Scanning Tunneling Microscopy Study on Surface Reactions of Carbon Negative Electrodes in Rechargeable Lithium Batteries

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Graphite is now widely used as negative electrode in rechargeable lithium batteries. It is generally recognized that a kind of passivating film, called solid electrolyte interface (SEI), is formed on carbon negative electrode during the first charging. Effective SET layer prevents further solvent decomposition and improves the safety and the cycleability of Li-ion cells. Solvent choice is very important to obtain excellent SEI.For example, propylene carbonate (PC) is a poor solvent for graphite anode because it decomposes at ca. 1 V vs. Li/Li^+, and thereby no intercalation takes place. However, the addition of 12-crown-4 to PC solution or the use of partially fluorinated PC suppresses solvent decomposition and enables lithium intercalation, in the present work, we observed the morphology changes of graphite surface in 12-crown-4/PC and 3-trifluoromethyl-2,5-dioxa- cyclopentan-1-one (TFPC) using electrochemical STM, and elucidated the mechanism of SEI formation on graphite negative ele … More ctrode in these electrolyte systems.In 1 M LiC1O_4/TFPC, exfoliation of graphite layers was observed at potentials around 0.9 V ; however, the exfoliation was not so severe as that observed in 1 M LiC1O_4/PC.Below 0.8 V, especially below around 0.5 V, such exfoliation was terminated by the formation of SEI along the newly formed step edges. In this case, the instability of TFPC against reduction enables rapid SEI formation and suppresses further exfoliation.The addition of 12-crown-4 into PC greatly suppressed the exfoliation. After potential was kept at 0.9 V, atomically flat, island-like structures with an enhanced height of 1 nm (referred to as "hills".) appeared on the surface. When the potential was kept below 0.8 V, part of the hills was swelled and changed to irregular-shaped structures with an enhanced height of 20-30 nm (referred to as "blisters"). Blister formation became more significant as the potential was lowered. The observed morphology changes were very similar to those observed in ethylene carbonate (EC)-based solutions in previous studies. These hills and blisters are considered to have been formed by the intercalation of Li^+/12- crown-4 complexes and their decomposition followed by accumulation of the decomposed products, respectively. Selective coordination of 12-crown-4 to Li^+ prevents PC from being co-intercalated within graphite, and thereby suppresses the exfoliation of graphite layers. Less