Silicon has a nominal gravimetric capacity of 4200 mAh/g, more than ten times that graphite anodes [1]. However, it has a couple of drawbacks. The lithiation of Si is accompanied by a large volume expansion (~300%), which may cause cracking, and subsequently fracturing and pulverization of the Si anode. To overcome this problem, Si can be prepared with low dimensions, like in the form of microwires, nanowires or nanotubes, thin films or nanoparticles [1]. Not less important is the low ionic conductivity of Si (it has a compact crystal structure). There have been a couple of works in which the use of slow pre-lithiation treatments of Si (for example, in 10 h) improve its cyclability and performance [2].

In Si nanowires, when Li is extracted from them, the expanded Si shrinks, but the final volume at the end of the discharge process is still larger than the original. This indicates the formation of empty space within the active material. In this way, pore formation directly depends on the volumetric expansion of the active material in the charge/discharge process [3]. This porosity could be beneficial for the diffusion of Li ions; in fact, it has been reported that mesoporous silicon (mp-Si) exhibits a superior electrochemical performance and long cycle life as anodic material in a Li ion battery (LIB) [4].

When Na ions are used as charge carriers instead of Li ions, the expansion during the incorporation of the charges is larger at same levels of charge storage. Nevertheless, first principles calculations indicate that Na forms alloys with Si with composition Na_xSi with , while the composition of the Li alloys is Li_xSi, where x could have a value of up to 4.4. The theoretical studies that have been carried out show that although the formation of NaSi alloys is thermodynamically favorable, the insertion of Na in c-Si is limited, this due to the large size of the Na compared to the Li (ionic radius of 0.098 nm and 0.068 nm, respectively), however, it is possible to sodiate Si [5]. Na is located below the Li in the periodic table and they share similar chemical properties in many aspects. The fundamental principles of ion sodium batteries (NIBs) and LIBs are identical, and like the Li, the Na form alloys with the Si. For all the above, it can be deduced that the chemistry of Na could be applied to Li battery systems (they are compatible).

The present work proposes to pre-condition Si anodes with Na, before using them in a Li ion battery. Electrochemical sodiation and desodiation will be performed. The pre-conditioning treatment of the anode is intended to facilitate the transport of Li ions by creating channels in the Si anode with similar size to the diameter of Na. In this way, the diffusivity of the Li ions will be increased in the subsequent charge and discharge cycles, since the ionic radius of Na is considerably larger than that of Li. It should be mentioned that the pore size formed by Li or Na is in the range of micropores (diameters less than 5 nm).

[1] E. Quiroga-González., J. Carstensen, H. Föll, Energies, 6, 5145 (2013).

[2] E. Quiroga-González, J. Carstensen, H. Föll, Electrochim. Acta, 101, 93 (2013).

[3] J.W. Choi, J. McDonough, S. Jeong, J.S. Yoo, C.K. Chan, Y. Cui, Nano Lett., 10, 1409 (2010).

[4] M. Ge, X. Fang, J. Rong, C. Zhou, Nanotechnology, 24, 422001 (2013).

[5] O.I. Malyi, T.L. Tan, S. Manzhos, Appl. Phys. Express, 6, 027301 (2013).