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A 74 wt% silicon composite electrode delivers 1000 cycles with 74% capacity retention against NMC811 cathodes and a cell stack energy density of 212 W h kg−1 in a standard carbonate electrolyte with two simple chemical and process improvements.

Silicon (Si) is a promising anode material for high-energy-density lithium-ion batteries (LIBs), but its short calendar life and poor cycling performance prevent its large-scale adoption. Introduci...

Nanoparticle silicon-graphite composite electrodes are a viable way to advance the cycle life and energy density of lithium-ion batteries. However, characterization of composite electrode architectures is complicated by the heterogeneous mixture of electrode components and nanoscale diameter of particles, which falls beneath the lateral and depth resolution of most laboratory-based instruments. In this work, we report an original laboratory-based scanning probe microscopy approach to investigate composite electrode microstructures with nanometer-scale resolution via contrast in the electronic properties of electrode components. Applying this technique to silicon-based composite anodes demonstrates that graphite, SiOx nanoparticles, carbon black, and LiPAA binder are all readily distinguished by their intrinsic electronic properties, with measured electronic resistivity closely matching their known material properties. Resolution is demonstrated by identification of individual nanoparticles as small as ∼20 nm. This technique presents future utility in multiscale characterization to better understand particle dispersion, localized lithiation, and degradation processes in composite electrodes for lithium-ion batteries.

Silicon oxides (SiOx) have been considered as promising alternatives to pure Si in high energy anodes in lithium-ion batteries (LIBs) due to their improved cycling stability. However, their fundamental lithiation mechanism has not yet been systematically investigated, and potential collateral downsides remain unclear. In this work, we report on the role of oxygen in lithiation/delithiation and solid electrolyte interphase (SEI) formation processes in SiOx thin film model electrodes with different oxygen contents. We show that the SiOx anodes with higher oxygen content experience smaller volume change and form a thinner and more stable SEI, both of which are beneficial for cycling stability. However, these SiOx anodes also show an irreversible lithiation at around 0.7 V attributed to the reduction of Si oxides, leading to lower first cycle coulombic efficiency that is undesirable for practical applications. Overall, these results offer a balanced perspective on the advantages and disadvantages that oxygen brings to Si-based anodes in LIBs.

Silicon (Si) anodes have the potential to greatly improve the energy density of lithium-ion batteries due to the greater specific capacity of Si relative to standard graphite (Gr) anodes (1). However, Si anode implementation is limited by issues such as volumetric expansion during cycling and an unstable solid-electrolyte interphase (SEI). Composite anodes with Si and Gr particles can increase capacity while mitigating Si anode degradation. However, these electrodes can be challenging to characterize (and therefore optimize) due to their multiple components, heterogeneous surfaces, and the complex processes occurring there. Scanning spreading resistance microscopy (SSRM) is a form of scanning probe microscopy that measures local resistivity in a sample. By removing material with the probe tip while measuring resistivity, three-dimensional resistivity maps can be determined (2). For Si-Gr composite anodes, the different resistivities of the electrode components (e.g., Si, Gr, carbon, and lithium polyacrylate binder) allows direct identification through SSRM. 3D nanoscale volumes can then be created for both the whole electrode and the individual components. This presentation will demonstrate 3D SSRM mapping of pristine and cycled anodes with three compositions – Si, Gr, and Si-Gr. By comparing changes in the morphology, thickness, and resistivity in the active material and SEI in each single-component electrode, the component distribution and evolution of the Si-Gr composite anode is better understood. SSRM results will be compared to chemical analysis techniques such as scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and electron energy loss spectroscopy to verify the component identification and to correlate chemical composition to resistivity. These data will then be correlated to electrochemical performance and used to further the development of Si-Gr electrodes with enhanced performance. W.-J. Zhang, Journal of Power Sources, 196, 13 (2011). C. Stetson, Z. Huey, A. Downard, Z. Li, B. To, A. Zakutayev, C. S. Jiang, M. M. Al-Jassim, D. P. Finegan, S. D. Han and S. C. DeCaluwe, Nano Lett (2020). Figure 1

Due to inherent properties of the silicon (Si)-electrolyte interphase (SEI)—complexity, high reactivity and continuous evolution—it remains a poorly understood topic in advanced Si-based Li-ion battery (LiB) research,1,2 and its detailed and real-time analysis is a great challenge. Vibrational spectroscopy, such as Raman and Fourier-transform infrared spectroscopy (FTIR), is one of the most important analytical tools for understanding and quantifying the interfacial chemical reactions. These techniques are used extensively by the battery community in their ex situ form, but developing in situ variants of these techniques and combing in situ analyses from both techniques can provide new insight and help to elucidate the mechanism of interfacial failure in battery systems. We have optimized in situ vibrational spectroscopy methods that show reproducible and stable performance over multiple cycles in terms of both electrochemistry and spectroscopy, which enables us to monitor the surface chemistry and evolution of the SEI. This is essential to gaining a better understanding of the electrochemical performance of Si-based LiBs.3-5 Further, the reactions leading to SEI growth and evolution in Si-based LiBs may be more comprehensively probed by monitoring changes in the gas and liquid phases. To this end, we are developing novel in situ gas chromatography (GC) techniques, which may be applied to both coin-type cells and larger-format pouch cells. Using custom cell designs, we track the evolution of electrolyte and degradation species in the gaseous and liquid6 phases at various stages of battery cycling. We achieve both qualitative identification and quantification through coupled mass-spectrometry (MS) and flame ionization detection (FID). This GC-MS-FID analysis complements our in situ vibrational spectroscopy studies, providing a true multi-modal and multi-phase approach to SEI analysis. Taken together, these studies can provide an in-depth understanding of the underlying chemistry and physics and mechanical explanation of various reactions/interactions within the SEI, thus enabling the rational design of electrolytes and electrodes to stabilize the SEI. Additionally, we have developed advanced techniques to assess the structural and electronic properties of Si-based composite electrodes. Comprehensive characterization of pristine and cycled composite electrode architectures is complicated by several inherent properties. During battery operation, Si-based composite electrodes experience localized lithiation and reaction rates due to heterogeneous conditions and distributions of electrode components. Further, (electro)chemical and structural properties continuously evolve during cycling. Finally, the nanoscale diameter of particles falls beneath the lateral and depth resolution of most laboratory-based instruments. To address these challenges, we have utilized scanning spreading resistance microscopy (SSRM) to image the three-dimensional nanostructure of composite anodes via contrast in the electronic properties of the distinct components.7 Specifically, Si-based composite anode components, such as SiOx nanoparticles, graphite, carbon black, and lithium polyacrylate binder, are all readily distinguished by their intrinsic electronic properties, with measured electronic resistivity closely matching known material properties. In combination with scanning electron microscopy/energy dispersive X-ray (SEM-EDX) and electron energy loss spectroscopy (EELS), we expect to compare the electrical, chemical and structural changes of Si-based composite electrodes before and after cycling. This can provide a fundamental understanding of localized degradation mechanisms, which informs our understanding of SEI formation and evolution. This technique may also be applied to assess heterogeneous aging within the electrode, and is more broadly applicable to other battery systems. In particular, it may be used to better understand particle dispersion, localized lithiation, electrolyte-electrode interphase formation/evolution, and degradation processes in composite electrodes for the development of next-generation batteries. References: Xu, C. Stetson, K. Wood, E. Sivonxay, C. Jiang, G. Teeter, S. Pylypenko, S.-D. Han, K. A. Persson, A. Burrell and A. Zakutayev, ACS Appl. Mater. Interfaces 2018, 10, 38558-38564. -D. Han,* K. N. Wood, C. Stetson, A. G. Norman, M. T. Brumbach, J. Coyle, Y. Xu, S. P. Harvey, G. Teeter, A. Zakutayev and A. K. Burrell, ACS Appl. Mater. Interfaces 2019, 11, 46993-47002. Ha, B. J. Tremolet de Villers, Z. Li, Y. Xu, P. Stradins, A. Zakutayev, A. Burrell and S.-D. Han,* J. Phys. Chem. Lett. 2020, 11, 286-291. Yin, R. Pekarek, C. Stetson, M. Schnabel, E. Arca, S. Harvey, B. Tremolet de Villers, S.-D. Han, S. DeCaluwe, G. Teeter, C. Ban and N. Neale, 2021, under revision. J. Tremolet de Villers, J. Yang, S.-M. Bak and S.-D. Han,* 2021, under revision. Ha, C. Stetson, S. P. Harvey, G. Teeter, B. J. Tremolet de Villers, C.-S. Jiang, M. Schnabel, P. Stradins, A. Burrell and S.-D. Han,* ACS Appl. Mater. Interfaces 2020, 12, 49563-49573. C. Stetson, Z. Huey, A. Downard, Z. Li, B. To, A. Zakutayev, C.-S. Jiang, M. Al-Jassim, D. P. Finegan, S.-D. Han* and S. C. DeCaluwe, Nano Lett. 2020, 20, 8081-8088.

We demonstrate that the addition of CO2 to a standard 1.0 M LiPF6 3:7 wt% ethylene carbonate:ethyl methyl carbonate electrolyte results in the formation of a thinner insoluble solid electrolyte interphase (SEI) that is dominated by the presence of LiF. In contrast, cells without CO2 result in a thicker insoluble SEI layer containing more organic constituents. The CO2 is incorporated in the dimethyl carbonate soluble part of the SEI composed primarily of polymeric poly(ethylene oxide) (PEO) on the surface of a thin inorganic layer. This combination of properties from CO2 addition provides an improved cycling performance through the reduction of irreversible side reactions, leading to higher coulombic efficiency. The results indicate that CO2 incorporates into the SEI and plays a role similar to additives like fluorinated ethylene carbonate and vinylene carbonate with respect to polymeric components.

Nanocomposite Si-graphite electrodes are a viable option for increasing the energy density and life of lithium-ion battery anodes. In comparison to model systems (e.g., Si wafer and Si thin film), Si nanoparticles, ranging in diameter from tens to hundreds of nanometers, have higher rate capacity and improved fracture toughness.1, 2 However, characterization efforts to better understand localized degradation mechanisms and heterogeneous aging behaviors of distinct components—Si, graphite, binder, and conductive carbon—in Si-based nanocomposite anodes through cycling are hindered by the limited information that can be achieved at the nanoscale using traditional microscopy techniques. To date, high resolution, three-dimensional structural visualization of the Si-based composite electrode has relied upon techniques like X-ray nano-computed tomography3, 4, that provide morphological information but not electronic or chemical information. In this work, we report a novel lab-scale scanning probe microscopy-based approach to map the 3-D nanostructure of composite anodes via contrast in their components’ intrinsic electronic properties. By incorporating experimental nanoscale electronic resistivity data through interpolation, volumes can be extracted to represent distinct constituent phases of the composite electrode. This approach presents an opportunity for relatively high throughput, nanoscale characterization of composite anodes and cathodes to investigate phenomena such as particle dispersion in electrode preparation and electrode evolution through cycling. Ultimately, this technique shows future utility for the multiscale understanding of structural evolution and degradation processes within lithium-ion batteries. Figure 1: Interpolated resistivity layers captured within a Si-Carbon(C) composite electrode, processed to extract C-rich and Si-rich volumes. R. C. de Guzman, J. Yang, M. M.-C. Cheng, S. O. Salley, and K. Y. Simon Ng, Journal of Materials Science, 48 (14), 4823-4833 (2013). S.-L. Chou, J.-Z. Wang, M. Choucair, H.-K. Liu, J. A. Stride, and S.-X. Dou, Electrochemistry Communications, 12 (2), 303-306 (2010). O. O. Taiwo, M. Loveridge, S. D. Beattie, D. P. Finegan, R. Bhagat, D. J. L. Brett, and P. R. Shearing, Electrochimica Acta, 253 85-92 (2017). J. Wu, F. Ma, X. Liu, X. Fan, L. Shen, Z. Wu, X. Ding, X. Han, Y. Deng, W. Hu, and C. Zhong, Small Methods, 3 (10), (2019). Figure 1

Silicon (Si) is a promising anode material for high energy density lithium-ion batteries (LIBs) but its poor cycling performance prevents its large-scale adoption. Introducing Mg salt into the electrolyte has shown to form a ternary Li-Mg-Si Zintl phase upon lithiation of Si and improve the cycling stability; however, its formation mechanism and impacts on the solid electrolyte interphase (SEI) are not yet well understood. Herein, we demonstrate the formation of a ternary Li-Mg-Si Zintl phase via a Magnesium (Mg) coated Si thin film electrode, where Mg diffuses into the Si film upon deposition and in the lithiation process. The Zintl phase alters the nature of SEI, suppresses the excess decomposition of electrolyte and improves the capacity retention of the Si anode. This study provides insights into the formation mechanism of ternary Zintl phase and guidelines for the future design of Si anodes.