Investigation of Self-healing Binders for Si/Gr-Anode Application

The topic of non-fossil based energy storage has gained considerable significance in recent years and is already an important issue in many areas of daily life, including energy supply and transportation. As a consequence, a variety of concepts were created. With the European Climate Law, the EU aims to reduce its net greenhouse emissions by at least 55% until 2030, while the European Green Deal follows the concept of net-zero greenhouse gas emissions by 2050 [1]. To achieve the goal of a carbon neutral society and reduce the dependency on foreign fuels, the EU has to make a transition from fossil fuels to renewable energy sources. However, the energy these generate must also be stored, in which batteries play a crucial role. Efforts are being made to boost the lithium-ion industry by supporting their sustainable processing and manufacturing. However, the high costs and the availability
active materials and solvents required to produce Lithium-Ion batteries (LiBs) considerably hinder progress. Consequently, the focus lies on developing lower-cost chemistries while aiming for high performance such as Li-ion chemistries [1].

The chemistries currently dominating the research field on the cathode side are lithium nickel manganese cobalt oxide (NMC), as well as lithium nickel manganese oxide (LixNi0.5Mn1.5O4). As for the anode, various types of graphite are being considered [2]. However, there is a growing demand for energy storage devices with high energy densities which can deliver more electrical energy per charge than the currently available LiBs [3]. Since conventional chemistries cannot meet these needs, silicon has been introduced for anodes. Due to its high theoretical capacity (≈ 3579 mAh g-1) and natural abundance, silicon is a promising candidate when it comes to anodes [4]. This is why one of the priorities is to develop high content (> 10%) silicon-based anodes [1]. However, silicone’s massive volume expansion (> 300 %) leads to particle deformation during cycling, which in turn leads to an unstable solid-electrolyte interphase (SEI), loss of electrical contact on electrode level, and poor electrochemical performance. All of this must be addressed to effectively implement the material as an anode [4], [5], [6].

The implementation of self-healing binders as a replacement for current state of the art binders represents a promising approach to overcoming the consequences of the unavoidable volume expansion of silicon during charging [1]. The underlying idea of this thesis was the exploration of flexible and putatively self-healing biomaterials as novel binders. For this purpose, a bioderived material of high self-healing ability was tested, namely the well-known S-layer proteins; in parallel, a well-known biopolymer shellac was tested as binder in the processing of Si/Gr anodes with a target silicon content of 15%. In addition, electrochemical performance tests were carried out in coin cell format. The active materials as well as the slurries were examined more closely by using advanced characterization techniques such as the rheometer and the transmission electron microscope (TEM) to obtain mechanical as well as optical information. The processing of these two materials as binders in Si/Gr anodes with high silicon content proved to be challenging as it resulted in unfavourable adhesion and cohesion forces for the electrodes. These challenges and the unknown interaction mechanism of the implemented binders with the electrolyte were reflected in the electrochemical performance tests and suggest that the choice of such novel biomaterials for flexible anode-layer material still
needs further study.

[1] MF, “BATT4EU SRIA European Partnership,” 2024.
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