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

石墨作为负极广泛应用于可充电锂电池中。一般认为，第一次充电时，在碳负极上形成一种钝化膜，称为固体电解质界面（SEI）。有效的SET层防止溶剂进一步分解，提高锂离子电池的安全性和可循环性。溶剂的选择是获得优良SEI的关键。例如，碳酸丙烯酯（PC）是石墨阳极的不良溶剂，因为它在约1 V时分解，而不是Li/Li^+，因此不会发生插层。然而，在PC溶液中加入12-冠-4或使用部分氟化的PC会抑制溶剂分解，从而使锂嵌入，本文利用电化学STM观察了12-冠-4/PC和3-三氟甲基-2,5-二氧基-环戊烷-1-one （TFPC）中石墨表面的形貌变化，并阐明了这些电解质体系中石墨负电极上SEI的形成机理。在1 M lic10_4 /TFPC中，在0.9 V左右的电位下，石墨层发生剥落；但脱皮现象不像1 M lico_4 /PC那样严重。在0.8 V以下，特别是在0.5 V左右，这种剥落被沿着新形成的台阶边缘形成的SEI所终止。在这种情况下，TFPC对还原的不稳定性使得SEI快速形成并抑制进一步的剥落。在PC中加入12-crown-4能明显抑制剥离。当电位保持在0.9 V时，表面出现了高度增强1 nm的原子扁平的岛状结构（称为“山丘”）。当电位保持在0.8 V以下时，部分小丘肿胀并变为不规则形状，高度增强20-30 nm（称为“水泡”）。随着电位的降低，水疱的形成变得更加明显。观察到的形貌变化与以往研究中在碳酸乙烯（EC）基溶液中观察到的非常相似。这些小山和水泡分别是由Li^+/12-冠-4配合物的插入和分解以及分解产物的积累形成的。12-crown-4与Li^+的选择性配位阻止了PC在石墨中的共插层，从而抑制了石墨层的剥落。少

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