|Image depicts the LiFePO4 particles anchored to the crimped unfolded graphene. Source: Jinli Yang. Click to enlarge.|
The research team from the Nanomaterials and Energy Group at the Western University led by Dr. Xueliang (Andy) Sun has reported that the specific capacity of LiFePO4 can be greatly boosted to up to 168 mAh g-1—98% of its theoretical capacity of 170 mAh g-1—by using unfolded graphene as a three dimensional (3D) conducting network for LiFePO4 nanoparticle growth. A paper on their work is published in the RSC journal Energy & Environmental Science.
Olivine-typed LiFePO4 is considered to be an attractive cathode material for lithium-ion bateries (LIBs) applied in the new generation of hybrid electric vehicles (HEVs) and electric vehicles (EVs). Other work has shown that LiFePO4 battery performance is strongly depended on the carbon coating, which can enhance the electronic conductivity of the electrodes. The Western University researchers found that the graphene with different thickness and morphology has a significant impact on the performance of LiFePO4.
Compared with stacked graphene, which has a wrinkled structure, the use of unfolded graphene enables better dispersion of LiFePO4 and restricts the LiFePO4 particle size at the nanoscale. More importantly, it allows each LiFePO4 particle to be attached to the conducting layer, which could greatly enhance the electronic conductivity, thereby realizing the full potential of the active materials. Based on its superior structure, after post-treatment for 12 hours, the LiFePO4–unfolded graphene nanocomposite achieved a discharge capacity of 166.2 mAh g−1 in the 1st cycle, which is 98% of the theoretical capacity (170 mAh g−1). The composite also displayed stable cycling behavior up to 100 cycles, whereas the LiFePO4–stacked graphene composite with a similar carbon content could deliver a discharge capacity of only 77 mAh g−1 in the 1st cycle. X-ray absorption near-edge spectroscopy (XANES) provided spectroscopic understanding of the crystallinity of LiFePO4 and chemical bonding between LiFePO4 and unfolded graphene.—Yang et al.
The use of an unfolded graphene matrix, which serves as a conducting 3D nano-network, enables both Li ions and electrons to migrate and reach each of LiFePO4 particles, hence realizing the full potential of the active materials.—Xueliang (Andy) Sun
The unfolded graphene synthesized by Sun’s team shows an even and flat structure with individual flakes, and the size is approximately 500 nm, whereas the stacked graphene consists of multiple wrinkled layers make the size up to 10 μm, which is 20 times larger than that of unfolded graphene.
The team also found that a 12 h annealed sample delivered the best discharge capacity (close to the theoretical capacity) and a superior rate capability.
This work is especially important for future research and development of graphene for lithium ion batteries because it presents a new direction on designing graphene based electrode system to realize the full potential of the active materials.—Xueliang (Andy) Sun
Jinli Yang, Jiajun Wang, Yongji Tang, Dongniu Wang, Xifei Li, Yuhai Hu, Ruying Li, Guoxian Liang, Tsun-Kong Sham and Xueliang Sun (2013) LiFePO4–graphene as a superior cathode material for rechargeable lithium batteries: impact of stacked graphene and unfolded graphene. Energy Environ. Sci., 2013 6, 1521-1528 doi: 10.1039/C3EE24163G