A Polypyrrole Hollow Sphere@Se Cathode for Rechargeable Lithium Batteries

The exploration of new electrode materials is indispensable for lithium ion batteries. Selenium, belonging to the same group as sulfur and with a high specific theoretical capacity of 675 Ah/kg or 3253 Ah/m³ was first demonstrated as a candidate of cathode material for lithium and sodium rechargeable batteries by Abouimrane et al.(1). Li-Se batteries are supposed to possess a large volumetric energy density that is of greater importance than gravimetric energy density in terms of lightweight and portability of batteries. Moreover, selenium exhibits a quite high intrinsic electronic conductivity (1×10^-3 S/m) that is approximately 20 orders of magnitude greater than sulfur(1, 2), which could afford Se better electrochemical activity and better rate capability.

Nevertheless, the shuttle effect in Li-Se system remains to be a problem that deteriorates the cycle performance due to the dissolution of polyselenide species in electrolyte. Here, we designed a novel polypyrrole(PPy) hollow sphere@Se composite, in which Se was confined in PPy hollow microspheres. Thus, the loss of active material and capacity fading was effectively suppressed.

Polypyrrole(PPy) hollow spheres were synthesized via a multi-step templating method. The PPy hollow sphere@Se composite was prepared by a melt-diffusion approach with a mixture of selenium and PPy hollow spheres in a certain weight ratio under vacuum.

The morphology of prepared PPy hollow spheres is shown in figure 1a and b. The hollow microspheres are self-assembled by smaller polypyrrole particles, with an approximate diameter of 300nm. The hollow PPy spheres with good shape and dispersibility are demonstrated. A part of selenium is infiltrated into the hollow spheres as shown in figure 1c. 

Figure 1. a) SEM image of PPy hollow spheres; TEM images of b) PPy hollow spheres and c) PPy hollow sphere@Se. d) Cycling performance and coulombic efficiency of PPy hollow sphere@Se and pristine Se; e) Nyquist plots of Li-PPy@Se and Li-Se cells after 50 cycles with 1M LiTFSI plus 0.1M LiNO₃ in DOL/DME as electrolyte.

Figure 1d compares the specific discharge capacity and coulombic efficiency of PPy hollow sphere@Se and pristine Se at 0.1C rate. Pristine Se cathode presents rather poor cycling stability with a low reversible capacity of less than 40 mAh/g after 50 cycles. The severe capacity fading is ascribed to the shuttle effect with dissolution of  polyselenide species generated during discharging process which also results in lower coulombic efficiency. By contrast, a specific discharge capacity of 250mAh/g is obtained for PPy hollow sphere@Se electrode after 50 cycles. Moreover, the coulombic efficiency almost reaches 100%, indicating that the shuttle effect has been inhibited effectively by the hollow structure of the PPy matrix.

Electrochemical impedance spectroscopy (EIS) provides further evidence for its superior cyclability as figure 1e indicates. The data show that both the electrolyte resistance R_e and the charge transfer resistance R_ct reduce when pristine Se is substituted by PPy hollow sphere@Se for cathode. Especially the value of R_ct  in Li-PPy@Se cell obviously decreases to just 40% of the one for Li-Se cell. The reduction of resistances is proposed to be owing to the restriction of the discharge products inside the hollow spheres and less shuttle effect which could also corrode the surface of lithium metal electrode. 

From the above, selenium cathode has great potential for the development of high energy density lithium batteries and PPy hollow sphere@Se composite is a favorable cathode in Li-Se system.

References

1. A. Abouimrane, D. Dambournet, K. W. Chapman, P. J. Chupas, W. Weng and K. Amine, Journal of the American Chemical Society, 134, 4505 (2012).

2. L. Liu and Y. Wu, Chem.Commun., 49, 11515 (2013).