Siloxene: A Potential Layered Silicon Intercalation Anode for Na, Li and K Ion Batteries
Authors: Loaiza Laura Cristina , Monconduit Laure , Seznec Vincent 
Published: February 2020

Silicon 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
[1] and 954 mAh/g for NaSi.[2] 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. In this work, the lamellar siloxene was obtained via topotactic deintercalation of Ca from CaSi2 and its electrochemical performance was evaluated with Li, Na and K. The results show the versatility of siloxene as anode for LIB, NIB and KIB, with delivered reversible capacities of 2300, 311 and 203 mAh/g for Li, Na and K, respectively. The material exhibits a noticeable structural stability after several cycles, deriving in a good capacity retention and coulombic efficiency.

The electrochemical mechanism taking place upon cycling is highlighted on the basis of ex situ Raman characterization combined with IR spectroscopy, SEM and TEM. The results are unsuccessful in explaining all the observed phenomena by means of a merely silicon alloying mechanism, therefore a possible alkali intercalation alternative has been proposed. Preliminary evidence of this approach can be found in the siloxene structural integrity after several cycles, the presence of Na in the discharged compound as observed by EDX, the absence of any of the diffraction peaks from NaSi/Li15Si4 (both crystalline and proper from the alloying mechanism) by XRD, and the lack of their respective Raman vibration bands [3-
4], at the end of the discharge. Indeed, by Raman spectroscopy it was possible to observe a reversible shift of the main Si-plane vibration band for the discharged and charged siloxene, accompanied by a loss of the –OH and Si-H vibrations observed in the pristine siloxene. Undoubtedly, these two last ones are likely related to a change in the bond nature of the Si-planes with the substituent group producing a different interlayer separation, probably the electrochemical cycling induces an exchange between –OH and –H with Li/Na. In fact, the intercalation of Na/Li into a layered Si-based materials has been theoretically predicted for a single layer of siloxene (silicene), with no experimental record. The calculations foresee a high coverage of the silicene with alkali ions like Na, Li and K due to the nature of their interactions. The full sodiated/lithiated state of silicene corresponds to X1Si1 (X=Li/Na), the predicted binding energies and diffusion barriers indicate that their intercalation is achievable without the kinetic limitations (higher diffusion coefficient for silicene), structure degradation and volume expansion of bulk Si. [5–9] This feasibility for alkali intercalation with such high structural stability introduces siloxene as a potential anode for LIB, NIB and KIB batteries. Nevertheless, a better understanding of its electrochemical mechanism is necessary to develop its maximum performance. To the best of our knowledge, it is the first time that a lamellar Silicon based material shows such high stable capacity without volume expansion, representing a real breakthrough for the batteries field and particularly for NIB.

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Figure 1