Formation of Nanosized Defective Lithium Peroxides through Si-Coated Carbon Nanotube Cathodes for High Energy Efficiency Li–O₂ Batteries

Abstract

The formation and decomposition of lithium peroxides (LiO) during cycling is the key process for the reversible operation of lithium-oxygen batteries. The manipulation of such products from the large toroidal particles about hundreds of nanometers to the ones in the scale of tens of nanometers can improve the energy efficiency and the cycle life of the batteries. In this work, we carry out an in situ morphology tuning of LiO by virtue of the surface properties of the n-type Si-modified aligned carbon nanotube (CNT) cathodes. With the introduction of an n-type Si coating layer on the CNT surface, the morphology of LiO formed by discharge changes from large toroidal particles (∼300 nm) deposited on the pristine CNT cathodes to nanoparticles (10-20 nm) with poor crystallinity and plenty of lithium vacancies. Beneficial from such changes, the charge overpotential dramatically decreases to 0.55 V, with the charge plateau lying at 3.5 V even in the case of a high discharge capacity (3450 mA h g) being delivered, resulting in the high electrical energy efficiency approaching 80%. Such an improvement is attributed to the fact that the introduction of the n-type Si coating layer changes the surface properties of CNTs and guides the formation of nanosized amorphous-like lithium peroxides with plenty of defects. These results demonstrate that the cathode surface properties play an important role in the formation of products formed during the cycle, providing inspiration to design superior cathodes for the Li-O cells.

Li2O2 Formation Electrochemistry and Its Influence on Oxygen Reduction/Evolution Reaction Kinetics in Aprotic Li–O2 Batteries

“…In this way, Li 2 O 2 decomposition could be facilitated and the charge overpotential could be lowered. [ 40 ]…”

“…For example, Guo and co‐workers fabricated a heavy n‐type Si coating layer onto aligned carbon nanotube (CNT) cathodes. [ 40 ] With the addition of such a layer on the CNT cathode surface, the discharge product featured a nanoparticle morphology (10–20 nm). The crystallinity was quite low with plenty of lithium vacancies.…”

Recent Progress on Catalysts for the Positive Electrode of Aprotic Lithium-Oxygen Batteries †

“…For example, a magnetron sputtering n-type Si nanoparticle on CNT composite positive electrode presented discharge product in the form of low crystalline accompanied with large amounts of defects and vacancies inside, which was in a striking contrast to the large toroid products formed on the bare CNT surface. Furthermore, the side reactions were inhibited by the Si coating, resulting in ultra-low cycling voltage gap of 0.72 V. While for the pristine CNT positive electrode, the gap was increased to 1.7 V under the same testing condition of 0.1 mA•cm −2 (Figure 6d,e) [117]. The battery with a highly ordered hierarchically porous honeycomb-like structured carbon skeleton could work under an ultrahigh discharging rate because of the greatly enhanced mass transfer.…”

Defect Chemistry in Discharge Products of Li–O2 Batteries

“…The computed adsorption energy indicated that LiO 2 was more favorably adsorbed at the interface of ZnO/VACNT than Li 2 O 2 and Li 3 O 4 ; thus, further disproportionation was inhibited. Several reports have shown that the formation of Li 2− x O 2 can be induced by using semiconducting catalysts, but similarly, no mechanism with concrete evidence has been proposed . Thus, more research is required to develop a clearer understanding of the function and catalytic mechanism of semiconductors in Li–O 2 batteries.…”

“…Several reports have shown that the formation of Li 2−x O 2 can be induced by using semiconducting catalysts, but similarly, no mechanism with concrete evidence has been proposed. [73,77] Thus, more research is required to develop a clearer understanding of the function and catalytic mechanism of semiconductors in Li-O 2 batteries.…”