Using first-principles density functional calculations, LiNiO2-related cathode materials are studied. It is found that in contrast to previous studies, the hole state in Li doped NiO shows predominately Ni character and is accompanied by a local Jahn-Teller distortion. We show that this is consistent with experiments. A new potential ground state LiNiO2 cell is found in which charge disproportionation Ni3+àNi2++Ni4+ occurs. However another cell in which the Jahn-Teller distortions of Ni3+ octahedral are in a zigzag ordering, is close in energy. Therefore we suggest that in real LiNiO2 samples, the two phases coexist. This explains the absence of long range ordering in LiNiO2. Rock-salt LiMO2 compounds crystallise in three different structures depending on the cation ordering. We show that this cannot be explained by the size effect and propose that the exchange interaction between M ions is responsible for the ordering. Both size difference between Li and M and the exchange interaction between nearest-neighbouring M ions favour the layered structure, whereas the exchange interaction between second-nearest-neighbouring M ions destabilises the layered structure. The defect formation energies are low in LiNiO2, consistent with the difficulty to synthesise truly stoichiometric LiNiO2. The tendency for Ni to be present in the Li layers can be explained by super-exchange interactions. Therefore with Co substitution for Ni, the nonmagnetic Co ions screen these interactions and destabilise the presence of Ni in the Li layer. The same effect is found with Al substitution from our calculations. We also show why substitution of Ni by Mn increases the concentration of the interlayer mixing defects worse compared to LiNiO2. In addition, a correlation between the oxygen charge and the defect formation of oxygen vacancy is found. It appears that the lower the effective oxygen charge, the smaller the defect formation energy.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:566323 |
| Date | January 2012 |
| Creators | Chen, Hungru |
| Contributors | Harding, John ; West, Anthony |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.whiterose.ac.uk/3323/ |
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