ABSTRACT
Lithium-sulfur batteries are attractive alternatives to lithium-ion batteries because of their high theoretical specific energy and natural abundance of sulfur. However, the practical specific energy and cycle life of Li-S pouch cells are significantly limited by the use of thin sulfur electrodes, flooded electrolytes and Li metal degradation. Here we propose a cathode design concept to achieve good Li-S pouch cell performances. The cathode is composed of uniformly embedded ZnS nanoparticles and Co-N-C single-atom catalyst to form double-end binding sites inside a highly oriented macroporous host, which can effectively immobilize and catalytically convert polysulfide intermediates during cycling, thus eliminating the shuttle effect and lithium metal corrosion. The ordered macropores enhance ionic transport under high sulfur loading by forming sufficient triple-phase boundaries between catalyst, conductive support and electrolyte. This design prevents the formation of inactive sulfur (dead sulfur). Our cathode structure shows improved performances in a pouch cell configuration under high sulfur loading and lean electrolyte operation. A 1-A-h-level pouch cell with only 100% lithium excess can deliver a cell specific energy of >300 W h kg-1 with a Coulombic efficiency >95% for 80 cycles.
ABSTRACT
The first-cycle behavior of layered Li-rich oxides, including Li2MnO3 activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the Li2MnO3 activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLi2MnO3·(1- x) LiNi0.3Co0.3Mn0.4O2 ( x = 0, 0.5, 1) cathode materials were studied with an in situ electrochemical quartz crystal microbalance (EQCM). The results demonstrate that a synergistic effect between the layered Li2MnO3 and LiNi0.3Co0.3Mn0.4O2 structures can significantly affect the activation pathway of Li1.2Ni0.12Co0.12Mn0.56O2, leading to an extra-high capacity. It is demonstrated that Li2MnO3 activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure Li2MnO3 governed by chemical decomposition (Li2O evolution). CEI evolution is closely related to Li+ extraction/insertion. The valence state variation of the metal ions (Ni, Co, Mn) in Li-rich materials can promote CEI formation. This study is of significance for understanding and designing Li-rich cathode-based batteries.
ABSTRACT
Rechargeable lithium-sulfur batteries are potential candidates for storing electrochemical energy because of their extremely high energy density. However, their practical applications are prohibited by the sluggish charge transfer, the retarding Li ion diffusion, and the shuttle effect of lithium polysulfides. We report here a high-performance cathode material in which a S submicrosphere with a mass fraction of 80% was encapsulated within a permeable Co(OH)2 nanoshell which functions as a physical barrier preventing the sulfur and polysulfides from leaking into the electrolyte and also contributes to the catalytic decomposition of polysulfides during the charge and discharge process. When an interlayer of carbon nanofibers is introduced between the S@Co(OH)2 cathode and the separator, the performance of the Li-S batteries can be further significantly enhanced. Specifically, the S@Co(OH)2 cathode possesses good cycling stability over 1000 cycles with an initial discharge capacity of 1100 mAh g-1 at 2 C and a reversible capacity of 606 mAh g-1. In particular, without the LiNO3 additive, this S@Co(OH)2 cathode also exhibits a Coulombic efficiency as high as 85%, just a little lower than that of commercial electrolyte with LiNO3 additive. Relevant mechanistic studies revealed that such superior performances are attributed to the enhanced internal electrical and ionic conductivity and suppressed shuttling effect, owing to the presence of the Co(OH)2 shell and the carbon-nanofiber interlayer. Theoretical simulations based on density functional theory were also carried out to figure out the interaction between the Co(OH)2 nanosheets and the polysulfides. It revealed that the Co(OH)2 nanoshell, rather than merely working as a physical barrier to trap the polysulfides, could also adsorb polysulfides and catalyze their decomposition during the cycling process, further helping to suppress the shuttling effect.
