Researchers in France have shown the versatility of siloxene, a 2D silicon structure (Si6O3H6), as an anode material for Li-, Na- and K-ion batteries.
In a paper in the Journal of Power Sources, the researchers report that lamellar siloxene obtained from the topotactic deintercalation of Ca from CaSi2 delivered reversible capacities of 2300, 311 and 203 mAh/g for Li, Na and K, respectively, with good capacity retention and coulombic efficiency for Li and Na.
Silicon is a crucial material for different technologies such as integrated circuits, photovoltaics, optoelectronics and most recently, batteries. It is one of the most promising anodes for lithium-ion (LIB) and sodium-ion (NIB) batteries due to its high theoretical capacity 3590 mAh/g for Li15Si4 and 954 mAh/g for NaSi. Nevertheless, its practical application is hindered by a series of obstacles. For lithium (Li), the access to such high lithiated phases causes extreme volume expansion (310%), resulting in a rapid capacity fade. For sodium (Na), the slow kinetics and the ionic radius restrict the sodiation of c-Si.
In an attempt to address these problems, recent attention has been given to the two-dimensional 2D silicon structures, comprising calcium silicide (CaSi2), polysilane (Si6H6) and siloxene (Si6O3H6), due to their potential ability to buffer the electrode volume changes during cycling and their facile synthesis through soft-chemical methods. CaSi2 has a Si-corrugated layered structure interconnected by Ca2+ ions. Upon reaction with concentrated HCl the Ca2+ is deintercalated while the integrity of the Si layers is preserved, producing the so-called Siloxene.—Loaiza et al.
Illustration of the transformation of CaSi2 into Siloxene upon reaction with HCl. Loaiza et al.
The obtained siloxene structure consisted of Si honeycombs planes, saturated with -H and -OH. The electrode material performed with excellent capacity retention and coulombic efficiency for Li-ion and Na-ion batteries.
The team proposed an intercalation mechanism for the siloxene cycling based on the experimental findings by Raman and IR spectroscopy complemented by SEM and TEM.
It is worth noting that it is the first time that a Silicon-based electrode material shows such high stable capacity without volume expansion and its associated consequences on the performance, representing a real breakthrough for the batteries field. Nevertheless, further experiments must be carried out for an advanced understanding, particularly the role of the different surface groups during the first discharge and the determination of the optimal cycling conditions for an improved performance.—Loaiza et al.
Laura C. Loaiza, Laure Monconduit, Vincent Seznec (2019) “Siloxene: A potential layered silicon intercalation anode for Na, Li and K ion batteries,” Journal of Power Sources, Volume 417, Pages 99-107 doi: 10.1016/j.jpowsour.2019.02.030