Film formation, side reactions and interactions in Si/C negative electrodes in Lithium ion batteries

Abstract

To ensure safe and long-term operation of Li-ion batteries (LIBs), one important key strategy is to understand the underlying aging mechanisms. In order to fulfill the demand of increased energy density, the introduction of high capacity active materi-als like Silicon (Si) is attractive, but this approach is accompanied by severe Si relat-ed aging mechanisms, which have to be investigated in further detail. This work presents the development and implementation of the glow discharge optical emis-sion spectroscopy (GD-OES) depth profiling to analyze aging phenomena in Si/C composites used for Li-ion battery negative electrodes. The GD-OES method revealed a new Si related film formation with parasitic Lithium ion (Li+) consumption. Moreover, GD-OES depth profiling meets high standards with regard to measurement time, reproducibility and depth resolution. These high stand-ards make the GD-OES method highly suitable for Post-Mortem analysis of interfac-es and interphases of battery electrodes. Detailed analysis of the atomic Si emission lines demonstrated that Hydrogen (H/H2) influences the specific emission intensity and the Si depth profiles, which needs to be considered within the correct analysis routine. In contact with ambient molecular oxygen, Si metal forms an oxide layer resulting in the formation of Si-O bonds on the Si particle surface. GD-OES depth profiling and Raman spectroscopy showed the important participation of these surface Si-O bonds to the electrochemical film formation during the initial cycle. The periodic vol-ume change of Si during cell operation leads to further electrolyte decomposition, supplying oxygen (O) for further Si corrosion. GD-OES is capable of tracking the progress of film formation, since the amount and the depth evolution of the film de-pend on external physical parameters, e.g. temperature and state-of-health. Moreo-ver, the elemental ratios of Li, Si and O, introduced by the formed Lithium silicates (LixSiyOz), can be analyzed depth resolved with GD-OES. Similar to the film for-mation on graphite, the film formation on Si/C composites was demonstrated to be partly reversible. Based on the correlation of negative electrode potentials and GD-OES depth profiles, a certain threshold of the negative electrode potential for the stability of the formed film was identified to be of high importance. The periodic vol-ume change of Si not only releases new active Si surface, but also leads to pore clogging in restrained cell housings, introducing uneven electrolyte salt distribution and inhomogeneous aging, since the external current load stays constant during operation. Additionally to the Si corrosion, the Li distribution between Si and Graphite provides a more detailed picture of the material aging behavior. This interaction results from different operating potentials, reaction mechanisms and intrinsic differences in bind-ing nature and interaction energies between the materials. Additional to GD-OES depth profiling, operando neutron diffraction was used to provide insights into the Li distribution in negative electrodes from aged cells. GD-OES depth profiling of pure graphite shows that the redistribution process of Li into graphite depends on the re-laxation time, while the addition of Si to pure Graphite materials results in an addi-tional Li redistribution process from Graphite to Si. The parasitic aging phenomena of Si described in this work need to be solved in order to increase the Si content in negative electrode blend materials, and hence develop advanced high-energy LIBs.