2.3. The adhesion strength of acrylic acid derivative terpolymer
Due to most of the binders being non-electrochemical active and non-electronic conductivity, it is desirable to develop binders with high adhesion capability to reduce the addition content (χ ) in the electrode to reach high electrochemical performance. As regards TBs, this is more important because the gas production of binders in thermal decomposition at TBs operation temperature may lead to the break of the battery bulk. LA136D is known for its excellent adhesion strength and is used in silicon and sulfur electrode manufacturing in secondary batteries.[29,30] In contrast to the traditional PVDF binder, which interacts with electrode materials via weak Van der Waals forces, LA136D interacts with electrode materials via covalent bonds.[31] The large amounts of high polarity groups (−C≡N, −COONH2, and −COOLi) in the side chain of LA136D act as the adsorption points to bond the electrode materials and current collector. In this work, we fabricated TBs thin film electrode by using LA136D as the binder and studied the electrode adhesion capability through an electrode peel-off test. Figure 4 (a)shows the preparation process of slurry coating and Figure 4 (b) is the prepared thin film cathode. Figure 4 (c) is the schematics of the electrode peel-off test process. Figure 4 (d)shows the peel-off force used to peel the different electrodes. For comparison, we also studied the electrode adhesion ability fabricated by PVDF and SBR. The content of the different binders in the electrode is kept to the same (1wt %), and the electrode thickness is controlled at 100 μm and with similar tap density (~3.5 mg cm−3). From Figure 4 (d), LA136D shows the highest peel-off force of 140 N, SBR shows a medium peel-off force of 98 N, and PVDF shows the lowest peel-off force of 60 N. The peel-off test indicates LA136D is capable to fabricate a mechanically robust electrode even at a low content. Figure 4 (e) is the digital pictures of electrodes after the peeling test. It can be seen that LA136D electrodes have the best surface integrity. This result is consistent with the peel-off force.
Traditional pressed-pellet technology is hard to produce electrodes with non-circular shapes and restricts the shape-accommodation capability of TBs.[32] Herein, it is shown that LA136D thin film electrode can be cut into various shapes randomly, as shown inFigure 4 (f) . In addition, the LA136D electrode also shows excellent handling properties (Figure 4(g) ). The electrode materials do not drop even at a high crinkle angle. It is worth noting that the content of LA136D used in the electrode is 1wt %. The “sticky” property of LA136D can be observed visibly in Figure 4 (h) , it is consistent with the high bonding capability of LA136D that few LA136D can bond to electrode materials efficiently through the high polarity groups.