ИСТИНА

The phase transformations occurring at the (de)intercalation processes make a crucial impact on the electrochemical performance of the electrode materials for metal-ion batteries. One of the most common examples of such compliance is LiFePO4 with the olivine structure [1]: in spite of its thermodynamically conditional two-phase transition between triphilite and heterosite phases, this material often exhibits rate capability properties which are usually peculiar to single-phase materials. The reason of such behavior is a well-known kinetic feature of appearing of the LixFePO4 solid solution mostly attributed to a small (<100 nm) particle size of LiFePO4 [2]. However, there are some other factors which affect the phase transformation behavior of the electrode materials being under kinetic control. Thus, cation disorder in the M2 site of the olivine crystal structure impedes the formation of two-phase boundary and therefore increases the single-phase contribution into overall (de)intercalation process. We studied the phase transformation behavior of different LiMPO4 (M = Fe, Mn, Co, Ni) compositions and observed this effect by independent electrochemical and in situ X-ray diffraction techniques [3]. As an example, the transformation of the 211/020 peak of the X-ray pattern of three studied materials during discharge is presented at the Figure 1: it is clear that increasing the number of d-cations in the structure leads to the appearing of the extended solid solution within triphilite-type LixMPO4 even without the formation of the heterosite phase. The discussion of this and other observed phenomena will be presented within the framework of the presentation. Figure 1. Transformation of the olivine 211/020 peak of the X-ray pattern during Li intercalation (discharge) for the LiFePO4, LiFe1/2Mn1/2PO4 and LiFe1/3Mn1/3Co1/3PO4 cathode materials. The type of the intercalating cation also plays crucial role in the phase transformation regime of the selected cathode materials. Thus, layered Na2FePO4F demonstrates two two-phase transitions in the Na-ion cell, but a noticeable contribution of the single-phase mechanism may be observed in the Li-ion system. The impact of the type of alkali ion and electrolyte on the phase transformations, diffusion and charge transfer resistances, rate capability and cycling stability of the fluorophospate cathode materials is also discussed in the present report.