Differentiating the Degradation Phenomena in Silicon-Graphite Electrodes for Lithium-Ion Batteries

Wetjen, Morten; Pritzl, Daniel; Jung, Roland; Solchenbach, Sophie; Ghadimi, Reza; Gasteiger, Hubert A.

doi:10.1149/2.1921712jes

Cited by 154 publications

(203 citation statements)

References 52 publications

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“…Hence, we investigate the use of lithium oxalate as capacity enhancer in combination with silicon/graphite (SiG) electrodes containing 35 wt% nano-Si and 45 wt% graphite. These electrodes, which have been investigated in more detail in a previous study by our group, 25 show a typical first cycle coulombic efficiency of ∼85%. Therefore, we combine them with LNMO cathodes containing 5 wt% lithium oxalate, as the amount of lithium oxalate in these electrodes should largely compensate the irreversible loss during the first cycle.…”

Section: Resultsmentioning

confidence: 97%

“…Previous studies have shown that the amount of consumed FEC correlates linearly with the cumulative irreversible discharge capacity (i.e., the sum of the differences between discharge and charge capacity over a certain amount of cycles), 22 which has recently also been demonstrated for SiG anodes with identical composition. 25 Therefore, the cumulative irreversible discharge capacity for each of the analyzed cells is shown on the y-axis of Figure 5, while the amount of consumed FEC in μmol is shown on the lower x-axis. The dashed lines show different e − /FEC ratios, among them the empirically found 4 e − /FEC (= 0.107 mAh irr /μmol) relationship observed by Jung et al 22 The offset of the e − /FEC ratios on the y-axis in Figure 5 can be explained by considering the following: As the lower cell cutoff potential is restricted to 4.0 V, the potential of the SiG anode is limited to a maximum of ∼0.7 V vs. Li/Li + (assuming a maximum cathode potential of 4.7 V vs Li/Li + ).…”

Section: Resultsmentioning

confidence: 99%

“…The ink (50% solid content) was coated onto copper foil (MTI, United States) using a 150 μm gap bar and dried at 50 • C for 6 h in a convection oven. Silicon-graphite electrodes were prepared from silicon nanoparticles (∼200 nm, Wacker Chemie AG, Germany), graphite, vapor grown carbon fibers (VCGF-H, Showa Denko, Japan) and lithium poly(acrylate) (LiPAA, Sigma-Aldrich, Germany) by a ballmilling routine as described in Wetjen et al 25 Graphite and SiG coatings were punched into 15 mm electrodes. The final loadings and compositions for all electrodes are given in Table I.…”

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confidence: 99%

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Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Solchenbach

Wetjen

Pritzl

et al. 2018

J. Electrochem. Soc.

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In the present study, we explore the use of lithium oxalate as a "sacrificial salt" in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO 2 (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li + . LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to ∼11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (∼1%) compared to cells without lithium oxalate. When CO 2 from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO 2 in the cells. Further analysis (e.g., FEC consumption by 19 F-NMR) indicate that CO 2 is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO 2 has a synergistic effect on the lifetime of full-cells with SiG anodes. Lithium nickel manganese spinel (LiNi 0.5 Mn 1.5 O 4 , LNMO) is a promising cathode material for high energy lithium ion batteries due to its high operating potential around 4.7 V vs. Li/Li + , its high rate capability, structural stability and the absence of cobalt. However, its lower specific capacity (146 mAh/g LNMO ) compared to layered oxide materials (e.g. lithium nickel manganese cobalt oxide (NMC), specific capacity 150-250 mAh/g NMC ) 1 is regarded as a major drawback. In full-cells, the practically achievable capacity of LNMO is even lower, as the formation of the solid-electrolyte interphase (SEI) on the graphite anode consumes active lithium. For many layered oxide cathodes, however, the first cycle irreversible capacity of the cathode is similar to the capacity needed for SEI formation (∼20 mAh/g NMC ), and hence the practical discharge capacity of the cathode and the remaining active lithium are more or less balanced again. 2,3 In contrast, the first cycle irreversible capacity of LNMO (∼6 mAh/g LNMO ) is much lower than the capacity needed for SEI formation. This leads to a mismatch between active lithium and practical cathode capacity, i.e., there is not enough active lithium available to fully discharge the LNMO cathode during subsequent cycling.In cells with silicon-based anodes, active lithium losses on the anode are even higher compared to graphite, as the expansion of the silicon particles during the first lithiation leads to a continuous exposure of fresh, unpassivated silicon surface. 4 On this new surface, electrolyte reduction occurs instantaneously, which reduces the total lithium reservoir in the cell. Therefore, different ideas to increase the amount of active lithium in lithium ion full-cells have been suggested, ...

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Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

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Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Solchenbach

Wetjen

Pritzl

et al. 2018

J. Electrochem. Soc.

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“…39 The LiPAA was prepared by diluting a 35 wt% poly(acrylic acid) solution (PAA, MW = 250,000 g mol −1 , Sigma-Aldrich, Germany) with deionized water and neutralizing it with lithium hydroxide (LiOH, Sigma-Aldrich, Germany) to a pH-value of ∼8. 43 The theoretical areal capacity of these electrodes was 1.8-1.9 mAh cm −2 (referenced to the theoretical specific capacities of 3579 mAh g (∼1280 mAh g −1 electrode ) could be utilized at a C-rate of 0.1 h −1 .…”

Section: Methodsmentioning

confidence: 99%

“…By use of scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS), we investigate the morphology and chemical composition of the silicon particles post mortem after different number of charge-discharge cycles in a SiG//LiFePO 4 pseudo-full cell setup (i.e., with a capacitively largely oversized cathode of ∼3.5 mAh cm −2 ) and a fluoroethylene carbonate (FEC)-based electrolyte. 39 Combining the microscopic characterization with the electrochemical analysis of the cycling stability, the electrode polarization, and the irreversible capacity losses upon cycling allows us to correlate the morphological changes of the silicon particles and the electrode with the observed cycling stability of the SiG electrodes. Electrochemical impedance spectroscopy (EIS) and cross-sectional scanning electron microscopy (SEM) images further complement the discussion by providing additional information about the electrode impedance and the morphological changes of the entire electrode structure as a function of the cycle number.…”

Section: −1mentioning

confidence: 99%

Morphological Changes of Silicon Nanoparticles and the Influence of Cutoff Potentials in Silicon-Graphite Electrodes

Wetjen

Solchenbach

Pritzl

et al. 2018

J. Electrochem. Soc.

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109

135

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Silicon-graphite electrodes usually exhibit improved cycling stability when limiting the capacity exchanged by the silicon particles per cycle. Yet, the influence of the upper and the lower cutoff potential was repeatedly shown to differ significantly. In the present study, we address this discrepancy by investigating two distinct degradation phenomena occurring in silicon-graphite electrodes, namely (i) the roughening of the silicon particles upon repeated (de-)lithiation which leads to increased irreversible capacity losses, and (ii) the decay in the reversible capacity which mainly originates from increased electronic interparticle resistances between the silicon particles. First, we investigate the cycling stability and polarization of the silicon-graphite electrodes in dependence on different cutoff potentials using pseudo full-cells with capacitively oversized LiFePO 4 cathodes. Further, we characterize postmortem the morphological changes of the silicon nanoparticles by means of scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) as a function of the cycle number. To evaluate the degradation of the entire electrode coating, we finally complement our investigation by impedance spectroscopy (EIS) with a gold-wire micro-reference electrode and post-mortem analyses of the electrode structure and coating thickness by cross-sectional SEM. Silicon is among the most promising anode materials for future lithium-ion batteries.1,2 For example, a prismatic hard case cell comprising a silicon-carbon anode with 1000 mAh gand an NMC811 cathode would offer a specific energy of up to ∼280 Wh kgcell . 3 In contrast to state-of-the-art graphite electrodes, where lithium is inserted into the interlayers between the graphene sheets, silicon reacts with lithium and forms Li x Si alloys.4-6 Because the (de-)alloying reaction allows a higher lithium uptake per silicon atom (3579 mAh g −1Si , Li 15 Si 4 ) compared to the intercalation of lithium into the graphite host structure (372 mAh g −1 C , LiC 6 ), silicon offers an about ∼10 times larger theoretical specific capacity. However, while the intercalation chemistry reveals excellent cycling stability with only minor irreversible changes of the graphite's morphology (ca. +10%), 8 the (de-)alloying reaction causes significant morphological and chemical changes to the silicon particles, including (i) a large volume expansion of up to +280% and (ii) repeated breakage and formation of Si-Si bonds, which leads to severe mechanical stress and particle fracturing. [9][10][11][12] Upon continued cycling, these morphological changes cause a rapid capacity decay of silicon-based electrodes, which is largely driven by the electrical isolation of the fractured silicon particles.13-17 Nanometer-sized structures, including nanoparticles and nanowires, were shown to mitigate the mechanical stress which results from volumetric changes during the (de-)alloying reaction.12,18-20 However, there exists a trade-off, because the reduction of the particle size als...

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Optimum Particle Size in Silicon Electrodes Dictated by Chemomechanical Deformation of the SEI

Jin

Guo

Xiao

et al. 2021

Adv Funct Materials

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Nanostructured alloy‐forming anode materials can resist fracture that is caused by extreme volume changes during cycling. However, the higher surface area per unit mass in nanomaterials increases exposure to the electrolyte reduction reactions that form a solid electrolyte interphase (SEI), which implies that capacity loss will increase as particle size decreases. This hypothesis is investigated with composite electrodes using different silicon nanoparticle sizes, and the expected particle size effect is not observed. Instead, there is an optimum particle size where capacity loss per volume is minimized. Finite element modeling demonstrates that the mechanical deformation of the SEI varies significantly with the silicon particle size. Smaller particles lead to the decrease of the tensile hoop strains in the outer portion of the SEI and simultaneously make the overall elastic strains in the inner portion more compressive. These results suggest that the SEI on smaller particles is more resistant to mechanical degradation, even though the higher specific surface areas increase initial SEI formation. The trade‐off between these effects leads to the observed optimum particle size.

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Differentiating the Degradation Phenomena in Silicon-Graphite Electrodes for Lithium-Ion Batteries

Cited by 154 publications

References 52 publications

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Morphological Changes of Silicon Nanoparticles and the Influence of Cutoff Potentials in Silicon-Graphite Electrodes

Optimum Particle Size in Silicon Electrodes Dictated by Chemomechanical Deformation of the SEI

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