Abstract:

Li-ion batteries (LIBs) have been considered as one of the most promising power sources for electric vehicles, portable electronics and electrical equipment because of their long cycle life and high energy density. The free-standing electrodes without binder, current collector and conductive agent can effectively obtain lager energy density as compared to the traditional electrodes where the addition of inactive components is required. In addition, the free-standing electrode plays an important role in developing flexible electronic devices. Currently, conventional graphite is still the main commercial anode material, but its theoretical specific capacity is limited, and the rate performance is poor. In recent years, the high temperature pyrolytic hard carbon has attracted wide attention due to its higher theoretical specific capacity and more defects than graphite carbon. Moreover, polymer polyacrylonitrile (PAN) can be used as the raw material for preparation of free-standing anodes without any conductive additives or binders by electrospinning technique. Meanwhile, it is beneficial to reduce the production cost and simplify the manufacturing procedures of electrode. However, PAN-based hard carbon anode materials also have certain problems, such as low conductivity, poor rate performance, unsatisfactory cycling stability, and inferior initial Coulombic efficiency (CE). In addition, soft carbon has advantages of high carbon yield, good conductivity, superior cycling stability, high initial CE and relatively low price, but its specific capacity is generally lower than that of hard carbon materials. Based on above analysis, carbon anode materials with good electrochemical performance can be obtained by combining hard carbon and soft carbon, but the specific capacity of carbon materials is still low. Tin (Sn), as an anode material for LIBs, has a high theoretical specific capacity (994 mAh∙g⁻¹) and a low lithium alloying voltage. Nonetheless, the practical use of Sn anode has been limited by its huge volume change (theoretically ∼260%) during the repeated alloying-dealloying process, resulting in large pulverization and cracking, which triggers the rapid capacity fading. Hence, in order to increase the specific capacity of carbon anode materials of LIBs, the C-Sn composite film with uniform Sn nanoparticles embedded in N-doped carbon nanofibers was prepared by electrospinning method following by a low-temperature carbonization process. The film was directly used as a free-standing electrode for LIBs and exhibited good electrochemical performance, and the introduction of Sn significantly improved the electrochemical properties of the carbon nanofiber film. The formed fibrous structure after Sn was uniformly coated with carbon can promote the conduction of ions and electrons, and effectively buffers the volume change of Sn nanoparticles during cycling, thus effectively preventing pulverization and agglomeration. The C-Sn-2 electrode with a Sn content of about 25.6% has the highest specific capacity and best rate performance among all samples. The electrochemical test results show that, the charge (discharge) capacity reaches 412.7 (413.5) mAh∙g⁻¹ at a current density of 2 A∙g⁻¹ even after 1000 cycles. Density functional theory (DFT) calculations show that N-doped amorphous carbon has good affinity with lithium, which is conducive to anchoring the Sn_xLi_y alloy formed after alloying reaction on the carbon surface, thereby relieving the volume change of Sn during charge-discharge. This article provides a feasible strategy for the design of high-performance lithium storage materials.

Key words: Free-standing electrode, Carbon nanofiber, Metallic Sn, Li-ion battery, Cycling stability