From Oxides to Ionically Conducting Polyanionic Frameworks as Positive Electrodes in Li Batteries 6553 2. The Early Days: The NASICON and Anti-NASICON Structures Used as Model Frameworks 6554 2.1. Structural Considerations 6554 2.2. The Inductive Effect: Tuning the M n+ /M (n−1)+ Redox Couple in the NASICON Structure by Changing the Chemical Nature of the XO 4 n− Groups 6555 2.3. Relative Positions of Various M n+ /M (n−1)+ Redox Couples (M = Fe, Ti, V, Nb) in NASICON-type Phosphates 6555 2.3.1. Position of the Ti 4+ /Ti 3+ Couple versus Li + /Li or Na + /Na 6555 2.3.2. Position of the Fe 3+ /Fe 2+ Couple versus Li + /Li or Na + /Na 6556 2.3.3. Position of the V n+ /V (n−1)+ Couples versus Li + /Li or Na + /Na 6556 2.4. Complex Redox Phenomena in Anti-NASI-CON Compositions Li x M 2 (PO 4 ) 3 (0 < x < 5 ; M = Fe, V) 6556 2.4.1. Li + Insertion into Monoclinic Fe 2 (SO 4 ) 3 , Li 3 Fe 2 (PO 4 ) 3 , and Li 3 Fe 2 (AsO 4 ) 3 6557 2.4.2. Li + Insertion/Extraction into Monoclinic Li 3 V 2 (PO 4 ) 3 6557 3. LiMPO 4 Compositions Based on the Olivine Structure: Fifteen Years of Great Achievements 6558 3.1. The Early Days: From an Academic Curiosity to First Industrial Realizations 6558 3.2. A Myriad of Synthesis Routes Developed for Optimal Electrochemical Response of LiMPO 4 Powders, Industrial Process Ability, and Cost Reduction 6559 3.2.1. Solid-State Syntheses 6559 3.2.2. Solution-Based Syntheses 6560 3.2.3. Carbon Coating and Purity Control of LiFePO 4 6562 3.3. The Highly Insulating LiMnPO 4 6562 3.4. Substitutions of Mn and/or Co for Fe in LiFePO 4 6563 3.5. Intrinsic Physicochemical Properties of LiFe-PO 4 and Mechanism of Li + Extraction 6565 3.5.1. Crystal Structure, Defects, e − Transport, Li + Diffusion 6565 3.5.2. Reactivity with Moisture and/or Air 6566 3.5.3. Mechanism 6567 4. Alternative Polyanionic Structures and Compositions: Hydrated Phosphates, Diphosphates, Alluaudites, Silicates, and Borates 6569 4.1. Fe 3+ /Fe 2+ Couple in Amorphous or Crystalline Iron Phosphates: FePO 4 •nH 2 O 6569 4.

Lithium iron phosphate is one of the most promising positive-electrode materials for the next generation of lithium-ion batteries that will be used in electric and plug-in hybrid vehicles. Lithium deintercalation (intercalation) proceeds through a two-phase reaction between compositions very close to LiFePO(4) and FePO(4). As both endmember phases are very poor ionic and electronic conductors, it is difficult to understand the intercalation mechanism at the microscopic scale. Here, we report a characterization of electrochemically deintercalated nanomaterials by X-ray diffraction and electron microscopy that shows the coexistence of fully intercalated and fully deintercalated individual particles. This result indicates that the growth reaction is considerably faster than its nucleation. The reaction mechanism is described by a 'domino-cascade model' and is explained by the existence of structural constraints occurring just at the reaction interface: the minimization of the elastic energy enhances the deintercalation (intercalation) process that occurs as a wave moving through the entire crystal. This model opens new perspectives in the search for new electrode materials even with poor ionic and electronic conductivities.