No abstract

The electrical conductivity and the impedance behavior of thin layers of amorphous silicon (a-Si), which are promising anode materials for lithium-ion batteries, were monitored in situ during the insertion/extraction of lithium in 1 M of a LiBOB (Li-bioxalato borate) propylene carbonate solution. In addition, Raman spectra of the same electrodes were recorded in situ and ex situ during lithiation/delithiation processes in the above-mentioned solutions. The conductivity of the a-Si electrode was increased by about 3.5 orders of magnitude during the course of lithium insertion. While the impedance response of these electrodes is complicated and cannot be resolved unambiguously, it is clear that the electrical conductivity influences strongly the electrodes' impedance:  a similar dependence of the electrical conductivity and the impedance of these electrodes on the potential are measured. The intensity of the Raman signal dropped significantly upon lithiation and recovered at a potential of 0.523 V vs Li/Li+. It is suggested that the drop in the intensity of the Raman signal of the silicon electrodes upon their lithiation is due to changes in the optical skin depth of the a-Si, which occur upon the formation of the Li−Si alloy.

No abstract

In this work, the behavior of composite graphite electrodes comprising synthetic graphite flakes in solutions based on a 1-methyl-1-propylpiperidinium ͓bis͑trifluoromethylsulfonyl͔͒ imide ͑MPP p TFSI͒ ionic liquid ͑IL͒ was investigated, using in situ Raman spectroscopy with microscopic lateral resolution, in conjunction with cyclic voltammetry. Both pure IL and IL solutions containing a LiN͑SO 2 CF 3 ͒ 2 ͑LiTFSI͒ salt were studied. Upon cathodic polarization, the IL cations ͑MPP p + ͒ are intercalated. This process is irreversible in a pure IL solution. When the solution comprises both IL and a Li salt ͑LiTFSI͒, the graphite electrodes can intercalate simultaneously the IL cations MPP p + and the Li cations at potentials ϳ0.5 V and below 0.3 V vs Li/Li + , respectively. The graphite electrodes become passivated due to the presence of the Li salt by the formation of surface films, which are Li-ion conducting, but electronically insulating. Hence, upon consecutive voltammetric cycling, the IL cation-intercalation is suppressed, while reversible Li intercalation becomes the dominant process. Raman spectroscopy enables one to distinguish among the various processes in these systems. Room-temperature ionic liquids ͑ILs͒ or molten salts have attracted considerable attention as alternative electrolytes for Li-ion secondary batteries due to their considerable advantages in terms of safety, namely, nonflammability and nonvolatility, together with reasonable conductivity and a wide electrochemical window.1 A basic possibility of IL application as electrolytes for Li-ion batteries is dependent on the solubility of the Li salts, which is defined by the structure of the anions of the molten salts. Two main types of ILs that dissolve Li salts are BF 4 − and bis͑trifluoromethylsulfonyl͒imide ͑TFSI͒-containing derivatives. The most available and investigated ILs that contain these two anions ͑in connection with batteries͒ are based on imidazolium and quaternary organic ammonium cations and their derivatives.As graphite and graphitized carbons have been used as major negative electrode materials for commercial Li-ion batteries, many research groups have investigated their behavior in Li-saltcontaining IL-based electrolytes.2-8 The use of pure imidazolium derivatives is restricted by their low cathodic stability, with their cathodic limit of about +1 V vs Li/Li + . 9 The more cathodically stable quaternary ammonium derivatives have another limitation at low potentials, namely, the possibility of cointercalation of the IL cations into the graphite structure at higher potentials than those of Li intercalation. 2,6,7 Imidazolium cations were proven to intercalate between graphene planes in graphite particles as well.10 Both these problems may be resolved by adding to the IL solutions additives or cosolvents, which form surface films on the graphite particles prior to organic cations intercalation ͑i.e., at higher potentials than those at which IL cations intercalate with graphite͒.2-7 Recently, it was reported that pure ILs containi...

In this paper, the study of three types of graphite electrodes in two types of ionic liquid solutions ͑ILs͒ using in situ Raman spectroscopy and X-ray diffraction in conjunction with electrochemical techniques such as voltammetry is described. The graphite materials included two types of synthetic flakes, differing from each other in their average particle size, and natural graphite flakes. The ILs included 1-methyl-1-propylpiperidinium bis͑trifluoromethyl sulfonyl͒imide ͑MPPp-TFSI͒ and 1-methyl-1-butyl pyrrolidinium bis͑trifluoromethyl sulfonyl͒imide ͑BMP-TFSI͒. The Li salt was Li TFSI. The graphite electrodes can intercalate with both Li ions and IL cations simultaneously. The latter intercalate with graphite at higher potentials ͑the onset potential is Ͼ0.7 V͒. The graphite electrodes develop passivation in the above Li TFSI/Li solutions upon their cathodic polarization, which blocks the intercalation of the IL cations but allows highly reversible intercalation with lithium. In situ Raman spectroscopy proved to be a very useful tool for studying both Li and IL cation intercalation processes with graphite electrodes and for determining their onset and reversibility. The effectiveness of the passivation of graphite electrodes in these solutions depends on both the type of graphite used and the structure of the IL cations. The most effective passivation, developed during a first cathodic polarization of the electrodes, was found for natural graphite electrodes and for MPPp ϩ -based solutions. The important factors that may determine the performance of graphite electrodes in these systems are discussed.