[1] Natl Univ Singapore, Dept Phys, Solid State Ion & Adv Batteries Lab, Singapore 117542, Singapore
[2] NUS High Sch Math & Sci, Singapore 129957, Singapore
[3] Natl Univ Singapore, Dept Mat Sci & Engn, Singapore 117576, Singapore
[4] Natl Univ Singapore, Graphene Res Ctr, Dept Chem, Singapore 117542, Singapore
[5] CSIR, ZA-0001 Pretoria, South Africa