Stress-Adaptive Binder Based on Shear-Thickening Properties for Highly Expansible LIB Electrodes
Authors: Kwon Ohhyun , Kang Jihyeon , Jang Seohyeon , Choi Seyoung , Eom Hojong , Shin Junhyeop , Park Jongkwon , Nam Inho 
Published: December 2024

In this study, we introduce a novel elastic binder incorporating an automatic stress-control mechanism for Li-ion batteries, particularly focusing on high-capacity electrode materials like silicon (Si) that suffer from significant volumetric expansion during lithiation. This expansion often leads to material degradation and reduced battery life. The design and optimization of elastic binders encapsulating nanoparticles has been one of the attractive approaches to relieving such stress. However, all these types of binders have a possibility of intrinsic detaching under dynamic shear stress conditions from the local surface of the particles in electrode. Although the effective management of the dynamic local stress is one of the essential factors to achieve structural robustness of electrode materials, the real-time automatic change in local stress by binders has not been considered up to now. Therefore, introducing a dynamic stress-adaptive attribute can provide novel insights into the binder characteristics, a domain that remains unexplored to the best of our knowledge.

Herein, we present a new type of binder that automatically controls the stress against changes in local surface shear modulus as a new factor of elastic binder properties. As a proof of concept, we selected starch as the main polymer binder because gelatinized starch is a representative shear-thickening fluid which means that the viscoelasticity of the surface can automatically change when under changes in stress. Starch can also create strong interactions with Si particles, therefore, we hypothesized that these characteristics of starch easily dissipate high mechanical stresses during the iterated reaction with huge volume changes of Si.

To demonstrate this concept of dynamic stress control, the study focused on the shear-thickening property, where polymers automatically amplify their viscoelasticity in response to local variation in surface stress. It incorporates an innovative stress-control mechanism that can adapt to dynamic local stress changes. Upon removal of the impact force, the shear-thickening materials instantly revert to their original elastic state. The shear-thickening properties have broad application prospects in shock absorption, human body protection, and surface polishing owing to their ability to absorb significant amounts of impact energy. However, they have not yet been adopted for microscale electrode protection.

Starch analogs, selected as the primary stress-adaptive elastic binders to validate our approach, exhibited shear-thickening behavior under varying stress levels owing to the unique alignment of their supramolecular bonds. When exposed to forces exceeding their critical shear rate, the starch polymer chains reorient, dissipate stress, and fortify their structure. This behavior suggests that the polymer is more inclined to strengthen than detach from the local surface of the electrode. This mechanism expedites the interactions among the polymer chains, effectively diminishing steric hindrance. Consequently, the stresses induced by the volumetric expansion of individual electrode particles can be mitigated, thereby preventing dynamic local structural breakdown.

Three types of starches with different contents of amylose (linear structure) and amylopectin (branched structure) were prepared for the binders, to compare their shear-thickening characteristics in detail. Commercially available potato starch (amylose with long-chain amylopectin, AMLAP), corn starch (amylose with short-chain amylopectin, AMSAP), and waxy maize (only composed of short-chain amylopectin, SAP) were selected to investigate the effect of the different amylose/amylopectin ratios on the stability of the Si electrode. The starches also have ready availability because it is environmentally benign and have a significantly low cost (0.4-0.5 USD kg-1) compared with commercial binder materials, such as CMC (2-5 USD kg-1) and PAA (10-12 USD kg-1).

The shear-thickening mechanism was comprehensively investigated using deep-learning based MD simulations and in situ TEM analysis, which determined the optimal conditions for effectively limiting dynamic local surface expansion. Among the starch analogs, the amylose and long-chain amylopectin (AMLAP) binder demonstrated improved high-rate capability (1710 mAh g-1 at 5 C) and superior reversible capacity (2025 and 1493 mAh g-1 after 100 and 500 cycles, respectively, at 1 C) with optimal shear-thickening properties. Furthermore, AMLAP exhibited favorable characteristics for affordable large-scale production. Hence, this study clearly demonstrates that the shear-thickening properties of binders can be considered a new factor in fabricating stable electrodes with extremely expandable materials.