Solid solutions of LiFe 1-y M y PO 4 /C (M = Co, Ni) within the whole substitution range (0 ≤ y ≤ 1) are systematically investigated as safe high-voltage positive electrode materials for Li-ion battery. Especially the similarities and differences between the Co-and Ni-substitution schemes are highlighted. With increasing substitution level y, the orthorhombic unit cell shrinks and the Li + -ion diffusion channel area decreases, more rapidly for the smaller-sized Ni substituent. While the Co 2+ /Co 3+ redox couple shows reversible electrochemical activity, only an initial, partial delithiation is achieved for the Ni 2+ /Ni 3+ couple at all y. However, both substitution schemes similarly affect the Fe 2+ /Fe 3+ redox couple by increasing its potential and enhancing the kinetics. Particularly, a mutual influence of Fe and Co/Ni on the delithation/lithiation characteristics is proposed: ex situ XRD analysis shows signs of a phase-composition change in the Fe 2+ /Fe 3+ region for higher y, and correspondingly, a changed Co 2+ /Co 3+ redox mechanism for lower y is indicated by cyclic voltammetry. We suggest that this behavior is related to a substitution-induced decrease in the volume change between the lithiated and delithiated phases. For the Co-substitution scheme, a composition around y ≈ 0.5 seems optimal for combining a high energy density and a kinetically beneficial, diffusional single-phase delithiation/lithiation mechanism. Li-ion batteries have been traditionally used for small, portable consumer electronics, but their utilization in large-scale applications is rapidly growing. As the size of the battery packs is increasing, the matters of safety, cost, and cycle-life become more crucial. In addition, for applications in transportation, high energy and power densities are demanded. Olivine-structured metal phosphates LiMPO 4 (M = Fe, Mn, Co, Ni) are an interesting material family to replace the layered metal oxides LiMO 2 as the positive electrode.1-4 Their safety owing to their intrinsic structural stability is very tempting for large-scale batteries. The orthorhombic olivine structure (space group Pnma) is composed of LiO 6 and MO 6 octahedra and PO 4 tetrahedra, such that Li + -ion diffusion during lithiation/delithiation takes place only in one dimension, along the b axis.
3,5The iron-based olivine LiFePO 4 is already commercially used in Li-ion batteries; it shows a theoretical capacity of 170 mAh g −1 , high safety and cycle stability, and is in addition cheap and environmentally friendly.1 The delithiation/lithiation (charge/discharge) reaction proceeds as a two-phase reaction with a varying LiFePO 4 /FePO 4 ratio, which brings about a kinetic limitation due to the single Fe valence (Fe 2+ or Fe 3+ ) in the two end-phases, resulting in low carrier density. 1,6 Additionally, the two-phase reaction pathway is restricted by nucleation and growth, when compared to a diffusional single-phase solidsolution mechanism. 7 However, narrow single-phase solid-solution regions are suggested to exist at room-tem...