1. Department of Energy and Resources Engineering, College of Engineering, Peking University , Beijing 100871 , China
2. Department of Materials Science and Engineering, Stanford University , Stanford, CA 94305 , USA
3. Department of Energy Science and Engineering, Stanford University , Stanford, CA 94305 , USA
4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory , Menlo Park, CA 94025 , USA