Lithium-Ion Capacitor Based on Electrodes ...

ECS Transactions, 72 (8) 45-53 (2016) 10.1149/07208.0045ecst ©The Electrochemical Society

Lithium-Ion Capacitor Based on Electrodes Constructed Via Electrostatic Spray Deposition R. Agrawala, C. Chena, and C. Wanga a

Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, USA Conventional Electrochemical double-layer capacitors (EDLCs) are well suited as power devices that can provide large bursts of energy in short time periods. However, their relatively poor energy densities hinder their application in devices that require a simultaneous supply of both high energy and high power. In the wake of addressing this shortcoming of EDLCs, the concept of hybridization of lithium-ion batteries (LIBs) and EDLCs has gained significant scientific interest in past few years. Such a device, generally referred to as the “lithium-ion capacitor” typically utilizes a lithium intercalating electrode along with a fast charging capacitor electrode in a lithium-containing electrolyte. Herein we have constructed a lithium ion capacitor comprising a Li4Ti5O12 –TiO2 (LTO-TiO2) anode and a graphene and carbon nanotube (G-CNT) composite cathode using electrostatic spray deposition (ESD). The morphology and material properties were studied using scanning and transmission electron microscopy and X-ray diffraction studies, respectively. Electrochemical characterization was thereafter carried out for both the half cells as well as full cells. Introduction

With the ever developing technology and automotive industry including electric vehicles (EVs) and hybrid electric vehicles (HEVs), the need for high performing electrochemical energy storage devices is at an all-time high. At the heart of electrochemical energy storage devices are lithium-ion batteries (LIBs) and electrochemical capacitors (ECs); the former have very high gravimetric energy densities reaching up to 250 Whkg-1 with generally lower power densities (10 Whkg-1) with very high power densities (~10kWkg-1) along with phenomenal cycle lives (105-106). Essentially ECs bridge the gap between rechargeable batteries and electrolytic capacitors with gravimetric energy densities typically one or two orders of magnitude larger than electrolytic capacitors given the high surface area carbons used in ECs. However, for a simultaneous supply of high energy and high power, neither of the aforementioned devices suffices and there is an urgent need to fabricate electrochemical devices which can provide both high energy and high power while still maintaining good cycle longevity. In the wake of creating such high energy supercapacitors, the concept of LIB and EC hybridization has garnered much attention in recent years. As noted by Cericola [1], battery and capacitor hybridization can be done in both external and internal configurations - the external hybridization involves hardwiring the two devices in either serial or parallel arrangement depending upon the energy and power requirements of the application, whereas the internal hybridization essentially is

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ECS Transactions, 72 (8) 45-53 (2016)

hybridization at the electrode level and can be also carried out in both serial and parallel arrangements. The internal serial arrangement typically involves a pristine battery electrode with a counter capacitor electrode as opposed to the internal parallel arrangement, which comprises bi-material electrodes, i.e. both battery and capacitive materials coexist within one electrode. One of the major challenges in designing such devices is optimizing the electrochemical performance of the hybridized device since the energy density is limited by the capacitive component whereas the power density and cycle life is dominated by the battery component. The sluggish lithium insertion and extraction to and from the crystal lattice of the battery material as opposed to the fast ion adsorption/de-adsorption at the electrode/electrolyte interface of the capacitive electrode makes hybridization of the two electrochemical devices a challenging phenomenon. The concept of hybridization of LIBs and EDLCs was pioneered by Amatucci et al in 2001 [2], in which they utilized a Li4Ti5O12 (LTO) as the lithium-intercalating electrode coupled with an activated carbon as the capacitive counter component in a LiBF4 electrolyte and the device exhibited an energy density of 25Whkg-1. In 2003, Pasquier et al [3] reported a 500F capacitor with nanostructured LTO and activated carbon and the device exhibited an energy density of 11Whkg-1 at a power density of 0.8 kWkg-1. Since then many systems have been investigated for lithium hybrid capacitors including LTO coupled with activated carbon [4], LTO-graphene coupled with activated carbon [5], LTO-carbon nanofibers coupled with activated carbon [6], graphite coupled with activated carbon [7], prelithiated graphite coupled with activated carbon [8], LTO coupled with trigol reduced graphene oxide [9], graphite and LTO coupled with activated graphene [10]. Herein, the internal serial hybrid arrangement has been investigated utilizing a Li4Ti5O12–TiO2 (LTO-TiO2) anode and a graphene and carbon nanotube (G-CNT) cathode created via electrostatic spray deposition (ESD). A schematic of the ESD technique is shown in Fig. 1 along with a picture of the ESD setup at Wang Lab. ESD is a versatile technique capable of producing a host of morphologies with varying the deposition parameters which include flow rate, distance between the needle and substrate, deposition temperature, deposition time, and the potential applied. Detailed discussion of ESD for battery applications is a subject of other works [11] but briefly, a precursor solution breaks into an aerosol spray with the application of a high potential, which then lands on a preheated substrate, where solvent evaporation and chemical reactions take place leaving behind a thin film. Thin films of different chemical compositions have been synthesized using ESD and electrospinning for energy storage purposes [12-15]. For the anode of the hybrid capacitor, a mixed phase LTO-TiO2 electrode was used. LTO has been widely investigated as an anode material for LIBs [16, 17]. LTO is a stable and safe redox material capable of enhancing the overall energy density of the system without giving up the interfacial characteristics. One of the biggest advantages of LTO is the negligible volume strain for lithium intercalation/deintercalation at 1.55 V vs Li/Li+ into and from the LTO lattice between the spinel and rock-salt phases. For a cutoff voltage of 1V vs. lithium, LTO exhibits a theoretical capacity of 175 mAhg-1 [17] and approximately 260 mAhg-1 for a cutoff voltage of 0V vs. lithium [18], which although is higher than typical double layer materials but is however quite low as compared to other battery materials. Moreover, the other shortcomings of LTO include sluggish lithium diffusion and low electronic conductivity and therefore a mixed LTO-TiO2 phase was chosen in order to utilize the advantage of higher theoretical capacity of TiO2. TiO2 polymorphs exhibit a theoretical capacity of 335 mAhg-1 [19] and quite akin to LTO,

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anatase TiO2 has been shown to intercalate and deintercalate lithium ions with very little volumetric strain [19]. Additionally, LTO-TiO2 composites have shown enhancement in lithium diffusion, improved charge transfer kinetics along with improvement in electrical conductivity as well [20]. For the cathode component of the hybrid capacitor, a reduced graphene oxide-carbon nanotube composite (rGO-CNT) electrode was used. Graphene and CNT are two nano-allotropes of carbon that have achieved a stellar status in the scientific community owing to their extraordinary mechanical, electrochemical, thermal and electrical properties. The very high specific surface area of graphene (~2600 m2/g), along with excellent electrical conductivity in addition to the mechanical strength makes it an ideal candidate for EC applications [21]. For the hybrid capacitor construction, the rGO-CNT electrodes were fabricated using the ESD technique. During the ESD process, graphene oxide which serves as the raw material for the cathode precursor reduces into rGO with some remnant oxygen-containing functional groups [22]. CNT on the other hand, not only adds to the overall electrical conductivity of the composite but also prevents the restacking of graphene sheets allowing for a larger accessible area for the electrolyte ions. Furthermore, CNT nanofillers have shown to enhance the mechanical properties of composites as well [23]. From the previous studies conducted in our group, a gravimetric composition of 90% graphene oxide and 10% CNT showed the best electrochemical performance and therefore was chosen as the cathode for the hybrid capacitor assembly [22].

Figure 1. A schematic of ESD (left), a picture of the ESD setup at the Wang Lab (right) Experimental Procedure Anode preparation: The LTO-TiO2 anodes were prepared using lithium acetate (Sigma Aldrich) and titanium butoxide (Sigma Aldrich) in a molar ratio of 4:5, respectively in ethyl alcohol and butyl carbitol (4:1 v:v). The solution was directly used as the precursor solution and deposited on a preheated nickel foam substrate using ESD. The substrate was heated at 250oC and the precursor solution flow rate was kept at 1-2 mlhr-1. The potential was kept between 5-7 kV and the distance between the needle and the substrate was approximately 3 cm. The deposition was carried out for 2 h. Post ESD deposition, the substrates with ESD films were calcined in an argon atmosphere at a temperature of 750oC for 2 h, and after which they were allowed to cool down to room temperature. After the thermal treatment the electrodes were directly used for

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electrochemical characterization without any further treatment or addition of binders or conducting additives. Cathode preparation: For rGO-CNT cathodes, single layer graphene oxide (GO) (Cheaptubes) and carboxyl-functionalized multi-walled carbon nanotubes (MWCNTCOOH) (Cheaptubes) were used as the raw material. Graphene oxide and CNTs were first dissolved in 1,2-propanediol (Sigma Aldrich) (1 mgml-1) and then sonicated for thorough dispersion for an hour after which the suspension was directly used for ESD on stainless steel substrates preheated at 250oC. The flow rate was 4-6 mlhr-1 and the potential was kept between 5-7 kV for approximately 4 hours. The electrodes were thereafter directly used for material and electrochemical characterization without adding any binders or conductive additives or any further thermal treatment. Electrochemical characterization: In order to characterize the electrode performance against lithium, the electrodes were assembled in half-cells with a lithium foil serving both as a counter and reference electrode. The electrolyte was 1M LiPF6 in EC:DEC (v:v) along with a polypropylene Celgard separator separating the electrodes. The cell assembly was carried out in an argon filled glove box with oxygen level < 1ppm. In order to study the full hybrid cells, the LTO-TiO2 anodes and rGO//CNT cathodes were used after both of the electrodes had stabilized. The same electrolyte was used along with the same separator as used for half-cell systems. CR 2032 type coin cells were used for both half and full cell assembly. Results and Discussion Material Characterization: The XRD patterns of the as-deposited and the powders calcinated at 750oC are shown in Fig. 2a. As evident, the untreated powder exhibits no discernable peaks whereas the diffraction data from the calcined powder validates presence of both LTO and anatase TiO2. The peaks at 18.3o, 35.5o, 43.2o, 57.2o and 62.8o correspond to (111), (311), (400), (333) and (440) planes from LTO (JCPDS card number 00-049-0207) whereas the peaks at 37.8o and 53.8o correspond to (004) and (105) planes from anatase TiO2 (JCPDS card number 00-021-1272). Figure 2b displays the TEM micrograph of the LTO-TiO2 powders calcined at 750oC. The Bragg planes from both LTO and anatase TiO2 crystals can be seen and the SAED patterns (the inset) confirmed the presence of both TiO2 and LTO. TiO2 with a d-spacing of 3.50 Å, 2.23 Å, 1.86 Å, 1.54 Å, and 1.19 Å had an orientation (hkl) of (120), (022), (132), (123), and (124), while LTO had d-spacings of 1.25 Å, 1.19 Å, and 1.09 Å and orientation (622), (444), and (731). Fig 2c and 2d show the typical morphology of the films before and after heat treatment, respectively. Prior to thermal treatment, the film particles are mostly flaky whereas upon thermal treatment the particles became more spherical with no appreciable changes in particle size. The SEM micrographs of the starting materials for rGO-CNT electrodes have been shown in Fig. 3a-b. The commercially procured single layer graphene oxide had a flaky/sheet like morphology as opposed to the tubular CNT used for the cathode deposition. The top view of the rGO-CNT electrodes has been shown in Fig. 3c. As evident the electrodes were porous and the cross section of the electrodes is seen is Fig 3d; the average thickness of the electrodes was ~8 µm. cross sectional view is seen in Fig 3f – CNTs are uniformly dispersed through the graphene sheets.

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Electrochemical Characterization: Fig. 4 displays the electrochemical characterization of the LTO-TiO2 anodes, rGO-CNT cathodes as well as the full hybrid cells. The CV curves of the LTO-TiO2 electrodes are shown in Fig. 4a. In this first cycle, in the cathodic branch of the scan two peaks are observed at 1.62V and 0.7V, whereas in the anodic branch one major peak at 2.21V is observed. The peak at 0.7V disappears in the subsequent cycles and can be attributed to the SEI formation. The redox peak pair at 1.62V and 2.21V is attributed to the lithium insertion and extraction in anatase TiO2. From the CV curves no discernable activity from LTO is noted which is an indication of TiO2 being the predominantly active electrochemical phase in the composite. Phase purity of the composite is a subject of future works. Fig. 4b shows the charge discharge curves of the first and second cycles of the LTO-TiO2 composite at a current density of 100 mAg-1. Two humps around 1.62V and 2.21V are observed which are consistent with the CV redox pairs. It should be noted that the first discharge capacity is ~267 mAhg-1 whereas the first charge capacity is ~127 mAhg-1 which results in a coulombic efficiency of ~47% resulting in an irreversible capacity loss of 140 mAhg-1, possibly a result of SEI foIn the second cycle however, the discharge capacity is ~124 mAhg-1 and the charge capacity is 110.5 mAhg-1 which results in an enhanced coulombic efficiency of ~89%. With subsequent cycles, high coulombic efficiency was maintained.

Figure 2. a) The X-ray diffraction patterns of the untreated and powders calcined at 750 o C; b) TEM micrograph of the calcined LTO-TiO2 powders showing different Bragg planes, the SAD pattern is shown in the inset; c) typical morphology of the LTO-TiO2 films before thermal treatment and d) SEM micrograph of the anodes after thermal treatment.

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Fig. 4c displays the CV curves of the rGO-CNT cathodes between a potential of 2-4.3V vs. lithium. As evident the curves deviate slightly from the ideal capacitive shape which can be ascribed to the presence of oxygen containing functional groups in the rGO and as well as the carboxyl functionalized CNT. The oxygen containing functional groups are expected to enhance the overall capacitance of the composite. Fig. 4d shows the charge-discharge curves of the rGO-CNT electrodes at different current densities. The curves are of “triangular” sloping-desloping nature indicating the predominance of surface based charge storage mechanism, which is expected for EDLC components. The electrodes display a capacity of ~63 mAhg-1 at a current density of 100 mAg-1.

Figure 3. The SEM micrographs of a) single layer graphene oxide, b) multiwalled carboxyl functionalized carbon nanotubes, c) top view of the rGO-CNT electrodes and d) cross-sectional view of the rGO-CNT electrodes The charge-discharge characteristics of the LTO-TiO2//rGO-CNT full cells have been displayed in Fig 4e. The curves deviate from the classic triangular charge-discharge shape and display rather asymmetric characteristics, which can be attributed to the composite nature of the cell. The two very disparate charge storage mechanisms, i.e. faradaic and nanofaradaic of the anode and cathode result in such asymmetric shape of the curves. The Ragone plot of the full cells is displayed in Fig. 4f. A maximum energy density of ~ 7Whkg-1 was achieved along with a maximum energy density of ~325 Wkg-1 was achieved. All the densities were normalized with the total mass of the anode and cathode active masses. Although not very high, the energy density of the system can be

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significantly improved if the system is balanced for optimal energy-power trade-off. Furthermore, as seen from the cathode performance against lithium, the iR drop increased significantly with increasing the current density, indicating relatively higher resistance, which can be monitored in order to enhance the overall performance of the full cell. Furthermore metal oxides typically have low electronic conductivity and much enhancement can be expected with the addition of nanocarbon fillers such as graphene or CNT to the anodes.

Figure 4. a) CV curves of the LTO-TiO2 anodes at a scan rate of 0.2 mVs-1, b) first and second cycle charge-discharge curves of the LTO-TiO2 anodes at a current density of 100 mAg-1, c) typical CV curves of the rGO-CNT electrodes at different scan rates; d) typical charge-discharge curves of the rGO-CNT electrodes at different current densities; e) typical charge-discharge curves of the LTO-TiO2//rGO-CNT full cells at different current densities and f) corresponding Ragone chart for the LTO-TiO2//rGO-CNT full cells

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Conclusions and Outlook A novel lithium ion capacitor utilizing LTO-TiO2 anode and rGO-CNT cathode constructed via electrostatic spray deposition was synthesized and characterized. The hybrid capacitor was able to deliver an energy density of 7 Whkg-1 and a maximum power density of 325 Wkg-1. Although the cell performance was quite modest, the energy-power performance of the full cell can be significantly enhanced by addressing issues like the relatively high resistance of the cathodic component of the cell, better halfcell performance, and balancing the electrodes for optimal cell performance. The feasibility of using ESD for construction of a hybrid capacitor utilizing ESD was thus demonstrated in this report. Future works include realizing an optimized balanced battery-capacitor system with high energy and high power along with high cycle longevity along with miniaturizing the system. Acknowledgments This work was partially supported by the National Science Foundation (NSF) projects (no. 1506640 and no. 1509735) and NERC ASSIST center seed funding. R.A. and C.C. acknowledge support through Doctoral Evidence Acquisition (DEA) fellowship and Dissertation Year Fellowship (DYF), respectively from Florida International University. The authors thank Dr. Yusuf Emirov and Samantha Dages for their help in TEM data procurement and analyses as well as the staff at AMERI at FIU.

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