Kim Kinoshita

The role of Li-ion battery electrolyte reactivity in performance decline and self-discharge

Abstract The purpose of this paper is to report on the reactivity of PF 5 and EC/linear carbonates to understand the thermal and electrochemical decomposition reactions of LiPF 6 in carbonate solvents and how these reactions lead to the formation of products that impact the performance of lithium-ion batteries. The behavior of other salts such as LiBF 4 and LiTFSI are also examined. Solid LiPF 6 is in equilibrium with solid LiF and PF 5 gas. In the bulk electrolyte, the equilibrium can move toward products as PF 5 reacts with the solvents. The Lewis acid property of the PF 5 induces a ring-opening polymerization of the EC that is present in the electrolyte and can lead to PEO-like polymers. The polymerization is endothermic until 170xa0°C and is driven by CO 2 evolution. Above this temperature the polymerization becomes exothermic and leads to a violent decomposition. The PEO-like polymers also react with the PF 5 to yield further products that may be soluble in the electrolyte or participate in solid electrolyte interphase (SEI) formation in real cells. GPC analysis of the heated electrolytes indicates the presence of material with M w up to 5000. More details on the polymerization reactions and further reactions with PF 5 are reported. Transesterification and polymer products are observed in the electrolytes of cycled and aged Li-ion cells. Formation of polymer materials which are further cross-linked by reaction with acidic species leads to degradation of the transport properties of the electrolyte in the composite electrodes with the accompanying loss of power and energy density. Generation of CO 2 in lithium-ion cells leads to saturation of the electrolyte and cessation of the polymerization reaction. However, CO 2 is easily reduced at the anode to oxalate, carbonate and CO. The carbonate contributes to the SEI layer while the oxalate is sufficiently soluble to reach the cathode to be re-oxidized to CO 2 thus resulting in a shuttle mechanism that explains reversible self-discharge. Irreversible reduction of CO 2 to carbonate and CO partially accounts for irreversible self-discharge.

Commercial Carbonaceous Materials as Lithium Intercalation Anodes

Commercial carbonaceous materials were examined as lithium intercalation anodes in propylene carbonate (carbons) and ethylene carbonate/dimethyl carbonate (graphites) electrolytes. The reversible capacity (180--355 mAh/g) and the irreversible capacity loss (15--200% based on reversible capacity) depends on the type of binder, carbon type, morphology, and phosphorus doping concentration. A carbon-based binder was chosen for electrode fabrication, producing mechanically and chemically stable electrodes and reproducible results. Several types of graphites had capacity approaching LiC{sub 6}. Petroleum fuel green cokes doped with phosphorus gave more than a 20% increase in capacity compared to undoped samples. Electrochemical characteristics are related to scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Brunauer, Emmett, and Teller method measurements.

Electrochemical Studies of Carbon Films from Pyrolyzed Photoresist

Carbon film electrodes were prepared by pyrolysis of photoresists on silicon wafers at temperatures ranging from 600 to 1,100 C. The physical properties of the carbon films were characterized by scanning and transmission electron microscopies, thermal gravimetric analysis, and four-point probe electrical resistivity measurements. The electrochemical properties of the carbon films were investigated by cyclic voltammetry to observe the kinetics of the Fe(CN){sub 6}{sup 4{minus}}/Fe(CN){sub 6}{sup 3{minus}} redox couple. The carbon film electrodes prepared at temperatures {ge} 700 C showed electrochemical behavior similar to that of glassy carbon. Better electrocatalytic behavior was obtained with carbon films prepared at the higher pyrolysis temperatures, which is attributed to different film compositions at different pyrolysis temperatures. The electrochemical properties of the carbon film electrodes are very stable, exhibiting reproducible behavior even after storing at room temperature in air for 3 months.

Electrochemical Measurements on Pt, Ir, and Ti Oxides as pH Probes

Donnees sur les mesures dimpedance et de potentiel en circuit ouvert doxydes dIr, de Pt et de Ti. On montre que la relation entre le potentiel en circuit ouvert et le pH des electrodes a oxydes suit un comportement nernstien et que le temps de reponse du potentiel a une variation de pH des oxydes les plus conducteurs sur Pt et Ir est comparable a celle dune electrode de verre pour pH metrie

Rate effect on lithium-ion graphite electrode performance

The electrochemical performance of lithium-ion graphite electrodes with particle diameter in the range of 6–44 µm was evaluated at different discharge (intercalation)/charge (deintercalation) rates (C to C/60). The electrode capacity depends on both the average particle size and rate. With a simple rate programme, the electrode performance is dependent on the cycling rate. The capacity of small graphite particles (6 µm) at C/2 rate was 80% of that achieved at C/24 rate (∼372 mAh g−1). The capacity of large graphite particles (44 µm) obtained at fast rates (C/2) was only 25% of that obtained under near-equilibrium conditions (C/24). The electrode capacity, however, is nearly independent of the charge rate when the electrode is fully intercalated using a modified rate programme containing a constant-voltage hold at 0.005 V (vs Li+/Li) for several hours. The electrochemical behaviour is related to the physicochemical properties of the graphite particles.

Development of a carbon-based lithium microbattery

Abstract A conceptual design for a carbon-based rechargeable Li microbattery and the progress in fabricating the electrode microstructures are described in this paper. The microstructures are produced from photoresists that are typically used by the semiconductor industry. The photoresist is spin coated on a silicon wafer, `patterned by photolithography and then heated in an inert environment to form carbon microstructures (

Microstructural Characterization of Lithiated Graphite

The microstructures of lithiated graphite were studied using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). HRTEM shows lattice images of the (001) layers of LiC{sub 6} with layer spacing of 3.70 {angstrom}, consistent with XRD. The morphology and distribution of the LiC{sub 6} and LiC{sub 12} phases were investigated by dark field image and selected-area electron diffraction in TEM. The results indicate that LiC{sub 6} and LiC{sub 12} phases can coexist in the lithiated graphite particle. The application is to lithium rechargeable batteries.

Surface modification of carbons for enhanced electrochemical activity

Abstract Graphitic materials have at least two distinct types of surface sites, namely the basal plane and edge plane sites. It is generally regarded that the active sites for electrochemical reactions are associated with the edge plane sites, while the basal plane is relatively inactive. In this study, controlled gas-phase reactions for oxidation/reduction were used to modify the surface of highly oriented pyrolytic graphite and carbon fibers by introducing edge plane sites. In some experiments, catalysts were used to enhance the formation of edge sites. The effects of the gas-phase treatments were monitored by electrochemical measurements of the double layer capacitance and the kinetics of oxygen reduction in alkaline electrolyte. The correlation between the electrochemical results and changes to the surface microstructure is discussed. The measurements clearly show that the electrochemical behavior is improved by introducing more edge sites or defects on the carbon surface. These results are consistent with other studies

Negative Electrodes for Li-Ion Batteries

Graphitized carbons have played a key role in the successful commercialization of Li-ion batteries. The physicochemical properties of carbon cover a wide range; therefore identifying the optimum active electrode material can be time consuming. The significant physical properties of negative electrodes for Li-ion batteries are summarized, and the relationship of these properties to their electrochemical performance in nonaqueous electrolytes, are discussed in this paper.

Lithium intercalation in heat-treated petroleum cokes

Abstract Petroleum needle cokes were processed by air-milling and heat treatment at three temperatures 1800, 2100 and 2350 °C, to produce a final average particle size of 10 p.m. The effects of air-milling (before and after heat treatment) on the physical and microstructural properties of the petroleum coke particles were examined. The results obtained for electrochemical lithium intercalation/de-intercalation in 0.5 M LiN(CF3SO2)2/EC:DMC electrolyte using these petroleum cokes after the different processing conditions are reported.