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Influence of Binders, Carbons, and Solvents on the Stability of Phosphorus Anodes for Li-ion Batteries Naoki Nitta, Danni Lei, Hong-Ryun Jung, Daniel Gordon, Enbo Zhao, Garrett Gresham, Jeremy Cai, Igor Luzinov, and Gleb Yushin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07931 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016
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ACS Applied Materials & Interfaces
Influence of Binders, Carbons, and Solvents on the Stability of Phosphorus Anodes for Li-ion Batteries Naoki Nittaa, Danni Leia, Hong-Ryun Junga, Daniel Gordona, Enbo Zhaob, Garrett Greshama, Jeremy Caia, Igor Luzinovc, Gleb Yushina,* a
-School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA
30332, USA. b
-School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332,
USA. c
- Department of Material Science and Engineering, Clemson University, Clemson, SC 29634,
USA KEYWORDS: batteries, phosphorus, polyacrylic acid, degradation, carbon nanotubes, FTIR, XPS.
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Abstract
Phosphorus (P) is an abundant element that exhibits one of the highest gravimetric and volumetric capacities for Li storage, making it a potentially attractive anode material for high capacity Li-ion batteries. However, while phosphorus carbon composite anodes have been previously explored, the influence of the inactive materials on electrode cycle performance is still poorly understood. Here we report and explain the significant impacts of polymer binder chemistry, carbon conductive additives and an under-layer between the Al current collector and ball milled P electrodes on cell stability. We focused our study on the commonly used polyvinylidene fluoride (PVDF) and polyacrylic acid (PAA) binders as well as exfoliated graphite (ExG) and carbon nanotube (CNT) additives. The mechanical properties of the binders were found to change drastically due to interactions with both the slurry and electrolyte solvents, significantly effecting the electrochemical cycle stability of the electrodes. Binder adhesion was also found to be critical in achieving stable electrochemical cycling. The best anodes demonstrated ~1400 mAh/g-P gravimetric capacity after 200 cycles at C/2 rates in Li half cells.
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Introduction Phosphorus (P) is an interesting alloying anode material for Li-ion batteries due to its promising features and differences in structure and behavior from other material candidates 1-2. The first and most advantageous feature of phosphorus is that its gravimetric (specific) and volumetric Li capacities are both among the highest of any element in the periodic table. Phosphorus is also highly abundant (even more than sulfur) and its fully lithiated phase has good Li+ transport properties 3-6. Admittedly, P has a relatively high average potential of delithiation, which reduces the full cell voltage when compared to Sn, Si or graphite anodes 1, and limits its energy density. However, this potential is lower than that of lithium titanate (LTO) anode, which has been commercialized due to its high rate capabilities 2. Meanwhile, the higher lithiation potential of P enables the use of a cheaper and lighter Al anode current collector over the more traditional and expensive Cu 7. In addition, P may possibly allow safer cells (compared to those built with graphite anodes) by preventing Li dendrite growth. We also acknowledge the known toxicity and high reactivity of elemental P, which may be undesirable for some applications. However, it is unlikely that any one anode chemistry will provide the solution to all energy storage needs in the long term. Therefore, in the view of the authors P provides an interesting but under-explored alternative chemistry with the potential to find multiple applications as we seek better solutions to our diverse and global energy needs. Differences in the chemical and physical properties of a material may cause differences in their interactions with the other components of the electrode and the battery as a whole. Understanding and controlling these interactions is key to developing a practical and long lasting battery. While all components indirectly interact in some way, the active material comes into direct contact with the current collector, carbon conductive additive, polymer binder, and organic
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electrolyte. Of these interactions, the solid electrolyte interphase (SEI) formed between the active material and electrolyte has received the most interest, as the chemical interactions there are the most dramatic and critical for the cell performance 8. However, given the low conductivity, high reactivity, and large volume change (315% and anisotropic based on Black P and Li3P density 914
) P undergoes during electrochemical cycling, its interactions with a polymer binder and a
conductive additive may prove to be just as important. In this study, the properties and interactions of the P, conductive carbon, and the binder have been systematically studied to understand how the differences in electrode performance arise from these material choices. As an active material, P provides a selection of phases which may be explored 15-17. We have selected P ball milled with carbon for an extended period of time because such an anode architecture demonstrated some of the most promising published results so far 18-23. The previously reported good cycle stability and high rate was linked to the highly intermeshed microstructure of the P-C composite particles, where black P forms strong bonds with the conductive C 19. In terms of binder chemistry - polyvinylidene fluoride (PVDF) is the most traditional choice in the battery industry. However, as we previously demonstrated with volume-changing Si alloying anodes 24-25, binders with significantly higher elastic modulus (when immersed in electrolyte) as well as oxygen containing functional groups capable of binding to and maintaining electrical contact between particles may prove to be advantageous 26-27. Polyacrylic acid (PAA) is a particularly convenient choice as it contains a carboxyl group which can bind well to many materials, while also possessing high mechanical properties due to very low swelling in electrolytes 26. In contrast to PAA, PVDF swells and loses much of its hardness in diethyl carbonate (DEC) 26-27, a common ingredient in Li ion battery electrolytes. It is conceivable that
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swelling of PVDF and PAA in other electrolyte solvents may also differ significantly. Therefore, we were interested to compare the performance of the PAA vs. PVDF-based electrodes. Because heat applied to dry the cast electrode may also change the binder chemical structure and its interactions with P and the current collector, we were further interested to reveal the impact of drying temperature. In order to minimize the uncontrolled reaction of water and oxygen with P 28
, we utilized the convenient ability of PAA to dissolve in n-methyl pyrrolidone (NMP) solvent
and casted both P-C/PVDF anodes and P-C/PAA anodes in an Ar filled glovebox. Exfoliated graphite (ExG) and carbon blacks are also some of the most traditional conductive additives used in battery electrodes. The known advantages of ExG (or, what can be now called “multi-layered graphene”) over carbon black are its lower irreversible capacity losses and larger dimensions, which allows a single ExG particle to electrically connect several large particles of active material (P-C composite particles, in our case). However, the steadily decreasing prices and growing applications of carbon nanotubes (CNTs) triggered our interest to compare the impact of CNTs vs ExG as both conductive additives have sufficiently long dimensions to link multiple active particles. While CNTs tend to entangle and agglomerate 29, they are significantly more compliant and flexible than ExG and may be dispersed in solvents by functionalization, sonication, and the addition of polar polymers. Experimental Section Electrode Construction To understand how binder and carbon choices translate to electrode performance, a series of electrodes were cast with ball milled P-C composites. Phosphorus was first ball milled with 30% wt. smaller diameter CNTs (inner diameter 2-5 nm, outer diameter
