hollow hemispheres,

which is favorable to the fast kinetics of electrode. Therefore, it is a promising strategy to design hollow hemispheres for energy storage material. In the family of hollow hemisphere, the folded hollow hemisphere is an outstanding one. It not only has the excellent characteristics of hollow hemisphere, but also possesses good pliability and mechanical strength. Both enable it an excellent flatform for electrode materials. However, the construction of hollow foldedhemisphere architecture is difficult to control and its application in rechargeable lithium-iodine battery is vacant until now. Therefore, it is necessary to develop facile strategies to controllable construct this unique architecture and anchor iodine into it to fabricate high-performance iodine/C electrode. Following this viewpoint, for the first time, we report the successful construction of highperformance iodine/N-FHS cathode through anchoring iodine to N-doped carbon hollow foldedhemisphere (N-FHS). Through carefully controlling the conditions, three different structured carbon frameworks, i.e. N-doped carbon hollow hemisphere (N-HS), hollow sphere (N-S) and Ndoped carbon hollow folded-hemisphere (N-FHS) are constructed. The formation mechanism of different architectures is analyzed and the relationship between the architectures of carbon host and the electrochemical performance of iodine/C cathodes are investigated. Moreover, the kinetic study is carried out on all the samples to clarify their capacity deterioration during cycling. This work not only provides a high performance iodine electrode for addressing the challenges of lithium-iodine battery, but also introduces a highly efficient architecture for functional materials.

2 Results and discussion 2.1 Controllable construction of N-doped hollow folded hemispherical carbon host



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Figure 1 Schematic illustration of the controllable strategy for synthesis of hollow sphere (N-S: top), hollow hemisphere (N-HS: meddle) and hollow folded hemisphere (N-FHS: bottom) shaped carbon frameworks.

The construction of N-doped hollow folded-hemisphere carbon framework is realized through carefully controlling the preparation conditions. As illustrated in Figure 1, the evolution of diverse carbon architectures base on a facile strategy. For example, the hollow sphere (N-S) is prepared based on a mixed gel@SiO2 (i.e. resorcinol/formaldehyde/ethylenediamine@SiO2) precursor (top line in Figure 1). The mixed gel of resorcinol/formaldehyde/ethylenediamine forms a uniform coating on the surface of central SiO2 sphere, which turns into N-doped carbon shell in the following calcination. After removing the SiO2 templates by HF acid etching, the integrated N-doped hollow spheres with uniform size (~200 nm) and good strength are successfully produced (Figure 2a~c). On the other hand, the hollow hemisphere (N-HS) was prepared by a more complicated precursor. As illustrated in the middle line of Figure 1,


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tetraethylorthosilicate (TEOS) is introduced into the mixed gel and produces a novel thin layer of SiO2/mixed gel on the surface of central SiO2 particles. After carbonization and etching processes, the TEOS-derived SiO2 nano-particles produce large amount of voids in the prepared carbon shell. They lead to the more porous and less strength of the carbon framework. As a result, the prepared hollow carbon shell is brickle and easy to crack into halves, and produce the hollow hemisphere shaped carbon host (N-HS, Figure 2d). As displayed in Figure 2d~f, the N-HS sample has uniform size (200 nm) and open single-layer hemisphere architecture.

Figure 2 Morphology of the hollow sphere (N-S: a~c), hollow hemisphere (N-FS: d~f) and hollow folded hemisphere (N-FHS: g~m) shaped carbon frameworks. SEM and TEM images of different carbon frameworks (a, b, g, h, m) with one enlarged particle (b, c, e, f, i~k) to clarify their difference. The bright-field TEM image of the hollow folded hemisphere (m).



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Figure 3 (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of the hollow sphere (green), hollow hemisphere (pink) and hollow folded hemisphere (blue) shaped carbon frameworks. Comparison of BET surfaces for different carbon frameworks are displayed as insert of b. (c) XPS N1s spectrum of the hollow folded hemisphere.

Based on above results, diverse architectures of carbon frameworks can be controllable fabricated via carefully controlling the preparation procedure and components of the precursors. To further characterize their differences, nitrogen adsorption/desorption analysis were employed.


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As displayed in Figure 3a, the N-FHS sample achieves the highest BET surface among the samples, which is about one and a half and two times higher than the N-HS and N-S samples, respectively. Moreover, the pore size analysis results confirm the wider particle size distribution of the N-FHS and N-HS samples than that of the N-S sample (Figure 3b). These differences are associated with the synthetic conditions of various carbon frameworks. As discussed above, after introduction of additional SiO2 into the mixed gel of precursors, both N-FHS and N-HS achieve more porous architecture, which results in their higher surface area and wider pore size distribution. Especially, the pliable, uniform and thin shells in N-FHS enable it to achieve the double-layer architecture with superior surface porosity. On the other hand, the XPS measurements are employed to characterize N-doping in the prepared carbon frameworks. As displayed in Figure 3c, the N1s spectrum consist three peaks located at 398.3, 400.1 and 401.3 eV, which corresponds to the pyridinic, pyrrolic, and quaternary nitrogen, respectively.25,26 The plenty of N-containing species provide more electrochemically active sites that are favourable to highly-efficient electrochemical reaction of prepared carbon frameworks. Furthermore, the tap densities of different carbon hosts are investigated. The tap density of N-S sample is 0.72 g cm-3, which is much lower than those of the N-FS (1.14 g cm-3) and N-FHS (1.05 g cm-3) samples. The slightly lower tap density of N-FHS than N-FS is associated with the existence of inner cavity between its folded walls. Therefore, the results demonstrate that the half spherical structure achieves a closer stacking capability than the hollow spherical one. Moreover, an additional hydrothermal treatment is employed in the synthetic route to construct a new architecture. As displayed in the bottom line of Figure 1, the hydrothermal treatment is employed to prepared SiO2/mixed-gel@SiO2 precursor. Through high-temperature



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and high-pressure treatment, a pliable shell is constructed. The good pliability, central hollow nature, and highly porous surface enable it to fold into a double-layer structure (Figure 2g, h, m). As a result, the carbon hollow folded hemispheres (N-FHS) with well-defined double-layer hemispherical architecture and uniform particle size of 200 nm are successfully constructed (Figure 2 i~l). Combined above results, diverse carbon frameworks with different outside architecture, internal microstructure and surface characteristics have been successfully constructed through carefully adjusting the synthetic conditions.

It enables the controllable design of carbon

framework with different architectures to fulfill their properties. In present work, the controllable construction of different hollow spherical architectures, i. e. hollow sphere (N-S), hollow hemisphere (N-HS) and hollow folded-hemisphere (N-FHS), provide clues to design and synthesize new architectures for functional carbon hosts. Particularly, the good physicochemical characteristics of N-FHS make it superior flatform architecture for high-performance electrodes. 2.2 Anchoring iodine to carbon framework and fabricate iodine/C electrodes The iodine/C composites were produced through impregnation of iodine into the asprepared carbon framework (Figure 4d). As illustrated in Figure 4a, the strong signal of iodine in EDX spectrum confirms the presence of iodine in the N-FHS based carbon framework. A clearer understanding is provided by EDS-STEM line-scan element mapping, which certifies the homogeneous distribution of iodine in carbon host (Figure 4b). Moreover, the nitrogen adsorption/desorption analysis was carried out to investigate the change of BET surface areas after iodine impregnation. As compared in Figure 4c, the iodine/C composites exhibit much lower surface areas than the pristine carbon frameworks, which demonstrates the uniformly filled of iodine on the surface of the pores in carbon framework (Figure 4d). Therefore, all the results


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demonstrate that the iodine is well absorbed in the highly porous carbon framework and the iodine/C composites with uniform element distribution have been successfully constructed.

Figure 4 (a) Morphology of the iodine/N-FHS composite. The EDX spectrum of iodine element is displayed as an inert of a. (b) TEM image and corresponding line scan of iodine element in iodine/N-FHS composite. (c) Comparison of BET surfaces of different carbon framework before and after iodine impregnation.

(d)

Schematic diagram of iodine impregnation in N-FHS carbon host.

In pristine carbonaceous hollow structured hosts, the porous carbon shell is nonpolar which only provides physical confinement on the adsorbent particles like sulfate or iodine. 27 When the heteroatom of nitrogen is employed, the defect sites were produced on the carbon host, resulting in the increased polar surface for the carbon cell. It facilitates the stronger interaction between the host and iodine.5,23,26 This influence becomes more significant as the higher polar redox products such as lithium polysulfide or lithium triiodide are produced. Therefore, in present study, the bond between the N-doped carbon host and iodine is a combination of physical



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and chemical effects. The physical absorption plays a major role on the immobilization of iodine, and moreover, chemical absorption also contributes to the interaction between iodine and the host. However, both theoretical calculation and experiment certification are needed to provide detailed certification, which will be carried out in the near work. The development of rechargeable lithium-iodine battery is greatly restricted by two major problems, i.e. the high dissolution and low stability of iodine electrodes.12-15 Both defects lead to easy capacity deterioration and inferior electrochemical properties of lithium-iodine battery. Employing carbon framework is considered to be a good strategy to improve the stability of iodine electrode. 17-24, 28-33 In order to investigate the influences of different carbon framework on iodine electrode, we compared the storage stability and dissolution capability of pure iodine and iodine/C composites. Firstly, we investigate the thermostability of the iodine based samples. TG measurements were used to investigate the weight loss processes of the iodine based samples. As illustrated in Figure 5a, a fast weight decrease is observed for pure iodine, which losses ~40 wt.% of its mass as it is elevated to 70 oC and losses all the mass before it reaches 100 oC. The results demonstrate the poor thermostability of pure iodine. On the contrary, the iodine/C composites exhibit no mass loss before 130 oC, certifying their improved thermostability. After reaching 400 oC, the weight of the iodine/C composite turns to stable. It is an indication of the totally consuming of the iodine and only carbon frameworks left in the composites. Based on the residual weight of the samples, the iodine contents in different iodine/C composites are calculated correspondingly. The iodine contents of 32 wt.%, 26 wt.%, 19 wt.% are obtained for the N-FHS based, N-HS based and N-S based composites, respectively. The highest iodine content is achieved for the N-FHS based composite, which is associated with its highly porous and unique hollow folded architecture.


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Figure 5 (a) TG curve and (b) storage property (at 35 oC) of the pure iodine and iodine/C composites. (c~e) Solubility of the pure iodine and iodine/C composites. (c) Digital pictures of the solution after different storage time. Bottom line: pure solvent; middle line: after one day storage; top line: after one week storage. (d) Iodine dissolved rate of different samples during storage in solvent. (e) Calculated iodine dissolved efficiency of the pure iodine and iodine/C composites.

Next, we investigate storage stability of the iodine based composites. On the consideration of both efficiency and the effectiveness of the experiments, we store all the samples at 35 oC for different time to clarify their differences. As displayed in Figure 5b, the pure iodine exhibits the fastest deterioration rate among the samples. It losses about one fourth of its weight after six hours and all of its weigh is lost after twenty-one hours. The result



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demonstrates its poor storage stability, which would lead to fast capacity deterioration. Great improvements are observed for the iodine/C composites. After a week, only a faction of weights is lost for the iodine/C composites. Even after a month, there are still 92%, 85% and 82% of the iodine remained in the N-FHS, N-HS and N-S based iodine/C composites. Therefore, the results demonstrate the excellent storage stability of the iodine/C composites, which is favourable to their electrochemical properties. To more clearly simulate the situation in electrochemical pack, the dissolution capability of all the samples in organic solvent (electrolyte) are carefully studied (Figure 5c). The pure iodine was dissolved immediately after it was put into the solvent (DOL:DME=1:1) and the solution turns to dark brown colour. On the contrary, the solutions in very slight colour were observed for the iodine/C composites after one day. Even after a week, only light brown solutions were observed. Especially the lightest colour was observed for the iodine/N-FHS composite than the others. The iodine dissolution rates of different samples are compared to provide clearer understanding. And the dissolved iodine ratio (ξ) is calculated based on the following equation, = ξ

c c max

× 100%

(1)

Where ξ is the dissolved iodine ratio, c is the iodine concentration of the solution after a certain time, and cmax is the maximum iodine concentration of the solution. As compared in the Figure 5d, the pure iodine reached its maximum concentration within extreme short time (less than two minutes). However, the iodine/C composites get their maximum iodine concentration after much longer time (four to five days). The results demonstrate the depressed iodine dissolution rate for iodine/C composite. Moreover, the


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dissolved iodine contents of the iodine/C composites are much lower than that of the pure iodine. To clarify the difference between the samples, the relative dissolution efficiency (κ) is calculated based on the following equation, = κ

c ×V × M × 100% m(1 − a %)

(2)

Where c is the maximum iodine concentration of the solution, V is the volume of the solution, M is molar weight of iodine, m is the weight of composites and a is the carbon content of the composite. As compared in Figure 5e, the pure iodine gets the highest dissolution efficiency near 100%, which certifies its total dissolution in the organic solvent. Much lower κ values are obtained for the iodine/C composites, which certify their depressed iodine dissolution capability. Particularly, the N-FHS based composite achieves the lowest κ value (~7%), which demonstrates its good stability and lowest dissolution capability in organic solvent. In fact, the organic electrolyte in the lithium iodine battery is only a few drops (100~200 μL), which is much lower than solution volume in present work (10 mL). Therefore, the actual dissolved iodine in the practical battery would be much lower than our results. Thus all above results demonstrate the good stabilities of iodine/C composites in organic solvent electrolyte. Moreover, the above results indicate that the stability of iodine/C composite depends on the architecture of carbon hosts. It is associated with the interaction between iodine and carbon hosts. As discussed above, the physical adsorption plays an important role on the immobilization of iodine to porous hosts. The higher porosity and larger surface area of the hosts ensure stronger interaction to absorb and stabilize iodine. In present study, the N-FHS based carbon host achieves higher porosity and larger surface area than the other samples, and iodine/N-FHS composite achieves the best



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stability among the samples. Therefore, the architecture of the carbon hosts greatly affects the stability of iodine/C composite. To further understand the effects of N-doping on the iodine loading of carbon host, we carried out some control experiments. The control samples include the N-doped and nitrogenfree carbon hosts. The nitrogen-free samples were prepared by the approaches that were similar to that used for N-doped samples without added ethylenediamine (EDA). For comparison, the iodine loading amounts on all the samples were investigated. As displayed in Figure S1, higher iodine contents were achieved by the N-doped samples as compared with those of the nitrogenfree ones. It is associated with heteroatoms induced in the carbon hosts,34-36 which provides additional defect sites for iodine loading. Therefore, the nitrogen doping in carbon nanomaterials modify the physicochemical properties of carbon hosts, which results in the enhanced capability to anchor iodine. Combined above results, the highly porous carbon framework not only effectively improves the thermostability and storage stability of iodine, but also protects it from fast dissolving in organic electrolyte. Both are favorable to improve the electrochemical properties of iodine electrodes and lithium iodine battery. Especially, N-FHS carbon framework with highly porous architecture exhibits the highest stability in both ambient environment and the organic solvent, which enable it a good flatform for iodine electrode. 2.3 Lithium intercalation chemistry Encouraged by the improved stability and depressed dissolution capability, the lithium intercalation chemistry of iodine/C composites is studied. Firstly, galvanostatic charge-discharge characteristics of the samples are investigated. Figure 6 displays the galvanostatic


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charge/discharge curves (a, c, e) and the corresponding differential capacity (dQ/dV) curves (b, d,

Figure 6 (a) Charge/discharge curves and (b) corresponding Raman spectra obtained at denoted points highlighted in the charge/discharge curves.

f) of the N-FHS, N-HS, N-S based iodine/C samples. The differential capacities curve provide similar information to the cyclic voltammetry results, where the oxidation/reduction peaks correspond to the plateaus in galvanostatic charge/discharge curves.37-39 Each oxidation peak is paired with a reduction peak, indicating the good reversibility of the electrochemical processes in lithium-iodine system. The major sharp O1/R1 redox pair in differential capacity curve corresponds to the flat plateaus at ~3.0 V in galvanostatic charge/discharge curve; and the minor broad O2/R2 redox pair is associated with the short sloping plateaus at high potential of ~3.4 V



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in charge/discharge curves. The similar integral area of redox peaks implies a good reversibility of redox reactions. Moreover, the above results also indicate the multiple lithium intercalation steps of iodine electrode, which can be described as following equations, I2 +

disch arg e 2 − 2 +  → 2 LiI 3 e + Li ←  ch arg e 3 3 3

disch arg e 2 4 4  → 2LiI LiI 3 + e − + Li + ←  ch arg e 3 3 3

(3) (4)

In order to clarify the existence of LiI3 in the intercalation process, we carry out Raman text to investigate the reaction mechanism of iodine electrode during galvanostatic charge/discharge. Figure 6a and b display the galvanostatic charge/discharge curves of the iodine/N-FHS electrode and corresponding Raman spectra measured at the point highlighted in the charge/discharge curves. Based on the previous study, the peak between 110~120 cm-1 is associated with the symmetric stretching mode of I3-.40,41 Thus it is a good indication to monitor the change of LiI3 during charge/discharge process. During initial stage of discharge from point 1 to 4, the peak of I3- gradually increases and becomes sharpened. It indicates the formation of LiI3 from I2 during the initial ion intercalation process (equation 3). As the discharging proceeds, the peak turns to weaken and finally disappear at the end of discharge (from point 4 to 8). It suggests the loss of I3- species during deep ion intercalation, which is associated with the transformation of LiI3 to LiI in the second step of discharge (Equation 4). In the charging process, the change of I3- species is fully reversible (Point 8~15). Therefore, the periodical change of I3- species demonstrates the multiple ion intercalation mechanism of iodine electrode. Moreover, the reversible transition during charge/discharge process demonstrates the good reversibility of ion de/intercalation processes for iodine electrode, which enables it a good electrode candidate for rechargeable battery.


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As displayed in Figure 7 (a, b), the N-FHS based composite exhibits sharper redox peaks and longer flat voltage plateaus than the others, indicating its superior lithium intercalation capability. However, the weaker redox peaks (Figure 7c, e) with sloping plateaus (Figure 7d, f) of the N-HS and N-S based composites certify their inferior lithium intercalation kinetics. The capacities of the iodine/C electrodes with N-FHS, N-FS and N-S carbon hosts are 293, 286 and 276 mAh g-1, which are calculated based on the mass of iodine. The capacities of these iodine/C electrodes are higher than the theoretical capacity of iodine electrode in lithium-iodine battery (211 mAh g-1). It is associated with the capacitance contribution of carbon host. As illustrated in Figure S2, the pure carbon hosts also contribute a part of capacity in terms of a capacitor effect. In our case, a capacitance capacity of 40, 33 and 24 mAh g-1 are obtained for the N-FHS, N-FS and N-S carbon hosts, respectively. Therefore, we achieve a higher specific capacity for iodine/C electrode than the theoretical one (Details are displayed in Supporting information S-3). If the contribution of carbon host is removed, the specific capacities of pure iodine is calculated to be 207, 192 and 174 mAh g-1 for N-FHS, N-FS and N-S based composites (Details are displayed in Supporting information S-4), which are all lower than iodine theoretical capacity. In Figure 7, the capacities are calculated based on the former capacity calculation strategy that has considered the contribution of the carbon host capacitance capacity (as described in supporting information S-3). Similar calculation strategy is also employed by previous reports on iodine-lithium battery.14 Among all the samples, the N-FHS based iodine/C composite achieves the highest capacity. It is associated with the highly porous structure of N-FHS carbon host, which facilitates highly efficient electrolyte penetration and effectively improves the redox reversibility of iodine. The cycling performance of all the samples is displayed in Figure 8d. The capacity stability of N-



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FHS based composite is significant higher than those of the other iodine/C composites. It retains 86% of the initial capacity after 200 cycles at 0.5 C, while only 71% and 60% of the initial capacities are obtained for the N-HS and N-S based samples, respectively.

Figure 7 (a, c, e) Charge/discharge curves and corresponding differential capacity curves (b, d, f) of the N-FHS (a, b), N-HS (c, d) and N-S (e, f) based composites. The capacities of the samples are calculated based on the mass of iodine.

To investigate the capacity deterioration process during cycling, electrochemical impedance (EIS) was carried out on all the iodine/C samples. Figure 8(a~c) display the Nyquist plots of all the samples after different charge/discharge cycles. The depressed arc in the high frequency of the spectra is associated with the charge-transfer resistance (Rct); and the inclined line in the low frequency is an indication of solid-state diffusion capability of lithium ions.42-48


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As increasing the cycle numbers, the high-frequency arcs of all the samples increase correspondingly. Therefore, the electrochemical impedance (Rct) was employed to characterize the deterioration during cycling. Figure 8e displays the relationship between the Rct values and the cycle numbers.

Figure 8 (a~c) Nyquist plots and (d) galvanostatic charge/discharge behaviors of the N-FHS (a), N-FS (b) and N-S (c) based composites. The relationships between (e) electrochemical resistance (Rct value), (f) apparent deteriorated rate and the cycle number of all the samples. (g) Schematic illustration of the highly efficient iodine redox reaction and fast electrochemical kinetics in the iodine/N-FHS composite during charge/discharge process.



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Among the samples, the N-FHS based sample shows the lowest Rct values in the whole range, which demonstrates its superior electrochemical capability. As the cycling number is increased, the Rct values of all the samples initially quickly increase, then turn to slowly increase. The changes of Rct values with cycling numbers indicate that the capacity deterioration is not uniform in the whole range. Therefore, an apparent deteriorate rate is calculated to further characterize the deterioration processes based on the following equation: v= ∆Rct / ∆n

(5)

where v is the apparent deterioration rate; R is the electrochemical resistance; n is the cycling number. Figure 8f shows the relationship between the apparent deterioration rates with the cycling numbers. The N-FHS based sample exhibits much lower deterioration rate than the N-HS and N-S based samples, certifying its lower deterioration rate than the others. Therefore, all above result demonstrates that the hollow folded hemispherical architecture facilitates the suppression of the capacity deterioration and improves the cycle stability of the iodine electrode. The superior electrochemical performance is associated with its unique architecture of N-FHS carbon host. On the one hand, the larger surface area and hollow folded hemispherical architecture of N-FHS carbon host facilitate better electrolyte penetration and larger interaction area between iodine and carbon framework (Figure 4d, left figure of Figure 8g). It enables strong immobilization of iodine in pores of carbon host and facilitates the depression of iodine dissolution. On the other hand, the enrichment of lithium ions on the surface of the carbon framework act as a buffer layer of high current density, which effectively accelerates the kinetics of iodine electrochemical reaction and improves its redox reversibility (middle figure of Figure


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8g). Moreover, the N atom doping can increase the overall electrical conductivity of the iodine/C composite, which is favourable to its capacitive behaviour. Therefore, the N-FHS based iodine/C composite achieves the highest iodine redox reversibility and best carbon host capacitive behavior among the samples, which results in its best galvanostatic charge/discharge characteristic. Combined above advantages, the N-doped hollow folded hemisphere facilitates fast lithium/electron transport capability and suppresses the iodine deterioration, which is beneficial to its fast electrochemical kinetics, high-rate capability and long-term cycling stability. Encouraged by the advantages of hollow spherical architecture, the high rate capability and long-term cycling stability of the iodine/C composites are investigated. A series of current densities, from 0.5 C to 20 C, are employed to evaluate their rate capability. Figure 9(a~c) display the discharge curves of the N-FHS, N-HS and N-S based composites at different current densities. The iodine/N-FHS composite exhibits higher capacities than the N-HS and N-S based samples at all current densities. As the current density increases, the difference between the samples becomes more significant. The results demonstrate the superior high rate capability of iodine/N-FHS composite, which is associated with the fast kinetics of the hollow folded hemisphere architecture. Moreover, the energy density and power density of the iodine/F-NHS composite are calculated based on cathode mass. It reaches an energy density of 156.4 Wh·kg-1 (0.5 C) and a power density of 11.6 kW·kg-1 (60 C). Detailed calculation processes are described in Supportingn information S-5, S-6 and Figure S3. The energy density of present iodine/F-NHS composite is higher than the of iodine/conductive carbon black composite in previous similar work (91.4 Wh·kg-1)7. And the power density obtained in our work is similar to the iodine/conductive carbon black of previous work (11.9 kW·kg-1)7. Therefore, the results indicate the good electrochemical properties of present iodine/N-FHS composite. Although the loading of



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iodine/C in present work is relative high, the pure iodine loading is much lower. It is associated with the low iodine content in the iodine/C composite. The large amount of conductive carbon enables the iodine/C composite achieve good rate capability. Therefore, the excellent rate performance is achieved for the iodine/C composite in present study.

Figure 9 (a~c) Charge/discharge curves and cycling properties (d) of the N-FHS (a), N-HS (b) and N-S (c) based iodine/C composites. (d) Rate cycling properties of the iodine/C composites based on different carbon hosts. (e) Long-term cycling properties of the N-FHS based composite.

Next, the cycling performances of all samples at various rates are investigated. As displayed in Figure 9d, obvious capacity deterioration are observed for the N-S and N-HS based samples, however, little change are observed for the N-FHS based sample during cycles. Thus the results demonstrate the good stability of N-FHS based composite, which can be attributed to


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its good iodine immobilization and highly efficient ion/electron transport capability as discussed above. Moreover, a long-term high rate charge/discharge cycling was carried out on the iodine/N-FHS composite. As displayed in Figure 9e, the N-FHS based sample achieves good long-term cycling stability, which retains 84% of the initial capacity after 200 cycles at 5 C. The results demonstrate the superior fast charge/discharge capability and long-term cycling stability of the N-FHS based iodine/C composite. Combined with above results, the advantages such as highly conductive and porous framework, large surface area and superior structure stability enable the N-doped hollow folded hemisphere a favorable architecture to construct highperformance iodine electrode.

Figure 10 Comparison of the electrochemical performance of the iodine/N-FHS samples with different iodine contents. (a) Discharge capacities of the samples at different rates; (b) Capacity retentions of the samples at different rates; (c) Cycling properties of the samples at different rates.

Moreover, we also investigate the electrochemical performance of iodine/N-FHS composite with higher iodine content of 47 wt.%, and compare it with that of previous iodine/N-



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FHS sample with the iodine content of 32 wt.%. As compared in Figure 10a, at low current density of 0.5 C, both composites exhibit similar capacities. When the current density is increased to 1 C, the higher iodine sample exhibits lower capacity than the previous sample. As the current increases, the differences between samples become more significant. Moreover, the cycling properties of both samples are investigated at the low, mediate and high rates (Figure 10b). The capacity retentions of both samples at different rates are summarized in Figure 10c. The previous sample with iodine content of 32 wt.% exhibits higher capacity retentions than the high iodine sample (iodine content 47 wt.%) at all current densities. The difference between the samples increases as the current density increases. Thus the results demonstrate that too much iodine amount in the composite deteriorate the high rate property of the iodine/N-FHS composite. Therefore, choosing appropriate iodine loading amount is crucial to achieve good electrochemical performance for iodine/C electrode. 3

Conclusions In summary, we have successfully designed different carbon architectures, i.e. N-doped

hollow sphere (N-S), N-doped hollow hemisphere (N-HS) and N-doped hollow folded hemisphere (N-FHS), to construct high-performance iodine/C cathodes for lithium-iodine batteries. The formation mechanism of different architectures is investigated, and their significant effects on the lithium intercalation chemistry of iodine/C composites are carefully studied. Compared with the N-HS and N-S hosts, the N-FHS carbon framework is a preferable host for its highly efficient immobilization of iodine. It enables the fast kinetics and slow cycling deterioration of iodine/N-FHS cathode, which are the key drawbacks of lithium-iodine battery. Taken the advantages of the highly porous and pliable framework, superior ion transport kinetics and strong iodine interconnection, the iodine/N-FHS composite achieves high iodine utilization,


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excellent cycling stability and superior high rate capability. Therefore, the present study promotes a significant research direction to solve the key problems in lithium-iodine batteries. Moreover, it also introduces a novel general and highly-efficient platform to construct functional carbon based materials for various applications, including energy storage, catalysis and so on. 4

Experimental Section

4.1 Synthesis Preparation of N-doped carbon hollow folded-hemisphere (N-FHS). Firstly, 35 mL ethanol, 20 mL DI water and 2 ml ammonia aqueous solution (25 wt.%) were mixed well to form a mixed solution, followed by the addition of 3.6 mL tetraethylorthosilicate (TEOS) to above solution. The mixture was stirred for one hour at ambient temperature. Secondly, 0.3 g resorcinol, 0.45 g formaldehyde, 1 mL TEOS and 0.64 mL ethylenediamine (EDA) were added to above solution. Then, the precursor mixture was strongly stirred for 24 hours at ambient temperature. Thirdly, it was sealed in an autoclave and heated at 100 oC for 24 hours. The obtained gel was washed by DI water and ethanol, followed by drying at 80 oC overnight to obtain the intermediate product. The intermediate products were heated at 800 oC for 3 hours with the heating rate of 0.5 oC/min under Ar atmosphere. Finally, the obtained product was etched firstly by diluted HF solution for one day, and then by hot dense HF solution (10 wt.%) for another day. Then, powders were washed by DI water until pH value of filtrate reaches 7. Preparation of N-doped carbon hollow hemisphere (N-HS) and N-doped carbon hollow sphere (N-S). The preparation processes of N-HS and N-S are similar to that of the N-FHS. The major difference between the N-HS and N-FHS is that the former one deletes the hydrothermal treatment to prepare the N-FHS sample. On the other hand, the major difference between the N-S



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and N-FHS is that the ratio of mixture solution is changed and only resorcinol and EDA are mixed in this step. Detailed preparation processes of N-HS and N-S are displayed in the supporting information S1 and S2. Preparation of iodine/C composites with different architectures. To prepare iodine/C composites, the as-prepared carbon frameworks were mixed with iodine particles. Then the mixture was sealed in a glass bottle, which was evacuated to vacuum and then heated at 180 oC for 48 hours. After washing the obtained products with water-ethanol mixed solution, the feeble iodine was removed and the iodine/C composites were achieved. 4.2 Materials characterization The morphology was observed with a scanning electron microscope (SEM, HITACHIS-4700) and a transmission electron microscope (TEM, JEOS-2010 PHILIPS). Nitrogen adsorptiondesorption isotherms were measured using a Micromeritics ASAP 2010 sorptometer and specific surface area were calculated correspondingly. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was employed to measure the element chemical state. The Raman spectra were collected on a Jobin-Yvon Labor Raman system by exciting a 514.5 nm Ar ion laser (Raman, HR-800 HORIBA). Thermogravimetric analysis (TGA, NETZSCH STA 449C) in flowing nitrogen was used to measure the iodine content in the iodine/C composite. The tap density of the carbon hosts was measured as follows: a certain quantity of powder was added into a dry measuring cylinder, and the cylinder was then taped at least two-hundred times to ensure the volume of the powder did not change. Then, the tap density was calculated by determining the ratio of mass-to-volume of the powders. 4.3 Electrochemical measurements


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Each iodine/C electrode was made from a mixture of the active material of carbon black and polytetrafluoroethylene (PTFE) in a weight ratio of 80: 10: 10. The mixture was rolled into a film and the electrode was punched from it. After dried in an oven, the iodine/C electrode was achieved. The 2032 coin cells were assembled in an argon filled glove box. Lithium foil was employed as counter and reference electrode. The electrolyte was 1M LiTFSI in a mixed solvent of DOL/DME (v/v=1:1) with LiNO3 (1 wt.%). Galvanostatic charge-discharge tests were performed on a Land battery testing system (Wuhan, China). EIS measurements were conducted using a Zivelab electrochemical workstation, and the applied frequency range is 100k~5 mHz. Associated Content Supporting Information The supporting information. Experimental Details (S-1, S-2), capacity calculation process (S-3, S-4), iodine loading in nitrogen-free sample (Figure S1) and charge/discharge curve of pure carbon host (Figure S2) and N-FHS based iodine/C composite at 60 C (Figure S3). Acknowledgements This work is supported by Natural Science Funds for Distinguished Young Scholar of Heilongjiang Province (No.JC2015001). References: 1

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TOC


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Anchoring Iodine to N-Doped Hollow Carbon Fold ... - ACS Publications

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