Mitigating Metal-dissolution in a High-voltage 15 wt% Si-Graphite‖Li-rich Layered Oxide Full-Cell Utilizing Fluorinated Dual-Additives

An effective solid-electrolyte interphase for stable solid-state batteries

Completing the Picture of the Solid Electrolyte Interphase

Electrolytes, consisting of salts, solvents, and additives, must form a stable solid electrolyte interphase (SEI) to ensure the performance and durability of lithium(Li)-ion batteries. However, the electric double layer (EDL) structure near charged surfaces is still unsolved, despite its importance in dictating the species being reduced for SEI formation near a negative electrode. In this work, a newly developed model was used to illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. Both electrolytes have fluoroethylene carbonate (FEC) as a common additive to form the beneficial F-containing SEI component (e.g., LiF). However, the role of FEC drastically differs in these electrolytes. FEC is an effective SEI modifier for the carbonate-based electrolyte by being the only F-containing species entering the EDL and being reduced, as the anion (PF6-) will not enter the EDL. For the ether-based electrolyte, both the anion (TFSI-) and FEC can enter the EDL and be reduced. The competition of the two species within the EDL due to the surface charge and temperature leads to a unique temperature effect observed in prior experiments: the FEC additive is more effective in modulating SEI components at a low temperature (-40 °C) than at room temperature (20 °C) in the ether-based electrolyte. These collective quantitative agreements with experiments emphasize the importance of incorporating the effect of the EDL in multicomponent electrolyte reduction reactions in simulations/experiments to predict/control the formation of the SEI layer.

There is a growing demand for high-rate rechargeable batteries for powering electric vehicles and portable electronics. Here, we demonstrate a strategy for improving electrode performance by controlling the formation of solid electrolyte interphase (SEI). A composite electrode consisting of hard carbon (HC) and carbon nanotubes (CNTs) was used to study the formation of the SEI at different charging rates in an electrolyte consisting of 1 M NaClO4 in a mixed solvent with ethylene carbonate (EC) and propylene carbonate (PC), as well as fluoroethylene carbonate (FEC) additive. The half-cell method was used to form the SEI at different charging rates (e.g., 1, 10, and 100 A/g). Symmetric capacitor cells were employed to study ion transport properties through the SEI. It was found that the SEI is a primary factor responsible for limiting the capacity of the composite anode material in conventional ester-based electrolytes. The electrode with the SEI formed at 100 A/g exhibited the lowest impedance and delivered nearly twice the capacity of the electrode with the SEI formed at 1 A/g. This significant difference is due to a thin SEI formed at the fast charging rate, as has been observed with ether-based electrolytes. An identical decay rate (0.11 mA h/g per cycle) was observed on the electrodes with SEIs formed at different charging rates in an ester electrolyte. No chemical difference among the three SEI layers was found. However, morphological differences of the SEI layers were observed. This difference is believed to account for the different electrochemical behaviors of the electrodes. This work shows that high charging rates can result in the formation of an optimal SEI layer, contradicting the widely accepted practice of using low charging rates during the SEI formation in alkali-ion batteries.

The formation of a solid electrolyte interphase (SEI) at the electrode/electrolyte interface substantially affects the stability and lifetime of lithium-ion batteries (LIBs). One of the methods to improve the lifetime of LIBs is by the inclusion of additive molecules to stabilize the SEI. To understand the effect of additive molecules on the initial stage of SEI formation, we compare the decomposition and oligomerization reactions of a fluoroethylene carbonate (FEC) additive on a range of oxygen functionalized graphitic anode to those of an ethylene carbonate (EC) organic electrolyte. A series of density functional theory (DFT) calculations augmented by ab-initio molecular dynamics (AIMD) simulations reveal that EC decomposition on an oxygen functionalized graphitic (1120) edge facet through an SN2 mechanism is spontaneous, even in an uncharged cell. Decomposition of EC through an SN2 reaction pathway results in alkoxide species regeneration which is responsible for continual oligomerization along the graphitic surface. In contrast, FEC prefers to decompose through an SN1 pathway, which does not promote alkoxide regeneration. The ability of FEC as an additive to suppress alkoxide regeneration results in a smaller and thinner SEI layer that is more flexible towards lithium intercalation during the charging/discharging process. In addition, the presence of different oxygen functional groups at the surface of graphite dictates the oligomerization products and LiF formation in the SEI.

In lithium ion battery (LIB), silicon is hopeful electrode having a high theoretical capacity of 3579 mAh/g, which is ten times higher than the currently used graphite anode (1). On the other hand, Si-based electrodes suffer from the poor capacity retention due to the large volume changes during the lithium lithiation and delithiation process. The consequence of the volume change will lead to continuous decomposition of the electrolyte, which results in the formation of solid electrolyte interphase (SEI). Herein, the SEI formed based on fluoroethylene carbonate (FEC) electrolyte additive on a Si-based anode was investigated using in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), which have been also used to analyze SEI species on different electrodes (2, 3). The SEI generated on Si-based anode using 1 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 1/2 (v/v)/ vinylene carbonate (VC), (2 wt%) was started to observe at the onset of 1.1 V in the potential region of 1.0 – 1.3 V due to the decomposition of VC, which preferentially reduce prior to EC and EMC (4). Formation of poly (VC), polycarbonates, and Li2CO3 were identified during first cycle discharging. The incorporation of 10 wt% FEC results with the formation of poly (FEC)/(VC), polycarbonates, Li2CO3, alkoxy species and alkyl phosphorous fluorides. Using FEC as an additive with the onset potetial of 1.4 V the SEI formation started at a higher potential region of 1.3 – 2.5 V. In the course of second discharging, incorporation of FEC also impedes the decomposition of both solvent and salt anion reduction. Notable enough, in-situ DRIFTS spectrum due to the impact of FEC additive after five cycles also show the formation of SEI-film species is in fact increase in the lithiation process. DRIFT spectrum of the SEI-film formed over Si-based anode with and without FEC electrolyte at the onset potential is shown in Fig. 1. Fig. 1 (a) The first cycle CV results of silicon-based electrodes with EC:EMC:VC/LiPF6 (black) and EC:EMC:VC + FEC/LiPF6 (blue). (b) DRIFT difference spectra obtained at the onset potential of silicon-based electrodes using (I) EC:EMC:VC/LiPF6 and (II) EC:EMC:VC + FEC/LiPF6based electrolytes.

Extending the cycle-life of state-of-the-art lithium-ion batteries (LiBs) is, amongst other aspects, predicated on optimizing the properties of the solid electrolyte interphase (SEI), such as its stability and resistance.[1] It is formed at the negative electrode during the first cycles by electrolyte decomposition and acts as a passivating layer between the electrode surface and the electrolyte.[2,3] Since the SEI formation is accompanied by an irreversible loss of cyclable lithium, its stability during cycling is crucial for improving the charge/discharge efficiency of LiBs. Furthermore, the arising resistance associated with the SEI formation, predominantly depending on the SEI thickness, its chemical composition, and its lithium-ion conductivity, all impact the rate performance of the anode.[4] Two of the most crucial factors impacting SEI formation are temperature and electrolyte composition (i.e., using electrolyte additives).[5] Additives are added to the electrolyte to inhibit the predominant reduction of other electrolyte constituents and thereby form an SEI with improved stability during cycling.[1,6,7] One of the most prominent additives for LiBs is fluoroethylene carbonate (FEC), which is preferentially reduced compared to common carbonate solvents (e.g., ethylene carbonate, ethyl methyl carbonate) due to its higher reduction potential.[7] In this study, we investigated the influence of the formation temperature between 10 and 60 °C, as well as the concentration of FEC (2 and 20 %wt in EC:EMC 3:7 wt/wt with 1 M LiPF6, Gotion, USA) on the characteristics of a synthetic graphite (SMG-A5, Resonac, Japan) anode, both in SMG/Li half-cells and in Ni-rich NCM/SMG full cells.We quantified the first-cycle irreversible capacity loss (ICL) in SMG/Li half cells with a lithium reference electrode after two 0.1 C formation cycles and compared the FEC containing electrolytes to the baseline- (LP-57) and an LP-572 (1 M LiPF6 in EC:EMC 3:7 wt/wt + 2 %wt vinylene carbonate (VC), Gotion, USA) electrolyte. Employing electrochemical impedance spectroscopy in an SMG/Li half-cell setup with a µ-reference electrode and a free-standing graphite electrode,[8,9] we could further quantify the intercalation resistance (R Int, representing the sum of the charge-transfer and the SEI resistance) after an identical formation protocol at 40 % SOC. Both quantities, the ICL and R Int, increase with increasing formation temperature and higher FEC content.The impact of an increasing R Int on the rate performance was tested in Ni-rich NCM/SMG full cells with a lithium reference electrode, revealing a decreased rate capability and higher susceptibility for lithium plating for cells that were formed at higher temperatures and contained 20 %wt FEC. Additionally, the cycling stability of Ni-rich NCM/SMG full cells at 25 °C was assessed in coin cells. Despite an increased ICL and R Int after formation, cells that were formed at elevated temperatures and contained 20 %wt FEC showed enhanced cycling stability compared to those that were formed at a lower temperature and contained less FEC.Finally, on-line electrochemical mass spectrometry (OEMS) was employed to quantify the evolved gases during the formation of a Ni-rich NCM/SMG full cell containing either of the FEC-containing electrolytes at different temperatures. In accordance with an increased ICL and R Int, a higher formation temperature, as well as a higher FEC content, gave rise to a more pronounced gas evolution during formation.

The electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are well known for increasing the lifetime of a Li-ion battery cell by supporting the formation of an effective solid electrolyte interphase (SEI) at the anode. In this study combined simultaneous electrochemical impedance spectroscopy (EIS) and operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) are employed together with in situ gas analysis (OEMS) to study the influence of VC and FEC on the passivation process and the interphase properties at carbon-based anodes. In small quantities both additives reduce the initial interphase mass loading by 30 to 50 %, but only VC also effectively prevents continuous side reactions and improves anode passivation significantly. VC and FEC are both reduced at potentials above 1 V vs. Li+/Li in the first cycle and change the SEI composition which causes an increase of the SEI shear storage modulus by over one order of magnitude in both cases. As a consequence, the ion diffusion coefficient and conductivity in the interphase is also significantly affected. While small quantities of VC in the initial electrolyte increase the SEI conductivity, FEC decomposition products hinder charge transport through the SEI and thus increase overall anode impedance significantly.

Due to its flame retarding effect, trimethyl phosphate (TMPa) was previously employed to suppress the flammability of the conventional LiPF6-organocarbonates electrolytes used in lithium ion batteries (LIBs). However, the presence of TMPa in conventional electrolytes interferes with the formation of effective solid electrolyte interphase (SEI) on graphite (Gr) electrodes. Consequently, the introduction of TMPa in electrolytes leads to significantly deteriorated electrochemical performance of LIBs. To resolve the incompatibility issue between TMPa and Gr electrode, the concept of localized high-concentration electrolyte (LHCE) was applied to TMPa. In the TMPa based LHCE (E-TMPa for abbreviation), the unique solvation structure of the ion sheaths facilitates the decomposition of anions of the conducting salt. The synergetic decompositions of TMPa and anions create an effective SEI on Gr that allows reversible lithiation/delithiation of the Gr electrodes. However, the SEI formed in E-TMPa only offers sufficient protection for <200 charge/discharge cycles. The Gr||NMC811 (LiNi0.8Mn0.1Co0.1O2) cells using E-TMPa suffer from a sudden capacity loss resulting from solvent co-intercalation and Gr exfoliation due to SEI failure. To further improve the SEI quality, electrolyte additives were introduced into E-TMPa. Among all the studied additives, fluoroethylene carbonate (FEC) stood out as the best electrolyte additive because of the formation of effective SEI in this FEC containing electrolyte (E-TMPa-F). Gr||NMC811 cells using E-TMPa-F achieved a capacity retention of 85.4% after 500 charge/discharge cycles within the voltage range of 2.5-4.4 V, which is significantly higher than that of cells using a conventional electrolyte (75.4%). In addition, the high TMPa content in LHCEs effectively suppressed the flammability of the electrolytes. Meanwhile, because of the reduced flammability and excellent compatibility with electrodes, E-TMPa-F is considered to be a promising electrolyte for constructing high energy-density LIBs with improved safety performance.

Lithium-metal batteries face more challenges than lithium-ion batteries (LIBs) with graphite-based negative electrodes. One significant challenge derives from the highly reactive nature of lithium metal, which can react with electrolytes upon contact to form the solid electrolyte interphase (SEI) and can develop detrimental dendrites upon cycling in contrast to graphite and silicon anodes, which require the application of current/voltage to form the SEI. Studies focusing on lithium metal SEI formation are less frequent than those for other anode materials. Through investigation of several electrolyte formulations including the film forming additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) we note multiple aspects of their respective SEI formation characteristics, as additives are consumed during interphase formation. Compared to VC, FEC shows a dependency on ethylene carbonate for equivalent interphase formation properties. Amounts of inorganic SEI compounds were found to be lowered after use of additives. Organic electrolyte-soluble compounds were not detectable if the additive is still present. This was achieved through an SEI accumulation process, leading to a sufficient analyte amount and complementary analytical methods ranging from quantitative gas analysis, over qualitative electrolyte analysis using NMR and mass spectrometry and solid residue analysis using XRD and quantitative gas chromatography.

The volume expansion/contraction of Si-based anodes during electrochemical lithiation/delithiation cycles causes a loss in mechanical integrity and accrued instability of the solid electrolyte interphase (SEI) layer, culminating into capacity fade. Electrolyte additives like fluoroethylene carbonate (FEC) improve SEI stability, but the associated causes still under debate. This work reveals some of the roles of FEC via post-mortem observations/analyses, operando stress measurements and a comprehensive study of the impedance associated with the formation/evolution of SEI during lithiation/delithiation. Usage of 10 vol.% FEC as electrolyte additive leads to significant improvements in cyclic stability, Coulombic efficiency and facilitates smoother/compact/crack-free surface/SEI, in contrast to the cracked/pitted/uneven surface upon non-usage of FEC. Operando stress measurements during SEI formation reveal compressive stress development, followed by loss in mechanical integrity, upon non-usage of electrolyte additive, in contrast to insignificant stress development associated with SEI formation upon usage of FEC. The EIS model proposed here facilitates good fit with the impedance data at all states-of-charges, with the SEI resistance and capacitance exhibiting expected variations with cycling and the SEI resistance progressively decreasing with cycle number in the presence of FEC. By contrast, in the absence of FEC, severe fluctuations observed with the SEI resistance and capacitance indicate instability.

Solid electrolyte interphase (SEI) formation upon reaction between electrode and electrolyte is an important phenomenon in metal anode-based batteries. Various components such as the nature of the electrode surface, solvent, additive, and salt contribute toward the SEI formation. In this work, we carried out ab initio molecular dynamics (AIMD) simulations on an electrolyte involving three different salts (LiPF6, LiBF4, and LiTFSI) in contact with the Al and LiAl anodes. We also checked the role played by the concentration of salt along with its nature during SEI formation. Different extents of salt decomposition are identified in different cases, and LiTFSI is found to be the most suitable salt leading to an inorganic-component-rich SEI layer. LiBF4 on the other hand decomposes minimally resulting in a solvent derived organic SEI. We highlight the importance of preferential reduction of salt before solvent to form inorganic-rich SEI which is preferable compared to organic SEI. The higher salt concentration is also found to support preferential salt decomposition. Overall, our work signifies the choice of proper salt along with its concentration to tune the SEI growth process in Al anode dual-ion batteries.

Intermetallic (Si, Sn, Sb etc.) electrodes offer significantly higher volumetric and gravimetric energy density compared to the widely used graphite-based electrodes, which make them promising candidates for next generation Li-ion cells for transportation applications. [1] However, numerous studies have demonstrated that the inherently non-passivating behavior of intermetallic electrodes in standard organic carbonate-based Li-ion electrolytes [2] is aggravated by significant volume changes during the charging and discharging processes. Mechanical stresses from volume change lead to particle decrepitation, resulting in electronic isolation of particles, loss of mechanical integrity of composite electrodes and interfacial instability. Particle factures expose fresh electrode surface to the electrolyte during cycling, leading to formation of a thick film of electrolyte reduction products, causing impedance rise, capacity loss and lithium inventory shift in the cell. [3] The solid electrolyte interphase (SEI) layer, which forms at the electrode/electrolyte interface during the initial charge/discharge cycles, is the key component that determines the long-term stability and cycling behavior of negative Li-ion battery electrodes. [4-6] Electrolyte reduction and SEI layer formation on these electrodes usually take place at potentials below 1.8V vs. Li/Li+ and accompany the formation of Li-Me phases, the so-called “Me-Li alloying” process at E <0.8 V vs. Li/Li+. The exact mechanism of the SEI formation processes on Si, Sn, Sb and their alloys with lithium, the SEI composition and the effect on the electrode electrochemical cycling performance is not well understood. Interestingly, model studies on Sn single crystal electrodes in organic carbonate electrolytes revealed a strong correlation between the crystal surface orientation and the SEI composition. [7,8] A similar study of the composition of the SEI on a silicon monocrystal electrode showed strong effects of different SEI formation protocols, presence/absence of intrinsic SiO2, electrolyte composition and impurities e.g., HF. On the other hand, SEI-forming electrolyte additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are known to alter the composition and properties of the SEI on Si and improve the electrode electrochemical performance. This study involves rigorous charcaterization studies of tin, silicon, Si-based alloys, and composite Si-based model electrodes to determine and understand the key thermodynamic and kinetic parameters, which enable their function and operation in Li-ion battery systems. Advanced diagnostic tools are used to gain fundamental insight into mechanism of intermatallic-based electrodes failure. An emphasis is placed on in situ methods that use multiple techniques at the same time e.g., imaging with spectroscopy. The diagnostic experimental strategies involve evaluations of model silicon and other intermetallic model composite electrodes as well as studies of the properties of the individual components and their interfaces in a carefully designed and well defined experimental systems. The interfacial instability of the electrolyte and the uncertainty associated with the formation of a stable solid electrolyte interphase (SEI) are the key problems being addressed. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Applied Battery Research for Transportation (ABR) Program and Award Number DE-EE0006443. References I.H. Son et al., Nat. Commun., 6 (2015) 7393\\C.K. Chan et al., Nat. Nanotechnol., 3 (2008) 31-35.B. Philippe et al., Chem. Mater., 24 (2012) 1107-1115.E. Peled, J. Electrochem Soc, 126, 2047 (1979).D. Aurbach et al., Electrochim. Acta, 45, 67 (1999).J. S. Gnanaraj etal., J. Electrochem. Soc, 154, A185 (2007)R. Qiao et al., Avanced Materials Interfaces, 1, n/a (2014).I. T. Lucas, J. Syzdek, and R. Kostecki, Electrochem Commun, 13, 1271 (2011).

Lithium-ion batteries (LIBs) are key to the transition from fossil fuels towards increased use of renewable energy sources. However, more widespread deployment requires improvements in energy density, cost and cycle-lifetime. Various cathode and anode materials are under consideration for next-generation LIBs, and the interfacial stability of these materials in contact with the electrolyte is a critical consideration. Interface-sensitive operando characterization techniques are thus urgently needed to reveal the reactions occurring in working batteries.1,2 The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC).3 Cross-referencing to cycling data, complementary bulk sensitive fluoresecent yield (FY) XAS measurements, and density functional theory (DFT) calculated spectra enables identification of the electrolyte and SEI species, and the dominant mechanisms of SEI formation. Without FEC present, LiF formation is detected at 0.6 V vs. Li/Li+ prior to significant lithiation of the a-Si, whilst at lower potentials the SEI grows in thickness with an increased contribution from organic components containing -C(=O)O- species. The observed sequential formation of inorganic and organic components is implicated in layering of the SEI. With FEC as an additive we see the onset of SEI formation at much higher potentials (1.0 V vs. Li/Li+), and attribute the improved cycle life seen with this additive to the rapid healing of SEI defects formed during delithiation. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations. References Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229.Weatherup et al. Top. Catal. 2018, 61, 2085.Swallow et al. Nature Commun. 2022, 13, 6070.

The electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are well known for increasing the lifetime of a Li-ion battery cell by supporting the formation of an effective solid electrolyte interphase (SEI) at the anode. In this study combined simultaneous electrochemical impedance spectroscopy (EIS) and operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) are employed together with in situ gas analysis (OEMS) to study the influence of VC and FEC on the passivation process and the interphase properties at carbon-based anodes. In small quantities both additives reduce the initial interphase mass loading by 30–50%, but only VC also effectively prevents continuous side reactions and improves anode passivation significantly. VC and FEC are both reduced at potentials above 1 V vs. Li+/Li in the first cycle and change the SEI composition which causes an increase of the SEI shear storage modulus by over one order of magnitude in both cases. As a consequence, the ion diffusion coefficient and conductivity in the interphase is also significantly affected. While small quantities of VC in the initial electrolyte increase the SEI conductivity, FEC decomposition products hinder charge transport through the SEI and thus increase overall anode impedance significantly.