Monitoring of the SEI-Evolution of Uncoated and Carbon-Coated Si Nanoparticles By Transmission Electron Microscopy and Electrochemical Impedance Spectroscopy

It is generally accepted that Li-ion batteries will become the most important energy storage solutions in the current energy transition, where hydrocarbons as privileged fuel are being replaced with sustainable energy sources. To replace the actual graphite based anode, several studies have been initiated over the past 15 years to implement alloy-based systems, but without success. Silicon is probably the most promising candidate because of its high specific capacity of 3572 mAh.g^-1 (10 times higher compared to graphite), resulting from an alloying reaction with lithium rather than an intercalation of lithium between graphene layers. The drawback of an alloying reaction is that it is accompanied by a drastic volume increase up to 300% (compared to 10% in graphite). Due to this volume change, during every lithiation and delithiation, freshly exposed silicon surfaces will continuously react with the electrolyte and create new decomposition products that accumulate at the surface, resulting in irreversible capacity loss. Because of these drawbacks, silicon-based anodes have not yet been able to convince on an industrial scale. A recent strategy is to focus on the influence of coatings and composite materials in order to control the volume changes and SEI-formation. To this end, the evolution of the SEI, as well as an applied carbon coating, on nanosilicon electrodes during the first electrochemical cycles is monitored.

Electrochemical impedance spectroscopy (EIS) is an ideal technique to study in situ interphase phenomena. Valuable information about the formation of surface coatings (decomposition layer and/or additional coatings), during the lithiation and delithiation process, can be gathered¹. However, a Li-ion battery is a very complex system and various electrode components (such as polymeric binder, conductive carbon, surface coatings, decomposition layers,…) contribute to the impedance signal, which is averaged over the entire electrode. Based on impedance spectroscopy solely and without prior knowledge, it is very difficult to assign every internal process to its corresponding impedance signal. Transmission electron microscopy (TEM) is well-suited to this challenge, because the surface coatings of individual silicon particles can be investigated after a certain cycling time. This is definitely no straightforward experiment, since contact with air and water needs to be avoided at all times during transfer from battery to electron microscope, in order to avoid side reactions of the surface coatings.

In this contribution, the evolution of surface coatings on silicon nanopowders will be presented through an electrochemical impedance spectroscopy and transmission electron microscopy study. Electrodes in three-electrode-type cells were cycled and studied in situ with EIS during the first electrochemical cycles. TEM measurements were performed ex situat both lithiated and delithiated state of the electrode. Samples were introduced inside the electron microscope using a dedicated vacuum-transfer sample holder. Electron energy-loss spectroscopy (EELS) and high-resolution transmission electron microscopy (HRTEM) imaging were performed at low acceleration voltage (80-120 kV) in an aberration-corrected instrument. A TEM-EELS fingerprint signal of carbonate structures from the SEI is discovered, which can be used to differentiate between SEI and a graphitic carbon matrix. Furthermore, the shielding effect of the carbon coating and the thickness evolution of the SEI is described. Finally, the EIS results provide valuable information on the degradation of the carbon-coating and pulverization of the electrode as well as on the SEI-evolution.

Figure 1: a) TEM image of the initial, carbon-coated silicon nanopowder; b) Electrochemical impedance spectra of both uncoated and carbon-coated nanomaterials; c) TEM composition map of carbon-coated Si, after the first lithiation and d) TEM composition map of carbon-coated Si, after the second lithiation.

¹ E. Radvanyi and K. Van Havenbergh et al, Electrochimica Acta 137 (2014) 751

^† K.V.H. gratefully acknowledges financial support from the Flemish agency for Innovation through Science and Technology (IWT).