UNIST teams develop high-rate iron oxide anode material, high-energy and high-rate cathode material for Li-ion batteries
16 August 2012
|The spindle-like porous α-Fe2O3 prepared from an iron-based metal organic framework (MOF) template showed high capacity and high rate capability. Credit: ACS, Xu et al. Click to enlarge.|
Teams at the Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST) in S. Korea, both led by Dr. Jaephil Cho, separately report on the development of a high-capacity and high-rate anode material for Li-ion batteries in the ACS journal Nano Letters and a high-rate and high-energy Li-ion cathode material in the journal Angewandte Chemie.
For the anode material, the UNIST team prepared spindle-like porous α-Fe2O3 from an iron-based metal organic framework (MOF) template. When tested as anode material for lithium batteries (LBs), this spindle-like porous α-Fe2O3 shows greatly enhanced performance of Li storage. The porous α-Fe2O3 retained 911 mAh g–1 after 50 cycles at a rate of 0.2 C. Even when cycled at 10 C, comparable capacity of 424 mAh g–1 could be achieved.
...graphite, the currently used anode material for commercial LIBs, has already approached its theoretical limit (372 mAh g−1) and seeking alternative anode materials has become an urgent task nowadays. Among those feasible anode materials, α-Fe2O3 has been proven to be a possible candidate because of its high theoretical capacity (1007 mAh g−1), low cost, ease of fabrication, and environmental benignity. The capacity of lithium storage is mainly achieved through the reversible conversion reaction between Li+ and Fe2O3, forming Fe nanocrystals dispersed in Li2O matrix.
Despite of those intriguing features, iron oxide still suffers from poor cyclability that is caused by the drastic volume change during charge−discharge process. The low conductivity of α-Fe2O3 also induces additional performance degradation, especially when charging and discharging at high current densities. To improve rate capability, different α-Fe2O3 nanomaterials with optimized shapes and particle sizes have been fabricated by various methods including thermal heating, controlled hydrolysis, templated synthesis, acidic etching, wrap-bake-peel approach, and so forth. However, there has been limited success in finding relatively simple ways to produce iron oxide electrodes with satisfactory high specific capacity and high rate capability.
...Herein, we report a new method to obtain spindle-like mesoporous α-Fe2O3 with higher surface area by using MOF as template. As fabricated from ordinary materials by a solvothermal process and subsequent calcination, this strategy is simple, inexpensive, tunable, and scalable. More importantly, when applied as anode material, the spindle-like porous α-Fe2O3 exhibits significantly improved electrochemical performance with a high specific capacity of 911 mAh g−1 after 50 cycles at a current rate of 0.2 C. The rate capability is also greatly enhanced, showing 861 and 424 mAh g−1 at 1 and 10 C, respectively.—Xu et al.
Using a typical iron-based MOF—MIL-88-Fe—as the template, the team utilized a two-step calcination method to prepare the spindle-like mesoporous α-Fe2O3. The resulting material has a relatively high BET surface area of 75 m2 g−1—twice that of mixed Fe2O3 prepared from Prussian blue.
The spindle-like structure consists of clustered porous Fe2O3 nanoparticles with size of <20 nm; pore size appears to be <10 nm. Both the small nanoparticles and pores are critical to the electrochemical performance of this material, the authors noted.
This approach may provide a general way for fabricating porous metal oxides for lithium-ion batteries, the authors suggested.
Cathode material. Conventional Li-ion batteries have the drawback of low rate capability, especially during the charging process (that is, the batteries require a long time to charge), Cho and his colleagues note, suggesting that for EVs to become popular, battery charging should be able to be completed in a few minutes—i.e., in a comparable time to that required for filling automobiles with gasoline.
Li-ion rate capability can be improved by reducing the dimensions of the active material; however, the LIBs would then have insufficient electrode density. To overcome this problem, The UNIST team synthesized carbon-coated single-crystal LiMn2O4 nanoparticle clusters as a cathode material; this material can be densely packed on the current collector.
...in the past decade, the synthesis of nanostructured LiMn2O4 having various morphologies has been intensively studied to enhance the rate capability...
The disadvantage of nanosized materials is that they cannot be packed as densely on the current collector as micrometer-sized materials; this fact means that electrodes made of nanosized materials have a high porosity, thus resulting in a decrease the cell capacity. Therefore, the best way to improve both the rate capability and electrode density would be to use micrometer-sized particles that consist of aggregated nanoparticles.
The disadvantage of this arrangement, however, is that the primary particles located around the center of a nanocluster exhibit a large electric resistance because these particles are not connected with the conducting agent, thus resulting in a high overpotential during high-rate charging and discharging. To overcome this disadvantage, we hypothesized that the primary particles in spinel LiMn2O4 nanoclusters could be coated with a thin carbon layer using sucrose as the carbon source. Sucrose carbonization on the single-crystal particle surface resulted in the formation of an electrical network within the secondary particle. The use of this proposed material in a cell afforded not only an extremely high rate capability but also a high energy density.
The resulting carbon-coated single-crystal LiMn2O4 nanoparticle cluster material itself exhibits a gravimetric energy of 300 Wh/kg of active material (kgam) while delivering a power of 45 kW/kgam and a volumetric energy of 440 Wh/liter of electrode (Le) while delivering 68 kW/Le of power. The use of this material would enable the lithium-ion battery to be charged up to 97% in 100 s and deliver more than 63% of the initial capacity after 2,000 cycles without changing power, at the same charge and discharge rates of 20 C (~3 min), according to the study.
The UNIST team evaluated the electrochemical performance of CSC-NPs in pouch-type half cells. First discharge capacity of the CSC-NPs was 120 mAhg-1. Compared with practical capacity at 0.5C, the CSC-NPs retained 95.3% capacity at 10 C (343 s) and 83.1% at 50 C (59 s). These results imply that CSC-NPs can be charged to 95.3% in 6 min and 83.1% in 1 min by the constant current mode, the team said.
The CSC-NPs exhibited discharge capacities of 121 mA h g-1, 108 mA h g-1, and 80 mA h g-1 at rates of 1 C, 50 C, and 100 C, respectively (1 C = 120 mA gg-1). Therefore, a cycle at 100 C takes only 48 s with the cell retaining 66% of its initial capacity.
To investigate cycle retention at high rates, these materials were cycled at a rate of 20C for 2000 cycles. The specific power of a 20 C cycle of an LIB is comparable with that of variable capacitors. With an initial capacity of 110 mA h g-1, 90 % of the initial capacity was maintained even after 400 cycles. Even after 2000 cycles, capacity retention was 63% and more importantly, the specific power did not change for different number of cycles because the average voltage was almost constant with respect to the number of cycles. In addition, a stepwise charging method was used for boosting charge. This charging method can allow the CSC-NP electrode to be charged to 98% of 1 C capacity in 100 s, and 90% of its initial capacity was delivered after 200 cycles.
By using this material, we overcame the low-electrode-density problem of nanosized active materials while maintaining excellent rate capability. The performance is attributed to: 1) fast electro-chemical reaction owing to the nanosize effect; 2) fast ion pathways to reach primary particles, such as grain boundary and particle surface; and 3) high electron conductivity in secondary particles as a result of the carbon coating of the constituent primary nanoparticles.
These efforts are an attempt to connect every single-crystal nanoparticle electrically and ionically in parallel, and we believe that these concepts offer a direct path to improving the rate capability of lithium-ion batteries without decreasing the electrode density.—Lee et al.
Xu, X., Cao, R., Jeong, S. and Cho, J. (2012) Spindle-like Mesoporous α-Fe2O3 Anode Material Prepared from MOF Template for High-Rate Lithium Batteries. Nano Letters doi: 10.1021/nl302618s
Lee, S., Cho, Y., Song, H.-K., Lee, K. T. and Cho, J. (2012b) Carbon-Coated Single-Crystal LiMn2O4 Nanoparticle Clusters as Cathode Material for High-Energy and High-Power Lithium-Ion Batteries. Angew. Chem. Int. Ed. doi: 10.1002/anie.201203581
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