Unoccupied 3d orbitals make Li-unalloyable transition metals usable as anode materials for lithium ion batteries†
Abstract
It is generally believed that metals which are unalloyable with Li are not active for Li storage. This work, however, demonstrates that transition metals with unoccupied 3d orbitals are usable as anode materials for Li storage although they are unalloyable with Li. Specifically, we synthesize spindle-shaped Fe@C (S-Fe@C) by the calcination of an Fe based metal–organic framework, which comprises carbon coated Fe subnanoparticles with an interconnected carbon network. When used in LIBs, S-Fe@C can exhibit a stable reversible capacity of 967.3 ± 16.2 mA h g−1 at 100 mA g−1. More interestingly, when S-Fe@C with smaller Fe nanoparticles and more spacing between the Fe nanoparticles and carbon shell is used as the anode for LIBs, the stable reversible capacity can reach up to 1091.9 ± 17.6 mA h g−1 at 100 mA g−1 after 100 cycles. The in operando XRD patterns exclude the contribution of capacities from the alloying between Fe and Li. DFT calculations indicate that the unoccupied 3d orbitals of Fe atoms can accept electrons from Li, which makes the surface of Fe nanoparticles highly active for Li adsorption. The Li adsorption can be multi-layered. Our findings can well explain why some transition metal oxides can exhibit extra capacities beyond their theoretical capacities.
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