Crystal Structures of Poly(Ethylene Oxide)₃:LiBF₄ and (Diglyme)_n:LiBF₄ (n = 1,2)

The crystal structures of PEO 3:LiBF 4, (diglyme) 2:LiBF 4, and (diglyme):LiBF 4 have been determined and are compared. The helical conformation of each polymer chain and the stacking of neighboring chains in the structure of the PEO complex is similar to that found in the complexes with Li imide and Li triflate salts of the same PEO:salt ratio. Both PEO 3:LiBF 4 and (diglyme):LiBF 4 have similar coordination around Li +, i.e., 3 ether oxygens and 2 fluorines from two BF 4 - anions. However, in the former each anion bridges two neighboring Li + cations located along the PEO chain axis forming a continuous chain −BF 4 - −Li + −BF 4 - −Li + − whereas in (diglyme):LiBF 4 two BF 4 - anions bridge between pairs of Li + forming discrete diglyme−Li + −(BF 4 - ) 2 −Li + −diglyme units that are linked to their neighbors predominantly by van der Waals bonding. The (diglyme) 2:LiBF 4 structure is compared with those of PEO 6:LiSbF 6 (no PEO 6:LiBF 4 exists) and (diglyme) 2:LiSbF 6 . In both cases the Li + cations are 6-coordinated by three ether oxygens from two different diglymes and there is no anion coordination.