Investigation of SEI Growth on Carbon-Coated ZnFe2O4 Li-Ion Battery Anode Material By I n Situ Raman Microscopy

Abstract

Lithium-ion batteries (LIBs) are electrochemical devices for energy storage used in portable electronics due to their high energy density, lightweight and flexible design. However, improvements in safety, cost and energy and power density to meet the requirements of automotive industry and large scale storage technology for the wide application of LIBs on these fields.[1, 2] The battery performance is affected by surface layers formed on the electrodes. In particular, the solid electrolyte interphase (SEI) is a protecting surface film formed on the negative electrode in organic-based electrolytes. It consists of inorganic and organic species resulting from electrolyte decomposition.[3, 4] This electron insulating layer allows the transport of lithium ions and it has an impact on the irreversible charge loss, rate capability, cyclability and safety.[3] Therefore, it is essential to understand the processes that occur at the electrode/electrolyte interfaces and the nature of the surface film to improve the efficiency and develop longer-life LIBs. ZnFe2O4 (ZFO) is a non-toxic and cheap material of great interest as anode for LIBs with a high specific theoretical capacity ca. 1000 mAhg-1 due to a conversion-alloying reaction that involves the intake of 9 Li+.[5] An environmental friendly method to encapsulate ZFO nanoparticles within a carbon matrix has been described to avoid the aggregation of particles and loss of electronic conductivity that leads to capacity fading.[5] This work presents in situ Raman measurements of carbon-coated ZnFe2O4(ZFO-C) material during the first discharge/charge cycle.[6] The Raman spectrum of ZFO-C electrode at the open-circuit potential (OCP) only shows the D and G bands characteristic of carbon materials. These peaks weaken during the discharge step suggesting that the lithiation also occurs in the carbon coating. The signal of the D and G bands is recovered at the end of the charge step. The absence of peaks related to ZFO material indicates that the carbon coating remains mechanically intact after the first discharge/charge cycle. New Raman bands are observed within a potential region that coincides with SEI layer formation. These bands have been assigned to poly(ethylene)oxide species and organic lithium alkyl carbonates from electrolyte side reactions. The detection of SEI compounds by conventional Raman microscopy may be the result of “temporary surface enhancement Raman effect” from Zn nanoparticles formed during the conversion reaction of ZnFe2O4 in a similar potential region as SEI formation. The observation of these polymeric species formed on the carbon coating is comparable to shell isolated nanoparticle Raman spectroscopy (SHINERS) reported by Tian and co-workers.[7]