Silicon anodes for Li-ion batteries - influence of binder, electrolyte and cathode

Improvements of current Li-ion technology requires new, or modified materials for battery cells. Silicon can potentially store up to 10 times as much lithium per weight compares to the state-of-the art anode material, which is graphite. It is also readily available at a low cost and non-toxic. Silicon cannot be used as the only anode material, as lithiation is associated with a massive expansion. For the next generation Li-ion batteries, socalled Gen 4a and 4b, the aim is to mix a certain fraction of silicon into the anode. Currently, up to 7-12 wt% silicon may be used in commercial anodes. In this project we are working with silicon powder fabricated at Elkem, in a cost-efficient process with a low CO2 footprint. We have studied different water-soluble solvents, both based on cellulose, and on so-called alginates. Cellulose-based binders are increasingly used in battery production in order to avoid the toxic solvents. We have shown that binders produced from alginates, made at the production site of FMC Biopolymers (now Dupont) in Haugesund, Norway, perform as well as the cellulose-based binders for the silicon anodes (SINTEF). Their performance appear to depend much on their molecular weight, and less on the exact composition of the alginate. Similarly, various modifications of alginates have been studies at SINTEF, but still the best results were obtained with commercial alginates. Furthermore, a screening of a range of commercial, water-soluble binders has been performed, of which one, polyacrylamid acid (PAA), turned out to give a very good performance (IFE, published in 2020). SINTEF has performed a series of experiments with different slurry chemistries with water soluble binders. The experiments show a huge variation in the cycling stability for the various slurry chemistries, which also provides valuable information on the mechanisms. The water soluble binders have turned out to be challenging to use in combination with the conventional cathode materials, but also with respect to these, promising results have been obtained with the same methods. We have synthesized cathode materials at NTNUs laboratories, and made small scale batteries by combining these cathodes with anodes made from 60 wt% silicon (from Elkem). The cathode of choice was LiNi0.8Co0.15Al0.05O2(NCA), as best performance was obtained with these. For this combination, a lab-scale battery cell with a lifetime of 245 cycles was demonstrated, which is a good result in view of the high silicon content of the anode. A PhD student at NTNU has focused on optimization of electrolytes for the silicon anodes, made at IFE. Quite early in the project it was verified that the silicon electrodes performed better with electrolytes based on the LiFSI salt, compared to the state-of-the-art salt LiPF6. Electrolytes of a range of concentrations of this salt, up to 10 M, was tested in combination with the silicon anodes. By use of a range of characterization techniques (FIB-SEM, TEM, XPS), differences in the surface films formed for the various electrolytes could be identified. The best-performing was 1M LiFSI, which turned out to have a layered surface film, with an inner layer composed of primarily inorganic components, and an outer primarily of organic. The latter is believed to explain the fact that the surface film appears more flexible, which is an advantage in combination with silicon anodes. Furthermore, this film has a better lithium conductivity, and a higher degree of lithiation is observed for silicon when cycled in this electrolyte. Full cells made from silicon anodes and LiNi0.4Mn0.4CoO2 (NMC) cathodes were also tested, with rather similar results for the LiPF6 based electrolyte compared to the one based on LiFSI. This could be attributed to the fact that the performance of NMC cathodes is slightly reduced with LiFSI electrolytes. Addition of a second salt could mitigate this performance degradation, and gave the best results upon cycling of full cells. During the last period of the project we have conducted full cell experiments with commercial cathodes (NMC) and commercial electrolytes based on LiPF6. The anode was a composite of 25 wt% silicon (Elkem, eSi 400) and graphite (commercial cells have today typically up to 10 wt % silicon or silicon oxide). The experiments were conducted at IFE, and a lifetime of 400 cycles was demonstrated for these cells, with a capacity of 1200 mAh/g for the silicon. The research partners of the project are NTNU, SINTEF and IFE. The industrial partners are Elkem, FMC Biopolymers (later Dupont) and CerPoTech. One Post Doc, one PhD (thesis defended in November 2021) and 7 master students have been affiliated to the project. The research activities related to binders and electrolyte are continued in other projects, including a so-called qualification project supported by the Research Council.

Based on the project results, Elkem has continued the optimization of silicon powders for the Li-ion battery market. For the research partners, research has continued in several directions, including further work on silicon materials, electrolytes for silicon materials, and strategies for improving the cathode performance in combination with LiFSI-based electrolytes. For IFE and SINTEF, the competence built has been developed further in the framework of national research and industrial projects (IPN), as well as in Horizon2020 projects. For NTNU, the research has been continued within the framework of national projects, and several master students have been affiliated to the project. The latter research has also been continued within the framework of a FORNY2020 commersialisation project (?kvalifiseringsprosjekt?), COLSHIELD.

Lithium-ion secondary cells are among the most advanced energy storage systems currently available, and the battery with the highest energy density. However, new materials are urgently required in order to address the need for higher energy density, longer cycle life, and improved safety at a reasonable cost. Silicon as anode material is among the most promising materials having a theoretical specific charge capacity of about 3800 mAh/g. In addition it is a low cost, non-toxic and readily available material. It is however generally agreed that the theoretical capacity of silicon cannot be reached in practical electrodes due to the high volume expansion of silicon during lithiation. Succesive lithiation/delithiation leads to high mechanical strains and destruction of the bulk particles, loss of contact between the particles, detachment from the current collector, and destabilization of solid-electrolyte interfaces and thereby continuous electrolyte decomposition. In addition to limiting the lithiation of the electrodes, the cycling stability may be improved by three main routes i) use of morphologies that can accomodate volume changes better (e.g. nanowires) ii) use only part of the available silicon or use of Si-composites, and iii) optimization of the surface and the interfaces (like binder and electrolyte) to accommodate the volume expansion without loss of capacity during repeated cycles. The proposed project will focus on the latter route for optimization of the silicon based anodes. The research will be conducted in close collaboration between the industry partners, Elkem, which will contribute by their expertise and providing their battery grade silicon materials, FMC with the expertise in binder materials, and the research partners. These are NTNU, which will host one PhD student and one Post Doc, SINTEF, which will have their main focus on binders and cathodes, and IFE which will focus mainly on electrolytes and binders.