New full concentration gradient high-energy cathode material from Hanyang and Argonne suited for EV batteries
23 October 2012
|Schematic diagram of the FCG lithium transition-metal oxide particle with the nickel concentration decreasing from the center towards the outer layer and the concentration of manganese increasing accordingly. Sun et al. Click to enlarge.|
A team from Hanyang University (Korea) and the US Department of Energy’s (DOE) Argonne National Laboratory have developed a full concentration gradient (FCG) nickel-rich lithium transition-metal oxide material with a very high capacity (215 mAh g−1) for use as a high-energy cathode in Li-ion batteries. Experimental results suggest that this nano-functional full-gradient cathode material is promising for applications that require high energy, long calendar life and excellent abuse tolerance such as electric vehicles, the researchers suggested in a paper published in the journal Nature Materials.
The nickel concentration decreases linearly whereas the manganese concentration increases linearly from the center to the outer layer of each particle. This nano-functional full-gradient approach allows the harnessing of the high energy density of the nickel-rich core and the high thermal stability and long life of the manganese-rich outer layers, according to the team. In addition, the micrometer-size secondary particles of this cathode material are composed of aligned needle-like nanosize primary particles, resulting in a high rate capability.
In the past decade, major efforts have been devoted to searching for high-capacity cathode materials based on LiNi1−xMxO2, mostly on account of their very high practical capacities (220–230 mAh g−1) at high voltages (4.4–4.6 V). However, at such high operating voltages, these materials react aggressively with the electrolyte owing to the instability of tetravalent nickel in the charged state, leading to very poor cycle and calendar life. Therefore, these materials operate reversibly only at a potential range below 4 V, resulting in low capacities of 150 mAh g−1.
To improve the stability of these materials, several researchers have investigated the effect of Mn substitution on cycle and calendar life. The introduction of Mn to the transition-metal layer can help stabilize the transition-metal oxide framework, because part of the Mn does not change valence state during charge and discharge.—Sun et al.
These efforts have faced some difficulties. While a core–shell approach delivered high capacity at high voltage, owing to the structural mismatch and the difference in volume change between the core and the shell, a large void forms at the core–shell interface after long-term cycling, leading to a sudden capacity drop, the authors noted.
Although the structural mismatch could be mitigated by nano-engineering of the core–shell material, where the shell exhibits a concentration gradient, because of the short shell thickness, the manganese concentration at the outer layer of the particle is low. Hence, its effectiveness in stabilizing the surface of the material is weak, especially during high-temperature cycling (55°C).
The new FCG material—LiNi0.75Co0.10Mn0.15O2—features a concentration of nickel that decreases gradually from the center towards the outer layer of the particle, while the concentration of manganese increases gradually so that the manganese-rich and nickel-poor outer layer can stabilize the material, especially during high-voltage cycling.
The researchers prepared the FCG material using a newly developed co-precipitation method involving the precipitation of transition-metal hydroxides from the precursor solutions, where the concentration ratio of Ni/Mn/Co changes continuously with the reaction time.
They assembled a pouch cell using the FCG material as the cathode and mesocarbon microbeads (MCMB, graphite) as the anode, then cycled it between 3.0 and 4.2 V with a constant current of 1C (33 A). The full-cell showed an outstanding capacity retention 17 after 1,000 cycles both at room and high temperature.
They also fabricated pouch-type full-cells; these were cycled to 4.3, 4.4 19 and 4.5 V at 1C rate. In all cases, the cells exhibited excellent cycling performance. The capacity of the cells increased with cutoff voltage owing to the higher lithium utilization at high voltage. The cell cycled to 4.5 V showed very minor capacity fade at 55 °C—possibly due to a limited reactivity between the charged cathode and the electrolyte, the team suggested.
The FCG material also seems to exhibit better safety characteristics than the inner composition (IC) materials.
We have developed a high-performance cathode material composed of lithium transition-metal oxide with FCG within each particle. The structure takes advantage of the high capacity from nickel-rich materials, the high thermal stability of manganese-rich materials and the high rate capability of highly percolated and aligned nano-rod morphology. This newly developed material can deliver a specific capacity of up to 215 mAh g−1 with outstanding cycling stability in a full-cell configuration, maintaining 90% capacity retention after 1,000 cycles.
Yang-Kook Sun, Zonghai Chen, Hyung-Joo Noh, Dong-Ju Lee, Hun-Gi Jung, Yang Ren, Steve Wang, Chong Seung Yoon, Seung-Taek Myung & Khalil Amine (2012) Nanostructured high-energy cathode materials for advanced lithium batteries. Nature Materials 11, 942–947 doi: 10.1038/nmat3435
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