Polyol-mediated carbon-coated Li4Ti5O12 nanoparticle/graphene composites with long-term cycling stability for lithium and sodium ion storages

•Microspherical composite composed of in situ carbon-coated Li4Ti5O12 and rGO.•Role of ethylene glycol as both a stabilizer to control particle growth and carbon source.•Improved rate capability by small sizes of oxide particles in the composite.•Improved cycling stability by uniform and complete in situ carbon coating on Li4Ti5O12.•Li4Ti5O12 as an anode material for lithium-ion batteries and sodium-ion batteries. Nano-sized oxides are investigated to improve rate capability by decreasing ion and electron travel length. However, extended contact area of nano-sized oxides with electrolyte causes undesirable side reactions and poor cycling stability. Interestingly, previous studies focus either on preparation of nano-sized oxides or on carbon coating to prevent side reactions. In this study, a microspherical composite of ethylene glycol-derived in situ carbon-coated Li4Ti5O12 nanoparticles and reduced graphene oxide is prepared by polyol-mediated spray drying method using ethylene glycol as a stabilizer to control particle growth and ethylene glycol coordinated with Ti precursor as a carbon source. The composite shows excellent rate capability as anode materials for lithium-ion and sodium-ion batteries. Most importantly, the composite shows 94% capacity retention after 3000 cycles at 10 C for Li+ storage and 95% capacity retention after 1000 cycles at 5 C for Na+ storage at room temperature. At 60 °C, furthermore, composite shows 93% capacity retention after 1000 cycles for Li+ storage and 95% capacity retention after 500 cycles for Na+ storage at 10 C. The post-mortem analysis confirms that in situ carbon coating on Li4Ti5O12 effectively prevents direct contact of Li4Ti5O12 nanoparticles with electrolyte, thus, blocking side reactions and greatly improving cycling stability.