Mechanism of Interactions between CMC Binder and Si Single Crystal Facets U. S. Vogl, †,‡ P. K. Das, ‡ A. Z. Weber, ‡ M. Winter,* ,† R. Kostecki, ‡ and S. F. Lux* ,†,‡ † MEET Battery Research Center, Westfä lische Wilhelms-Universitä t Mü nster, Corrensstraße 46, 48149 Mü nster, Germany ‡ Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, 94611 Berkeley, California 94720, United States ABSTRACT: Interactions of the active material particles with the binder are crucial in tailoring the properties of composite electrodes used in lithium-ion batteries. The dependency of the protonation degree of the carboxyl group in the carboxymethyl cellulose (CMC) structure on the pH value of the preparation solution was investigated by Fourier transform infrared spectroscopy (FTIR). Three different distinctive chemical states of CMC binder were chosen (protonated, deprotonated, and half-half), and their inter- actions with different silicon single crystal facets were investigated. The different Si surface orientations display distinct differences of strength of interactions with the CMC binder. The CMC/Si adhesion forces in solution and Si wettability of the silicon are also strongly dependent on the protonation degree of the CMC. This work provides an insight into the nature of these interactions, which determine the electrochemical performance of silicon composite electrodes. ■ INTRODUCTION Currently, graphite is the most common negative electrode material (theoretical capacity of 372 mAh g -1 ) in lithium-ion batteries, but new alternative materials that offer higher energy density are sought for the next-generation Li-ion battery technology. 1 A promising candidate to replace graphite is silicon, which offers an exceptionally high theoretical specific capacity of 4200 mAh g -1 and a low operating potential. 2-6 However, serious intrinsic problems of silicon, which include the large volumetric expansion (ca. 300%) during the lithiation processes, poor electronic conductivity of silicon, and the interfacial instability of Si electrodes in organic electrolytes, have to be tackled in order to utilize them in commercial lithium-ion batteries. 7-12 These detrimental effects result in low coulombic charge/discharge efficiency, especially during the first few cycles, and a poor electrochemical long-term performance. 11-15 To suppress these detrimental effects and to improve the cycling performance, multiphase composites, 4 nanostructured architec- tures, 7 electrolyte additives, 16 and new types of binder in composite electrodes have been applied to Si and other Li storage metals Si, Sn, Pb, Al, Pt, Au, or Mg. 4,11,12,17,18 The binder chemical, mechanical, and electronic conductivity properties have a strong influence on the active material utilization and cycling stability of intermetallic composite electrodes. 19,20 Depending on the binder chemistry and the processing method used in electrode slurry preparation, the binder distribution can vary significantly, and it can produce serious barriers for mass transport and charge transfer within the composite electrode (Figure 1). 19 In general, one may postulate three different possibilities to connect lithium storage metal particles, such as silicon particles with the binder. The first one, illustrated in Figure 2a, is a simple physical “sticking function” that connects the single silicon particles with each other (and with the current collector and the conductive additive). One may describe the binding mechanism as “particles that are stuck together with a flexible paste”. This type of binding mechanism can be found for instance with the polyvinylidene difluoride (PVdF) binder. During the volume expansion of the silicon particles due to lithiation, mechanical stress in the electrode will occur. The silicon particles will be pushed and pulled in several directions during the lithiation/ delithiation processes. As the “binder glue” is not able to compensate these extreme tensions, the expanded silicon particles will be separated from each other, resulting in a (partial) isolation of the particles. From Sn-based materials particle fracture and particle movement within the composite electrode are well-known as a result of lithiation and large volume expansion. 21 Another thought leads to a binding approach where the silicon particles are embedded in a flexible binder matrix. This binder matrix consists of a flexible cross-linked network of single polymer units. The silicon particles are no longer stuck together by only weak physical forces, but the formation of strong bonds with a chemical nature occurs between the particles and the binder matrix. These bonds may be even permanent for instance covalent bonds leading to a binding mechanism, shown in Figure 2b. During volume expansion/shrinkage, the flexible binder matrix can compensate for a part of the tensions within the electrode, but the Received: May 12, 2014 Revised: July 31, 2014 Published: August 10, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 10299 dx.doi.org/10.1021/la501791q | Langmuir 2014, 30, 10299-10307