| Lithium-Excess High-Capacity Electrode Materials with 4d-Transition Metals for Rechargeable Lithium Batteries |
| Naoaki YABUUCHI |
| Tokyo Denki University, Japan |
|
In the past three decades, intensive research efforts have been made to explore new positive electrode materials for rechargeable lithium batteries. Lithium-excess manganese oxides, Li2MnO3, and their solid solution phases with LiMeO2 (Me = Co, Ni, Mn etc.) have recently attracted interest as high-capacity electrode materials. It has been proposed that the large reversible capacity partly originates from the participation of oxide ions in the crystal lattice in redox reaction.[1,2] The use of analogy is anticipated to be the important strategy to design new high-capacity electrode materials.
In this study, lithium niobium oxide, Li3NbO4, which is classified as a cation ordered rocksalt structure with a cubic-close packed (ccp) oxygen array similar to Li2MnO3, is targeted as a potential new end-member for high-capacity electrode materials. Framework structure of Li3NbO4 consists of Nb6O14 clusters, which further form a body-centered cubic lattice, and lithium ions are located at remaining octahedral sites. Since Li3NbO4 is an electrical insulator, transition metals are partly substituted for Li+ and Nb5+. After several trials, it is found that Co2+, Ni2+, Fe3+, and Mn3+ are substituted for Li+ and Nb5+.[3] Such transition metal substitution is achieved in the series of Li3NbO4-MeO (Ni2+ and Co2+) and Li3NbO4-LiMeO2 (Fe3+ and Mn3+) systems with the common ccp oxygen lattice. The metal substitution influences the clustering of niobium in the ccp oxygen array, leading to the formation of cation disordered rocksalt phases. In general, well-crystallized cation disordered rocksalt phases are known to be electrochemically inactive. Nevertheless, a manganese-substituted sample (0.43Li3NbO4–0.57LiMnO2 or Li1.3Nb0.3Mn0.4O2 as a layered formulation) shows large reversible capacity with small initial irreversible capacity at 60 oC.[3] Initial charge/discharge capacity reaches 300 mAh g-1, which is clearly larger than that of the expected capacity (118 mAh g-1) based on the Mn3+/Mn4+ redox reaction. Therefore, the charge compensation is expected to be achieved by the contribution of oxide ions in the crystal lattice, similar to Li2MnO3-based materials which is further supported by first-principles calculation and hard/soft X-ray absorption spectroscopy.[3]
In addition to Li3NbO4, lithium molybdenum (VI) oxide, which is also classified as the cation ordered rocksalt structure with the ccp oxygen array, has been studied as a new host structure for high-capacity positive electrode materials. From these results, we will discuss the possibility of new series of lithium-excess electrode materials containing 4d-transition metal ions with high oxidation states.
References
[1] T. Ohzuku et al., J. Mater. Chem., 21, 10179 (2011).
[2] M. Sathiya et al., Chem. Mater., 25, 1121 (2013).
[3] N. Yabuuchi et al., submitted |
| Acknowledgements : This study was partly supported by the ALCA project of the Japan Science and Technology Agency. |
|
