Surface structures and crystal morphologies of LiFePO4: relevance to electrochemical behaviour

Advanced simulation techniques are used to provide atomic-scale insight into the surface structures and crystal morphologies of the lithium battery cathode material LiFePO4. Relaxed surface structures and energies are reported for 19 low index planes. The calculated equilibrium morphology takes on a rounded, isometric appearance, with {010}, {201}, {011}, and {100} faces prominent. Almost all of the low energy surfaces are lithium-deficient relative to the bulk lattice, requiring Li vacancies at the surface. The calculated growth morphology exhibits the {010}, {100} and {101} faces, with an elongated hexagonal prism-like shape; this morphology is more consistent with experimentally observed LiFePO4 particles. The exposure of the (010) surface in our calculated equilibrium and growth morphologies is significant since it is normal to the most facile pathway for lithium ion conduction (along the [010] channel), and hence important for the reversible insertion/de-insertion of lithium ions. SEM images of plate-like crystallites from hydrothermal synthesis are also simulated by our methods, and exhibit large (010) faces.