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Redox-Active Porous Organic Polymers as Novel Electrode Materials for Green Rechargeable Sodium Ion Batteries K. Shamara Weeraratne, Ahmed A. Alzharani, and Hani M. El-Kaderi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05956 • Publication Date (Web): 10 Jun 2019 Downloaded from http://pubs.acs.org on June 10, 2019
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ACS Applied Materials & Interfaces
Redox-Active Porous Organic Polymers as Novel Electrode Materials for Green Rechargeable Sodium Ion Batteries K. Shamara Weeraratne†, Ahmed A. Alzharani†‡, and Hani M. El-Kaderi*† †Department ‡Department
of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, United States. of Chemistry, AlBaha University, AlBaha 1988-65411, Saudi Arabia.
ABSTRACT: The use of redox-active organic materials in rechargeable batteries has the potential to transform the field by enabling lightweight, flexible, green batteries while replacing lithium with sodium would mitigate the limited supplies and high cost of lithium. Herein, we report the first use of highly porous azo-linked polymers (ALPs) as a new redox-active electrode material for rechargeable sodium ion batteries. ALPs are highly cross-linked polymers and therefore eliminate the solubility issue of organic electrodes in common electrolytes, which is prominent in small organic molecules and leads to fast capacity fading. Moreover, the high surface area coupled with the π-conjugated microporous nature of ALPs facilitates electrolyte adsorption in the pores and assists in fast ionic transport and charge transfer rates. An average specific capacity of 170 mA h g-1 at 0.3 C rate was attained while maintaining 96% coulombic efficiency over 150 charge/discharge cycles.
KEYWORDS: azo-linked polymers, sodium batteries, electrochemical energy storage, redox active polymers, organic electrodes
INTRODUCTION High demand on electrochemical energy storage systems like rechargeable batteries is on the rise and there is a need to find alternatives for lithium ion batteries (LIBs) and conventional electrodes that employ transition metals.1 While LIBs technology is well-established, it suffers from limited lithium supplies and the high toxicity of metals used in the electrodes places a huge burden on the environment during metal extraction and battery disposal.2,3 Therefore, replacing lithium with sodium in rechargeable batteries (i.e. sodium ion batteries, SIBs) has garnered considerable attention in recent years because of sodium’s wider accessibility and lower price, yet similar electrochemical properties to lithium.4,5 Due to the larger ionic radius of Na+ (1.02 Å), its intercalation in conventional electrode materials remains a significant challenge that needs to be addressed through material design.6 One of the emerging approaches that addresses these concerns is the use of organic materials (i.e. small molecules and polymers) which endow batteries with desirable features such as light weight, flexibility, and sustainability.79 Among the advantages of organic materials are their abundance, relatively inexpensive cost, tailorable redox properties, and convenient synthetic routes. Several organic materials including small molecules and polymers have been studied as electrodes.7-19 However, there are several drawbacks and practical issues of using small molecules as electrode materials due to their high solubility in organic electrolytes. This greatly limits the diversity of simple molecules that can be applied for battery chemistry. Alternatively, porous organic polymers (POPs) offer a unique opportunity that integrates several important features into one material that can advance battery stability and performance.20,21 POPs can be tailored at the molecular level to have multiple redox-active sites, π-conjugated
frameworks, adjustable pore sizes, and high surface areas which makes them ideal platforms for electrochemical energy storage application. Such features enable high charge storage, fast ion transport, and physicochemical integrity. While most studies involving POPs and their crystalline counterparts; covalent organic frameworks (COFs) have been geared towards LIBs, the use of POPs and COFs in SIBs remains very scarce.7,20,22-24 Scheme 1. Synthesis of ALP-8 and its redox mechanism with sodium ions.
H2N
NH2
H2N
NH2
N N
N N
N N
N N
N N
N N
N N
N N
N N
N N
N N
N N
CuBr, Pyridine Toloune/THF
N N
N N
Na+ N- NNa+
Na+ - N N
Na+ - N N Na+
N- NNa+
Na+
+ nNa+, ne- nNa+, neN N
N N
Na+
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Herein, we report the first use of highly porous azo-linked polymers (ALPs)25-31 as a new redox active electrode material for SIBs. ALPs are highly cross-linked polymers and thus would eliminate the solubility issue, which is prominent in small organic molecules and leads to fast capacity fading. Moreover, the high surface area coupled with the π-conjugated microporous nature of the polymer facilitates electrolyte adsorption in the pores and therefore assists in fast ionic transport.32 The azo-linked polymer ALP-8 (Scheme 1) investigated in this study was specifically chosen because it has high azo-linkage density which serves as the redox-active site in the framework leading to a theoretical specific capacity of 278 mA h g-1 based on a 4electron reduction per repeating unit. According to our electrochemical analysis, the use of ALP-8 in SIBs leads to a specific capacity as high as 170 mA h g-1 at 0.3 C rate while maintaining over 96% coulombic efficiency over 150 charge/discharge cycles.
EXPERIMENTAL SECTION Materials. All chemicals were purchased in reagent grade or higher purity from commercial suppliers (Fisher Scientific, Alfa Aesar, or TCI America) and used without further purification, unless otherwise noted. The compound 1,1,2,2-tetrakis(4-aminophenyl)ethene and ALP-8 were prepared according to literature.26 Preparation of ALP-8 Composite Electrodes. ALP-8 composite electrodes were prepared using 60 wt% active material (ALP-8), 30 wt% conductive carbon (Super P), and 10 wt% sodium alginate binder in an aqueous medium. The resulting slurry was casted onto an aluminum current collector using a doctor blade and dried at 90 °C overnight in a vacuum oven. The dried material was then cut into circular discs of 15 mm diameter to afford an areal mass loading of 0.5 - 1 mg cm-2. Physical and Spectral Characterization. N2 adsorption/desorption isotherms were collected using 3Flex Surface Characterization Analyzer (Micrometrics) after degassing the samples at 120 °C for 24 hours under vacuum. For Scanning Electron Microscopy (SEM) imaging, the samples were prepared by mounting the prepared electrode on sticky carbon attached to Al sample holder. The samples were coated with platinum at a pressure of 1 x 10-5 mbar in a N2 atmosphere for 60 seconds before SEM imaging. The images were taken using a Hitachi SU-70 FESEM. Ex-situ X-ray photoelectron microscopy (XPS) was carried out using PHI VersaProbe III Scanning XPS Microprobe. Raman spectroscopy was carried out using Thermo Scientific DXR SmartRaman (532 nm) at 5 mW. The structural changes of electrodes on cycling were examined by dismantling the half cells in an Ar-filled glovebox using a decrimping device and the working electrodes were carefully removed and stored under N2 until ready for examination. Electrochemical measurements. The prepared ALP-8 electrode was used as the working electrode in the half cell. 1M NaPF6 in diethylene glycol dimethyl ether (DEGDME) was used as the electrolyte (30 μL mg-1 of active material) while polypropylene (Celgard 3501, LLC, Corp., USA) was used as the separator. Sodium metal cut into circular discs (thickness = 0.6 mm) was used as the counter and reference
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electrode in these Na cells. CR2032-type coin half-cells were assembled in an Ar-filled glovebox using an electric crimper (MTI Corp.). The assembled coin cells were tested using CHI600 C electrochemical workstation for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. Galvanostatic charge and discharge data were obtained using a battery analyzer (MTI Corp.). All measurements were carried out at room temperature.
RESULTS AND DISCUSSION The synthesis of ALPs is well-established in literature25,26 and the reported methods provide convenient routes for low-cost and efficient preparation of a library of polymers that have a great potential to advance SIBs. The nitrogennitrogen double bonds (azo-bonds) in such porous materials provide the redox-active sites that are needed for the charge storage in the framework without compromising the chemical and physical stability of the electrodes. In this study, ALP-8 was prepared according to a previously reported method via a homocoupling reaction of 1,1,2,2tetrakis(4-aminophenyl)ethene using a CuBr/pyridine catalyst.26 ALP-8 has an amorphous microporous structure (Figure S1) with a high Brunauer-Emmett-Teller (BET) surface area (SABET = 550 m2 g-1) and a dominant pore size of ~1.05 nm (Figure S2). The gradual increase in the N2 uptake at higher relative pressures and the hysteresis can be attributed to the small amount of mesoporosity in the material and framework flexibility.33-35 The ALP-8 electrodes show high stability and remain intact in various solvents used conventionally in SIBs such as diethyleneglycoldimethylether, ethylene carbonate, diethyl carbonate, 1,2-dimethoxyethane and 1,3-dioxalone (Figure S3). To assess the electrochemical activity of ALP-8 in SIBs, the fabricated electrodes were tested in CR2032 coin cells using 1M NaPF6 in diethylene glycol dimethyl ether as the electrolyte, and Na metal as both the counter and reference electrode. Cyclic voltammetry (CV) analysis was carried out at a scan rate of 0.1 mV s-1 in the potential range 0.01 – 3.0 V (Figure 1a).
Figure 1. CV profiles of ALP-8 battery at (a) 0.1 mV s-1 in the potential range 0.01 – 3 V and (b) Different scan rates in the potential range 0.9 – 2.5 V (vs Na+/Na).
The CV at very slow scan rates show a distinct redox peak below 0.1 V vs Na/Na+ which most likely corresponds to Na+ ion insertion into the porous structure of the electrode (Figure 1a).36,37 The smaller broad peak between 1.0 V to 0.6 V is due to the irreversible formation of the solid electrolyte interface (SEI) on the electrode during the first discharge which gradually decreases with the number of cycles.
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ACS Applied Materials & Interfaces Consistent with recent literature, the azo bond reversibly reacts with Na+ ions in the potential range 1.2-1.5 V vs Na/Na+.11,12 The CV shows a broad cathodic peak around 1.4 V and a broad anodic peak above 1.5 V which shifts with the number of cycles. The decrease in the anodic peak current and the shift of the peak potential indicate the presence of a partially irreversible reaction at a low scan rate. However, these shifts are not apparent at higher scan rates (Figure 1b) demonstrating that the side reactions occur only due to slow electron transfer rates.38 The anodic and cathodic peaks correspond to quasi-reversible reduction (sodiation) and oxidation (desodiation) processes at the azo center. The single but broad anodic and cathodic peaks suggest nonequivalent electroactive sites on the polymer due to defects in the conjugation. The broad CV bands may also suggest that the sodiation is a mixed process involving Na+ insertion into the polymer as well as surface pseudocapacitive Na+ storage.39 This is confirmed by the CVs obtained at different scan rates (Figure 1b) where the log current increases linearly with log scan rate (Figure S4). The cathodic and anodic logarithmic curves produced slopes of 0.82 and 0.79, respectively which are close to 1 and indicate that the Na+ ion diffusion is not ratedetermining but rather a surface-limiting redox process.40,41 To determine the effectiveness of the electrode in reversible Na+ ion storage, galvanostatic charge-discharge studies were carried out in the potential range 0.01 – 3.0 V verses Na+/Na reference electrode. In this study, discharge corresponds to sodiation and charge corresponds to desodiation. Figure 2a shows the charge-discharge profile at a current density 0.3 C (1 C = 278 mA g-1). A higher discharge capacity of 315 mA h g-1 was observed in the first cycle. This value is greater than the theoretical capacity for ALP-8 (278 mA h g-1), which was calculated for a fourelectron redox reaction (Scheme 1). Thus, the first discharge capacity can be attributed to the sum of irreversible capacity as a result of the formation of the SEI and the reversible capacity. The sloping charge/discharge curves (Figure 2a) arise due to the complex electronic properties and redox reactions in the amorphous framework as well as Na+ insertion.42-45 It is worth mentioning that the charge discharge plateaus are in good agreement with the CV profiles obtained (Figure 1a, S5). Furthermore, Na+ insertion into the pores has minimal contribution to the capacity (
