ABSTRACT
Li-ion batteries have empowered consumer electronics and are now seen as the best choice to propel forward the development of eco-friendly (hybrid) electric vehicles. To enhance the energy density, an intensive search has been made for new polyanionic compounds that have a higher potential for the Fe²âº/Fe³âº redox couple. Herein we push this potential to 3.90 V in a new polyanionic material that crystallizes in the triplite structure by substituting as little as 5 atomic per cent of Mn for Fe in Li(Fe(1-δ)Mn(δ))SO4F. Not only is this the highest voltage reported so far for the Fe²âº/Fe³âº redox couple, exceeding that of LiFePO4 by 450 mV, but this new triplite phase is capable of reversibly releasing and reinserting 0.7-0.8 Li ions with a volume change of 0.6% (compared with 7 and 10% for LiFePO4 and LiFeSO4F respectively), to give a capacity of ~125 mA h g⻹.
ABSTRACT
A novel hydrated fluoroselenate NaCoSeO(4)F·2H(2)O has been synthesized, and its structure determined. Like its sulfate homologue, NaCoSO(4)F·2H(2)O, the structure contains one-dimensional chains of corner-sharing MO(4)F(2) octahedra linked together through F atoms sitting in a trans configuration with respect to each other. The magnetic properties of the two phases have been investigated using powder neutron diffraction and susceptibility measurements which indicate antiferromagnetic ordering along the length of the chains and result in a G-type antiferromagnetic ground state. Both compounds exhibit a Néel temperature near 4 K, and undergo a field-induced magnetic phase transition in fields greater than 3 kOe.
ABSTRACT
Li-ion batteries have contributed to the commercial success of portable electronics, and are now in a position to influence higher-volume applications such as plug-in hybrid electric vehicles. Most commercial Li-ion batteries use positive electrodes based on lithium cobalt oxides. Despite showing a lower voltage than cobalt-based systems (3.45 V versus 4 V) and a lower energy density, LiFePO(4) has emerged as a promising contender owing to the cost sensitivity of higher-volume markets. LiFePO(4) also shows intrinsically low ionic and electronic transport, necessitating nanosizing and/or carbon coating. Clearly, there is a need for inexpensive materials with higher energy densities. Although this could in principle be achieved by introducing fluorine and by replacing phosphate groups with more electron-withdrawing sulphate groups, this avenue has remained unexplored. Herein, we synthesize and show promising electrode performance for LiFeSO(4)F. This material shows a slightly higher voltage (3.6 V versus Li) than LiFePO(4) and suppresses the need for nanosizing or carbon coating while sharing the same cost advantage. This work not only provides a positive-electrode contender to rival LiFePO(4), but also suggests that broad classes of fluoro-oxyanion materials could be discovered.
