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Energy, Environmental, and Catalysis Applications
Comparative Analysis of Aqueous Binders for High-Energy Li-rich NMC as Lithium-Ion Cathode and the Impact of Adding Phosphoric Acid Arefeh Kazzazi, Dominic Bresser, Agnese Birrozzi, Jan von Zamory, Maral Hekmatfar, and Stefano Passerini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03657 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018
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ACS Applied Materials & Interfaces
Comparative analysis of aqueous binders for high-energy Li-rich NMC as lithium-ion cathode and the impact of adding phosphoric acid Arefeh Kazzazi,a,b Dominic Bresser,a,b,* Agnese Birrozzi,a,b Jan von Zamory,a,b Maral Hekmatfar,a,b and Stefano Passerini a,b,* a b
Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany
Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
Abstract Even though electrochemically inactive, the binding agent in lithium-ion electrodes contributes substantially to performance metrics like the achievable capacity, rate capability, and cycling stability. Herein, we present an in-depth comparative analysis of three different aqueous binding agents, allowing for the replacement of the toxic N-methyl-2-pyrrolidone (NMP) as processing solvent, for high-energy Li1.2Ni0.16Mn0.56Co0.08O2 (Li-rich NMC or LR-NMC) as potential next generation cathode material. The impact of the binding agents, sodium carboxymethyl cellulose (CMC), sodium alginate (ALG), and commercial TRD202A (TRD), and the related chemical reactions occurring during the electrode coating process on the electrode morphology and cycling performance is investigated. In particular, the role of phosphoric acid (PA) in avoiding the aluminum current collector corrosion and stabilizing the Lirich/electrolyte interface as well as its chemical interaction with the binder are investigated, providing an explanation for the observed differences in electrochemical performance.
Keywords: aqueous electrode processing, binder, phosphoric acid, Li-rich NMC, lithium-ion cathode, battery * Corresponding authors:
[email protected] ;
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1. Introduction Lithium-ion batteries are the electrochemical energy storage technology of choice for a variety of applications, ranging from portable electronics to electric bikes, scooters, vehicles, and, increasingly, stationary storage.1–3 A successful penetration of the automotive mass market, however, requires further improvement with respect to the charging time and particularly the driving range, i.e., energy density.4 Focusing on the cathode side, an attractive candidate for next generation, high-energy lithium-ion batteries is certainly lithium (rich) nickel manganese cobalt oxide (LR-NMC), i.e., a nano-composite of x Li2MnO3 (1x) LiMO2 (with M = Ni, Mn, Co), theoretically providing energy densities in the range of 1,000 Wh kg-1.5–7 Moreover, these materials are commonly characterized by a rather low cobalt content, thus, offering also advantages in terms of environmental benignity and cost.7 Nonetheless, their application in commercial cells remains hindered due to the relatively high capacity fading and voltage decay upon cycling and, to a lesser extent, the large structural changes occurring upon the electrochemical activation of the monoclinic Li2MnO3 phase.4–7 The voltage decay upon cycling is not yet fully understood, although great progress has been achieved recently in identifying and understanding the underlying redox processes and structural transitions upon cycling and especially upon the first charge to sufficiently high potentials (i.e., >4.4 V) as well as the impact of the anodic cut-off for the subsequent cycles and the relative ratio of the different transition metal cations.8–10 These processes and transitions include the release of oxygen and its partial oxidation, the (superficial) transition to the spinel phase,10 as well as transition metal segregation.7,11–19 As these reactions are substantially surface-driven, detrimentally affecting also the electrolyte stability at the interface,20,21 several groups have tried to address these issues by tailoring the transition metal composition,8–10 incorporating suitable electrolyte additives,22–25 or applying stabilizing surface coatings like AlF326,27 or lithium phosphorus oxynitride (LiPON),28 resulting, indeed, in significant improvement compared to the therein studied reference samples. In this regard, our group recently investigated vanadium oxide coated LRNMC,29 which eventually allows for the aqueous processing of these electrodes. In fact, little has been done so far to investigate the impact of aqueous electrode processing for LR-NMC cathodes, despite the great
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chance to reduce the processing cost by avoiding the need for the recovery of the processing solvent, facilitate the recyclability, and decrease the environmental impact of such potentially fluorine-free cathodes.30 Wu et al.,31 for instance, performed a comparison of TRD (fluorinated acrylate polymer latex) and PVdF (poly(vinylidene difluoride)) as binders for LR-NMC cathodes, showing that the use of TRD results in slightly reduced area specific impedance and improved rate capability. Li et al.32 reported the replacement of PVdF by CMC to yield enhanced cycling stability and hinder the evolution of the low potential (ca. 3.1 V) oxidation peak, presumably related to the occurrence of a new crystalline phase. Similarly, Han et al.33 observed better cycling stability and, interestingly, decreased voltage decay when comparing PVdF, CMC, and ALG as binders for LR-NMC cathodes. Also Zhang et al.34 reported the beneficial effect of guar gum as aqueous binder on the interfacial stability of the LR-NMC particles towards the organic electrolyte. One critical aspect when dealing with the aqueous electrode processing of water-sensitive lithium-ion cathode materials, however, has been overlooked so far. This is the dramatic pH increase upon dispersion of the electrode components in water, leading to substantial corrosion of the aluminum current collector.35–37 To overcome this drawback, we have recently developed a modified electrode preparation, including the addition of phosphoric acid to the slurry. The pH buffering and formation of a protective transition metal phosphate surface layer on the active material particle, as confirmed for NMC11138 and high-voltage LiNi0.5Mn1.5O439, resulted in outstanding electrodes performance. Following these considerations and previous results, we report herein an in-depth comparison of TRD, CMC, and ALG as water-soluble binders for high-energy LR-NMC cathodes. Furthermore, we investigated the impact of adding phosphoric acid to the aqueous LR-NMC electrode slurry, studying the effect on the electrode morphology, current collector corrosion, and eventually the electrochemical performance – once again in a comparative manner for the three different aqueous binding agents.
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2. Experimental 2.1 Active Material Synthesis Li1.2Ni0.16Mn0.56Co0.08O2 (Li-rich NMC, LR-NMC) was synthesized by a co-precipitation method, followed by a solid state reaction, using the corresponding metal acetates, i.e., Mn(CH3COO)2·4H2O (99%, Acros Organics), Ni(CH3COO)2·4H2O (98%, Aldrich), and Co(CH3COO)2·4H2O (98%, Alfa Aesar) as well as Li(OH)·H2O (98%, Alfa Aesar) as precursors, according to the earlier reported procedure.40 The detailed information concerning the synthesis is provided in the Supporting Information.
2.2 Physicochemical Characterization The Li, Co, Ni, and Mn contents in the as-synthesized LR-NMC were analyzed using inductively coupled plasma optical emission spectrometry (ICP-EOS) to confirm the targeted elemental composition. Scanning electron microscopy (SEM) was conducted to investigate the particle and electrode morphology as well as the impact of the aqueous electrode processing on the aluminum current collector. The structural investigation of pristine LR-NMC and cathodes based thereon was performed by means of X-ray diffraction (XRD) employing a Bruker D8 Advance (Cu-Kα radiation with a wavelength of 1.5406 Å). For the ex situ study of cycled LR-NMC cathodes a specific sample holder was used in order to avoid any contact to air and moisture. The electrodes were extracted from cycled cells under argon atmosphere and rinsed with dimethyl carbonate (to remove residual lithium salt) prior to the XRD analysis. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy were also carried out to investigate the impact of adding phosphoric acid upon electrode preparation. For the latter, CMC and ALG were carefully dried prior to the measurement at 120 °C under vacuum for approximately 24 hours. TRD, CMC+PA, ALG+PA, and TRD+PA were dried at 100 °C. The TRD-comprising samples were dried for several days until all the water stemming from the as-received emulsion had been removed. The XPS spectra were obtained by means of a PHI 5800 MultiTechnique ESCA System, using monochromatized Mg-Kα (1253.6 eV) radiation. The spectra were acquired at a detection angle of 45°, using pass energies of 187.85 and 29.35 eV for the survey and detailed spectra, respectively. All XPS spectra were calibrated based on the C1s peak at 284.8 eV and 4 ACS Paragon Plus Environment
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analyzed utilizing the CasaXPS software. FTIR was conducted by means of a Bruker VERTEX 70v FTIR spectrometer.
2.3 Electrode Preparation The overall electrode composition was 85 wt% LR-NMC, 10 wt% conductive carbon (C-NERGY™ Super C45, Imerys), and 5 wt% of the corresponding binding agent, i.e., sodium carboxymethyl cellulose (CMC; Dow Wolff Cellulosics, Walocel CRT 2000), sodium alginate (ALG; Sigma Aldrich), or TRD202A (TRD; JSR Micro). For this latter binder, a 1:1 mixture of CMC and TRD was used to facilitate the electrode preparation and ensure suitable mechanical properties of the resulting electrodes. Generally, the binder was dissolved in deionized water prior to the addition of the conductive carbon and LR-NMC. The resulting mixture was dispersed by means of a planetary ball mill (Vario-planetary ball mill “PULVERISETTE 4 classic line) for 3h. For each binding agent, two electrode slurries were prepared – one with and one without the addition of 1 wt% phosphoric acid (PA; >99%, Bernd Kraft; 50% in aqueous solution; the wt-ratio refers to the content of LR-NMC) prior to the dispersion of the active material. The obtained slurries were cast on aluminum foil (thickness: 20 µm; battery grade), utilizing a laboratory-scale doctor blade with a wet film thickness of 60 µm. The electrode sheets were dried for 10 min at 60 °C and afterwards at room temperature overnight. Disk-shaped electrodes were punched with a diameter of 1.2 cm, pressed at 4 tons cm-2 and finally dried once more at 180 °C under vacuum for 14 h. For the determination of the pH value of the abovementioned cathode slurries, a Lab 860 pH meter (SI Analytics) equipped with a Blue Line 14 pH electrodes (Schott Instruments) was used. The experiments were conducted at room temperature.
2.4 Electrochemical Characterization
For the electrochemical characterization, three-electrode Swagelok®-type cells made of stainless steel were used. Cell assembly was carried out in an argon-filled glove box with controlled level of oxygen (
