New Li0.8M0.1Ti2(PO4)3 (M=Co, Mg) Electrode Materials for Lithium‐Ion Batteries: In Operando X‐Ray Diffraction and Ex Situ X‐ray Photoelectron Spectroscopy Investigations

Abstract

AbstractPure Li0.8M0.1Ti2(PO4)3 (M: Co, Mg) phosphates with the NASICON‐type structure were synthesized using a solid‐state reaction. Depending on the cation, M, the synthesis temperature and the annealing duration were optimized. The electrochemical performances of Li0.8M0.1Ti2(PO4)3 were evaluated as negative electrode materials for Li‐ion batteries over two different voltage ranges, 1.5–3.0 V and 1.0–3.0 V. The best electrochemical results were obtained for the voltage range 1.5–3.0 V with discharge capacities of 100 mAh g−1 and 80 mAh g−1 for Li0.8Co0.1Ti2(PO4)3 and Li0.8Mg0.1Ti2(PO4)3, respectively, over 20 cycles with capacity retentions exceeding 80 %. Raman spectroscopy was used to confirm the formation of the carbon layer after the carbon‐coating procedure. In operando XRD was performed to investigate the structural changes upon the first discharge/charge cycle, showing the progressive evolution of Li‐poor and Li‐rich phase fractions. Ex situ XPS analysis was conducted to follow the oxidation state of titanium during the insertion/de‐insertion of lithium ions, demonstrating that the redox couple (Ti4+/Ti3+) is reversibly active during the electrochemical cycling.