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● 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]

Due to the limited resources of lithium source and their high price, lithium-ion batteries (LIBs) cannot meet the demands of future large-scale energy storage. Sodium-ion batteries (SIBs) with advantages of the abundant sodium resources and low cost, are one of the most promising alternatives to LIBs. Here, we firstly synthesize a new manganese-based polyanionic compound (Na 3 Mn 2 P 3 O 11) through a simple sol-gel method. The resolved crystal structure indicates that single-phase Na 3 Mn 2 P 3 O 11 compound belongs to the orthorhombic structure. When tested as for cathodes, it displays a poor electrochemical performance with a potential window of 1.8–4.3 V (versus Na/Na+), caused by the Jahn-Teller effect of Mn3+ generated during charging process. In this context, a series of Fe-doped Na 3 Mn 2-x Fe x P 3 O 11 (0.1 ≤ x ≤ 0.5) are prepared to improve the electrochemical performance. Results indicate that Fe substitution of Mn in Na 2 Mn 2-x Fe x P 3 O 11 structure can not only enhance the electrochemical performance, but also increase conductivity and achieve fast reaction kinetics. Noticeably, when iron-doped amount is 0.4, Na 3 Mn 1.6 Fe 0.4 P 3 O 11 exhibits the best cycling (62.7 mA h g−1 at 0.1 C over 100 cycles) and rate performance (19.7 mA h g−1 at 5 C). A novel manganese-based polyanionic compound (Na 3 Mn 2 P 3 O 11) was successfully prepared via a simple sol-gel way, which belongs to orthorhombic system (space group P 2 1 2 1 2 1). The reasonably small Rp and Rwp factors of the Rietveld refinement reveal that single phase can be obtained, and no impurity phases can be detected in Fe-doped Na 3 Mn 2-x Fe x P 3 O 11 (0 ≤ x ≤ 0.5). Moreover, when iron-doped amount is 0.4, it exhibited the best cycling and rate performance. Image 1 • Na 3 Mn 2 P 3 O 11 (space group P 2 1 2 1 2 1) was firstly prepared via a simple sol-gel method. • Rietveld refinement proved existence of Na 3 Mn 2-x Fe x P 3 O 11 (x = 0–0.5) solid solution. • Carbon-coated Fe-doped samples were prepared to improve electrochemical property. • Na3Mn1.6Fe0.4P3O11@C shows a capacity of 62.7 mA h g−1 at 0.1 C over 100 cycles. [ABSTRACT FROM AUTHOR]