Xianying Qin

Suppressing Self-Discharge and Shuttle Effect of Lithium–Sulfur Batteries with V2O5-Decorated Carbon Nanofiber Interlayer

V2 O5 decorated carbon nanofibers (CNFs) are prepared and used as a multifunctional interlayer for a lithium-sulfur (Li-S) battery. V2 O5 anchored on CNFs can not only suppress the shuttle effect of polysulfide by the strong adsorption and redox reaction, but also work as a high-potential dam to restrain the self-discharge behavior in the battery. As a result, Li-S batteries with a high capacity and long cycling life can be stored and rested for a long time without obvious capacity fading.

Ultrafine TiO2 Decorated Carbon Nanofibers as Multifunctional Interlayer for High-Performance Lithium–Sulfur Battery

Although lithium-sulfur (Li-S) batteries deliver high specific energy densities, lots of intrinsic and fatal obstacles still restrict their practical application. Electrospun carbon nanofibers (CNFs) decorated with ultrafine TiO2 nanoparticles (CNF-T) were prepared and used as a multifunctional interlayer to suppress the volume expansion and shuttle effect of Li-S battery. With this strategy, the CNF network with abundant space and superior conductivity can accommodate and recycle the dissolved polysulfides for the bare sulfur cathode. Meanwhile, the ultrafine TiO2 nanoparticles on CNFs work as anchoring points to capture the polysulfides with the strong interaction, making the battery perform with remarkable and stable electrochemical properties. As a result, the Li-S battery with the CNF-T interlayer delivers an initial reversible capacity of 935 mA h g(-1) at 1 C with a capacity retention of 74.2% after 500 cycles. It is believed that this simple, low-cost and scalable method will definitely bring a novel perspective on the practical utilization of Li-S batteries.

Electrospun core–shell silicon/carbon fibers with an internal honeycomb-like conductive carbon framework as an anode for lithium ion batteries

Core–shell silicon/carbon (Si/C) fibers with an internal honeycomb-like carbon framework are prepared based on the coaxial electrospinning technique. For this hierarchical structure, the fibers core is composed of a porous carbon framework and embedded Si nanoparticles, which is further wrapped by a compact carbon shell. The well-defined Si/C composite anode shows high specific capacities, good capacity retention, and high accessibility of Si in lithium-ion batteries. An initial reversible capacity of 997 mA h g−1 and a capacity retention of 71% after 150 cycles are demonstrated with a current density of 0.2 A g−1. At a higher current density of 0.5 A g−1, a reversible capacity of 603 mA h g−1 can be maintained after 300 cycles. The accessibility of Si in the Si/C anode is up to 3612 mA h g−1 in the 1st cycle. The excellent electrochemical properties are attributed to the hierarchical structure of Si/C fibers. The porous carbon framework in the core region could not only accommodate the volume expansion of Si, but also enhance the conductivity inside these fibers. The compact carbon shell is able to prevent the electrolyte from permeating into the core section, therefore a stable solid-electrolyte interphase can be formed on the fiber surface.

Highly Crystalline Lithium Titanium Oxide Sheets Coated with Nitrogen‐Doped Carbon enable High‐Rate Lithium‐Ion Batteries

Sheets of Li4Ti5O12 with high crystallinity are coated with nitrogen-doped carbon (NC-LTO) using a controlled process, comprising hydrothermal reaction followed by chemical vapor deposition (CVD). Acetonitrile (CH3 CN) vapor is used as carbon and nitrogen source to obtain a thin coating layer of nitrogen-doped carbon. The layer enables the NC-LTO material to maintain its sheet structure during the high-temperature CVD process and to achieve high crystallinity. Doping with nitrogen introduces defects into the carbon coating layer, and this increased degree of disorder allows fast transportation of lithium ions in the layer. An electrode of NC-LTO synthesized at 700 °C exhibits greatly improved rate and cycling performance due to a markedly decreased total cell resistance and enhanced Li-ion diffusion coefficient (D(Li)). Specific capacities of 159.2 and 145.8 mA h g(-1) are obtained using the NC-LTO sheets, at charge/discharge rates of 1 and 10 C, respectively. These values are much higher than values for LTO particles did not undergo the acetonitrile CVD treatment. A capacity retention value as high as 94.7% is achieved for the NC-LTO sheets after 400 cycles in a half-cell at 5 C discharge rate.

Three-Dimensional Porous Graphene Aerogel Cathode with High Sulfur Loading and Embedded TiO2 Nanoparticles for Advanced Lithium–Sulfur Batteries

Three-dimensional graphene aerogel/TiO2/sulfur (GA/TiO2/S) composites are synthesized through a facile, one-pot hydrothermal route as the cathode for lithium-sulfur batteries. With a high sulfur content of 75.1 wt %, the conductive, highly porous composite electrode delivers a high discharge capacity of 512 mA h/g after 250 cycles at a current rate of 1 C with a low capacity decay of 0.128% per cycle. The excellent capacities and cyclic stability arise from several unique functional features of the cathode. (i) The conductive graphene aerogel framework ameliorates ion/electron transfer while accommodating the volume expansion induced during discharge, and (ii) TiO2 nanoparticles play an important role in restricting the dissolution of polysulfides by chemical bonds with sulfur.

Investigation of cyano resin-based gel polymer electrolyte: in situ gelation mechanism and electrode–electrolyte interfacial fabrication in lithium-ion battery

Cyanoethyl polyvinyl alcohol (PVA-CN)-based gel polymer electrolyte (GPE) is a high-performance electrolyte for lithium-ion batteries (LIBs), which is in situ synthesized from a stable monomer without using additional initiators. Unfortunately, the gelation mechanism of PVA-CN is still unclear. Furthermore, for general GPEs prepared by in situ polymerization, the electrode–GPE interface in batteries remains to be further optimized. Here we present the gelation mechanism of the PVA-CN-based GPE and fabricate an electrode–GPE interface with less resistance during battery formation. The cross-linkable PVA-CN-based organogel is formed via in situ cationic polymerization of the cyano resin initiated by PF5, a strong Lewis acid produced by the thermo-decomposition of LiPF6. It is interesting to find that the battery formation process completed in the precursor solution instead of gel can greatly reduce the interfacial resistance of graphite–GPE and benefit the formation of a more stable solid electrolyte interface (SEI) on the anode, which contributes to a dramatic improvement in battery performance. This work gives useful guidance towards designing new GPE materials and promoting their practical application in LIBs.

Cyclized-polyacrylonitrile modified carbon nanofiber interlayers enabling strong trapping of polysulfides in lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries are seriously constrained by the diffusion and crossover of intermediary product polysulfides and their further reductions on the anode surface. Although carbon-based interlayers have been widely used to inhibit the detrimental shuttle effect in Li–S batteries, the weak physical adsorption of pure carbon materials for trapping polysulfides still leads to low recycle efficiency of active species and short cycle life for cells. Herein, we report a cyclized-polyacrylonitrile-cast carbon nanofiber (CP@CNF) film as an interlayer in Li–S batteries. By exploiting the CP@CNF interlayer, the batteries assembled with bare sulfur cathodes deliver superior rate capability and cycle stability. The reversible capacity could be maintained at 710 mA h g−1 after 200 cycles at 0.3C and a capacity of 560 mA h g−1 can be obtained even at a 2C rate. The improved performances are attributed to both the abundant pyridine groups in the cyclized polyacrylonitrile matrix, which can entrap polysulfides by strong interatomic attraction, and the three-dimensional porous conductive network composed of the carbon nanofiber skeleton and conjugated polymer matrix, giving rise to highly effective transfer pathways for electrons and ions.

Synthesis of Lithium Iron Phosphate/Carbon Microspheres by Using Polyacrylic Acid Coated Iron Phosphate Nanoparticles Derived from Iron(III) Acrylate

Lithium iron phosphate/carbon (LiFePO4 /C) microspheres with high rate and cycling performance are synthesized from iron phosphate/polyacrylic acid (FePO4 /PAA) nanoparticles. Iron(III) acrylate is used as a precursor for both the iron and carbon sources. FePO4 nanoparticles are first produced by a coprecipitation reaction. The byproduct, acrylic acid ions, is polymerized in situ to form a uniform PAA layer on the surface of the FePO4 nanoparticles. The as-prepared LiFePO4 /C microspheres are composed of primary nanoparticles with sizes of 40-50 nm. The nanoparticles are fully coated with a thin, uniform carbon layer derived from the decomposition of the PAA layer. The uniform carbon-coating layer cooperates with interstitial and boundary carbon derived from sucrose successfully to construct an excellent interconnecting conductive network in the microspheres. As a result of the unique structure, the as-prepared LiFePO4 /C microspheres display both high electronic and ionic conductivities, which contribute to their high rate performance (162.9 mAh g(-1) at 0.1C and 126.1 mAh g(-1) at 5C) and excellent cycling stability (97.1% of capacity retention after 500 cycles at 5C/5C).

Large Polarization of Li4Ti5O12 Lithiated to 0 V at Large Charge/Discharge Rates

The ability to enhance the specific capacity of Li4Ti5O12 (LTO) is of practical significance and offers the opportunity to increase the energy density of a full LTO-based battery. Widening the discharge cutoff voltage to 0 V is an effective way to increase the capacity of LTO at low current density. However, whether the specific capacity of LTO at large current rates can be enhanced remains largely unknown. Herein, intriguingly, we found that, when cycled down to 0 V (i.e., from 1.0-2.5 to 0-25 V), LTO exhibited greatly enhanced specific capacity at low rates (<5 C), while it showed a rapid capacity fading and a greatly increased charge/discharge potential gap at high rates (>10 C). The decreased lithium-ion diffusion caused by extra lithium-ion intercalation together with increased internal impedance significantly enhances the polarization and reduces the electrochemical reaction kinetics, which, in turn, hinders the lithiation reaction from LTO to Li7Ti5O12 and further to Li9Ti5O12.

Electrosprayed porous Fe3O4/carbon microspheres as anode materials for high-performance lithium-ion batteries

Porous Fe3O4/carbon microspheres (PFCMs) were successfully fabricated via a facile electrospray method and subsequent heat treatment, using ferrous acetylacetonate, carbon nanotubes (CNTs), Ketjen black (KB), polyvinylpyrrolidone (PVP), and polystyrene (PS) as raw materials. The porous carbon sphere framework decorated with well-dispersed CNTs and KB exhibits excellent electronic conductivity and acts as a good host to confine the Fe3O4 nanoparticles. The abundant mesopores in the carbon matrix derived from polymer pyrolysis can effectively accommodate the volume changes of Fe3O4 during the charge/discharge process, facilitate electrolyte penetration, and promote fast ion diffusion. Moreover, a thin amorphous carbon layer on the Fe3O4 nanoparticle formed during polymer carbonization can further alleviate the mechanical stress associated with volume changes, and preventing aggregation and exfoliation of Fe3O4 nanoparticles during cycling. Therefore, as anode materials for lithium-ion batteries, the PFCMs exhibited excellent cycling stability with high specific capacities, and outstanding rate performances. After 130 cycles at a small current density of 0.1 A·g–1, the reversible capacity of the PFCM electrode is maintained at almost 1,317 mAh·g–1. High capacities of 746 and 525 mAh·g–1 were still achieved after 300 cycles at the larger currents of 1 and 5 A·g–1, respectively. The optimized structure design and facile fabrication process provide a promising way for the utilization of energy storage materials, which have high capacities but whose performance is hindered by large volume changes and poor electrical conductivity in lithium or sodium ion batteries.