X. Song

Electrochemical analysis for cycle performance and capacity fading of a lithium-ion battery cycled at elevated temperature

Laboratory-size LiNi0.8Co0.15Al0.05O2/graphite lithium-ion pouch cells were cycled over 100% DOD at room temperature and 60 8 Ci n order to investigate high-temperature degradation mechanisms of this important technology. Capacity fade for the cell was correlated with that for the individual components, using electrochemical analysis of the electrodes and other diagnostic techniques. The high-temperature cell lost 65% of its initial capacity after 140 cycles at 60 8C compared to only a 4% loss for the cell cycled at room temperature. Cell ohmic impedance increased significantly with a elevated temperature cycling, resulting in some of loss of capacity at the C/2 rate. However, as determined with slow rate testing of the individual electrodes, the anode retained most of its original capacity, while the cathode lost 65%, even when cycled with a fresh source of lithium. Diagnostic evaluation of cell components including X-ray diffraction (XRD), Raman, CSAFM and suggest capacity loss occurs primarily due to a rise in the impedance of the cathode, especially at the end-of-charge. The impedance rise may be caused in part by a loss of the conductive carbon at the surface of the cathode and/or by an organic film on the surface of the cathode that becomes non-ionically conductive at low lithium content. Published by Elsevier Science B.V.

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.

Surface studies of carbon films from pyrolyzed photoresist

Abstract Positive and negative photoresists, which are commonly used in the semiconductor industry, were deposited on silicon wafers by spin coating and then pyrolyzed at temperatures of 600–1100°C in an inert environment to produce thin carbon films. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning probe microscopy involving current-sensing atomic force microscopy (CS-AFM) were utilized to characterize the properties of the carbon films. Raman spectroscopy showed two broad bands at approximately 1360 cm −1 and 1600 cm −1 , which deconvoluted to four Gaussian bands. The origin of these bands is discussed. CS-AFM showed that the surface conductance increased with increased pyrolysis temperature, and the results are consistent with measurements by a four-point probe method. The XPS spectra revealed the presence of oxygen functional groups (Cue605O and Cue5f8O) on the carbon surface. The relative fraction of oxygen, O/C ratio, decreased as the pyrolysis temperature increased, in agreement with published results. The full-width at half-maximum of the C 1s peak obtained by XPS also decreased with increasing pyrolysis temperature.

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.

Selection of Conductive Additives in Li-Ion Battery Cathodes A Numerical Study

The lithium-ion cell has been successively improved with adoption of new cathode electrochemistries, from LiCoO 2 to higher-capacity LiNi 1-x CO 2 O 2 to lower cost LiNi 1-x Co x O 2 . The addition of conductive additives to cathode materials has been demonstrated to improve each type. Four systems have emerged as important cathodes in recent studies: (i) the spinel LiMn 2 O 4 , (ii) LiFePO 4 , (iii) the Gen 2 material, Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 , and (iv) the Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 system. The architectures of model composite cathodes were generated using our prior approach in simulating packing of polydisperse arrangements; conductivity was then simulated for several realizations of each case. A key finding was that the conductive coatings significantly improve overall conductivity. Percolation was achieved for the volume fraction of active material (≥30%) in studied cases, which was larger than the percolation threshold (29%) for a 3D spherical particulate system. Neither surface nor bulk modifications of active-material particle conductivities seem desirable targets for improvement of laminate conductivity at present. As part of future work, trade-offs between conductivity and capacity will be considered.

In situ studies of SEI formation

Abstract Electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer occur during the initial charge/discharge cycles of carbon in electrolytes used in Li-ion batteries. This paper describes our approach to characterize the formation of SEI layers on various carbonaceous materials by in situ ellipsometry. Five types of carbon samples (carbon films on glass, pyrolyzed photoresist on silicon, highly oriented pyrolytic graphite and natural graphite) with specular surfaces were characterized by Raman spectroscopy and in situ ellipsometry/electrochemical studies in ethylene carbonate–dimethyl carbonate containing a lithium salt. Raman spectroscopy showed that the carbons films deposited on glass contain broad overlapping peaks from 900 to 1700xa0cm −1 , which is indicative of the highly disordered nature of the carbon films. Changes in the ellipsometric parameters, Δ and ψ , were correlated with the formation of the SEI layer during the initial charge (intercalation) process.

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 (

Purification process of natural graphite as anode for Li-ion batteries: chemical versus thermal

Abstract The intercalation of Li ions in natural graphite that was purified by chemical and thermal processes was investigated. A new chemical process was developed that involved a mixed aqueous solution containing 30% H2SO4 and 30% NHxFy heated to 90xa0°C. The results of this process are compared to those obtained by heating the natural graphite from 1500 to 2400xa0°C in an inert environment (thermal process). The first-cycle coulombic efficiency of the purified natural graphite obtained by the chemical process is 91 and 84% after the thermal process at 2400xa0°C. Grinding the natural graphite before or after purification had no significant effect on electrochemical performance at low currents. However, grinding to a very small particle size before purification permitted optimization of the size distribution of the particles, which gives rise to a more homogenous electrode. The impurities in the graphite play a role as microabrasion agents during grinding which enhances its hardness and improves its mechanical properties. Grinding also modifies the particle morphology from a 2- to a 3-D structure (similar in shape to a potato). This potato-shaped natural graphite shows high reversible capacity at high current densities (about 90% at 1C rate). Our analysis suggests that thermal processing is considerably more expensive than the chemical process to obtain purified natural graphite.

Thermal analysis of the oxidation of natural graphite: isothermal kinetic studies

Abstract The oxidation kinetics of natural graphite particles (2–40xa0μm average particle size) with a flake-like morphology were investigated at 933, 982 and 1033xa0K. The reaction rate in air increased with increase in temperature and with a decrease in particle size of natural graphite. The activation energy, derived from the classical Arrhenius relationship, was 188±2.2xa0kJ/mol, in good agreement with published results. The activation energies of the natural graphite did not show any systematic trend with particle size. The rate constant, normalized for the area of active sites, is independent of the particle size and Brunauer–Emmet–Teller (BET) surface area, which strongly suggests that the edge sites play a significant role in the oxidation kinetics. This observation is consistent with conclusions reported in the literature that the oxidation kinetics of carbonaceous materials decreases with a decrease in active surface area.

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.