In situ SEM study of the interfaces in plastic lithium cells
Introduction
Rechargeable Li0-batteries present safety problems because of the growth of dendrites at the anode/electrolyte interface. To alleviate this problem, lithium metal was replaced by an insertion electrode, such as graphite or coke [1]. However the energy density of these Li-ion batteries is lower than for the Li0-cells which theoretically stay the most promising rechargeable batteries. In order to solve this dendrite issue, we decided to study the way by which lithium deposition occurs by means of electron microscopy techniques. More specifically, we observed the lithium deposit on Li, Cu and graphite at different cycling rates, in order to correlate the morphology to the current density.
Although lithium deposits have been already observed by Scanning Electron Microscopy (SEM) [2], these experiments were performed with samples which were exposed to air during the transfer to the microscope, so that reactions between air and lithiated compounds could mask the reality. In this proceeding, we report cross-section micrographs from plastic cells of various configurations obtained by means of a specific transfer system, designed by Philips, preventing any air exposure during the transfer of the cell from a glove box into the SEM. Besides, the sample was cooled down to limit the solvent evaporation due to the vacuum in the antechamber during the observation.
Section snippets
Experimental
The plastic laminates of the cells were based on the Bellcore plastic technology [3]. The separator film consisted of a copolymer matrix (polyvinylidene fluoride hexafluoropropylene PVdF-HFP) mixed with SiO2 as filter, and dibutyl phtalate (DBP) as plasticizer. The plastic cathode film was composed of LiMn2O4, PVdF-HFP, DBP, and carbon black. These cathode and separator films were laminated with an aluminum collector to form the positive electrode used for the three kinds of cells studied. This
Lithium batteries
The influence of the current densities on the capacity retention is shown in Fig. 2. As expected, the larger the current density, the quicker the capacity fades upon cycling. For instance, the C/5 cycling rate is characterized by a dramatic capacity loss after 15 cycles. This phenomenon will be explained by the following SEM micrographs depicting the state of the interfaces.
Cross-section images of a not cycled battery are shown in Fig. 3. The cathode is identified by the LiMn2O4 particles
Discussion
SEM micrographs of lithium cells were obtained by means of a new technique which presents the following advantage: the batteries were never exposed to air during the transfer from the glove box to the microscope. Moreover, they are cooled down to limit the evaporation of the volatile electrolyte compounds during the observation under vacuum. Other hermetic transfer techniques have been previously used to study lithium surfaces in many electrolytes [5]. However the cells were dismantled, and the
Conclusion
We have shown a direct correlation between current density and dendrite formation, with more dendritic deposits formed at high currents. Further investigations are in progress to determine a solution to suppress the dendrites by conceiving high surface area Li-anodes, leading thereby to low current densities.
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