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Aqueous Lithium-Ion Batteries Using Polyimide-Activated Carbon Composites Anode and Spinel LiMn2O4 Cathode Zhaowei Guo, Long Chen, Yonggang Wang, Congxiao Wang, and Yongyao Xia* Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433, People’s Republic of China

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S Supporting Information *

ABSTRACT: Polyimide/activated carbon (PI/AC) composites were prepared by in situ polymerization of 1,4,5,8naphthalenete-tracarboxylic dianhydride (NTCDA) and ethylene diamine (EDA) on activated carbon with various mass ratios varying from 50:50 to 70:30. These composites were examined as anode materials in 5 M LiNO3 solution in the potential window from −0.75 to 0 V vs Ag/AgCl. With an optimal composition PI/AC 50:50 in mass ratio, the composite delivers a specific capacity of 87 mAh g−1 at a current density of 0.2 A g−1, and it also shows excellent cycling stability and rate capability. A sealed full cell containing a PI/ AC composite anode and LiMn2O4 cathode delivers a specific capacity of 42 mAh g−1 and energy density of 51Wh kg1− (based on the total weight of both active materials) at a current density of 0.2 A g−1. The full cell exhibits good cycling stability with a specific capacity of 35 mAh g−1 after 450 cycles, corresponding to a capacity retention of 89%. KEYWORDS: Aqueous lithium-ion battery, Polyimide, Activated carbon, Composite anode, LiMn2O4

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INTRODUCTION As one of the most potential stationary power sources for sustainable energies, for instance, wind and solar power, the aqueous rechargeable lithium-ion battery (ARLB) has attracted more and more attention from the public.1−3 The ARLB may not only solve the safety problem due to inevitable use of highly toxic and flammable organic solvents for lithium-ion batteries but may also improve the poor cycling performance associated with commercialized aqueous rechargeable batteries containing nickel metal−hydride systems and lead acid.2−4 However, an aqueous electrolyte proposes more requirements on the cathode and anode materials as a result of the narrow electrochemical window.5,6 Over the past few decades, many more efforts have been made to improve the electrochemical performance of ARLB since it was first proposed by Dahn’s group in 1994.7 Layered LiCoO2, spinel LiMn2O4, or olivine LiFePO4 was used as the cathode, while V- or Ti-based oxides and compounds, such as VO2, LiV3O8, LixV2O5, V2O5, TiP2O7, or LiTi2(PO4)3, were used as the anode for ARLB in most previous research.8−10 In spite of plenty of improvements in energy density having been achieved during the research of ARLB, cycling stability is still limited because of the poor stability of the anode in the aqueous electrolyte, which is derived from the side reactions of the anode with oxygen and/ or water, dissolution of active materials in the electrolyte, and adverse structural change during the charge/discharge process.10−12 To a large extent, the development process of the © 2017 American Chemical Society

ARLB systems depends on the electrochemical performance of the anode in the aqueous electrolyte. Organic electrode materials have already been examined in organic lithium-ion batteries mostly, while they have been implemented in ARLB recently. These materials, especially carbonyl group-conjugated compounds, possess certain advantages of structure diversity, flexibility, tunable redox property, and probable high energy density.13−16 Compared with previously reported anode materials for ARLB, the advantages of a totally distinct redox mechanism10 and better insolubility in aqueous electrolyte over small molecules16 are presented by polyimide. Besides, during the redox process, the framework of polyimide keeps stable. Polyimide has been examined as a potential anode material for ARLB owing to its superior electrochemical performance.17 As other organic materials, polyimide is a typically poor electrical conductor. Large amounts of conductive additive are inevitably used for polyimide as an anode in ARLB. Song et al. developed nanocomposite combining polyimide with functionalized graphene sheets to improve rate performance of polyimide in rechargeable lithium batteries.18 However, graphene delivered very low capacity, limiting the capacity of the composite anode. Herein, owing to possessing certain Received: September 5, 2016 Revised: November 6, 2016 Published: January 4, 2017 1503

DOI: 10.1021/acssuschemeng.6b02127 ACS Sustainable Chem. Eng. 2017, 5, 1503−1508

Research Article

ACS Sustainable Chemistry & Engineering capacity and good electrical conductivity, activated carbon (AC) was intimately mixed with polyimide (synthesized by a simple polycondensation reaction from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) and ethylene diamine(EDA)). The PI/AC mass ratio was examined through the comparison of capacity, rate capability, and cycling performance of these composites comprising blended polyimide and activated carbon in a mass ratio varying from 50:50 to 70:30. Afterward, PI/AC in a 50:50 mass ratio was used as anode for ARLB with a LiMn2O4 cathode, which was examined as a cathode with good electrochemical performance,19−21 to assemble the PI-AC/LiNO3 /LiMn2O4 full cell. The electrochemical property of the full cell was studied in terms of charge/discharge and cycling tests.

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EXPERIMENTAL SECTION

Materials Synthesis. The composites comprising blended PI and activated carbon were prepared by the above-mentioned method in the mass ratio varying from 50:50 to 70:30. A certain amount of activated carbon was dispersed in the solvent of N-methylpyrrolidone (NMP), followed by stoichiometric NTCDA and EDA added into the suspension. The mixture reacted under reflux at 300 °C for 6 h. Following being filtrated and washed with ethanol and NMP several times in turn, the product was dried at 120 °C in an air oven for 12 h. After that, the product was calcined in a furnace at 300 °C for 8 h in nitrogen atmosphere. As a comparison, pure polyimide was also prepared by the same process mentioned above. Characterization and Electrochemical Measurement. All samples were characterized by Fourier transform infrared spectroscopy (FT-IR) measurements through a NICOLET 6700 FI-TR spectrometer using KBr pellet. The morphologies of the samples were examined by scanning electron microscopy (SEM, JEOL JSM-6390). Transmission electron microscope (TEM) images were taken using a JEOL JEM-2010 microscope. The PI-containing electrodes were produced by vigorously mixing PI with activated carbon, acetylene black (AB), and binder (polytetrafluoroethylene) (PTFE) in isopropanol in approximately a 60:30:10 mass ratio. The electrode film was vacuum-dried at 80 °C for 10 h to remove the solvent. Then, the electrode film was dried at 120 °C in an air oven for 12 h, followed by pressing the electrode film on a stainless steel grid. The typical active material mass loading was about 3 mg cm−2. The cathode electrode consisted of LiMn2O4, acetylene black, and PTFE in an 80:15:5 mass ratio. For the three-electrode cell, the counter electrode was produced with activated carbon, acetylene black, and PTFE in an 85:10:5 mass ratio. Both the LiMn2O4 electrode and AC electrode were treated in a similar process. The cyclic voltammetry (CV) tests of all the samples were carried out on a CH Instruments electrochemical workstation (CHI 660D). All the electrodes described above were tested in a 5 M LiNO3 aqueous solution. The Ag/AgCl electrode (E = 0.1971 V vs SHE) was used as the reference electrode. The CV scan of LiMn2O4 was performed between 0.5 and 1.05 V vs Ag/AgCl at a scan rate of 0.5 mV s−1, while that of PI was between −0.85 and −0.1 V vs Ag/AgCl. Galvanostatic charge−discharge and cycle performance tests were performed between −0.75 and 0 V vs Ag/AgCl on a Hukuto Denko battery charge/discharge system HJ series (Japan) controlled computer with continuous bubbling nitrogen to remove oxygen. The electrochemical performance of the full cell was tested between 0 and 1.75 V at the current density of 0.2 A g−1.

Figure 1. FT-IR spectra (KBr pellets) of PI and PI/AC composites.

These results from FT-IR are consistent with previous reports.16 There are no significant differences among SEM images of composites. As shown in Figure 2a−d, PI agglomerates are approximately varied from a few tenths of a micrometer to a few micrometers in diameter, and PI/AC composites are a little bigger than pure PI in size. Figure 2e shows the TEM image of pure PI, which is agglomerated together. As observed in Figure 2f and Figure S1, activated carbon is coated by irregular PI. Results of the cyclic voltammetry of PI are presented in Figure 3. As shown in the CV curves, both oxidation and reduction processes consist of two continuous steps associated with two electrons transfer, respectively. Two redox peaks appear at −0.22 V/−0.38 V and −0.62 V/−0.69 V vs Ag/AgCl, indicating the expected redox reactions at the proper potential. The electrochemical performance results of PI and PI/AC composites in different PI: AC mass ratio anodes are depicted in Figure 4 and Figure 5. These data show that capacity, rate, and cycling performance is correlated to the PI/AC mass ratio. The charge−discharge profiles of PI and PI/AC composites at different current densities are presented in Figure 4a−d. Without activated carbon blended, as Figure 4a shows, PI electrode presents very low specific capacity at even low current density, only 20 mAh g−1 at the current density of 0.2 A g−1, and delivers almost no capacity at the current density of 0.5 A g−1. The delivered specific capacity increases with more activated carbon included in the composites anode. As observed in Figure 4b, reversible capacity of the composite in a 50:50 PI/ AC mass ratio decreases from 88 to 73 mAh g−1 as the current density increases from 0.1 to 2.0 A g−1, with a 17% capacity drop, while reversible specific capacity of the composite in a 60:40 PI/AC mass ratio decreases from 78 to 53 mAh g−1 under the same current density range, as Figure 4c shows, with a 32% capacity drop. Meanwhile, Figure 4d shows that the composite in a 70:30 PI/AC mass ratio delivers a 47% capacity drop from 80 to 42 mAh g−1. Figure 4e compares the rate performance of PI and PI/AC composite electrodes in a pronounced difference. It is obvious that the PI/AC composite in a 50:50 mass ratio presents the best rate performance and highest capacity among these composites. Theoretically, increasing the relative amount of PI will result in a composite anode of higher energy density since activated carbon delivers lower capacity than polyimide, which is seemingly not in line with the above results. However, that is because that polyimide

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RESULTS AND DISCUSSION Figure 1 shows the FT-IR spectra data collected from asprepared PI and PI/AC composites. The characteristic absorption peaks at 1702, 1671, and 771 cm−1 correspond to νas (imide CO), νs (imide CO), and δ (imide CO), respectively. The absorption peaks at 1583 and 1351 cm−1 can be assigned to naphthalene and ν (imide C−N), respectively. 1504

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Figure 2. SEM image of (a) PI, (b) PI/AC composite in a 50:50 mass ratio, (c) PI/AC composite in a 60:40 mass ratio and (d) PI/AC composite in a 70:30 mass ratio; TEM image of (e) PI/AC composite in a 50:50 mass ratio and (f) PI.

evident that the composite in a 50:50 PI/AC mass ratio presents the highest capacity, best rate, and cycling performance among these composites. Compared with traditional anode materials of ARLB, the composite in a 50:50 PI/AC mass ratio presents higher capacity and better cycling performance, improving the electrochemical performance of ARLB. Considering the capacity, rate capability, and cycling performance, the composite in a 50:50 PI/AC mass ratio was used as the anode in concert with the cathode LiMn2O4 for ARLB. LiMn2O4 has been regarded as one of the most promising cathode materials in the ARLB system. Cyclic voltammetry, charge−discharge, and cycling tests were carried out in three-electrode cells. The cyclic voltammetry profile at a scan rate of 0.5 mV s−1, as shown in Figure 6a, presents two redox peaks of LiMn2O4 at 0.82 V/0.72 and 0.97 V/0.86 V vs Ag/AgCl, corresponding to two stages of Li+ intercalation/ deintercalation. LiMn2O4 delivers good rate performance and high capacity as shown in Figure 6b and c, with a discharge capacity of 107 mAh g−1 at the current density of 0.1 A g−1 and 92 mAh g−1 at the current density of 0.5 A g−1, namely, the capacity retention of 86%. As Figure 6d shows, during the galvanostatic cycling between 0 and 1.05 V vs Ag/AgCl at the current density of 0.2 A g−1, 96% of the initial discharge capacity of 96 mAh g−1 is retained after 100 cycles, with the Coulombic efficiency remained at above 97% for the duration of the cycling. All these data demonstrates that LiMn2O4 is a potential cathode material with good rate and cycling performance for ARLB. The mass ratio of LiMn2O4: PI/AC (in a 50:50 mass ratio) is set as 1:1.2 according to the charging capacity of LiMn2O4 and the discharging capacity of PI/AC composite in a 50:50 mass ratio (88:106) of the first cycle. As is observed in Figure 7a, the PI-AC/LiNO3/LiMn2O4 full cell shows an average discharging voltage of 1.21 V. An initial capacity of 42 mAh g−1 and a specific energy of 51Wh kg1− (based on the total weight of both active materials), with a capacity retention of 89% after 450 cycles, are illustrated at the current density of 0.2 A g−1 in Figure 7b. In addition, the Coulombic efficiency is above 99% after the initial cycle. The cycling stability of the full cell for ARLB is much superior to traditional ARLB systems.

Figure 3. CV profile of PI between −0.85 and −0.1 V vs Ag/AgCl at a scan rate of 0.5 mV s−1.

cannot deliver its capacity completely with insufficient activated carbon, as the composites in the 60:40 and 70:30 mass ratios did. There are no significant differences in cycling stability among PI and PI/AC composites in Figure 5, which shows the cycling performance at the current density of 0.2 A g−1 of PI and PI/ AC composites but different cycling capacity. As Figure 5 shows, the PI electrode delivers quite low reversible capacity, about 20 mAh g−1 at the current density of 0.2 A g−1, but excellent cycling performance with little capacity fade. The capacity of the composite in a 50:50 PI/AC mass ratio at the first cycle is 87 mAh g−1. Then, it slowly drops to 81 mAh g−1 at the 100th cycle with the capacity retention of 93%. While the capacity of the composite in a 60:40 PI/AC mass ratio delivers the capacity of 70 mAh g−1 at the 95th cycle with the capacity retention of 92%, within 100 cycles, only 11% capacity drop is observed on the composite in a 70:30 PI/AC mass ratio electrode with the capacity of 67 mAh g−1 at the 100th cycle. Through the comparison of cycling performance of PI/AC composites, the composite in a 50:50 PI/AC mass ratio presents the highest capacity among these three composites with excellent cycling performance. From the above results, it is 1505

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Figure 4. Charge−discharge profiles of (a) PI, (b) PI/AC composite in a 50:50 mass ratio, (c) PI/AC composite in a 60:40 mass ratio, and (d) PI/ AC composite in a 70:30 mass ratio and (e) rate performance of PI and PI/AC composites at different current densities between −0.75 and 0 V vs Ag/AgCl in 5 M LiNO3.

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CONCLUSIONS In summary, polyimide and activated carbon composites were produced and studied as anodes for aqueous rechargeable lithium-ion batteries. Stable structure, low solubility, and excellent cycling stability in aqueous solution make polyimide very attractive for use in ARLB. To enhance the poor electrical conductivity, the PI/AC composites in the mass ratio varying from 50:50 to 70:30 were produced by a simple polycondensation reaction between NTCDA and EDA. These PI/AC

composites present different capacity, rate and cycling performance. By comparison, the composite in a 50:50 PI/AC mass ratio delivers the highest discharge capacity of 87 mAh g−1 at the current density of 0.2 A g−1, the best rate and cycling performance among them, with a capacity retained of 93% after 100 cycles. LiMn2O4 has been examined to be a potential cathode material with good rate and cycling performance for ARLB. A sealed aqueous lithium-ion battery based on a PI/AC composite (in a 50:50 mass ratio) anode and LiMn2O4 cathode 1506

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density and excellent cycling performance, better than traditional ARLB systems. The PI/AC composite in a 50:50 mass ratio is demonstrated to be a potential anode with superior electrochemical performance for ARLB.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02127. Information as mentioned in text. TEM of PI/AC composites in a mass ratio of 60:40 and 70:30; charge− discharge profiles of PI electrode (PI: AB: PTFE = 30:60:10); cycling profiles of PI electrode (PI: AB: PTFE = 30:60:10). (PDF)

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Figure 5. Cycling performance of PI and PI/AC composites at the current density of 0.2 A g−1 between −0.75 and 0 V vs Ag/AgCl in 5 M LiNO3.

AUTHOR INFORMATION

Corresponding Author

*Yongyao Xia. E-mail: [email protected]. Tel. and Fax: 008621-51630318.

has been implemented. The PI-AC/LiNO3/LiMn2O4 full cell delivers a special capacity of 42 mAh g−1 and energy density of 51Wh kg1− (based on both active materials), with a capacity retention of 89% after 450 cycles at a current density of 0.2 A g−1. The PI-AC/LiMn2O4 ARLB system presents high power

ORCID

Yonggang Wang: 0000-0002-2447-4679 Yongyao Xia: 0000-0001-6379-9655

Figure 6. (a) Cyclic voltammetry profiles of LiMn2O4 at a scan rate of 0.5 mV s−1; (b) charge−discharge profiles, and (c) rate capability of LiMn2O4 at different current densities between 0.5 and 1.05 V vs Ag/AgCl in 5 M LiNO3; (d) cycling performance of LiMn2O4 at the current density of 0.2 A g−1 between 0.5 and 1.05 V vs Ag/AgCl in 5 M LiNO3. 1507

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Figure 7. (a) Charge−discharge profile and (b) cycling profiles of PI-AC/LiNO3/LiMn2O4 full cell at a current density of 0.2 A g−1 between 0 and 1.75 V vs Ag/AgCl in 5 M LiNO3.

Notes

ybenzoquinone: toward the development of a sustainable Li-ion battery. J. Am. Chem. Soc. 2009, 131, 8984−8988. (14) Armand, M.; Grugeon, S.; Vezin, H.; Laruelle, S.; Ribiere, P.; Poizot, P.; Tarascon, J. M. Conjugated dicarboxylate anodes for Li-ion batteries. Nat. Mater. 2009, 8, 120−125. (15) Chen, L.; Li, W. Y.; Wang, Y. G.; Wang, C. X.; Xia, Y. Y. Polyimide as anode electrode material for rechargeable sodium batteries. RSC Adv. 2014, 4, 25369−25373. (16) Song, Z. P.; Zhan, H.; Zhou, Y. H. Anthraquinone based polymer as high performance cathode material for rechargeable lithium batteries. Chem. Commun. 2009, 4, 448−450. (17) Chen, L.; Li, W. Y.; Guo, Z. W.; Wang, Y. G.; Wang, C. X.; Che, Y.; Xia, Y. Y. Aqueous lithium-ion batteries using O2 self-elimination polyimides electrodes. J. Electrochem. Soc. 2015, 162, 1972−1977. (18) Song, Z. P.; Xu, T.; Gordin, M. L.; Jiang, Y. B.; Bae, I.; Xiao, Q. F.; Zhan, H.; Liu, J.; Wang, D. H. Polymer-graphene nanocomposites as ultrafast-charge and -discharge cathodes for rechargeable lithium batteries. Nano Lett. 2012, 12, 2205−2211. (19) Alias, N.; Mohamad, A. A. Advances of aqueous rechargeable lithium-ion battery: A review. J. Power Sources 2015, 274, 237−251. (20) Zeng, J. S.; Li, M. S.; Li, X. F.; Chen, C.; Xiong, D. B.; Dong, L. T.; Li, D. J.; Lushington, A.; Sun, X. L. A novel coating onto LiMn2O4 cathode with increased lithium ion battery performance. Appl. Surf. Sci. 2014, 317, 884−891. (21) Kim, H.; Hong, J.; Park, K. Y.; Kim, H.; Kim, S. W.; Kang, K. Aqueous rechargeable Li and Na Ion batteries. Chem. Rev. 2014, 114, 11788−11827.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (No. 21333002), National Key Research and Development Plan (2016YFB0901503), and Shanghai Science & Technology Committee (13JC1407900).

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REFERENCES

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