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2017-11-03

Lithium-carbon battery could be cost effective, safe and have five to eight times the energy density of lithium ion batteries




Ragone plot, comparing Li-CNT-F batteries with other batteries in terms of weight of cathode materials.

The advantages of using carbon are that it is cost-effective and safe to use, and the energy output is five to eight times higher than lithium-ion batteries currently on the market. The new battery technology also performs better than two other future technologies: lithium-sulfur batteries, currently in the prototype stage, and lithium-air batteries, now under development. For example, the induced-fluorination technology could be used to produce cellphone batteries that would charge faster and last longer. The research team developed the new battery technology for energy storage using carbon nanomaterials and a process called induced fluorination.
High performance rechargeable batteries are urgently demanded for future energy storage systems. Here, we adopted a lithium-carbon battery configuration. Instead of using carbon materials as the surface provider for lithium-ion adsorption and desorption, we realized induced fluorination of carbon nanotube array (CNTA) paper cathodes, with the source of fluoride ions from electrolytes, by an in-situ electrochemical induction process. The induced fluorination of CNTA papers activated the reversible fluorination/defluorination reactions and lithium-ion storage/release at the CNTA paper cathodes, resulting in a dual-storage mechanism. The rechargeable battery with this dual-storage mechanism demonstrated a maximum discharging capacity of 2174 mAh gcarbon-1 and a specific energy of 4113 Wh kgcarbon-1 with good cycling performance.
Although Li-ion batteries (LIBs) have transformed portable electronics, the energy density and cycle life of existing LIBs, even if fully developed, remain insufficient. Reaching beyond the horizon of LIBs requires the exploration of new electrochemistry and/or new materials1. The recent popular attempts are Li-sulfur (Li-S) and Li-air (Li-O2) batteries. However, there are some formidable challenges for Li-S and Li-O2 batteries, e.g., dissolution of discharging products, poor cathode electrical conductivity, and large volume expansion upon lithiation.
Li-CFx batteries have the highest energy density among all primary lithium batteries with a theoretical specific energy of 2180 Wh kg(Li+CF)-1. A high capacity of 615 mAh gCFx-1 was also reported for the pre-synthesized CFx cathodes. It is well known that defluorination of carbon fluorides can be achieved with the assistance of lithium cations during discharging in Li-CFx batteries. However, Li-CFx batteries have attracted limited interest because of their strictly non-rechargeable nature16 and the non-environmental-friendly synthesis process for carbon fluorides, e.g., the use of F2 gas and/or catalysts under extreme temperature condition
The complex anion of [F-TPFPB]- was previously found to be reversibly intercalated in graphite with limited capacity, 60 ~ 80 mA gcarbon-1. It is the intercalation of the bulky complex anion of [F-TPFPB]- that will sterically hinder further anion intercalation and worsen the cathode specific capacity. However, in this report, the intercalation of [F-TPFPB]- was successfully suppressed, as suggested in Figure 2f, which may explain the high capacity achieved in Figure 4. The suppression of bulky [F-TPFPB]- intercalation is due to the particular induction temperature (70C) conducted, which reduces the energy barrier for the F- release from [F-TPFPB]-, and therefore, promotes the intercalation of free F- in CNTA papers. It is also worth to note that the free fluoride ions released from [F-TPFPB]- are originally from the dissolved LiF salts, rather than from the decomposition of TPFPB molecules. It has been calculated that the energy barrier for the fluoride anion release from TPFPB (59.2 kcal/mol) is much lo
lower than the breakdown of a true covalent bond (typically on the order of 100 kcal/mol) in TPFPB
In conclusion, they realized the induced fluorination of CNTA paper cathodes by an in-situ electrochemical induction process at 70C and in the presence of TPFPB. The induced fluorination of CNTA papers activated the reversible fluorination/defluorination reactions and lithium-ion storage/release at the CNTA paper cathodes, resulting in a dual-storage mechanism. It is the first time that the reversible fluorination/defluorination reactions were realized at pure carbon and non-fluoride materials. In addition, the induced fluorination destructed the graphitic carbon to defective nanostructures, which further facilitated the two reversible reactions at both 70C and 22C. The rechargeable battery with this dual-storage mechanism demonstrated a maximum discharging capacity of 2174 mAh gcarbon-1 and a specific energy of 4113 Wh kgcarbon-1 with good cycling performance. This paper uncovers the significance of energy storage by carbon materials at high voltages, and demonstrates the Li-C-F battery system a new promising candidate for the future energy storage systems


Dual-storage mechanism with reversible fluorination/defluorination reactions and lithium-ion storage/release occurring at CNTA paper cathode.