Key Points
- Development of Li-air battery with the capacity of ～4-6 times higher than Li-ion battery with more than 100 cycling lifetime
- Expectation of one full charge providing +500 km travel distance for all electric vehicles
- Expectation of further development of Li-air battery by combination of high performance nanoporous graphene and catalysts
The research team headed by Prof. Mingwei Chen in AIMR in Tohoku University successfully developed a novel three-dimensional (3D) nanoporous cathode by encapsulating RuO2 nanoparticles into nitrogen-doped (N-doped) nanoporous graphene for high-performance and rechargeable Li-air batteries. This research is sponsored by JST-CREST.
For standard electric vehicles (EV) powered by Li-ion batteries, the one-charge travel distance is usually 200-300 km, much shorter than conventional ones fueled by gasoline. The inconvenience caused by insufficient energy density of Li-ion batteries is one of the key factors that restrict the popularization of EV. Therefore, there is a high demand of high capacity batteries for longer driving distance of EV.
The 3D nanoporous graphene encapsulated with RuO2 nanoparticles allows the Li-air batteries to achieves high capacity of 2000 mAh/g with low charge potential of 3.7 V and high stability up to 100 charge/discharge cycles. The hybrid 3D nanoporous graphene cathode well preserves the 2D graphene characters of high conductivity and extremely large porosity and surface area for a large rechargeable capacity at a low charge voltage. This work shines a light for practical implementations of Li-air battery by utilizing 3D nanoporous graphene based materials as high performance cathodes.
Introduction
High capacity lithium-air (Li-air) battery is a new type of secondary battery which has attracted great attention in recent years because of the high theoretical capacity and energy density. However, the practical implementation of the new battery is limited by the low cycling stability and poor energy efficiency. To solve these challenging issues, developing new cathode materials with high electrochemical stability and low charge/discharge overpotentials is critical and the current topical of intense discussion. As shown in Figure 1(a), the principle of Li-air battery is based on a simple redox reaction that takes place at the three-phase (i.e. solid, liquid and gas) interfaces in the vicinity of a conductive cathode. Therefore, the cathode is required to be highly conductive and porous, which is capable of transporting charge, gas and liquid, storing and dissolving solid discharge products. Figure 1(b) is the photo of a coin-cell type Li-air battery using the free-standing 3D nanoporous graphene with encapsulated RuO2 catalysts as the cathode. Figure 1(c) illustrates the reaction scheme of the redox reaction on catalytically active sites and the formation and decompositions of the Li2O2 during the operation of the Li-air battery.
Results and discussion
In this report, the highly conductive nanoporous graphene based cathode with large surface area and high porosity and loaded with a low content of encapsulating RuO2 catalysts (< 5 vol%) has been developed for Li-air batteries, which shows a high reversible capacity of 2000 mAh/g, low charge potential of 3.7 V, high energy efficiency of 72% and long cycling stability >100 cycles (Figure 2). The conductive and nanoporous graphene cathode promotes the transport of the Li ions, O2 gas and liquid electrolyte. The high porosity (> 95%) plays a critical role in the storage of the discharge products Li2O2 for a large rechargeable capacity (Figure 2(a)) while the large surface area together with the loaded RuO2 catalysts benefits the full decomposition of the solid reaction products at a low charging potential (Figure 2(b)). Moreover, the nano-sized RuO2 catalysts are stabilized by the surrounding graphene for high cycling stability (Figure 2(c)).
The cycling stability of the nanoporous nitrogen-doped graphene with encapsulated RuO2 in Li-air battery has been tested (Figure 3). The battery shows the high cycling stability of more than 100 cycles with the high reversible capacity of 2000 mAh/g. For the rate performances, due to the high conductivity and catalytic activity, the graphene-based cathode displays excellent rate performance with relatively low charge/discharge overpotentials at high current densities, which offers the high energy efficiency of 72%. This is one of the best results achieved in Li-air batteries. The performances of the reported electrodes for Li-air batteries are summarized in Table 1.
Future plan
Compared to the conversional carbon based electrodes with low conductivity and low energy efficiency, the 3D nanoporous graphene cathode shows the higher energy efficiency of 72% and high rechargeable capacity. However, the noble catalyst RuO2 used in the nanoporous cathode may lead to the high material costs. In near future, the research will be focused on the development of low-cost catalysts which can offer better battery performances in terms of rechargeable capacity, charge/discharge overpotentials and cycling lifetimes.
Figures and Table
Figure 1: Li-O2 battery and mechanism of the electrode reactions
- (a) The working mechanism of the Li-O2 battery
- (b) The digital photo of the coin-cell type Li-O2 battery used in the study
- (c) The redox reactions on the active sites of the RuO2/nanoporous graphene based cathode
Figure 2: The nanoporous graphene based cathode with encapsulated RuO2 nanoparticles
- (a) SEM image of the discharged nanoporous cathode. The solid product Li2O2 can be seen.
- (b) SEM image of the fully charged nanoporous cathode. The pore size is 100-300 nm.
- (c) TEM image of the RuO2 nanoparticles on the nanoporous graphene based cathode after 50 cycles. The RuO2 nanoparticles are still encapsulated by 2-3 layers of graphene and are highly stable during the charge/discharge cycling.
Figure 3: The cycling stability of the RuO2/nanoporous graphene based Li-O2 battery
Experimental conditions: 1.0 M LiTFSI in TEGDME electrolyte, Current density: 400 mA/g, Capacity: 2000 mAh/g.
| Cathode materials | Surface area (m2/g) |
Operating voltage (V) discharge/charge |
Energy efficiency (%) |
Capacity and Cycling life | |
|---|---|---|---|---|---|
| This work | Nanoporous N-doped graphene | 772 | 2.65/4.55 | 58 | 1000 mAh/g, 100 ycles |
| RuO2/nanoporous N-doped graphene | 188 | 2.65/3.7 | 72 | 2000 mAh/g, 100 cycles | |
| State-of-the-art | Porous graphene | 309 | 2.75/3.8 | 74 | 1000 mAh/g, 20 cycles |
| Ru/porous graphene | 254 | 2.79/3.6 | 78 | up to 800 mAh/g, 100 cycles | |
| PtAu/carbon black | - | 2.7/3.5 | 77 | 2000 mAh/g, 1 cycle | |
| RuO2/Carbon nanotube | 72.7 | 2.64/3.62 | 73 | up to 500 mAh/g, 20 cycles |
Table 1: Comparison of the Li-O2 battery performances between our nanoporous cathode and other reported results
Information
Researcher Information
JST CREST
Research Area “Phase Interface Science for Highly Efficient Energy Utilization”
Research Theme “Interface science inspired nanoporous composites for next-generation energy devices”
Journal Information
Xianwei Guo, Pan Liu, Jiuhui Han, Yoshikazu Ito, Akihiko Hirata, Takeshi Fujita, and Mingwei Chen. “3D Nanoporous Nitrogen-Doped Graphene with Encapsulated RuO2 Nanoparticles for Li-O2 Batteries”. Advanced Materials, Published online 1 September 2015, doi: 10.1002/adma.201503182.
Contact
[About Research]
Mingwei Chen, Ph.D
Professor, Advanced Institute for Materials Research, Tohoku University
E-mail: 
URL: http://www.wpi-aimr.tohoku.ac.jp/chen_labo/
[About Program]
Masashi Furukawa
Green Innovation Group, Department of Innovation Research, JST
E-mail: 