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Lithium-Organic Nanocomposite Suspension for High-Energy-Density Redox Flow Batteries Hongning Chen, Yucun Zhou, and Yi-Chun Lu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01257 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Energy Letters

Lithium-Organic Nanocomposite Suspension for High-Energy-Density Redox Flow Batteries Hongning Chen, Yucun Zhou and Yi-Chun Lu * Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong Shatin N.T., Hong Kong SAR, China. AUTHOR INFORMATION Corresponding Author * Yi-Chun Lu. E-mail: [email protected]

ABSTRACT: Organic redox-active materials are promising for redox flow batteries (RFBs) owing to their inherently low-cost, vast abundance, and high structure tunability. However, many organic RFBs suffer from low energy density owing to low solubility. We demonstrate a facile lithium-organic nanocomposite suspension (LIONS) by melting solid organic materials into the void of carbon networks in the semi-solid posolyte to achieve high-energy-density Li-RFBs. We demonstrate the first organic based semi-solid Li-RFBs using 10-methylphenothiazine (MPT), delivering a high volumetric capacity (55 Ah L-1) and energy density (190 Wh L-1) with high coulombic efficiency (>98%) and cycling stability (>100 cycles). The demonstrated volumetric capacity is 8 fold that of the liquid-MPT RFB, achieving the highest energy density among Liorganic based RFBs. LIONS is a universal approach to enable the use of low solubility organic

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materials in RFBs, providing a new direction for all insoluble organic active material to achieve low-cost and high-energy-density RFBs.

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ACS Energy Letters

Developing low-cost, high-energy-density and stable energy storage technologies is imperative to the widespread of renewable energy and growth of electrical vehicle applications110

. Redox flow battery (RFB) is a promising technology for large-scale energy storage owing to

its unique advantage of decoupling power and energy11-13. However, the application and competitiveness of RFBs are strongly limited by its low energy density due to the low solubility and narrow cell voltage of active materials14-17. In 2011, Lu and Goodenough have proposed a hybrid flow battery using lithium metal as anode and redox-active species (e.g., ferricyanide) in aqueous solution as posolytes, separated by a glass ceramic membrane18, 19. This cell configuration takes advantages of both high cell voltage of Li-ion battery and scale-up flexibility of RFBs20-22, which shows the promising performance to improve the voltage of battery. However, the energy density of Li-aqueous RFBs is limited due to the low solubility of active materials in aqueous electrolyte and potential safety issues associated with the incompatibility between the aqueous solution and the lithium metal should be addressed. Developing non-aqueous based posolytes for Li-RFBs becomes important to address the incompatibility between aqueous solution and lithium metal23-25. For example, Wei et al. have developed a Li-TEMPO non-aqueous flow battery which can achieve 126 Wh L-1 energy density with 2.0 M TEMPO posolyte26. These types of organic active species have relatively low cost and high redox potentials, which are attractive for RFBs compared with inorganic active materials in the future27-29. Unfortunately, the low solubility of many other organic active species is still a limiting factor for their applications in Li-organic non-aqueous RFBs and strongly influenced the competitiveness in many fields 30-32. Chiang and co-workers have proposed the concept of Li-ion semi-solid flow batteries (SSFBs)33. By combining insulating solid active materials with a carbon (Ketjen black (KB))


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percolating conducting network to form a flowable suspension, this method breaks the limitation of solubility and increases the energy density of RFBs. They have demonstrated a combination of 20 % LiCoO2 (10.2 M) and 10 % Li4Ti5O12 (2.3 M) suspensions to achieve the energy density more than 100 Wh L-1. Recently, our group has demonstrated a sulfur-impregnated carbon (S/C) composite flow posolyte to further increase the performance of SSFBs34, 35. Some other active materials have also been demonstrated in semi-solid flow batteries such as LiNi0.5Mn1.5O433, LiFePO436, 37 , organosulfur38 and silicon39, 40 etc. Recently, a new concept of deep eutectic solvent (DES) has been proposed to increase the energy density of RFBs, which also showed a promising direction for novel RFBs41-44. For example, by using low cost and highly concentrated Al DES and Fe DES, a Fe-Al hybrid battery delivered a high energy density of 166.2 Wh L−1.43 In this work, we propose a facile lithium-organic nanocomposite suspension (LIONS) by utilizing low-melting-point organic active species in the semi-solid posolyte to achieve highenergy-density Li-organic based RFBs. This method integrates low-cost and abundant organic active materials with high-energy-density semi-solid suspension process, providing a promising direction for all insoluble organic active materials for RFB applications. Here, we used 10methylphenothiazine (MPT) as an example to demonstrate a Li-MPT semi-solid RFBs, achieving a volumetric capacity of 55 Ah L-1 and energy density of 190 Wh L-1 with high coulombic efficiency (>98%) and long cycle life (>100 cycles). The achieved volumetric capacity is 8 times higher than the Li-MPT liquid based RFB, representing the highest energy density among all Liorganic based RFBs to date. This approach directly addresses low solubility and low conductivity of organic active materials for redox flow battery applications. Our successful demonstration of LIONS provides a new approach to apply insoluble organic active material for high-energy RFB applications.


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ACS Energy Letters

LIONS flow battery enables the use of low-cost, reversible, abundant but insoluble organic materials for high-energy-density RFBs, as shown in Figure 1a. 10-methylphenothiazine (MPT), a reported overcharge protection molecule in Li-ion battery45,

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, has a high redox potential

around 3.45 V vs Li/Li+ (Figure 1b) and a low melting point of 100 centigrade, which can be easily melted and recrystallized with conducting carbon to form effective nanocomposite. In theory, all the insoluble organic active materials with high redox reversibility can be utilized by this method. However, for organic materials that have a low-density and large-molecular-weight, the improvement in volumetric capacity by suspension method will be limited. The lower the melting point of the organic molecules, the easier to be incorporated by this method. However, to avoid melting of the organic material and decomposition of the composite during battery operation as a result of temperature fluctuation, choosing organic active materials with melting point between 60 – 120 oC can be considered. Figure 2a shows the cyclic voltammogram of dissolved MPT at various scanning rates, performing a reversible one-electron redox reaction at 3.45 V vs Li/Li+. The MPT achieves excellent reversibility between oxidation capacity and the reduction capacity (Qoxi/Qred = 1.044). The MPT posolyte shows stable cycle performance when dissolved in liquid electrolyte. Here we used 50 mM MPT in 1 M LiPF6 in EC/DEC (1:1 v/v) for the charge/discharge tests at various current densities as shown in Figure 2c. In the 1st cycle, the specific capacity of MPT reached 122 mAh g-1 at 0.1 mA cm-2, which is 97% of its theoretical capacity (125.8 mAh g-1). The rate capability of the Li-MPT cell was evaluated between 0.1 – 0.8 mA cm-2, showing stable cycling with high coulombic efficiency (99.68 %) (Figure S1). Long term cycling of the Li-MPT was evaluated at 0.1 mA cm-2 (Figure 2d) and the capacity retention achieved more than 95 % after 1000 cycles.


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Figure 1. Concept of lithium-organic nanocomposite suspension (LIONS): (a) Schematic illustration of LIONS flow battery. (b) chemical structure and redox reaction of 10-Methylphenothiazine (MPT). (c) schematic representation of MPT-KB composite preparation process.

Figure 2. MPT electrochemical stability and reversibility tests: (a) Cyclic voltammetry (CV) results of 50 mM MPT posolyte in 1 M LiPF6 EC/DEC (v/v 1:1) at various scanning rates. (b) capacity integration of


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ACS Energy Letters

50 mM MPT posolyte during oxidation and reduction process at 20 mV s-1. (c) first galvanostatic charge/discharge profiles of 50 mM MPT posolyte between 0.1 – 0.8 mA cm-2. (d) capacity retention and coulombic efficiency of 50 mM MPT posolyte at 0.1 mA cm-2. A specific capacity of 120 mAh g-1 corresponds to1.09 Ah L-1 at 50 mM of MPT.

Organic-carbon composite achieves higher electrical conductivity and cell stability compared with non-conducting organic raw materials mixed with carbon. We prepare the MPT-KB composite as active material by facile melting and recrystallization of MPT with conducting carbon Ketjenblack (KB). The low-melting-point of MPT (100 oC) enables homogenous mixing between MPT and KB (Figure 1c). The energy-dispersive X-ray spectroscopy (EDX) images of the MPT-KB composite show uniform mixture of MPT (source of sulfur) and KB carbon (Figure S2). The high-resolution SEM images (Figure S3) reveal significantly different morphologies between the MPT pristine powder (film-like) and the MPT-KB (particle-based network), which is more similar to the morphology of the KB network (Figure S3b). In addition, FT-IR shows that the chemical nature of MPT is preserved after mixing with KB in the MPT-KB composite (Figure S4). The as-prepared MPT composite is used as active material in semi-solid suspension. We evaluated suspensions made by nanocomposite method (20 vol% MPT-5 vol% KB (20MPT5KB), 40 vol% MPT-5 vol% KB (40MPT-5KB) in comparison with the same concentration prepared by mechanical mixing method with KB additives (20MPT-5KB-MM) or pure solid MPT particles (20MPT-MM). Since saturated MPT electrolyte (0.3 M MPT in 1 M LiPF6 EC/DEC) was used to prepare the suspension, the dissolution of MPT from the composite in the electrolyte can be minimized. The MPT composite posolyte achieves larger volumetric capacity and higher capacity retention compared with the posolyte prepared via mechanical mixing (Figure 3a). The achieved


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volumetric capacity of 20MPT-5KB posolyte (27 Ah L-1) was higher than that of 20MPT-5KBMM (24 Ah L-1) with a superior capacity retention (78 % vs 50 %) after 70 cycles (Figure 3c). The stable cycle performance of MPT composite suspension could be attributed to higher electrical conductivity and effective contact between MPT and KB after impregnation. As evidenced by electrochemical impedance spectroscopy (EIS) comparison in Figure S5, the 20MPT-5KB composite suspension showed lower ohmic resistance and interfacial resistance compared to the mechanical mixed 20MPT-5KB-MM after cycling. The achieved volumetric capacity of 20MPT-MM posolyte was only ~6.8 Ah L-1, which is similar to the 0.3 M liquid MPT electrolyte (7.0 Ah L-1, Figure 3a). This shows that no additional capacity from solid MPT can be accessed if no carbon additives is provided, even if excess amount of solid MPT is included. This confirms that the composites are not just the MPT reservoirs and that carbon additives and composite are critical in accessing the reversible capacity of the solid MPT. We note that the oxidized MPT (MPT+) exhibits similar solubility (0.34 M, see Experimental) as the neutral MPT (0.3 M). The essential role of nanocomposite suggests that the electron transfer do reversibly occur to the composites (major part of capacity) in addition solvated compound in solution (small part of capacity).


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ACS Energy Letters

Figure 3. Electrochemical characterization of LIONS: (a) 1st and 40th galvanostatic charge/discharge profiles of MPT electrolytes prepared with different methods: 0.3 M MPT (pure liquid, saturated), 20MPT-MM, 20MPT-5KB-MM and 20MPT-5KB at 0.2 mA cm-2 in static cell. (b) first galvanostatic charge/discharge profiles of various MPT posolytes comparison at 0.1 mA cm-2. (c) cycling retention in volumetric capacity and coulombic efficiency of 40MPT-5KB, 20MPT-5KB and 20MPT-5KB-MM posolyte at 0.2 mA cm-2. The cell was continued to cycle after the lithium was replaced at 70 cycles for 40MPT-5KB and 20MPT-5KB.

We exploited scanning electron microscope (SEM) to investigate the morphological changes of the organic composite posolyte (20MPT-5KB) and the mechanically mixed posolytes (20MPT-5KB-MM) upon cycles (Figure 4). The SEM image of 20MPT-5KB-MM pristine sample showed large aggregates of organic material accumulated on the surface of KB (Figure 4a), which impedes the utilization of active material in the suspension. In contrast, the MPT phase is homogenously integrated into the KB network in the 20MPT-5KB composite


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suspension (Figure 4c). After cycling, thick MPT aggregates covered the entire KB network in 20MPT-5KB-MM (Figure 4b) whereas uniform and porous structures were largely maintained in 20MPT-5KB (Figure 4d). Notably, some of the 20MPT-5KB composite underwent morphological changes forming small aggregates (Figure 4d), which can be phase separation between the dissolved (and re-precipitated) MPT and the KB network. This can be one of the major degradation mechanisms of the MPT-KB composite upon cycling. This SEM results confirmed that our composite method effectively improves the uniformity between insulating organic material and KB in suspension, alleviates the solid phase aggregation thereby increasing the utilization and stability of organic materials.

Figure 4. Morphology comparison of LIONS: SEM images of 20MPT-5KB-MM posolytes at (a) pristine stage and (b) after 40 cycles (0.2 mA cm-2). SEM images of 20MPT-5KB posolytes at (c) pristine stage and (d) after 40 cycles (0.2 mA cm-2).

LIONS significantly increases the energy density of Li-organic based RFBs. The solubility of MPT in 1 M LiPF6 EC/DEC electrolyte is only around 0.3 M, which only translate to a volumetric capacity of 7 Ah L-1. This small volumetric capacity dramatically decreases the


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ACS Energy Letters

competitiveness of organic active materials in RFBs. In order to fully utilize the low-cost advantage of organic active materials, it is important to increase the volumetric capacity. Some methods such as molecule modification were proposed to improve the solubility of organic active material47-49, however the improvement was still very limited. We evaluated the MPT posolyte from 20% (20MPT-5KB) to 40% (40MPT-5KB) and achieved 27 Ah L-1 and 55 Ah L-1, respectively, which is 8 times higher than the soluble MPT posolyte (Figure 3b). The volumetric capacity of 40MPT-5KB (55 Ah L-1) is about twice of that of 20MPT-5KB (27 Ah L-1), suggesting that the utilization of active materials preserves even at a much higher concentration. Considering the voltage of MPT at 3.45 V vs. Li/Li+, the achieved energy density of MPT by LIONS is more than 190 Wh L-1 which is the highest in all Li-organic based RFBs to date. Owing to the high level of effective concentration and low cost of the raw material (MPT), the MPT suspension posolyte exhibits competitive price ($43 kWh-1, including the cost of Li anode) compared to other related Li-organic flow system and Li-intercalation suspension system. We compare the cost, rate performance, and the energy density of various Li-suspension systems in the Supporting Information (Table S1). Since the MPT has a relatively low density (~1.1 g cm-3), there is still large room for further improvement in the volumetric capacity by using organic active materials with higher densities. We compare the cycling stability between 20MPT-5KB and 40MPT-5KB, as shown in Figure 3c. The capacity of the 40MPT-5KB cell decayed faster than that of the 20 MPT-5KB cell, especially after 20 cycles. We examine the contribution of capacity decay from Li anode for both 20MPT-5KB and 40MPT-5KB cells. The capacity of the 40MPT-5KB cell recovered from 25 to 43 Ah L-1 (72 %), which is much higher than the recovery observed in the 20MPT-5KB cell (21 to 25 Ah L-1, 19 %). These observations suggest that 1) the decay of the 40MPT-5KB cell can be largely attributed to Li anode; 2) the degree of


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Li decay is much more serious for 40MPT-5KB, which can be related to the fact that the cycling capacity is much higher for 40MPT-5KB (~55 Ah L-1) than 20MPT-5KB (~27 Ah L-1); 3) the capacity of both cells were not fully recovered after replacing Li, which could be ascribed to the instability of MPT upon cycling as revealed by changes of chemical nature of MPT after cycling (Figure S4). We evaluate the electrochemical performance of the MPT posolyte at both low and high concentrations of 50 mM MPT liquid posolyte and 20MPT-5KB semi-solid posolyte at the continuous flow mode. A total of 4 mL posolyte was used for galvanostatic discharge/charge tests at 0.1 mA cm-2, as shown in Figure S6. The limiting factor of the current density of the system can be attributed to the solid-state electrolyte membrane LAGP including its ionic resistance and its interfacial resistance between anode and cathode. In addition, the charge transfer and interfacial resistance of MPT-KB suspension are considerably higher than soluble organic electrolyte. Therefore, using three-dimensional porous current collectors (e.g. porous metal foam) that effectively integrate the solid composite to the electrical network is an important research direction to further improve the rate performance the system. The overpotentials of both posolytes were reduced at the increased flow rate from 3.4 mL min-1 to 13.6 mL min-1. The overpotential of the 50 mM MPT liquid posolyte decreased by 30 mV when the flow rate increased from 3.4 mL min-1 to 13.6 mL min-1. Interestingly, the overpotential decreased by 120 mV for the 20MPT-5KB semi-solid posolyte at the same changes in flow rate (Figure S6). The different influences of flow rate on overpotential could be attributed to different flow field distribution in the cell channel between the pure liquid and semi-solid fluids. We applied a simplified flow modeling method to investigate the flow field distribution of different


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ACS Energy Letters

rheological property fluids (Newtonian and non-Newtonian posolytes). The control equations can be expressed by incompressible Navier-Stokes equation and non-Newtonian power law as follows 39. Incompressible Navier-Stokes equation: ρቆ

߲‫ݑ‬ ሬԦ +‫ݑ‬ ሬԦ ∙ ∇‫ݑ‬ ሬԦ ቇ = −∇‫ ݌‬+ ∇ ∙ ൫ߤሺ∇‫ݑ‬ ሬԦ + ሺ∇‫ݑ‬ ሬԦ ሻ் ሻ൯ ߲‫ݐ‬

Non-Newtonian Power Law: ߤ = ܰሺߛሶ ሻ௡ିଵ where µ is the non-Newtonian fluid viscosity, N and n is the rheological constant which can be obtained from the viscosity measurement experiment, ρ is the negolyte density which can be calculated from the volume ratio and density of the posolyte components. We examined the flow field distribution inside the cell channel for different rheological properties at an inlet flow rate of 0.0142 m s-1 (3.4 mL/min) as show in Figure S7. The flow field distribution inside the channel is much more uniform for non-Newtonian fluid compared with Newtonian fluid. We also investigated flow rate change at midplane of the cell channel under different inlet flow rate (Figure S8). For Newtonian fluid, as the flow rate increases, the uniformity of flow rate is reduced and the thickness of velocity boundary layer is increased. On the contrary, for non-Newtonian fluid, the uniformity of flow rate inside the channel is maintained at an increasing flow rate. These differences in the flow rate uniformity inside the channel could contribute to the greater reduction of overpotential for semi-solid MPT posolyte compared to liquid MPT at an increasing flow rate.35, 50.


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Figure 5. Comparison of achieved volumetric capacity versus cell potential of reported Li-organic based RFBs including the work of Lu et al.19, Wei et al.26, 51, Ding et al.27, 52, Cong et al.29, Takechi et al.30, Kim et al.53

In summary, we demonstrated a facile method to enable the use of low-cost and insoluble organic materials for high-energy RFB applications. We impregnated low-melting-point organic material, 10-methylphenothiazine (MPT), into conducting carbon networks forming lithiumorganic nanocomposite suspension (LIONS) as the electrolyte in Li-RFBs. This approach successfully improved the volumetric capacity of MPT from 7 Ah L-1 (soluble MPT posolyte) to 55 Ah L-1 (semi-solid MPT posolyte), achieving the highest energy density (190 Wh L-1) among all Li-organic RFBs reported (Figure 5)19, 26, 27, 29, 30, 51-53 with a stable cycle life (> 100 cycles). We performed LIONS flow battery at the continuous flow mode and examined the uniformity of the flow rate inside the channel. This approach provides a promising direction to increase the energy density of organic based RFBs towards a low-cost organic flow battery system. .


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ACS Energy Letters

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Including: Detailed experimental methods, rate capability tests of 50 mM MPT posolyte, EDX mapping of the MPT-KB composite, SEM images of pristine MPT powder, KB and 20MPT5KB composite, FTIR spectra of MPT powder, MPT-KB composite and MPT-KB composite after cycle, EIS comparison between 20MPT-5KB-MM and 20MPT-5KB posolytes, continuous flow mode tests of LIONS flow battery, flow field distribution of Newtonian fluid and nonNewtonian fluid, inlet flow rate influence on the flow field distribution, schematic illustration and photographs of static cell and flow cell. AUTHOR INFORMATION Corresponding Author * Yi-Chun Lu. E-mail: [email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT H.C. and Y.C.L. conceived the project. H.C. designed the experiments. H.C. performed experiments and analyzed the data. Y.Z. conducted the LAGP ceramic membrane synthesis. Y.C.L. and H.C. wrote the manuscript. All authors edited the manuscript. The work described herein was fully supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).


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ACS Energy Letters

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ACS Energy Letters

(53) Kim, H.-S.; Yoon, T.; Kim, Y.; Hwang, S.; Ryu, J. H.; Oh, S. M. Increase of Both Solubility and Working Voltage by Acetyl Substitution on Ferrocene for Non-Aqueous Flow Battery. Electrochem. Commun. 2016, 69, 72-75.


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