Exploring New Storage Mechanisms to Improve Capacity Performance of Na-Based Batteries

Room temperature sodium-based batteries are thought to be one of the most promising energy storage systems beyond lithium ion batteries due to the abundant reserve of sodium in nature and its low cost. As the ion radius (1.02 Å) of Na⁺ is by 34% larger than that (0.76 Å) of Li⁺, Na transport and storage are quite sluggish in most of the Li-insertable host structures. Recently open framework strategy has proved to be successful to construct kinetically favorable Na-ion channels with increased size and dimension.[1,2] However the reversible capacity is still limited in view of the preservation of structure integrity.

In this work, we use a structurally stable Li₄Ti₅O₁₂ spinel thin film as insertion-type model material to investigate its intrinsic Na-ion transport kinetics and coupled pseudocapacitive charging.[3,4] It is found that the latter effect is remarkably activated by the nanocrystalline microstructure full of defect-rich surface, which can simutaneously promote Na-ion and electron accessibility to the surface/subsurface. It is proposed that imposing a pseudocapacitance effect on typical insertion electrodes is a potential solution to break through capacity limitation for Na-based batteries without the cost of collapsing host structure. Therefore a highly reversible charge capacity of 225 mAh g^-1 (exceeding the theoretical value 175 mAh g^-1based on insertion reaction) at 1C is achievable.

To further improve the capacity performance of Na-based batteries, phase transformation reactions including alloying or conversion ones are often resorted to. However they tend to induce larger volume change and more sluggish Na-ion transport at multiphase solid interfaces than for Li-ion batteries, leading to inefficiency of mixed conductive networks and thus degradation of reversibility, polarization or rate performance. 

O₂-NaO₂ phase transformation however occurs at kinetically favorable gas-solid interfaces and yields more conductive superoxide product as shown in the investigation of Na-O₂ batteries.[5] Metal-air batteries are thought to be the ultimate solution to energy storage systems owing to their super-high energy density. Here we report a long-life Na-O₂ battery with a high capacity of 750 mAh g^-1_carbon by manipulating the nucleation and growth of nano-sized NaO₂ particles in a vertically aligned carbon nanotube network with large surface area.[6] With a low overpotential of  ~0.2 V, the electrical energy efficiency is as high as 90 % up to 100 cycles. A good rate performance (~1500 mAh g^-1_carbon at 667 mA g^-1_carbon) can be achieved through pre-depositing a thin NaO₂layer. 

References

[1] Li C L, Yin C L, Gu L, Dinnebier R E, Mu X K, van Aken P A, Maier J. J. Am. Chem. Soc., 2013, 135, 11425.

[2] Li C L, Yin C L, Mu X K, Maier J. Chem. Mater., 2013, 25, 962.

[3] Sun Y, Zhao L, Pan H, Lu X, Gu L, Hu Y S, Li H, Armand M, Ikuhara Y, Chen L, Huang X. Nat. Commun. 2013, 4, 1870.

[4] Yu P F, Li C L, Guo X X. Submitted.

[5] Hartmann P, Bender C L, Vracar M, Garsuch A, Durr A K, Janek J, Adelhelm P. Nat. Mater. 2013, 12, 228.

[6] Zhao N, Li C L, Guo X X. Submitted.