Poly(aniline-co-anthranilic acid) as an electrically conductive and mechanically stable binder for high-performance silicon anodes
Graphical abstract
Introduction
The energy density levels of currently available lithium-ion batteries (LIBs) are sufficient for small mobile devices, but not for high-capacity applications such as electric vehicles and energy-storage devices. One approach used to overcome this problem has been to replace the currently used graphite electrodes with silicon. Theoretically, silicon has about ten times the capacity of graphite, enough to be used as a high-energy-density anode material for lithium-ion batteries required in high-capacity storage devices [[1], [2], [3], [4]]. But this use of silicon has not shown commercial success, in large part as a result of silicon undergoing large volume changes, of about 400%, in the course of storing lithium ions. This volume change causes the silicon to break into small pieces during the charge-discharge process, with the surfaces of the newly exposed silicon pieces becoming coated with a solid-electrolyte interphase (SEI), and the efficiency of the silicon hence becoming reduced. This issue along with the low electrical conductivity of Si itself, contribute to a rapid deterioration in the cycle life of its batteries.
Various studies have been conducted to solve these problems regarding silicon. One approach has been to modify silicon into a form allowing the silicon to accommodate the volume change without developing the above-described problems. For this purpose, the main pursuits have been producing nanosized forms of silicon, specifically nanoparticles [5], nanowires [6,7], nanotubes [8,9] and porous nanonetworks [10,11]. However, these methods significantly increase the cost of producing the otherwise low-cost silicon, and result in a decreased efficiency caused by the expansion of the underlying surface and the lower energy density per volume.
Another method has been to use polymer binder materials with high mechanical strength [3,12,13]. Polymer binders are used to fabricate LIB electrodes, including for bonding the active material, conductive agent and current collector to each other. Introducing various functionalities into the polymeric binder as well as deploying polymers having excellent physical properties have been advantageous, including for mitigating the problems such as volume changes associated with silicon, and doing so at a lower cost than conventional approaches.
Quite a few polymers have been investigated as binder materials for the Si anode. Carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and poly (vinylidene fluoride) (PVdF) [[14], [15], [16], [17], [18]], for example, have been tested due to their high levels of thermal and electrical stability, but while such conventional polymeric binders exhibit excellent properties for electrodes showing little physical stress due to volume expansion during charging and discharging, they are not sufficient for solving the problems associated with the volume change of silicon, and hence cannot be used for silicon anodes [3,12,13]. Polymers with hydrophilic functional groups have been reported to be effective at solving the various problems occurring when silicon serves as the active material: for example, poly(acrylic acid) (PAA), which has a high density of carboxylic acid functional groups, has been reported to form strong hydrogen bonds with various polar functional groups present on the surface of the silicon, greatly improving both the physical properties of the electrode and the cycle life of the silicon battery [19,20], and polymers containing a high density of hydroxy (OH) groups, such as poly(vinyl alcohol) (PVA) [21,22] as well as various polysaccharides including alginate [[23], [24], [25]], chitosan [26,27] and pullulan [28], have also been actively studied as binders for the silicon anode. And studies have been conducted to further enhance the physical properties of polymer binders by introducing additional molecular interactions between the polymers, for example, along with crosslinking methods for chemically forming covalent bonds between CMC and PAA [29,30] and between PAA and PVA [31] using a condensation reaction between functional groups, various methods using reversible physical crosslinking via strong hydrogen bonds occurring between the polymer main chains having many polar functional groups [[32], [33], [34]] and acid-base links between PAA and poly(benzimidazole) [35] have been reported.
In addition to these studies aimed at producing polymers with improved adhesion to silicon and hence mechanically robust Si electrodes, other studies aimed at endowing polymer binders with additional functionalities – especially conductivity – have also been undertaken. Such studied polymers have included lithium ion-conducting polymers such as Nafion® [36,37] and poly(ethylene glycol) (PEG) [38], and electrically conductive polymers such as poly(phenylenes) (PPs) [39], poly(3,4-ethylenedioxythiophene) (PEDOT) [40], poly(9-dioctylfluorene-co-fluorenone) (PFFO) [41,42] to which electrical conductivity was granted to complement the limitations of insulative silicon - have also been studied. In many of these studies, the electrically conducting polymers can also serve as a conducting agent so that some of the conductive agent in the electrode can be replaced with an active material. Doing so yields the additional advantage that the energy density can be further increased by increasing the relative amount of active material. To synthesize typical electrically conducting polymers, however, the use of various harmful organometallic intermediates is required and the cost of, for example, Pd catalysts used in the polymerization is very high as well. Furthermore, these conducting polymers interact only weakly with silicon. As a result, much work remains to be done before these conducting polymers can be used in practical applications. Therefore, we have been trying to produce a new easy-to-synthesize conducting polymer and use it as a binder of the silicon anode.
We have previously developed a physically crosslinked binder system by adding small amount of the conductive poly(aniline) (PANI) to the poly(acrylic acid) (PAA) to form acid-base interactions and, at the same time, impart the electrical conductivity of PANI to the binder, for the purpose of producing mechanically robust PANI-PAA-based electrodes showing high-level cell performances [43]. Despite the advantages of the PANI-PAA system, PANI has, however, no functional groups capable of interacting with silicon, and has very poor solubility, making it difficult to use substantial amounts of it. It is, therefore, problematic to manufacture a silicon electrode by using PANI alone, and hence blending it with other polymers would be necessary.
We report herein a new approach using only one type of polymer without blending it with other polymers such as PAA, with this approach involving the use of poly(aniline-co-anthranilic acid) copolymer (PAAA). Specifically, introducing the polar carboxylic acid group into PANI endowed the product with electrical conductivity, greater solubility, and improved physical properties as a result of interactions with silicon (Fig. 1). Moreover, unlike other conducting polymers, we were able to synthesize PAAA inexpensively. The effect of acid group content in PAAA on the physical properties, electrical and ion conductivities, and cell performances of the electrode were fully investigated.
Section snippets
Materials
Aniline, anthranilic acid and ammonium persulfate (APS) were purchased from Sigma-Aldrich. Silicon nanoparticles (50 nm) were purchased from Alfa-Aesar. The electrolyte (1 M LiPF6 in ethylene carbonate/ethylmethyl carbonate (EC:EMC = 3:7 v/v) with 10 wt% fluoroethylene carbonate (FEC)) was purchased from PanaxEtec.
Synthesis of the poly(aniline-co-anthranilic acid) copolymer (PAAA)
The aniline was distilled and anthranilic acid was recrystallized before use. For the synthesis of PAAA with different compositions of anthranilic acid, aniline and anthranilic acid
Preparation of the poly(aniline-co-anthranilic acid) copolymer (PAAA)
As shown in Scheme 1, the aniline and anthranilic acid monomers were mixed and dissolved in a HCl solution. APS was used as an initiator and these monomers were oxidatively polymerized to form PAAA.
Various PAAA copolymers, with different relative amounts of anthranilic acid, specifically 10%, 25%, 50%, and 75%, were synthesized, and are denoted in this manuscript as 0.10-PAAA, 0.25-PAAA, 0.50-PAAA and 0.75-PAAA, respectively. The structure of each PAAA copolymer was confirmed by using FT-IR
Conclusion
In this study, we produced PAAA, a new electrically conductive and mechanically robust copolymer binder material for silicon anodes. The synthesis of PAAA was inexpensive, in contrast to the high costs of previously reported electrically conductive polymers. In contrast to the polymeric aniline, PANI, which cannot be used independently for the production of silicon electrodes due to its weak adhesion to silicon, the PAAA copolymer was designed to include COOH-containing anthranilic acid units
Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF-2017R1A6A1A06015181).
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