Novel nanosheet-assembled compact CaV4O9 microflowers exhibit high areal capacity and stable cycling performance at high mass loadings
Novel nanosheet-assembled compact CaV4O9 microflowers exhibit high areal capacity and stable cycling performance at high mass loadings

Recently, scientists from Wuhan University of Technology in China have discovered a new anode material, which is promising to produce better lithium-ion and sodium-ion batteries [Xu et al., Nano Energy (2018), doi:10.1016/j.nanoen.2018.06.012].

The increasing demand for energy density of batteries requires new electrode materials with higher capacity. Recently, scientists reported a series of new anode materials beyond commercialized graphite anodes, including Si, SnO2, Fe2O3, etc. These anode materials exhibit much higher initial capacity compared with graphite. However, large volume change of these high-capacity electrodes during charge/discharge process is a new issue, which will result in fast capacity fading. Previous work mainly focused on designing nanostructured materials to address the large volume change issue, but nanomaterials also have limitations, such as low tap density, which is not beneficial for practical applications.

“The identified new anode material, CaV4O9 microflower, shows very promising and exciting properties,” says Xiaoming Xu, first author of the study.

The researchers find that the CaV4O9 microflower anode can exhibit reversible gravimetric capacity about 700 mAh g-1 when used for a lithium-ion battery, about twice of that of commercial graphite. When at a high mass loading of 4.4 mg cm-2, it can display a high areal capacity of ~2.5 mAh cm-2. More importantly, it shows excellent cycling stability. The researchers demonstrate a stable cycling over 400 cycles with the areal capacity over 1.5 mAh cm-2.

“Mass loading and areal capacity are very important parameters for the electrodes. A high mass loading and high areal capacity are necessary for the practical applications. However, most previous investigations about high-capacity anodes only provide the electrochemical performance at very low mass loading level (<1.0 mg cm-2). In that case, the obtained data such as cycling stability and rate capability looks very good and very promising, but actually low mass loading are accompanied with low areal capacity far from the level for practical application,” explains Xu. “Our results at high mass loadings with high areal capacity indicate that CaV4O9 microflower is really a promising new anode material.”

In addition, CaV4O9 microflower is also able to store Na ions in a sodium-ion battery, with reversible gravimetric capacity of ~320 mAh g-1. The areal capacity reaches to ~1.0 mAh cm-2 at a high mass loading of 3.65 mg cm-2. After 100 cycles, the capacity retention is 82%.

An important intrinsic property of the CaV4O9 is the small volume change, as demonstrated by the researchers based on ex-situ TEM and SEM characterizations. This is different from most of other high-capacity anodes. The small volume change properties provide the CaV4O9 electrode good stability even at high mass loadings.

Another advantage of the CaV4O9 microflower is the micron-sized compact morphology. Compared with the nanomaterials, especially porous or hollow nanomaterials, micron-sized structure will effectively increase the tap density of the electrodes and decrease the thickness of the electrodes at high mass loading and the nanosheet assemblies maintain the nano-effects of the electrodes. These aspects are highly beneficial to the final electrochemical performance.

This newly reported anode material suggests that CaV4O9 or other alkaline earth metal vanadates are promising candidates for Li/Na-ion battery anodes. In addition, the design of compact microflowers also provides an increase in the tap density of electrodes whilethe nano-effects assure good electrochemical performance.