Study on Solid-Electrolyte-Interphase of Si and C-Coated Si Electrodes in Lithium Cells

Abstract

The solid-electrolyte-interphase ͑SEI͒ layers formed on the electrodes of pristine Si and carbon-coated Si ͑C-Si͒ particles in Li cells have been studied. The counter electrode is Li, and the electrolyte is LiPF 6 in the mixture of ethylene carbonate and ethyl methyl carbonate. Other than those, such as Li carbonates and fluoride, already known to the SEI of graphite electrode, there were detected significant amounts of SEI species unique to each of the Si electrodes. On the pristine Si electrode, there was concurrence of abundance of C and Si fluorides after long cycles. Coating the Si particles with a graphitized carbon layer has significant effects on the SEI formation. It helps to keep the Si particles remaining integrated after cycling, resulting in a smooth superficial SEI layer. It removes the native oxide layer not only to reduce humidity contamination but also to significantly change the SEI compositions. The SEI of the C-Si electrode shows the absence of Si and C fluorides but the presence of siloxane species. Reaction mechanisms leading to the formation of the fluoride and siloxane species have been proposed, elucidating an important role played by the native Si oxide layer. Si possesses a maximum capacity exceeding 3000 mAh/g for being a negative electrode for Li-ion batteries.1,2 Two issues are considered critical to realize this application. The first critical issue is the dramatic volume expansion and shrinkage of the Si particles during lithiation and delithiation, [3][4][5] respectively. Such cyclic volumetric variations tend to cause fast mechanical failure of the electrode structure, resulting in a very poor cycle life. A fairly large amount of literature adopting different approaches to enhance the structural robustness of the electrode has been dedicated to tackle this crucial problem. In the case of the conventional thick-film electrode made of particulate materials, for example, studies 3-8 have coated the Si particles with different conducting materials, which may serve either to enhance the conductivity of the electrode or to act as a buffer to partially accommodate the volumetric variations during cycling. In particular, coating with a carbon/graphite surface layer 3-6 has shown a significant beneficial effect on enhancing cycle life.The second critical issue is the properties of the surface layer on Si in contact with the Li + -containing electrolyte, also known as the solid-electrolyte-interphase ͑SEI͒. The SEI properties, on either cathode or anode, have been well recognized to play an important role in, among others, the safety, power capability, and cycle life of Li-ion-based batteries. It is believed to be equally important to the electrochemical performance of Si anode. Unfortunately, study on this critical issue is scarce. Choi et al.9,10 once reported preliminary results on the effects of certain electrolyte additive and salt on the SEI compositions of Si thin-film electrodes.In this work, the morphology and composition of the SEI formed on the electrodes containing either pris...

Solution-Grown Silicon Nanowires for Lithium-Ion Battery Anodes

“…After the carbon coating, the highenergy peak increased in intensity, indicating that the SiNWs were coated with a film. The C 1s peak looked very similar before and after the carbon coating, but the O 1s peak showed a shift to higher energies after the coating, suggesting that CAO bonds 30 may have been formed ( Figure S2, Supporting Information). Different slurry compositions were made for galvanostatic cycling using either carbon black (CB) or multiwalled carbon nanotubes (MWNTs) as the conducting additive.…”

Interphase chemistry of Si electrodes used as anodes in Li-ion batteries

“…Si can also form lower Li content Li-Si phases in the very early stages of silicon lithiation such as Li 13 Si 4 (Li/Si = 3.25), Li 7 Si 4 (Li/Si = 1.75), and Li 12 Si 7 (Li/Si = 1.71). Theoretically, Li 22 Si 5 can deliver a specific capacity of 4200 mAh/g, which is 10 times higher than graphite electrode (372 mAh/g). Nevertheless, volume variation during lithiation is the major drawback of Li-Si ).…”

“…These elements (M) are well known to form very rich-lithium compounds such as Li 4.4 M and Li 3 M, respectively [1], at very low potentials close to the lithiation of graphite, which also contributes to the increase of the energy density of the battery. Among them, silicon is the most attractive one due to its low molecular weight (28 g/mol) and the possibility of forming Li 22 Si 5 alloy (Li/Si = 4.4) according to the Li-Si phase diagram [2,3]. Si can also form lower Li content Li-Si phases in the very early stages of silicon lithiation such as Li 13 Si 4 (Li/Si = 3.25), Li 7 Si 4 (Li/Si = 1.75), and Li 12 Si 7 (Li/Si = 1.71).…”

A hard X-ray photoelectron spectroscopy study on the solid electrolyte interphase of a lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide based electrolyte for Si-electrodes

“…A lot of work has been done to understand the SEI on graphitic negative electrodes while the field of SEI formation on silicon is still a young area with only a rather small number of publications. [4,[7][8][9][10][11][12][13][14][15][16][17][18][19] Philippe et al have…”