New metastable cathode materials for lithium-ion batteries
This PhD work is dedicated to the discovery and study of new cathode materials for lithium-ion batteries. To obtain new materials, a well-known strategy based on ion-exchanging alkali metals within stable crystalline frameworks was used. Ion-exchange procedures between sodium and lithium ions were performed on known sodiated materials, NaMnTiO4 with the Na0.44MnO2 structure and NaFeTiO4 and Na2Fe3-xSn2xSb1-xO8 (0 ≤ x ≤ 1) with the calcium-ferrite structure. A combination of Energy-Dispersive X-ray Spectroscopy (EDS), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray (XRD) and Neutron (NPD) diffractions was used to determine the crystal structure of the samples obtained via ion-exchange and confirmed that LiMnTiO4 and LiFeTiO4 and Li2Fe3-xSn2xSb1-xO8 (0 ≤ x ≤ 1) were obtained with a 1:1 ion-exchange between sodium and lithium. LiMnTiO4 has the orthorhombic Pbam space group, with a = 9.074(5), b = 24.97(1) and c = 2.899(2) Å. The shapes and dimensions of the channels are modified compared to NaMnTiO4, with displaced alkali metal positions and occupancies. LiMnTiO4 was cycled vs Li and up to 0.89 lithium ions can be reversibly inserted into the structure, with a discharge capacity of 137 mAh/g after 20 cycles at C/20 and room temperature. At 60°C, all the lithium is removed at the end of the first charge at C/20, with subsequent cycles showing reversible insertion of 1.06 Li-ions when cycled between 1.5 and 4.6 V. The electrochemistry of calcium-ferrite LiFeTiO4 and Li2Fe3SbO8 was investigated in half cells versus lithium and up to 0.63 and 1.35 lithium ions can be reversibly inserted into the structure after 50 cycles at a C/5 rate, respectively. LiFeTiO4 showed good cyclability with no capacity fade observed after the second cycle while Li2Fe3SbO8 exhibited a constant capacity fade with a 60 % capacity retention after the 50th cycle. Doping Li2Fe3SbO8 with tin reduces the capacity. However, the capacity retention is significantly enhanced. For Li2Fe2.5Sb0.5SnO8 after 20 cycles at C/5, the capacity is stable and comparable with that observed for Li2Fe3SbO8 after the same number of cycles. Using ion-exchange procedures has allowed new metastable materials to be obtained which have the potential to be used as cathodes in lithium-ion batteries. Doping these families of materials with different atoms has been shown to improve their electrochemical performance. Ex situ XRD was used to demonstrate that the original structures of LiMnTiO4, LiFeTiO4 and Li2Fe3SbO8 are retained during cycling. The volume change observed for Li2Fe3SbO8 upon delithiation was particularly noteworthy with a small decrease of 0.9 % at the end of charge when cycled at C/100 and room temperature, indicating structural stability upon lithium insertion/de-insertion.
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