Stabilizing an amorphous V2O5/carbon nanotube paper electrode with conformal TiO2 coating by atomic layer deposition for lithium ion batteries†
Amorphous V2O5 (a-V2O5) thin films were conformally coated onto the surface of hydroxyl (–OH) functionalized multi-walled carbon nanotubes (CNTs) and carbon nanotube (CNT) paper using atomic layer deposition (ALD). In order to achieve 3 Li+ intercalation (442 mA h g−1) and prevent V2O5 dissolution at 1.5 V, a conformal TiO2 protective layer is coated on the surface of V2O5/CNT. A free-standing paper electrode can be made by vacuum filtration or coating pre-fabricated CNT paper directly. The electrochemical characteristics of the TiO2/V2O5/CNT paper electrode were then determined using cyclic voltammetry and galvanostatic charge/discharge curves. Because the TiO2 and V2O5 ALD films were ultrathin, the poor electrical conductivity and low ionic diffusivity of V2O5 did not limit the ability of the V2O5 ALD films to display high specific capacity and high rate capability. A high discharge capacity of ∼400 mA h g−1 is obtained for 15 cycle ALD TiO2 coated 50 cycle ALD V2O5/CNT samples by depositing pre-fabricated CNT paper. We believe that this is the highest capacity for V2O5 cathodes reported in the literature. The capacities of the a-V2O5/CNT nanocomposites are higher than the bulk theoretical values. The extra capacity is attributed to additional interfacial charge storage resulting from the high surface area of the a-V2O5/CNT nanocomposites. These results demonstrate that metal oxide ALD on high surface-area conducting carbon substrates can be used to fabricate high power and high capacity electrode materials for lithium ion batteries. In addition, ultrathin and conformal TiO2 ALD coating can be used to mitigate the dissolution and capacity fading of the cathode.