Na-ion batteries attract much attention at present owing to its significant cost advantages. It is very important to develop new kinds of electrode materials for NIBs with high energy density and good cyclability. In previous work by Komaba et al., O3-type NaNi_0.5Mn_0.5O₂ (NNMO) was synthesized with good electrochemical performances. Herein, a new kind of Li-substituted material, NaLi_0.1Ni_0.35Mn_0.55O₂ (NLNMO), was prepared by a simple solid-state method and showed great electrochemical performance. A series of techniques (i.e. In-situ XRD and ss-NMR) were used to characterize structural changes during electrochemical cycling.

Experimental section

NLNMO was synthesized by a solid-state method. Stoichiometric amounts of Na₂CO₃, Li₂CO₃, NiCO₃ and Mn₂O₃ were mixed with acetone as a dispersant via ball milling for 6 h and dried at 120 ℃. The mixture was pressed into pellets, calcined at 900 ℃ for 12 h in air and then cooled down to room temperature in furnace. The electrode was prepared by mixing 80% active materials with 10% acetylene black and 10% PVDF. 1M NaClO₄ dissolved in a 98:2 mixture of PC and FEC was used as the electrolyte.

Results and discussion

We obtained the powder X-ray diffraction patterns of the as-synthesized material together with the Rietveld refinement profile. It can be found from the result that all the major diffraction peaks can be indexed to a Rhombohedral O3-type structure with a space group of Rm. A few impurities can also be found, corresponding to an O’3 phase (a distorted O3 phase).

Electrochemical performance of NLNMO cycled between 2.0-4.2 V at a current density of 12 mAg^-1 was tested. The discharge capacity is ~130 mAh g^-1 in the first cycle. The result shows great cyclability with capacity retention at 94% after 50 cycles, reaching 86% at the end of 100 cycles. When the voltage range is widened to 1.5-4.3 V, the specific capacity can reach up to 160 mAh g^-1 also with good structural stability.

In situ XRD data of NLNMO in the first cycle shows that the phase transformation process is O3-O’3-P3. In contrast, the structural change in NNMO is O3-P3 without an intermediate state, and severe amorphization process can be found. The formation of O’3 phase can prevent big structural damage as that of direct O3-P3 transformation.

⁷Li NMR spectrum of as-prepared NLNMO shows the specific position of Li⁺ in the material. The peaks above and below 1200 ppm represent Li⁺ in TM sites and Na sites, respectively. This phenomenon can explain why the O’3 phase is easily formed, as the atomic number in Na sites is more than that of TM sites and vacancies would thus form in TM layers [(NaLi_x)(Li_0.1-xNi_0.35Mn_0.55□_2x)O₂].²³Na NMR spectra of NLNMO electrodes at different state of charge were also obtained. One broad peak is found in the pristine electrode, which can be attributed to O3 or O’3 phase with small interslab space and thus strong interactions between Na⁺ and other ions. In contrast, the peaks become sharp when charged to 3.6 V, which indicates a larger interslab space and corresponds to the P3 structure. This result corresponds to the in situ XRD data well.

Conclusion

Li-substituted NaLi_0.1Ni_0.35Mn_0.55O₂ with an O3 phase is successfully prepared by a solid-state method. The material delivers a specific capacity of ~130 mAh g^-1, and the cyclability is highly improved after Li-substitution with capacity retention at 86% after 100 cycles at a voltage range of 2.0-4.2V. When the voltage range is widened to 1.5-4.3 V, the specific capacity can reach up to 160 mAh g^-1. It is proved by the in situ XRD data that the phase transformation of NLNMO with formation of an intermediate state of O’3 structure has a beneficial effect on improving the structural stability, compared to direct O3-P3 structural change of NNMO. ⁷Li NMR result shows that Li⁺ can be found in both Na layers and TM layers, which can cause the formation of O’3 phase in NLNMO.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NNSFC, Grant No. 21233004 and No. 21473148)

References

1)      S. Komaba et al.,Inorg. Chem. 2012, 51, 6211-6220

2)      J. Xu et al.,Chem. Mater. 26(2), 2014, 1260-1269