Unusual Improvement of Pseudocapacitance of Nanocomposite Electrodes: Three-Dimensional Amorphous Carbon Frameworks Triggered by TiO₂ Nanocrystals

Both nanocrystals and carbon materials have attracted considerable attention in lithium-ion batteries (LIBs) because of their fast kinetics for lithium storage or long-life cycles. However, the easy aggregation of nanocrystals and high-temperature doping process of carbon materials seriously hindered their application in LIBs. Here, we report the development of unprecedented TiO_2-@C nanocomposite electrodes through a unique "melting low-temperature pyrolysis" strategy. It is found that the continuous and interconnected three-dimensional amorphous carbon frameworks (3DCFs) in the composites are closely connected with TiO₂ nanocrystals by Ti-O-C covalent bonding, forming robust 3D framework architectures. Interestingly, we find that TiO₂ nanocrystals can greatly improve the pseudocapacitance of TiO_2-@C nanocomposite electrodes with increasing cycles, which significantly exceeds previously reported TiO₂-based anodes and carbon materials. Furthermore, for the first time, the unusual improvement of pseudocapacitance of TiO_2-@C electrodes is carefully investigated by means of dQ/dV curves and electrochemical kinetic analysis to reveal the extra contribution of lithium storage. 3DCF, a "lithium-ion reservoir", possesses an unexpected capacity enhancement behavior that is triggered by TiO₂ nanocrystals and exhibits bicontinuous pathways for both rapid ion and electron transport. In this case, TiO₂ nanocrystals stabilizing the 3DCF acted as a conductive agent during charge and discharge. Our findings confirm that the 3DCF triggered by TiO₂ nanocrystals boosted the electrochemical performance of TiO_2-@C nanocomposite electrodes, especially the pseudocapacitance enhancement. The unique characteristics of ingenious combination of TiO₂ nanocrystals and amorphous carbon materials make them attain superior electrochemical properties in all known TiO₂- and carbon-based anodes (289 mA h g^-1 at 5 A g^-1 after 4000 cycles). Above all, our findings reveal previously ignored fundamental aspects of pseudocapacitance improvement of nanocomposite electrodes and offer new hope for structural design and carbon coating process of high-performance anode materials.