On the structural integrity and electrochemical activity of a 0.5Li(2)MnO(3)center dot 0.5LiCoO(2) cathode material for lithium-ion batteries

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Structural changes in a 0.5Li2MnO3·0.5LiCoO2cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li2MnO3and LiCoO2components of the material exist as separate domains, however, with some exchange of transition metal (TM) ions in their slab layers. A large irreversible capacity observed during activation of the material in the 1stcycle can be attributed to an irreversible oxygen release from Li2MnO3domains during lithium extraction. The average valence state of manganese ions remains unchanged at 4+ during charge and discharge. In the absence of conventional redox processes, lithium extraction/reinsertion from/into Li2MnO3domains occurs with the participation of oxygen anions in redox reactions and most likely involves the ion-exchange process. In contrast, lithium deintercalation/intercalation from/into LiCoO2domains occurs topotactically, involving a conventional Co3+/Co4+redox reaction. The presence of Li2MnO3domains and their unusual participation in electrochemical processes enable LiCoO2domains of the material to sustain a higher cut-off voltage without undergoing irreversible structural changes.EC/EFRE/200720132-35/EU//BATMA

Al2O3 coating on anode surface in lithium ion batteries: Impact on low temperature cycling and safety behavior

Commercial 18650-type lithium ion cells employing an Al2O3 coating on the anode surface as a safety feature are investigated regarding cycling behavior at low temperatures and related safety. Due to irreversible lithium metal deposition, the cells show a pronounced capacity fading, especially in the first cycles, leading to a shortened lifetime. The amount of reversibly strippable lithium metal decreases with every cycle. Post-mortem analysis of electrochemically aged anodes reveals a thick layer of lithium metal deposited beneath the coating. The Al2O3 coating on the electrode surface is mostly intact. The lithium metal deposition and dissolution mechanisms were determined combining electrochemical and post-mortem methods. Moreover, the cell response to mechanical and thermal abuse was determined in an open and adiabatic system, revealing a similar behavior of fresh and aged cells, thus, demonstrating no deterioration in the safety behavior despite the presence of a thick lithium metal layer on the anode surface

A Step toward High-Energy Silicon-Based Thin Film Lithium Ion Batteries

The next generation of lithium ion batteries (LIBs) with increased energy density for large-scale applications, such as electric mobility, and also for small electronic devices, such as microbatteries and on-chip batteries, requires advanced electrode active materials with enhanced specific and volumetric capacities. In this regard, silicon as anode material has attracted much attention due to its high specific capacity. However, the enormous volume changes during lithiation/delithiation are still a main obstacle avoiding the broad commercial use of Si-based electrodes. In this work, Si-based thin film electrodes, prepared by magnetron sputtering, are studied. Herein, we present a sophisticated surface design and electrode structure modification by amorphous carbon layers to increase the mechanical integrity and, thus, the electrochemical performance. Therefore, the influence of amorphous C thin film layers, either deposited on top (C/Si) or incorporated between the amorphous Si thin film layers (Si/C/Si), was characterized according to their physical and electrochemical properties. The thin film electrodes were thoroughly studied by means of electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. We can show that the silicon thin film electrodes with an amorphous C layer showed a remarkably improved electrochemical performance in terms of capacity retention and Coulombic efficiency. The C layer is able to mitigate the mechanical stress during lithiation of the Si thin film by buffering the volume changes and to reduce the loss of active lithium during solid electrolyte interphase formation and cycling