Free-standing hybrid film of less defective graphene coated with mesoporous TiO₂ for flexible lithium ion batteries with fast charging/discharging capabilities

Benefiting from extremely high conductivity, graphene sheets (GS) with very low defect density are preferable to reduced graphene oxide sheets for constructing the free-standing hybrid electrodes of flexible electrochemical energy storage devices. However, due to the hydrophobic nature and deficiency of nucleation sites, how to uniformly and intimately anchor electrochemically active materials onto less defective GS is a challenge. Herein, a free-standing and mechanically flexible hybrid film with two-layer structure, mesoporous TiO₂ anchored less defective GS hybrid (mTiO₂-GS) upper-layer and graphene under-layer, denoted as mTiO₂-GS/G, is fabricated. The hydrolysis of a Ti glycolate aqueous sol solution were applied to form mTiO₂. The decoration of less defective GS with sodium lignosulfonate (SLS) surfactant is crucial for anchoring TiO₂ nanoparticles (NPs). The aromatic rings of SLS favor a non-destructive functionalization of GS through the π-π stacking interaction. The sulfonic acid groups and hydroxyl groups of SLS, respectively, greatly improve the dispersity of GS in water and trigger the nucleation of TiO₂ through the oxolation in the hydrolysis of Ti glycolate sol solution. The following characteristics of free-standing mTiO₂-GS/G electrode benefit the fast charging/discharging capabilities: highly conductive graphene framework, ultra-small NPs (∼5.0 nm) in mTiO₂ anchored, high specific surface area (202.5 m² g^-1), abundant mesopores (0.32 cm³ g^-1), intimate interfacial interaction between mTiO₂ and GS, robust contact between the mTiO₂-GS upper-layer and an under-layer of bare graphene as the current collector. In coin half-cells, the mTiO₂-GS/G electrode delivers a capacity of 130 mA h g^-1 at 50 C, and 71 mA h g^-1 at 100 C, and it also exhibits excellent cycle stability up to 10 000 cycles under 10 C, with a degradation rate of 0.0033% per cycle. When packed in flexible cells, the mTiO₂-GS/G electrode maintains fast charging/discharging capabilities regardless of being flat or bent. Furthermore, because of the high durability of mTiO₂-GS/G electrode, repeated deformations do not cause extra capacity degradation.