Dual-doped mesoporous carbon/SWCNT nanoshells for Li-ion battery electrode materials

Abstract

There is no doubt that rechargeable lithium-ion batteries (LIBs) have been developed as an electrical power source in a wide variety of energy storage systems including portable electronics, electric vehicles (EVs) and plug-in hybrid electric vehicle (PHEVs). Continuous advanced demands for higher energy density energy storage systems strongly push us to develop breakthroughs for improved cathode materials of LIBs. Recently, over-lithiated layered metal oxides (xLi2MnO3•(1-x)LiMO2, [M=Ni, Co, Mn], OLOs) materials, which are widely used in cathode materials of LIBs for high discharge capacity (over 250 mAh/g), are struggling with fast capacity fading and Mn dissolution on the unstable lithium-rich layered metal oxides surface. The poor electrical conductivity of lithium-rich layered metal oxides and Mn dissolution-triggered by-products are known to induce serious capacity fading during charge/discharge cycling. As a newly-synthesized polymeric ionic liquid (PIL)-driven material/single-walled carbon nanotube (SWCNT) nano-architecture strategy to develop an ion/electron-conductive nanoshells far beyond traditional surface modification, we have demonstrated single-walled carbon nanotube (SWCNT)-embedded and dual atom (nitrogen (N) and sulfur (S))-doped mesoporous carbon shells (referred to as “SMC” shell) on the LNMO (LiNi0.5Mn1.5O4) as well as OLO (0.49Li2MnO3·0.51LiNi0.37Co0.24Mn0.39O2) surface. The SMC-coated electrode materials are fabricated via simple mixing of pristine active materials in the SWCNT/PIL mixture solution and the subsequent one-pot carbonization process of the coated PIL on active materials surface. The PIL synthesized herein consists of poly(1-vinyl-3-ethylimidazolium) cations and dodecyl sulfate counter anions, of which chemical structures are purposeful designed to achieve multifunctional roles as: i) precursor of conformal/continuous carbon shell, ii) dual (N and S)-doping source, iii) porogen, and iv) SWCNT dispersant. Driven by such chemical/structural originality, the SMC effectively reduces unwanted interfacial side reactions between the cathode materials and liquid electrolyte while charge/discharge process. As a consequence, the SMC-coated cathode materials provided unprecedented improvements in the high-performance (rate capability, cycling performance and thermal stability) of lithium-ion batteries. The SMC-coated cathode materials hold a great deal of promise as a facile and versatile platform surface modification technology for high-performance batteries and also open a new opportunity for next-generation multifunctional molecularly-designed, ion/electron-conductive nanoshells on electrode materials that are in strong pursuit of progress in electrochemical performance.