Recently, sodium-ion batteries (SIBs), as the promising candidate for grid energy storage, have attracted worldwide interest owing to the natural abundance and low cost of sodium. Especially, SIBs shares the identical intercalation chemistry as lithium-ion batteries (LIBs), which triggers extensive studies on the new electrode materials analogous to those of LIBs. Among various cathode materials, compounds with NASICON-type (Na super-ionic conductor) structure have received particular attention because the three-dimensional open framework gives rise to plenty of interstices, capable of allowing fast Na⁺ insertion and extraction with little lattice strain, such as Na₃V₂(PO₄)_3¹, Na₃V₂(PO₄)₂F_3² and etc.

In this work, a new NASICON-structured compound, Na₂(VTi)(PO₄)₃@C (hereafter donated as NVTP@C), was successfully synthesized by sol-gel method and its structural property and morphology were examined in terms of XRD, EDX and TEM. Rietveld analysis confirmed the single phase of the as-prepared sample with monoclinic symmetry (space group:). SEM and TEM images showed that NVTP nanoparticles were well dispersed inside the mesoporous amorphous carbon matrix. Also, a dense layer of carbon was found to be coated on the surface of NVTP particles. Though composting with carbon cannot optimize intrinsic electron conductivity of NVTP, such architecture can greatly accelerate electrons transport via the coated carbon and the amorphous carbon matrix. Furthermore, the nano size of the as-prepared particles could greatly shorten the transport pathway of sodium ions. Thus, it is highly expected that NVTP@C nanocomposite would demonstrate superior Na⁺storage performance.

The electrochemical properties of NVTP@C nanocomposite were evaluated using sodium-ion half cell in the voltage window of 1.5 - 4.5 V. The initial charge profile showed a working plateau at around 3.4 V due the oxidized process of V³⁺ into V⁴⁺. The initial charge capacity reached 59 mAh g^-1. The subsequent discharge/charge curves demonstrated a series of symmetrical plateaus at around 3.4, 2.1 and 1.6 V, corresponding to the redox of V⁴⁺/V³⁺, Ti⁴⁺/Ti³⁺, and V³⁺/V²⁺, respectively. The reversible capacity was achieved as 147 mAh g^-1 with high coulombic efficiency of nearly 100%, one of the highest capacities among the state-of-the-art NASICON-structured compounds. Moreover, NVTP@C electrode demonstrateed superior rate capability with specific capacities of nearly 88 and 56 mA h g^-1 under high current densities of 10 C (1.25 A g^-1) and 20 C (2.5 A g^-1), respectively. More importantly, the symmetric full cell was constructed by using NVTP@C as both cathode and anode. This full cell shows an average working plateau of 1.1 V, a revesible specific capacity of 62 mAh g^-1under high current density of 10 C-rate, 85% capacity retention after 1000 cycles and superior rate capability. The full symmetric cell using the same active materials as both cathode and anode will strongly benefit to the design of sodium-ion batteries with high safety, low cost and long-term cycle life.

References:

¹Dongxue Wang, Nan Chen, Malin Li, Chunzhong Wang, helmut Ehrenberg, Xiaofei Bie, Yingjin Wei, Gang Chen, and Fei Du, J. Mater. Chem. A 2015, 3, 8636

²Qiang Liu, Dongxue Wang, Xu Yang, Nan Chen, Chunzhong Wang, Xiaofei Bie, Yingjin Wei, Gang Chen, and Fei Du, J. Mater. Chem. A 2015, 3, 21478