Solid Electrolyte Interface Components Research Articles

Li+ Ion-Dipole Interaction-Enabled a Dynamic Supramolecular Elastomer Interface Layer for Dendrite-Free Lithium Metal Anodes.

The unstable lithium (Li)/electrolyte interface, causing inferior cycling efficiency and unrestrained dendrite growth, has severely hampered the practical deployment of Li metal batteries (LMBs), particularly in carbonate electrolytes. Herein, we present a robust approach capitalizing on a dynamic supramolecular elastomer (DSE) interface layer, which is capable of being reduced with Li metal to spontaneously form strong Li+ ion-dipole interaction, thereby enhancing interfacial stability in carbonate electrolytes. The soft phase in the DSE structure enables fast Li+ transport via loosely coordinated Li+-O interaction, while the hard phase, rich in electronegative lithiophilic sites, drives the generation of fast-ion-conducting solid electrolyte interface components, including Li3N and Li2S. Furthermore, the dynamically resilient DSE network composed of soft and hard phases protects Li anodes from electrolyte corrosion and accommodates volume changes during cycling. All features of the DSE layer synergistically facilitate uniform Li+ deposition and suppress Li dendrite propagation, ensuring a stable and dendrite-free Li anode. Consequently, the symmetric Li||Li cell incorporating the DSE layer achieves cycling stability exceeding 6000 h under 1 mA cm-2 and 1 mA h cm-2 conditions. Furthermore, full cell pairing DSE/Li anode with LiFePO4 (LFP) or high-voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes exhibits high-efficiency Li deposition and cycling stability, even under constrained conditions of limited Li (40 μm) and ultrahigh loading NMC811 cathode (21.5 mg cm-2). This study underscores the effectiveness of the ion-dipole interaction-enabled DSE network in developing stable, high-energy-density LMBs.

Engineering Moderately Lithiophilic Paper-Based Current Collectors with Variable Solid Electrolyte Interface Films for Anode-Free Lithium Batteries.

Compared to traditional lithium metal batteries, anode-free lithium metal batteries use bare current collectors as an anode instead of Li metal, making them highly promising for mass production and achieving high-energy density. The current collector, as the sole component of the anode, is crucial in lithium deposition-stripping behavior and greatly impacts the rate of Li depletion from the cathode. In this study, to investigate the lithiophilicity effect of the current collector on the solid electrolyte interface (SEI) film construction and cycling performance of anode-free lithium batteries, various lightweight paper-based current collectors were prepared by electroless plating Cu and lipophilic Ag on low-dust paper (LDP). The areal densities of the as-prepared LDP@Cu, LDP@Cu-Ag, and LDP@Ag were approximately 0.33 mg cm-2. The use of lipophilic Ag-coated collectors with varying loadings allowed for the regulation of lipophilicity. The impacts of these collectors on the distribution of SEI components and Li depletion rate in common electrolytes were investigated. The findings suggest that higher loadings of lipophilic materials, such as Ag, on the current collector increase its lipophilicity but also lead to significant Li depletion during the cycling process in full-cell anode-free Li metal batteries. Thus, moderately lithiophilic current collectors, such as LDP@Cu-Ag, show more potential for Li deposition and striping and stable SEI with a low speed of Li depletion.

Mechanistic Study on Artificial Stabilization of Lithium Metal Anode via Thermal Pyrolysis of Ammonium Fluoride in Lithium Metal Batteries.

The use of the "Holy Grail" lithium metal anode is pivotal to achieve superior energy density. However, the practice of a lithium metal anode faces practical challenges due to the thermodynamic instability of lithium metal and dendrite growth. Herein, an artificial stabilization of lithium metal was carried out via the thermal pyrolysis of the NH4F salt, which generates HF(g) and NH3(g). An exposure of lithium metal to the generated gas induces a spontaneous reaction that forms multiple solid electrolyte interface (SEI) components, such as LiF, Li3N, Li2NH, LiNH2, and LiH, from a single salt. The artificially multilayered protection on lithium metal (AF-Li) sustains stable lithium stripping/plating. It suppresses the Li dendrite under the Li||Li symmetric cell. The half-cell Li||Cu and Li||MCMB systems depicted the attributions of the protective layer. We demonstrate that the desirable protective layer in AF-Li exhibited remarkable capacity retention (CR) results. LiFePO4 (LFP) showed a CR of 90.6% at 0.5 mA cm-2 after 280 cycles, and LiNi0.5Mn0.3Co0.2O2 (NCM523) showed 58.7% at 3 mA cm-2 after 410 cycles. Formulating the multilayered protection, with the simultaneous formation of multiple SEI components in a facile and cost-effective approach from NH4F as a single salt, made the system competent.

Unambiguous molecular characterization of solid electrolyte interphase species on graphite negative electrodes

Performances of lithium-ion batteries (LIBs) are closely related to the control of solid-electrolyte interface (SEI) stability. To decrease the capacity losses linked to the build-up of this interface or potentially reverse such losses, electrolyte formulations have been continuously optimized over years to evaluate how they affect SEI. However, direct molecular characterization of the diverse interphases remains extremely challenging. Herein, we report the molecular imaging of SEI components formed on graphite electrodes by laser desorption ionization (LDI) coupled to ultrahigh-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). By exploiting the surface analysis offered by LDI combined with the molecular formula attributions provided by the FT-ICR MS, it is possible to unambiguously identify/exclude suspected molecules in the SEI such as lithium ethylene dicarbonate and lithium ethylene mono carbonate. Moreover, thousands of unknown species were observed, which could be exploited for further understanding of the surface composition. We believe this methodology to be a critical advance for the design of high-performance LiBs.

In-situ pyrolysis based on alkaline medium removes fluorine-containing contaminants from spent lithium-ion batteries

Pyrolysis is an effective method for removing organic contaminants (e.g. electrolytes, solid electrolyte interface (SEI), and polyvinylidene fluoride (PVDF) binders) from spent lithium-ion batteries (LIBs). However, during pyrolysis, the metal oxides in black mass (BM) readily react with fluorine-containing contaminants, resulting in a high content of dissociable fluorine in pyrolyzed BM and fluorine-containing wastewater in subsequent hydrometallurgical processes. Herein, an in-situ pyrolysis process is proposed to control the transition pathway of fluorine species in BM using Ca(OH)2-based materials. Results show that the designed fluorine removal additives (FRA@Ca(OH)2) can effectively scavenge SEI components (LixPOFy) and PVDF binders from BM. During the in-situ pyrolysis, potential fluorine species (e.g. HF, PF5, and POF3) are adsorbed and converted to CaF2 on the surface of FRA@Ca(OH)2 additives, thereby inhibiting the fluorination reaction with electrode materials. Under the optimal experimental conditions (temperature = 400 °C, BM: FRA@Ca(OH)2 = 1: 4, holding time = 1.0 h), the dissociable fluorine content in BM was reduced from 3.84 wt% to 2.54 wt%. The inherent metal fluorides in BM feedstock hinder the further removal of fluorine with pyrolysis treatment. This study provides a potential strategy for source control of fluorine-containing contaminants in the recycling process of spent LIBs.

Hierarchical porous Co-CoO@NC hollow microspheres with capacity growth by reactivation of solid-electrolyte interface films

Transition metal oxide (TMO)-based electrodes exhibit increased capacities, yet the mechanism behind the true cause of capacity in such materials remains unclear. Herein, hierarchical porous and hollow Co-CoO@NC spheres assembled by nanorods with refined nanoparticles and amorphous carbon have been synthesized by a two-step annealing approach. A temperature gradient-driven mechanism is revealed for the evolution of the hollow structure. Compared with the solid CoO@NC spheres, the novel hierarchical of Co-CoO@NC can fully utilize the interior active material by exposing both ends of each nanorod into electrolyte. The hollow interior provides extra space for the volume variation, leading to an up-trend capacity of 919.3 mAh g−1 at 200 mA g−1 over 200 cycles. Differential capacity curves disclose that solid electrolyte interface (SEI) films reactivation partly contributes to increasing reversible capacity. The introduction of nanosized Co particles benefits the process by participating in the transformation of SEI components. This study provides a guide for constructing anodic material with exceptional electrochemical performance.

In Situ Measurements of the Mechanical Properties of Electrochemically Deposited Li2CO3 and Li2O Nanorods.

Solid-electrolyte interface (SEI) is "the most important but least understood (component) in rechargeable Li-ion batteries". The ideal SEI requires high elastic strength and can resist the penetration of a Li dendrite mechanically, which is vital for inhibiting the dendrite growth in lithium batteries. Even though Li2CO3 and Li2O are identified as the major components of SEI, their mechanical properties are not well understood. Herein, SEI-related materials such as Li2CO3 and Li2O were electrochemically deposited using an environmental transmission electron microscopy (ETEM), and their mechanical properties were assessed by in situ atomic force microscopy (AFM) and inverse finite element simulations. Both Li2CO3 and Li2O exhibit nanocrystalline structures and good plasticity. The ultimate strength of Li2CO3 ranges from 192 to 330 MPa, while that of Li2O is less than 100 MPa. These results provide a new understanding of the SEI and its related dendritic problems in lithium batteries.

Engineering Solid Electrolyte Interface at Nano‐Scale for High‐Performance Hard Carbon in Sodium‐Ion Batteries

AbstractEngineering the structure and chemistry of solid electrolyte interface (SEI) on electrode materials is crucial for rechargeable batteries. Using hard carbon (HC) as a platform material, a correlation between Na+ storage performance, and the properties of SEI is comprehensively explored. It is found that a “good” SEI layer on HC may not be directly associated with certain kinds of SEI components, such as NaF and Na2O. Whereas, arranging nano SEI components with refined structures constructs the foundation of “good” SEI that enables fast Na+ storage and interface stability of HC in Na‐ion batteries. A layer‐by‐layer SEI on HC with inorganic‐rich inner layer and tolerant organic‐rich outer flexible layer can facilitate excellent rate and cycling life. Besides, SEI layer as the gate for Na+ from electrolyte to HC electrode can modulate interfacial crystallographic structures of HC with pillar‐solvent that function as “pseudo‐SEI” for fast and stable Na+ storage in optimal 1 m NaPF6‐TEGDME electrolytes. Such a layer‐by‐layer SEI combined with a “pseudo‐SEI” layer for HC enables an outstanding rate of 192 mAh g−1 at 2 C and stable cycling over 1100 cycles at 0.5 C. This study provides valuable guidance to improve the electrochemical performance of electrode materials through regulation of SEI in optimal electrolytes.

(Invited) Approaches to Understanding and Predicting the Stability of Liquid Electrolytes and the Early Formation of the Solid-Liquid Electrolyte Interphase

Electrolytes with improved stability have the potential to transform the current state-of-the-art Li-ion architectures and enable future technologies such as multivalent systems (e.g., Mg2+, Ca2+ and Zn2+), as well as Li-S and Li-O2 conversion systems. A remaining challenge in the pursuit of rational design of novel and optimized electrolytes is understanding and ultimately predicting the reaction cascade responsible for the creation of a functional liquid-anode solid-electrolyte interface (SEI). We here present an overview of the challenge, coupled with several examples of systems where minority as well as majority speciation of the electrolyte is imperative to understand the origin of the electrochemical window. Furthermore, to address the complexity of the resulting reaction cascade once these species start to decompose, we advocate the need for data-driven approach coupled with a high-throughput quantum chemistry computational infrastructure. To this end, we have constructed a chemical reaction network with tens of thousands of elementary reactions which allow for bond breaking/formation as well as oxidation/reduction reactions. The reaction energetics are obtained from our computational framework which automate geometry optimization and vibrational frequency calculations for molecules, including radical, charged, metal-coordinated, and solvated species. To analyze possible reaction pathways, we apply a graph representation to our network, leveraging an iterative reweighting strategy that solves for all prerequisite branching such that traditional pathfinding algorithms correctly capture coordinated pathways. Machine-learning algorithms operating on bond-formation and breaking energetics aid in rapid evaluation of highly reactive processes. We show that without any apriori chemical intuition, our automated framework can recover favorable reaction paths to form key SEI components, which have been carefully identified over the past two decades. This data-driven approach and infrastructure show promise to accelerate the understanding of chemical reactivity in complex environments, in the aid of novel electrolyte design.

Enhanced Solid Electrolyte Interface (SEI) on Li Metal Surfaces through Catalytic Electrolyte Decomposition

Lithium metal batteries have seen a rise in attention as the next generation battery is sought. Li metal is considered the “Holy grail” anode material with a high theoretical capacity of 3860 mAh g-1.[1] However, Li metal is unstable during cycling leading to issues with dendrite formation leading to cell short or “dead” Li, and increase in cell impedance due to the excess formation of SEI on the “dead” Li..[2–4] The SEI plays an important role in altering the ion transport which alters the structure evolution of the Li metal anode during repeated cycling. While the understanding of the fundamental properties of the SEI has advanced continuously in recent years, effectively tuning the SEI components, especially the inorganic constituents, is still challenging. In this work, tungsten oxide, WO 3, is found to promote the formation of inorganic salts e.g., LiF/Li2CO3 in SEI layers and thus enhancing the SEI properties allowing for the smoother plating of Li metal. Additionally, WO3 is reduced to W during electrochemical process, the addition of W within the SEI increases the electronic conductivity, reducing the formation of “dead” Li which otherwise is completely wrapped by insulating SEI layers. The possibility of WO3 to catalyze electrolyte decomposition through favored reaction pathway to produce more robust SEI layers is discussed, providing new insights to control the SEI formation on Li metal. Xu, W., Wang, J., Ding, F., Chen, X., Nasybulin, E., Zhang, Y. and Zhang, J.G., 2014. Energy & Environmental Science, 7(2), pp.513-537.Pei, A.; Zheng, G.; Shi, F.; Li, Y.; Cui, Y. Nano Lett 2017, 17, 1132–1139.Wu, B.; Lochala, J.; Taverne, T.; Xiao, J. Nano Energy 2017, 40, 34–41.Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H. J. Solid State Ionics 2002, 148, 405–416.

Stable Li Metal Anode through Designed Solution Chemistry

Lithium metal is being extensively studied as an anode to replace graphite due to its high capacity and the lowest negative electrochemical potential. Current research focus is to mitigate lithium dendrite growth and to increase coulomb efficiency during electrochemical cycling by using various surface coatings, electrolyte additives.1, 2 In addition, a 3D host can be beneficial due to the reduced volume change.3 We have recently focused on designing novel electrolyte solution chemistry that enables dendrite free Li metal cycling. The structure, physical and chemical properties of the designed electrolyte solution are well in accordance with the quantum chemistry calculations. The XPS analysis of the SEI (solid electrolyte interface) components on the Li metal surface reveals that a LiF rich protection layer forms. As a result, a high average coulombic efficiency of 99.4% has been achieved at 0.5 mA cm-2 for 1 mAh cm-2 over 900 cycles. Even at a high current density of 10 mA cm-2, the coulombic efficiency of Li metal in this novel electrolyte is above 98.5%. A Li metal full cell with PANS (Poly(acrylonitrile) Sulfur) as cathode realizes a capacity retention of >98% over 900 cycles. This study offers a promising approach to enable highly stable Li metal battery applications. Acknowledgements This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Part of the work used the UCSD-MTI Battery Fabrication Facility and the UCSD-Arbin Battery Testing Facility. H. Liu, X. Wang, H. Zhou, H.-D. Lim, X. Xing, Q. Yan, Y. S. Meng and P. Liu, ACS Applied Energy Materials, 2018, DOI: 10.1021/acsaem.8b00348.H. Liu, H. Zhou, B.-S. Lee, X. Xing, M. Gonzalez and P. Liu, Acs Applied Materials & Interfaces, 2017, 9, 30635-30642.H. Liu, X. Yue, X. Xing, Q. Yan, J. Huang, V. Petrova, H. Zhou and P. Liu, Energy Storage Materials, 2018, DOI: https://doi.org/10.1016/j.ensm.2018.09.021.

(Invited) One Step Synthesis of Li-Alkyl Carbonates and Their Applications As Coatings on Li Anode

Lithium metal is being extensively studied as an anode to replace graphite due to its high capacity and the lowest negative electrochemical potential.1 Current research focus is to mitigate lithium dendrite growth and to increase coulomb efficiency during electrochemical cycling by using various surface coatings or electrolyte additives. Due to the complex reduction reactions of electrolyte components on lithium metal surface, it is highly desirable to develop coating materials with high electrochemical stability. In this work, we report the synthesis of several Li-alkyl carbonates via a simple one step reaction. These carbonates are known components of solid electrolyte interface (SEI) layers on lithium and graphite anode surfaces. The chemical composition and structural information of these synthetic Li-alkyl carbonates are identified by X-ray diffraction and Infrared Spectroscopy. In one implementation of the reaction, a single-component coating is formed on Li metal in a LiI/organic carbonates (dimethyl carbonate (DMC) and ethylene carbonate (EC)) electrolyte. LiI chemically reacts with dimethyl carbonate to form lithium methyl carbonate, which precipitates and forms a chemically homogeneous coating layer on Li surface. This coating layer is shown to enable dendrite free Li cycling in a symmetric Li||Li cell even at a current density of 3 mA cm-2. Adding EC to DMC modulates the formation of the lithium methyl carbonate, resulting in a stable coating layer which is essential for long term Li cycling stability. Fig.1 shows the morphology of deposited Li using LiI-EC/DMC electrolytes.2 Furthermore, the coating can enable dendrite free, high coulomb efficiency cycling after being transferred to common LiPF6/carbonates electrolytes which are compatible with metal oxide cathodes. We will show that LiI reacts with a broad range of organic carbonates and ethers to form lithium compounds relevant to high efficiency operations of battery anodes. Acknowledgements This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Reference W. Xu, J. L. Wang, F. Ding, X. L. Chen, E. Nasybutin, Y. H. Zhang and J. G. Zhang, Energ Environ Sci, 2014, 7, 513-537.H. D. Liu, H. Y. Zhou, B. S. Lee, X. Xing, M. Gonzalez and P. Liu, ACS Appl Mater Inter, 2017, 9, 30635-30642. Figure 1

Multiscale Chemical Evolution of Si Electrode upon Cycling in Li-Ion Cell Using Advanced Characterization Tools

Continuous solid electrolyte interface (SEI) formation remains the limiting factor of the lifetime of silicon nanoparticles (SiNPs) based negative electrodes. Methods that could provide clear diagnosis of the electrode degradation are of utmost necessity to streamline further developments. We show that a combination of selected techniques, such as 7Li, 19F MAS NMR, XPS, TOF-SIMS and STEM-EELS, provides an in-depth characterization at different scales of the SEI layer forming on the surface of silicon particles as well as its evolution upon cycling. We demonstrate that STEM-EELS can be used to quickly map SEI components and quantify LixSi alloys from single experiments with nanoscale resolutions down to 5 nm. Exploiting the low-loss part of the EEL spectrum allowed us to circumvent the degradation phenomena that have so far crippled the application of this technique on such beam-sensitive compounds. Our results provide unprecedented insight into silicon failure mechanisms explored at the nanometer scale in full cell configuration. We observe the morphology of the SEI to be extremely heterogeneous at the particle scale but with clear chemical evolutions with extended cycling coming from both SEI accumulation and a transition from lithium-rich carbonate-like compounds to lithium-poor ones. In addition, we were able to correlate local discrepancies in lithiation to the initial crystallinity of silicon as well as to the local SEI chemistry and morphology. Electrode failure mechanisms at the particle scale were directly observed to be related to SiNP disconnection from both electronic and ionic conduction pathways, creating strong heterogeneous lithiation processes leading to some overlithiated SiNPs. It leads to differential swelling, amplified pulverization and enhanced SEI formation. These findings suggest that capacity retention could be improved through a better control of the nanoscale homogeneity of the electrical wiring and accessibility of porosity in these highly electrolyte-reactive electrodes. Such results emphasize the importance of achieving multiprobe, multiscale and nanoscale analysis by using various advanced characterization tools.

Nanoscale Mechanics of the Solid Electrolyte Interphase on Lithiated-Silicon Electrodes

The ∼300% volume changes of lithiated silicon electrodes (LixSi) during electrochemical cycling lead to cracking of the solid electrolyte interface (SEI). Here, we report how strain is transferred from LixSi to two primary inorganic SEI components: LiF and Li2O. Our first principle calculations show that LiF, effectively bonded on LixSi at x > 1, enables the entire interface structure to deform plastically by forming delocalized stable voids. In contrast, Li2O tightly bonded to LixSi is stiffer, and deforms rigidly across all x. Our results explain the significantly improved ductility of SEI with higher LiF versus Li2O content observed experimentally.

Density Functional Theory Research into the Reduction Mechanism for the Solvent/Additive in a Sodium-Ion Battery.

The solid-electrolyte interface (SEI) film in a sodium-ion battery is closely related to capacity fading and cycling stability of the battery. However, there are few studies on the SEI film of sodium-ion batteries and the mechanism of SEI film formation is unclear. The mechanism for the reduction of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), ethylene sulfite (ES), 1,3-propylene sulfite (PS), and fluorinated ethylene carbonate (FEC) is studied by DFT. The reaction activation energies, Gibbs free energies, enthalpies, and structures of the transition states are calculated. It is indicated that VC, ES, and PS additives in the electrolyte are all easier to form organic components in the anode SEI film by one-electron reduction. The priority of one-electron reduction to produce organic SEI components is in the order of VC>PC>EC; two-electron reduction to produce the inorganic Na2 CO3 component is different and follows the order of EC>PC>VC. Two-electron reduction for sulfites ES and PS to form inorganic Na2 SO3 is harder than that of carbonate ester reduction. It is also suggested that the one- and two-electron reductive decomposition pathway for FEC is more feasible to produce inorganic NaF components.

Ultrathin Layered Hydroxide Cobalt Acetate Nanoplates Face‐to‐Face Anchored to Graphene Nanosheets for High‐Efficiency Lithium Storage

The dramatically increasing demand of high‐energy lithium‐ion batteries (LIBs) urgently requires advanced substitution for graphite‐based anodes. Herein, inspired from the extra capacity of lithium storage in solid‐electrolyte interface (SEI) films, layered hydroxide cobalt acetates (LHCA, Co(Ac)0.48(OH)1.52·0.55H2O) are introduced as novel and high‐efficiency anode materials. Furthermore, ultrathin LHCA nanoplates are face‐to‐face anchored on the surface of graphene nanosheets (GNS) through a facile solvothermal method to improve the electronic transport and avoid agglomeration during repeated cycles. Profiting from the parallel structure, LHCA//GNS nanosheets exhibit extraordinary long‐term and high‐rate performance. At the current densities of 1000 and 4000 mA g−1, the reversible capacities maintain ≈1050 mAh g−1 after 200 cycles and ≈780 mAh g−1 after 300 cycles, respectively, much higher than the theoretical value of LHCA according to the conversion mechanism. Fourier transform infrared spectroscopy confirms the conversion from acetate to acetaldehyde after lithiation. A reasonable mechanism is proposed to elucidate the lithium storage behaviors referring to the electrocatalytic conversion of OH groups with Co nanocatalysts. This work can help further understand the contribution of SEI components (especially LiOH and LiAc) to lithium storage. It is envisaged that layered transition metal hydroxides can be used as advanced materials for energy storage devices.

Nanoscale Chemical Evolution of Silicon Negative Electrodes Characterized by Low-Loss STEM-EELS.

Continuous solid electrolyte interface (SEI) formation remains the limiting factor of the lifetime of silicon nanoparticles (SiNPs) based negative electrodes. Methods that could provide clear diagnosis of the electrode degradation are of utmost necessity to streamline further developments. We demonstrate that electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM can be used to quickly map SEI components and quantify LixSi alloys from single experiments with resolutions down to 5 nm. Exploiting the low-loss part of the EEL spectrum allowed us to circumvent the degradation phenomena that have so far crippled the application of this technique on such beam-sensitive compounds. Our results provide unprecedented insight into silicon aging mechanisms in full cell configuration. We observe the morphology of the SEI to be extremely heterogeneous at the particle scale but with clear chemical evolutions with extended cycling coming from both SEI accumulation and a transition from lithium-rich carbonate-like compounds to lithium-poor ones. Thanks to the retrieval of several results from a single data set we were able to correlate local discrepancies in lithiation to the initial crystallinity of silicon as well as to the local SEI chemistry and morphology. This study emphasizes how initial heterogeneities in the percolating electronic network and the porosity affect SiNPs aggregates along cycling. These findings pinpoint the crucial role of an optimized formulation in silicon-based thick electrodes.

Lithium difluorophosphate as an additive to improve the low temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite cells

Lithium difluorophosphate (LiPO2F2) was used as an electrolyte additive to promote the low temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite cells. The impact of LiPO2F2 on the solid electrolyte interface (SEI) film-formation on electrodes was demonstrated by various electrochemical methods and microscopy techniques, such as transmission electron microscopy (TEM), scanning electron microscope (SEM) as well as X-ray photoelectron spectroscopy (XPS), in the pouch cells and half cells. The results showed that the cells containing 1% LiPO2F2 performed 71.9% (−20°C) and 57.93% (−30°C) of initial capacity, while the cells without LiPO2F2 discharged only 49.41% and 9.6% of initial capacity under the same condition. In addition, the enhancement of cyclic performance at 0°C was attributed to a conductive and stable SEI film formed on the graphite by the sacrifice of LiPO2F2, which led to a low impedance and richer content of LiF and Li2CO3 in SEI components, as depicted in XPS.

Surface Analysis of Li-Ion Electrode-Electrolyte Interfaces: Insights By a Surface Science Approach

Electrode-electrolyte interfaces play a key role for the performance and degradation of electrochemical devices such as lithium-ion cells. Side reactions at cathode-electrolyte interfaces result in the formation of a solid-electrolyte interface (SEI) layer, resulting in loss or performance due to lithium loss and increase in surface layer resistance. Over the past years, the cathodic SEI layer has been subject to extensive investigation, focused on the correlation between composition and battery cycling parameters. Yet, open questions remain with regard to formation mechanisms and properties. Most investigations on SEI layers have been performed by post mortem analysis of composite electrode surfaces by photoelectron spectroscopy (XPS). XPS gives information on the chemical structure by way of core level electron binding energies. Despite of the standard use of XPS for SEI analysis, unambiguous identification of the different phases is often not straightforward as binding energy shifts are encountered. Moreover, the post mortem analysis of composite materials is not ideally suited to investigate the SEI on the active material, as emissions from the other materials in the electrode (carbon, binder) add complexity. Nevertheless, such data on the SEI composition of composite electrodes are often used to propose reaction mechanisms on SEI-formation and side reactions (electrolyte oxidation) occurring at the surface of the active materials. Improved insights into SEI-composition and side reactions require experiments that address binding energies and interaction of single material surfaces with components of the electrolyte. We prepare model interfaces of LCO (LiCoO2) thin film electrodes with components of the SEI, artificial SEI-layers, as well as with solvents, and analyze them with surface analysis methods [1]. In such experiments, the SEI or electrolyte-related components are deposited as top phase either by thin film deposition or low temperature adsorption, and analysis is performed by photoelectron spectroscopy (XPS, UPS, SXPS) and high resolution electron energy loss spectroscopy (HREELS). Usually, the preparation is performed stepwise with intermediate analysis in order to investigate the interface formation in detail with respect to chemical structure, electronic structure and space charge effects. In this contribution, we present our results with respect to the deposition of several solid compounds such as lithium oxide and LiPON, as well as the exposure to several solvents (water, diethyl carbonate, etc.). In most cases, the formation of interface and/or gradient layers is observed. For the substrate-related binding energies we usually note a shift to higher values which is attributed to space charge layer formation in the LiCoO2 (band bending downwards). Also, we find indications that binding energies from nominally identical overlayer compounds and those related to the substrate are differently positioned against each other depending on the preparation method. Possible causes for this are charging effects, differences in the overlayer compound´s Fermi-level, or presence of interfacial dipoles. Mechanistic insights regarding the formation of cathode-electrolyte interfaces and of ionic interfaces in general are discussed [2][3]. [1] R. Hausbrand, D. Becker, and W. Jaegermann, Progress in Solid State Chemistry 42 (2014) [2] A. Schwöbel, W. Jaegermann, and R. Hausbrand, Solid State Ionics, in press. [3] D. Becker, et al., Journal of Physical Chemistry C, 118 (2014)

Degradation Analyses of Commercial Lithium-Ion Cells by Temperature/C-rate Controlled Cycle Test

Cycle life tests of commercial 18650-type lithium-ion cells were conducted at temperatures of 0°C, 25°C, and 45°C and current rates of 1 C and 2 C. Surprisingly, the discharge capacity rapidly decreased at 0°C regardless of the current rate. On the other hand, the discharge capacity decreased faster at 45°C than at 25°C for a rate of 1 C, and this decrease was faster at 25°C than 45°C for a rate of 2 C. Electrodes and electrolytic solutions of the degraded cells were characterized by several analytical methods. X-ray photoelectron spectroscopy revealed differences between solid electrolyte interface components according to test temperature. The carbonate ratio was high at low temperature while the phosphate ratio was high at elevated temperature. The observed cell degradations were caused by several factors exhibiting various temperature and current rate dependencies.