Comprehensive study of the CuF<inf>2</inf> conversion reaction mechanism in a lithium ion battery

Conversion materials for lithium ion batteries have recently attracted considerable attention due to their exceptional specific capacities. Some metal fluorides, such as CuF2, are promising candidates for cathode materials owing to their high operating potential, which stems from the high electronegativity of fluorine. However, the high ionicity of the metal–fluorine bond leads to a large band gap that renders these materials poor electronic conductors. Nanosizing the active material and embedding it within a conductive matrix such as carbon can greatly improve its electrochemical performance. In contrast to other fluorides, such as FeF2 and NiF2, good capacity retention has not, however, been achieved for CuF2. The reaction mechanisms that occur in the first and subsequent cycles and the reasons for the poor charge performance of CuF2 are studied in this paper via a variety of characterization methods. In situ pair distribution function analysis clearly shows CuF2 conversion in the first discharge. However, few structural changes are seen in the following charge and subsequent cycles. Cyclic voltammetry results, in combination with in situ X-ray absorption near edge structure and ex situ nuclear magnetic resonance spectroscopy, indicate that Cu dissolution is associated with the consumption of the LiF phase, which occurs during the first charge via the formation of a Cu1+ intermediate. The dissolution process consequently prevents Cu and LiF from transforming back to CuF2. Such side reactions result in negligible capacity in subsequent cycles and make this material challenging to use in a rechargeable battery.We acknowledge the funding from the U.S. DOE BES via funding to the EFRC NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294 (support for Rosa Robert and Lin-Shu Du) and EPSRC via the “nanoionics” programme grant (support for Xiao Hua). Use of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.This is the final published version of the article. It first appeared at http://pubs.acs.org/doi/abs/10.1021/jp503902z and is posted here under the terms of ACS's Editors' Choice scheme (http://pubs.acs.org/page/policy/authorchoice_termsofuse.html)

Cycling Behavior of a High Voltage Spinel Using an Original Three Electrodes Li1-xNi0.4Mn1.6O4//Li//LiNi0.4Mn1.6O4 Symmetric Cell: Application to LiNi0.4Mn1.6O4 Electrolyte Interface Degradation Studies

International audienceThe interface between LiNi0.4Mn1.6O4 and alkylcarbonate-based electrolytes is investigated by ab initio calculations, ICP-AES measurements and electrochemical tests. Interface degradation is known to occur by both the electrolyte oxidation and the Mnn+ and Nin+ ion dissolution. Nevertheless, EC or PC oxidation, leading to a polymeric film formation, is able to contribute to the interface stabilization. Besides Li//LiNi0.4Mn1.6O4 half-cells, Li1-xNi0.4Mn1.6O4//LiNi0.4Mn1.6O4 symmetric cells are used in order to eliminate the effects from the strong reducing nature of lithium on the electrolyte. Systematic comparisons of fading and coulombic efficiency show that the main degradation mechanism in half-cells is the electrolyte oxidation, as a consequence of the continuous precipitation of metal ion-based compounds on the lithium electrode. The symmetric cell studies indicate that redox shuttles (Mn+ ↔ M(n-1)+, M=Mn or Ni) are mainly responsible for the LiNi0.4Mn1.6O4/electrolyte interface degradation despite the possible presence of a polymeric film. Symmetric cells also confirm EC superiority over other alkylcarbonates at the LiNi0.4Mn1.6O4 interface

Evaluate Sulfone-Based Reduction Sensitive Electrolytes with Lithium Li4Ti5O12/Li and Symmetric Li4+XTi5O12/Li4Ti5O12 Cells

International audienceBinary mixtures of cyclic or acyclic sulfones with EMC or DMC are used in electrolytes containing LiPF6 (1M) in both Li4Ti5O12/Li half-cells and Li4+xTi5O12/Li4Ti5O12 symmetrical cells and compared with standard EC/EMC or EC/DMC mixtures. In half-cells, sulfone-based electrolytes cannot be satisfactorily cycled owing to the formation of a resistive layer at the lithium interface, which is not stable and generates species (RSO2-, RSO3-) able to migrate toward the titanate electrode interface. Potentiostatic and galvanostatic tests in Li4Ti5O12/Li half-cells show that charge transfer resistance increases drastically when sulfones are used in the electrolyte composition. Moreover, cycling ability and coulombic efficiency are low. At the opposite, when symmetrical Li4+xTi5O12/Li4Ti5O12 cells are used, it is demonstrated that MIS- (methyl isopropyl sulfone) and TMS- (tetra methyl sulfone) based electrolytes exhibit reasonable electrochemical performances compared to the EC/DMC or EC/EMC standard mixtures. Surface analysis by XPS of both the Li4+xTi5O12 (partially oxidized) and Li7Ti5O12 (reduced) electrodes taken from symmetrical cells reveals that sulfones do not participate in the formation of surface layers. Alkylcarbonates (EMC or DMC), used as co-solvents in sulfone-based binary electrolytes, ensure the formation of surface layers at the titanate interfaces

Chemical and Structural Stability of Lithium-Ion Battery Electrode Materials under Electron Beam

The investigation of chemical and structural dynamics in battery materials is essential to elucidation of structure-property relationships for rational design of advanced battery materials. Spatially resolved techniques, such as scanning/transmission electron microscopy (S/TEM), are widely applied to address this challenge. However, battery materials are susceptible to electron beam damage, complicating the data interpretation. In this study, we demonstrate that, under electron beam irradiation, the surface and bulk of battery materials undergo chemical and structural evolution equivalent to that observed during charge-discharge cycling. In a lithiated NiO nanosheet, a Li(2)CO(3)-containing surface reaction layer (SRL) was gradually decomposed during electron energy loss spectroscopy (EELS) acquisition. For cycled LiNi(0.4)Mn(0.4)Co(0.18)Ti(0.02)O(2) particles, repeated electron beam irradiation induced a phase transition from an [Image: see text] layered structure to an [Image: see text] rock-salt structure, which is attributed to the stoichiometric lithium and oxygen removal from [Image: see text] 3a and 6c sites, respectively. Nevertheless, it is still feasible to preserve pristine chemical environments by minimizing electron beam damage, for example, using fast electron imaging and spectroscopy. Finally, the present study provides examples of electron beam damage on lithium-ion battery materials and suggests that special attention is necessary to prevent misinterpretation of experimental results