|SEI live formation on a graphite surface in electrolyte. Credit: ACS, Cresce et al. Click to enlarge.|
A team at the US Army Research Laboratory in Adelphi, Maryland reports on a diagnostic approach to study and to study and to characterize the solid electrolyte interphase (SEI) on graphitic anodes of Li-ion batteries. A paper on their work, which combines in situ AFM (atomic force microscopy) with ex situ XPS (X-ray photoelectron spectroscopy), is published in the ACS journal Nano Letters.
The SEI plays a critical role in electrochemical reversibility and cell chemistry kinetics of Li-ion batteries; however, it is not well understood due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative tools to characterize it, the authors note.
In state-of-the-art lithium ion batteries, the graphitic anode typically operates at potentials (−3.03 V vs SHE) where nearly no organic electrolyte component could remain thermodynamically stable against electrochemical reduction. To enable reversible Li+ intercalation/deintercalation with graphitic anodes, electrolyte solvents are chosen to maximize lithium salt solubility and Li+ conductivity, but more importantly for their ability to stabilize the electrolyte−graphite interface through the formation of a new subcomponent.
This new phase, consisting of sacrificial reduction products from the electrolyte solution, prevents continuous electrolyte decomposition and is known as the solid electrolyte interphase (SEI) due to its electrolyte-like nature. The physical and chemical properties of the SEI not only ensure the reversibility of Li+ intercalation at the graphitic anode but also strongly influence the kinetics of Li+ transport across the electrolyte−electrode interface. The former manifests as cycle/calendar life stability of Li-ion batteries as well as resilience against chemical and thermal degradation; the latter dictates rate capability and power density.
Given the importance of the SEI, numerous efforts are dedicated to understanding its chemistry, physical properties, and formation process. Many aspects of the interphase are still under debate, including the electrochemical potential at which it forms, whether it consists of a singular or multiple components from different mechanisms, or if these components are arranged in a homogeneous manner or distributed hierarchically in a multilayer structure.—Cresce et al.
The Army researchers used in situ electrochemical atomic force microscopy to provide topographic images and quantitative information about the structure, hierarchy, and thickness of interphases as function of electrolyte composition. They complemented this with the ex situ chemical analysis via XPS to form a comprehensive and dynamic picture of interphase formation during the first lithiation cycle of the graphitic anode.
The AFM observations showed that interphasial species start to accumulate along graphite edge sites at ∼1.5 V, a process probably dictated by the intercalation of solvated Li+ and accompanied with early stage solvent reduction.
As the potential became more negative, in situ AFM detected that the interphase actually consists of two distinct parts:
- a soft/polymeric upper layer; and
- a hard/salt-like lower layer, covering both edges and basal planes of graphite.
The organic nature of the soft upper layer and the salt-like nature of the hard under layer was confirmed by complementary ex situ XPS analysis.
Force-displacement spectroscopy enabled an accurate and statistical estimate of the thickness of the soft upper layer, which was measured to be as thick as 480 nm with significant variation in thickness over the sample surface.
This thickness could be reduced through the use of available electrolyte additives, implying that additives might play a role in the earliest stages of SEI formation, the team suggested.
This combined diagnostic approach could potentially serve as a universal quantitative tool to evaluate the properties and quality of interphases formed on different electrodes from diverse electrolytes. It is hoped that this technique, properly applied, will lead to a detailed and predictive understanding of the crucial electrolyte−electrode interaction in all advanced battery chemistries.—Cresce et al.
Arthur v. Cresce, Selena M. Russell, David R. Baker, Karen J. Gaskell, and Kang Xu (2014) “In Situ and Quantitative Characterization of Solid Electrolyte Interphases,” Nano Letters doi: 10.1021/nl404471v