The solid electrolyte interphase (SEI) in lithium-ion batteries separates the highly reductive lithiated graphite from reducible electrolyte components. It is critical for the performance, durability, and safe operation of batteries. In situ imaging of the SEI is demonstrated using the feedback mode of scanning electrochemical microscopy (SECM) with 2,5-di-tert-butyl-1,4-dimethoxy benzene as mediator. The formation of the SEI is indicated by a decrease of the mediator regeneration rate. Prolonged imaging of the same region revealed fluctuation of the passivating properties on time scales between 2 min and 20 h with an inhomogeneous distribution over the sample. The implications of the approach for in situ assessment of local SEI properties on graphite electrodes are discussed with respect to studying the influence of mechanical stress on SEI reliability and the mode of action of electrolyte additives aiming at improving SEI properties.

The passivating properties of solid electrolyte interphases (SEI) at metallic lithium were characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB) as redox mediator at OCP. The SEI at Li allows electron transfer toward DBDMB with finite rate. In comparison to charged graphite composite electrodes, the electron transfer rate tends to be smaller at Li. Both, graphite composite and Li electrodes, show a local variation of electron transfer rates and temporal changes within a time span of hours. The long-term changes of SEI passivity at metallic Li are dependent on the solvents in the liquid electrolyte. In addition, significant short-term changes of SEI passivity occur at both electrodes. However, the frequency of such events is smaller for metallic Li compared to graphite. A strong decrease of SEI passivity and a strong increase of fluctuations in the passivating properties are observed when the microelectrode mechanically touches the metallic Li and damages the SEI. The changes of SEI passivity by a mechanical touch are orders of magnitude larger compared to spontaneous changes. A local SEI damage by the microelectrode decreases not only the SEI passivity locally, but also a few hundreds of μm apart. Li metal is currently an electrode material of interest for rechargeable lithium-air and lithium-sulfur batteries.1,2 It is promising because of its high theoretical specific capacity 3 of 3860 mA h g −1 and smallest electrochemical potential of −3.040 V vs. SHE. Li metal reductively decomposes electrolyte molecules upon contact, because the potential of Li exceeds the stability window of the electrolytes. 4 The decomposition products form a solid electrolyte interphase (SEI) on top of the metallic Li. 5 The properties of the SEI are very important for the performance of the Li negative electrode, because the SEI affects Li dendrite growth. 4 The tendency of Li to form dendrites or high surface area lithium during galvanic deposition is one major drawback of Li metal and causes safety concerns regarding this electrode. The second major drawback is the low coulombic efficiency caused by the ongoing lithium corrosion and sensitivity to SEI passivity. 6 Thus, SEI passivity is a relevant parameter for practical applications.Since the SEI properties are significant for battery performance, substantial ex situ, in situ and in operando techniques were applied for SEI investigation in general.7 SEI formation on graphite occurs mainly in the first cycle because of the rather stable graphite host structure. 4 In contrast, the SEI on metallic Li is subject to continuous reformation upon cycling. Despite this significant difference, both graphite and Li metal are covered by similar SEIs. 8 In this study the SEI passivity is characterized by in situ scanning electrochemical microscopy (SECM) 9 using 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB) as a redox mediator. DBDMB was introduced by Dahn et al. 10 as overcharge protection agent for lithium ...

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The physical swelling of uncharged graphite composite electrodes due to electrolyte-binder interactions is investigated by scanning electrochemical microscopy (SECM) using 2,5-di-tert-butyl-1,4,-dimethoxybenzene as a redox mediator. A series of approach curves at the same location is conducted in order to quantify in situ and locally the physical swelling. The film thickness change δ film amounted to 9.1 μm on average for a 80 μm thick uncharged graphite composite electrode in LP40 electrolyte between 1.1 and 5.9 h. Curves of δ film vs. t usually reach a saturation within 12 h. The swelling ratio χ varies from 0.3% to 17.6% for uncharged graphite composite electrodes from the same batch in the same electrolyte. In contrast, the 8 μm thick polyvinylidene fluoride (PVDF) model sample swelled by χ = 99%. Approach curves demonstrate that swelling of the PVDF is the main cause for the physical swelling of uncharged graphite composite electrodes. Both PVDF model sample and uncharged graphite composite electrodes show locally different swelling ratios by SECM imaging. Based on these results a swelling model is proposed, where the uncharged graphite composite electrode swells physically on average by at least χ = 11% and the local topography is changing during swelling. Li-ion batteries (LIBs) are currently the most rapidly developing commercial rechargeable batteries. They are mainly used for portable electronics but increasingly find application for electrified vehicles and stationary storage applications because of the high practical energy density, good cyclability and low self-discharge.

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