Carbon Quantum Dots Deposed on TiO2 Hollow Spheres for Lithium-Ion Battery Anodes

Abstract

Lithium-ion batteries (LIBs) have been studied for decades and play an important part in many of our daily activities. However, the demand for higher energy capacity and power density continues to increase, particularly in applications such as electric vehicles (EVs). To satisfy the demands of applications operating at high currents and high power, the development of electrode materials with high power density is required. Owing to its abundance, long cycle life, and low cost, titanium dioxide (TiO2) has been intensively investigated as an anode material. Moreover, TiO2–based anodes exhibit two advantageous properties: 1) a relatively high lithium insertion/extraction voltage (higher than 1.5 V vs. Li+/Li); and, 2) the prevention of lithium plating on the anode. Both characteristics improve the safety of the batteries compared to the use of a lithium metal anode. However, because of the low conductivity of TiO2, it shows poor performance as an anode material for lithium ion batteries. To overcome this limitation, graphene substrates can be used to boost the electrochemical performance of TiO2 since they can enhance the electronic conductivity in the electrode. Unfortunately, the drastic volume expansion and disintegration of graphene surface during charge/discharge is a main obstacle to this approach. Therefore, the particle size and dispersion of graphene substrates are key factors for improving anode conductivity and reliability. Herein, we report a strategy to improve the electrochemical performance of TiO2 and prevent the volume expansion suffered by graphene substrates by combining TiO2 hollow spheres with carbon quantum dots (CQDs) that possess high conductivity and good dispersion. Mesoporous TiO2 hollow spheres are prepared by a sol-gel method using SiO2 spheres as sacrificial templates. The TiO2 hollow spheres provide not only the porosity, but also the large surface area needed to enhance the attachment of CQDs. The CQDs are attached on the surface of TiO2 hollow spheres by a hydrothermal treatment (CQDs@TiO2). These hybrid nanoparticles exhibit higher conductivity and less volume expansion due to their porous . The size of the CQDs@TiO2 was determined to be about 200 nm by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The crystallinity of CQDs@TiO­2 were confirmed by X-ray diffraction (XRD), showing anatase crystal structure. The surface area and pore size distributionwere characterized CR2032-type coin cells are assembled by using the CQDs@TiO2 active materials and LiFePO4 (as the cathode material). Electrochemical properties are characterized by cycling under different C-rates and charging/discharging profiles.