Silicon has proven to have a great potential as anode material for lithium ion batteries due to its high theoretical capacity, however; there are several obstacles that need to be overcome in order to make it a commercially viable option. Two of the main issues stem from the fact that silicon undergoes a large volume change during lithiation and delithiation (1). This makes forming a stable solid electrolyte interphase (SEI) difficult, resulting in a continuous loss mechanism of electrolyte and lithium as SEI is formed and broken each cycle. Uneven expansion and contraction causes the silicon to fracture, both exposing new surface on which more SEI might form, as well as electrically disconnecting material from the electrode, rendering it inactive.

In this work we investigate the use of amorphous silicon nitride as an alternative to pure silicon anodes. Silicon nitride is believed to form lithiated silicon and lithium nitride or one of several lithium silicon nitride ternary phases during the initial lithiation (2, 3). The resulting material is therefore believed to combine the high lithium ion conductivity of lithium containing nitrides with the high capacity of silicon. In a sufficiently fine structure, we hypothesize the silicon would be able to expand and contract with little to no fracturing owing to dimensional stabilization, short diffusion distances, and fast lithium diffusion in the surrounding lithium nitride, resulting in a high cycling stability.

We investigate this theory using a thin film electrode system. For this purpose, a-SiN_x:H thin films are deposited on copper foil substrates using plasma enhanced chemical vapor deposition (PECVD) with silane (SiH₄) and ammonia (NH₃) as precursors. By changing the flow ratio of the precursor gases in the plasma, SiN_xfilms with five different compositions were made (x = 0, 0.36, 0.63, 0.88 and 1.00). Films of different thicknesses were also made to be able to separate surface and bulk irreversible capacities, and to evaluate the kinetics of the material. Ellipsometry and transmission electron microscopy (TEM) were used to determine the thickness, composition, structure and quality of the pristine films. Electrochemical tests were conducted in 2032 coin cells, with a lithium metal counter electrode, using a commercial electrolyte with 5% FEC and 1% VC, and cycling is conducted between 50 mV and 1 V.

The variation of the first cycle specific charge capacity as function of different stoichiometries is studied, as well as the stability of the resulting material. Figure 1 shows the charge capacity and Coulombic efficiency of three 40 nm thick SiN_1.00 thin film electrodes which were primed at C/6 for 6 cycles and cycled at 1C for 750 cycles. These had an average charge capacity of 1126±40 mAh/g on the first 1C cycle, and experienced only a slight increase in capacity during the first 230 cycles, peaking at 1259±33 mAh/g before leveling out. After finishing 750 cycles the cells still retained an average capacity of 1245±19 mAh/g. Voltage-capacity curves for a selection of cycles can be seen in the figure 2, showing the large difference between the first conversion cycle and the subsequent cycles.

By comparing the first cycle irreversible capacities of SiN_1.00 films with different thicknesses (40 nm, 80 nm, 120 nm, 160 nm, and 200 nm), it has been determined that the conversion reaction of this nitride composition consumes approximately 565 mAh/g, while the initial formation of SEI consumes 0.021 mAh/cm². These experiments also demonstrated that increasing the thickness of the films had no significant adverse effects on their performance, but rather improved the first cycle irreversible capacity and increased the long term (cycles 10-100) average Coulombic efficiency from 99.6% (40 nm) to >99.9% (80 nm, 120 nm, 160 nm and 200 nm).

1.            Kasavajjula U, Wang C, Appleby AJ. Nano-and bulk-silicon-based insertion anodes for lithium-ion secondary cells. Journal of Power Sources. 2007;163(2):1003-39.

2.            Ahn D, Kim C, Lee J-G, Park B. The effect of nitrogen on the cycling performance in thin-film Si1−xNx anode. Journal of Solid State Chemistry. 2008;181(9):2139-42.

3.            Suzuki N, Cervera RB, Ohnishi T, Takada K. Silicon nitride thin film electrode for lithium-ion batteries. Journal of Power Sources. 2013.