Formation of the Solid Electrolyte Interphase on the Graphite Anode in Lithium-Ion Batteries – an Operando Neutron Depth Profiling Study

Apart from increasing the battery size of electric vehicles (EVs), another approach to reduce range anxiety and to achieve mass market penetration is to increase the charging power and thus significantly lower charging times. To date, commercial lithium-ion battery (LIB) cells are almost exclusively based on graphite as anode material.[1] Here, owing to the low potential of the graphite electrode during charge, the charging rate must be limited in order to avoid lithium plating, which can thermodynamically occur in regions of the electrode where the potential drops below 0 V vs. Li+/Li. Plated lithium is very reactive, leading to irreversible reactions with the electrolyte and therefore strongly reduces lifetime and safety of a battery.[2] Studying lithium plating in a battery cell is very challenging since post-mortem diagnostics do not reflect the electrode state during operation, since plated lithium will reintercalate into the active graphite material on a timescale of minutes at room temperature.[3] , [4] Hence, non-destructive operando techniques[5] , [6] are necessary to elucidate and follow lithium plating mechanisms in an operating cell.In this study, we present data collected via Neutron Depth Profiling (NDP) during lithium plating induced by fast charging of a graphite electrode at a rate of 2 C. NDP is a lithium-sensitive, non-destructive technique utilizing the neutron capture reaction of 6Li atoms to quantitatively measure the lithium distribution over the thickness of an electrode by probing the energy loss of charged particles formed by the neutron capture reaction.[7] We employ a recently developed operando NDP cell that shows a cycling performance comparable to a commercial cell even at high charging rates, and at the same time allows charged particles to exit the cell through a thin Kapton® window.[8] We were able to resolve the lithium depth distribution changes during the open circuit voltage (OCV) phase after a constant current charge with a time resolution of 2 min. This exceptionally high time resolution was achieved by combining the high cold neutron flux of 2.7×109 ncm-2s-1 at the NIST Center for Neutron Research (Washington D.C., USA) with a 6Li enriched electrolyte and a 6LiFePO4 cathode, which enhances the signal by an additional factor of 12. In order to achieve lithium plating already at a rate of 2 C, the LiPF6 salt concentration in the EC/EMC = 3/7 (w/w) electrolyte was reduced to 0.3 M.Figure 1 shows the NDP signal intensity as a function of the energy of the detected tritium particles, which translates into a depth profile: signals at an energy of ≈2400 keV represent lithium at the interface between the graphite electrode and the Cu current collector, and signals at decreasing energies represent lithium from increasing depth into the graphite electrode, with signals at an energy of ≈800 keV deriving from the graphite anode / separator interface. It is clearly visible that lithium plating starts towards the separator facing side of the graphite anode, caused by the lithium-ion concentration gradient forming over the thickness of the electrode during fast charging (dark blue profile); during the subsequent OCV phase, lithium re-distributes over the anode thickness, whereby this equilibration process is observed to be completed within ≈24 min.[1] C. Pillot, “The Rechargeable Battery Market and Main Trends 2014-2025”, 32nd International Battery Seminar and Exhibit, 3/9/2015, Presentation.[2] J. C. Burns, D. A. Stevens, J. R. Dahn, J. Electrochem. Soc. 2015, 162, A959.[3] Z. Y. Tang, J. J. Xue, C. Y. Liu, X. G. Zhuang, Acta Phys. - Chim. Sin. 2001, 17, 388.[4] M. Park, X. Zhang, M. Chung, G. B. Less, A. M. Sastry, J. Power Sources 2010, 195, 7904.[5] J. Wandt, P. Jakes, J. Granwehr, R. A. Eichel, H. A. Gasteiger, Mater. Today 2018, 21, 231.[6] R. Bhattacharyya, B. Key, H. Chen, A. S. Best, A. F. Hollenkamp, C. P. Grey, Nat. Mater. 2010, 9, 504.[7] M. Trunk, M. Wetjen, L. Werner, R. Gernhäuser, B. Märkisch, Z. Révay, H. A. Gasteiger, R. Gilles, Mater. Charact. 2018, 146, 127.[8] F. Linsenmann et al., A Lithium-Ion Battery Cell Design for Operando Neutron Depth Profiling, to be submitted. Figure 1

Fast charging of commercial lithium-ion batteries severely compromises long-term cycle durability, particularly in cells using high mass-loading thick electrodes. Such performance decay originates from interfacial kinetic limitations in the graphite anode as follows: (i) a sluggish Li+ desolvation at the electrolyte-graphite interface, (ii) a hindered Li+ diffusion across the solid electrolyte interphase (SEI), and (iii) a restricted Li+ insertion into the graphite, which collectively lead to an undesirable Li plating. Herein, we introduce an ultrathin and uniform MoO2/Mo2C biphasic passivation layer, achieved through a sequential cationic polyelectrolyte-assisted molybdate adsorption approach. The outer MoO2 layer does not only suppress an excessive SEI formation but also stabilizes the electrolyte interface by promoting the formation of Li2O and LiF-rich SEI that are both ionically conductive and chemically robust. The inner Mo2C layer provides a low Li+ adsorption energy (-0.97 eV), a reduced surface diffusion barrier (43 meV), and a high electrical conductivity (∼104 S cm-1), consequently enabling capacitive behavior and fast intercalation kinetics at the edge plane. The biphasic layer-passivated graphite anode delivers a fast-charging capability, reaching the 80% state of charge in just 7.4 min at a current density of 6 C and retaining 78.3% of its initial capacity after 600 fast-charge cycles with a practically viable high areal capacity of 3.2 mAh cm-2. These results represent a notable advancement over previously reported surface-engineered graphite anodes, particularly under industrially demanding conditions including high mass-loading and fast-charging.

In this study, we present a facile formic acid treatment to rejuvenate aged graphite anodes from spent lithium-ion batteries (LIBs) without damaging the electrode structure. This method effectively removes the interfacial blocking layer, improving capacity and rate performance. Our approach contributes to sustainable battery recycling strategies for spent graphite anodes in LIBs.

The development of high–capacity anode materials for lithium ion batteries (LIBs) is a key step toward achieving large–scale energy storage applications for portable electronic devices, electrical vehicles, and hybrid electrical vehicles.1 Since its first discovery, the current commercial LIBs have been using graphite as anode material. Although this carbon–based anode is cheap, abundant and reliable, however it provides a limited theoretical capacity of 372 mAh/g, hence hampering the establishment of high–energy density LIBs to meet huge technological demands in the near future.2 To replace the incumbent anode, intensive research has been devoted on the alloy–based materials such as silicon, germanium, tin, and tin oxide (SnO2). Among these candidates, SnO2 is a promising anode material attributing to its low cost, nontoxicity, low operating potential (0.25 V vs. Li/Li+), high theoretical capacity (782 and 1493 mAh/g for alloying/de–alloying and both alloying/de–alloying and conversion reaction, respectively), and easy synthesis in regard to nanostructures.3 It was widely studied that employing the pristine SnO2 anode for LIBs may face a significant barrier because of a severe volume change during the electrochemical cycle, causing pulverization and cracks and eventually the loss of electrical conduction paths. In the meantime, the new–fresh cracks that are exposed to the electrolyte may form a continuous solid electrolyte interphase (SEI)–filming process resulting in a thickened SEI layer on the surface of SnO2 anode. Consequently, the cell cannot avoid a rapid capacity fading.4 To overcome these problems, we synthesized the rational anode material consisting of hierarchical structure and thin double carbon layers. The hierarchical structures of SnO2 anode are aimed at increasing the active surface area, allowing more Li ions can be stored in the spaces, thus promoting a faster Li ion diffusivity. In addition, thin double carbon layers are aimed at alleviating the volume change during charge-discharge reaction, leading to mechanically keep the structure of electrode stable and enhance the ionic conductivity at the interfaces. As the results, our C@SnO2@C anode demonstrated a significant electrochemical performance such as long cycle life and high reversible capacity, hence postulating a solution for commercialization of Sn-based anode for LIBs in the market. [References] Chen, Y. M.; Lu, Z. G.; Zhou, L. M.; Mai, Y. W.; Huang, H. T. Triple–coaxial electrospun amorphous carbon nanotubes with hollow graphitic carbon nanospheres for high– performance Li ion batteries. Energy Environ. Sci. 2012, 5, 7898–7902. Ko, Y. N.; Park, S. B.; Kang, Y. C. Design and fabrication of new nanostructured SnO2–carbon composite microspheres for fast and stable lithium storage performance. Small 2014, 10, 3240–3245. Thomas, R.; Rao, G. M. SnO2 nanowire anchored graphene nanosheet matrix for the superior performance of Li–ion thin film battery anode. J Mater Chem A 2015, 3, 274–280. Zhu, C. R.; Xia, X. H.; Liu, J. L.; Fan, Z. X.; Chao, D. L.; Zhang, H. et al. TiO2 nanotube@SnO2 nanoflake core–branch arrays for lithium–ion battery anode. Nano Energy 2014, 4, 105–112.

Silicon has a high potential to replace commercial graphite anodes in lithium-ion batteries (LiBs). It can facilitate achieving excellent energy densities because of its high theoretical specific lithiation capacity (Chan et al. 2008). However, major challenges such as rapid capacity fading and low Coulombic efficiency due to the huge volume change (∼300 %) during cycling have seriously hindered its commercialization (Wu et al. 2013). Although nanostructuring has been successful in minimizing volume expansion issues, the electrochemical performance of nano-sized silicon is still limited due to unstable solid-electrolyte interphase, low coating density and overall poor electrical properties due to the higher interparticle resistance (Liu et al. 2014). To tackle those challenges, here we introduce a new concept of post-synthesis spray drying to produce hierarchically structured micro-agglomerates from silicon/carbon (Si/C) composite nanoaggregates synthesized in the gas phase (Amin et al. 2023; Adil Amin et al.).The resulting agglomerates were characterized i) on the powder level by scanning electron microscopy (Fig. 1a) and N2 sorption, ii) on the dispersion level by rheometry and analytical centrifugation, iii) on the electrode level by atomic force microscopy for structure and iv) via electrochemical testing on half-cells for electrochemical performance (Fig. 1b). These results show that electrodes from Si/C supraparticles with the highest concentration of stabilizer exhibit better redispersion stability and excellent first cycle specific discharge capacity as well as better cycling stability as compared to the Si/C composite nanoaggregates (4 wt.% carbon content). Furthermore, compared to the reference electrodes made of nanoaggregates, the more stable supraparticles (3 wt.% stabilizer) showed the highest first Coulombic efficiency. This is due to the reduction of their surface to volume ratio which helps in forming less volume of solid electrolyte interface.To conclude, our investigation suggests how an established industrial process (gas phase synthesis of nanoparticle in a hot-wall reactor) can be combined with scalable one-step spray drying to get dense active materials for LiBs. This enables to fully utilize the new materials’ potential by the right packaging into optimum electrode structures with high performance and longevity. Furthermore, we are of the opinion that this pioneering method can be utilized as a broadly relevant design principle to enhance other (anode) materials that encounter analogous problems of volume expansion. Publication bibliography Amin, Adil; Özcan, Fatih; Loewenich, Moritz; Kilian, Stefan O.; Wiggers, Hartmut; Segets, Doris; Wassmer, Theresa; Bade, Stefan; Lyubina, Julia: Hierarchically Structured Si/C Agglomerates by Spray-Drying. Patent no. PCT / EP 2022/ 052453.Amin, Adil; Loewenich, Moritz; Kilian, Stefan O.; Wassmer, Theresa; Bade, Stefan; Lyubina, Julia et al. (2023): One-Step Non-Reactive Spray Drying Approach to Produce Silicon/Carbon Composite-Based Hierarchically Structured Supraparticles for Lithium-Ion Battery Anodes. In J. Electrochem. Soc. 170 (2), p. 20523. DOI: 10.1149/1945-7111/acb66b.Chan, Candace K.; Peng, Hailin; Liu, Gao; McIlwrath, Kevin; Zhang, Xiao Feng; Huggins, Robert A.; Cui, Yi (2008): High-performance lithium battery anodes using silicon nanowires. In Nature nanotechnology 3 (1), pp. 31–35. DOI: 10.1038/nnano.2007.411.Liu, Nian; Lu, Zhenda; Zhao, Jie; McDowell, Matthew T.; Lee, Hyun-Wook; Zhao, Wenting; Cui, Yi (2014): A pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes. In Nature nanotechnology 9 (3), pp. 187–192. DOI: 10.1038/nnano.2014.6.Wu, Hui; Yu, Guihua; Pan, Lijia; Liu, Nian; McDowell, Matthew T.; Bao, Zhenan; Cui, Yi (2013): Stable Li-ion battery anodes by in-situ polymerization of conducting hydrogel to conformally coat silicon nanoparticles. In Nature communications 4, p. 1943. DOI: 10.1038/ncomms2941. Figure 1

Metal oxide is being researched as a potential replacement for the graphite anode in lithium-ion batteries. This material offers several advantages as an alternative anode for advanced rechargeable batteries, including higher theoretical capacity, superior discharge potential, affordability, natural availability, and safety characteristics [1].Furthermore, when formed as nanoparticles, increased surface area enhances electrode activity and facilitates faster ion transport, which improves lithium mobility in lithium-ion batteries. However, the stability of the nanoparticles is considered a significant challenge.In this work, we present the synthesis conditions and characterization of stable spinel manganese oxide nanoparticles with various sizes and morphologies and high crystallinity [2]. We demonstrate that it is possible to control the size and shape of these nanocrystals under specific conditions by using the polyol process [3]. These nanomaterials were tested in half-cell configurations using lithium metal as the counter and reference electrodes. Figure 1 (a) presents the TEM images of Mn₃O₄ nanocrystals, which exhibit uniform particle sizes in the range of approximately 7–10 nm. A nanocomposite anode was fabricated by combining these Mn₃O₄ nanoparticles with graphite. Figure 1 (b) displays the first formation cycle of the composite anode, demonstrating enhanced specific capacity relative to conventional graphite which has specific capacity of 350 mAh/g, indicative of improved electrochemical performance. In this study, we are developing composite anode materials with varying weight percentages of metal oxides and different sizes and morphologies to enhance capacity, rate capability, and stability.

An integrated separator/anode assembly based on electrospinning technique for advanced lithium-ion batteries

3D-structured Co9S8@NSG prepared using deep eutectic solvents as high-performance anode material of lithium-ion batteries

High-temperature treatment induced carbon anode with ultrahigh Na storage capacity at low-voltage plateau

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6 by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by 13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using 29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

Lithium-ion batteries (LIBs) are one of the most promising power sources for electric vehicles because of their high energy density and long lifetime. Graphite is widely used as anode electrode material in the state-of-the-art LIBs. However, a graphite anode is usually only compatible with an ethylene carbonate (EC)-based electrolyte. When contacting with a propylene carbonate (PC)-based electrolyte, which is advantageous over the EC-based one for its wider temperature range and higher low-temperature conductivity, the graphite anode may suffer from substantial exfoliation problem during the initial lithium intercalation step. Consequently, a PC-based electrolyte cannot be used in LIBs with graphite as the anode unless some solid electrolyte interphase (SEI) film-formation additives are introduced into the electrolytes. Usually this type of additives is reduced predominantly on the graphite electrode surface before PC is intercalated and reduced. These additives mainly contain some functional groups such as vinylene or cyclic unsaturated compounds. For example, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) have been widely studied and used as additives for PC-based electrolytes. They play an important role in the protection of the structure of the graphitic anode from destruction by PC, but they also have some shortcomings. Usually these additives result in a thick SEI protective layer, which could significantly reduce the rate capability and limit low-temperature performance and cycling stability at elevated temperatures because of its additional impedance and poor thermal stability. Recently, we developed a novel electrolyte additive that can suppress the PC reductive decomposition on the graphite anode without a thick and resistant SEI film. The additive may provide a novel protective mechanism for the graphite anode in the PC-containing electrolytes without or with suppressed SEI film formation. The SEI-film-suppression additive can not only significantly improve the compatibility between graphite anode and PC-containing electrolyte, but also exhibit superior rate capability and excellent cycling stability at elevated temperatures.We believe that this new electrolyte additive can be applied to LIBs and other electrochemical systems using PC-containing electrolytes and graphite anodes. The details of the results will be reported. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technology of the U.S. Department of Energy (DOE). The electron microscopy and XRD measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL.

Lithium-metal batteries, owing to their remarkable energy density, represent a promising solution for future energy storage needs. However, the widespread adoption of lithium-metal batteries has been impeded by the inherent instability that exists between lithium metal and traditional liquid lithium electrolytes, initially designed for graphite anodes in lithium-ion batteries. Recent insights underscore the efficacy of electrolyte engineering as a strategic avenue to realize the potential of lithium-metal batteries. A notable approach involves the fluorination of solvent molecules, particularly those of the ether class. Nonetheless, a comprehensive understanding of the various factors governing solvent molecular design remains elusive. Here, we examine four solvents derived from 1,2-dimethoxylethane (DME) via molecular dynamics simulation. These solvents are engineered with the introduction of additional alkyl groups or through fluorination. We particularly scrutinize two critical facets: steric effects, arising from the incorporation of bulkier alkyl chains, and electronic effects, originating from fluorination. Our inquiry delves deeply into the stability, ion transport characteristics, and solvation behavior exhibited by these five distinct solvents. Our study underscores the profound impact of adjusting the steric and electronic attributes of solvent molecules on Li+ solvation behavior. This, in turn, influences the coordination strength and the mode of association between Li+ and solvation sites within the first solvation shell, providing key insights into the disparities in ion transport properties within electrolytes.

A structural descriptor XBP was proposed to establish the relationship between the molecular structure of the precursor and the kinetics of lithium storage, providing a new strategy for the design of fast-charging anodes.

A flexible anode was developed with graphite as active material for improving the rate capability of lithium-ion batteries (LIB) by targeting the choice of secondary solvents that are immiscible with primary solvents (aqueous). The ink consists of graphite as active material along with C-45 carbon black as conductive additive and carboxymethyl cellulose (CMC) as aqueous binder along with styrene-butadiene rubber (SBR). Octanol was chosen as secondary solvent as it is immiscible in water (primary solvent). Half-cell LIBs were assembled with graphite electrodes with 1 vol% octanol (Oct-1) and control electrode with 0 vol% octanol (Oct-0). Rate performance and capacity retention tests at different current rates were performed for Oct-1 and Oct-0 graphite electrodes. Oct-1 graphite electrodes delivered an enhanced specific capacity of 145 mAh/g when compared to Oct-0 graphite electrode with specific capacity of 115 mAh/g at current rate of 4C.

Organic carbonyl compounds have been widely investigated as a promising candidate for graphite anodes in lithium-ion batteries (LIBs) because of their high capacity, low cost, and diverse structure. Here, a multicarbonyl polyimide derivative bearing an anhydride with benzoquinone was designed and synthesized by a simple hydrothermal method. Integrating flexible structural design with careful regulation of polymerization conditions, the polyimide derivative exhibits an unprecedented three-dimensional microsphere-like superstructure constructed by interconnected two-dimensional nanosheets, which is beneficial to trigger interfacial energy storage and improve storage capacity. When explored as an anode material for LIBs, the polyimide derivative delivers an outstanding reversible capacity (992 mAh/g at 100 mA/g), good rate capability (268 mAh/g at 2 A/g), and excellent cycle stability (465 mAh/g after 150 cycles at 0.5 A/g). Based on density functional theory calculations and experimental results, a 24 Li-storage mechanism with a five-step lithiation/delithiation process is proposed.