Improved electrochemical performance of LiMO2 (M=Mn, Ni, Co)-Li2MnO3 cathode materials in ionic liquid-based electrolyte

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in N-butyl-N- methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) (1:9 in molar ratio) is successfully tested as electrolyte for the high voltage LiMO 2-Li2MnO3 (cathode)/lithium (anode) cells at elevated temperature (40 C). Compared to conventional electrolytes, such as 1 M LiPF6 solution in the mixed solvent of ethylene and dimethyl carbonate (EC:DMC = 1:1), the use of PYR14FSI-LiTFSI electrolyte results in a net improvement of LiMO2-Li2MnO3 cycling stability while granting comparable initial capacity. In addition, the ionic conductivity of the ionic liquid-based electrolyte at 40 C is high enough to sustain the excellent rate capability of this cathode material. Li/LiMO 2-Li2MnO3 cells delivered initial capacity exceeding 200 mA h g-1 at high current rate (2 C) while retaining 94% of the initial capacity after 100 cycles. Differential capacity versus potential analysis and post-mortem characterization by scanning electron microscope, X-ray diffraction and were carried out to explain the improved performance of LiMO2-Li2MnO3 in the IL-based electrolyte. © 2013 Elsevier B.V. All rights reserved

Truncated octahedral LiNi0.5Mn1.5O4 cathode material for ultralong-life lithium-ion battery: Positive (100) surfaces in high-voltage spinel system

So far, it has not yet reached an agreement that (111) surfaces or (100) surfaces are more positive to electrochemical performance in the spinel system. Herein, we present the synthesis of regular truncated octahedral high-voltage spinel LiNi0.5Mn1.5O4 single crystals with preferred growth of (100) surfaces, which incredibly exhibit the best long-term cycling stability compared with the state-of-art spinel material. The capacity retention is about 90% after 2000 cycles at 1 C. The extraordinary performance is mostly attributed to the highly regular truncated octahedral microstructure with large portions of stable (100) facets, which can stabilize the spinel structure to effectively suppress the side reactions with the electrolyte at high operating voltage and are also orientated to support Li+ transport kinetics. Therefore, our work further promotes the practical application of LiNi0.5Mn1.5O4 cathode material in next generation Lithium-ion batteries with high energy density and power performance

How do reactions at the anode/electrolyte interface determine the cathode performance in lithium-ion batteries?

Today, it is common knowledge, that materials science in the field of electrochemical energy storage has to follow a system approach as the interactions between active materials, electrolyte, separator and various inactive materials (binder, current collector, conductive fillers, cell-housing, etc.) which are of similar or even higher importance than the properties and performance parameters of the individual materials only. In particular, for lithium-ion batteries, it is widely accepted that the electrolyte interacts and reacts with the electrodes. Here, we report how reactions at a graphite anode (involving electrolyte decomposition and solid electrolyte interphase (SEI) formation), affect the performance of a LiCoO2 (LCO) cathode and the full lithium-ion cell during cycling. We discuss effects of the SEI-forming electrolyte additive vinylene carbonate (VC) and the influence of graphite anodes with different surface areas on the cycling stability, end of charge (EOC) and end of discharge (EOD) potentials of the LCO cathode. We will thus elucidate the failure mechanism of LCO/graphite cells by showing that the formation and growth of SEI on the anode, resistance increase in the cathode, electrode and electrolyte degradation in general, as well as capacity and power fade of the lithium ion cell are in fact strongly interrelated processes. © 2013 The Electrochemical Society

Best Practice: Performance and Cost Evaluation of Lithium Ion Battery Active Materials with Special Emphasis on Energy Efficiency

In order to increase the energy content of lithium ion batteries (LIBs), researchers worldwide focus on high specific energy (Wh/kg) and energy density (Wh/L) anode and cathode materials. However, most of the attention is primarily paid to the specific gravimetric and/or volumetric capacities of these materials, while other key parameters are often neglected. For practical applications, in particular for large size battery cells, the Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) have to be considered, which we point out in this work by comparing numerous LIB active materials. For all presented active materials, energy inefficiency is mainly caused by a voltage inefficiency, which in turn is affected by the voltage hysteresis between the charge and discharge curves. Hence, this study could show that materials with larger voltage hysteresis such as the ZnFe2O4 (ZFO) anode or the Li-rich cathode material exhibit also a lower VE and EE than for instance graphite and LiNi0.5Mn1.5O4. Furthermore, from the accumulated EE losses the resulting "extra energy costs" are calculated based on industry and domestic electricity costs in Germany, in Japan and in the U.S.A. In particular, in countries with higher electricity costs such as Germany, the accumulated extra energy, which is necessary to compensate the energy inefficiency while retaining a certain energy level in the electrode material, has a stronger impact on the extra energy costs and thus on the total cost of ownership of the battery cell system

On the structural integrity and electrochemical activity of a 0.5Li 2MnO3·0.5LiCoO2 cathode material for lithium-ion batteries

Structural changes in a 0.5Li2MnO3·0. 5LiCoO2 cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li2MnO3 and LiCoO2 components 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 1 st cycle can be attributed to an irreversible oxygen release from Li2MnO3 domains 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 Li2MnO3 domains 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 LiCoO2 domains occurs topotactically, involving a conventional Co3+/Co4+ redox reaction. The presence of Li2MnO3 domains and their unusual participation in electrochemical processes enable LiCoO2 domains of the material to sustain a higher cut-off voltage without undergoing irreversible structural changes. © 2014 the Partner Organisations

Nanostructured ZnFe2O4 as anode material for lithium-ion batteries: Ionic liquid-assisted synthesis and performance evaluation with special emphasis on comparative metal dissolution

In this work, a ZnFe2O4 anode material was successfully synthesized by a novel ionic liquid-assisted synthesis method followed by a carbon coating procedure. The as-prepared ZnFe2O4 particles demonstrate a relatively homogeneous particle size distribution with particle diameters ranging from 40 to 80 nm. This material, which is well known to offer an interesting combination of an alloying and conversion mechanism, is capable of accommodating nine equivalents of lithium per unit formula, resulting in a high specific capacity (≥ 1,000 mAh g-1). The resulting composite anode material displayed a stable capacity of ca. 1,091 mAh g-1 for 190 cycles at a medium de-lithiation potential of 1.7 V and at a charge/discharge rate of 1C. Furthermore, the material displays an excellent high rate capability up to 20C, displaying a reversible capacity of still 216 mAh g-1. Studies on Fe and Zn losses of the ZnFe2O4 active material by dissolution in the electrolyte were performed and compared to those of silicon-, germanium- and tin-based high-capacity anode materials. In conclusion, ion dissolution from metal containing anode materials should not be underestimated in view of its impact on the overall cell performance and cycling stability

Ionic liquid-assisted solvothermal synthesis of hollow Mn2O3 anode and LiMn2O4 cathode materials for Li-ion batteries

Mn-based Mn2O3 anode and LiMn2O4 cathode materials are prepared by a solvothermal method combined with post annealing process. Environmentally friendly ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate as both structure-directing agent and fluorine source is used to prepare hollow polyhedron MnF2 precursor. Both target materials Mn2O3 anode and LiMn2O4 cathode have the morphology of the MnF2 precursor. The Mn2O3 anode using carboxymethyl cellulose as binder could deliver slight better electrochemical performance than the one using poly (vinyldifluoride) as binder. The former has an initial charge capacity of 800 mAh g-1 at a current density of 101.8 mA g-1, and exhibits no obvious capacity decay for 150 cycles at 101.8 mA g-1. The LiMn2O4 cathode material prepared with molten salt assistant could display much better electrochemical performance than the one prepared without molten salt assistance. In particular, it has an initial discharge capacity of 117.5 mAh g-1 at a current density of 0.5C and good rate capability. In the field of lithium ion batteries, both the Mn2O3 anode and LiMn2O4 cathode materials could exhibit enhanced electrochemical performance due to the well formed morphology based on the ionic liquid-assisted solvothermal method

Structural changes in a Li-rich 0.5Li2MnO3∗0.5LiMn0.4Ni0.4Co0.2O2 cathode material for Li-ion batteries: A local perspective

Local structural changes in a Li-rich 0.5Li2MnO3∗0.5LiMn0.4Ni0.4Co0.2O2 cathode material are investigated using X-ray absorption spectroscopy (XAS). The element-selective nature of XAS revealed the composite structure of the material, where both Li2MnO3 and LiMn0.4Ni0.4Co0.2O2 components exist as separate domains and also exhibit a distinct electrochemical response. An irreversible oxygen release from Li2MnO3 domains contributes to a large irreversible capacity delivered by the material during activation and gives rise to the formation of a layered MnO2-type structure. Lithium reinsertion into this layered MnO2-type structure during discharge reforms the original Li2MnO3-type structure, which is lithium and oxygen deficient. The average valence state of Mn in Li2MnO3 domains remains unchanged at 4+ during charge and discharge, suggesting an unusual participation of oxygen anions of Li2MnO3 domains in redox processes. On the contrary, electrochemical processes in LiMn0.4Ni0.4Co0.2O2 domains involve conventional redox processes of transition-metal (TM) ions. In addition to Ni2+/Ni4+ and Co3+/Co4+ redox reactions, a small amount of Mn3+ detected in LiMn0.4Ni0.4Co0.2O2 domains also participates in electrochemical processes via a Mn3+/Mn4+ redox reaction. All structural modifications introduced into the material during activation are recovered upon discharge to 2.5 V, except those caused by the permanent removal of oxygen from Li2MnO3 domains

Aging of Li2FeSiO4 cathode material in fluorine containing organic electrolytes for lithium-ion batteries

The stability vs. aging of Li2FeSiO4 (LFS) cathode material in fluorine-based electrolytes, especially at elevated temperature, was studied in this work. The LFS powder was initially synthesized using a hydrothermal route and then aged at 60 °C for 40 days in LiPF6 and LiBF4-based electrolytes. The residual powder and the electrolyte were investigated afterwards. In the case of LiPF6, a structural and compositional change of LFS to Li2SiF6 was observed by XRD. SEM images confirmed that this change led to a morphology change of the aged material. XPS, EDX and ICP-OES measurements showed a large increase of fluorine content inside the residual powder. NMR investigations indicated an accelerated decomposition of electrolyte in the presence of LFS compared to the electrolyte aged without LFS. Our results suggest a degradation of LFS to Li2SiF6 in the fluorine-based electrolyte at elevated temperatures while the electrolyte decomposition is accelerated. © 2011 Elsevier Ltd