Felix Joho

The complex electrochemistry of graphite electrodes in lithium-ion batteries

This paper discusses the interrelated phenomena of solid electrolyte interphase (SEI) formation and the irreversible charge consumption which occurs during the first cycle of a graphite electrode, as well as their relevance to the cycling stability of lithium-ion batteries. Thus, results from relevant characterization methods, namely, in situ mass spectrometry, in situ infrared spectroscopy, in situ Raman and video microscopy, in situ scanning probe microscopy, in situ quartz crystal microbalance, and differential scanning calorimetry were combined for a more thorough understanding of observations made in cycling experiments. From electrochemical cycling tests, we have learned that a high specific charge (∼360 Ah/kg of carbon), satisfactory cycle life of the graphite electrodes (1000 deep cycles), and an irreversible charge of <7% during SEI formation can only be obtained when water contamination of the cell is avoided. Under such conditions, a good-quality SEI film is formed on the carbon surface. We conclude that during SEI film formation, at first the carbonate solvent(s) are reduced, forming ethylene gas, organic radicals, oligomers, and polymers. Then a SEI film is precipitated on the surface via a nucleation and growth mechanism. The irreversible charge consumption due to SEI formation is proportional to the BET specific surface area of the graphite and rapidly increases with increasing water content in the cell.

Advanced in situ methods for the characterization of practical electrodes in lithium-ion batteries

In this paper, an overview of the progress recently achieved in our laboratory in the development and application of four in situ methods, namely in situ X-ray diffraction measurements (XRD), differential electrochemical mass spectrometry (DEMS), infrared spectroscopy, and Raman microscopy, is presented. For bulk investigations during cycling, in situ XRD measurements are used, and explained here in the instance of the reaction of graphite with lithium. Next, three surface-sensitive in situ methods are discussed, namely DEMS, infrared spectroscopy, and Raman microscopy. As an illustration of in situ infrared spectroscopy and DEMS, we show results for both the oxidative and reductive decomposition of electrolyte solutions. Finally, a cell for in situ Raman microscopy is described. This method is well suited, e.g., for an analysis of the structural properties of carbon materials, for a characterization of positive electrodes, and electrolytes of lithium-ion cells. As an example, Raman spectra measured in situ at a single LiCoO2 particle selected on the surface of a commercial electrode are discussed.

FTIR and DEMS investigations on the electroreduction of chloroethylene carbonate-based electrolyte solutions for lithium-ion cells

Abstract Chloroethylene carbonate (ClEC) is decomposed to CO 2 at graphite electrodes. We assume that the CO 2 participates in the formation of an effective solid electrolyte interphase (SEI) on the electrode. Two in-situ techniques, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) and differential electrochemical mass spectrometry (DEMS), were applied in order to detect CO 2 formation and possible secondary reactions. The applied analytical methods provided conforming information about the onset of CO 2 formation (2.2–2.1 V vs. Li/Li + ).

SNIFTIRS investigation of the oxidative decomposition of organic-carbonate-based electrolytes for lithium-ion cells

Abstract A new cell developed for in situ FTIR investigations was used to investigate the oxidative decomposition of propylene carbonate with different amounts of water. Further subtractively normalized interfacial (SNIFTIRS) spectra were obtained for the oxidation products of 1 M LiPF 6 in ethylene carbonate (EC) and of 1 M LiPF 6 in dimethylcarbonate (DMC). Finally, the decomposition products of a solution of 1 M LiPF 6 in a 1:1 EC–DMC mixture were investigated and the corresponding spectra assigned to decomposition products of the single-solvent electrolyte solutions.

In situ investigation of the interaction between graphite and electrolyte solutions

The formation of a solid electrolyte interphase (SEI) before and during lithium intercalation was studied on graphite electrodes in ethylene carbonate based electrolytes. We demonstrated by using in situ mass spectrometry that during the first charge of the graphite electrode ethylene gas is evolved in a potential window that corresponds to the formation of a SEI. Moreover, development of hydrogen gas was detected even in dry electrolytes containing <10 ppm H2O. No CO2 is developed however, as confirmed by two in situ methods, mass spectrometry and infrared spectroscopy. We conclude that the formation of the SEI is a complex process which depends among other things on the amount of trace water present in the cell. In addition, in situ Raman mapping experiments revealed that lithium intercalation into graphite does not proceed homogeneously.

In Situ Characterization of a Graphite Electrode in a Secondary Lithium-Ion Battery Using Raman Microscopy

A Raman microscopy study of lithium intercalation into the graphite electrode of a lithium-ion battery is presented. An in situ spectro-electrochemical cell was designed for direct observation of the electrode/electrolyte interface. The performance of this cell is discussed in terms of the results of a calibration experiment performed at a single defined point on the electrode surface. The electrode was made of TIMREX SFG 44 synthetic graphite with polyvinylidene fluoride binder. The electrolyte was lithium perchlorate, LiClO4, dissolved in a mixture of ethylene carbonate and dimethyl carbonate. In the region of the carbonyl stretching vibrational modes of the electrolyte components, changes in the band profile have been observed. At electrode potentials negative to 180 mV vs. Li/Li+, a new band evolved at about 1850 cm−1. This band has tentatively been assigned to a complex between lithium ions and decomposition products of the ethylene carbonate electrolyte component. The maximum intensity of this new band is observed at 5 mV vs. Li/Li+; its intensity decreases with increasing potential upon lithium de-intercalation. Raman mapping of the graphite electrode under potentiostatic conditions indicates that lithium intercalation does not proceed homogeneously over the graphite electrode surface at a potential of 200 mV vs. Li/Li+. An additional Raman mapping study was performed under galvanostatic conditions. With this method, the presence of “blind spots” on the electrode surface can be detected. These points lag behind in the process of lithium intercalation. Furthermore, information on changes in the carbon component of the electrode can be inferred from these measurements.

Safety Aspects of Graphite Negative Electrode Materials for Lithium-Ion Batteries

Safety aspects of different graphite negative electrode materials for lithium-ion batteries have been investigated using differential scanning calorimetry. Heat evolution was measured for different types of graphitic carbon between 30 and 300°C. This heat evolution, which is irreversible, starts above 100°C. From the values of energy evolved, the temperature rise in complete lithium-ion cells was estimated. The heat evolved between 80 and 220°C is a linear function of the irreversible charge capacity of the carbon. The specific Brunauer, Emmett, and Teller method surface area measured by nitrogen gas adsorption, which is usually also a linear function of irreversible charge capacity, may be used with certain reservations to calculate approximately the heat evolution of graphitic carbon negative electrode materials in lithium-ion batteries. Graphite materials are usually safer if their irreversible charge capacity during the first cycle is low.

Purely Hexagonal Graphite and the Influence of Surface Modifications on Its Electrochemical Lithium Insertion Properties

High-temperature treatment of the highly crystalline synthetic graphite TIMREX® SLX50 under inert gas atmosphere led to an increased crystallinity with no evidence of rhombohedral stacking defects in the hexagonal graphite crystal structure as well as a significantly lower specific BET (Brunauer-Emmett-Teller) surface area. The first electrochemical Li + insertion in this purely hexagonal graphite indicated coinsertion of solvated lithium ions which caused significant exfoliation of the graphite structure and an increased irreversible capacity compared to the untreated graphite. A progressive oxidation treatment of the heat-treated TIMREX® SLX50 in air preserved its purely hexagonal crystal structure. However, the exfoliation effects during the first electrochemical Li + insertion disappeared gradually with the oxidation temperature and finally vanished at oxidation temperatures above 800°C. Surface analysis investigations on TIMREX® SLX50 before and after heat-treatment indicated a surface curing effect. The amounts of prismatic surfaces (polar edges), low-energy defects located on the graphite basal planes, disordered carbon on the graphite particle surface, as well as the superficial oxygen concentration decreased as a result of the heat-treatment. A progressive oxidation of the heat-treated hexagonal graphite tends to increase the amount of disordered carbon atoms, the oxygen atom concentration, as well as the amount of prismatic surfaces, but keeps the number of low-energy defects unchanged. These results indicated that not the graphite crystal structure hut the surface properties are the responsible parameters for the exfoliation of the graphite structure and the irreversible capacity observed during the first electrochemical Li + insertion.

Key factors for the cycling stability of graphite intercalation electrodes for lithium-ion batteries

Aspects of the charge loss during the first cycle and the cycling stability of lithium-ion batteries are discussed as functions of water and oxygen impurities in their electrolyte solutions. Differential electrochemical mass spectrometry revealed different decomposition products depending on the water content. Carbon black, copper particles, and nickel particles were added to the graphite electrodes in order to improve their cycling stability.

Influence of graphite surface modifications on lithium intercalation properties

Abstract The first galvanostatic reduction curve of the lithium intercalation into several treated graphite samples is discussed with respect to the effect of surface treatment on the intercalation process.