Precipitation Revisited: Shape Control of LiFePO₄ Nanoparticles by Combinatorial Precipitation

Abstract

Tunable precipitation strategy to control the shape of nanoparticles of a three-component system is presented. The strategy is devised from understanding the effects of precursor addition sequences on the morphology of resultant precipitates. LiFePO4, one of the most potential candidate as a cathode material of lithium ion batteries for electric vehicles, was used as a representative model of the three (Li, Fe, and PO4)-component system. According to the precursor addition sequence, three different precipitation methods were adopted: coprecipitation (Copr) and two different types of sequential precipitations (Seq1 and Seq2). Solubility product (Ksp) of intermediate precipitates (Li3PO4 and Fe3(PO4)2) is the key parameter to help the precipitation processes understood. In Copr, the intermediate precipitates are formed simultaneously under Ksp-governed competition. In Seq1 and Seq2, Li3PO4 precipitates prior to Fe3(PO4)2. When Fe2+ is introduced into the suspension of Li3PO4, the preformed precipitate is sacrificed to supply PO43– for Fe3(PO4)2 precipitation due to the stronger tendency (smaller value of Ksp) of precipitation of Fe3(PO4)2. Also, the interaction between a cationic surfactant and PO43– makes the difference between Seq1 and Seq2. As a conclusion of the effects of precursor sequence, the shape of particles spans from spherical nanoparticles through a hollow sphere secondary structure of the same nanoparticles to nanoplates. Each own morphology developed by different precipitation methods leads to different intercalation/deintercalation behavior of lithium ions in conventional rechargeable battery cells.