Iodine and nitrogen co-doped carbon microspheres for ultrahigh

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Iodine and nitrogen co-doped carbon microspheres for ultrahigh volumetric capacity of Li-ion batteries Dong Wang, Junshuang Zhou, Junkai Li, Yuanzhe Wang, Li Hou, and Faming Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04588 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Sustainable Chemistry & Engineering

Iodine and nitrogen co-doped carbon microspheres for ultrahigh volumetric capacity of Li-ion batteries Dong Wang, Junshuang Zhou, Junkai Li, Yuanzhe Wang, Li Hou and Faming Gao* Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, No.438 Hebei Street, Qinhuangdao 066004, China

E-mail: [email protected] (F. Gao)

ABSTRACT: Due to high density, carbon microsphere is a good candidate for ultrahigh volumetric capacity electrode materials. We find that the synergistic effects of nitrogen and iodine element can tremendously improve the carbon electrode capacity. Here, we present a high volumetric capacity anode consisting of iodine and nitrogen

co-doping

carbon

microspheres

synthesized

by

low-temperature

solvothermal route. A reversible capacity of 1418 mAh cm-3, which is much larger than that of commercial graphitic carbon anode, is achieved at current density of 0.05 mA cm-2 by using these carbon microspheres as anode material. After 100 charge–discharge cycles at 0.1 mA cm-2 and 500 charge–discharge cycles at 1 mA cm-2, carbon microspheres retained volumetric capacities of 1335mAh cm-3 and 1090mAh cm-3, respectively. Keywords: anode, solvothermal, synergistic effects, high density, heteroatoms

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Introduction Since their commercialization by Sony in 1991, rechargeable lithium ion batteries have relied on carbons as the main sources of anode active materials due to their low cost, excellent electrical conductivity and long lifespan.[1-7] However, most common graphitic carbons have relatively low theoretical capacity (372 mAh g-1).[8-11] Moreover,

the

unwanted lithium

dendrites

will

inevitably

separate

out on the surface of graphite based anode because of its working potential of around 0 V versus Li+/Li.[12] Thus, massive efforts have been made to develop new carbon materials as energy storage devices with large gravimetric and volumetric capacity to meet the growing capacity and rate demand of the portable electronic equipment. The various carbon based materials such as fullerenes,[13] carbon nanotubes,[14] carbon nanosheets,[15] graphene[16] and porous carbon[17] have been paid much attention. The gravimetric capacities of these two-dimension or porous nano materials can reach to 1000~1500 mAh g−1. However, these carbon anodes generally have a low volumetric capacity owing to their low packing density(< 0.5 g cm−3 ).[18,19] As is known, volumetric capacity is more significant in miniaturized and integration development of electronic products. Since the volumetric capacity is the product of gravimetric capacity and tap density of electrode,[20,21] to gain high volumetric capacity, both gravimetric capacity and electrode density should be considered. Compare to the large specific surface area and porous structure, doping and surface functional groups have a less influence on electrode material density. Doping with heteroatoms such as nitrogen, boron, phosphorus and sulfur has been

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demonstrated to improve the lithium ion storage and binding ability by replacing carbon atom to introduce defects.[22] Unfortunately, blindly pursuing the high electrode density generally decreases the lithium ion-accessible surface area, which inevitably reduces its gravimetric capacity. To circumvent the above issues, keeping effective and enough lithium ion access in the process of increasing electrode density seems like a solution. Wang et al.[23] recently reported a carbon monolith made from nitrogen doped graphene with a density of 1.1 g cm-3. And the carbon monolith delivers an ultrahigh volumetric capacity of 1052 mAh cm-3 as an anode. Though the holey structure in carbon monolith provides lots of edges to enhance Li intercalation, its two-dimensional feature will also be a limiting factor at electrode density. So finding a “hand” without decrease of density to catch lithium ion into high density electrode material to increase the volumetric capacity is necessary. In this work, we present a solution to improve the poor electrochemistry of high density carbon materials without forming pores. That is, iodine as an effectively lithium ion-accessible approach doped in high density carbon material to improve capacity and rate performance. Iodine doping as functional groups at the edge or surface of carbon layers, instead of replacing carbon atom because the atomic radius of iodine is too large, can form electron transfer to improve the charge density and electrochemical activity.[24-26] We employ iodine and nitrogen co-doped carbon microspheres (INCM) with high packing density as anode electrode. The INCM shows a high electrode density of 1.59 g/cm-3 and an ultrahigh volumetric capacity of

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1418 mAh cm-3. The capacity of INCM is much higher than that of nitrogen doped carbon microspheres (NCM, 160mAh cm-3) and commercial graphite (550 mAh cm-3). The iodine ions in carbon microspheres could not only provide efficient attraction on lithium ions for the highly electronegative of iodine functional groups but also provide active sites for storage of lithium ions. And similar effects of iodine ion have been verified in other commercial carbon. Results and Discussions SEM images of the INCM are shown in Figure 1 and SEM images of the NCM are shown in Figure S2. The image of the products synthesized thorough a simple low-temperature solvothermal route indicates that the final products consist of a large quantity of carbon microspheres and the diameters of the microspheres typically range between 2 µm and 4µm. The carbon spheres are in nearly-perfect spherical morphology whose surface is smooth without cracks. The morphology is beneficial for the safety of lithium-ion battery due to even current distribution resulting in reduced dendrite.[27] Energy dispersive spectral (EDS) mapping images of the INCM were performed to show the homogeneous dispersion of carbon, nitrogen and iodine within carbon microspheres.

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Figure 1. a) low-magnification SEM image of the INCM; b) high-magnification SEM image showing spherical morphology of INCM; c) a SEM image of INCM, and the corresponding EDS elemental mappings of (c) oxygen(green) (d) carbon(magenta); (e) nitrogen (Cyan); and (f) iodine (yellow).

The crystal structure of the INCM and NCM is analyzed by X-ray diffraction (XRD), with the result shown in Figure 2a. Broadened diffraction peaks (002) at 2-theta of 25.8 are observed in both samples, implying pronounced features of amorphous carbon.[28] After doping with iodine the (002) peak position has not shift obviously, illustrating that the doping causes no changes of the carbon lattice structure. Compared with nitrogen doped carbon microspheres the intensity of the peak of the iodine and nitrogen co-doped microspheres increased, suggesting more graphitic-like ordered structure are formed with the doping of iodine. Same results are obtained from Raman spectra to characterizing structural defects. As can be seen from Figure 2b, both NCM and INCM have two peaks located at 1350 and 1590 cm-1, which can be attributed to the D band and G band, respectively. The D band is associated with 5



the disorder or defects in the carbon structure and the G band is related to sp2-bonded carbon atoms in graphite-related mode.[29] The intensity ratio of these two peaks (IG/ID) is always used as a parameters to assess the crystalline degree of the carbon materials.[30] The D/G intensity ratio (ID/IG) of INCM (1.165) is lower than that of NCM (1.297), which suggests that a larger proportion of graphitic structure in INCM.

Figure 2. a) XRD patterns and b) Raman spectra of the INCM and NCM.

Elemental chemical states of the INCM and NCM were determined from XPS analysis are shown in Figure 3a. The C1s, O1s, N1s and I3d peaks without any other impurities can be seen in the XPS spectra of INCM. As plotted in Figure 3b, a typical C1s peak at 284.8 eV, which contains four typical peaks at 284.6 eV (C atoms in the sp2 aromatic bonds), 285.2 eV (C–I ), 286.7 eV (C-N), and 289 eV (O-C=O), respectively.[31,32] The XPS N1s spectra in Figure 3c at 398.6eV of INCM showed three defined peaks located around 398.3, 399.8 and 400.7 eV. These different peaks are attributable to the pyridinic N (edges of graphitic planes), pyrrolic N (bond with two carbon atoms), graphitic N (substitute within the graphene plane), respectively.[33,34] The O content in INCM (O1s peak at 532 eV) is due to partial surface oxidation.[35] In Figure 3d, the two representative I3d peaks located at ~ 619.3

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and ~631.0 eV can be assigned to I3d5/2 and I3d3/2 states, respectively. The peak signals of I3d5/2 can be further deconvoluted into two peaks. One is located at 617.97 eV for triodide (I3-), another is locate at 620.22 eV for pentaiodide (I5-). The peak signals of I3d3/2 can be further deconvoluted with two peaks locating at 630.1 and 632.1 eV, which are associated with the I- and C-I.[25,36,37] The specific contents of every component (C,N,I,O) in INCM derive from XPS were shown in Table S1.

Figure 3. a) XPS survey spectrum of INCM; and b–d) high-resolution XPS spectra of C 1s, N 1s and I3d of INCM, respectively.

To further determine the porosity of INCM, N2 adsorption/desorption isothermal analysis was executed. The INCM performed a low SSA of 7.1 m2 g-1, owing to their few micropores and stacked structures. The micropores in the nano range can be

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observed from the pore size distribution (Figure S3a), and the total pore volume of INCM is only 0.006 cm3 g-1. The NCM performed a similar SSA and pore size distribution with INCM (Figure S3b), demonstrating the doping of iodine causes a not pronounced change. The low SSA and nearly-nonporous structure are major contributors to gain high density carbon electrodes. The performance of the iodine and nitrogen co-doped carbon microspheres and nitrogen doped carbon microspheres are investigated as anode for LIBs. In Figure 4a and Figure S4, both INCM and NCM show typical CV curves containing the first three cycles at a scan rate of 0.1 mV s-1. As can be seen in Figure 4a, the scan of the iodine and nitrogen co-doped carbon microspheres shows a reduction peak at ~2.6V in the first cycle may be assigned to reduction of the tri-iodine ion to iodine ion. The peak close to 0 V is often considered intercalation of lithium ion into the carbon anodes. The extra part of first cycle is result of electrolyte decomposition and SEI layer formation of both INCM and NCM electrodes.[38] Figure 4b shows the 1st, 2nd, 50th, and 100th discharge/charge profiles for the INCM electrode between 3 V and 0.01 V versus Li+/Li under a low current. The reversible capacity of INCM is approximate 1400 mAh cm-3 in the first cycle at 0.1mA cm-2 and 1335 mAh cm-3 after 100 cycles, which are much higher than that commercial graphite. A slightly increased capacity is observed in Figure 4c from the tenth cycle indicates the enhancement of volumetric capacity and stability. To shows the rate performance of INCM and NCM, the materials were cycled at different current densities from 0.05 to 2.5 mA cm-2 (for 10 cycles at each rate), and

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the corresponding rate capacities of INCM and NCM are displayed in Figure 4d. With the increase of charge/discharge current densities, the reversible capacities of INCM are approximately 1418, 1263, 1122, 1004, 840, 664 and 504 mAh cm-3 at 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5 mA cm-2, respectively. After 70 cycles discharging/charging at the increased current densities, the reversible capacity can still be recovered to 1400 mAh cm-3 when the current density goes back to 0.05 mA cm-2, implying excellent stability and reversibility. The areal capacities of INCM are approximately 2.8, 2.5, 2.2, 2.0, 1.7, 1.3 and 1.0 mAh cm-2 at 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5 mA cm-2, respectively. By contrast, NCM exhibits a poor rate performance. The reversible capacities of NCM are approximately 167, 127, 109, 94, 81, 67 and 36 mAh cm-3 at 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5 mA cm-2, respectively. So the doping of iodine also greatly increased the capacities under high current densities. To understand the improved kinetics of lithium ion transport in INCM electrode, the electrochemical impedance spectroscopy (EIS) measurements were carried out before and after cycling. As can be seen in Figure 4e, the cycled impedance spectra obtained from INCM and NCM are composed of two parts: a semicircle at high-to-medium frequency region and an inclined line at low-frequency region.[39] It is obviously that the doping of iodine makes the charge-transfer resistance of INCM a greatly reduction when compares to that of NCM for the smaller semicircle, indicating that the SEI and contact resistance of INCM was reduced after I-doping. Besides, the inclined lines INCM have a bigger slope, which means that the lithium-ion diffusion in the INCM electrode is improved. These improvements can be

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proved by the fitted values, which were shown according to the equivalent circuit in Figure 4e. Here, Re represents the resistance of electrolyte and separator, Rf and CPE1 stand for the migration of Li+ through the SEI film on the surface. Rct and CPE2 represent the charge-transfer resistance and double layer capacitance, respectively. The ZW is related to the solid state diffusion of Li+ in the active material. The Rf and Rct of INCM are 12.8 and 49.4 Ω, respectively, which are much less than those of NCM (55.5 and 196.2 Ω). It can thus clearly be seen that iodine-doping makes the charge-transfer and Li-ion diffusion easier. Comparing with the impedance spectra of INCM and NCM before cycling in Figure S5, the SEI and contact resistance of cycled electrodes have a reduction after cycling. For the INCM, the Rf and Rct (12.8 and 49.4 Ω) of cycling are less than those of before cycling (41.8 and 150.5Ω). The trend of NCM is the same, the Rf and Rct (55.5 and 196.2 Ω) of cycling are less than those of before cycling (163.9 and 698.5Ω), which are both benefit from stable SEI and batter wetting between the electrode and electrolyte. Figure 5a exhibits cyclic performance of INCM and NCM at the current density of 1 mA cm-2. A much higher reversible capacity of INCM anode was performed than that of NCM. From 10th cycle to 350th cycle, the reversible capacity of INCM shows a significant increase, indicating a gradually activation of electrode. From 350th cycle to 500th cycle, the reversible capacity of INCM maintains nearly constant and finally settles around 1090 mA cm-3. The excellent performance has proved that the method of iodine doping can efficiently improve the reversibility of insertion/extraction of lithium ion, which is similar to that previously reported.[25] To prove the

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stability of these carbon material, XRD pattern and Raman spectra were measured before and after cycling. As can be seen in Figure S6, the structures of INCM and NCM both have no obvious changes. Only crystallinity decrease was found in pattern, which was result from the insertion/extraction of lithium.

-1

Figure 4. a) Cyclic voltammograms of INCM at a scan rate of 1 mV s . b) Voltage profiles of INCM collected at 0.1 mA cm-2 during the 1st, 2nd, 50th, and 100th cycles. c) The discharge capacity retention and Coulombic efficiency of INCM for the 100 times cycling test at 0.1 mA

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cm-2. d) Rate capacities of INCM at the current densities varying from 0.05 to 5 mA cm-2. e) Electrochemical impedance spectra of cycled INCM and NCM.

Compared with NCM, the I, N co-doped carbon microspheres can reduce the transportation resistance of Li ion, which effectively promote the insertion and extraction of lithium ion. In order to prove this point, we conducted a series of additional experiments.

1.

NCM

was mechanically ground by a

short-term

ball milling to increase the electrode/electrolyte interface without any chemical activation for eliminating new defects and functional groups. The ball-milling NCM is fluffier viewed with the naked eye. Figure 5c shows the discharging and charging curves in first cycle of NCM, ball-milling NCM and INCM between 3 V and 0.01 V versus Li+/Li under a low current. Along with the increase of the electrode/electrolyte interface, gravimetric capacity of ball-milling NCM effect, but the electrode density obviously decreased relative to NCM and INCM. That is, Nitrogen doped carbon is used to being anode of lithium ion batteries, but the too low surface area of NCM limited the intercalation of lithium ion. 2. Different content of iodine doped NCM was synthesized and evaluated through half cells. We now named the INCM mentioned above as INCM-2 for distinction. INCM-1 and INCM-3 represent the quantity of iodine during synthesis were 1g and 3g respectively. After measuring, the content of iodine in carbon spheres has no significant influence on electrode density. Figure 5d shows the discharging and charging curves in first cycle. INCM-1 has a relatively low gravimetric capacity for their is not enough iodine ion to catch lithium ion. But the content of iodine in INCM-3 is excess. The unwanted wild fluctuation at ~2.8V on

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charging curve may result in shuttling of excess iodine like polysulfide in lithium-sulphur batteries. 3. To prove the attraction and storage to lithium ion of iodine ion in carbon, we delivered a similar experiment to obtain iodine doped carbon microspheres (ICM). Figure 5e exhibits gravimetric capacities of carbon microspheres (CM) and ICM at different current density. It is apparent that the ICM performed a better gravimetric capacity and rate performance indicating that the dopant of iodine has positive effects on high packing density carbon electrode.

Figure 5. a) Galvanostatic cycling performance of INCM at 1 mA cm-2. b) Schematic illustration of the intercalation of lithium ion across the carbon microspheres. c) Discharge and charge curves of NCM, ball-milling NCM and INCM. d) Discharge and charge curves of INCM-1, INCM-2 and INCM-3. e) Discharge and charge curves of CM and ICM at different C rates (1C=372mA g-1).

Every microsphere can be regarded as many carbon sheets densely stacked.[40] A

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schematic of the lithuim ion-intercalation across the carbon microspheres is shown in Figure 5b. The groups of I3- and I5- were introduced on the surface or edges of carbon sheets during the reaction process. The present of the negative charge will facilitate the attraction and catch of lithium ion.[25] Through these comparisons of NCM, ICM and INCM, we can see that nitrogen doping can induce a large number of defects to store Li+, but the too low surface area of the high density NCM electrode decreases the lithium ion access. In contrast, although iodine doping can not induce a large number of Li+ storage sites, it can improve the lithium ion-accessible approach in carbon electrode. The nitrogen and iodine co-doping exhibits synergistic effects on the capacity of high density electrode. During the synthesis stage, the increased stress of reaction still because of the volatile of iodine under high temperature may lead to a greater density of electrodes material.[41] According to the properities of other anode materials included in Table S2, the ultrahigh volumetric capacity of iodine and nitrogen co-doping carbon microspheres benefited from both increase of gravimetric capacity and density of electrode, which make the INCM an promising anode material for Li-ion batteries. In conclusion, high volumetric capacity electrodes based on iodine and nitrogen co-doped carbon microspheres have been successfully synthesized by the hydrothermal method. As anode in LIBs, the iodine and nitrogen co-doping carbon microspheres exhibit high volumetric capacity, excellent cyclic stability and rate performance. After doping of iodine and nitrogen, the negative charge and defects are simultaneously introduced to the electrode materials, which lead to a more effective

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attraction and storage of lithium ion. Besides, the inexpensive raw materials and easily synthetic method and superior Li-ion storage performance enable its potential for practical applications in miniaturization energy storage devices.

Experimental Section Material synthesis: All the chemical reagents are analytically pure and purchased from

Alading Chemical Reagent Co., Ltd. INCM: 2 g iodine (as the iodine source) was dissolved in mixture of 10 mL of acetonitrile and 20 mL benzene, then 1 g of CTAB (as the stabilizer) was added and stirred for 30 min. Pour the obtained solution

in a

stainless steel autoclave (total capacity: 50 mL) under the protection of argon atmosphere. The autoclave was closed and move into a furnace at 400℃ for 16h. When the reaction is over, open the lid of furnace and cooled down naturally. The black powders were collected from steel autoclave, and followed by washing with ethanol and demonized water, respectively. Finally, the powder was dried in drying oven at 80℃. NCM: nitrogen doped carbon microspheres were synthesized with a same operation without iodine source. ICM: 2 g iodine was dissolved in 30 mL benzene. Then 2 g mesophase carbon microbeads was added in above solution and stirred for 30min. The mixture was transferred into a same autoclave. The autoclave was treated as the same at 400℃ for 16 h, and then cooled down. The products of ICM were collected from steel autoclave, and followed by washing with ethanol and deionized water, respectively. Finally, the product was dried in drying oven. Materials characterization: The phases of sample was measured on X-ray diffraction

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(XRD, D/max-2500/PC, Cu-Ka radiation (λ=0.15418 nm)). The morphology of sample was collected on scanning electron microscopy (SEM, JEOL-JSM-7001F). Raman spectra of INCM and NCM were performed using a Princeton Instruments Spectrograph (SP2750) with 532 nm excitation source. The surface chemical content of sample was detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 spectrometer (Perkin-Elmer)). N2 adsorption and desorption isotherms were measured at 77 K on a TriStar 3020 system. Surface area and pore size distributions were obtained by the BET and the BJH method, respectively. Electrochemical characterization: The electrochemical properties of the samples were

tested through half cells (2032 type). For preparing working electrode, the slurry mixed with the carbon microspheres (80 wt%), carbon black (10 wt%) and poly(vinylidene difluoride) (PVDF) (10 wt%) in NMP was coated on a copper foil. Dried it at 80 ℃ for 12 h in a vacuum oven. The half cells were composed of lithium metal counter electrode, Celgard 2400 separator and working electrode. 1 M LiPF6 in a 1:1 v/v mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. Galvanostatic charge–discharge was tested by Land battery tester (Wuhan, China) in the voltage range of 3–0.01 V vs. Li+/Li. Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) were carried out on a CHI660e electrochemical workstation.

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ASSOCIATED CONTENT Supporting Information

The arithmetic of volumetric capacity; low-magnification SEM image of the NCM, N2 isotherms and pore-size distribution of INCM and NCM; cyclic voltammograms of NCM; electrochemical impedance spectra of before cycling INCM and NCM; XRD pattern of INCM and NCM electrodes before and after cycling; Raman spectra of INCM and NCM electrodes before and after cycling; low-magnification SEM image of ICM and EDS elemental mappings; galvanostatic cycling performance of NCM collected at 0.1 mA cm-2; composition of INCM, NCM and ICM; performance of other anode materials reported.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

ACKNOWLEDGMENT The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21371149, 21671168) and the Natural Science Foundation of Hebei (Grant No. B2016203498, 17964403D, GCC2014009). REFERENCES [1] Yoshio, M.; Wang, H.; Fukuda, K. Spherical Carbon‐ Coated Natural Graphite as a Lithium‐ Ion Battery‐ Anode Material. Angew. Chem. 2003, 115, 4335-4338. 17



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Y.

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in nanostructured

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For Table of Contents Use Only

Synopsis:

Ultrahigh

volumetric

capacity

anode

is

synthesized

for

LIBs,

which benefits from the synergistic effects of nitrogen and iodine in carbon microspheres.

23


Iodine and nitrogen co-doped carbon microspheres for ultrahigh

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