Date Published: May 12, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Jeongyim Shin, Junghoon Yang, Chernov Sergey, Min‐Sang Song, Yong‐Mook Kang.
Fast lithium ion and electron transport inside electrode materials are essential to realize its superb electrochemical performances for lithium rechargeable batteries. Herein, a distinctive structure of cathode material is proposed, which can simultaneously satisfy these requirements. Nanosized Li3V2(PO4)3 (LVP) particles can be successfully grown up on the carbon nanofiber via electrospinning method followed by a controlled heat‐treatment. Herein, LVP particles are anchored onto the surface of carbon nanofiber, and with this growing process, the size of LVP particles as well as the thickness of carbon nanofiber can be regulated together. The morphological features of this composite structure enable not only direct contact between electrolytes and LVP particles that can enhance lithium ion diffusivity, but also fast electron transport through 1D carbon network along nanofibers simultaneously. Finally, it is demonstrated that this unique structure is an ideal one to realize high electron transport and ion diffusivity together, which are essential for enhancing the electrochemical performances of electrode materials.
In the current society, rechargeable lithium‐ion batteries (LIBs) have been considered as one of the most important energy storage systems since they were invented.1, 2 The development of advanced LIBs is still going on in order to satisfy the growing demand of portable devices, electric vehicles (EV), and hybrid electric vehicles (HEV), which commonly require higher energy density and more durable cycle life.1, 2, 3 As the performance of LIBs heavily fluctuates with materials that they are composed of, intensive studies on key materials look essential for future advanced battery system.4, 5, 6 Bearing it in mind, the phosphate (PO4)3− based materials are considered as a highly promising cathode candidate because these materials contain both mobile lithium cations and redox‐active metal sites shrouded within a rigid phosphate network.6, 7, 8 Especially, compared with lithium transition metal oxides, it displays remarkable electrochemical and thermal stability, as well as comparable energy density.6, 7
Figure1a shows the schematic diagram for the growth mechanisms of LVP/carbon nanofibers with controlled heat‐treatments conditions. In here, LVP component and carbon nanofiber component are represented by black and light gray color, respectively. When we annealed LVP precursor fiber at 800 °C for 1 h, smooth‐surfaced fiber was only observed as indicated in the first part of Figure 1a. However, when the heat‐treatment time was increased up to 4 h, oval‐shaped LVP particles appeared on the surface of carbon nanofiber. When the heat‐treatment time is maintained for 12 h, carbon nanofiber almost disappeared and only LVP particles remained. Each sample maintained for 1, 4, and 12 h at 800 °C was each named as LVP‐fiber, LVP‐particle/fiber, and LVP‐particle, which well represents the structural characteristics of samples. When considering the way how the morphology of samples is changing during the heat‐treatment, it seems to predominantly affect the grain or particle growth by facilitating movement of atoms or molecules through the several mechanisms such as surface diffusion, evaporation–condensation, and lattice diffusion. The increase of heat‐treatment time at elevated temperature is also accompanied by the reduction of carbon nanofiber not only making its diameter thinner but also reducing V5+ to V3+. This series of process looks very important to regulate the peculiar structure of LVP/carbon nanofiber composite. Figure 1b illustrates lithium ion and electron movement behavior inside this composite structure. As aforementioned, LVP‐fiber is unfavorable for fast lithium ion diffusion due to the carbon layer that covers LVP parts. Actually, the presence of carbon layer suppresses or delays ion diffusion because it acts as an additional barrier which lithium ions should penetrate through. However, this uniform carbon layer for LVP‐fiber is beneficial in terms of electron transport. In comparison, LVP‐particle/fiber with exposed LVP surface can be much easily accessed by lithium ions due to the secured direct contact between LVP particles and electrolyte. Small size of LVP particles in LVP‐particle/fiber is regarded as another merit of this sample that can significantly shorten the distance for lithium ion diffusion and electron transport. Furthermore, LVP‐particle/fiber provides the interconnected electron pathways alongside 1D carbon nanofiber network. However, when the particle size of LVP particle is further increased, this carbon nanofiber network can be disconnected and finally disappear together with the growth of LVP particle as shown in LVP‐particle. The corresponding structure of LVP‐particle looks completely unfavorable for electron transport even if the contact area between LVP‐particle and electrolyte can be extended finally facilitating lithium ion diffusion a little more. More detailed relationship between structures and electrochemical properties in these LVP‐based samples will be discussed as follows.
In summary, we successfully designed and realized LVP/carbon nanofiber comprised of LVP particles anchored in carbon nanofibers to simultaneously increase electronic conductivity as well as lithium ion diffusivity. Herein, the morphology or microstructure of LVP/carbon nanofiber composites was well modulated according to the typical sintering mechanisms at initial stage site such as surface diffusion, evaporation–condensation, etc. Interestingly, the apparent changes for LVP particle size and carbon nanofiber thickness were observed with heat‐treatment time finally giving the clue for the way how the unique LVP/carbon nanofiber structure is formed. By adjusting synthetic condition, we could grow up small LVP particles on carbon nanofibers in which some part of particle was protruded out of nanofiber. The unique microstructural features provided the chance to simultaneously secure fast lithium ion diffusion and electron transfer, which are essential for the enhanced electrochemical properties of electrode materials. Thanks to the synergistic effect of regulated LVP particle and carbon nanofiber, LVP/carbon fiber composite shows superb electrochemical performances. Actually, the optimized LVP/carbon fiber composite delivers high discharge capacities of 126 mAh g−1 (3.0–4.3 V) and 172 mAh g−1 (3.0–4.8 V) at 0.1 C, an impressive cyclic retentions (98.73% of the initial capacity was maintained after 500 cycles between 3.0 and 4.3 V) and a superb rate capability (the capacity at 20 C corresponded to 72.09% of that at 0.1 C between 3.0 and 4.3 V). Therefore, the unique structure reported here looks very promising to improve the kinetic properties of electrode materials. We demonstrated that the size and crystallinity of protruded LVP particles as well as the thickness and crystallinity of carbon nanofiber are very important physical parameters dominating the electrochemical properties of the corresponding composite cathode. This optimized unique structure can provide not only good electron pathways through 1D carbon nanofiber network but also showed good lithium ion diffusivity through the direct contact between electrolyte and LVP particles. Finally, this inventive idea can be considered as one of the most creative ones to not only overcome the limitation of conventional carbon‐coating strategies in terms of ionic diffusivity but also provide a clear solution for enhancing the kinetic properties of oxide‐based electrode materials including LVP.
Sample Preparation: LVP samples were prepared by electrospinning method. First, stoichiometric amount of NH4VO3, NH4H2PO4, and CH3COOLi·2H2O were mixed in 5 mL of 14 wt% citric acid solution and magnetically stirred for 4 h in 60 °C oil bath. In here, citric acid was employed as a chelating agent and carbon source. After that, the mixed precursor was dropped slowly into the mixture solution of PVP 1.2 g (Mw: 1,300,000) and 4.0 g deionized water and stirred for another 4 h. The mixed solution was loaded into a plastic syringe equipped with a 27‐gauge plastic needle. The needle was then connected to a high‐voltage power supply that generates DC voltage of 22.0 kV. The precursor solution was fed in the rate of 0.2 mL h−1 with syringe pump, and the distance between needle point and aluminum collector was 10 cm. The electrospun fibers were annealed at 800 °C maintaining for 1, 4, and 12 h at ramp rate of 2 °C min−1.