1. Department of Applied Physics, KTH Royal Institute of Technology, Hannes Alfvéns väg 12, 114 19, Stockholm, Sweden
2. Shanghai Institute of Space Power-Sources, Shanghai 200245, China
3. College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, P. R. China
4. Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China