Molybdenum-based NASICON Li2M2(MoO4)3 (M = Zn, Cu): Understanding structural evolution and lithium storage mechanism.

● A facile sol-gel method is first proposed to synthesize NASICON-type Li 2 Zn 2 (MoO 4) 3 and Li 2 Cu 2 (MoO 4) 3. ● Rietveld refinement indicated the structure evolution in Cu-doped Li 2 Zn 2−x Cu x (MoO 4) 3. ● The Li 2 Zn 2 (MoO 4) 3 and Li 2 Cu 2 (MoO 4) 3 electrode display excellent lithium storage performance. ● In-situ XRD confirmed the existence of mesophase Li 4 MoO 5 in Li 2 M 2 (MoO 4) 3 (M=Zn, Cu) during the initial lithium insertion. NASICON (Na superionic conductor) type electrode materials are known for their wide range of electrochemical potentials, high ionic conductivity, and most importantly their structural and thermal stabilities. Li 2 M 2 (MoO 4) 3 (M=Zn, Cu), belonging to molybdenum-based NASICON family, are successfully synthesized by simple sol-gel method. The structure evolution caused by the substitution of Zn2+ by Cu2+ within Li 2 Zn 2- x Cu x (MoO 4) 3 (0 ≤ x ≤ 2) has been investigated. Rietveld refinement results reveals that the priority order of copper atoms replacing zinc atoms in Li 2 Zn 2 (MoO 4) 3. Furthermore, both Li 2 Zn 2 (MoO 4) 3 and Li 2 Cu 2 (MoO 4) 3 electrodes deliver excellent electrochemical performance, and high reversible capacities of 864 mAh g−1 and 747 mAh g−1 can be acquired at a current density of 0.1 A g−1, respectively. In addition, the in-operando X-ray diffraction measurements indicate the formation of cubic mesophase Li 4 MoO 5 both in Li 2 Zn 2 (MoO 4) 3 and Li 2 Cu 2 (MoO 4) 3 during the initial lithium insertion, which irreversibly converts into the amorphous phases on further discharge at subsequent discharge and charge process. [ABSTRACT FROM AUTHOR]