C.H. Chen

Symmetric cell approach and impedance spectroscopy of high power lithium-ion batteries

High power lithium-ion cells are a very promising energy source for practical hybrid vehicles. It is found that the impedance of the 18650 high-power cells using LiNi{sub 0.8}Co{sub 0.2}O{sub 2} chemistry increases with time during the beginning period of storage. A symmetric cell approach is developed to distinguish the anode and cathode effects on the impedance rise. Cathode impedance, especially charge-transfer resistance, is identified as the main component of the cell impedance and is most responsible for the rise of the cell impedance during storage at room temperature. With analysis of impedance spectra from a variety of cells, the charge-transfer process is thought to take place at the interface between the electrolyte solution and the surface of surface layers on the electrode. We also propose that the surface layers might be mixed conductors of electrons and lithium ions, instead of pure lithium-ion conductors. The nature of the surface layers on the cathode is likely different from that of the surface layers on the anode.

Factors responsible for impedance rise in high power lithium ion batteries

Abstract High-power, 18,650 lithium-ion cells have been designed and fabricated in order to understand the factors limiting the calendar life of the lithium-ion system. Each cell consisted of a LiNi0.8Co0.2O2 positive electrode, a blend of MCMB-6 and SFG-6 carbon negative electrode, and a LiPF6 in EC:DEC (1:1) electrolyte. These cells, which initially meet the power requirement set by the partnership for a new generation of vehicles (PNGV), were subjected to accelerated calendar life and cycle life testing. After testing at elevated temperatures, the cells experienced a significant impedance rise and loss of power. The fade rate of power in these cells was dependent of the state of charge and the temperature of testing. Micro-reference electrode and ac-impedance studies on symmetrical cells have confirmed that the interfacial resistance at the positive electrode was the main reason behind the impedance rise in the high power cell.

Diagnosis of power fade mechanisms in high-power lithium-ion cells☆

Abstract Hybrid electric vehicles (HEV) need long-lived high-power batteries as energy storage devices. Batteries based on lithium-ion technology can meet the high-power goals but have been unable to meet HEV calendar-life requirements. As part of the US Department of Energy’s Advanced Technology Development (ATD) Program, diagnostic studies are being conducted on 18650-type lithium-ion cells that were subjected to accelerated aging tests at temperatures ranging from 40 to 70xa0°C. This article summarizes data obtained by gas chromatography, liquid chromatography, electron microscopy, X-ray spectroscopy and electrochemical techniques, and identifies cell components that are responsible for the observed impedance rise and power fade.

Nanoporous cuprous oxide/lithia composite anode with capacity increasing characteristic and high rate capability

Reticular nanoporous thin films of Cu2O?Li2O (Cu:Li = 1:1) composite electrodes supported on nickel foam and copper foil substrates were prepared under optimized conditions at 250??C by the electrostatic spray deposition (ESD) technique. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) indicate that the as-deposited films are composed of Cu2O and Li2O. It is found that such a film as an electrode in lithium cells exhibits abnormal characteristics of strong and continuous capacity rise (up?to 0.81% per cycle) and outstanding rate capability. Even before a long-time cycling, the specific capacity at 1, 5, 10 and 15?C is 400, 320, 310 and 305?mA?h?g?1 for the Cu2O?Li2O film supported on nickel foam substrate. XPS analysis and AC impedance spectroscopy suggest two mechanisms, i.e.?valence-related capacity rise and surface-related capacity rise. These composite films may be useful in high power and high energy density lithium-ion batteries.

Symmetric cell approach towards simplified study of cathode and anode behavior in lithium ion batteries

Abstract It was observed that the impedance of 18xa0650 high-power cells using LiNi 0.8 Co 0.2 O 2 chemistry increased with time during the beginning period of storage. A symmetric cell approach was adopted to isolate the effects of the positive and negative electrodes on the impedance of a laboratory-scale lithium ion cell using the same chemistry. The AC impedance spectra of a normal cell and symmetric cells were measured and compared. The cathode processes were found to be most responsible for the cell impedance and the impedance rise. In addition, the impedance spectra provide indirect evidence that surface layers might be formed on both the positive and negative electrodes.

Material characterization and electrochemical study on LiNi{sub 0.95}Ti{sub 0.05}O{sub 2} materials.

Layered LiNi{sub 0.95}Ti{sub 0.05}O{sub 2} cathodes were prepared by solid-state reaction and characterized by various techniques. X-ray diffraction shows that the samples have highly ordered and phase-pure layered compounds after heating at 750{sup o}C in an O{sub 2} atmosphere. Scanning electron microscopy and particle size distribution analysis reveal that the LiNi{sub 0.95}Ti{sub 0.05}O{sub 2} material has spherical morphology with particle size of about 12 {mu}m. The LiNi{sub 0.95}Ti{sub 0.05}O{sub 2} electrodes exhibited large capacity and excellent electrochemical behavior. X-ray photoelectron spectroscopy confirms the existence of tetravalent titanium as a substituent. Indications are that the structural integrity of the LiNi{sub 1-x}Ti{sub x}O{sub 2} materials was preserved because the Ti{sup 4+} ions prevented impurity Ni{sup 2+} migration into the lithium sites. Hybrid pulse power characterization testing shows the area specific impedance of 22 {Omega}cm{sup 2}, indicating that the materials could provide more than enough power to level the load on the main engine of the hybrid electric vehicle. Aging tests also show stable impedance values as a function of time (days) at 55{sup o}C, which indicates that the battery based on this material could exhibit long calendar life.