In situ Defektcharakterisierung von LiFePO4-Kathoden mit der Positronenannihilationsspektroskopie PAS
Zusammenfassung der Projektergebnisse
Today lithium ion batteries (LIB) are one of the most promising candidates for energy storage. This project provides further insight into the generation of defects and their properties in cathode materials for LIBs. The focus is laid on the development suitable techniques of the positron annihilation spectroscopy (PAS) for the investigation of battery electrode materials, in particular LiFePO4 cathode materials. LiFePO4 belongs to the class of phospho-olivines and is one of the commonly used cathode materials. It is favourable for the large-scale applications and substitution of cobalt-containing materials due to the following aspects: stability, safety, availability, environmental compatibility and moderate costs. In LiFePO4 the de-/lithiation (i.e. charging /discharging) is governed by a phase transition between a Li-rich (Li0.89FePO4) and a Li-poor (Li0.05FePO4) phase. Phase stability and interface processes in LiFePO4 cathodes during charging /discharging processes is studied by ex-situ and in-situ positron annihilation lifetime spectroscopy (PALS). For this purpose, an in-situ cell for collecting PAS / PALS data during dis-/charging in a coin-type Li ion battery cells is designed and constructed. LiFePO4 powders were successfully synthesized under hydrothermal conditions and the structure of LiFePO4 was characterised by the Rietveld refinement of X-ray diffraction data. Large volume of LiFePO4 unit cell indicates an incomplete transformation from Fe0.5FePO4 to LiFePO4. The dehydrated iron (II) phosphate phase with the sum formula Fe3(PO4)2 can be rewritten as Fe0.5FePO4 in which the half of the cation (M1) sites is occupied by Fe2+. In LiFePO4 all M1 sites are occupied by Li+. This finding displays that the structural disorder in LiFePO4 is not an antisite defect (Fe on Li sites and vice versa) but an excess of Fe on the M1 site. Time-resolved in-situ heating XRD characterisation was performed on the synthesized materials to identify the structural defects and to investigate their thermal stability. With heating the sample under inert gas atmosphere we observe an expansion of the LiFePO4 unit cell. Cooling of the samples back to the room temperature results in a slightly decreased cell volume. These experiments were repeated on different materials as well heating / cooling conditions with the same result – in all these experiments under the given conditions an abrupt decrease in the cell volume of LiFePO4 was not observed. These data indicate that the synthesized LiFePO4 powder had almost no antisite defects, the structural defects in LiFePO4 are due to the excess of Fe on Li sites resulted from the transformation vivianite => sarcopside => triphylite. LiFePO4 single crystals were successfully synthesized by the optical floating zone technique in the flowing Ar by using the pressed powders in the form of the long rods which were sintered under argon atmosphere at 850°C for 10 hours. After sintering the rod exhibits reddish/ brown colour, indicating surface oxidation of Fe2+ to Fe3+. XRD revealed only small additional diffraction peaks close to detection limit, indexed as hematite phase. In order to avoid the hematite side phase, the sinter process was repeated under forming gas (1 % H2 in Ar). The SEM-EDX characterisation reveals the molar ratio Fe:P:O = 1.1:1.2:4 which is under consideration of quantification errors close to theoretical ratio of Fe:P:O = 1:1:4 for LiFePO4. Synthesized LiFePO4 materials were analyzed by ex-situ PALS in order to determine the influence particle size and defect chemistry on free annihilation lifetimes. Heat treatment, accompanied with crystals growth and densification lead to reduced lifetimes between 240 for sintered pellets and 250 ps for single crystals. The characteristics of the sintered powdered samples with grains in µm range do not differ much from the single crystal. The lifetime from the lifetime decomposition approach gave values in the same range, i.e. between 235 and 255 ps. These results indicate a free annihilation lifetime of positrons in LiFePO4 of around 240 ps. Two possible causes are conceivable higher lifetime: (1) the small crystallite size of 100 nm enables the positrons to leave a crystallite and annihilate somewhere in the inter-granular region with its high amount surface area, (2) the longer lifetime could arise from side phases, e.g. from the above mentioned amorphous phases and Li2SO4xH2O residuals.
