VASP Calculation on Lithium Reaction to Tin Phosphides

Studies on electrochemical reaction of SnP_x materials were  carried out by VASP ab initio calculation, where. SnP_x  having  four different crystalline compositions, SnP_0.75(Sn₄P₃), SnP_0.94(SnP_17/18), SnP and SnP₃. The reaction potential of the lithiation of SnP_0.94(SnP_17/18) material was compared between intercalation and conversion reaction. The total-energy calculation and full structural optimization of SnP_x, SnP_x related materials and Li materials were performed using the Vienna ab initio Simulation Package (VASP)[1]. The possibility of intercalation of lithium to SnP_0.94 was analyzed to obtain reaction potential of 1.853 V (Li/Li⁺) accommodating lithium to 6i site. 6i site is a vacant site in Sn₁₈P₁₇ (SnP_0.94). Through further intercalation of lithium, reaction potential of Li_0.5SnP_0.94, Li_1.5SnP_1.56 and Li_6.06SnP_0.94 can also be calculated as 0.9026, 0.705 and -1.5 V(Li/Li⁺), respectively.Considering conversion reaction, SnP_0.94 can react to produce 2 kinds of products such as LiP and Li₃P as shown in Eq. 1. 

SnP_0.94 + (0.94z+y) = 0.94Li_zP + Li_ySn ---------(1) 

At Li_ySn formation, y of zero means that only conversion reaction takes place without the formation of Li_ySn. Reaction potential of y=0 were 1.35 V and 0.948 V for the formation of LiP and Li₃P.  Reaction potential of the hole reaction [SnP_0.94 + xLi = products], including intercalation and conversion, was plotted against reacted lithium extent of x in Fig. 1. It is observed that the reaction potential of conversion is higher than that of the lithium intercalation. In case of conversion reaction to form LiP with reacted lithium extent of 0.94 was 1.35 V which was higher than 0.937 V of lithium intercalation to form Li_0.56SnP_0.94 .  Therefore, conversion reaction is labile compared to intercalation. Moreover, the reaction potential of conversion to form Li₃P with reacted lithium extent of 2.833 was 0.948 V which is higher than that of the conversion reaction potential of 0.704 V to form LiP with reacted lithium extent of 3.28. Therefore conversion reaction to form Li₃P is easier than to form LiP. We will discuss VASP calculation on lithium reaction to Tin Phosphides in details

[1] Kresse,G.; Hafner, J. Phys. Rev. B, 47 (1993) 558; 49 (1994) 14251.