Nickel-Salen Type Polymer as Conducting Agent and Binder for

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Nickel-Salen Type Polymer as Conducting Agent and Binder for Carbon-Free Cathodes in Lithium-Ion Batteries Cody O’Meara, Iuliia A. Polozhentceva, Mikhail P. Karushev, Sajith Dharmasena, Hanna Cho, Benjamin J. Yurkovich, Sam Kogan, and Jung-Hyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13742 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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ACS Applied Materials & Interfaces

Nickel-Salen Type Polymer as Conducting Agent and Binder for Carbon-Free Cathodes in Lithium-Ion Batteries

Cody O’Meara,a Iuliia A. Polozhentceva,b Mikhail P. Karushev,b Sajith Dharmasena,a Hanna Cho,a Benjamin J. Yurkovich,a,b Sam Kogan,b Jung-Hyun Kima,* aCenter

for Automotive Research, Department of Mechanical and Aerospace Engineering, The

Ohio State University, Columbus, Ohio 43210, USA bPowermer

Inc., Westerville, Ohio 43082, USA

*Corresponding author. E-mail: [email protected] 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Systematic physical and electrochemical characterizations revealed unique positive multifunction of a polymeric salen-type nickel(II) complex, poly[Ni(CH3-Salen)], as an additive for conventional cathodes in lithium-ion (Li-ion) batteries. Due to its promising electrochemical and mechanical properties, combined with its unique three-dimensional (3D) web-like electron-network structure, the redox-active-organometallic polymer can eliminate conductive carbon and replace a significant portion of the polyvinylidene fluoride (PVdF) binder that has been used in conventional LiFePO4 cathodes. By replacing such electrochemically inactive components (i.e., carbon and PVdF), LiFePO4 cathodes with poly[Ni(CH3-Salen)] deliver improved energy density compared with the conventional LiFePO4 cathode. Facile electron transfer via large-area contact at polymer/LiFePO4 interfaces significantly accelerates charge-transfer reactions and consequently improves the ratecapability of the cathodes. In addition, unlike PVdF, poly[Ni(CH3-Salen)] retains steady Young’s modulus values after immersing in an electrolyte solvent, which enhances the mechanical integrity of the cathodes during the cycling of battery cells and thereby improves their cycle life. The unique multifunction of the poly[Ni(CH3-Salen)] will be of broad interest for its application in nextgeneration energy storage devices.

KEYWORDS: cathode, LiFePO4, conductive polymer, carbon-free, binder, nickel-salen

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INTRODUCTION The use of Lithium-ion (Li-ion) batteries has increased significantly since Sony first commercialized them in the early 1990s. With the expansion of laptop and handheld device usage, Li-ion batteries have become a staple in the mobile technology revolution. Growing environmental concerns has led to newer applications for Li-ion batteries, such as powering electric vehicles and grid-scale energy storage. These newer applications require higher performance batteries that are not only safer, low cost, and environmentally friendly, but also have improved energy density, power density, and lifetime.1–3 Numerous groups have been studying new electrode and electrolyte materials to help meet these goals,4 but in order to fully achieve them, other battery components need to be studied as well.5 Conventional Li-ion battery electrodes consist of active material, conductive additives (carbon black), and binders, which are coated on a current collector (typically Al or Cu foil). One way to improve the battery performance is to replace any electrochemically inactive materials (those that do not contribute to energy storage) with materials that are electrochemically active. Examples of electrochemically inactive materials in conventional electrodes include conductive additives and binders. Not only do these electrochemically inactive components reduce the volumetric and gravimetric energy density of Li-ion cells by occupying volume and mass without contributing to energy storage, but side reactions can be involved at the interface between the components and electrolyte, which can reduce the lifetime of the battery.6–8 One group of electrochemically active materials that have shown promise for battery applications are electronically conducting polymers. These conducting polymers take up or give off ions during electrochemical redox reactions in order to maintain electroneutrality.9 Attractive



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features of electronically conducting polymers include their electroactive properties and their ability to allow the electrolyte to penetrate the polymer mass10–13, as well as their mechanical properties such as flexibility, elasticity, and ease of treatment.14 An interesting type of an electronically conducting polymer can be produced by oxidative electrochemical polymerization of transition metal complexes with salen-type N2O2 Schiff base ligands (Salen is N,N'-bis(salicylidene)ethylenediamine). Such polymeric salen-type metal complexes, often denoted as poly[M(Schiff)] (M is a transition metal, Schiff is a salen-type N2O2 Schiff base), are usually obtained in the form of thin films on the electrode surface. Among all poly[M(Schiff)] films, the ones formed from [Ni(Schiff)] monomers have been most intensively investigated.15–26 Most poly[Ni(Schiff)] films feature a three-dimensional structure wherein polymer chains are additionally organized in stacks. In the polymer chains, monomeric units are joined together by carbon-carbon single bonds between adjacent aromatic rings, as schematically shown in Figure 1(a).16,24,25 The stacks can be stabilized through donor-acceptor interactions between the metal ion of one fragment and the ligand of another (Figure 1(b))18 or through π-π interactions between the phenyl rings of adjacent monomeric units (Figure 1(c))16. The oxidation/reduction of poly[Ni(Schiff)] films is a ligand-based process, wherein the charge transfer occurs along the polymer chains and between stacked chains through phenyl rings of the ligands, with the metal ion acting as an innocent bridge between phenolate moieties.16 Polymeric nickel complexes with salen-type ligands have been proven to have similar performance to other conducting polymers in terms of conductivity, while often surpassing them in terms of stability, capacity, and charge transfer parameters15–19. These polymers have already found an application in energy storage devices with their use in supercapacitors.15,17,20–23 and have also been suggested for use as cathodes for rechargeable Li batteries.24 For example, poly[Ni(CH3-Salen)] showed 4 ACS Paragon Plus Environment

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promising properties as cathodes in Li-ion batteries due to stability and electroactive properties in the potential range of 2.1 – 4.2 Vvs.Li.24 However, its lower specific capacity (~ 50 mAh g-1) reported in literature24 would not make it very attractive for next generation cathodes in Li-ion batteries. Alternatively, we presented here promising electrochemical and mechanical properties of poly[Ni(CH3-Salen)], which replaces the carbon conductor and portion of PVdF binder and offers positive multifunction in LiFePO4 cathodes. EXPERIMENTAL SECTION Ni(CH3-Salen) monomeric complexes were synthesized following the procedure described elsewhere.18 The monomeric complexes were recrystallized for future use. LiFePO4 (MTI), PVdF (HSV 900), and Super P carbon black (MTI) were used as received. LiFePO4 cathodes were prepared with Super P (denoted hereafter as “Carbon-LFP”) and Ni(CH3-Salen) (denoted hereafter as “Ni(CH3-Salen)-LFP”). For Carbon-LFP cathodes, LiFePO4, Super P, and PVdF were mixed according to the electrode formulation of 90:5:5 wt ratio. For Ni(CH3-Salen)-LFP cathodes, LiFePO4, Ni(CH3-Salen), and PVdF were mixed according to the electrode formulation of 93:5.5:1.5 wt ratio. N-Methyl-2-pyrrolidone (NMP) solvent were added to give the mixtures the appropriate viscosity, and slurries were mixed at 240 rpm for 24 hours. The obtained slurries were cast onto carbon-coated aluminum (Al) foil (MTI) using the doctor blade method. The resulting electrodes were dried for 1 hour at 95°C in air. The resulting foil was cut into disks of 13.7mm diameter and pressed at 310 kg/cm2. The cathodes were dried at 90°C overnight in vacuum to remove moisture absorbed during handling. Li metal was used as anodes for half cells. Composite electrodes were assembled with polypropylene separators (Celgard 2500) using 1 M LiPF6 EC/DEC (1:1 vol ratio, Aldrich) electrolyte solution in 2032 stainless steel cells



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(MTI). All the cell fabrications were carried out in a highly pure argon atmosphere in glove boxes. The assembled cells were charged to 4.3 V at a constant current (CC) of C/10 and maintained 4.3V. After that, the cell was discharged to 3.0 V at C/10 rate. The loading amounts of LiFePO4 active material in either Ni(CH3-Salen)-LFP or Carbon-LFP cathodes are in a range of 4.4 – 5.25 mg/cm2. These values can be translated into areal capacities of 0.67 – 0.73 mAh/cm2 for the halfcells. The SEM characterization of materials was carried out with a scanning electron microscope (SEM) (FEI Apreo LoVac Analytical Microscope) operating at 20 kV. Polymerized samples (denoted hereafter as “poly[Ni(CH3-Salen)]-LFP”) were prepared by cycling the Ni(CH3-Salen)LFP half cells three times, before being disassembled. The cathodes were rinsed in a pure DMC solution, and dried in a vacuum oven to remove any remaining moisture. All SEM samples were sputtered with a 12 nm-thick layer of carbon using a sputter coater (Leica ACE600). Fourier transform infrared spectra (FT-IR, Renishaw/Smith Detection) were collected by attenuated total reflection for monomer and polymerized electrode samples under an Ar-purge. The atomic force microscopy (AFM, MFP 3D, Asylum Research) measured Young’s modulus of poly[Ni(CH3-Salen)]-LFP. The Fluid Lite Cell (Asylum Research) were used to provide a closed, liquid environment for the sample. The AC160TS-R3 cantilever (Asylum Research) was used to gather scan and force data. This cantilever's tip is made of silicon, with a nominal radius of 7 nm and a measured stiffness of ~26 N/m stiffness, and ~300 Hz frequency. For inverse optical lever sensitivity (InvOLS) calibration, force-distance curves were obtained separately on a hard-sapphire surface in both dry and liquid environments for which zero indentation was assumed. Once the calibration had been performed, force maps were obtained on the poly[Ni(CH3-Salen)] surface to determine its Young’s modulus. Force map measurements 6 ACS Paragon Plus Environment

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were restricted to low force ranges to prevent damage to the sample and reduce any substrate induced effects. The Young’s modulus was determined by converting the force-distance curves into force-indentation curves and fitting the data into the Johnson-Kandall-Roberts (JKR) model.27 Battery testing was carried out using multichannel battery testers (Arbin) in voltage ranges of 3 – 4 V for LiFePO4/Li half cells. Electrochemical impedance spectroscopy (EIS) of the twoelectrode coin cells was measured using the Gamry Interface 2050E Potentiostat/Galvanostat in galvanostatic mode with an AC current of 300 μA in the frequency range of 100 kHz to 0.1 Hz. The resulting spectra were fitted using Echem Analyst software (Gamry). RESULTS AND DISCUSSION Microstructure It has been reported that poly[Ni(CH3-Salen)] films have a porous structure in thin films,24 but the polymer structure in the presence of other materials such as conventional cathodes (e.g., LiFePO4) in Li-ion batteries has been unknown. Therefore, the microstructure of Ni(CH3-Salen)LFP cathode was observed by using SEM before and after the electrochemical polymerization process. Figure 2(a) shows SEM images of as-prepared Ni(CH3-Salen)-LFP cathode containing Ni(CH3-Salen) monomer (i.e., before the polymerization) that has an even distribution of LiFePO4 particles. In contrast, conventional Carbon-LFP cathode had some domains that consists of agglomeration of nanoscale carbon conductive additives, as shown in Figure 2(b). The carbon conductor plays a major role in electron conduction of cathodes, particularly for LiFePO4 that suffers from its intrinsically poor conductivity. However, it has been known that even distribution of the carbon conductor with nano-sized LiFePO4 particles is difficult to achieve, and bulky carbon particles can compromise volumetric energy density of the cells.28–30 7 ACS Paragon Plus Environment


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Figures 3 and 4 show SEM images of poly[Ni(CH3-Salen)-LFP cathode sample collected after the polymerization and repeated electrochemical cycling. SEM images from a top layer of cycled poly[Ni(CH3-Salen)]-LFP cathode are shown in Figure 3. Figure 3(a,c) were taken with an in-column detector that could better image the polymer, while Figure 3(b,d) were taken with an Everhart-Thornley detector that could better image LiFePO4 particles. By comparing theses SEM images it is apparent that LiFePO4 particles are encapsulated by poly[Ni(CH3-Salen)]. In addition, the outer poly[Ni(CH3-Salen)] films construct web-like structure that network individual LiFePO4 particles. These results suggest that, during polymerization, the poly[Ni(CH3-Salen)] creates unique 3D structure that interconnects the LiFePO4 particles seamlessly, which will serve as efficient electron conduction pathways. Cross-sectional images of the polymerized electrode in Figure 4 show the presence of poly[Ni(CH3-Salen)] throughout the depth of the electrode. For example, Figure 4(b)-(d) reveal that the conductive polymer maintains the web-like structure and interconnect LiFePO4 particles at different depths vertically through the cathode layer. This result suggests that the conductive polymer effectively binds the LiFePO4 particles in entire cathode layers. However, compared with the top cathode layer (see, Figure 3(a)), the amount of polymer coated at the surface of LiFePO4 particles was smaller inside the cathode. This can be explained by considering following processes. During the cell fabrication, Ni(CH3-Salen) monomers in Ni(CH3-Salen)-LFP cathode are partially dissolved into the electrolyte, followed by their re-deposition onto LiFePO4 particles during the electrochemical polymerization process. Because the top surface layer of cathode is facing an abundance of monomer dissolved in electrolyte solution (i.e., larger electrolyte/LiFePO4 ratio), it can form the dense polymer coatings. On the other hand, the amount of monomer inside the cathode is relatively small because of reduced amount of electrolyte existing inside cathode pores 8 ACS Paragon Plus Environment

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(i.e., smaller electrolyte/LiFePO4 ratio), which results in sparse coating on the LiFePO4 surface. However, electrochemical characterizations in later sections demonstrates that poly[Ni(CH3Salen)-LFP cathodes still deliver promising battery performances that surpass the conventional Carbon-LFP cathodes (i.e., consisting of carbon conductor and PVdF binder). In addition, repeated elemental mapping of the cathode showed well-distributed Ni across all areas in the cathodes, as shown in Figure 4(e). Our result suggests that the density of polymer deposition and its microstructure can be affected by electrode design such as porosity, tortuosity, and electrode thickness. Future studies will be necessary to understand the governing parameters that can control and optimize the distribution of the poly[Ni(CH3-Salen)] across the cathodes. FT-IR Analysis The FT-IR spectra of Ni(CH3-Salen)-LFP cathodes were collected before and after polymerization. Due to the low amount of the conductive polymer (5.5 wt %) in the Ni(CH3Salen)-LFP cathodes, its absorption intensity is much lower than that of LiFePO4. However, we can still distinguish major absorption peaks from the monomer and polymer. Figure 5 compares various FT-IR spectra from Ni(CH3-Salen)-LFP and poly[Ni(CH3-Salen)-LFP] cathodes, Ni(CH3Salen) monomer, LiFePO4, and PVdF. The Ni(CH3-Salen) monomer demonstrated an absorption from C=N imine stretching band at around 1614 cm-1, which is split by the asymmetric stretching vibration of C=N–C absorption.25,31 For the monomer spectrum, absorption bands between 1300 – 1550 cm-1 are indicative of interaction between phenoxide and Ni, and split peak at 1095 cm-1 is from C–O stretch vibrations, which agrees very well with literature.20,31 After the electrochemical polymerization at 4.3 Vvs. Li and continuous cycling in a range of 3.0 – 4.0 Vvs. Li, the poly[Ni(CH3Salen)] in the cathode maintained the major bands that are observed from the monomers, which is indicative of the stable polymerization process and its reversible redox capability during repeated 9 ACS Paragon Plus Environment


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cycling. As reported in earlier study, absorption peak at 737 cm-1 from monomer sample disappears after the polymerization process,31 of which characteristic is currently unknown and remains for future studies. Mechanical Properties The SEM images showed that poly[Ni(CH3-Salen)] not only serves as a conducting agent but also serves as an effective binder for LiFePO4 particles; either totally encapsulating particles at the top cathode layer (see, Figure 3) or partially covering the particles inside the cathode (see, Figure 4). Therefore, we used a minimal amount (1.5 wt %) of PVdF binder in the Ni(CH3-Salen)LFP cathode, which was only sufficient to hold the LiFePO4 particles during the cell fabrication and polymerization process. By this way, we could reduce the electrochemically inactive phase (i.e., PVdF) and maximize the energy density of battery cells at the cathode level. In this regard, understanding the mechanical properties of poly[Ni(CH3-Salen)] and comparing it with conventional binder (i.e., PVdF) will be critical for its future applications in Li-ion batteries, which is currently unknown. Among various properties, Young’s modulus is an important character of binders because a binder with higher modulus (i.e., stiffness) can handle larger stresses in the electrode caused by volume expansions and can improve the cycle life of the cathode. Therefore, we measured Young’s modulus of a poly[Ni(CH3-Salen)]-LFP cathode at (1) dried state and (2) wet state by immersing the cathode in dimethyl carbonate (DMC) electrolyte solvent by using AFM. Here, the dried poly[Ni(CH3-Salen)]-LFP cathode sample was collected from cycled battery cells, followed by washing with DMC solvents and drying. As shown from SEM images (see, Figure 3), AFM analyzed the top layer of the poly[Ni(CH3-Salen)]-LFP cathode where the poly[Ni(CH3-Salen)] created a dense polymer coating that encapsulates LiFePO4 particles. Figure 6 shows that the Young’s modulus of the dry poly[Ni(CH3-Salen)] is found to be 10 ACS Paragon Plus Environment

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~ 1.7 GPa. Coupled with a first principle calculations showing that the Young’s Modulus of LiFePO4 to be greater than 100 GPa,32 the result confirmed that the AFM collected the data from the poly[Ni(CH3-Salen)] layer. Then, we monitored the variation of modulus values with time after soaking the poly[Ni(CH3-Salen)]-LFP cathode into DMC electrolyte solvent. Figure 6 shows that the wet poly[Ni(CH3-Salen)] has steady modulus values in a range of 1.74 – 2.83 GPa, which is similar to the value of the dry polymer. This is to be expected, as the Ni(CH3-Salen) is polymerized in an electrolyte environment, and therefore should maintain its elastic properties over time. In stark contrast, PVdF binder experienced significant decrease in Young’s modulus value (approximately one order of magnitude smaller) when its dry sample was immersed in an electrolyte solution.33 The stable mechanical property of poly[Ni(CH3-Salen)] at wetted state is desirable for binders since it offers high resistance to elastic and plastic deformation of electrodes that occurs due to repeated volume expansion of active materials (i.e., LiFePO4). Battery Performance Poly[Ni(CH3-Salen)] and LiFePO4 deliver their major redox capacities in a voltage range of 3 – 4 Vvs.Li.24 Therefore, their combination will be ideal in terms of materials utilization (i.e., specific capacity) and electrochemical stabilities during long-term cycling. Figure S1 shows a representative CV profile of the poly[Ni(CH3-Salen)]. Cycle life analyses were performed on the poly[Ni(CH3-Salen)]-LFP and Carbon-LFP cells, where both half-cells were cycled 150 times between 3.0 and 4.0V with a C/5 charge rate and C/2 discharge rate. The first three cycles are formation cycles, with a charge and discharge C-Rate of C/10, in order to form a stable SEI layer. Specific discharge capacities in Figure 7(a) were calculated from the mass of sole LiFePO4. In Figure 7(a), polyNi(CH3-Salen)-LFP delivers a discharge capacity (153 mAh g-1 at 3rd cycle) 11 ACS Paragon Plus Environment


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higher than that of the Carbon-LFP cathode (147 mAh g-1 at 3rd cycle). Considering that pure poly[Ni(CH3-Salen)] delivers a specific capacity of ~ 50 mAh g-1,24 5.5 wt% of poly[Ni(CH3Salen)] can provide extra capacity value of ~ 3 mAh g-1. In other words, specific capacity of poly[Ni(CH3-Salen)]-LFP cathode is delivered by redox from both Li1-xFePO4 and poly[Ni(CH3Salen)], respectively, using Li+ and PF6- as charge-carriers. Since the actual capacity increase is ~ 6 mAh g-1, the slight increase in capacity (~ 3 mAh g-1) comes from an increase in performance of LiFePO4. The LiFePO4 is known to suffer from poor conductivity and slow diffusion of Li-ions, which is a general characteristic of materials containing polyanions such as PO42-.34,35 In order to account for this, LiFePO4 is often coated with conducting agents, such as carbon nanoparticles. The addition of the electronically conductive poly[Ni(CH3-Salen)] will help to further improve the electronic conductivity of the LiFePO4 particles based on its intimate contact with particles as evidenced by SEM images. Figure 7(b) shows the specific discharge capacities calculated from the masses of the entire cathodes (i.e., including LiFePO4, PVdF, poly[Ni(CH3-Salen)], or carbon), excluding the current collector. Since masses of some electrochemically inactive components are now included, specific discharge capacities of the cathodes decrease compared with that shown in Figure 7(a). For example, poly[Ni(CH3-Salen)]-LFP cathode delivers 142 mAh g-1 while Carbon-LFP cathode delivers 132 mAh g-1 at the 3rd cycle. This improvement can be explained by that the poly[Ni(CH3Salen)] eliminates conductive carbon (i.e., Super P) and replaces a significant portion of PVdF binder, in addition to its own redox capacity. We also confirmed the improved specific capacity of the poly[Ni(CH3-Salen)]-LFP cathode over the Carbon-LFP cathode in full-cells, using the graphite anodes. Figure S2 plots two first consecutive C/10-rate charge-discharge cycles for full cells with poly[Ni(CH3-Salen)]-LFP and Carbon-LFP electrodes. The formation of SEI on the 12 ACS Paragon Plus Environment

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graphite can clearly be observed during 1st charge (at around 2.8 V full-cell voltage). The specific capacity values are calculated based on the masses of the entire cathodes (excluding Al-foil). The 2nd cycle capacities are, respectively, 132.8 mAh/g for the poly[Ni(CH3-Salen)]-LFP and 123.3 mAh/g for the Carbon-LFP. After 150 cycles, the poly[Ni(CH3-Salen)]-LFP cathode retained 96.0% of its capacity at the 4th cycle, while the Carbon-LFP cell retained 90.5% of its capacity at the 4th cycle of half-cells. Coulombic efficiency (CE) is an important indicator of the presence of unwanted interfacial reactions during repeated electrochemical charge/discharge cycling. Figure 7(c) compares CEs of poly[Ni(CH3-Salen)]-LFP and Carbon-LFP cells. The poly[Ni(CH3-Salen)]-LFP cell delivers an initial CE value of 89.2% (at C/10-rate formation cycle), followed by an average CE value of 99.4% for the remaining 147 cycles (at C/2-rate cycles). In comparison, the Carbon-LFP cell delivers an initial CE value of 88.8% (at C/10-rate formation cycle), followed by an average CE value of 98.9% for the remaining 147 cycles (at C/2-rate cycles). An earlier study suggested that capacity fading from Carbon-LFP cells stem from parasitic reactions involving Fe2+ dissolution from LiFePO4.36 Therefore, the improved CE and cycle life of poly[Ni(CH3-Salen)]-LFP might be explained by a role of poly[Ni(CH3-Salen)], passivating the top surficial layer of LiFePO4, as evidenced by SEM images in Fig. 3, where electrode/electrolyte interfacial reactions can occur dominantly. The promising binding properties of the poly[Ni(CH3-Salen)] can also help maintain the mechanical integrity of the cathode during repeated cycling, which would suppress particle isolation and provide stable cycle-life of the poly[Ni(CH3-Salen)]-LFP compared with that of the Carbon-LFP cathode. High-rate capability of the poly[Ni(CH3-Salen)]-LFP cathode was assessed during both fast-charging and fast-discharging conditions, in comparison with the conventional Carbon-LFP 13 ACS Paragon Plus Environment


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cathode. Since the LiFePO4 has been used mainly for high-power-cell applications in Li-ion batteries, the effect of poly[Ni(CH3-Salen)] on the rate capability of the cathode is important in practical applications. For the fast-charging tests, The C-rate was varied from C/10 to 5C during charging, with a constant C/10 discharge rate. Similarly, for the fast-discharging tests, C-rate was varied from C/10 to 5C during discharge, with a constant C/10 charge rate. Here, the specific capacity is calculated from the mass of the sole LiFePO4. The voltage profiles and normalized capacity retentions were plotted in Figure 8(a) and (b), respectively, for fast-charging and fastdischarging tests. The poly[Ni(CH3-Salen)]-LFP cathode delivers improved capacities compared with the Carbon-LFP cathode during both fast-charging and fast-discharging tests. The normalized capacities vs. C-rate plot clearly demonstrates that the improvement is greater during the fastcharging. The conductivity of poly[Ni(CH3-Salen)] increases as the operating voltage of the poly[Ni(CH3-Salen)] increases, due to the polymer having higher doping levels,36 which is a typical property of conductive polymers. During charging, the cathode experiences a higher operating voltage, which leads to increased conductivity of the poly[Ni(CH3-Salen)]. This may explain the improved C-rate performance during the fast-charging of the poly[Ni(CH3-Salen)]LFP cathode. EIS Characterization EIS characterization was performed on poly[Ni(CH3-Salen)]-LFP and Carbon-LFP cathodes to compare and understand their conduction mechanisms. Prior to the EIS analysis, each cell was cycled three-times to form a stable SEI layers at electrode/electrolyte interfaces. Symmetrical cells consisting of two identical electrodes has been known to offer highly reproducible EIS data compared with two- or three-electrode cells using Li metal as the counter and/or reference electrode.37–39 Therefore, two pairs of half-cells (cathode/Li) were cycled under the same condition 14 ACS Paragon Plus Environment

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(three-cycles) and then opened at 50% state-of-charge (SOC), followed by reassembling into symmetrical cells by paring cathode/cathode. Figure 9 shows the Nyquist plots obtained from the resulting symmetrical cells. Recent study by Schmidt et. al40 reported that Nyquist plot of CarbonLFP cathodes comprised two RC elements and a Warburg-type diffusion: a higher frequency RC response associated with impedance at the cathode/current collector interface, and lower frequency RC response associated with charge transfer in the cathode. Since this earlier study40 used excess amount of carbon conductor (24 wt%) with only small amount of PVdF binder (6 wt%), its cathode/current-collector impedance was significantly large (> 100 Ω cm2) indicating a poor binder adhesion. Since we used more reasonable combination of carbon conductor and PVdF binder with 1:1 wt ratio, our Carbon-LFP cathode has an interfacial impedance of 3.4 Ω cm2 (i.e., the high-frequency RC element) that is much lower than the literature value (i.e., > 100 Ω cm2) by fitting the Nyquist plots using the same electrical circuit model discussed above.40 The poly[Ni(CH3-Salen)]-LFP cathode showed even smaller interfacial impedance of 1.97 Ω cm2 that suggests a much improved cathode/cathode cohesion and/or cathode/current-collector adhesion due to the promising role of the poly[Ni(CH3-Salen)] as a binder. In addition, the earlier study40 revealed that the lower frequency RC circuit, which corresponds to charge-transfer reactions of LiFePO4, could significantly overlap with Warburg diffusion, and could be distinguished and analyzed by the distribution of relaxation times (DRT) method only after carefully processing the data (i.e., defining and subtracting the Warburg-type diffusion). By using the same approach, we also found that both Carbon-LFP and poly[Ni(CH3Salen)]-LFP cathodes follow the same characteristics. The Carbon-LFP cathode at 50% SOC delivered charge-transfer resistance value of 7.9 Ω cm2, which is very close to the literature value (16.5 Ω cm2)40 collected at 100% SOC; it should be noted that charge-transfer resistance of 15 ACS Paragon Plus Environment


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LiFePO4 (also other cathodes) usually increases at its fully charged state. In comparison, charge transfer resistance from the poly[Ni(CH3-Salen)]-LFP cathode only showed 0.49 Ω cm2, which is much lower than that of the Carbon-LFP cathode. Due to the low electronic conductivity of LiFePO4, its charge-transfer reaction would be governed by the effectiveness of conduction network. Carbon conductor (i.e., Super P) forms a point-contact with LiFePO4 and other carbon particles by mixing with PVdF binder in conventional cathodes (i.e., Carbon-LFP). In contrast, we demonstrated that poly[Ni(CH3-Salen)] had an ability to create the web-like electron network at the cathode surface that covering the surface of LiFePO4 particles very well (also even encapsulate at the top cathode layer), which in turn increased their contact area significantly. As a result, facile electron transfer through the large surface contact between conductive polymer and LiFePO4 surfaces leads to the significantly enhanced charge-transfer rate, which accounts for the improved rate capabilities of the poly[Ni(CH3-Salen)]-LFP cathodes. In this study, we newly found multiple positive roles of poly[Ni(CH3-Salen)] for carbonfree cathodes: (i) formation of 3D electronic network, (ii) accelerating charge-transfer reaction of cathode, (iii) providing extra capacity from the redox of poly[Ni(CH3-Salen)], thus increasing energy density at cathode level, (iv) enhanced mechanical property as a binder for cathodes, and (v) passivation of active materials at top surface, which are illustrated in Scheme 1. Future studies will be necessary to investigate materials/electrode parameters that can control the local distribution of polymers in the cathode microstructure, which can further improve the battery performances.


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CONCLUSION We have presented multiple positive roles of poly[Ni(CH3-Salen)] in cathodes, as illustrated in Scheme 1. The poly[Ni(CH3-Salen)] forms web-like electron network on top of the LiFePO4 particles: either encapsulating LiFePO4 particles on the top cathode layer or creating a complex electron network inside the cathode microstructure, as evidenced by SEM. FT-IR analysis showed the stable chemical structure of the polymer after cycling, which demonstrated its good electrochemical reversibility. AFM analysis also presented its steady Young’s modulus values after immersion in electrolyte solutions, which proves its potential as a binder based on its ability to improve mechanical property of cathode during extended cycling compared with conventional PVdF binder. As a result, the poly[Ni(CH3-Salen)]-LFP cathodes delivered much improved battery performances compared with the conventional Carbon-LFP cathodes, in terms of specific capacity, cycle life, and rate capabilities. EIS data presented that the poly[Ni(CH3-Salen)]-LFP cathodes had much lower interfacial contact resistance and charge-transfer resistance, compared with those of Carbon-LFP cathodes, which can be explained by an effective adhesion/cohesion function and a facile electron transfer via the polymer’s unique 3D network structure. ACKNOWLEDGMENT This work was supported by OSU Materials Research Seed Grant Program and Powermers Inc.



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Figure 1. Structures of (a) poly[Ni(Schiff)] chain; (b) d-π stabilized poly[Ni(Schiff)] stack; (c) ππ stabilized poly[Ni(Schiff)] stack; (d) Ni(CH3-Salen) monomer. Images (a), (b), and (c) show the structure of poly[Ni(CH3-Salen)] as the simplest representative of poly-[Ni(Schiff)] films.


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a (a)

b

3 µm

Figure 2. SEM images of as-prepared (a) Ni(CH3-Salen)-LFP cathode and (b) conventional Carbon-LFP cathode.



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Figure 3. SEM images collected from the top layer of poly[Ni(CH3-Salen)]-LFP cathodes. The same areas (spot A or spot B) were imaged by using two-different detectors; (a,c) better focusing on polymer shells and (b,d) better focusing on LiFePO4 at cores.


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Figure 4. (a) Cross sectional image of poly[Ni(CH3-Salen)]-LFP cathode with (b-d) zoomed in portions at different regions of cross-section. (e) Representative Ni element mapping from the poly[Ni(CH3-Salen)]-LFP cathode.



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Figure 5. FT-IR spectra of (a) Ni(CH3-Salen)-LFP cathode, (b) Ni(CH3-Salen) monomer, (c) poly[Ni(CH3-Salen)]-LFP cathode recovered after cycling, (d) LiFePO4 powder, and (e) PVdF powder samples. 28 ACS Paragon Plus Environment

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Figure 6. (Top)Topography, Young’s modulus map, and its histogram for dry and wetted poly[Ni(CH3-Salen)]-LFP cathode collected under different immersion time in DMC solvent. (Bottom) Evolution of Young’s modulus values with time after the immersion.



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a

b

c

Figure 7. (a,b) Cycle life and (c)Coulombic efficiency of poly[Ni(CH3-Salen)]-LFP and CarbonLFP cells. Specific discharge capacity was calculated based on (a) mass of sole LFP and (b) mass of entire cathode excluding current collector. The data was collected at room-temperature.


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a

b

Figure 8. (a) Fast-charging and (b) fast-discharging capability of poly[Ni(CH3-Salen)]-LFP and Carbon-LFP cathodes. The data was collected at 25oC. 31 ACS Paragon Plus Environment


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Figure 9. Nyquist plots of cathode/cathode symmetrical cells with observed (circles) and simulated (solid lines) data at 25oC. Each cathode was at 50% SOC.


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Scheme 1. Multiple roles of poly[Ni(CH3-Salen)] in carbon-free cathodes: (i) formation of 3D electronic network, (ii) accelerate charge-transfer reaction of cathode, (iii) providing extra capacity from the redox of poly[Ni(CH3-Salen)], thus increasing energy density at cathode level, (iv) enhanced mechanical property as a binder for cathodes, and (v) passivation of active materials at top surface. The direction of electrochemical reactions is for a battery charging process.



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Table of Contents


Nickel-Salen Type Polymer as Conducting Agent and Binder for

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