HKUST-1, a Surface Metal-Organic Framework, As a Protective Back-End Coating Material for Si Anode

Abstract

Si, the high-capacity anode for Li-ion battery (LIB), has intrinsic 300% volume changes limiting its commercial application. The volume change leads to particle pulverization that result in loss of electrical contacts. Various nanostructures are proposed to avoid the pulverization, but the commercialization is still a distant future. It has been known that the Al2O3 has demonstrated its ability to enhance electrochemical cycling performance. Recently, a comprehensive mechanistic role of the Al2O3 has been studied. By combining electrochemical and chemical agitation test, two novel mechanisms are proposed: Si agglomeration and a protective role of the Al2O3. LiPF6, the common Li salt of the LIB electrolyte, decomposes and form HF that etches the native oxide layer then form labile Si-H surface. Because of the labile Si-H surfaces, the Si particles agglomerate during the volume changes. The Si agglomeration has a detrimental effect on the cycling performance associated with the loss of electrical contacts. On the other hand, in the presence of the Al2O3, the Al2O3 consumes the HF, protecting the native oxide layer that resists the agglomeration. Thus, the Si particles with Al2O3 are better dispersed. The Al2O3 allows the better Si dispersion during electrochemical cycles, resulting in improved capacity retention. However, this study incorporated the Al2O3 via simple mixing. It is difficult to assume a homogeneous distribution of the additive. The homogeneous distribution of the additive will be crucial in effectively protecting the Si particles. To achieve such distribution of the additive, a back-end coatings are effective methods. The back-end coating methods involve applying coating on a prefabricated electrode surface. The back-end coating is typically done with atomic layered deposition (ALD) or molecular layered deposition (MLD). These techniques require high temperature, vacuum, and large-sized apparatus. To reduce the cost of the back-end coating, surface metal-organic frameworks (MOFs) are utilized in this work. The surface MOFs are a class of MOFs those have a self-limiting reaction and can be a coating layer on carboxylate surface. The surface of Si anode with carboxymethyl cellulose (CMC) binder can act as the substrate for the surface MOFs. Utilizing the carboxylate groups in CMC, a MOF can be coated at back-end of the electrode fabrication. Furthermore, the back-end coating with the surface MOF is done in room temperature, atmospheric condition, and requires only beakers, which significantly brings down the cost of the coating. Since the protective role can be achieved with various inorganic materials, HKUST-1 is chosen as a coating material. HKUST-1 is composed of 1,3,5-benzenetricarboxylate as an organic linker and Cu2+ ion as a metal cluster. Upon coating HKUST-1 on the Si anode, scanning electron microscopy, infrared, transmission electron microscopy, and energy dispersive X-ray spectroscopy are utilized to confirm the uniformity of HKUST-1 coating. The electrochemical performance as expected has improved with the HKUST-1 coating. Upon electrochemical cycling, X-ray photoelectron spectroscopy has been utilized to study the solid electrolyte interphase composition of the electrode. Compared to the ALD or MLD method, surface MOF achieves the goal of improving the Si anode performance with low cost. To the best of authors' knowledge, this work reports, for the first time, the utilization of the surface MOF on the Li-ion battery.