- Email: [email protected]
4 V Aqueous hybrid supercapacitors based on dual electrolyte technologies
Available online at www.sciencedirect.com
Review Article 4 V Aqueous hybrid supercapacitors based on dual electrolyte technologies Sho Makino 1 , Dai Mochizuki 1,2 and Wataru Sugimoto 1,2,∗ Aqueous hybrid supercapacitors have the advantage of utilizing high pseudo-capacitive electrodes, but suffer from low cell voltage due to the narrow voltage window of water decomposition. This disadvantage can be overcome by combining aqueous electrochemical capacitor technologies with a water-stable anode that is protected with a solid electrolyte. In this short review, we summarize recent advances in hybrid supercapacitors with a rated voltage of ∼4 V using an aqueous catholyte and pseudo-capacitive cathode combined with a protected Li or Li-pre-doped carbon anode. Record high specific energy of >700 Wh (kg-cathode)−1 were obtained with a pseudo-capacitive positive electrode and a protected Li-pre-doped carbon anode. The newly developed hybrid supercapacitor outperforms present Li-ion capacitor technology and even has the potential to compete with rechargeable batteries. Addresses 1 Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan 2 Center for Energy and Environmental Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan ∗
Corresponding author: Sugimoto, Wataru ([email protected])
Current Opinion in Electrochemistry 2017, 6:127–130 This review comes from a themed issue on Batteries and Super capacitors Edited by S.M. Oh For a complete overview see the Issue and the Editorial Available online 26 October 2017 https://doi.org/10.1016/j.coelec.2017.10.022 2451-9103/© 2017 Elsevier B.V. All rights reserved.
Introduction Electrochemical capacitors (also referred as supercapacitors) are classified into electrical double layer capacitors that rely on non-Faradaic charge storage or pseudocapacitors which utilize fast surface redox process. Electrical double layer capacitors are composed of two activated carbon electrodes sandwiched between organic electrolytes comprised of a quaternary ammonium salt dissolved in non-aqueous solvents. The energy density of supercapacitor is defined by CV2 /2 (C is capacitance www.sciencedirect.com
and V is the cell voltage), hence increasing C and V is the design principle for better performance. Specific capacitance can be increased by introducing surface-confined redox processes (pseudo-capacitance or redox capacitance) while the cell voltage can be extended by using ionic liquids or an asymmetric configuration (also known as hybrid supercapacitors). Pseudo-capacitive oxides based on ruthenium, manganese, vanadium, etc. can deliver specific capacitance much larger than activated carbon owing to the surface-confined redox processes. For example, RuO2 nanosheets and nanoparticles deliver specific capacitance of 600–800 F g−1 in sulfuric acid [1–6], which is a significant advantage compared to typical activated carbons (∼100 F g−1 ). Unfortunately, this high capacitance can be obtained only in aqueous electrolytes thereby limiting the voltage window to 1 V. A lithium-ion capacitor is a typical hybrid supercapacitor that uses activated carbon as the positive electrode, a Lidoped graphitic carbon (Lix C6 ) as the negative electrode, and a Li salt dissolved in an organic solvent as the electrolyte [2,7]. The low standard electrode potential of the Li intercalation/deintercalation reaction (E° = −3.045 V vs. SHE) results in a maximum cell voltage of ∼3.8 to 4 V, much larger than those for electrical double layer capacitors. Owing to this high cell voltage, the energy density of lithium-ion capacitors are typically a few times higher than electrical double layer capacitors. The energy density of hybrid supercapacitors are governed by the capacitive electrode, thus in the case of lithium-ion capacitor with an activated carbon cathode (100 F g−1 or capacity of 56 mAh g−1 ) and a 2 V window (1.8–3.8 V), an energy density of 20 Wh (kg-device)−1 can be obtained. The Lix C6 anode can be replaced with other battery-type negative electrodes, for example Li4 Ti5 O12 and TiO2 (B) [8–10]. Novel hybrid configurations need to be developed in order to realize further increase in the energy density of hybrid supercapacitors to compete or compliment lithiumion batteries.
Hybrid supercapacitors using dual electrolytes Since the energy density of hybrid supercapacitors is governed by the charge storage capability of the capacitive electrode, high capacitance electrodes that utilize pseudo-capacitance will be advantageous. In order to utilize pseudo-capacitance, aqueous electrolytes need to be used, which is not compatible with Li electrochemistry. Current Opinion in Electrochemistry 2017, 6:127–130
128
Batteries and Super capacitors
Figure 1
Schematic representation of AdHiCapTM cell (Advanced Hybrid Capacitors) using dual electrolytes.
We have devised a novel hybrid device, which we call AdHiCapTM (Advanced Hybrid Capacitor), that can take benefit from both the high capacitance of fast surface redox processes and the low standard electrode potential of Li by separating the aqueous catholyte and the nonaqueous anolyte with a water-stable Li+ conducting solid electrolyte, as shown in Figure 1 [11•• ]. A multi-layered anode (protected anode), which has been designed for use in aqueous Li–air rechargeable batteries [12,13], is used. The protected anode is a pouched cell composed of a Li foil or Lix C6 powder, polymer or gel electrolyte, and Li+ conducting solid electrolyte. For the Li+ conducting solid electrolyte, a commercially available NASICONtype glass ceramic Li1+ x + y (Ti,Ge)2−x Alx Siy P3−y O12 (x ∼ 0.25, y ∼ 0.3) (LTAP) with a thickness of 150 μm is used. Our proof-of-concept AdHiCapTM utilized a protected Li anode developed initially for Li–air rechargeable batteries, which can be expressed as (Li anode | PEOLiTFSI catholyte | LTAP separator) [11•• ]. The anolyte PEO-LiTFSI (polyethylene oxide polymer doped with Li(CF3 SO2 )2 N) acts as a buffer layer to avoid contact of Li with LTAP. For the catholyte, aqueous LiCl or Li2 SO4 was used, as LTAP is stable only near neutral pH. Combining this with positive electrodes such as activated carbon, MnO2 or RuO2 completes the hybrid supercapacitor, which can be expressed as (Li | PEO-LiTFSI | LTAP | aq. LiCl or Li2 SO4 | AC, MnO2 or RuO2 ). Shortly after our report, a similar device was reported by Chou et al., using LiNTf2 -[C3 mpyr][NTf2 ] (lithium bis(trifluoromethanesulfonyl)imide in N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide) as the anolyte [14]. Lower resistance cells were realized by using protected Lix C6 anodes and ionic liquid additives to the anolyte [15• ]. We then used acetic acid–lithium acetate buffered solutions as an alternative to Li2 SO4 , which afforded higher pseudo-capacitance [16• ]. Current Opinion in Electrochemistry 2017, 6:127–130
Figure 2
Charge/discharge curves of aqueous hybrid supercapacitor with protected Li anode and (a) activated carbon, (b) MnO2 , and RuO2 nanosheet cathode and aqueous LiCl or Li2 SO4 catholyte. Current density at 0.255 mA cm−2 and 60 °C. Shaded region represents the specific energy [11•• ].
Hybrid supercapacitor with protected Li anode (proof-of-concept)
Figure 2 shows galvanostatic charge/discharge curves of one of our initial hybrid supercapacitor with protected Li anode and various positive electrodes (Li | PEOLiTFSI | LTAP | aq. LiCl or Li2 SO4 | cathode) at j = 0.255 mA cm−2 . Three positive electrodes were studied, i.e., AC, MnO2 , and RuO2 ·nH2 O. The cell needed to be warmed to 60 °C due to the rather high resistance of the protected Li anode. In the case where AC was used as the positive electrode, linear charge/discharge curves with no plateau typical of capacitive charge storage was observed between 2.9 and 3.9 V. The discharge capacity was 108 Wh kg−1 based on the mass of the positive electrode. This is comparable with the discharge capacity of 100 Wh (kg-AC)−1 for lithium-ion capacitors [8,17], demonstrating the validity of the concept. By using MnO2 as the positive electrode for AdHiCapTM , the cell voltage can be extended even further to 4.3 V due to the high overpotential for oxygen evolution. The energy storage capability of the MnO2 positive electrode will depend on the crystal structure, particle size, and electrode thickness, but specific energy as high as 750 Wh (kgMnO2 )−1 can be achieved [18]. The cyclability of the hybrid supercapacitor in a pouched cell was also studied. As shown in Figure 3, 85% of the capacity was retained after 2000 cycles (normalized by the 50th cycle). Extrapolation of the degradation rate gives an estimated 15,000 cycles for 70% retention [18]. The use of RuO2 nanoparticles and nanosheets with high pseudo-capacitance will also lead to higher energy density. RuO2 nanomaterials can give capacitance up to 800 F g−1 in H2 SO4 . This value is unfortunately considerably reduced to 542 F g−1 (177 mAh g−1 ) in neutral www.sciencedirect.com
4-V Aqueous hybrid supercapacitors Makino, Mochizuki and Sugimoto
Figure 3
129
Figure 4
Consecutive cycling data on full pouched cell with an electrodeposited MnO2 /carbon paper positive electrode [18].
electrolytes such as Li2 SO4 , due to absence of surface redox processes. Nonetheless, it is a few times larger than AC electrodes. A cell voltage of 3.8 V with specific energy of 544 Wh kg−1 , was achieved [11•• ]. High capacitance can be attained with a weakly acidic buffered solution (pH = 5.8) of acetic acid–lithium acetate and even bioelectrolytes such as phosphate buffered saline and fetal bovine serum [16• ,19]. Remarkably, capacitance higher than that in H2 SO4 was achieved in acetic acid–Li acetate, reaching 1040 F g−1 and specific energy of 724 Wh kg−1 [16• ]. Hybrid supercapacitor with protected Lix C6 anode
The proof-of-concept hybrid supercapacitors using protected Li anode (Li | PEO-LiTFSI | LTAP) described in the preceding paragraph suffers from high impedance originating from the high resistance of the solid electrolyte, polymer electrolyte and the fact that there are many interfaces. This is the reason that hybrid supercapacitor with protected Li anode had to be operated at 60 °C. By changing from Li foil to Li-predoped graphitic carbon (Lix C6 ) particles, the interfacial resistance between the anode and the buffer layer could be decreased. The resistance could be further decreased by adding N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI) ionic liquid into the polymer electrolyte. The resistance of the improved protected Lix C6 anode (Li x C6 | PEO-LiTFSIPP13TFSI | LTAP) was decreased to 1/6 compared to the initial protected Li anode (Li | PEO-LiTFSI | LTAP), allowing room temperature operation of the cell [15• ]. With this protected Lix C6 anode with enhanced conductivity, maximum capacitance of 196 mAh g−1 and specific energy of 625 Wh kg−1 based on the positive electrode mass was achieved (Figure 4) [15• ]. www.sciencedirect.com
Charge/discharge curves at room temperature (25 °C) for an aqueous hybrid supercapacitor with protected Lix C6 anode, RuO2 nanosheet cathode, and acetic acid–lithium acetate buffered solution for the catholyte. Current density at 0.255 mA cm−2 and 60 °C (Lix C6 |PEO-LiTFSI-PP13TFSI|LTAP|2.0 M AcOH-AcOLi|RuO2 nanosheet) [15• ].
Related hybrid devices based on dual electrolyte concept and future outlook Despite the short period since the success of our proof-ofconcept study in 2012, enhancement in performance has slowly but surely progressed. Reduction in resistance of more than 10 times now allows cell operation at room temperature. We now have a better understanding of the merits as well as complications of dual electrolyte hybrid supercapacitors. The advantage of Li–air rechargeable batteries is the high theoretical in protected Li anode. In the case of hybrid supercapacitors, the use of massive Li is not necessary, and thus the use of Lix C6 is beneficial. Another important point is that unlike battery technology, power performance and cycle life are required for supercapacitors. Therefore, decreasing resistance of the protected anode is essential. Clearly more work is necessary to meet the market demands for higher power and longer life. Nonetheless, the concept of dual electrolyte energy storage is quickly catching on which should accelerate realization into practical applications. Following our work, hybrid devices based on similar concepts have been presented. The protected Li anode used in our AdHiCapTM aqueous hybrid supercapacitor was borrowed from aqueous Li–air rechargeable battery technology. The protected Li anode can be used for high voltage aqueous rechargeable batteries [20–22]. For example, ∼450 Wh (kg-electrode mass)−1 was achieved with Current Opinion in Electrochemistry 2017, 6:127–130
130
Batteries and Super capacitors
a LiMn2 O4 or LiCoO2 cathode [20,21]. Another route to increasing cell voltage with dual electrolytes with different pH has been demonstrated by combining anion and cation exchange membranes [23] or a glass fiber separator soaked in KNO3 or LiNO3 solution [24,25].
Acknowledgments This work was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •
Paper of special interest. Paper of outstanding interest.
••
1.
Long JW, Bélanger D, Brousse T, Sugimoto W, Sassin MB, Crosnier O: Asymmetric electrochemical capacitors—stretching the limits of aqueous electrolytes. MRS Bull 2011, 36:513–522. https://doi.org/10.1557/mrs.2011.137.
2.
Brousse T, Bélanger D, Chiba K, Egashira M, Favier F, Long J, Miller JR, Morita M, Naoi K, Simon P, Sugimoto W: Materials for electrochemical capacitors. In: Breitkopf C, Swider-Lyons K. Springer Handbook of Electrochemical Energy. Springer-Verlag Berlin Heidelberg; 2017:495–561.
3.
Zheng JP, Jow TR: A new charge storage mechanism for electrochemical capacitors. J Electrochem Soc 1995, 142:L6–L8. https://doi.org/10.1149/1.2043984.
4.
Zheng JP, Cygan PJ, Jow TR: Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J Electrochem Soc 1995, 142:2699–2703. https://doi.org/10.1149/1.2050077.
5.
6.
Sugimoto W, Iwata H, Yasunaga Y, Murakami Y, Takasu Y: Preparation of ruthenic acid nanosheets and utilization of its interlayer surface for electrochemical energy storage. Angew Chem Int Ed Engl 2003, 42:4092–4096. https://doi.org/10.1002/anie.200351691. Fukuda K, Saida T, Sato J, Yonezawa M, Takasu Y, Sugimoto W: Synthesis of nanosheet crystallites of ruthenate with an alpha-NaFeO2 -related structure and its electrochemical supercapacitor property. Inorg Chem 2010, 49:4391–4393. https://doi.org/10.1021/ic100176d.
7.
Naoi K, Simon P: New materials and new configurations for advanced electrochemical capacitors. Electrochem Soc Interface 2008, 17:34–37.
8.
Amatucci GG, Badway F, Du Pasquier A, Zheng T: An asymmetric hybrid nonaqueous energy storage cell. J Electrochem Soc 2001, 148:A930. https://doi.org/10.1149/1.1383553.
9.
Naoi K, Naoi W, Aoyagi S, Miyamoto J-I, Kamino T: New generation “Nanohybrid Supercapacitor”. Acc Chem Res 2012, 46:1075–1083. https://doi.org/10.1021/ar200308h.
10. Naoi K, Ishimoto S, Miyamoto J, Naoi W: Second generation “nanohybrid supercapacitor”: evolution of capacitive energy storage devices. Energy Environ Sci 2012, 5:9363. https://doi.org/10.1039/c2ee21675b. 11. Makino S, Shinohara Y, Ban T, Shimizu W, Takahashi K, Imanishi N, •• Sugimoto W: 4V class aqueous hybrid electrochemical capacitor with battery-like capacity. RSC Adv 2012, 2:12144–12147. https://doi.org/10.1039/c2ra22265e. First paper on aqueous supercapacitor with 4 V operating voltage, made possible by utilizing a protected Li anode.
Current Opinion in Electrochemistry 2017, 6:127–130
12. Zhang T, Imanishi N, Shimonishi Y, Hirano A, Takeda Y, Yamamoto O, Sammes N: A novel high energy density rechargeable lithium/air battery. Chem Commun 2010, 46:1661–1663. https://doi.org/10.1039/b920012f. 13. Zhang T, Imanishi N, Shimonishi Y, Hirano A, Xie J, Takeda Y, Yamamoto O, Sammes N: Stability of a water-stable lithium metal anode for a lithium–air battery with acetic acid–water solutions. J Electrochem Soc 2010, 157:A214–A218. https://doi.org/10.1149/1.3271103. 14. Chou S-L, Wang Y-X, Xu J, Wang J-Z, Liu H-K, Dou S-X: A hybrid electrolyte energy storage device with high energy and long life using lithium anode and MnO2 nanoflake cathode. Electrochem Commun 2013, 31:35–38. https://doi.org/10.1016/j.elecom.2013.03.003. 15. Makino S, Yamamoto R, Sugimoto S, Sugimoto W: Room • temperature performance of 4V aqueous hybrid supercapacitor using multi-layered lithium-doped carbon negative electrode. J Power Sources 2016, 326:711–716. https://doi.org/10.1016/j.jpowsour.2016.04.058. The use of lithiated graphite in place of Li foil as the anode material combined with improvement in the polymer electrolyte buffer layer was reported. This afforded a large decrease in resistance and allowed room temperature operation of the hybrid supercapacitor. 16. Makino S, Ban T, Sugimoto W: Electrochemical capacitor • behavior of RuO2 nanosheets in buffered solution and its application to hybrid capacitor. Electrochemistry 2013, 81:795–797. https://doi.org/10.5796/electrochemistry.81.795. First report on the use of buffered solutions to replace the conventional sulfuric acid electrolyte. Acetic acid-lithium acetate buffered was shown to outperform sulfuric acid, thereby opening the gate for safer electrolytes for aqueous supercapacitors. 17. Sivakkumar SR, Pandolfo AG: Evaluation of lithium-ion capacitors assembled with pre-lithiated graphite anode and activated carbon cathode. Electrochim Acta 2012, 65:280–287. https://doi.org/10.1016/j.electacta.2012.01.076. 18. Shimizu W, Makino S, Takahashi K, Imanishi N, Sugimoto W: Development of a 4.2 V aqueous hybrid electrochemical capacitor based on MnO2 positive and protected Li negative electrodes. J Power Sources 2013, 241:572–577. https://doi.org/10.1016/j.jpowsour.2013.05.003. 19. Makino S, Ban T, Sugimoto W: Towards implantable bio-supercapacitors: pseudocapacitance of ruthenium oxide nanoparticles and nanosheets in acids, buffered solutions, and bioelectrolytes. J Electrochem Soc 2015, 162:A5001–A5006. https://doi.org/10.1149/2.0021505jes. 20. Wang X, Hou Y, Zhu Y, Wu Y, Holze R: An aqueous rechargeable lithium battery using coated Li metal as anode. Sci Rep 2013, 3:1401. https://doi.org/10.1038/srep01401. 21. Wang X, Qu Q, Hou Y, Wang F, Wu Y: An aqueous rechargeable lithium battery of high energy density based on coated Li metal and LiCoO2 . Chem Commun 2013, 49:6179–6181. https://doi.org/10.1039/c3cc42676a. 22. Chang Z, Li C, Wang Y, Chen B, Fu L, Zhu Y, Zhang L, Wu Y, Huang W: A lithium ion battery using an aqueous electrolyte solution. Sci Rep 2016, 6:28421. https://doi.org/10.1038/srep28421. 23. Wang X, Chandrabose RS, Jian Z, Xing Z, Ji X: A 1.8 V aqueous supercapacitor with a bipolar assembly of ion-exchange membranes as the separator. J Electrochem Soc 2016, 163:A1853–A1858. https://doi.org/10.1149/2.0311609jes. 24. Fic K, Meller M, Menzel J, Frackowiak E: Around the thermodynamic limitations of supercapacitors operating in aqueous electrolytes. Electrochim Acta 2016, 206:496–503. https://doi.org/10.1016/j.electacta.2016.02.077. 25. Menzel J, Fic K, Frackowiak E: Hybrid aqueous capacitors with improved energy/power performance. Prog Nat Sci Mater Int 2015, 25:642–649. https://doi.org/10.1016/j.pnsc.2015.12.001.
www.sciencedirect.com
Review Article 4 V Aqueous hybrid supercapacitors based on dual electrolyte technologies Sho Makino 1 , Dai Mochizuki 1,2 and Wataru Sugimoto 1,2,∗ Aqueous hybrid supercapacitors have the advantage of utilizing high pseudo-capacitive electrodes, but suffer from low cell voltage due to the narrow voltage window of water decomposition. This disadvantage can be overcome by combining aqueous electrochemical capacitor technologies with a water-stable anode that is protected with a solid electrolyte. In this short review, we summarize recent advances in hybrid supercapacitors with a rated voltage of ∼4 V using an aqueous catholyte and pseudo-capacitive cathode combined with a protected Li or Li-pre-doped carbon anode. Record high specific energy of >700 Wh (kg-cathode)−1 were obtained with a pseudo-capacitive positive electrode and a protected Li-pre-doped carbon anode. The newly developed hybrid supercapacitor outperforms present Li-ion capacitor technology and even has the potential to compete with rechargeable batteries. Addresses 1 Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan 2 Center for Energy and Environmental Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan ∗
Corresponding author: Sugimoto, Wataru ([email protected])
Current Opinion in Electrochemistry 2017, 6:127–130 This review comes from a themed issue on Batteries and Super capacitors Edited by S.M. Oh For a complete overview see the Issue and the Editorial Available online 26 October 2017 https://doi.org/10.1016/j.coelec.2017.10.022 2451-9103/© 2017 Elsevier B.V. All rights reserved.
Introduction Electrochemical capacitors (also referred as supercapacitors) are classified into electrical double layer capacitors that rely on non-Faradaic charge storage or pseudocapacitors which utilize fast surface redox process. Electrical double layer capacitors are composed of two activated carbon electrodes sandwiched between organic electrolytes comprised of a quaternary ammonium salt dissolved in non-aqueous solvents. The energy density of supercapacitor is defined by CV2 /2 (C is capacitance www.sciencedirect.com
and V is the cell voltage), hence increasing C and V is the design principle for better performance. Specific capacitance can be increased by introducing surface-confined redox processes (pseudo-capacitance or redox capacitance) while the cell voltage can be extended by using ionic liquids or an asymmetric configuration (also known as hybrid supercapacitors). Pseudo-capacitive oxides based on ruthenium, manganese, vanadium, etc. can deliver specific capacitance much larger than activated carbon owing to the surface-confined redox processes. For example, RuO2 nanosheets and nanoparticles deliver specific capacitance of 600–800 F g−1 in sulfuric acid [1–6], which is a significant advantage compared to typical activated carbons (∼100 F g−1 ). Unfortunately, this high capacitance can be obtained only in aqueous electrolytes thereby limiting the voltage window to 1 V. A lithium-ion capacitor is a typical hybrid supercapacitor that uses activated carbon as the positive electrode, a Lidoped graphitic carbon (Lix C6 ) as the negative electrode, and a Li salt dissolved in an organic solvent as the electrolyte [2,7]. The low standard electrode potential of the Li intercalation/deintercalation reaction (E° = −3.045 V vs. SHE) results in a maximum cell voltage of ∼3.8 to 4 V, much larger than those for electrical double layer capacitors. Owing to this high cell voltage, the energy density of lithium-ion capacitors are typically a few times higher than electrical double layer capacitors. The energy density of hybrid supercapacitors are governed by the capacitive electrode, thus in the case of lithium-ion capacitor with an activated carbon cathode (100 F g−1 or capacity of 56 mAh g−1 ) and a 2 V window (1.8–3.8 V), an energy density of 20 Wh (kg-device)−1 can be obtained. The Lix C6 anode can be replaced with other battery-type negative electrodes, for example Li4 Ti5 O12 and TiO2 (B) [8–10]. Novel hybrid configurations need to be developed in order to realize further increase in the energy density of hybrid supercapacitors to compete or compliment lithiumion batteries.
Hybrid supercapacitors using dual electrolytes Since the energy density of hybrid supercapacitors is governed by the charge storage capability of the capacitive electrode, high capacitance electrodes that utilize pseudo-capacitance will be advantageous. In order to utilize pseudo-capacitance, aqueous electrolytes need to be used, which is not compatible with Li electrochemistry. Current Opinion in Electrochemistry 2017, 6:127–130
128
Batteries and Super capacitors
Figure 1
Schematic representation of AdHiCapTM cell (Advanced Hybrid Capacitors) using dual electrolytes.
We have devised a novel hybrid device, which we call AdHiCapTM (Advanced Hybrid Capacitor), that can take benefit from both the high capacitance of fast surface redox processes and the low standard electrode potential of Li by separating the aqueous catholyte and the nonaqueous anolyte with a water-stable Li+ conducting solid electrolyte, as shown in Figure 1 [11•• ]. A multi-layered anode (protected anode), which has been designed for use in aqueous Li–air rechargeable batteries [12,13], is used. The protected anode is a pouched cell composed of a Li foil or Lix C6 powder, polymer or gel electrolyte, and Li+ conducting solid electrolyte. For the Li+ conducting solid electrolyte, a commercially available NASICONtype glass ceramic Li1+ x + y (Ti,Ge)2−x Alx Siy P3−y O12 (x ∼ 0.25, y ∼ 0.3) (LTAP) with a thickness of 150 μm is used. Our proof-of-concept AdHiCapTM utilized a protected Li anode developed initially for Li–air rechargeable batteries, which can be expressed as (Li anode | PEOLiTFSI catholyte | LTAP separator) [11•• ]. The anolyte PEO-LiTFSI (polyethylene oxide polymer doped with Li(CF3 SO2 )2 N) acts as a buffer layer to avoid contact of Li with LTAP. For the catholyte, aqueous LiCl or Li2 SO4 was used, as LTAP is stable only near neutral pH. Combining this with positive electrodes such as activated carbon, MnO2 or RuO2 completes the hybrid supercapacitor, which can be expressed as (Li | PEO-LiTFSI | LTAP | aq. LiCl or Li2 SO4 | AC, MnO2 or RuO2 ). Shortly after our report, a similar device was reported by Chou et al., using LiNTf2 -[C3 mpyr][NTf2 ] (lithium bis(trifluoromethanesulfonyl)imide in N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide) as the anolyte [14]. Lower resistance cells were realized by using protected Lix C6 anodes and ionic liquid additives to the anolyte [15• ]. We then used acetic acid–lithium acetate buffered solutions as an alternative to Li2 SO4 , which afforded higher pseudo-capacitance [16• ]. Current Opinion in Electrochemistry 2017, 6:127–130
Figure 2
Charge/discharge curves of aqueous hybrid supercapacitor with protected Li anode and (a) activated carbon, (b) MnO2 , and RuO2 nanosheet cathode and aqueous LiCl or Li2 SO4 catholyte. Current density at 0.255 mA cm−2 and 60 °C. Shaded region represents the specific energy [11•• ].
Hybrid supercapacitor with protected Li anode (proof-of-concept)
Figure 2 shows galvanostatic charge/discharge curves of one of our initial hybrid supercapacitor with protected Li anode and various positive electrodes (Li | PEOLiTFSI | LTAP | aq. LiCl or Li2 SO4 | cathode) at j = 0.255 mA cm−2 . Three positive electrodes were studied, i.e., AC, MnO2 , and RuO2 ·nH2 O. The cell needed to be warmed to 60 °C due to the rather high resistance of the protected Li anode. In the case where AC was used as the positive electrode, linear charge/discharge curves with no plateau typical of capacitive charge storage was observed between 2.9 and 3.9 V. The discharge capacity was 108 Wh kg−1 based on the mass of the positive electrode. This is comparable with the discharge capacity of 100 Wh (kg-AC)−1 for lithium-ion capacitors [8,17], demonstrating the validity of the concept. By using MnO2 as the positive electrode for AdHiCapTM , the cell voltage can be extended even further to 4.3 V due to the high overpotential for oxygen evolution. The energy storage capability of the MnO2 positive electrode will depend on the crystal structure, particle size, and electrode thickness, but specific energy as high as 750 Wh (kgMnO2 )−1 can be achieved [18]. The cyclability of the hybrid supercapacitor in a pouched cell was also studied. As shown in Figure 3, 85% of the capacity was retained after 2000 cycles (normalized by the 50th cycle). Extrapolation of the degradation rate gives an estimated 15,000 cycles for 70% retention [18]. The use of RuO2 nanoparticles and nanosheets with high pseudo-capacitance will also lead to higher energy density. RuO2 nanomaterials can give capacitance up to 800 F g−1 in H2 SO4 . This value is unfortunately considerably reduced to 542 F g−1 (177 mAh g−1 ) in neutral www.sciencedirect.com
4-V Aqueous hybrid supercapacitors Makino, Mochizuki and Sugimoto
Figure 3
129
Figure 4
Consecutive cycling data on full pouched cell with an electrodeposited MnO2 /carbon paper positive electrode [18].
electrolytes such as Li2 SO4 , due to absence of surface redox processes. Nonetheless, it is a few times larger than AC electrodes. A cell voltage of 3.8 V with specific energy of 544 Wh kg−1 , was achieved [11•• ]. High capacitance can be attained with a weakly acidic buffered solution (pH = 5.8) of acetic acid–lithium acetate and even bioelectrolytes such as phosphate buffered saline and fetal bovine serum [16• ,19]. Remarkably, capacitance higher than that in H2 SO4 was achieved in acetic acid–Li acetate, reaching 1040 F g−1 and specific energy of 724 Wh kg−1 [16• ]. Hybrid supercapacitor with protected Lix C6 anode
The proof-of-concept hybrid supercapacitors using protected Li anode (Li | PEO-LiTFSI | LTAP) described in the preceding paragraph suffers from high impedance originating from the high resistance of the solid electrolyte, polymer electrolyte and the fact that there are many interfaces. This is the reason that hybrid supercapacitor with protected Li anode had to be operated at 60 °C. By changing from Li foil to Li-predoped graphitic carbon (Lix C6 ) particles, the interfacial resistance between the anode and the buffer layer could be decreased. The resistance could be further decreased by adding N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI) ionic liquid into the polymer electrolyte. The resistance of the improved protected Lix C6 anode (Li x C6 | PEO-LiTFSIPP13TFSI | LTAP) was decreased to 1/6 compared to the initial protected Li anode (Li | PEO-LiTFSI | LTAP), allowing room temperature operation of the cell [15• ]. With this protected Lix C6 anode with enhanced conductivity, maximum capacitance of 196 mAh g−1 and specific energy of 625 Wh kg−1 based on the positive electrode mass was achieved (Figure 4) [15• ]. www.sciencedirect.com
Charge/discharge curves at room temperature (25 °C) for an aqueous hybrid supercapacitor with protected Lix C6 anode, RuO2 nanosheet cathode, and acetic acid–lithium acetate buffered solution for the catholyte. Current density at 0.255 mA cm−2 and 60 °C (Lix C6 |PEO-LiTFSI-PP13TFSI|LTAP|2.0 M AcOH-AcOLi|RuO2 nanosheet) [15• ].
Related hybrid devices based on dual electrolyte concept and future outlook Despite the short period since the success of our proof-ofconcept study in 2012, enhancement in performance has slowly but surely progressed. Reduction in resistance of more than 10 times now allows cell operation at room temperature. We now have a better understanding of the merits as well as complications of dual electrolyte hybrid supercapacitors. The advantage of Li–air rechargeable batteries is the high theoretical in protected Li anode. In the case of hybrid supercapacitors, the use of massive Li is not necessary, and thus the use of Lix C6 is beneficial. Another important point is that unlike battery technology, power performance and cycle life are required for supercapacitors. Therefore, decreasing resistance of the protected anode is essential. Clearly more work is necessary to meet the market demands for higher power and longer life. Nonetheless, the concept of dual electrolyte energy storage is quickly catching on which should accelerate realization into practical applications. Following our work, hybrid devices based on similar concepts have been presented. The protected Li anode used in our AdHiCapTM aqueous hybrid supercapacitor was borrowed from aqueous Li–air rechargeable battery technology. The protected Li anode can be used for high voltage aqueous rechargeable batteries [20–22]. For example, ∼450 Wh (kg-electrode mass)−1 was achieved with Current Opinion in Electrochemistry 2017, 6:127–130
130
Batteries and Super capacitors
a LiMn2 O4 or LiCoO2 cathode [20,21]. Another route to increasing cell voltage with dual electrolytes with different pH has been demonstrated by combining anion and cation exchange membranes [23] or a glass fiber separator soaked in KNO3 or LiNO3 solution [24,25].
Acknowledgments This work was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: •
Paper of special interest. Paper of outstanding interest.
••
1.
Long JW, Bélanger D, Brousse T, Sugimoto W, Sassin MB, Crosnier O: Asymmetric electrochemical capacitors—stretching the limits of aqueous electrolytes. MRS Bull 2011, 36:513–522. https://doi.org/10.1557/mrs.2011.137.
2.
Brousse T, Bélanger D, Chiba K, Egashira M, Favier F, Long J, Miller JR, Morita M, Naoi K, Simon P, Sugimoto W: Materials for electrochemical capacitors. In: Breitkopf C, Swider-Lyons K. Springer Handbook of Electrochemical Energy. Springer-Verlag Berlin Heidelberg; 2017:495–561.
3.
Zheng JP, Jow TR: A new charge storage mechanism for electrochemical capacitors. J Electrochem Soc 1995, 142:L6–L8. https://doi.org/10.1149/1.2043984.
4.
Zheng JP, Cygan PJ, Jow TR: Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J Electrochem Soc 1995, 142:2699–2703. https://doi.org/10.1149/1.2050077.
5.
6.
Sugimoto W, Iwata H, Yasunaga Y, Murakami Y, Takasu Y: Preparation of ruthenic acid nanosheets and utilization of its interlayer surface for electrochemical energy storage. Angew Chem Int Ed Engl 2003, 42:4092–4096. https://doi.org/10.1002/anie.200351691. Fukuda K, Saida T, Sato J, Yonezawa M, Takasu Y, Sugimoto W: Synthesis of nanosheet crystallites of ruthenate with an alpha-NaFeO2 -related structure and its electrochemical supercapacitor property. Inorg Chem 2010, 49:4391–4393. https://doi.org/10.1021/ic100176d.
7.
Naoi K, Simon P: New materials and new configurations for advanced electrochemical capacitors. Electrochem Soc Interface 2008, 17:34–37.
8.
Amatucci GG, Badway F, Du Pasquier A, Zheng T: An asymmetric hybrid nonaqueous energy storage cell. J Electrochem Soc 2001, 148:A930. https://doi.org/10.1149/1.1383553.
9.
Naoi K, Naoi W, Aoyagi S, Miyamoto J-I, Kamino T: New generation “Nanohybrid Supercapacitor”. Acc Chem Res 2012, 46:1075–1083. https://doi.org/10.1021/ar200308h.
10. Naoi K, Ishimoto S, Miyamoto J, Naoi W: Second generation “nanohybrid supercapacitor”: evolution of capacitive energy storage devices. Energy Environ Sci 2012, 5:9363. https://doi.org/10.1039/c2ee21675b. 11. Makino S, Shinohara Y, Ban T, Shimizu W, Takahashi K, Imanishi N, •• Sugimoto W: 4V class aqueous hybrid electrochemical capacitor with battery-like capacity. RSC Adv 2012, 2:12144–12147. https://doi.org/10.1039/c2ra22265e. First paper on aqueous supercapacitor with 4 V operating voltage, made possible by utilizing a protected Li anode.
Current Opinion in Electrochemistry 2017, 6:127–130
12. Zhang T, Imanishi N, Shimonishi Y, Hirano A, Takeda Y, Yamamoto O, Sammes N: A novel high energy density rechargeable lithium/air battery. Chem Commun 2010, 46:1661–1663. https://doi.org/10.1039/b920012f. 13. Zhang T, Imanishi N, Shimonishi Y, Hirano A, Xie J, Takeda Y, Yamamoto O, Sammes N: Stability of a water-stable lithium metal anode for a lithium–air battery with acetic acid–water solutions. J Electrochem Soc 2010, 157:A214–A218. https://doi.org/10.1149/1.3271103. 14. Chou S-L, Wang Y-X, Xu J, Wang J-Z, Liu H-K, Dou S-X: A hybrid electrolyte energy storage device with high energy and long life using lithium anode and MnO2 nanoflake cathode. Electrochem Commun 2013, 31:35–38. https://doi.org/10.1016/j.elecom.2013.03.003. 15. Makino S, Yamamoto R, Sugimoto S, Sugimoto W: Room • temperature performance of 4V aqueous hybrid supercapacitor using multi-layered lithium-doped carbon negative electrode. J Power Sources 2016, 326:711–716. https://doi.org/10.1016/j.jpowsour.2016.04.058. The use of lithiated graphite in place of Li foil as the anode material combined with improvement in the polymer electrolyte buffer layer was reported. This afforded a large decrease in resistance and allowed room temperature operation of the hybrid supercapacitor. 16. Makino S, Ban T, Sugimoto W: Electrochemical capacitor • behavior of RuO2 nanosheets in buffered solution and its application to hybrid capacitor. Electrochemistry 2013, 81:795–797. https://doi.org/10.5796/electrochemistry.81.795. First report on the use of buffered solutions to replace the conventional sulfuric acid electrolyte. Acetic acid-lithium acetate buffered was shown to outperform sulfuric acid, thereby opening the gate for safer electrolytes for aqueous supercapacitors. 17. Sivakkumar SR, Pandolfo AG: Evaluation of lithium-ion capacitors assembled with pre-lithiated graphite anode and activated carbon cathode. Electrochim Acta 2012, 65:280–287. https://doi.org/10.1016/j.electacta.2012.01.076. 18. Shimizu W, Makino S, Takahashi K, Imanishi N, Sugimoto W: Development of a 4.2 V aqueous hybrid electrochemical capacitor based on MnO2 positive and protected Li negative electrodes. J Power Sources 2013, 241:572–577. https://doi.org/10.1016/j.jpowsour.2013.05.003. 19. Makino S, Ban T, Sugimoto W: Towards implantable bio-supercapacitors: pseudocapacitance of ruthenium oxide nanoparticles and nanosheets in acids, buffered solutions, and bioelectrolytes. J Electrochem Soc 2015, 162:A5001–A5006. https://doi.org/10.1149/2.0021505jes. 20. Wang X, Hou Y, Zhu Y, Wu Y, Holze R: An aqueous rechargeable lithium battery using coated Li metal as anode. Sci Rep 2013, 3:1401. https://doi.org/10.1038/srep01401. 21. Wang X, Qu Q, Hou Y, Wang F, Wu Y: An aqueous rechargeable lithium battery of high energy density based on coated Li metal and LiCoO2 . Chem Commun 2013, 49:6179–6181. https://doi.org/10.1039/c3cc42676a. 22. Chang Z, Li C, Wang Y, Chen B, Fu L, Zhu Y, Zhang L, Wu Y, Huang W: A lithium ion battery using an aqueous electrolyte solution. Sci Rep 2016, 6:28421. https://doi.org/10.1038/srep28421. 23. Wang X, Chandrabose RS, Jian Z, Xing Z, Ji X: A 1.8 V aqueous supercapacitor with a bipolar assembly of ion-exchange membranes as the separator. J Electrochem Soc 2016, 163:A1853–A1858. https://doi.org/10.1149/2.0311609jes. 24. Fic K, Meller M, Menzel J, Frackowiak E: Around the thermodynamic limitations of supercapacitors operating in aqueous electrolytes. Electrochim Acta 2016, 206:496–503. https://doi.org/10.1016/j.electacta.2016.02.077. 25. Menzel J, Fic K, Frackowiak E: Hybrid aqueous capacitors with improved energy/power performance. Prog Nat Sci Mater Int 2015, 25:642–649. https://doi.org/10.1016/j.pnsc.2015.12.001.
www.sciencedirect.com