Nitrogen‐doped coral-like carbon fiber arrays shown to be highly efficient air electrodes for high-performance Li-air batteries
|a) SEM image of a VA-NCCF array grown on a piece of Si wafer. (b) TEM image of an individual VA-NCCF. (c) The sketch of Li2O2 grown on a coral-like carbon fiber. Credit: ACS, Shui et al. Click to enlarge.|
Researchers at Case Western Reserve University and Kent State University have developed highly efficient oxygen electrodes for nonaqueous Li-O2 batteries by using vertically aligned nitrogen-doped coral-like carbon fiber (VA-NCCF) arrays supported with a stainless steel cloth as the current collector.
In a paper in the journal ACS Nano, they reported obtaining a narrow voltage gap (0.3 V) between the charge and discharge plateaus and an unusually high energy efficiency of 90%. Electrolyte decomposition—a problem with Li-air batteries—was minimized due to the low overpotential, and the battery could run for more than 150 cycles with a good reversibility under considerably high specific capacities (up to 1,000 mAh g-1) per cycle.
A non-aqueous Li-air (Li-O2) battery comprises a metallic lithium anode; a porous cathode (usually carbon-based materials with or without catalysts); and nonaqueous electrolyte (Liþ-containing solution). During discharging, oxygen molecules are reduced by electrons from the current collector to combine with Li+ ions dissolved in the electrolyte to produce solid Li2O2 on the cathode. Subsequent charging causes a reverse reaction. However, large polarizations are commonly observed during charging, with a typical 1-2 V voltage gap between the charge and discharge plateaus.
Because of the instability of nonaqueous electrolytes, the decomposition of electrolytes to form an insulating coating on the electrode surface increases the overpotential, which, in turn, accelerates the decomposition of electrolytes. So far, no stable nonaqueous electrolyte has been found. Besides, carbon materials in oxygen electrodes have been proven to decompose accompanying the electrolyte decomposition, particularly at high overpotentials. On the other hand, the poor electron transportation between the electrode and Li2O2, due to the charging-induced formation of the Li2O2/electrode interfacial barrier contributes also to the high polarizations for nonaqueous Li-O2 batteries.
…Generally speaking, an ideal oxygen electrode requires a highly conductive porous structure to facilitate both electron and oxygen transportations. A large specific surface area is also desirable for the electrode to show a high Li2O2 uptake. … However, most fibrous electrodes [tried in other research projects] still suffered from multiple drawbacks.
… Although recent efforts have reduced the voltage gap between charge/discharge plateaus from 1.5 to 0.7 V and extended the number of reversible cycles up to 100, oxygen electrodes (including fibrous ones) that simultaneously possess a high specific capacity, high energy efficiency, and long cycling life are still scarce.—Shui et al.
The researchers prepared vertically aligned nitrogen-doped coral-like carbon nanofiber (VA-NCCF) arrays by chemical vapor deposition (CVD) and then transferred the material onto a piece of microporous stainless steel cloth as a binder-free oxygen electrode for nonaqueous Li-O2 batteries.
They used the VA-NCCF electrode in Li-O2 batteries with a piece of Li foil as anode and 1 M bis(trifluoromethane)sulfonimide lithium salt (LiCF3SO3) in tetraethylene glycol dimethyl ether (TEGDME) as electrolyte. Three batteries were tested separately under current densities of 100, 600, and 1000 mA g-1 at a controlled capacity of 1000 mAh g-1.
They observed a low overpotential of 0.3 V at the middle of discharge/charge plateaus at a current density of 100 mA g-1. Increasing the current density still resulting in reasonably low overpotentials. The cutoff voltage at the end of charge was 3.6 V at the current density as high as 1000 mA g-1.
To the best of our knowledge, such a low overpotential (0.3 V) was the first time reported for nonaqueous Li-O2 batteries, which is even lower than those of noble metal-based catalysts. The corresponding energy efficiency was calculated to be 90%, the highest value for nonaqueous Li-O2 batteries reported so far.
… The observed outstanding battery performance resulted from multiple factors played together, including the N-doping-induced catalytic activity to lower the charging overpotential for minimizing the electrolyte decomposition and facilitating Li2O2 deposition along the VA-NCCF fibers, the unique vertically aligned coral-like fiber structure to provide a large free space for efficient Li2O2 deposition and enhanced electron/electrolyte/ reactant transport, and the highly conductive micro-porous SS cloth support with a minimized contact resistance.
This work clearly demonstrates that the performance of Li-O2 batteries could be dramatically improved by using rationally designed oxygen electrodes with well-defined hierarchical structures and heteroatom-doping induced catalytic activities, which represents a significant advance in the research and development of Li-O2 batteries and other energy devices.—Shui et al.
Jianglan Shui, Feng Du, Chenming Xue, Quan Li, and Liming Dai (2014) “Vertically Aligned N-Doped Coral-like Carbon Fiber Arrays as Efficient Air Electrodes for High-Performance Nonaqueous Li–O2 Batteries,” ACS Nano doi: 0.1021/nn500327p