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 (Sn 4 P 3 ), SnP 0.94 (SnP 17/18 ), SnP and SnP 3 . 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 Sn 18 P 17 (SnP 0.94 ). Through further intercalation of lithium, reaction potential of Li 0.5 SnP 0.94 , Li 1.5 SnP 1.56 and Li 6.06 SnP 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 Li 3 P as shown in Eq. 1. SnP 0.94 + (0.94z+y) = 0.94Li z P + Li y Sn ---------(1) At Li y Sn formation, y of zero means that only conversion reaction takes place without the formation of Li y Sn. Reaction potential of y=0 were 1.35 V and 0.948 V for the formation of LiP and Li 3 P. 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 0.56 SnP 0.94 . Therefore, conversion reaction is labile compared to intercalation. Moreover, the reaction potential of conversion to form Li 3 P 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 Li 3 P 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.