The properties, insertion mechanisms, and electrochemical performances of the Na1.5Fe0.5Ti1.5(PO4)3/C composite as electrode material for Na-ion batteries are reported. The composite was obtained by solid-state reaction and consists of porous secondary particles of submicron-sized particles coated by carbon. Detailed characterizations were performed by combining theoretical and experimental tools. This includes the determination of the crystal structure of Na1.5Fe0.5Ti1.5(PO4)3 from both first-principles calculations and X-ray diffraction providing Na distribution over M1 and M2 interstitial sites, which is of importance for ionic conductivity. Na1.5Fe0.5Ti1.5(PO4)3/C was used as an electrode material at 2.2 V versus Na+/Na0, exhibiting good Na-storage ability with a specific capacity of 125 mAh g–1, close to the theoretical value, for the first discharge at C/10, good capacity retention, and Coulombic efficiency of 95% and 99.5% at the 60th cycle, respectively, and high power rate with a decrease of the specific capacity of only 14% from C/10 to 2C. These good performances have been related to the morphology of the composite and the substitution of Fe for Ti, leading to an insertion mechanism that differs from that of NaTi2(PO4)3. This mechanism was quantitatively analyzed from operando 57Fe Mössbauer spectroscopy used for the first time in both galvanostatic and GITT modes.
NaSn2(PO4)3/C composites obtained by solid-state and pyrolysis reactions present high capacity retention and high-rate capability as anode materials for Li-ion batteries. The structure of NaSn2(PO4)3 was analyzed by combining X-ray diffraction and density functional theory to confirm the R3̅ space group. The composite is formed by submicrometer particles with a cube-like shape coated by pyrolytic carbon that improves the electronic percolation. The 119Sn Mössbauer spectroscopy shows the existence of Sn4+ with a more ionic character than SnO2, which can be related to the inductive effect of the phosphate groups. This technique was used in the operando mode to follow the reaction with lithium during the first discharge that is a crucial step for improving the performance. A two-step mechanism has been identified that consists of the irreversible transformation of the pristine material into Sn0 species for the first half of the discharge followed by reversible Li–Sn alloying reactions. The Mössbauer spectra of the Sn0 species differ from the spectrum of βSn because of their nanosize and the existence of chemical bonds with the sodium phosphate matrix formed during the conversion reaction. The first part of the discharge should be considered as a restructuration of the pristine material leading to the dispersion of Sn0 small particles in strong interaction with the phosphate matrix and providing good cyclability. Such a mechanism strongly differs from the insertion mechanism of NaTi2(PO4)3/C that contains a transition metal with the same oxidation state as Sn4+.
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