Effect of Water Concentration in LiPF₆-Based Electrolytes on the Formation, Evolution, and Properties of the Solid Electrolyte Interphase on Si Anodes

Abstract

A trace amount of water in an electrolyte is one of the factors detrimental to the electrochemical performance of silicon (Si)-based lithium-ion batteries that adversely affect the formation and evolution of the solid electrolyte interphase (SEI) on Si-based anodes and change its properties. Thus far, a lack of fundamental and mechanistic understanding of SEI formation, evolution, and properties in the presence of water has inhibited efforts to stabilize the SEI for improved electrochemical performance. Thus, we investigated the SEI formed in a Gen2 electrolyte (1.2 M LiPF 6 in ethylene carbonate/ethyl methyl carbonate, 3:7 wt %, water content: <10 ppm) with and without additional water (50 ppm) at varying potentials (1.0, 0.5, 0.2, and 0.01 V vs Li/Li + ). The impact of additional water on the morphological, (electro)chemical, and structural properties of SEI was studied using microscopic (atomic force microscopy and scanning spreading resistance microscopy) and spectroscopic (X-ray photoelectron spectroscopy, attenuated total reflection Fouriertransform infrared spectroscopy, and time-of-flight secondary ion mass spectrometry) techniques. The SEI exhibits both potential-and water concentration-dependent trends in its morphology and chemical composition. The presence of additional water in the electrolyte causes parasitic reactions, which onset at ∼1.0 V, resulting in a reduction of electrolyte components and result in the formation of an insulating, fluorophosphate-rich SEI. In addition, hydrolysis of LiPF 6 creates hydrofluoric acid, which reacts with the surface oxide layer on the Si electrode, leading to a pitted and inhomogeneous SEI structure.

The solvation structure, transport properties and reduction behavior of carbonate-based electrolytes of lithium-ion batteries

“…Finally, water contamination has been reported to cause detrimental parasitic reactions that affect the formation, evolution, and properties of the SEI. 95 In addition, the hydrolysis of LiPF 6 creates hydrofluoric acid, leading to a pitted and inhomogeneous SEI structure. 96 Therefore, we note the importance of considering specific surface conditions as well as possible impurities when investigating the interfacial reactions between anodes and electrolytes.…”

Microscale Silicon-Based Anodes: Fundamental Understanding and Industrial Prospects for Practical High-Energy Lithium-Ion Batteries

“…Typical organic electrolytes are composed of Li salts, solvents, and additives. LiPF 6 is the mainstream Li salts for Si-based LIBs due to the combination of a series of balanced characteristics. , Although other Li salts such as lithium bis­(fluorosulfonyl)­imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)­imidazolide, and lithium bis­(oxalate)­borate have been investigated, other issues such as solubility and reactivity have ultimately prevented these electrolytes from successfully replacing LiPF 6 . Since an individual component can hardly meet the various requirements of battery applications, two or more solvents are usually utilized together to achieve various functions simultaneously.…”

Solid Electrolyte Interphase Layer Formation during Lithiation of Single-Crystal Silicon Electrodes with a Protective Aluminum Oxide Coating

“…According to Ha et al, moisture may be the reason for the side reactions. 36 As a consequence of the voltage plateau, the current was increased to 500 μA (part 3b). Again, a voltage plateau was observed, but the voltage decreases slowly this time.…”

“…This observation indicates ongoing side reactions, which should be suppressed by the aluminum oxide coating . Ha et al proved traces of humidity within the electrolyte that may lead to side reactions at these voltages, including the formation of HF . Once the current was increased, the voltage plateau dropped to lower voltage values and ended after roughly 5 h.…”

“…A lack of contrast prevents the observation of further morphology changes here. According to Ha et al, moisture may be the reason for the side reactions …”