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.