Completing the Picture of the Solid Electrolyte Interphase

Abstract

Although solid electrolyte interphase (SEI) formation is an important factor in battery performance, difficulties in characterizing the SEI mean that it is still not well understood. In this issue of Joule, Nanda and coworkers utilize tip-enhanced Raman spectroscopy to paint a more complete picture of the SEI on silicon electrodes. Although solid electrolyte interphase (SEI) formation is an important factor in battery performance, difficulties in characterizing the SEI mean that it is still not well understood. In this issue of Joule, Nanda and coworkers utilize tip-enhanced Raman spectroscopy to paint a more complete picture of the SEI on silicon electrodes. Lithium-ion batteries (LIBs) have been commercially available for nearly 30 years. Even though LIBs have allowed for the development of portable electronics, such as laptops and cell phones, and paved the way for electric vehicles, there are still some parts of LIBs that are not well understood. One of these components is the solid electrolyte interphase (SEI). During the charging and discharging of a battery, Li ions are transferred from one electrode to the other and an electrical potential is applied across the battery. In between the two electrodes lies the electrolyte. Most commonly in commercial LIBs, organic molecules such as carbonates and a salt such as LiPF6 are used as the liquid electrolyte. As an electric potential is generated during cycling, the molecules that comprise the electrolyte decompose at the surface of the electrode. These decomposition products then can become attached to the surface of the electrode and form the SEI. At first the formation of the SEI may sound detrimental to the performance of the battery because, by its very nature, it leads to a depletion of electrolyte; however, the formation of SEI plays an important role in the long-term performance of batteries. The SEI is ionically conductive, meaning Li ions are able to pass through the SEI to the electrode, but the SEI is simultaneously electronically insulating, meaning that the passage of electrons is blocked.1Verma P. Maire P. Novák P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries.Electrochim. Acta. 2010; 55: 6332-6341Crossref Scopus (2099) Google Scholar These characteristics of the SEI help to prevent the further decomposition and subsequent depletion of the electrolyte in later cycles of the battery, which in turn leads to reduced capacity loss in the battery. Capacity loss in batteries is a quite familiar phenomenon. When a laptop battery or cell phone battery gets older, it experiences capacity loss and as a result needs to be charged more frequently. Understanding SEI and the role it plays in preventing capacity loss could help to solve one of the modern dilemmas of frequently needing to charge portable electronic devices. Additionally, with the advent of electric vehicles, the consequences of having to charge a car more frequently with time significantly increase the risk of the vehicle running out of charge and becoming stranded far from a charging station. Although the formation of SEI in LIBs with a liquid electrolyte and the critical role that the SEI plays in battery performance are well known, accurately characterizing the SEI, particularly on silicon electrodes, has been a challenge.2Winter M. The Solid Electrolyte Interphase – The Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries.Z. Phys. Chem. 2009; 223: 1395-1406Crossref Scopus (634) Google Scholar During lithiation, Si can undergo volume changes of up to 300%, which can lead to the mechanical failure of the rather rigid SEI.3Kumar R. Tokranov A. Sheldon B.W. Xiao X. Huang Z. Li C. Mueller T. In Situ and Operando Investigations of Failure Mechanisms of the Solid Electrolyte Interphase on Silicon Electrodes.ACS Energy Lett. 2016; 1: 689-697Crossref Scopus (93) Google Scholar As an added complication, the chemical composition of the SEI is dependent on the electrolyte composition that is used. Because there is not a standard electrolyte for LIBs, each combination of electrolyte components results in a different variety of SEI. Additionally, the chemical composition and thickness of the SEI have presented challenges in the characterization of the SEI.1Verma P. Maire P. Novák P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries.Electrochim. Acta. 2010; 55: 6332-6341Crossref Scopus (2099) Google Scholar As a result of these obstacles, there is a lack of consensus among battery researchers as to what the SEI looks like.4Peled E. Golodnitsky D. Ardel G. Eshkenazy V. The sei model—application to lithium-polymer electrolyte batteries.Electrochim. Acta. 1995; 40: 2197-2204Crossref Scopus (190) Google Scholar, 5Zaban A. Zinigrad E. Aurbach D. Impedance Spectroscopy of Li Electrodes. 4. A General Simple Model of the Li−Solution Interphase in Polar Aprotic Systems.J. Phys. Chem. 1996; 100: 3089-3101Crossref Scopus (147) Google Scholar Part of this disagreement stems from the various techniques used to study the SEI. Each technique has its own strengths and limitations, and as a result each provides a different perspective of the SEI. The various techniques used to characterize the SEI are analogous to looking at a painting in a museum. When standing close to a painting, it is possible to see a small section of the painting as well as the brush strokes used to create the image. When standing at a greater distance from the painting, the entire image that the painting depicts becomes visible, but the brush strokes themselves, though they are the building blocks upon which the entire painting is created, are no longer discernable. In the same way, each characterization technique looks at the SEI from a different perspective and therefore is sensitive to certain aspects of the SEI, such as chemical composition or morphology. Though there is value in each perspective, re-constructing the complete picture from the different perspectives has not resulted in a clear conclusive picture of the SEI. For example, to gain a macroscopic view of the SEI, Dahn and coworkers used operando pressure measurements to detect SEI growth and determine the relationship between SEI growth and battery performance.6Louli A.J. Ellis L.D. Dahn J.R. Operando Pressure Measurements Reveal Solid Electrolyte Interphase Growth to Rank Li-Ion Cell Performance.Joule. 2019; 3: 745-761Abstract Full Text Full Text PDF Scopus (104) Google Scholar Focusing in on the building blocks of the SEI, Cui and coworkers determined the average chemical composition of the SEI on Si nanowires using X-ray photoelectron spectroscopy (XPS) and the morphology of the SEI using scanning electron microscopy (SEM).7Chan C.K. Ruffo R. Hong S.S. Cui Y. Surface chemistry and morphology of the solid electrolyte interphase on silicon nanowire lithium-ion battery anodes.J. Power Sources. 2009; 189: 1132-1140Crossref Scopus (529) Google Scholar Looking at the SEI from a different angle, Toney and coworkers utilized X-ray reflectivity (XRR) to characterize the layers of SEI that form on native oxide-terminated Si.8Cao C. Abate I.I. Sivonxay E. Shyam B. Jia C. Moritz B. Devereaux T.P. Persson K.A. Steinrück H.-G. Toney M.F. Solid Electrolyte Interphase on Native Oxide-Terminated Silicon Anodes for Li-Ion Batteries.Joule. 2019; 3: 762-781Abstract Full Text Full Text PDF Scopus (138) Google Scholar As insightful as these and many other studies of the SEI are, they have not provided the up-close view of the chemical components of the SEI that is needed in order to fully understand the SEI. Nanda and coworkers, in this issue of Joule, take a very close look at the SEI formed on amorphous Si thin film electrodes using tip-enhanced Raman spectroscopy (TERS) (Figure 1).9Nanda J. Yang G. Hou T. Voylov D.N. Li X. Ruther R.E. Naguib M. Persson K. Veith G.M. Sokolov A.P. Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman Spectroscopy.Joule. 2019; 3 (this issue): 2001-2019Scopus (77) Google Scholar TERS allows for the nanoscale probing of Raman signals from molecules. The identification of the chemical components of the SEI are possible because each molecule that comprises the SEI has signature Raman and infrared (IR) spectra. As a result, TERS maps the nanoscale chemical composition of the SEI. This is distinct from previous work that determined the average chemical composition of the SEI across a region of the electrode.7Chan C.K. Ruffo R. Hong S.S. Cui Y. Surface chemistry and morphology of the solid electrolyte interphase on silicon nanowire lithium-ion battery anodes.J. Power Sources. 2009; 189: 1132-1140Crossref Scopus (529) Google Scholar Through this mapping of the SEI composition, Nanda and coworkers determine that the SEI has a mosaic structure, meaning that the chemical composition of the SEI is not uniform across the electrode.9Nanda J. Yang G. Hou T. Voylov D.N. Li X. Ruther R.E. Naguib M. Persson K. Veith G.M. Sokolov A.P. Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman Spectroscopy.Joule. 2019; 3 (this issue): 2001-2019Scopus (77) Google Scholar Additionally, because of the heterogeneous composition of the SEI on the surface of the SEI, the authors hypothesize that the layers of the SEI are also not uniform in composition.9Nanda J. Yang G. Hou T. Voylov D.N. Li X. Ruther R.E. Naguib M. Persson K. Veith G.M. Sokolov A.P. Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman Spectroscopy.Joule. 2019; 3 (this issue): 2001-2019Scopus (77) Google Scholar As a result, the SEI model proposed is that of a nanomosaic-multilayer hybrid SEI.9Nanda J. Yang G. Hou T. Voylov D.N. Li X. Ruther R.E. Naguib M. Persson K. Veith G.M. Sokolov A.P. Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman Spectroscopy.Joule. 2019; 3 (this issue): 2001-2019Scopus (77) Google Scholar This new model completes the nanoscale component that was missing in the two SEI models previously proposed by Peled and Aurbach.4Peled E. Golodnitsky D. Ardel G. Eshkenazy V. The sei model—application to lithium-polymer electrolyte batteries.Electrochim. Acta. 1995; 40: 2197-2204Crossref Scopus (190) Google Scholar, 5Zaban A. Zinigrad E. Aurbach D. Impedance Spectroscopy of Li Electrodes. 4. A General Simple Model of the Li−Solution Interphase in Polar Aprotic Systems.J. Phys. Chem. 1996; 100: 3089-3101Crossref Scopus (147) Google Scholar Although this new nanoscale model provides a new perspective of the SEI, as with any technique, there are drawbacks to TERS that may distort the picture that it provides. The nature of TERS requires an ex situ measurement of the electrode. During the process of disassembling the cell, the fragile SEI could be damaged from the exposure to the atmosphere and from the rinsing of the electrode to remove excess electrolyte left behind on the electrode.1Verma P. Maire P. Novák P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries.Electrochim. Acta. 2010; 55: 6332-6341Crossref Scopus (2099) Google Scholar However, the other standard SEI characterization techniques have similar flaws that could also affect the accuracy of their measurements. The insight that the SEI is heterogeneous in composition is not surprising, but conclusively showing the nanoscale heterogeneity is an important and needed component of the SEI picture. TERS has allowed the brush strokes that comprise the SEI to finally be seen. With the insights that Nanda and coworkers have provided, the picture of the SEI has become much more complete. This new insight into the chemical composition of the SEI naturally leads to additional questions. For instance, how do the heterogeneities in the SEI affect the uniformity of lithiation, and is the resistance of the SEI different in areas with varied chemical compositions? In their study, Nanda and coworkers looked at the SEI formation on amorphous-Si. Naturally, questions arise as to whether the SEI formed on graphite, on Li metal, or even on the anode of a Na-ion battery is a mosaic as well. There are many paintings in the world to admire, and similarly there are many varieties of SEI to examine. Nanda and coworkers have provided a tool for the characterization of the SEI and a new set of questions to be asked. Equipped with TERS and these new questions, the SEI in all batteries is one step closer to being understood, and possibly the complete pictures of the SEI for all batteries will turn out to be mosaics. Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman SpectroscopyNanda et al.JouleJune 20, 2019In BriefThis article probes one of the key fundamental factors that determines the safety and life of Lithium-ion batteries known as “solid electrolyte interphase” (SEI). In this work, we provided a mechanistic picture on how the SEI evolves with battery cycling by investigating the SEI chemical composition and topography by using state-of-the-art tip-enhanced Raman spectroscopy (TERS). Full-Text PDF Open Archive