Stabilizing an amorphous V2O5/carbon nanotube paper electrode with conformal TiO2 coating by atomic layer deposition for lithium ion batteries

Abstract

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.