Reduction of Carbonate Electrolytes and the Formation of Solid-Electrolyte Interface (SEI) in Lithium-Ion Batteries. 1. Spectroscopic Observations of Radical Intermediates Generated in One-Electron Reduction of Carbonates

The capacity fade mechanisms of LiCoO 2 /Si-alloy:graphite pouch cells filled with a 1M LiPF 6 EC:EMC:FEC (27:63:10) electrolyte were studied using galvanostatic cycling, electrochemical impedance spectroscopy on symmetric cells, gas-chromatography and differential voltage analysis. Analysis of the gas generated during the first cycle indicated that FEC reacts at the negative electrode following a 1-electron reduction pathway and other pathways that do not lead to the formation of gaseous products. An analysis of the electrolyte showed that FEC is continuously consumed during the first 80% of the first charge (formation cycle). Typical cells charged and discharged at 40 • C showed a gradual capacity loss for the first 250 cycles followed by a sudden capacity drop associated with a large polarization growth. Analysis of the electrolyte showed that this sudden failure is associated with the depletion of FEC. The capacity loss as well as the consumption of FEC prior to the sudden failure was fitted using a model that includes a time dependence and a cycle number dependence. The time dependence was associated with the thickening solid electrolyte interface at the surface of the negative electrode particles (Si-alloy and graphite) and the cycle number dependence was associated with the solid electrolyte interface repair at the surface of Si-alloy particles during repeated expansion and contraction. Cost reduction and an increase in the volumetric energy density of Li-ion cells can possibly be attained using silicon-based negative electrodes.1-3 While these benefits have been known for many years, commercial implementation remains limited due to silicon's inherent challenges.Si-based electrodes can suffer from particle pulverization upon cycling (e.g. micron sized silicon), 1,4-7 can have loss of electronic contact between particles after repeated expansion and contraction during cycling, 2,8-10 can have high irreversible first cycle coulombic efficiency 11,[12][13][14][15] and can have low coulombic efficiency (compared to graphite electrodes) during cycling. 1,16,17 The root cause of these challenges is the large volume expansion 1,18 of the material during lithiation and subsequent contraction during delithiation. Several issues have been solved through the use of either nanosized Si or nanostructured Si. For instance the use of nanosized Si particles solves some of the pulverization problems encountered by micron sized Si particles. 1,4,11,[19][20][21] However, nanosized Si still suffers from inter-particle contact loss during cycling as well as low coulombic efficiency due to large specific surface area. 1,16In a recent publication, Chevrier et al. 16 presented a rational way to design commercially relevant Si-based negative electrodes. They showed that confining nano-domains of silicon in an alloy matrix suppresses particle pulverization and greatly reduces the surface area of Si exposed to the electrolyte. This approach yields materials with lower first cycle irreversible capacity (IRC) loss, better coulombic e...

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“…21,51,52,67,[72][73][74] Figure 2a shows the reaction mechanism of the alkoxide-catalyzed transesterification of linear carbonates.…”

Section: Resultsmentioning

confidence: 99%

Studies of the Capacity Fade Mechanisms of LiCoO₂/Si-Alloy: Graphite Cells

Petibon

Chevrier

Aiken

et al. 2016

J. Electrochem. Soc.

120

134

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“…[52][53][54] 55 These open-chain carbonate species (and alkoxide anions, generated via RO − + CO 2 equilibrium with ROCO 2 − ) can react further with EC molecules in the electrolyte, initiating anionic polymerization, and causing internalization of the carbonate groups into the polymer chains. Cross-linking of these chains is possible by incorporation of other recombination products, so the outer SEI is a network consisting of interlinked polymer chains and open-chain carbonates (with other products) trapped in this matrix.…”

Section: Discussionmentioning

confidence: 99%

Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells

Gilbert

Shkrob

Abraham

2017

J. Electrochem. Soc.

Self Cite

383

500

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Continuous operation of full cells with layered transition metal (TM) oxide positive electrodes (NCM523) leads to dissolution of TM ions and their migration and incorporation into the solid electrolyte interphase (SEI) of the graphite-based negative electrode. These processes correlate with cell capacity fade and accelerate markedly as the upper cutoff voltage (UCV) exceeds 4.30 V. At voltages ≥4.4 V there is enhanced fracture of the oxide during cycling that creates new surfaces and causes increased solvent oxidation and TM dissolution. Despite this deterioration, cell capacity fade still mainly results from lithium loss in the negative electrode SEI. Among TMs, Mn content in the SEI shows a better correlation with cell capacity loss than Co and Ni contents. As Mn ions become incorporated into the SEI, the kinetics of lithium trapping change from power to linear at the higher UCVs, indicating a large effect of these ions on SEI growth and implicating (electro)catalytic reactions. We estimate that each Mn II ion deposited in the SEI causes trapping of ∼10 2 additional Li + ions thereby hastening the depletion of cyclable lithium ions. Using these results, we sketch a mechanism for cell capacity fade, emphasizing the conceptual picture over the chemical detail. Increasing the energy and power density of Li-ion batteries (LIBs) for transportation applications is desirable for extending driving range and for providing the burst of power required for vehicle acceleration.1-3 Layered transition metal (TM) oxides containing Co, Mn, and Ni are promising high-energy electrode materials, as they exhibit theoretical oxide-specific capacities >200 mAh/g and can be cycled to potentials exceeding 4.50 V vs. Li/Li + . 4,5 However, the performance deterioration of full cells, in which these oxides are paired with a graphite (Gr) negative electrode, increases markedly at potentials exceeding 4.30 V, limiting their wider use as high-energy cathode materials.6,7 Therefore, it is imperative to examine the causes for their performance loss at high voltages. In this study we aim at understanding both the phenomenology and mechanism of capacity loss; mitigation of this loss will be the focus of future articles.Several possible causes for cell performance degradation have been identified over the years. These causes include, but are not limited to (i) TM oxide structure changes leading to voltage fade, 8 (ii) buildup of electrode surface films, especially the solid-electrolyte interphases (SEIs) 9,10 at the negative electrode, leading to Li + inventory depletion and/or impedance rise, 11-13 (iii) decomposition of the electrolyte, [14][15][16] (iv) pitting corrosion of aluminum current collectors, 17-19 (v) binder destabilization leading to delamination of electrode coatings, and (vi) dissolution of electrode-active materials. [20][21][22][23] This last source, especially the dissolution of Mn and its deposition in the negative electrode SEI, has been shown to be particularly detrimental to cell performance. [24][25][26] All steps o...

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“…This layer may also contain some oligomerized products from the 1 e − of EC which initiates with an electrochemically generated radical that then polymerizes with unreacted electrolyte compounds. 27,28 The second region (II) initiates at 1.2 V and is marked by the continual increase in the thickness of the inner layer. This increase can be attributed to Eqns.…”

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confidence: 99%

Evaluation of the SEI Using a Multilayer Spectroscopic Ellipsometry Model

Dufek

2014

ECS Electrochemistry Letters

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A multilayer spectroscopic ellipsometry (SE) model has been developed to characterize SEI formation. The model, which consists of two Cauchy layers, is constructed with an inner layer meant to model primarily inorganic compounds adjacent to an electrode and an outer layer which mirrors polymeric, organic constituents on the exterior of the SEI. Comparison of 1:1 EC:EMC and 1:4 EC:EMC with 1.0 M LiPF 6 shows distinct differences in the two modeled layers. The data suggest that the thickness of both layers change over a wide potential range. These changes have been linked with other reports on the growth of the SEI. In Li-ion batteries (LIBs) the electrolyte has a unique role, not only for transport of Li, but also for the formation of the solid electrolyte interphase (SEI) which provides cell stability and practical existence. 1As a film formed from electrolyte components, SEI growth is related to the reduction of electrolyte components at the anode interface. 2Generally the SEI is viewed as consisting of multiple components with a primarily inorganic inner layer (adjacent to the electrode) with a more organic outer layer (adjacent to the electrolyte).3-8 Spectroscopic ellipsometry (SE), by following changes in the polarization of light as it interacts with surface films, provides an in situ route to follow thickness changes in the SEI. [9][10][11][12][13][14] In typical experiments, the amplitude ratio ( ) and the phase difference ( ) are used to describe the orientation of the reflected light based on Fresnel amplitude reflection coefficients. As the optical properties and thicknesses of surface films change, so do and .To appropriately analyze experimental data, optical models for SE analysis are constructed. A model which uses realistic refraction index (n), extinction coefficient (k) and other measures of physicality such as the number of films present is important when interpreting SE data. When applied to monitoring SEI growth, prior SE works have largely relied on describing the SEI as a single layer. 10,12,14 This is partially due to the desire to obtain a fit with the fewest variables possible. Keeping this tenant in mind, the present work looks to build on prior studies of SEI formation using SE by looking at an optical model for SEI growth which incorporates two layers to more closely align with non-SE studies of SEI composition and growth. 3-8Experimental SE experiments used an M2000V spectroscopic ellipsometer (JA Woollam) and a custom designed cell (Figure 1) which consisted of a fused silica tube (19 mm od) with one end enclosed and the other end capped using a Swagelok vacuum fitting. 12,15 No loss or change in electrolyte appearance was observed during evaporation studies of up to two weeks indicating the cell maintained an inert atmosphere. Two Teflon blocks were used to maintain parallel electrode placement (5.5 mm spacing). This spacing allows acquisition of SE data at angles near the Brewster angle for many solid-liquid interfaces (70-75• ) while minimizing electrolyte volume (nominally 1 mL...

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Reduction of Carbonate Electrolytes and the Formation of Solid-Electrolyte Interface (SEI) in Lithium-Ion Batteries. 1. Spectroscopic Observations of Radical Intermediates Generated in One-Electron Reduction of Carbonates

Cited by 168 publications

References 69 publications

Studies of the Capacity Fade Mechanisms of LiCoO₂/Si-Alloy: Graphite Cells

Studies of the Capacity Fade Mechanisms of LiCoO₂/Si-Alloy: Graphite Cells

Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells

Evaluation of the SEI Using a Multilayer Spectroscopic Ellipsometry Model

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Reduction of Carbonate Electrolytes and the Formation of Solid-Electrolyte Interface (SEI) in Lithium-Ion Batteries. 1. Spectroscopic Observations of Radical Intermediates Generated in One-Electron Reduction of Carbonates

Cited by 168 publications

References 69 publications

Studies of the Capacity Fade Mechanisms of LiCoO2/Si-Alloy: Graphite Cells

Studies of the Capacity Fade Mechanisms of LiCoO2/Si-Alloy: Graphite Cells

Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells

Evaluation of the SEI Using a Multilayer Spectroscopic Ellipsometry Model

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Product

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Studies of the Capacity Fade Mechanisms of LiCoO₂/Si-Alloy: Graphite Cells

Studies of the Capacity Fade Mechanisms of LiCoO₂/Si-Alloy: Graphite Cells