Aging of Li2FeSiO4 cathode material in fluorine containing organic electrolytes for lithium-ion batteries

The stability vs. aging of Li2FeSiO4 (LFS) cathode material in fluorine-based electrolytes, especially at elevated temperature, was studied in this work. The LFS powder was initially synthesized using a hydrothermal route and then aged at 60 °C for 40 days in LiPF6 and LiBF4-based electrolytes. The residual powder and the electrolyte were investigated afterwards. In the case of LiPF6, a structural and compositional change of LFS to Li2SiF6 was observed by XRD. SEM images confirmed that this change led to a morphology change of the aged material. XPS, EDX and ICP-OES measurements showed a large increase of fluorine content inside the residual powder. NMR investigations indicated an accelerated decomposition of electrolyte in the presence of LFS compared to the electrolyte aged without LFS. Our results suggest a degradation of LFS to Li2SiF6 in the fluorine-based electrolyte at elevated temperatures while the electrolyte decomposition is accelerated. © 2011 Elsevier Ltd

O3-type Na[Fe1/3Ni1/3Ti1/3]O2 cathode material for rechargeable sodium ion batteries

A Na[Fe1/3Ni1/3Ti1/3]O2 cathode material for sodium-ion batteries has been synthesized by a solid-state reaction method. The obtained Na[Fe1/3Ni1/3Ti1/3]O2 shows an O3-type structure, and delivers a discharge capacity of 117 mA h g-1 at a current density of 10 mA g-1 in a range of 1.5-4.0 V at 20 °C. Furthermore, the Na[Fe1/3Ni1/3Ti1/3]O2 cathode material shows good rate capability and cycling stability. The working and structural transition mechanisms of the Na[Fe1/3Ni1/3Ti1/3]O2 material are examined by ex situ X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD) methods. The valence state of Fe ions in the Na[Fe1/3Ni1/3Ti1/3]O2 material is estimated to be 2.67+. The main redox couple is Ni2+/Ni4+, but the Fe2+/Fe3+ contributes a little as well at voltages below 2.0 V. The original O3 phase transforms to a P3 phase during sodium extraction with good reversibility, but a slightly irreversible change of lattice parameters may lead to capacity decay during long-term cycling. Moreover, the gas evolution during the first charge/discharge process is analyzed by using an operando mass spectrometry technique. The obvious release of CO2 gas at the end of the charge process may be the other origin of the capacity decay. Nevertheless, the absence of O2 evolution indicates an improved safety of the Na/Na[Fe1/3Ni1/3Ti1/3]O2 cell