The Challenge of Probing Lithium Insertion Mechanisms in Cathode Materials

Abstract: The Li-ion battery has, from its commercialisation in the early 1990's, now become the most widely used power source for portable low-power electronics: laptops, cellular phones and MP3-players are a few examples. To further develop existing and find new electrode materials for these batteries, it is vital to understand the lithium insertion/extraction mechanisms taking place during battery operation. In this thesis, single-crystal X-ray diffraction has been used to investigate lithium insertion/extraction mechanisms in the cathode materials V6O13 and LiFePO4. A novel single-crystal electrochemical cell for in situ single-crystal X-ray diffraction studies has also been developed.The phases Li3V6O13 and Li3+xV6O13, 0<x<1, both contain a disordered lithium ion. A low-temperature study of Li3.24V6O13 (at 95 K) shows that this disorder is static rather than dynamic; the lithium ion is equally distributed above and below an inversion centre in the centrosymmetric V6O13 host structure. Short-range-ordering between this disordered lithium ion and the lithium ion inserted into Li3V6O13 gives rise to solid-solution behaviour not observed earlier in the LixV6O13 system. A model is proposed for the lithium insertion mechanism up to the end-member composition Li6V6O13.Lithium has also been electrochemically extracted from LiFePO4 single crystals. On the basis of the shapes of the LiFePO4 and FePO4 reflections, it is concluded that FePO4 is formed at the crystal surface and that the LiFePO4/FePO4 interface propagates into the crystal. This is in agreement with an earlier proposed model for lithium extraction from LiFePO4 particles.Initial experiments with the newly developed single-crystal electrochemical cell for in situ single-crystal X-ray diffraction demonstrate that it is possible to insert lithium into a single crystal of V6O13 and then collect single-crystal X-ray diffraction data. The method needs further development but promises to become a powerful tool for studying lithium insertion/extraction mechanisms.