主题：充电电池介绍 -- 积吉

Our synthesis strategy has been to create an appropriate off-stoichiometry in the starting materials so that the coating constituents phase-separate from LiFePO4 as it forms during the heat treatment, thereby creating the active storage material and coating in a single process. Here we describe results with an iron:phosphorus deficiency ratio of 2:1 (for example LiFe1-2yP1-yO4-delta, y = 0.05), as indicated by arrow A in Supplementary Fig. 1. We note that the more common one-to-one iron:phosphorus deficiency (arrow B in Supplementary Fig. 1, equivalent to lithium excess22) creates a mixture of Li3PO4 and iron oxides, which are not likely to conduct well under the synthesis conditions used to prepare LiFePO4.

LiFe0.9P0.95O4-delta was synthesized by ball-milling Li2CO3, FeC2O4dot2H2O and NH4H2PO4 in appropriate amounts, heating the mixture at 350 °C for 10 h and then heating at 600 °C for 10 h under argon. X-ray diffraction (Fig. 1a and Supplementary Fig. 2) shows that despite the off-stoichiometric starting mixture, stoichiometric LiFePO4 forms with lattice parameters (a = 10.3134 , b = 6.002 and c = 4.691 ) very similar to those reported in the literature23. No crystalline Fe2P can be observed in the X-ray pattern of the material synthesized at 600 °C, but a small amount of Fe2P is present in the material synthesized at 700 °C (Fig. 1a). However, amorphous FeP or Fe2P created by the reducing atmosphere cannot be excluded24. Mssbauer spectroscopy (Supplementary Fig. 4) indicates that apart from the major LiFePO4 component, around 10% of the Fe is present in some other environment. The isomer shift (0.464 mm s-1) and quadrupole splitting (0.798 mm s-1) of this second component fall in the region of values given in the literature for Fe3+ in pyrophosphate (P2O7-containing) glasses, although recent work24 argues that iron monophosphides also give a Mssbauer signal in this range. To distinguish between the two possibilities, as-made material was discharged. The large discharge capacity found (Supplementary Fig. 5) is consistent with the presence of reducible Fe3+ rather than FeP in the material. Furthermore, in subsequent charge–discharge cycles we consistently find 15–18 mA h g-1 capacity in the 3.2–2.0 V voltage window, in agreement with the approx10% proportion of iron