K. Amine

Electrochemical performance of nano-sized ZnO-coated LiNi0.5Mn1.5O4 spinel as 5 V materials at elevated temperatures

ZnO-coated LiNi0.5Mn1.5O4 powders with excellent electrochemical cyclability and structural stability have been synthesized. The electrochemical performance and structural stability of ZnO-coated LiNi0.5Mn1.5O4 electrodes in the 5 V region at elevated temperature has been studied as function of the level of ZnO coating. The 1.5 wt% ZnO-coated LiNi0.5Mn1.5O4 electrode delivers an initial discharge capacity of 137 mAh g−1 with excellent cyclability at elevated temperature even at 55 °C. The reason for the excellent cycling performance of ZnO-coated LiNi0.5Mn1.5O4 electrode is largely attributed to ZnO playing an important role of HF getting in the electrolyte.

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.

Development of a high-power lithium-ion battery

Safety is a key concern for a high-power energy storage system such as will be required in a hybrid vehicle. Present lithium-ion technology, which uses a carbon/graphite negative electrode, lacks inherent safety for two main reasons: (1) carbon/graphite intercalates lithium at near lithium potential, and (2) there is no end-of-charge indicator in the voltage profile that can signal the onset of catastrophic oxygen evolution from the cathode (LiCoO{sub 2}). Our approach to solving these safety/life problems is to replace the graphite/carbon negative electrode with an electrode that exhibits stronger two-phase behavior further away from lithium potential, such as Li{sub 4}Ti{sub 5}O{sub 12}. Cycle-life and pulse-power capability data are presented in accordance with the Partnership for a New Generation of Vehicles (PNGV) test procedures, as well as a full-scale design based on a spreadsheet model.

Interpreting the structural and electrochemical complexity of 0.5Li2MnO3·0.5LiMO2 electrodes for lithium batteries (M = Mn0.5−xNi0.5−xCo2x, 0 ≤x≤ 0.5)

The structural and electrochemical features of layered 0.5Li2MnO3·0.5LiMO2 electrodes, in which M = Mn0.5−xNi0.5−xCo2x (0 ≤ x ≤ 0.5), have been studied by powder X-ray diffraction, electrochemical differential-capacity measurements, 7Li magic-angle-spinning nuclear magnetic resonance, and X-ray absorption near-edge spectroscopy. Li2MnO3-like regions in the as-prepared samples were observed for all values of x, with transition-metal cation disorder between the LiMO2 and Li2MnO3 components increasing with cobalt content (i.e., the value of x). The structural disorder and complexity of the electrochemical redox reactions increase when the Li2MnO3-like regions within the electrode are activated to 4.6 V in lithium cells; interpretations of structural and electrochemical phenomena are provided.

Surface changes on LiNi0.8Co0.2O2 particles during testing of high-power lithium-ion cells

LiNi0.8Co0.2O2 particles from high-power lithium-ion cells were examined to determine material changes that result from accelerated aging tests. X-ray absorption spectroscopy (XAS) and transmission electron microscope (TEM) data indicated a LixNi1−xO-type layer on the particle surfaces. The greater thickness on particles from high-power fade cells indicate that these surface layers are a significant contributor to cathode impedance rise observed during cell tests.

Synthesis and Electrochemical Properties of ZnO-Coated LiNi0.5Mn1.5 O 4 Spinel as 5 V Cathode Material for Lithium Secondary Batteries

ZnO-coated LiNi 0 . 5 Mn 1 . 5 O 4 powders with excellent electrochemical cyclability and structural stability at elevated temperature have been synthesized by a sol-gel method. The structural degradation of the as-preparedLiNi 0 . 5 Mn 1 . 5 O 4 and ZnO-coated LiNi 0 . 5 Mn 1 . 5 O 4 electrodes before and after cycling in the 5 V region has been studied. The ZnO-coated LiNi 0 . 5 Mn 1 . 5 O 4 electrode showed almost no capacity loss and retained its original cubic spinel structure after 50 cycles. We found that ZnO played an important role in reducing the HF content in the electrolyte solution.

The Effects of Acid Treatment on the Electrochemical Properties of 0.5 Li2MnO3 ∙ 0.5 LiNi0.44Co0.25Mn0.31O2 Electrodes in Lithium Cells

The electrochemical properties of 0.5 Li 2 MnO 3 ·0.5 LiNi 0.4 Co 0.25 Mn 0.31 O 2 electrodes, when preconditioned and activated with acid for 2-24 h, have been studied in lithium cells. Powder X-ray diffraction data and electrochemical measurements provide supporting evidence for an intergrown, composite electrode structure from which Li 2 O can be leached from the Li 2 MnO 3 (Li 2 O MnO 2 ) component with acid, thereby mimicking the electrochemical charge process at high potentials (>4.5 V). The MnO 2 -rich domains generated by acid treatment are reduced during electrochemical discharge at a lower potential than electrochemically generated MnO 2 -rich domains. With prolonged cycling between 4.6 and 2.0 V, dQ/dV plots of untreated and acid-treated electrodes develop similar, but not identical, character, suggesting a coalescence and redox interaction of the manganese ions in MnO 2 -rich and Ni 0.44 Co 0.25 Mn 0.31 O 2 regions of the structure. Acid treatment eliminates the first-cycle capacity loss of the electrodes, consistent with earlier reports for related systems, but it damages their cycling stability and rate capability.

AlF3-Coating to Improve High Voltage Cycling Performance of Li [ Ni1 ∕ 3Co1 ∕ 3Mn1 ∕ 3 ] O2 Cathode Materials for Lithium Secondary Batteries

Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 powders were modified by coating their surface with amorphous AlF3 as a new coating material. The AlF 3 -coated Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 electrode showed improved cycle performance and rate capability under a high cutoff voltage range of 4.5 and 4.6 V. AC impedance results showed that the AlF 3 -coated Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 has stable charge transfer resistance (R ct ) regardless of the cycle number. Electron diffraction analysis also showed that no structural transition of the primary particles was observed for the AlF 3 -coated electrode. Electrochemical impedance spectroscopy and electron microscopy indicate that AlF 3 coating plays an important role of stabilizing the interface between cathode and electrolyte.

Microscopy and spectroscopy of lithium nickel oxide-based particles used in high power lithium-ion cells

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy (XAS) revealed trivalent Ni and Co ions in the bulk LiNi{sub 0.8}Co{sub 0.2}O{sub 2} powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a <5 nm thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered R{bar 3}m structure. This structural change was accompanied by oxygen loss and a lowering of the Ni- and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells.

On the Safety of the Li4Ti5O12 ∕ LiMn2O4 Lithium-Ion Battery System

The aim of this work is to investigate the inherent safety characteristics of the Li 4 Ti 5 O 12 /LiMn 2 O 4 cell chemistry in a real battery. For this purpose, the reactivity of the Li 4 Ti 5 O 12 anode material with the electrolyte was first studied upon its electrochemical lithiation in a Li-metal half-cell. Results obtained by differential scanning calorimetry show that the total heat associated with this reaction increased when the lithium amount inserted in Li 4 Ti 5 O 12 increased, with no noticeable change in the onset temperature (125°C). It was also found that the total heat of the fully lithiated Li 4 Ti 5 O 12 (383 J/g) was much smaller compared to that of the fully lithiated graphite (2700 J/g), the latter having a lower onset temperature (100°C). The thermal and structural stability of Li 6.5 Ti 5 O 12 and Li o.2 Mn 2 O 4 phases was investigated after the chemical lithiation of Li 4 Ti 5 O 12 with butylithium and the chemical delithiation of LiMn 2 O 4 with nitronium tetrafluoroborate. Data from thermal gravimetric analysis show that the Li o-2 Mn 2 O 4 cathode released less than 2 wt % oxygen below 400°C, while the L1 6.5 Ti 5 O 12 anode gained 4 wt % at the same temperature. The accelerated rate calorimetry test performed on 18650-cells containing L i4 Ti 5 O 12 /LiMn 2 O 4 chemistry showed no thermal runaway, explosion, or fire. These results clearly demonstrate that the Li 4 Ti 5 O 12 /LiMn 2 O 4 battery could be one of the safest Li-ion battery systems.