Defect engineering on inorganic oxides for Lithium-ion batteries

Abstract

Inorganic oxides have received wide attention as a new type of functional energy storage material, but their application prospects are usually limited due to their intrinsic properties, such as poor conductivity, limited active sites, and unstable crystal structure. Defect engineering is able to fine tune the concentration, mobility, and spatial distribution of carriers and electrons within crystalline structures, and these changed features radically optimize the electronic structure and physicochemical properties of the oxides. These optimized properties of oxides help to solve the kinetic problems of ion diffusion during the charge-storage process. However, defect engineering in oxides still faces the restriction of bottlenecks, which includes but is not limited to, defect preparation, defect preservation, and defect concentration adjustment. Defect preparation refers to the complicated process and harsh conditions required in the process of defect formation; these unstable defects will gradually recover or be refilled if the oxide is exposed to air or water leading to the disappearance of optimized properties; Controllable defect concentration is a further requirement for orderly regulation of oxide intrinsic properties in the development of defect engineering. In the first study, vacancy concentration varies with temperature exponentially according to the formation mechanism of defects. But these vacancies will gradually recover or be refilled in the slow cooling process. Based on this, commercial spinel-type lithium titanium oxide (LTO) was selected and quenched in this work, and we found the rapid cooling could interrupt the recovery of defects and preserve the defect state at high temperatures in the crystal lattice. More importantly, these preserved defects (oxygen vacancies and Li/Ti redistributions) can allow for an extraordinary capacity of 201.7 mAh g-1 beyond the theoretical value of pristine bulk LTO (175 mAh g-1) at a rate of 1 C with the voltage range of 1.0-2.5 V. This work establishes a new mechanism for Li+charge storage and opens up a new avenue for the control of oxygen vacancy and Li/Ti redistribution in electrode materials for the enhancement of electrochemical performance of LTO anode materials. Traditional defect introduction involves high-temperature calcination, vacuum or inert environments and expensive reducing agents, which are not cost-effective and have great energy consumption. Hence, we proposed a universal defect preparation strategy via a simple redox reaction at room temperature. N-Butyl lithium, a powerful nucleophile and reducing agent, was selected in this work and easily realized defect engineering construction in oxide lattices by a simple soaking method. More importantly, such a simple defect preparation method is suitable for common oxides but not limited to LTO, TiO2, SnO2, CeO2, Sb2O5, and Nb2O5. Also, the controllable defect concentration was achieved by adjusting the reaction time. Here, the defective LTO was selected to further study its electrochemical lithium storage at low temperatures. A highly reversible discharge specific capacity of 109.9 mAh g-1 can be obtained at a rate of 10 C in a severe environment of -10 oC after 200 cycles, which is significantly better than the pristine LTO. Such a simple, efficient, and universal defect method is a breakthrough innovation in terms of the current status of defect engineering. Compared with silicon anodes, silicon oxide (SiO2) possesses relatively smaller volume expansion. In addition, the O atoms in silicon oxides can generate an inert buffer matrix phase (Li2O and Li4SiO4) around the silicon particles, which can further effectively alleviate the stress caused by volume expansion. However, crystalline SiO2 cannot be used for Li+ insertion and is excluded as a promising anode for Li-ion batteries due to inherently poor conductivity. Hence, we extract crystalline SiO2 from natural precursors (montmorillonite and sand) and introduce defects into crystalline SiO2 via quenching and carbon-reduction strategies. Such a simple process enables defective SiO2 to possess a higher electrochemical activity for Li+ reversible deintercalation. The defective SiO2 from montmorillonite delivers a high initial capacity of 2774.3 mAh g-1 at 100 mA g-1 and an excellent cycle stability (734.8 mAh g-1 at 500 mA g-1 after 200 cycles), while the defective SiO2 from sand also shows a highly reversible capacity of 558.1 mAh g-1 at 500 mA g-1 after 500 cycles with superior cycling stability. To the best of our knowledge, this work is the first to demonstrate such outstanding lithium storage performance in crystalline SiO2 anodes. More importantly, the wide availability of SiO2 in the natural environment indicates considerable potential for commercial applications. In summary, several techniques developed for defect engineering on inorganic oxide materials are explored to enhance the electrochemical performance of LIBs through optimizing the intrinsic properties of oxides. The simple operation, low cost, moderate condition, abundant resources, and excellent lithium storage performance in the series of defective oxides presented in this thesis provide a wide range of application prospects.