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A novel itaconic acid-graphite composite anode for enhanced lithium storage in lithium ion batteries
Carbon 152 (2019) 671e679
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Carbon journal homepage: www.elsevier.com/locate/carbon
A novel itaconic acid-graphite composite anode for enhanced lithium storage in lithium ion batteries Ruitian Guo 1, Yan Wang*, 1, Xiaojian Shan, Yongkang Han, Zhang Cao, Honghe Zheng** College of Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu, 215006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2019 Received in revised form 14 June 2019 Accepted 15 June 2019 Available online 18 June 2019
An itaconic acid-graphite composite anode is prepared for lithium ion batteries. The composite anode exhibits much improved electrochemical performances compared to the traditional graphite anode. A high specific capacity of 571 mAh g1 can be obtained at 25 mA g1 in the voltage range of 0.01e3.0 V vs. Li/Liþ, and retained 96.8% at 10 A g1. After cycled 200 cycles at 100 mA g1 charge/200 mA g1 discharge, a reversible capacity of 693 mAh g1 still can be achieved. At 0.01e1.5 V vs. Li/Liþ, it still delivers a specific capacity of 460 mAh g1, higher than the theoretical capacity of graphite (372 mAh g1). The reversible lithiation/delithiation process of both itaconic acid and graphite contribute the high capacity. The excellent rate performances are related to the buffering of the itaconic acid matrix combining its superior lithium storage capability. And the stable solid electrolyte interphase formed on the electrode during electrochemical cycling is the main factor for the stable cycle. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Organic conjugated carbonyl compounds have been considered as promising candidates for next generation electrode material in lithium ion batteries (LIBs) because of high theoretic capacity, fast reaction kinetics, abundant existence, low cost, and environmental friendly [1e3]. However, the low electrical conductivity of the organic materials still is one of the main factors restrict the development of the organic electrodes [4e6]. In order to resolve this problem, extensive studies have been carried out, such as: Song [7] found that composite polyimide with 11 wt% graphene could increase the capacity from 156 to 205 mAh g1 at 36.7 mA g1, corresponding to the number of electrons transferred in each structure unit elevated from 1.7 to 2.5 per unit. Zhang [8] composited 9,10anthraquinone with 30.0 wt% CMK-3, the capacity increased from 37.6 to 146 mAh g1 at 2 C. Lee [9] reported that after mixing 4.5 mg lumiflavine with 5 mg single-walled carbon nanotubes (SWCNTs), the lithium storage capability of lumiflavine increased from 148 mAh g1 (1.42 Liþ/molecule) to 204 mAh g1 (1.95 Liþ/molecule) at 20 mA g1. Although composite organic materials with these
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (H. Zheng). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.06.065 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
conductive carbons could improve the utilization of organic materials, there are still need adding a large amount of conductive carbon black (normally at 20e40 wt%). And considering CMK-3 and SWCNTs etc. are inactive materials in the scientific sense, the total inactive materials of the electrodes still attain to ca. 30e50 wt%. Graphite (G), is widely used as anode material in commercial LIBs because of its long cycle life, low lithium-intercalation potential, stable solid-electrolyte interface (SEI), and high electrical conductivity [10,11]. However, the G anode has its limitations of low energy and power density. To solve these problems, G particle, conductive carbon additive, polymeric binder, electrolyte and separator were optimized [12e15]. However, these optimization approaches are still far from enough due to the very limited theoretical capacity of G (372 mAh g1). In recent years, composite silicon (Si) with G is known to be an important way to improve the battery performance due to the high theoretical capacity of Si (4200 mAh g1) [16e18]. Still, it is quite challenging to devise a stable Si/G anodes because of the huge volume expansions (>300%) and the instability of the SEI layer of Si, which cause mechanical failure and result in severely degraded electrochemical performances of the electrodes [19,20]. In addition, the synthetic processes involve the use of toxic chemicals for generating Si, thereby limiting their commercial use [21]. In this study, we prepared a novel itaconic acid (IA)-G composite anode for LIBs. IA is an unsaturated binary carboxyl acid with one of which conjugated to the methylene group (see Scheme S1). G is a
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kind of artificial graphite with carboxylic acid and hydroxyl groups on the surface. This composite system combines the excellent lithium storage capability of the organic IA with the high electrical conductivity of the electro-active G, in which G supplies continuous electronic transport pathway for the IA and provides active sites for charge-transfer reactions, and IA compensates for the shortcomings of low capacity and poor rate for G. The structure, electrochemical properties, and the lithium storage mechanisms of the composite anode were investigated. The IA-G composite anode could deliver a first reversible capacity of 551 mAh g1 with the improved the first coulombic efficiency (CE) of 78.5% (most of the reported organic-based anodes have a first CE around 40e60%) [22,23]. At 10 A g1 discharge, it retained a high capacity of 553 mAh g1. In addition, after 200 cycles at 100 mA g1 charge/ 500 mA g1 discharge, the IA-G composite anode retained a specific capacity of 642 mAh g1. This IA-G composite electrode exhibits high capacity, high-rate capability and long cycle life, which is a significant progress for the development of electro-active organic materials and LIBs anodes. 2. Experimental 2.1. The IA-G composite preparation 0.1 g IA (Adamas-beta®) was mixed with 0.9 g G (Code No AGP-8, Shenzhen Beterui New Energy Materials Group Co., Ltd. China) in 10 g Milli-Q water at room temperature. While vigorous stirring, the mixture was kept at 60 C until the solution was completely dried. Finally, the obtained IA-G composite was dried at 60 C for 12 h in drying oven before use. Similarly, the IA-G composites with different IA:G weight ratios were prepared with the similar procedure. The weight ratio of IA:G is controlled at 1:9, 2:8, 3:7, 4:6 and 5:5, respectively. 2.2. Materials characterizations X-ray diffraction (XRD, PANalytical X'Pert PRO, Netherlands) was adopted to study the crystalline structure of the composite, the diffraction angle (2q) was stepped between 10 and 80 with an increment of 6 min1. Fourier transform infrared (FTIR, TENSOR 27, BRUKER OPTICS, Germany) was employed to study the chemical bonding states of the composite in the frequency range of 4000e400 cm1. Raman (JobinYvonLabRAM HR 800) with a 632.8 nm excitation laser was performed to characterize the surface structure of the samples. X-ray photoelectron spectroscopy (XPS) spectroscopic studies were carried out on an Escalab 250 Xi (Thermo Fisher, America) instrument. 2.3. Electrode preparation and electrochemical characterizations The slurries were prepared by thoroughly mixing the asprepared IA-G composites with acetylene black (AB, particle size of ca. 40 nm, Denka Singapore Private Co., Ltd.) and Carboxymethyl cellulose sodium (CMC, 2200, DoDoChem)-Styrene butadiene rubber (SBR, ZEON, Japan) binder in Milli-Q water. Disperse the mixture evenly with a FA25 superfine homogenizer at a rate of 4000 rpm for 30 min, then cast the uniform slurry onto 15 mm-thick copper foil. After drying for 6 h under 60 C in drying oven, the electrode laminates were then calendered and punched out into electrode discs. The anode discs were thoroughly dried at 120 C under vacuum for 16 h prior to use. To optimize the electrochemical performances of the IA-G composite anode, the effect of the IA:G weight ratio (IA:G ¼ 0.2:9.8, 1:9, 2:8, 3:7, 4:6 and 5:5, IA-G:AB:CMC:SBR ¼ 7:2:0.5:0.5), IA-G:AB weight ratio (IA-G:AB ¼ 6:3, 7:2 and 8:1, IA:G ¼ 1:9) and CMC:SBR
weight ratio (CMC:SBR ¼ 1:2, 1:1, 2:1 and 1:0, IA:G:AB:CMCSBR ¼ 0.7:6.3:2:1) were studied. IA-G composite anodes (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%) with different thicknesses were also prepared by varying the doctor blade height (from 75 to 300 mm). G and IA anodes were prepared by dispersing G or IA, AB, and CMC-SBR binder in Milli-Q water with a weight ratio of 7:2:1. Besides, NCM cathode was prepared by thoroughly mixing NCM (811, Haian Zhichuan Battery Materials Technology Co., Ltd. China) and AB and poly(vinylidene fluoride) (PVDF, KF1100, Kureha, Japan) binder in N-methylpyrrolidone (NMP, Aldrich) solvent with the weight ratio of 8:1:1, the uniform slurry was cast onto 18 mm-thick aluminum foil, the subsequent preparation process is as the same as the anode. Electrochemical characterization of the electrodes was conducted by using CR2032 coin cells. The cells were assembled in an Ar-filled glove box (<0.5 ppm of oxygen and water, OMNI-LAB, VAC). The electrolyte is 1 mol L1 LiPF6 (99.99% purity) dissolved in ethylene carbonate/diethylene carbonate/dimethyl carbonate (EC/DEC/DMC, v/v/v ¼ 1:1:1) (Shenzhen Capchem Technology). Celgard 3501 was adopted as the separator. Galvanostatic tests of the cells were performed on a Maccor battery cycler (S4000, Maccor Instruments, USA) at 30 C. For IA-G half-cell, lithium foil was used as the counter electrode. Three formation cycles at a current density of 25 mA g1 charge/discharge rate were first applied between 0.01 and 3.0 V vs. Li/Liþ. Rate capability of the IA-G composite anode was evaluated through full charge (lithiation) at a constant rate of 100 mA g1 and full discharge (de-lithiation) at various rates from 100 mA g1 to 10 A g1, respectively. Long-term cycling of the Li//IA-G half cells was tested at 100 mA g1 charge/ 200 mA g1 discharge rate and 100 mA g1 charge/500 mA g1 discharge, respectively. All the specific capacities (mAh g1) obtained for IA-G anodes are based on the IA-G active material. Cyclic voltammetry (CV) was conducted on an Autolab potentiostat (PGSTAT302 N, Autolab Instruments, Switzerland) at a scan rate of 0.1 mV s1 in a voltage range of 0.01e3 V. Galvanostatic intermittent titration technique (GITT) was measured on an electrochemical workstation (VMP3, Bio-Logic, France), the cells were charged/ discharged at 25 mA g1 with a current pulse duration of 15 min and an interval time of 15 min. For NCM//IA-G full-cell, three formation cycles at 18 mA g1 charge/discharge rate were first applied between 2.8 and 4.3 V vs. Li/Liþ. Rate capability was evaluated at a constant charge rate of 90 mA g1, and discharge rates from 18 mA g1 to 1.8 A g1. Long-term cycling was tested at 36 mA g1 charge and 180 mA g1 discharge. 3. Results and discussion 3.1. Structure characterizations of the IA-G composite The XRD pattern of the as-prepared IA-G composite (IA:G ¼ 1:9 wt%) is shown in Fig. 1a. For the IA-G composite, two typical diffraction peaks at 19.2 and 20.5 are consistent with the IA characteristic peaks. Other weak peaks at 23.1, 24.5, 30.0, 32.1, 33.2, 34.3 and 39.1 are also observed for the IA sample. And the two distinct peaks at 26.6 and 54.7, and the three week peaks at 42.4, 44.7 and 77.6 correspond well to the (002), (100), (101), (004) and (110) planes of G from low to high 2q angle [24]. These results show that the crystalline structures of both IA and G are not destroyed after composited them. FTIR spectra of the IA, G and IA-G composite are compared in Fig. 1b. For the IA sample, the broad absorption centered at 3052 cm1 is attributed to the CeOeH vibration, the strong absorption peak at 1711 cm1 is assigned to the C]O stretching vibration of carboxylic acid (COOH) group for the IA [25]. The assignments of other absorption bands are given as follows [26e28]: 2932 cm1 (yCeH in CH2), 1441 cm1 (dCeH in CH2), 1306 cm1 (yC-C), 1216 cm1 (yC-O in COOH) and 910 cm1 (gO-H in
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Fig. 1. (a) XRD patterns, (b) FTIR spectra and (c) Raman spectra of IA, G and the as-prepared IA-G composite (IA:G ¼ 1:9 wt%). The fitting results of XPS spectra of C1s spectra for (d) IA, (e) G and (f) the as-prepared IA-G composite (IA:G ¼ 1:9 wt%). (A colour version of this figure can be viewed online.)
COOH). For the G sample, the broad peak at 3432 cm1 and 1634 cm1 indicated the presence of OH, the narrow peak at 1716 cm1 and 1147 cm1 demonstrated the presence of C]O and CeOH, respectively [29]. The peak at 1070 cm1 is assigned to the CeC skeletal vibrations in six-membered ring forms of G [30]. For the IA-G composite, all the characteristic peaks of IA and G are well preserved, and it should be noted that the OH frequency showed an upshift from 3432 cm1 to 3450 cm1, and the CeOH vibration also exhibited a upshift of 20 cm1 to give a frequency of 1167 cm1, indicating the formation of hydrogen bond between IA and G [31]. Raman spectra of the IA, G and IA-G composite (Fig. 1c) validates the presence of D-band (1332 cm1), G-band (1579 cm1), and D*band (2683 cm1) [32] bonds of G and all the characteristic peaks of IA in the IA-G composite, indicating the typical structures of both IA and G are well preserved in the composite. XPS was further used to analyze the elemental composition of the IA-G composite (IA:G ¼ 1:9 wt%). XPS spectra of C1s for the IA, G and the as prepared IA-G composite are compared in Fig. 1def. Fig. 1d shows IA has five types carbon located at 284.3, 285.3, 288.6, 289.3 and 291.8 eV, corresponding to C]C, CeH/CeC, CeO, COOH and satellite (p-p*), respectively [33]. Fig. 1e shows the fitting results of G, it is easy to discern the typical graphite peak at 283.6 eV, the other peaks at 283.8, 285.2, 287.1 and 290.3 eV are attributed to CHx, C] O/CeO, COO and CO2 3 /p-p*, respectively [34]. Fig. 1f suggests all the characteristic peaks of IA and G preserved in the IA-G composite, namely graphitic (283.4 eV), C]C/CHx (284.4 eV), CeO (285.3 eV), COOR (286.4 eV), COOH (287.9 eV) and CO2 3 /p-p* (289.9 eV) [33,34]. These results further confirm that the primeval chemical structures for both IA and G are not significantly changed after composite them, and the slightly shift can be ascribed to the intermolecular interaction. 3.2. Electrochemical properties of the IA-G composite anodes Fig. 2 compares the first reversible specific capacity and the CE of the Li//IA-G half cells in the 0.01e3.0 V voltage range with different compositions and electrode thicknesses. Fig. 2a presents the first electrochemical performances of the IA-G anode (IA-
G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt%) with different IA:G weight ratios (IA:G ¼ 0:10, 0.2:9.8, 1:9, 2:8, 3:7, 4:6 and 5:5). As shown in this figure, the first specific capacity increased with the increase of the IA:G weight ratios, and the electrode with IA:G ¼ 4:6 wt% has the highest capacity of 765 mAh g1, indicating the significantly enhanced capacity resulted from IA. The capacity decrease of the IA:G ¼ 5:5 wt% is resulted from the lack of the electronic conductivity of the IA. On the contrary, the first CE decreased with the increase of the IA:G weight ratios. For the IA-G composite anode, IA:G ¼ 0.2:9.8 wt% has the highest CE of 82.0%, which is lower than the pure G anode (denoted as IA:G ¼ 0:10 wt% in Fig. 2a) of 87.4%. The decrease of the first CE can be attributed to the increased irreversible lithium consumption resulting from the increasing IA amount. Fig. 2b presents the first reversible specific capacity and the CE of the IA-G anode with different IA-G:AB weight ratios (IAG:AB:CMC:SBR ¼ 9-x:x:0.5:0.5, x ¼ 3, 2, 1, IA:G ¼ 1:9). Obviously, the first specific capacity decreased with the decrease of the AB weight ratios, indicating electronic conductivity plays a crucial role controlling the utilization rate of active materials. It worth noting that IA-G:AB ¼ 7:2 wt% has the highest CE, this is because too much AB (30 wt%) amounts result in a large irreversible lithium consumption, while lake of AB (10 wt%) amounts lead to difficult delithiation process of the electrode. Fig. 2c indicates the CMC:SBR weight ratio (CMC:SBR ¼ 1:2, 1:1, 2:1 and 1:0) has ignorable effect on the first electrochemical performance, and CMC:SBR ¼ 1:1 wt% slightly higher than others. The first reversible specific capacity and CE values of the IA-G anode with different electrode components are shown in Table S1. And Figure S(1-3) exhibit the corresponding charge/discharge curves and the rate capabilities of IA-G composite anodes with different composites. Fig. 2d shows the effect of the electrode thickness on the first electrochemical performance for the IA-G anode (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%). All the electrode thicknesses are final thickness with copper foil. It is worth noting that, even at 75 mm electrode thickness with an active material loading of 2.70e2.76 mg cm2, the IA-G anode still delivers a reversible capacity of 369 mAh g1. And the first CE of 74.6% is still much higher than most of the reported organic anodes [22,23,35]. The accurate capacity and CE values of Li//IA-G half cells with
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Fig. 2. The first specific capacity (red column) and the corresponding CE (blue polyline) of Li//IA-G half cells with different IA:G:AB:CMC:SBR weight ratios in the voltage range of 0.01e3.0 V: (a) IA-G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt% with different IA:G weight ratios, (b) IA-G:AB:CMC:SBR ¼ 9-x:x:0.5:0.5 wt%, x ¼ 3, 2, 1, IA:G ¼ 1:9 wt%, (c) IA:G:AB:CMCSBR ¼ 0.7:6.3:2:1 wt% with different CMC:SBR weight ratios, and (d) IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt% with different electrode thickness. (A colour version of this figure can be viewed online.)
different electrode thickness (and the corresponding active loading) are shown in Table S2. The corresponding first charge/ discharge curves and the rate capabilities can be seen in Figure S4. Considering the IA-G composite anode with IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt% has the relatively high first CE (78.5%) and the high reversible capacity of 551 mAh g1, more electrochemical tests of this anode have been conducted. And Li//G half cell (G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt%) was used as a comparison. Fig. 3a presents the initial three charge/discharge profiles of the Li//IA-G half cell and Li//G half cell at 25 mA g1 between 0.01 and 3 V. The first charge and discharge capacities of Li//IA-G half cell are obtained to be 702 and 551 mAh g1, respectively, corresponding to the first CE of 78.5%. The first discharge capacity of Li//IA-G half cell is significantly higher than the Li//G half cell of 364 mAh g1, while the first CE of IA-G composite anode is lower than the pure G anode of 87.4%, this is mainly because of the irreversible lithium consumption of IA. But it should be noted that the first CE is still dramatically improved than the reported organic and organic/inorganic composite anodes in the same voltage range [22,36]. And it's worth point out that at 0.01e1.5 V, the specific capacity of the Li//IA-G half cell (460 mAh g1) is still higher than the Li//G half cell (344 mAh g1). In the second cycle, the reversible capacity increased to 561 mAh g1 and the corresponding CE attained 95.6%. In the third cycle, the reversible capacity attained to 571 mAh g1 with the high CE of 97.2%. The rate behaviors of the Li//IA-G half cell and Li//G half cell were compared at a series of discharging current densities. As shown in Fig. 3b, at the high current density of 1, 2.5, 5, and 10 A g1, the IA-G composite electrode still exhibits the high capacities of 569, 567, 562, and 553 mAh g1, respectively. While the pure G anode exhibits ca. 350 mAh g1 at these current densities. When the current density turns back to 25 mA g1, the capacity is totally recovered, suggesting that the IA-G composite anode is tolerant of high-rate operation. Fig. 3c exhibits the long-term cycling performance of the Li//IA-G half cell at different current densities. When cycled at 100 mA g1 charge/200 mA g1 discharge, a stable reversible
capacity of 693 mAh g1 is obtained after 200 cycles. When cycled at 100 mA g1 charge/500 mA g1 discharge, the IA-G composite anode retained a reversible capacity of 642 mAh g1 after 200 cycles. The continuous increase of the specific capacity before 200 cycles can be ascribed to the created more charge transfer channels within the electrode in the subsequent cycles. It should be pointed out that the significant capacity reduction of the electrode after ca. 210 cycles is not caused by the electrode material itself. This can be verified by switching a fresh Li metal counter electrode (marked in black arrow), we can see both electrodes can be charged and discharged normally. We ascribed the capacity decrease after ca. 210 cycles to the electrolyte dryness, because after disassembling the cell, we found that the electrode sheet was very dry (see the inset of Fig. 3c). Both of the electrodes exhibit an average CE of ca. 99% during the whole cycles, indicating the IA-G anode has an excellent reversible lithium storage capability. All these results proved that the IA-G composite is suitable for constructing long-life and highrate anode for LIBs. The electrochemical performance of the IA-G composite was further examined using NCM//IA-G full cells. The IA-G anode was pre-treated according to a modified reported method [37] as shown in Fig. 4a. Specifically, the Li//IA-G half cell is charged and discharged at 25 mA g1 current density for three cycles, and then charged to 0.01 V and stand still for 12 h. Next, the half cell is disassembled in glove box, and the IA-G anode is taken out for assembling the NCM//IA-G full cell. The electrochemical performances of NCM//IA-G full cells are presented in Fig. 4bed. Fig. 4b shows the initial three charge/discharge curves for full cells between 2.8 and 4.3 V at 18 mA g1. The full cell shows an initial reversible capacity of 182 mAh g1 with 74% CE. And the capacity increases to 192 mAh g1 after three cycles with an improved CE of 95%. Fig. 4c shows the rate capability of the NCM//IA-G full-cell. Surprisingly, the full cell exhibits high reversible capacity of 135 mAh g1 at 0.9 A g1 and 118 mAh g1 at a high rate of 1.8 A g1, which are much higher than reported NCM-based full-cells [37,38]. Moreover, the cycling stability of the full-cell at 36 mA g1 charge
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Fig. 3. Electrochemical performances of the Li//IA-G half cell (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%) and Li//G half cell (G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt%) in the voltage of 0.01e3.0 V: (a) charge/discharge curves for the initial three cycles at 25 mA g1 current density and the (b) rate capability at various discharge current densities; and the (c) longterm cycling performances of the Li//IA-G half cell at different current densities. (A colour version of this figure can be viewed online.)
Fig. 4. (a) The pre-treated IA-G composite anode used in the full-cells. The electrochemical performances of NCM//IA-G full-cells between 2.8 and 4.3 V: (b) the initial three charge/ discharge curves at 18 mA g1, (c) rate properties at various rates, and (d) the cycling life. (A colour version of this figure can be viewed online.)
and 180 mA g1 discharge is exhibited in Fig. 4d. The full-cell shows a capacity of 95 mAh g1 after 100 cycles, and the CE is above 99.5% in each cycle. The capacity fade of NCM//IA-G full-cell is mainly associated with electrolyte decomposition, active material dissolution and phase changes of electrode materials in both cathode and anode [38,39]. The cyclic stability of NCM//IA-G full-cell is better than the reported full-cell with G as anode and 3,4,9,10-
perylene-tetracarboxylicacid-dianhydride/graphene aerogel as cathode [40]. In addition, we also prepared the NCM//IA-G full-cell with fresh IA-G anode, it shows a reversible capacity of 74 mAh g1 after 100 cycles at 36 mA g1 charge and 180 mA g1 discharge (see Figure S5a). And at 0.9 and 1.8 A g1, it exhibits a reversible capacity of 106 and 104 mAh g1, respectively (see Figure S5b). It's obvious that NCM//IA-G full-cell with fresh IA-G anode still exhibits an
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attractive rate capability, which mainly because of the excellent lithium conductivity of IA.
3.3. Li storage mechanisms for the IA-G composite anode To investigate the lithiation/de-lithiation mechanism of the IA-G composite, CV curves of the Li//IA-G half cell at a scan rate of 0.1 mV s1 between 0.01 and 3.0 V are plotted in Fig. 5a. Fig. 5b is the selected cathodic process from 3.0 to 0.3 V. In the first cathodic process, three reduction peaks at 1.72, 1.32 and 0.93 V are observed. The first peak at 1.72 V slightly shifts to a lower potential of 1.64 V in the second cycle, suggesting that there was an initial adaptation of the IA to lithiation. While the latter two peaks completely disappeared in the following cycles, indicating these two processes are irreversible. Specifically, the cathodic peak at around 1.32 V is ascribed to the irreversible replacement of the hydrogen on the carboxyl group of IA with lithium ions, which verified by ex-situ FTIR spectroscopy and ex-situ XPS analysis for the IA anode at different charging states in the first cycle (see Figure S6 and S7). The cathodic peak at around 0.93 V is related to the formation of the SEI film on the materials, which is a common character for anodes of LIBs [35,41]. From the second cathodic process, four typical peaks centered at 2.45, 1.64, 1.12 and 0.64 V are related to the lithiation process of the IA. And all these peaks retained in following cycles, indicating all these categories of reactions are reversible. Fig. 5c is the selected cathodic and the corresponding anodic processes from 0.3 to 0.01 V. As seen in this figure, toward negative potential, four typical cathodic peaks centered at 0.18, 0.13, 0.07 and 0.03 V are attributed to the formation of stage-4, stage-3, stage-2 and stage-1 graphite intercalation compound (GIC), respectively. In the following anodic process, three strong lithium extraction peaks centered at 0.13, 0.19 and 0.24 V are related to the de-lithiation of G. All these peaks well overlapped in subsequent CV curves, reflecting excellent reversibility of the G in the composite electrode. Fig. 5d is the selected anodic process from 0.3 to 3.0 V. In the first anodic
process, two obvious oxidation peaks at 0.94 and 1.49 V and one weak oxidation peak at 2.34 V are related to the de-lithiation processes of the IA. In the following anodic scan, the three peaks slightly shift to higher potential and stabilized at 1.06, 1.55 and 2.68 V in the third cycle. All these three categories of reactions are reversible, which explains the appearance of the anodic peaks in the following cycles. And the reversible cathodic and anodic peaks of IA can also be verified by Figure S8, which shows that the peaks ascribed to IA increase with the IA amount. Fig. 5 and S8 verified that both IA and G could exhibit excellent reversibility in the IA-G electrode, this contributes the high capacity of the composite anode. And it's worth pointing out that the specific capacity of IA-G composite is not a simple superposition of IA and G, but has a synergistic effect, which exhibits a significantly enhanced specific capacity than the calculated specific capacity (see Figure S9 and Table S3). Fig. 6a shows discharge profiles of the Li//IA-G half cell at various discharge current densities in the voltage range of 0.01e3.0 V. As shown in this figure, the total specific capacity of the Li//IA-G half cell has no obvious decay with increasing current densities, a slight elevation of the discharge plateau due to the polarization of the cell resulting mainly from the internal resistance of the cell. To study the de-lithiation capability of the G in the composite, the discharge profiles in the voltage range of 0.01e0.3 V have been selected and analyzed (Fig. 6b). It can be seen, that at low discharge current densities (from 25 to 250 mA g1), the electrode displays the typical plateaus of G. When the discharge rate exceeds 500 mA g1, a sudden capacity loss of the G is observed, which is mainly due to lithium ion diffusion limitation of the G in the composite electrode. Conversely, the total specific capacity of the Li//IA-G half cell still retained a high capacity of 553 mAh g1 even at 10 A g1, indicating IA could effectively facilitate the lithium ion diffusion in the composite electrode even there is a large gradient of the lithium ion concentration within the electrode. This can also be verified by comparison of diffusion coefficient of lithium ion
Fig. 5. (a) CV curves of the Li//IA-G half cell in the voltage range of 0.01e3.0 V, (b) the cathodic process in the voltage of 3.0e0.3 V, (c) the cathodic and anodic process in the voltage of 0.3e0.01 V, and (d) the anodic process in the voltage of 0.3e3.0 V. (A colour version of this figure can be viewed online.)
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Fig. 6. (a) Discharge profiles of Li//IA-G half cell at various discharge current densities in the voltage range of 0.01e3.0 V and (b) the selected discharge profiles in the voltage of 0.01e0.3 V. (A colour version of this figure can be viewed online.)
(DLi) in IA-G composite anode and pure G anode. According to GITT (calculation schematic see Figure S10) test, IA-G exhibits higher DLi than G until 0.42 V, especially being nearly three times higher than G at 0.85 V (see Figure S11), verified IA indeed could promote lithium ion transport. These results verify that composite IA with G could compensate for the poor large rate performance of G anode, and contributes to a high-rate anode. Fig. 7 presents the XPS spectra of C1s, O1s, Li1s and F1s core levels recorded for the IA-G electrode at OCV state and after three cycles. For C1s core levels (Fig. 7a), the electrode at OCV state can be decomposed in five components at 284.3, 285.2, 286.0, 288.4 and 289.2 eV assigned to graphite/CHx, CeO, COOR, COOH, and CO2 3 /pp*, respectively [34]. In the case of the electrode after three cycles, similar peaks at 284.5 and 286.3 eV are found. The new peak at 286.9 eV is assigned to ReOLi. And the significant increased peak at 288.9 eV is due to the formation of carbonate-like species on the electrode surface [42,43]. For O1s core levels (Fig. 7b), three
components at 531.6, 533.0 and 534.0 eV assigned to CO 2 , CeO and CO2 3 , respectively [43]. After three cycles, all three peaks retained except slightly shift, and a new peak at 529.6 eV is assigned to ROLi/ LiOH appeared [44]. It should be noted that, no Li1s (Fig. 7c) and F1s (Fig. 7d) signal observed for the electrode at OCV state, while significant Li1s and F1s signals can be observed after three cycles. Specifically, the Li1s peak includes Li2CO3/ROCO2Li (55.3 eV), LiF (56.1 eV) and LiPF6 (56.6 eV) [45]. The F1s peak includes LiF (684.6 eV) and LiPF6 (686.8 eV) [42]. As ReOLi, ROCO2Li, Li2CO3 and LiF are known to be the important components for the passivation film on anodes, XPS study verified that a stable SEI film formed on the electrode after three cycles, which is important for the stable cycling performance of the electrode. 4. Conclusions In summary, a novel IA-G composite anode with high-rate
Fig. 7. XPS high resolution spectra of (a) C1s, (b) O1s, (c) Li1s and (d) F1s for the IA-G composite anode at OCV state and after three cycles. (A colour version of this figure can be viewed online.)
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capability and long-cycle life was first adopted for LIBs. After optimization, the IA-G anode (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%) is able to deliver ca. 570 mAh g1 reversible capacity in the voltage range of 0.01e3.0 V vs. Li/Liþ, with the first CE of 78.5%. At a high discharge rate of 10 A g1, the IA-G electrode still retained 96.8% capacity compared the electrode cycled at 25 mA g1, showing excellent high-rate performance. Besides, the IA-G anode retained ca. 700 mAh g1 specific capacity after 200 electrochemical cycles at 100 mA g1 charge and 200 mA g1 discharge rate. And at 0.01e1.5 V vs. Li/Liþ, IA-G anode still delivers a higher specific capacity (460 mAh g1) compared to the theoretical capacity of graphite (372 mAh g1). This composite anode can realize the effective utilization of organic electrode materials on the basis of reducing the proportion of inactive conductive carbon in the electrode, provide a new way for the development of the electro-active organic materials.
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Acknowledgements [23]
This work was financially supported by National Natural Science Foundation of China (NSFC, contract no. 21875154 and 21473120) and 863 project of the Department of Science and Technologies, China (contract no. 2015AA034601).
[24]
[25]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.06.065.
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A novel itaconic acid-graphite composite anode for enhanced lithium storage in lithium ion batteries Ruitian Guo 1, Yan Wang*, 1, Xiaojian Shan, Yongkang Han, Zhang Cao, Honghe Zheng** College of Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu, 215006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2019 Received in revised form 14 June 2019 Accepted 15 June 2019 Available online 18 June 2019
An itaconic acid-graphite composite anode is prepared for lithium ion batteries. The composite anode exhibits much improved electrochemical performances compared to the traditional graphite anode. A high specific capacity of 571 mAh g1 can be obtained at 25 mA g1 in the voltage range of 0.01e3.0 V vs. Li/Liþ, and retained 96.8% at 10 A g1. After cycled 200 cycles at 100 mA g1 charge/200 mA g1 discharge, a reversible capacity of 693 mAh g1 still can be achieved. At 0.01e1.5 V vs. Li/Liþ, it still delivers a specific capacity of 460 mAh g1, higher than the theoretical capacity of graphite (372 mAh g1). The reversible lithiation/delithiation process of both itaconic acid and graphite contribute the high capacity. The excellent rate performances are related to the buffering of the itaconic acid matrix combining its superior lithium storage capability. And the stable solid electrolyte interphase formed on the electrode during electrochemical cycling is the main factor for the stable cycle. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Organic conjugated carbonyl compounds have been considered as promising candidates for next generation electrode material in lithium ion batteries (LIBs) because of high theoretic capacity, fast reaction kinetics, abundant existence, low cost, and environmental friendly [1e3]. However, the low electrical conductivity of the organic materials still is one of the main factors restrict the development of the organic electrodes [4e6]. In order to resolve this problem, extensive studies have been carried out, such as: Song [7] found that composite polyimide with 11 wt% graphene could increase the capacity from 156 to 205 mAh g1 at 36.7 mA g1, corresponding to the number of electrons transferred in each structure unit elevated from 1.7 to 2.5 per unit. Zhang [8] composited 9,10anthraquinone with 30.0 wt% CMK-3, the capacity increased from 37.6 to 146 mAh g1 at 2 C. Lee [9] reported that after mixing 4.5 mg lumiflavine with 5 mg single-walled carbon nanotubes (SWCNTs), the lithium storage capability of lumiflavine increased from 148 mAh g1 (1.42 Liþ/molecule) to 204 mAh g1 (1.95 Liþ/molecule) at 20 mA g1. Although composite organic materials with these
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (H. Zheng). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.carbon.2019.06.065 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
conductive carbons could improve the utilization of organic materials, there are still need adding a large amount of conductive carbon black (normally at 20e40 wt%). And considering CMK-3 and SWCNTs etc. are inactive materials in the scientific sense, the total inactive materials of the electrodes still attain to ca. 30e50 wt%. Graphite (G), is widely used as anode material in commercial LIBs because of its long cycle life, low lithium-intercalation potential, stable solid-electrolyte interface (SEI), and high electrical conductivity [10,11]. However, the G anode has its limitations of low energy and power density. To solve these problems, G particle, conductive carbon additive, polymeric binder, electrolyte and separator were optimized [12e15]. However, these optimization approaches are still far from enough due to the very limited theoretical capacity of G (372 mAh g1). In recent years, composite silicon (Si) with G is known to be an important way to improve the battery performance due to the high theoretical capacity of Si (4200 mAh g1) [16e18]. Still, it is quite challenging to devise a stable Si/G anodes because of the huge volume expansions (>300%) and the instability of the SEI layer of Si, which cause mechanical failure and result in severely degraded electrochemical performances of the electrodes [19,20]. In addition, the synthetic processes involve the use of toxic chemicals for generating Si, thereby limiting their commercial use [21]. In this study, we prepared a novel itaconic acid (IA)-G composite anode for LIBs. IA is an unsaturated binary carboxyl acid with one of which conjugated to the methylene group (see Scheme S1). G is a
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kind of artificial graphite with carboxylic acid and hydroxyl groups on the surface. This composite system combines the excellent lithium storage capability of the organic IA with the high electrical conductivity of the electro-active G, in which G supplies continuous electronic transport pathway for the IA and provides active sites for charge-transfer reactions, and IA compensates for the shortcomings of low capacity and poor rate for G. The structure, electrochemical properties, and the lithium storage mechanisms of the composite anode were investigated. The IA-G composite anode could deliver a first reversible capacity of 551 mAh g1 with the improved the first coulombic efficiency (CE) of 78.5% (most of the reported organic-based anodes have a first CE around 40e60%) [22,23]. At 10 A g1 discharge, it retained a high capacity of 553 mAh g1. In addition, after 200 cycles at 100 mA g1 charge/ 500 mA g1 discharge, the IA-G composite anode retained a specific capacity of 642 mAh g1. This IA-G composite electrode exhibits high capacity, high-rate capability and long cycle life, which is a significant progress for the development of electro-active organic materials and LIBs anodes. 2. Experimental 2.1. The IA-G composite preparation 0.1 g IA (Adamas-beta®) was mixed with 0.9 g G (Code No AGP-8, Shenzhen Beterui New Energy Materials Group Co., Ltd. China) in 10 g Milli-Q water at room temperature. While vigorous stirring, the mixture was kept at 60 C until the solution was completely dried. Finally, the obtained IA-G composite was dried at 60 C for 12 h in drying oven before use. Similarly, the IA-G composites with different IA:G weight ratios were prepared with the similar procedure. The weight ratio of IA:G is controlled at 1:9, 2:8, 3:7, 4:6 and 5:5, respectively. 2.2. Materials characterizations X-ray diffraction (XRD, PANalytical X'Pert PRO, Netherlands) was adopted to study the crystalline structure of the composite, the diffraction angle (2q) was stepped between 10 and 80 with an increment of 6 min1. Fourier transform infrared (FTIR, TENSOR 27, BRUKER OPTICS, Germany) was employed to study the chemical bonding states of the composite in the frequency range of 4000e400 cm1. Raman (JobinYvonLabRAM HR 800) with a 632.8 nm excitation laser was performed to characterize the surface structure of the samples. X-ray photoelectron spectroscopy (XPS) spectroscopic studies were carried out on an Escalab 250 Xi (Thermo Fisher, America) instrument. 2.3. Electrode preparation and electrochemical characterizations The slurries were prepared by thoroughly mixing the asprepared IA-G composites with acetylene black (AB, particle size of ca. 40 nm, Denka Singapore Private Co., Ltd.) and Carboxymethyl cellulose sodium (CMC, 2200, DoDoChem)-Styrene butadiene rubber (SBR, ZEON, Japan) binder in Milli-Q water. Disperse the mixture evenly with a FA25 superfine homogenizer at a rate of 4000 rpm for 30 min, then cast the uniform slurry onto 15 mm-thick copper foil. After drying for 6 h under 60 C in drying oven, the electrode laminates were then calendered and punched out into electrode discs. The anode discs were thoroughly dried at 120 C under vacuum for 16 h prior to use. To optimize the electrochemical performances of the IA-G composite anode, the effect of the IA:G weight ratio (IA:G ¼ 0.2:9.8, 1:9, 2:8, 3:7, 4:6 and 5:5, IA-G:AB:CMC:SBR ¼ 7:2:0.5:0.5), IA-G:AB weight ratio (IA-G:AB ¼ 6:3, 7:2 and 8:1, IA:G ¼ 1:9) and CMC:SBR
weight ratio (CMC:SBR ¼ 1:2, 1:1, 2:1 and 1:0, IA:G:AB:CMCSBR ¼ 0.7:6.3:2:1) were studied. IA-G composite anodes (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%) with different thicknesses were also prepared by varying the doctor blade height (from 75 to 300 mm). G and IA anodes were prepared by dispersing G or IA, AB, and CMC-SBR binder in Milli-Q water with a weight ratio of 7:2:1. Besides, NCM cathode was prepared by thoroughly mixing NCM (811, Haian Zhichuan Battery Materials Technology Co., Ltd. China) and AB and poly(vinylidene fluoride) (PVDF, KF1100, Kureha, Japan) binder in N-methylpyrrolidone (NMP, Aldrich) solvent with the weight ratio of 8:1:1, the uniform slurry was cast onto 18 mm-thick aluminum foil, the subsequent preparation process is as the same as the anode. Electrochemical characterization of the electrodes was conducted by using CR2032 coin cells. The cells were assembled in an Ar-filled glove box (<0.5 ppm of oxygen and water, OMNI-LAB, VAC). The electrolyte is 1 mol L1 LiPF6 (99.99% purity) dissolved in ethylene carbonate/diethylene carbonate/dimethyl carbonate (EC/DEC/DMC, v/v/v ¼ 1:1:1) (Shenzhen Capchem Technology). Celgard 3501 was adopted as the separator. Galvanostatic tests of the cells were performed on a Maccor battery cycler (S4000, Maccor Instruments, USA) at 30 C. For IA-G half-cell, lithium foil was used as the counter electrode. Three formation cycles at a current density of 25 mA g1 charge/discharge rate were first applied between 0.01 and 3.0 V vs. Li/Liþ. Rate capability of the IA-G composite anode was evaluated through full charge (lithiation) at a constant rate of 100 mA g1 and full discharge (de-lithiation) at various rates from 100 mA g1 to 10 A g1, respectively. Long-term cycling of the Li//IA-G half cells was tested at 100 mA g1 charge/ 200 mA g1 discharge rate and 100 mA g1 charge/500 mA g1 discharge, respectively. All the specific capacities (mAh g1) obtained for IA-G anodes are based on the IA-G active material. Cyclic voltammetry (CV) was conducted on an Autolab potentiostat (PGSTAT302 N, Autolab Instruments, Switzerland) at a scan rate of 0.1 mV s1 in a voltage range of 0.01e3 V. Galvanostatic intermittent titration technique (GITT) was measured on an electrochemical workstation (VMP3, Bio-Logic, France), the cells were charged/ discharged at 25 mA g1 with a current pulse duration of 15 min and an interval time of 15 min. For NCM//IA-G full-cell, three formation cycles at 18 mA g1 charge/discharge rate were first applied between 2.8 and 4.3 V vs. Li/Liþ. Rate capability was evaluated at a constant charge rate of 90 mA g1, and discharge rates from 18 mA g1 to 1.8 A g1. Long-term cycling was tested at 36 mA g1 charge and 180 mA g1 discharge. 3. Results and discussion 3.1. Structure characterizations of the IA-G composite The XRD pattern of the as-prepared IA-G composite (IA:G ¼ 1:9 wt%) is shown in Fig. 1a. For the IA-G composite, two typical diffraction peaks at 19.2 and 20.5 are consistent with the IA characteristic peaks. Other weak peaks at 23.1, 24.5, 30.0, 32.1, 33.2, 34.3 and 39.1 are also observed for the IA sample. And the two distinct peaks at 26.6 and 54.7, and the three week peaks at 42.4, 44.7 and 77.6 correspond well to the (002), (100), (101), (004) and (110) planes of G from low to high 2q angle [24]. These results show that the crystalline structures of both IA and G are not destroyed after composited them. FTIR spectra of the IA, G and IA-G composite are compared in Fig. 1b. For the IA sample, the broad absorption centered at 3052 cm1 is attributed to the CeOeH vibration, the strong absorption peak at 1711 cm1 is assigned to the C]O stretching vibration of carboxylic acid (COOH) group for the IA [25]. The assignments of other absorption bands are given as follows [26e28]: 2932 cm1 (yCeH in CH2), 1441 cm1 (dCeH in CH2), 1306 cm1 (yC-C), 1216 cm1 (yC-O in COOH) and 910 cm1 (gO-H in
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Fig. 1. (a) XRD patterns, (b) FTIR spectra and (c) Raman spectra of IA, G and the as-prepared IA-G composite (IA:G ¼ 1:9 wt%). The fitting results of XPS spectra of C1s spectra for (d) IA, (e) G and (f) the as-prepared IA-G composite (IA:G ¼ 1:9 wt%). (A colour version of this figure can be viewed online.)
COOH). For the G sample, the broad peak at 3432 cm1 and 1634 cm1 indicated the presence of OH, the narrow peak at 1716 cm1 and 1147 cm1 demonstrated the presence of C]O and CeOH, respectively [29]. The peak at 1070 cm1 is assigned to the CeC skeletal vibrations in six-membered ring forms of G [30]. For the IA-G composite, all the characteristic peaks of IA and G are well preserved, and it should be noted that the OH frequency showed an upshift from 3432 cm1 to 3450 cm1, and the CeOH vibration also exhibited a upshift of 20 cm1 to give a frequency of 1167 cm1, indicating the formation of hydrogen bond between IA and G [31]. Raman spectra of the IA, G and IA-G composite (Fig. 1c) validates the presence of D-band (1332 cm1), G-band (1579 cm1), and D*band (2683 cm1) [32] bonds of G and all the characteristic peaks of IA in the IA-G composite, indicating the typical structures of both IA and G are well preserved in the composite. XPS was further used to analyze the elemental composition of the IA-G composite (IA:G ¼ 1:9 wt%). XPS spectra of C1s for the IA, G and the as prepared IA-G composite are compared in Fig. 1def. Fig. 1d shows IA has five types carbon located at 284.3, 285.3, 288.6, 289.3 and 291.8 eV, corresponding to C]C, CeH/CeC, CeO, COOH and satellite (p-p*), respectively [33]. Fig. 1e shows the fitting results of G, it is easy to discern the typical graphite peak at 283.6 eV, the other peaks at 283.8, 285.2, 287.1 and 290.3 eV are attributed to CHx, C] O/CeO, COO and CO2 3 /p-p*, respectively [34]. Fig. 1f suggests all the characteristic peaks of IA and G preserved in the IA-G composite, namely graphitic (283.4 eV), C]C/CHx (284.4 eV), CeO (285.3 eV), COOR (286.4 eV), COOH (287.9 eV) and CO2 3 /p-p* (289.9 eV) [33,34]. These results further confirm that the primeval chemical structures for both IA and G are not significantly changed after composite them, and the slightly shift can be ascribed to the intermolecular interaction. 3.2. Electrochemical properties of the IA-G composite anodes Fig. 2 compares the first reversible specific capacity and the CE of the Li//IA-G half cells in the 0.01e3.0 V voltage range with different compositions and electrode thicknesses. Fig. 2a presents the first electrochemical performances of the IA-G anode (IA-
G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt%) with different IA:G weight ratios (IA:G ¼ 0:10, 0.2:9.8, 1:9, 2:8, 3:7, 4:6 and 5:5). As shown in this figure, the first specific capacity increased with the increase of the IA:G weight ratios, and the electrode with IA:G ¼ 4:6 wt% has the highest capacity of 765 mAh g1, indicating the significantly enhanced capacity resulted from IA. The capacity decrease of the IA:G ¼ 5:5 wt% is resulted from the lack of the electronic conductivity of the IA. On the contrary, the first CE decreased with the increase of the IA:G weight ratios. For the IA-G composite anode, IA:G ¼ 0.2:9.8 wt% has the highest CE of 82.0%, which is lower than the pure G anode (denoted as IA:G ¼ 0:10 wt% in Fig. 2a) of 87.4%. The decrease of the first CE can be attributed to the increased irreversible lithium consumption resulting from the increasing IA amount. Fig. 2b presents the first reversible specific capacity and the CE of the IA-G anode with different IA-G:AB weight ratios (IAG:AB:CMC:SBR ¼ 9-x:x:0.5:0.5, x ¼ 3, 2, 1, IA:G ¼ 1:9). Obviously, the first specific capacity decreased with the decrease of the AB weight ratios, indicating electronic conductivity plays a crucial role controlling the utilization rate of active materials. It worth noting that IA-G:AB ¼ 7:2 wt% has the highest CE, this is because too much AB (30 wt%) amounts result in a large irreversible lithium consumption, while lake of AB (10 wt%) amounts lead to difficult delithiation process of the electrode. Fig. 2c indicates the CMC:SBR weight ratio (CMC:SBR ¼ 1:2, 1:1, 2:1 and 1:0) has ignorable effect on the first electrochemical performance, and CMC:SBR ¼ 1:1 wt% slightly higher than others. The first reversible specific capacity and CE values of the IA-G anode with different electrode components are shown in Table S1. And Figure S(1-3) exhibit the corresponding charge/discharge curves and the rate capabilities of IA-G composite anodes with different composites. Fig. 2d shows the effect of the electrode thickness on the first electrochemical performance for the IA-G anode (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%). All the electrode thicknesses are final thickness with copper foil. It is worth noting that, even at 75 mm electrode thickness with an active material loading of 2.70e2.76 mg cm2, the IA-G anode still delivers a reversible capacity of 369 mAh g1. And the first CE of 74.6% is still much higher than most of the reported organic anodes [22,23,35]. The accurate capacity and CE values of Li//IA-G half cells with
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Fig. 2. The first specific capacity (red column) and the corresponding CE (blue polyline) of Li//IA-G half cells with different IA:G:AB:CMC:SBR weight ratios in the voltage range of 0.01e3.0 V: (a) IA-G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt% with different IA:G weight ratios, (b) IA-G:AB:CMC:SBR ¼ 9-x:x:0.5:0.5 wt%, x ¼ 3, 2, 1, IA:G ¼ 1:9 wt%, (c) IA:G:AB:CMCSBR ¼ 0.7:6.3:2:1 wt% with different CMC:SBR weight ratios, and (d) IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt% with different electrode thickness. (A colour version of this figure can be viewed online.)
different electrode thickness (and the corresponding active loading) are shown in Table S2. The corresponding first charge/ discharge curves and the rate capabilities can be seen in Figure S4. Considering the IA-G composite anode with IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt% has the relatively high first CE (78.5%) and the high reversible capacity of 551 mAh g1, more electrochemical tests of this anode have been conducted. And Li//G half cell (G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt%) was used as a comparison. Fig. 3a presents the initial three charge/discharge profiles of the Li//IA-G half cell and Li//G half cell at 25 mA g1 between 0.01 and 3 V. The first charge and discharge capacities of Li//IA-G half cell are obtained to be 702 and 551 mAh g1, respectively, corresponding to the first CE of 78.5%. The first discharge capacity of Li//IA-G half cell is significantly higher than the Li//G half cell of 364 mAh g1, while the first CE of IA-G composite anode is lower than the pure G anode of 87.4%, this is mainly because of the irreversible lithium consumption of IA. But it should be noted that the first CE is still dramatically improved than the reported organic and organic/inorganic composite anodes in the same voltage range [22,36]. And it's worth point out that at 0.01e1.5 V, the specific capacity of the Li//IA-G half cell (460 mAh g1) is still higher than the Li//G half cell (344 mAh g1). In the second cycle, the reversible capacity increased to 561 mAh g1 and the corresponding CE attained 95.6%. In the third cycle, the reversible capacity attained to 571 mAh g1 with the high CE of 97.2%. The rate behaviors of the Li//IA-G half cell and Li//G half cell were compared at a series of discharging current densities. As shown in Fig. 3b, at the high current density of 1, 2.5, 5, and 10 A g1, the IA-G composite electrode still exhibits the high capacities of 569, 567, 562, and 553 mAh g1, respectively. While the pure G anode exhibits ca. 350 mAh g1 at these current densities. When the current density turns back to 25 mA g1, the capacity is totally recovered, suggesting that the IA-G composite anode is tolerant of high-rate operation. Fig. 3c exhibits the long-term cycling performance of the Li//IA-G half cell at different current densities. When cycled at 100 mA g1 charge/200 mA g1 discharge, a stable reversible
capacity of 693 mAh g1 is obtained after 200 cycles. When cycled at 100 mA g1 charge/500 mA g1 discharge, the IA-G composite anode retained a reversible capacity of 642 mAh g1 after 200 cycles. The continuous increase of the specific capacity before 200 cycles can be ascribed to the created more charge transfer channels within the electrode in the subsequent cycles. It should be pointed out that the significant capacity reduction of the electrode after ca. 210 cycles is not caused by the electrode material itself. This can be verified by switching a fresh Li metal counter electrode (marked in black arrow), we can see both electrodes can be charged and discharged normally. We ascribed the capacity decrease after ca. 210 cycles to the electrolyte dryness, because after disassembling the cell, we found that the electrode sheet was very dry (see the inset of Fig. 3c). Both of the electrodes exhibit an average CE of ca. 99% during the whole cycles, indicating the IA-G anode has an excellent reversible lithium storage capability. All these results proved that the IA-G composite is suitable for constructing long-life and highrate anode for LIBs. The electrochemical performance of the IA-G composite was further examined using NCM//IA-G full cells. The IA-G anode was pre-treated according to a modified reported method [37] as shown in Fig. 4a. Specifically, the Li//IA-G half cell is charged and discharged at 25 mA g1 current density for three cycles, and then charged to 0.01 V and stand still for 12 h. Next, the half cell is disassembled in glove box, and the IA-G anode is taken out for assembling the NCM//IA-G full cell. The electrochemical performances of NCM//IA-G full cells are presented in Fig. 4bed. Fig. 4b shows the initial three charge/discharge curves for full cells between 2.8 and 4.3 V at 18 mA g1. The full cell shows an initial reversible capacity of 182 mAh g1 with 74% CE. And the capacity increases to 192 mAh g1 after three cycles with an improved CE of 95%. Fig. 4c shows the rate capability of the NCM//IA-G full-cell. Surprisingly, the full cell exhibits high reversible capacity of 135 mAh g1 at 0.9 A g1 and 118 mAh g1 at a high rate of 1.8 A g1, which are much higher than reported NCM-based full-cells [37,38]. Moreover, the cycling stability of the full-cell at 36 mA g1 charge
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Fig. 3. Electrochemical performances of the Li//IA-G half cell (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%) and Li//G half cell (G:AB:CMC:SBR ¼ 7:2:0.5:0.5 wt%) in the voltage of 0.01e3.0 V: (a) charge/discharge curves for the initial three cycles at 25 mA g1 current density and the (b) rate capability at various discharge current densities; and the (c) longterm cycling performances of the Li//IA-G half cell at different current densities. (A colour version of this figure can be viewed online.)
Fig. 4. (a) The pre-treated IA-G composite anode used in the full-cells. The electrochemical performances of NCM//IA-G full-cells between 2.8 and 4.3 V: (b) the initial three charge/ discharge curves at 18 mA g1, (c) rate properties at various rates, and (d) the cycling life. (A colour version of this figure can be viewed online.)
and 180 mA g1 discharge is exhibited in Fig. 4d. The full-cell shows a capacity of 95 mAh g1 after 100 cycles, and the CE is above 99.5% in each cycle. The capacity fade of NCM//IA-G full-cell is mainly associated with electrolyte decomposition, active material dissolution and phase changes of electrode materials in both cathode and anode [38,39]. The cyclic stability of NCM//IA-G full-cell is better than the reported full-cell with G as anode and 3,4,9,10-
perylene-tetracarboxylicacid-dianhydride/graphene aerogel as cathode [40]. In addition, we also prepared the NCM//IA-G full-cell with fresh IA-G anode, it shows a reversible capacity of 74 mAh g1 after 100 cycles at 36 mA g1 charge and 180 mA g1 discharge (see Figure S5a). And at 0.9 and 1.8 A g1, it exhibits a reversible capacity of 106 and 104 mAh g1, respectively (see Figure S5b). It's obvious that NCM//IA-G full-cell with fresh IA-G anode still exhibits an
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attractive rate capability, which mainly because of the excellent lithium conductivity of IA.
3.3. Li storage mechanisms for the IA-G composite anode To investigate the lithiation/de-lithiation mechanism of the IA-G composite, CV curves of the Li//IA-G half cell at a scan rate of 0.1 mV s1 between 0.01 and 3.0 V are plotted in Fig. 5a. Fig. 5b is the selected cathodic process from 3.0 to 0.3 V. In the first cathodic process, three reduction peaks at 1.72, 1.32 and 0.93 V are observed. The first peak at 1.72 V slightly shifts to a lower potential of 1.64 V in the second cycle, suggesting that there was an initial adaptation of the IA to lithiation. While the latter two peaks completely disappeared in the following cycles, indicating these two processes are irreversible. Specifically, the cathodic peak at around 1.32 V is ascribed to the irreversible replacement of the hydrogen on the carboxyl group of IA with lithium ions, which verified by ex-situ FTIR spectroscopy and ex-situ XPS analysis for the IA anode at different charging states in the first cycle (see Figure S6 and S7). The cathodic peak at around 0.93 V is related to the formation of the SEI film on the materials, which is a common character for anodes of LIBs [35,41]. From the second cathodic process, four typical peaks centered at 2.45, 1.64, 1.12 and 0.64 V are related to the lithiation process of the IA. And all these peaks retained in following cycles, indicating all these categories of reactions are reversible. Fig. 5c is the selected cathodic and the corresponding anodic processes from 0.3 to 0.01 V. As seen in this figure, toward negative potential, four typical cathodic peaks centered at 0.18, 0.13, 0.07 and 0.03 V are attributed to the formation of stage-4, stage-3, stage-2 and stage-1 graphite intercalation compound (GIC), respectively. In the following anodic process, three strong lithium extraction peaks centered at 0.13, 0.19 and 0.24 V are related to the de-lithiation of G. All these peaks well overlapped in subsequent CV curves, reflecting excellent reversibility of the G in the composite electrode. Fig. 5d is the selected anodic process from 0.3 to 3.0 V. In the first anodic
process, two obvious oxidation peaks at 0.94 and 1.49 V and one weak oxidation peak at 2.34 V are related to the de-lithiation processes of the IA. In the following anodic scan, the three peaks slightly shift to higher potential and stabilized at 1.06, 1.55 and 2.68 V in the third cycle. All these three categories of reactions are reversible, which explains the appearance of the anodic peaks in the following cycles. And the reversible cathodic and anodic peaks of IA can also be verified by Figure S8, which shows that the peaks ascribed to IA increase with the IA amount. Fig. 5 and S8 verified that both IA and G could exhibit excellent reversibility in the IA-G electrode, this contributes the high capacity of the composite anode. And it's worth pointing out that the specific capacity of IA-G composite is not a simple superposition of IA and G, but has a synergistic effect, which exhibits a significantly enhanced specific capacity than the calculated specific capacity (see Figure S9 and Table S3). Fig. 6a shows discharge profiles of the Li//IA-G half cell at various discharge current densities in the voltage range of 0.01e3.0 V. As shown in this figure, the total specific capacity of the Li//IA-G half cell has no obvious decay with increasing current densities, a slight elevation of the discharge plateau due to the polarization of the cell resulting mainly from the internal resistance of the cell. To study the de-lithiation capability of the G in the composite, the discharge profiles in the voltage range of 0.01e0.3 V have been selected and analyzed (Fig. 6b). It can be seen, that at low discharge current densities (from 25 to 250 mA g1), the electrode displays the typical plateaus of G. When the discharge rate exceeds 500 mA g1, a sudden capacity loss of the G is observed, which is mainly due to lithium ion diffusion limitation of the G in the composite electrode. Conversely, the total specific capacity of the Li//IA-G half cell still retained a high capacity of 553 mAh g1 even at 10 A g1, indicating IA could effectively facilitate the lithium ion diffusion in the composite electrode even there is a large gradient of the lithium ion concentration within the electrode. This can also be verified by comparison of diffusion coefficient of lithium ion
Fig. 5. (a) CV curves of the Li//IA-G half cell in the voltage range of 0.01e3.0 V, (b) the cathodic process in the voltage of 3.0e0.3 V, (c) the cathodic and anodic process in the voltage of 0.3e0.01 V, and (d) the anodic process in the voltage of 0.3e3.0 V. (A colour version of this figure can be viewed online.)
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Fig. 6. (a) Discharge profiles of Li//IA-G half cell at various discharge current densities in the voltage range of 0.01e3.0 V and (b) the selected discharge profiles in the voltage of 0.01e0.3 V. (A colour version of this figure can be viewed online.)
(DLi) in IA-G composite anode and pure G anode. According to GITT (calculation schematic see Figure S10) test, IA-G exhibits higher DLi than G until 0.42 V, especially being nearly three times higher than G at 0.85 V (see Figure S11), verified IA indeed could promote lithium ion transport. These results verify that composite IA with G could compensate for the poor large rate performance of G anode, and contributes to a high-rate anode. Fig. 7 presents the XPS spectra of C1s, O1s, Li1s and F1s core levels recorded for the IA-G electrode at OCV state and after three cycles. For C1s core levels (Fig. 7a), the electrode at OCV state can be decomposed in five components at 284.3, 285.2, 286.0, 288.4 and 289.2 eV assigned to graphite/CHx, CeO, COOR, COOH, and CO2 3 /pp*, respectively [34]. In the case of the electrode after three cycles, similar peaks at 284.5 and 286.3 eV are found. The new peak at 286.9 eV is assigned to ReOLi. And the significant increased peak at 288.9 eV is due to the formation of carbonate-like species on the electrode surface [42,43]. For O1s core levels (Fig. 7b), three
components at 531.6, 533.0 and 534.0 eV assigned to CO 2 , CeO and CO2 3 , respectively [43]. After three cycles, all three peaks retained except slightly shift, and a new peak at 529.6 eV is assigned to ROLi/ LiOH appeared [44]. It should be noted that, no Li1s (Fig. 7c) and F1s (Fig. 7d) signal observed for the electrode at OCV state, while significant Li1s and F1s signals can be observed after three cycles. Specifically, the Li1s peak includes Li2CO3/ROCO2Li (55.3 eV), LiF (56.1 eV) and LiPF6 (56.6 eV) [45]. The F1s peak includes LiF (684.6 eV) and LiPF6 (686.8 eV) [42]. As ReOLi, ROCO2Li, Li2CO3 and LiF are known to be the important components for the passivation film on anodes, XPS study verified that a stable SEI film formed on the electrode after three cycles, which is important for the stable cycling performance of the electrode. 4. Conclusions In summary, a novel IA-G composite anode with high-rate
Fig. 7. XPS high resolution spectra of (a) C1s, (b) O1s, (c) Li1s and (d) F1s for the IA-G composite anode at OCV state and after three cycles. (A colour version of this figure can be viewed online.)
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capability and long-cycle life was first adopted for LIBs. After optimization, the IA-G anode (IA:G:AB:CMC:SBR ¼ 0.7:6.3:2:0.5:0.5 wt%) is able to deliver ca. 570 mAh g1 reversible capacity in the voltage range of 0.01e3.0 V vs. Li/Liþ, with the first CE of 78.5%. At a high discharge rate of 10 A g1, the IA-G electrode still retained 96.8% capacity compared the electrode cycled at 25 mA g1, showing excellent high-rate performance. Besides, the IA-G anode retained ca. 700 mAh g1 specific capacity after 200 electrochemical cycles at 100 mA g1 charge and 200 mA g1 discharge rate. And at 0.01e1.5 V vs. Li/Liþ, IA-G anode still delivers a higher specific capacity (460 mAh g1) compared to the theoretical capacity of graphite (372 mAh g1). This composite anode can realize the effective utilization of organic electrode materials on the basis of reducing the proportion of inactive conductive carbon in the electrode, provide a new way for the development of the electro-active organic materials.
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Acknowledgements [23]
This work was financially supported by National Natural Science Foundation of China (NSFC, contract no. 21875154 and 21473120) and 863 project of the Department of Science and Technologies, China (contract no. 2015AA034601).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.06.065.
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