The origin of heavy element doping to relieve the lattice thermal vibration of layered materials for high energy density Li ion cathodes†
To achieve high energy density in Li-ion batteries, the limit of lithium extractable from layered structure cathodes needs to be increased, which can be realized by raising the cut-off potential. However, repeated charge/discharge up to a high voltage typically generates the spinel phase from the parent layered structures and thereby deteriorates their electrochemical performances. There have been few effective strategies to date which can solve this problem. After a systematic review on the doping strategies reported, we realized that many heavy element dopants including 5d transition metals, lanthanides and p-block elements favor the structural stability of the layered crystal. Here, we offer a comprehensive study on Sn-doped LiCoO2 to reveal how these heavy-element dopants contribute to the structural stability especially at high charging voltages involving detrimental deintercalation. The introduction of the Sn dopant can pin the lattice vibration in the layered structure cathode and thereby stabilizes the CoO6 octahedra against the irreversible transformation into the cubic spinel structure under high-voltage operation and over extended cycling. Further analyses based on Debye theory suggest that the relief of the lattice thermal vibration must be a common feature of heavy element dopants in these layered structures unless the bonding characters in the parent structure are significantly changed. The precise investigation here shows clearly the detailed role of the Sn dopant in the layered LiCoO2, thereby providing a new insight into innovating the electrochemical performance of layered structure cathodes.