So-called Solid Electrolyte Interphase Research Articles

Bulk SEI: Investigating the Instability of Li6PS5Cl at the Lithium Metal Anode

Among the multitude of lithium ion conducting materials, thiophosphate-based solid electrolytes (thio-SEs) attract great attention from academia and industry alike. Due to their high ionic conductivities and increased safety, thio-SEs – such as lithium argyrodite Li6PS5Cl – are considered suitable to be employed in solid-state batteries (SSBs). Thus, by substituting liquid organic electrolytes of lithium-ion batteries (LIBs), higher energy and power densities (in combination with lithium metal anodes (LMAs)) as well as simplified cell designs are promoted.[1] Unfortunately, one major drawback of thio-SEs is the inadequate reduction stability in contact with LMAs,[2] which impedes device-level applications and has to be overcome.Due to its thermodynamic instability, Li6PS5Cl is reduced upon contact with highly reactive lithium metal to a mixture of compounds, i.e., Li2S, Li3P, and LiCl. Thus, a so-called solid electrolyte interphase (SEI) is formed, which is predominantly ion-conducting. Its electronically insulating nature is suspected to limit the interphase formation and further growth.[3] Additionally, the chemical and structural properties differ from that of pure SE. Unfortunately, the SEI only evolves at the Li | Li6PS5Cl interface and its thickness is in the nanometer range.[4] Suitable methods to examine the characteristics of SEI layers are therefore scarce and mostly limited to surface sensitive characterization techniques.Here, we employ a novel approach to gain further insights into the nature of SEI layers of thiophosphate-based SE in contact with lithium metal, demonstrated with Li6PS5Cl as model solid electrolyte. Bulk material corresponding to the composition of the SEI is synthesized from SE and Li-metal in a direct manner and hence, examined in respect to different properties. The successful synthesis of bulk-phase SEI is confirmed by X-ray photoelectron spectroscopy measurements, validating the chemical composition. This is complemented by structural analysis. Electrochemical characterization (electrochemical impedance spectroscopy and Wagner-Hebb) is utilized to quantify the ionic and electronic conductivity of the obtained bulk material, respectively, and to further determine transport properties and SEI growth at the Li | Li6PS5Cl interface. The combined results of our studies allow comprehensive understanding of the SEI nature and offer boundary conditions to approximate the nanoscale properties of the SEI at the Li | Li6PS5Cl interface from bulk material synthesis and analysis.

Evolution of the Interphase between Argyrodite-Based Solid Electrolytes and the Lithium Metal Anode─The Kinetics of Solid Electrolyte Interphase Growth

Argyrodite-based solid electrolytes (SEs) are promising candidates for application in solid-state batteries (SSBs) due to their high ionic conductivity and mechanical malleability. However, they are reduced by lithium and form interphases when they are in contact with a lithium metal anode, which are needed to construct cells with high energy density. If the interphase is electronically insulating, a so-called solid electrolyte interphase (SEI) is formed that can protect the solid electrolyte from further degradation and grows only slowly. Careful evaluation of individual lithium metal anode|SE interface reactions and their growth kinetics is necessary to advance the concept of lithium metal batteries. Here, the interphase growth in symmetric Li|Li6PS5Cl|Li cells is studied quantitatively by impedance spectroscopy using a three-electrode cell setup. Unidirectional plating experiments show that the three-electrode cell is well suited to study the SEI evolution. Passivated and freshly prepared lithium foils are investigated, and the impedance evolution is studied to explore the influence of the lithium metal anode surface on SEI growth. The study reveals that an inherent passivation layer, present on most commercial lithium foils, influences the rate of SEI formation and causes high internal cell resistance. The lithium reservoir-free anode (“anode-free concept”) is recommended to overcome the issues caused by chemically poorly defined lithium foil surfaces.

(Digital Presentation) Investigation of Solid Electrolyte Interphase Formation on Highly Oriented Pyrolytic Graphite Anodes in Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are widely used in modern society. Since LIBs represent a substantial fraction and sometimes even majority of the device cost, extending the lifetime of LIBs and understanding their degradation mechanisms draw an increasing attention. Graphite, commonly used as the negative electrode in LIBs, suffers from two main degradation mechanisms: loss of active material and loss of lithium-ion inventory. Formation of the so-called solid electrolyte interphase (SEI) layer is known as the key factor for functioning of the anode. On the one hand, its growth covering the electrode surface slows down Li-ion intercalation. On the other hand, the SEI layer both protects the graphite particles from exfoliation upon intercalation and the electrolyte from decomposition by preventing organic molecules from entering graphite. Due to surface defects of the graphite particles used in common LIBs, it is challenging to observe the SEI layer formation. Therefore, highly oriented pyrolytic graphite (HOPG), which has a perfectly smooth surface, is commonly used in SEI formation experiments.In this work, we have applied electrochemical analysis, as well as morphology and structure characterization to investigate processes involved into the formation of SEI layer.A three-electrode electrochemical cell setup was installed in a glove box under inert argon atmosphere. HOPG was used as the working electrode and lithium metal was employed as both the reference electrode (RE) and counter electrode (CE). The electrolyte was composed of 1M lithium hexafluorophosphate (LiPF6) dissolved in 1:1 v/v mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Cyclic voltammetry (CV) was performed in voltage ranges of 2, 1.5, and 0.5 V versus Li/Li+. canning Electron Microscopy (Zeiss XB1540) and Atomic Force Microscopy (Ntegra Aura) were used to characterize the morphology and topography of the HOPG electrode after CV. X-ray Photoelectron Spectroscopy (XPS, Phoibos 150 1D-DLD, XR 50, Specs) was employed to measure the elemental composition and structure at the HOPG surface.When the cycling was performed between 3 – 2 V, small particles were observed on the surface of the HOPG, which can be assigned to the deposition of electrolyte. Homogeneous microspheres started to appear when the voltage reached below 1.5 V, which can be attributed to the formation of SEI particles. As the CV range increased, which also meant an increase in the depth of discharge (DOD), the density of microspheres on the surface increased as well. The diameter of the microspheres was found to be around 80 – 100 nm at a low lithiation level, and as the HOPG electrode became fully lithiated, the size of the particles slightly increased. This finding was supported by XPS measurements where the integral of the LixPFy and LixPFyOy peak, which is related to the main constituents of SEI layer, was found to be the largest (57.68% of the full F 1s spectra peak area) in the 3 – 0 V cycle range, while a lower peak (54.56%) was observed in the 3 – 0.5 V range (in the attached Figure).These results provide an indication of how the charging method affects the capacity fade and lifetime of the LIBs. The stage from 0.5 to 0 V on HOPG is approximately equivalent to 80-100% SOC for a full cell. As was found, in this stage, the SEI film grows thicker, thereby aggravating the aging of the cell. The lifetime of LIBs can therefore be extended if cycling at high SOC is avoided. Based on this, our future work will focus on the development of an optimized multi-step fast-charging method, which can reduce degradation and extend the lifetime of commercial lithium-ion batteries. Figure 1

New Technique for Probing the Protecting Character of the Solid Electrolyte Interphase as a Critical but Elusive Property for Pursuing Long Cycle Life Lithium-Ion Batteries.

The formation of a protecting nanolayer, so-called solid electrolyte interphase (SEI), on the negative electrode of Li-ion batteries (LIBs) from product precipitation of the cathodic decomposition of the electrolyte is a blessing since the electrically insulating nature of this nanolayer protects the electrode surface, preventing continuous electrolyte decomposition and enabling the large nominal cell voltage of LIBs, e.g., 3.3–3.8 V. Thus, the protection performance of the nanolayer SEI is essential for LIBs to achieve a long cycle life. Unfortunately, the evaluation of this critical property of the SEI is not trivial. Herein, a new, cheap, and easily implementable methodology, the redox-mediated enhanced coulometry, is presented to estimate the protecting quality of the SEI. The key element of the methodology is the addition of a redox mediator in the electrolyte during the degassing step (after the SEI formation cycle). The redox mediator leads to an internal self-discharge process that is inversely proportional to the protecting character of the SEI. Also, the self-discharge process results in an easily measurable decrease in Coulombic efficiency. The influence of vinylene carbonate as an electrolyte additive in the resulting SEI is used as a case study to showcase the potential of the proposed methodology.

Accessing the solid electrolyte interphase on silicon anodes for lithium-ion batteries in-situ through transmission soft X-ray absorption spectroscopy

Silicon offers nine times higher theoretical storage capacity than commercial graphite anodes for Li-ion batteries. For cycling stability, the electrolyte needs to be kinetically stabilized by the so-called Solid Electrolyte Interphase (SEI), a layer which ideally forms once from decomposition products of the electrolyte. While it works for graphite, the SEI on silicon fails to stabilize the electrolyte sufficiently, partly due to the large volume changes upon de-/lithiation. To investigate the SEI on silicon anodes, we developed a novel approach for X-ray Absorption Spectroscopy (XAS), that puts a twist to conventional SiNx window-based liquid cells by utilizing a deliberately induced gas-bubble to form a soft X-ray transparent electrolyte layer. We demonstrate our approach to allow transmission XAS in the soft X-ray regime on liquids and electrode thin-film materials under in-situ conditions. In our case XAS study of the SEI on silicon anodes, we reveal the main SEI constituents as Li acetate, Li ethylene di-carbonate or Li ethylene mono-carbonate, Li acetylacetonate, LiOH, and LiF. Additionally, we see evidence for aldehyde species which we attribute to possible liquid inclusions within a porous SEI morphology. We consider our method an appropriate tool for the successful engineering of a stable, efficient SEI in the future.

Investigations of the Solid Electrolyte Interphase Using X‐Ray Photoelectron Spectroscopy In situ Experiment on the Lithium‐Based Solid Electrolyte LiPSON

Lithium phosphorus sulfuric oxide nitride (LiPSON) is prepared by radio frequency (RF) sputtering using a sputter target with compositions of 50 wt% Li3PO4 and 50 wt% Li2SO4. Knowledge about the present phases at the solid electrolyte–lithium anode interface, the so‐called solid electrolyte interphase (SEI), is achieved by performing an in situ sputtering of metallic lithium on the electrolyte and subsequently measuring the chemical bonding by X‐ray photoelectron spectroscopy (XPS). Information about the SEI is of essential interest to understand the process of lithium‐ion conduction in the half cell. Therefore, the information about the formation of lithium phases in the SEI is crucial.

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

To date, commercial LIB cells are mostly based on graphite as anode material. During the first inter­calation of lithium into graphite, the electrolyte gets reduced at the anode, forming a nm-thick surface layer, the so-called solid electrolyte interphase (SEI). The SEI stops further electrolyte reduction, but as it consumes lithium during its formation, there is a loss of active lithium inventory, which causes the irreversible capacity loss during the first cycle of an LIB.[1] Neutron depth profiling (NDP) is a non-destructive technique and a suitable tool to measure lithium concentrations as a function of depth.[2,3] When irradiating the sample with a cold neutron beam, 6Li nuclides emit charged particles after neutron capture. The residual energy and the signal rate of the emitted 3H particles are correlated to depth and amount of lithium, respectively. Thus, SEI growth and lithium (de-)intercalation in graphite anodes can be studied up to a depth of ca. 30 µm. So far, in-situ NDP measurements in LIBs have been limited to thin solid-state cells or to cells with sub-mm thick anodes.[4,5] Here, we present operando NDP data for the first charge/discharge cycle of a 14 mm thick graphite anode (4 mm particles) vs. a capacitively over­sized LiFePO4 cathode, using a custom-designed coin-cell casing with 0.5 mm diameter holes which are sealed with a 7.5 µm-thick Kapton® window. Fig. 1 shows the voltage profile during cycling, demonstrating that the operando cell yields the expected first-cycle discharge capacity of 350 mAh/ggraphite. The higher irreversible capacity of ca. 98 mAh/g (ca. 22%) compared to conventional graphite used in commercial cells (ca. 10%) is due to the very high BET area of ca. 14 m2/g of the 4 µm sized graphite particles. Fig. 2 shows the NDP signals at three different states-of-charge (SOC): i) for the pristine graphite anode (0 h, black line), reflecting the lithium-containing electrolyte (1 M LiPF6 in EC/EMC 3:7 v:v) within the electrode pores; ii) for 100% SOC at the end of the first charge (16 h, red line); and, iii) at the end of the first discharge at again 0% SOC (32 h, blue line). The difference between the red and the blue line indicates that ca. ¼ of the active lithium has been consumed irreversibly for the SEI formation, in good agreement with the electrochemical data.

In Situ Mechanical Analysis of the Nanoscopic Solid Electrolyte Interphase on Anodes of Li-Ion Batteries.

The interfacial decomposition products forming the so‐called solid–electrolyte interphase (SEI) significantly determine the destiny of a Li‐ion battery. Ultimate knowledge of its detailed behavior and better control are required for higher rates, longer life‐time, and increased safety. Employing an electrochemical surface force apparatus, it is possible to control the growth and to investigate the mechanical properties of an SEI in a lithium‐ion battery environment. This new approach is here introduced on a gold model system and reveals a compressible film at all stages of SEI growth. The demonstrated methodology provides a unique tool for analyzing electrochemical battery interfaces, in particular in view of alternative electrolyte formulations and artificial interfaces.

Synthesis, Water‐Removing Method and Influences of Trace Water for LiBF4

AbstractRecent works show the promise for lithium tetrafluoroborate (LiBF4) as an electrolyte salt for lithium‐ion batteries. In this paper, LiBF4 is synthesized by a novel reaction between the as‐prepared anhydrous Li2SO3 and (C2H5)2O⋅BF3 in an aprotic solvent. The moisture sensitivity and water‐removing method of LiBF4 materials which has been dried under vacuum for 2 h at different temperature, is studied by Fourier transform infrared spectroscopy measurements. Results show that almost all of the adsorbed water would be removed at above 65 °C. With the increase of drying temperature, crystal water will decrease slowly, and almost disappear completely at 170 °C. However, due to the thermal decomposition of LiBF4, LiF and BF2OH impurities are formed as the temperature reaches 165 °C. So, we determine that the proper drying temperature is 155 °C, though the trace water cannot be removed thoroughly. Besides, influences of trace water on the electrochemical performance of Li/MCMB half‐cell with LiBF4 as electrolyte salt has been investigated. Results show that discharge plateaus at about 1.5 V vs. Li/Li+ will be lengthened with the increase of water content. Subsequently, the so‐called solid‐electrolyte interphase (SEI) film will become thicker and more resistant, which leads to poor Li+ ion migration and bad cycling performance.

Molecular Modeling of Reaction and Diffusion Processes at Electrochemical Interfaces in Lithium Ion Batteries

Lithium batteries are well-known as a critical technology, but they are also a fundamentally interesting chemical system do to the presence of complex phenomena such as chemical and electrochemical reactions, phase changes, and ion transport across interfaces, to name a few. Such complexity, however, provides opportunity for computational approaches which are able to resolve size- and time-scales unreachable through experimental methods. Many of the practical hurdles moving forward in battery technologies lie at the interfaces between the electrolyte and the electrodes, making study of these interfaces an important application of computational tools. However, no single computational approach is robust enough to encapsulate the entirety of the interface while also providing the appropriate resolution to study charge transfer and chemical bond breaking/formation. For this reason, it is critical that a multi-scale approach is taken in order to incorporate both accurate models and accurate methods into the characterization of important interfacial phenomena. Our work focuses on the formation of the so-called solid electrolyte interphase (SEI) by the reductive decomposition of ethylene carbonate (EC) at a graphite electrode surface prior to the initial lithiation during its first charge cycle. The SEI is a solid film consisting of decomposed electrolyte which permits the conduction of Li+ ions to and from the electrode, while passivating further electron transfer. Thusly, its formation process consists of electrochemical reduction, subsequent chemical reactions, reactant/intermediate/product transport, and solid precipitation onto the electrode surface. In order to encapsulate all of these phenomena, multiple simulation tools must be used. In particular, we utilize molecular dynamics (both classical force field and ab initio), enhanced sampling techniques, and mixed MD/Monte-Carlo methods to model reaction, transport, and phase transformation phenomena which contribute to the formation of the SEI. In this talk, we will present some of our recent findings from our investigation of the SEI and the methods we use to study it. In particular, we will discuss our framework for modeling reactions and diffusion processes near the electrode and the importance of explicit consideration of the electrolyte in predicting kinetics. Interfaces are critical to electrochemical systems from lithium ion batteries, to electrocatalysts, to sensors and computational methods possess great potential in shaping our understanding of them.

(Invited) The Crucial Role of Nano-Sized Porosity in the Lithiation and Stability of Methylated Amorphous Si Electrodes for Li-Ion Batteries

Silicon electrodes are attracting for lithium-ion batteries but their use is impeded by their poor mechanical stability upon operation (huge volume variation takes place upon electrochemical Li insertion in and extraction from the material) and a poorly-controlled growth of surface layers resulting from the electrolyte decomposition products (the so-called solid-electrolyte interphase, SEI). A thin layer of methylated amorphous silicon a-Si(1-x)(CH3) x :H has a much better resistance to electrochemical lithiation/delithiation cycles than amorphous silicon a-Si:H [1]. Methylated amorphous silicon is a carbon-silicon alloy, deposited using PECVD, in which carbon is incorporated in the bulk material under the form of methyl groups CH3 only. We have studied the lithiation processes in the material for various methyl contents x ranging from 0 (a-Si:H) to 0.12. The lithiation process has been investigated using operando infrared spectroscopy in geometry of attenuated total reflection (ATR) where the active thin layer (30 nm) is directly deposited at the surface of the ATR prism. For all the investigated values of the methyl content x in the material, it was observed that the first lithiation proceeds according to a two-phase mechanism [2]: a (highl²y) lithiated phase with a well-defined Li content z (Li z Si phase) progressively invades the active-material film; if lithiation is performed in galvanostatic mode, this regime corresponds to the observation of a potential plateau. When lithiation is pursued beyond the potential plateau, a single-phase behavior is observed: the Li concentration in the active film becomes uniform and increases as long as the lithiation current is maintained. Delithiation and subsequent lithiation/delithiation cycles proceed homogeneously according to this single-phase behavior. When the electrode is thicker (200 nm), extensive electrochemical cycling results in the cracking of the film. The appearance of the crack network can be followed by operando optical microscopy. When the active film is deposited on a reflecting substrate such as stainless steel, the lithiation can also be followed using the same technique by monitoring of interference colors, since the lithiation results in a thickening of the film and in a change of its refractive index. The quantitative analysis of the two-phase regime of the first lithiation shows that the Li content z in the invading phase depends in a non-trivial way upon the methyl content x of the material: z first decreases upon increasing x, then for values of x larger than 0.05, z increases when increasing x. This behavior can be rationalized in the framework of a model describing the first lithiation of a-Si(1-x)(CH3) x :H. Noteworthy, the regime in which z increases upon increasing x can be ascribed to the existence of a methyl-induced porosity of the material at the nanometric scale. Moreover, the careful analysis of color patterns during lithiation demonstrates that the locations at which the lithiation occurs is not homogeneous at the very beginning of the second cycle: color change can be identified at the locations where future cracks will appear (at the end of the second delithiation of the electrode or later). Possible implications regarding the optimization of the material for its use as electrodes in Li-ion batteries will be discussed.

Structure formation and surface chemistry of ionic liquids on model electrode surfaces-Model studies for the electrode electrolyte interface in Li-ion batteries.

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]-) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO2(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate-adsorbate and adsorbate-adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li3N, Li2S, LixSOy, and Li2O. In the absence of a [BMP]+[TFSI]- adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO2) above a critical temperature, forming LiOx and Ti3+ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li+ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

Non-linearity of the solid-electrolyte-interphase overpotential

In today’s modeling and analysis of electrochemical cycling of Li- and Na-ion batteries, an assumption is often made regarding the interphase that forms between the active material and liquid electrolyte at low potentials, the so-called solid-electrolyte interphase (SEI). The SEI is generally assumed to act like an Ohmic resistor despite its complex chemical composition and porosity distribution. Here, one reports that this assumption does not hold for alkali-ion batteries. The SEI possesses a non-linear overpotential characteristic which saturates already at low current density of 0.1mAcm−2 giving only 3.3±1mV for Li-metal electrodes in different electrolytes. For Na- and K-metal electrodes, these SEI overpotentials become dominating with 31mV and 72mV at the same low current densities giving significant disadvantages over Li-ion batteries for commercial applications. With the introduction of a new term, one achieves agreement between the parameters from galvanostatic cycling and electrochemical impedance spectroscopy for the first time. The discovery of the non-linear SEI overpotential disrupts the general believes about the role of the SEI for today’s batteries as it is basically negligible for Li-ion batteries at room temperature.

Myth and Reality about the Origin of Inductive Loops in Impedance Spectra of Lithium-Ion Electrodes — A Critical Experimental Approach

Electrochemical Impedance Spectroscopy (EIS) is a sophisticated analyzing tool in battery research. Though, there are still phenomena subject to speculation. As an example, for a long time the formation of the so-called Solid Electrolyte Interphase (SEI) has been made responsible for inductive loop formation in impedance spectra. In our opinion, there are, however, no satisfying evidences supporting this attempt to explain the occurrence of inductive loops. The present study is, thus, aimed at answering the question whether other origins such as cell design ought to be considered to reveal the electrical sources for this phenomenon. Therefore, two commonly used anode materials for lithium-ion batteries were investigated with respect to their tendency to the formation of SEIs. Besides the influence of the SEI on the overall impedance response, also the impact of the cell design on electrical response was investigated. We identified four different sources of (i) inductive and (ii) negative capacitance loop formation: springs, reference electrodes, drift, and corrosion. In conclusion, any interrelationship between inductive loops and SEI formation could be ruled out. This finding disproves an earlier, commonly known attempt that traced the presence of inductive loops back to the formation of passivating surface layers because of electrolyte decomposition.

Alumina-coated silicon-based nanowire arrays for high quality Li-ion battery anodes

Amongst other requirements, a good anode for Li-ion battery applications must exhibit dimensional stability upon Li insertion, as well as chemical inertness with respect to the electrolyte. This latter characteristic is usually provided by the so-called solid electrolyte interphase (SEI), a passivating film that is formed at the end of the first lithiation step, originating from the partial reduction of the electrolyte and Li salt. However, silicon, which exhibits the highest known capacity for Li alloying, possesses none of the above attributes when used as an anode material. Actually, the large volume expansion of Si upon alloying with Li induces a mechanical instability of the SEI film, which therefore fails to provide its protective role. In this paper, we have studied the effect of thin alumina deposits on top of Si-based nanowires. A thin alumina deposit will act as a substitute for the SEI, preventing electrolyte decomposition. We observe that even if alumina films crack during lithiation–delithiation steps of the Si-based nanowires, they still provide some kind of protection, prolonging the lifetime of the anode. Using Al2O3-coated Si-based nanowires, we have been able to obtain a lifetime of 1280 cycles when the capacity of the anode was limited to 1200 mA h g−1. We also show that uncoated Si nanowires degrade more rapidly when cycled under identical conditions.

Self-discharge behavior and its temperature dependence of carbon electrodes in lithium-ion batteries

Self-discharging characteristics of negative electrodes with different carbon materials have been investigated by monitoring the open circuit potential (OCP), the capacity loss and the ac impedance change during the storage at different temperatures. The OCP change with the storage time reflected state-of-charge (SOC), which depended on both the carbon material and the storage temperature. Higher specific surface area of the material and higher storage temperature lead to higher self-discharging rate. The activation energy for self-discharging was estimated from the temperature dependence of the self-discharging rate. Although small difference was observed among the materials, the value of the activation energy suggests that the self-discharging reaction at each electrode is controlled by a diffusion process. Changes in the interfacial resistance with the storage time reflected the growth of so-called Solid Electrolyte Interphase (SEI) at carbon surface. The rate of SEI formation at lower temperature does not depend on the carbon material, but at higher storage temperature the rate on spherical graphite was much higher than those on the other carbon materials.

The cathode–electrolyte interface in the Li-ion battery

The same experimental techniques as used earlier to characterize the composition and properties of the so-called solid electrolyte interphase (SEI) layer formed at the graphite-anode–electrolyte interface of a Li-ion battery are used here to acquire some degree of understanding of interface phenomena occurring on the cathode side of the cell, even though the validity of the SEI-layer concept is still somewhat tenuous in this “cathode” context. We here probe cathode-related SEI phenomena for the three cases: LiMn 2O 4, LiCoO 2/LiNi 0.8Co 0.2O 2, and carbon-coated LiFePO 4. The various layer types formed have been analyzed systematically for different salts, solvents, cycling modes, storage times, temperatures, etc., using photoelectron spectroscopy (PES). Depth-profiling of the layers formed was achieved using Al Kα radiation in conjunction with Ar-ion sputtering; non-destructive depth-profiling was made possible using synchrotron radiation, and applied to the important case of carbon-coated LiFePO 4. A number of trends have emerged from our studies, and some general models are proposed to reflect features characteristic of the various systems studied. Our results are related to the more familiar SEI-layer formed on graphite.

X-ray photoemission studies of surface pre-treated graphite electrodes

Surface pre-treatment of graphitic electrode materials for lithium ion cells has recently been shown to significantly reduce the irreversible consumption of material and charge due to the formation of the so-called solid electrolyte interphase, during battery charging. In this paper we compare graphite powders and carbon fibres as model materials for studies of the effects of CO 2 pre-treatments on solid electrolyte interphase (SEI) formation using X-ray photoemission spectroscopy (XPS). We observe significant modification in the surface elemental composition as a function of CO 2 treatment temperature for carbon fibres, but no strong changes for graphite powders. We conclude that the predominance of the relatively unreactive basal plane in the surface area of the powders makes them far less suitable for detailed XPS studies than the fibres, where the more reactive prismatic planes predominate.

XPS studies of graphite electrode materials for lithium ion batteries

Surface pre-treatment of graphitic electrode materials for lithium ion cells has recently been shown to significantly reduce the irreversible consumption of material and charge due to the formation of the so-called solid electrolyte interphase (SEI) during battery charging. In this paper, we compare graphite powders and carbon fibres as model materials for X-ray photoemission spectroscopy (XPS) studies of the effects of surface pre-treatments. For carbon fibres, the surface carbon percentage was found to vary from 70–95% depending on the surface treatment, with corresponding changes in the relative proportion of graphitic compared to CO bonds, as determined from C 1s curve fits. In contrast, results from the graphite powders show very little change in surface chemical composition and an essentially constant C 1s lineshape dominated by graphitic carbon. SEM data show the carbon fibre cross-section to be composed of a radial array of layered graphite, leaving a surface consisting largely of prismatic planes, while the graphite powder consists of graphite platelets with the surface area predominantly of basal planes. We conclude that the chemical modification occurs at the prismatic planes, and that the powders are unsuitable as models for XPS studies of electrode surface modification, while the fibres are very well suited.

In situ investigation of electrochemical lithium intercalation into graphite powder

The reaction kinetics for the electrochemical insertion–extraction of lithium into and out of graphite and the electrode intrinsic resistance have been studied using galvanostatic intermittent titration with applied microcurrent pulses (GIT), electrochemical impedance spectroscopy (EIS) and in situ intrinsic (i.e. physical) resistance measurements. The formation of a so-called solid electrolyte interphase (SEI) or ‘passivation layer’ on the graphite surface at about +0.8 V versus Li + /Li increases the total intrinsic resistance within the electrode on the first cycle. The gradual growth (i.e. change in volume) of the SEI film during repeated lithium insertion–extraction results in a continued increase in intrinsic resistance. The good agreement between EIS and in situ intrinsic resistance measurements indicates that the latter is a simple and powerful method for investigating the change in this parameter for graphite electrodes. GIT showed a gradual increase in kinetic rate in an initial single-phase region, followed by a decrease in a two-phase transformation region, which was in turn followed by an increase in kinetic rate in a second single-phase region. This is explained by a shrinking unreacted core model. The controlling step of lithium insertion–extraction during the two-phase transformation region was investigated by EIS and GIT.