A good candidate for the next generation of batteries has to be abundant, environmentally friendly, cheap and safe; silicon fulfills these conditions. Silicon, which represents the second most abundant element in the earth’s crust is a light element and can accommodate 3.75 Li atoms per Si atom at room temperature, resulting in a maximum capacity of 3579 mAh/g.

However, electrochemical cells using silicon are still suffering from a low Coulombic efficiency and a constant decrease in capacity. The main reason is due to the large volume expansion occurring upon lithiation of the silicon. Successive expansions/ contractions of the Si particles can lead to cracking, partial, and/or full disconnection between the Si particles, the conductive additive (carbon black), and the current collector, and a part of the active material will thus be lost. A lot of different strategies have allowed to improve the stability of the cells by avoiding or limiting this volume expansion by, for example, reducing the size of the particles, by using specific cycling conditions of the battery, and more recently, by the development of nanostructured Si materials.

An additional reason for the observed irreversible capacity is the result of the electrolyte decomposition and the formation of a protective film: the solid electrolyte interphase (SEI). For Si-based electrodes, the formation of the SEI is affected by cracks during the volume expansion/contraction during cycling, and it has to be continuously reformed consuming extra amounts of lithium. The stability of the SEI layer is crucial for maintaining good performance of a Li-ion battery.

To improve the stability of the SEI layer, electrolyte additives such as VC or silanes have been added to the classical electrolyte (LiPF₆ in nonaqueous carbonate solvents, e.g. EC, DEC, dimethyl carbonate (DMC)). However, the role of the electrolyte salt itself and its interface reactivity have attracted less attention and in the current Li-ion batteries, it is LiPF₆ which is used as conducting salt.

—Phillippe et al.

While LiPF₆ presents a high ionic conductivity and good electrochemical stability when it is solved in carbonate solvents, it also presents many problems, the authors noted. It is thermally instable and also extremely sensitive to traces of water, moisture and alcohol, leading to the formation of hydrofluoric acid (HF). Additionally, the risk of release of gaseous HF in case of thermal runaway of the battery presents a safety problem that needs to be solved. Therefore, they suggested, the replacement of the classical LiPF₆ salt has to be considered.

They selected LiFSI as a candidate. LiFSI presents a better ionic conductivity than LiPF₆, and shows good anticorrosive properties toward aluminum. The salt outperforms LiPF₆ and exhibits good electrochemical performances when used as salt in nonaqueous carbonates solvents in half-cells. Its better stability toward hydrolysis than LiPF₆ is another advantage—this can be a crucial point for the good cyclability of lithium-ion batteries, they observed.

In their study, the researchers showed that LiFSI allows avoiding the fluorination process of the silicon particles surface upon long-term cycling, which is observed with the common salt LiPF6. As a result, the composition in surface silicon phases is modified, and the favorable interactions between the binder and the active material surface are preserved.

They also found that the reduction products deposited at the surface of the electrode act as a passivation layer which prevents further reduction of the salt and preserves the electrochemical performances of the battery.

Resources

• Bertrand Philippe, Rémi Dedryvère, Mihaela Gorgoi, Håkan Rensmo, Danielle Gonbeau, and Kristina Edström (2013) Improved Performances of Nanosilicon Electrodes Using the Salt LiFSI: A Photoelectron Spectroscopy Study. Journal of the American Chemical Society doi: 10.1021/ja403082s