Solid-State NMR Investigation of Electrolyte Effects on Silicon-Graphite Composite Anode: Solid Electrolyte Interphase Formation and Failure Mechanisms.

1,3,2-Dioxathiolane 2,2-dioxide additive in carbonate-based gel polymer electrolyte enables dual-Interface stabilization for high-performance long-cycling sodium metal batteries.

Measuring the pore size and pore-size distributions and exploring the fluid-exchange dynamics between different types of pores in porous materials remains a significant experimental challenge but is critical to understanding catalysis, chromatography, nutrient cycling, and a whole range of geochemical phenomena, including shale gas and tight gas extraction. Here, we present the results of 1D 13C NMR and 2D exchange spectroscopy (EXSY) NMR investigations of a porous silica using supercritical methane (scCH4) as a direct probe of pore size and fluid exchange between pore types. The results show that the 13C chemical shift of scCH4 adsorbed in nanometer-scale silica pores becomes more negative with increasing pore diameter, in agreement with trends reported for gas hydrates, zeolites, MOFs, and clays and other microporous (<10 nm pores) geochemical materials. These 13C chemical shifts follow a natural log–linear trend with pore size in the vacuum-dried porous silicas studied here, allowing one to predict pore size based on the 13C chemical shift in dry silica nanopores. The EXSY and 1D 13C NMR results both show that CH4 exchanges between pore and bulk fluid environments over rate scales from 0.01 to 1 kHz in vacuum-dried samples and that exchange dynamics must be considered when interpreting 13C NMR of pore-adsorbed CH4. Introducing H2O to the system causes the 13C chemical shift to become more negative with increasing H2O content as a result of H2O preferentially filling small pores, forcing CH4 to occupy larger pores that are better connected to the bulk environment. Likewise, the 1D 13C data show a decreased distribution of exchange rates between pore-adsorbed and bulk CH4 in the presence of H2O.

Behaviour of highly crystalline graphites in lithium-ion cells with propylene carbonate containing electrolytes

Given concerns about the low earth abundance of lithium (Li) in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) are a compelling alternative due to the ~2000-fold higher concentration of Ca in the earth’s crust, along with attractive theoretical metrics (2073 mAh/cm3, 1337 mAh/g, -2.87 V vs. SHE) if Ca is used as a metallic anode. When most common battery electrolytes come into contact with Ca, however, a passivating solid electrolyte interphase (SEI) forms that prevents Ca2+ transport,[1] and only a handful of electrolytes have been discovered that allow for reversible Ca plating/stripping to any degree. Ca(BH4)2 in tetrahydrofuran (THF) is one of the more widely-tested electrolytes and a current benchmark given its highest-to-date cycling Coulombic efficiency (CE) of ~95%.[2] Yet, there is limited understanding of the reactions forming the SEI and the resulting degree of SEI stability, which impedes further understanding into this electrolyte’s function. To begin to address these knowledge gaps, this study employs gas chromatography (GC) analysis to track gas evolution markers of SEI formation in Ca(BH4)2/THF across a range of cell conditions and timescales.Both H2 and linear hydrocarbon (RH) gases are detected as the major evolved gas species, with the former predominantly evolving from reaction between Ca(BH4)2 and residual water, and RH products evolving from reaction between Ca and THF. RH evolution is found to be dynamic for cells at rest, with distinct timescales attributed to initial and extended SEI formation and spanning several days. When cells are polarized during Ca plating, RH evolution arrests after a certain plated capacity is reached, signaling the formation of a protective yet functional SEI that continues to be permissive to Ca plating to higher capacities. Yet, this SEI is metastable, as further chemical changes are observed when polarization is relaxed. Given the significant role of water in contributing to H2 evolution, we further systematically investigate the impact of water on SEI formation dynamics and composition. Increased water content is observed to mainly shorten SEI formation timescales, but its impact diminishes over extended cycling, and the SEI composition ultimately remains largely independent of initial water content. This work probes the highly-dynamic reactions between Ca, Ca(BH4)2/THF, and residual water, improving understanding into both the fundamentals of this electrolyte system and the unique methodologies required for its study. While H2 evolution presents severe challenges to the practical implementation of any borohydride-based electrolyte, addition of small amounts of Ca(BH4)2 to other Ca electrolyte systems, if compatible, may offer longer term benefits to the chemical stability of a battery, given the salt’s ability to scavenge trace water and accelerate early SEI formation to passivate the anode at a shorter timescale.[1] Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society 138, 3536-3545 (1991).[2] Wang, D. et al. Plating and stripping calcium in an organic electrolyte. Nature Materials 17, 16 (2017).

Rechargeable aluminum metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric and gravimetric capacities. Commercialization of rechargeable aluminum battery systems has been limited due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal and (ii) positive electrode materials that are (electro)chemically within those electrolytes while providing high capacity and cycle life. Metal-chalcogen (e.g., sulfur, selenium) batteries are among the most promising candidates for low-cost, high-energy-density electrochemical energy storage systems; however, to date, rechargeable aluminum-chalcogen batteries have seldom been explored scientifically or technologically.Here, rechargeable aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were prepared with a chloroaluminate ionic liquid electrolyte (AlCl3:[EMIm][Cl], molar ratio of 1.5:1) and their electrochemical reaction mechanisms and products were studied at a molecular-level with solid-state nuclear magnetic resonance (NMR) spectroscopy. Galvanostatic cycling of both Al-S and Al-Se cells showed similar initial capacities and overpotentials to previously reported systems. For Al-S systems, the higher specific capacity upon charge, compared to discharge, points towards either an unwanted side reaction or deleterious polysulfide-shuttle-like behavior. This phenomenon is heavily prevalent at low current densities and affects reversibility. For Al-Se systems, two different reaction mechanisms that have been previously reported were found to be dependent on the applied current density and the crystallinity of the selenium. We were able to consolidate both reaction mechanisms during a single charge/discharge step, significantly increasing specific capacity and energy density. For both Al-S and Al-Se systems, significant capacity fade was also observed over multiple galvanostatic cycles. Solid-state 27Al single-pulse magic-angle-spinning (MAS) NMR spectra acquired on sulfur electrodes cycled to different states-of-charge reveal the presence of both solid and liquid discharge products. Solid-state 2D 27Al{27Al} multiple-quantum (MQ-)MAS measurements reveal the amorphous nature of the solid discharge product, while 2D 27Al{27Al} exchange spectroscopy (EXSY) NMR spectra reveal chemical exchange between electrolyte-soluble sulfur species coordinating with AlCl4 - and Al2Cl7 - after discharge. For Al-Se systems, solid-state 27Al and 77Se single-pulse MAS NMR measurements also reveal liquid-state reaction products in the electrolyte, leading to loss of active material from the cathode, while revealing compositional changes in the selenium electrode. Scientific perspectives on these aluminum-chalcogen battery chemistries will be discussed. This work also highlights the importance that molecular-level analytical tools, such as solid-state NMR spectroscopy, can have in clarifying electrochemical mechanisms in emerging electrochemical systems.

Silicon is one of the most attractive anode materials for next generation high-capacity lithium-ion batteries (LIBs) due to its high specific capacity. However, the large volume change (up to 300%) of silicon during lithiation/delithiation process leads to coupled mechanical and chemical degradation, such as pulverization of silicon particles, formation of unstable solid-electrolyte interphase (SEI), and loss of electrical connectivity, resulting in high irreversible capacity loss and rapid capacity fading. Pre-lithiation is one of the effective approaches to compensate for the loss of active lithium and extend battery cycle life. In this work, various approaches of pre-lithiation for Si electrodes are systematically studied and compared. Different levels of pre-lithiation dosage are applied to the Si electrodes and the pre-lithiated Si electrodes are evaluated in NMC622/Si full cell configuration. After pre-lithiation, the cell capacity and cycling stability are significantly improved. The impacts of operating voltage window, and N/P ratio on electrochemical performance are also investigated. The morphology of pre-lithiated Si electrodes before and after cycling are also compared via cross section scanning electron microscopy (SEM) to understand the electrode expansion and SEI formation/growth during cycling.

Superior long-term cycling of high-voltage lithium-ion batteries enabled by single-solvent electrolyte

The solid-electrolyte interphase (SEI) is a phase formed on an electrode by the decomposition of an electrolyte containing a lithium salt [1]. The SEI is thought to prevent further decomposition of the electrolyte, while the Li+ conductivity of the SEI enables the electrode reaction involving Li+. The Li metal anode has been investigated as an ideal anode for rechargeable Li batteries due to its negative electrode potential and high specific capacity. The cyclability of the Li anode is known to be related to the morphology of the Li metal during charge and discharge processes. The morphology of Li metal has been considered to be influenced by the properties of the SEI formed on Li metal. Therefore, the study of the SEI is important to improve the cyclability of the Li metal anode. We have previously reported that the SEI formation can be evaluated by monitoring the redox reaction of ferrocenium (Fc+)/ferrocene (Fc) as a redox probe [2-4]. The rate constant for Fc+/Fc is affected by the thickness of the SEI formed on an electrode. In the case of Fc, the SEI formation on a Cu electrode cannot be evaluated because the anodic dissolution of Cu occurs at a potential more negative than the redox potential of Fc+/Fc [5]. The redox potential of nickelocenium (Nc+)/nickelocene (Nc) is known to be about 0.5 V more negative than that of Fc+/Fc [6]. Therefore, it is expected to evaluate the SEI formation on a Cu electrode using Nc as a redox probe. In the present study, the evaluation of SEI formation on a Cu electrode was attempted in an ionic liquid, 1-buty-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) containing LiFSA.An anodic and cathodic current peak were observed at –0.8 V vs. Ag|Ag(I) in the cyclic voltammogram of a Cu electrode in 0.5 M LiFSA/BMPFSA containing 10 mM Nc. The redox peaks were unchanged after keeping the electrode at –1.5 V vs. Ag|Ag(I) for 12 h, whereas some decomposition products were identified in the F 1s spectrum of a Cu electrode kept at –1.5 V vs. Ag|Ag(I) for 3 h, suggesting that the redox reaction of Nc+/Nc is so fast that the thickness of the SEI formed at this potential is insufficient to induce a peak shift [7]. On the other hand, a peak shift was observed at –2.0 V due to an increase in the growth rate of the SEI at the more negative potential. Thus, SEI formation on a Cu electrode was successfully observed using Nc as a redox probe.[1] E. Peled and S. Menkin, J. Electrochem. Soc., 164, A1703 (2017).[2] S. Kato, N. Serizawa, and Y. Katayama, J. Electrochem. Soc., 169, 076509 (2022).[3] S. Kato, N. Serizawa, and Y. Katayama, J. Electrochem. Soc., 170, 056504 (2023).[4] S. Okazaki, N. Serizawa, and Y. Katayama, Electrochemistry, 92, 043006 (2024).[5] N. Serizawa, T. Hisada, and Y. Katayama, Electrochemistry, 92, 043009 (2024).[6] S. Momose, M. L. Thomas, N. Serizawa, and Y. Katayama, PRiME 2024, L02-3825, Honolulu, Oct. 7 (2024).[7] J. D. Gribble and S. Wherland, Inorg. Chem., 29, 1130 (1990).

Electrolyte optimization, such as using fluoride-bearing electrolytes, is regarded as an effective way to improve the cycle performance of lithium metal batteries (LMBs), but the promotion mechanisms of the electrolytes are in controversy due to the lack of quantitative understanding of the reaction products during cycling. Here, taking several fluorinated electrolytes as models, we use mass spectrometry titration (MST) and solid state nuclear magnetic resonance (NMR) techniques to quantify the evolution of dead Li metal, solid electrolyte interphases (SEI) and lithium hydride (LiH) during cycling. Our quantitative results clearly disclose that lithium difluoro(oxalato)borate (LiODFB) is able to inhibit the formation of SEI and LiH while fluoroethylene carbonate (FEC) mainly inhibits the formation of dead Li metal. Furthermore, we surprisingly observe a linear correlation between LiH and SEI formation, whereas the commonly mentioned lithium fluoride (LiF) shows a weak correlation with either dead Li metal or SEI. Guided by the clear failure mechanism, we can provide a reasonable explanation for the synergistic effect with the combination of LiODFB and FEC from a quantitative perspective. We believe that a quantitative insight of electrolytes on the failure mechanism of LMBs will guide us to explore the functional electrolytes to achieve the practical application of LMBs.

As the world is running out of fossil fuels, sustainable energy sources and efficient forms of energy storage are studied nowadays. Hydrogen is an energy carrier and has advantages that it is abundant and does not pollute the environment. Metal hydrides are studied for their reversible hydrogen storage properties. MgH2 is a promising candidate for light-weight hydrogen storage material. It can store up to 7.7 wt-% of hydrogen. However, it suffers from high thermodynamic stability and poor sorption kinetics. To improve its performance, Mg is doped with transition metals (TM). The resulting complex alloys are not fully understood on an atomistic level. This doctoral dissertation therefore examines the following central research questions: Is it possible to distinguish between hydrogen atoms with different metal coordination, Qn = H-MgnTM4-n, where n is the number of Mg atoms in the first coordination sphere. Do hydrogen atoms from one metal-coordination environment move to all other coordination environments? How are the different metal coordinations arranged within the lattice? What is a typical length scale separation between different metal coordinations? A single technique alone is not adequate to characterize these complex materials completely. Therefore, we have used a combination of techniques such as fast Magic Angle Spinning (MAS) solid state Nuclear Magnetic Resonance (ssNMR) spectroscopy, powder X-ray diffraction (XRD) and Neutron diffraction (ND) to elucidate the nanostructure of Mg-based hydrogen-storage materials. Diffraction–based techniques are applicable to materials which possess long-range structural ordering. Long-range ordering is not required for NMR. Information about the local environment of the energy carrier i.e hydrogen (deuterium) is necessary to optimize the hydrogen-storage materials. ssNMR is unique and powerful method in this respect. Advanced pulse sequences can be employed to obtain information about the local environment of the energy carrier itself, directly. Moreover, motion of the energy carrier can be monitored which is essential for hydrogen-storage materials. Chapter 2 gives the theoretical understanding of the methodologies that are used in this study to investigate Mg-TM based hydrides. We have studied gas-phase deuterated Mg0.65Sc0.35 and Mg0.65Ti 0.35 materials. In Mg0.65Sc0.35D2.2, with a novel 2 H double-quantum NMR with 45 Sc irradiation and TRAPDOR NMR techniques, we were able to observe that Mg and Sc are not randomly distributed within the XRD determined coherent lattice, instead we found Mg-rich and Sc-rich clusters. With Two-Dimensional Exchange Spectroscopy (2D Exsy), the length separation between these clusters was found to be within few unit cells. Sc is a valuable element and hence, its neighbor in the periodic table, Ti, was investigated. Mg and Ti are immiscible under equilibrium conditions. Therefore, we have investigated Mg0.65Ti0.35 alloy prepared with non-equilibrium methods, namely ball-milling and magnetron sputtering. Chapter 3 describes MgTi alloy prepared by ball-milling and subsequently deuterated at 175 °C and 70 bars. Both NMR and XRD showed that the alloy, after deuteration, phase separates into MgD2 and TiD2. Additionally, NMR indicates the presence of another TiDy phase that is not visible with XRD. Two- Dimensional Exchange Spectroscopy (2D Exsy) reveals deuterium exchange between the XRD invisible TiDy phase and the MgD2. With One-Dimensional Exsy, a weak temperature dependence is found corresponding to an effective activation barrier for deuterium exchange of approximately 12 kJ/mol. This low effective activation energy is probably the result of a broad deuterium-mobility distribution. Comparing Mg0.65Sc0.35D2.2 and Mg0.65Ti0.35D0.65, we propose that the stabilization of the nano-structure of the later maybe a possible outcome from the coherent coupling of individual crystal lattices of MgD2 and TiD2. XRD homogenous MgTi films can be prepared by rf magnetron sputtering. Chapter 4 elucidates the hydrogen siting and dynamics in Mg0.65Ti0.35 prepared by this method. After gas-phase deuterium loading at room temperature, we did not observe a macro-phase separated TiD2 phase unlike in ballmilled material. We do observe, partly resolved signals of deuterium located in non-conductive domains at tetrahedral Mg4 and, possibly, mixed MgnTi4-n sites (4 ppm), and deuterium at Ti4 sites in conducting TiD2 nanodomains (-29 and -68 ppm). No bulk-TiD2 signal at -150 ppm is observed, in contrast to what we find in ball-milled Mg0.65Ti0.35D0.65, which is largely phase separated. 2D Exsy indicates deuterium exchange between deuterium states resonating at position 4 and –29 ppm, but not with those giving rise to the resonance at –68 ppm. The signal at –68 ppm probably represents deuterium atoms which are stably bound to Ti. The observed deuterium exchange and the reduced Knight shift compared to bulk TiD2 are explained using a model with TiD2 nano-slabs. At temperatures T ?? 300 K, the intensity of the signal from the Mg-rich sites decreases and a new signal appears at –10 ppm. This is a reversible phase transition and the upfield shift indicates that deuterium is in contact with Ti, probably at Mg-Ti interface. Since we study the energy carriers, i.e. hydrogen or deuterium atoms directly with NMR, it is of most importance to know the visibility of all the energy carriers. Chapter 5 investigates the visibility of deuterium atoms in the melt-cast Mg0.65Sc0.35D2.2.The loss of overall signal intensity in 1D Exsy indicate the presence of NMR-invisible or "dark" deuterium states. We explain the invisibility on the basis of second-order quadrupolar line broadening arising from the unequal charge distribution in the mixed co-ordination states, Q n = H-MgnSc4-n, (1 £ n £ 3) where n is the number of Mg atoms in the first coordination sphere. Approximately 30% of deuterium atoms are invisible. With this correction for deuterium visibility, the distribution of Mg and Sc over the metal within the lattice tends to be closer to statistical distribution rather than clustering of Mg and Sc rich domains. Nano-sizing and confinement is another approach employed to overcome the disadvantages of MgH2. Carbon supported MgH2 nano-composites prepared by Mg-melt infiltration is studied in Chapter 6. The susceptibility of the nano-porous carbon support results in broad resonances in 1 H MAS NMR spectra. For highly packed samples at 11.7 T (500 MHz) the NMR visibility is severely affected by the conductivity of the carbon. We show that the problems associated with conductivity and susceptibility of nano porous carbon can be overcome by working at a lower magnetic field, 4.7 T (200 MHz). The NMR visibility at a lower field is not affected by the packing density of the materials, which implies quantitative NMR is still feasible. MgH2 within the nano-pores of carbon are not detected with XRD. Static and MAS 1 H NMR of a series of MgH2 carbon nanocomposites with different MgH2 content, indicate the presence of two hydride phases with different spin-lattice relaxation time and chemical shift. The component with the broad static 1 H NMR lineshape and long relaxation time (~10 2 s) is assigned to bulk MgH2. The second component has a narrower static 1 H NMR lineshape and a shorter relaxation time (~10 -1 s) and is tentatively assigned to a nanophase consisting of MgH2 and Mg(OH)2. The 1 H NMR lineshape of MgH2/Mg(OH)2 nano-phase is narrower than that of bulk-MgH2, which indicates a higher mobility of the hydrogen atoms in the MgH2/Mg(OH)2 nano-phase. We have tried to separately identify 1 H NMR signals from MgH2 and Mg(OH)2 in the nanophase by using 20-kHz MAS. However, the susceptibility broadening by the nanoporous carbon is too strong. Assuming that the length scale of the susceptibility variation might be longer than the typical distance between MgH2 and Mg(OH)2 we have further tried to enhance the chemical resolution in the inhomegeneous local field caused by susceptibility by use of 2D 1 H MAS NMR Exsy. However, 2D Exsy shows only non specific broadening as a function of mixing time. At the timescale of 10 -1 s, there is a complete non-specific spin exchange or hydrogen exchange over the susceptibility broadened resonance. This indicates that the length scale of susceptibility variation is smaller or equal to that of the average distance between MgH2 and Mg (OH)2 spin- or the susceptibility determined NMR chemical shifts. Finally, chapter 7 summarizes the main findings of the study described in previous chapters.

Formation and evolution of the microscopic solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface are less reported and need to be completely understood to overcome the compatibility challenges at the Mg anode-electrolyte. In this paper, SEI evolution at the Mg electrolyte/electrode interface is investigated via an in situ electrochemical quartz crystal microbalance with dissipation mode (EQCM-D), electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR). Results reveal remarkably different interfacial evolutions for the two Mg electrolyte systems that are studied, a non-halogen Mg(TFSI)2 electrolyte in THF with DMA as a cosolvent (nhMg-DMA electrolyte) versus a halogen-containing all-phenyl complex (APC) electrolyte. The nhMg-DMA electrolyte reports a minuscule SEI formation along with a significant Coulomb loss at the initial electrochemical cycles owing to an electrolyte reconstruction process. Interestingly, a more complicated SEI growth is observed at the later electrochemical cycles accompanied by an improved reversible Mg deposition attributed to the newly formed coordination environment with Mg2+ and ultimately leads to a more homogeneous morphology for the electrochemically deposited Mg0, which maintains a MgF2-rich interface. In contrast, the APC electrolyte shows an extensive SEI formation at its initial electrochemical cycles, followed by a SEI dissolution process upon electrochemical cycling accompanied by an improved coulombic efficiency with trace water and chloride species removed. Therefore, it leads to SEI stabilization progression upon further electrochemical cycling, resulting in elevated charge transport kinetics and superior purity of the electrochemically deposited Mg0. These outstanding findings augment the understanding of the SEI formation and evolution on the Mg interface and pave a way for a future Mg-ion battery design.

Lithium–sulfur (Li–S) batteries have become a powerful alternative for lithium-ion batteries due to their high energy density and high specific capacity. However, several problems still hinder its practical application, such as poor electron conductivity, large volumetric expansion, and severe shuttle effect. To overcome these obstacles, a metallic and polar sphere-like Co9S8 with high electron conductivity is designed and prepared. Merited from a large chemical affinity and improved integration with polar polysulfides (LiPSs), Co9S8 enables chemically immobilizing LiPSs effectively with an improved cycle life of Li–S batteries. Besides, the hollow sphere structure can physically block the diffusion of LiPSs into electrolyte. Sufficient internal space can accommodate more active materials and mitigate the volume expansion. The high electron conductivity of Co9S8 facilitates improving of electrochemical reaction kinetics. Thus, serving as host material for Li–S batteries, the Co9S8 electrode exhibits a high initial specific capacity of ∼1200 mAh/g at 0.1 C and the discharge specific capacity retains 570 mAh/g after 100 cycles at 0.5 C.

The effects of different binders, polyvinylidene difluoride (PVdF), poly(acrylic acid) (PAA), sodium carboxymethyl cellulose (CMC), and cross-linked PAA-CMC (c-PAA-CMC), on the cycling performance and solid electrolyte interphase (SEI) formation on silicon nanoparticle electrodes have been investigated. Electrodes composed of Si-PAA, Si-CMC, and Si-PAA-CMC exhibit a specific capacity ≥3000 mAh/g after 20 cycles while Si-PVdF electrodes have a rapid capacity fade to 1000 mAh/g after just 10 cycles. Infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) reveal that PAA and CMC react with the surface of the Si nanoparticles during electrode fabrication. The fresh Si-CMC electrode has a thicker surface coating of SiOx than Si-PAA and Si-PAA-CMC electrodes, due to the formation of thicker SiOx during electrode preparation, which leads to lower cyclability. The carboxylic acid functional groups of the PAA binder are reactive toward the electrolyte, causing the decomposition of LiPF6 and dissolution of SiOx during the electrode wetting process. The PAA and CMC binder surface films are then electrochemically reduced during the first cycle to form a protective layer on Si. This layer effectively suppresses the decomposition of carbonate solvents during cycling resulting in a thin SEI. On the contrary, the Si-PVDF electrode has poor cycling performance and continuous reduction of carbonate solvents is observed resulting in the generation of a thicker SEI. Interestingly, the Lewis basic -CO2Na of CMC was found to scavenge HF in electrolyte.

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm−3 led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm−3 solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI− anions to form LiF from Li+-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth.

Deciphering the degradation mechanisms of nano-Si and micro-SiO anodes in lithium-ion battery full-cells using distribution relaxation times analysis