Xiangxin Guo

Monodispersed Carbon-Coated Cubic NiP2 Nanoparticles Anchored on Carbon Nanotubes as Ultra-Long-Life Anodes for Reversible Lithium Storage

In search of new electrode materials for lithium-ion batteries, metal phosphides that exhibit desirable properties such as high theoretical capacity, moderate discharge plateau, and relatively low polarization recently have attracted a great deal of attention as anode materials. However, the large volume changes and thus resulting collapse of electrode structure during long-term cycling are still challenges for metal-phosphide-based anodes. Here we report an electrode design strategy to solve these problems. The key to this strategy is to confine the electroactive nanoparticles into flexible conductive hosts (like carbon materials) and meanwhile maintain a monodispersed nature of the electroactive particles within the hosts. Monodispersed carbon-coated cubic NiP2 nanoparticles anchored on carbon nanotubes (NiP2@C-CNTs) as a proof-of-concept were designed and synthesized. Excellent cyclability (more than 1000 cycles) and capacity retention (high capacities of 816 mAh g-1 after 1200 cycles at 1300 mA g-1 and 654.5 mAh g-1 after 1500 cycles at 5000 mA g-1) are characterized, which is among the best performance of the NiP2 anodes and even most of the phosphide-based anodes reported so far. The impressive performance is attributed to the superior structure stability and the enhanced reaction kinetics incurred by our design. Furthermore, a full cell consisting of a NiP2@C-CNTs anode and a LiFePO4 cathode is investigated. It delivers an average discharge capacity of 827 mAh g-1 based on the mass of the NiP2 anode and exhibits a capacity retention of 80.7% over 200 cycles, with an average output of ∼2.32 V. As a proof-of-concept, these results demonstrate the effectiveness of our strategy on improving the electrode performance. We believe that this strategy for construction of high-performance anodes can be extended to other phase-transformation-type materials, which suffer a large volume change upon lithium insertion/extraction.

Equilibrium voltage and overpotential variation of nonaqueous Li–O2 batteries using the galvanostatic intermittent titration technique

The Li–air (or Li–O2) battery has attracted wide attention, since it has the highest theoretical specific gravimetric energy density. In spite of the rapid progress made on improving its cyclic performance and reducing its voltage polarization, many key issues on thermodynamics and kinetics in nonaqueous Li–O2 batteries are still unresolved. In this study, by using the galvanostatic intermittent titration technique, several novel phenomena have been observed, such as zero voltage gap for the open circuit voltage (OCV) between charging and discharging, asymmetrical polarization behaviours at different current densities and temperatures, a continuous increase of overpotential during charging, and a negative temperature coefficient of the cells thermodynamic equilibrium voltage. These results could inspire other researchers to comprehensively investigate the complicated reaction mechanisms, thermodynamics, and kinetic properties of the Li–air battery, as well as other advanced batteries.

High-voltage and free-standing poly(propylene carbonate)/Li6.75La3Zr1.75Ta0.25O12 composite solid electrolyte for wide temperature range and flexible solid lithium ion battery

Solid electrolyte is regarded as a perfect way to enhance safety issues and boost energy density of lithium batteries. Herein, we developed a type of free-standing poly(propylene carbonate)/Li6.75La3Zr1.75Ta0.25O12 composite solid electrolyte for ambient temperature and flexible solid-state lithium batteries. The composite solid electrolyte exhibited excellent comprehensive performance in terms of high ionic conductivity (5.2 × 10−4 S cm−1) at 20 °C, a wide electrochemical window (4.6 V), high ionic transference number (0.75) and satisfactory mechanical strength (6.8 MPa). When evaluated as solid electrolyte for an ambient-temperature solid lithium battery, such a composite electrolyte delivered excellent rate capability (5C) at 20 °C. This superior performance can be comparable to a liquid electrolyte-soaked PP separator-based lithium battery at room temperature. To our knowledge, this is the best rate capability of a solid composite electrolyte for a solid lithium battery at ambient temperature. Moreover, such a composite electrolyte-based flexible LiFePO4/Li4Ti5O12 lithium ion battery delivered excellent rate capability and superior cycling stability. All these fascinating features make poly(propylene carbonate)/Li6.75La3Zr1.75Ta0.25O12 a very promising all-solid-state electrolyte for flexible solid lithium batteries. Our study makes a big step into addressing the challenges of ambient-temperature solid lithium batteries.

Positive Role of Surface Defects on Carbon Nanotube Cathodes in Overpotential and Capacity Retention of Rechargeable Lithium-Oxygen Batteries

Surface defects on carbon nanotube cathodes have been artificially introduced by bombardment with argon plasma. Their roles in the electrochemical performance of rechargeable Li-O2 batteries have been investigated. In batteries with tetraethylene glycol dimethyl ether (TEGDME)- and N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI)-based electrolytes, the defects increase the number of nucleation sites for the growth of Li2O2 particles and reduce the size of the formed particles. This leads to increased discharge capacity and reduced cycle overpotential. However, in the former batteries, the hydrophilic surfaces induced by the defects promote carbonate formation, which imposes a deteriorating effect on the cycle performance of the Li-O2 batteries. In contrast, in the latter case, the defective cathodes promote Li2O2 formation without enhancing formation of carbonates on the cathode surfaces, resulting in extended cycle life. This is most probably attributable to the passivation effect on the functional groups of the cathode surfaces imposed by the ionic liquid. These results indicate that defects on carbon surfaces may have a positive effect on the cycle performance of Li-O2 batteries if they are combined with a helpful electrolyte solvent such as PP13TFSI.

Charge carrier accumulation in lithium fluoride thin films due to Li-ion absorption by titania (100) subsurface.

The thermodynamically required redistribution of ions at given interfaces is being paid increased attention. The present investigation of the contact LiF/TiO(2) offers a highly worthwhile example, as the redistribution processes can be predicted and verified. It consists in Li ion transfer from LiF into the space charge zones of TiO(2). We not only can measure the resulting increase of lithium vacancy conductivity in LiF, we also observe a transition from n- to p-type conductivity in TiO(2) in consistency with the generalized space charge model.

In Situ Fabrication of CoS and NiS Nanomaterials Anchored on Reduced Graphene Oxide for Reversible Lithium Storage

CoS and NiS nanomaterials anchored on reduced graphene oxide (rGO) sheets, synthesized via combination of hydrothermal with sulfidation process, are studied as high-capacity anode materials for the reversible lithium storage. The obtained CoS nanofibers and NiS nanoparticles are uniformly dispersed on rGO sheets without aggregation, forming the sheet-on-sheet composite structure. Such nanoarchitecture can not only facilitate ion/electron transport along the interfaces, but also effectively prevent metal-sulfide nanomaterials aggregation during the lithium reactions. Both the rGO-supported CoS nanofibers (NFs) and NiS nanoparticles (NPs) show superior lithium storage performance. In particular, the CoS NFs-rGO electrodes deliver the discharge capacity as high as 939 mA h g(-1) after the 100th cycle at 100 mA g(-1) with Coulombic efficiency above 98%. This strategy for construction of such composite structure can also synthesize other metal-sulfide-rGO nanomaterials for high-capacity lithium-ion batteries.

Colossal magnetoresistance effect in perovskite-type La-Sn-Mn-O epitaxial films

La–Sn–Mn–O (LSnMO) thin films epitaxially grown on single-crystal substrates by pulsed-laser deposition are reported. The films have a perovskite structure and perform the colossal magnetoresistance effect with the maximum magnetoresistance (MR) ratio of 103% (at 233 K and 6 T). The dependence of electrical transport and magnetic properties on the film thickness has been studied. The analyses reveal that the electrical transport, in contrast with the magnetic phase transition, is more sensitive to the thickness of the films.

Novel one-step gas-phase reaction synthesis of transition metal sulfide nanoparticles embedded in carbon matrices for reversible lithium storage

This report presents a novel one-step method based on the gas-phase reaction between metallocenes and sulfur for synthesizing the nanocomposites of transition metal sulfide nanoparticles embedded in carbon matrices (TMS@C). Various nanocomposites including FeS@C, Cr2S3@C and NiS2@C have been successfully synthesized by using ferrocene, chromocene and nickelocene, respectively. The SEM investigations evidence that the TMS nanoparticles are evenly distributed in the in situ formed carbon matrices, demonstrating that this novel method is an easy way to synthesize homogenous TMS-based nanocomposites with well-controlled nanostructures. As the anodes for lithium ion batteries (LIBs), the as-prepared TMS@C electrodes exhibit excellent rate capability and high reversible capacity. For example, a high reversible capacity of 550 and 480 mA h g−1 can be retained for the FeS@C anode even after 350 cycles at a current density of 0.1 A g−1 and 500 cycles at 0.5 A g−1, respectively. The TEM investigations on the 100th discharged and recharged electrodes demonstrate superior structural stability against repeated lithiation/delithiation of the FeS@C. These impressive results indicate that this novel approach is a promising way to synthesize high-performance TMS electrodes for highly reversible lithium storage.

Reaction pathway and wiring network dependent Li/Na storage of micro-sized conversion anode with mesoporosity and metallic conductivity

Micro-sized or monolithic electrode materials with sufficient mesoporosity and a high intrinsic conductivity are highly desired for high-energy batteries without the trade-off of electrolyte infiltration and accommodation of volume expansion. Here metallic nitrides consisting of mesoporous microparticles were prepared based on a mechanism of solid–solid phase separation and used as conversion anodes for Li and Na storage. Their superior capacity and rate performance during thousands of cycles benefit from the preservation or self-reconstruction of hierarchically conductive wiring networks. The conversion efficiency is also highly dependent on the reaction pathway and product. Exploring more conductive and percolating mass/charge transport networks particularly in a deep sodiation state is a potential solution for activation of Na-driven conversion electrochemistry.

A Rechargeable Li-Air Fuel Cell Battery Based on Garnet Solid Electrolytes

Non-aqueous Li-air batteries have been intensively studied in the past few years for their theoretically super-high energy density. However, they cannot operate properly in real air because they contain highly unstable and volatile electrolytes. Here, we report the fabrication of solid-state Li-air batteries using garnet (i.e., Li6.4La3Zr1.4Ta0.6O12, LLZTO) ceramic disks with high density and ionic conductivity as the electrolytes and composite cathodes consisting of garnet powder, Li salts (LiTFSI) and active carbon. These batteries run in real air based on the formation and decomposition at least partially of Li2CO3. Batteries with LiTFSI mixed with polyimide (PI:LiTFSI) as a binder show rechargeability at 200 °C with a specific capacity of 2184 mAh g−1carbon at 20 μA cm−2. Replacement of PI:LiTFSI with LiTFSI dissolved in polypropylene carbonate (PPC:LiTFSI) reduces interfacial resistance, and the resulting batteries show a greatly increased discharge capacity of approximately 20300 mAh g−1carbon and cycle 50 times while maintaining a cutoff capacity of 1000 mAh g−1carbon at 20 μA cm−2 and 80 °C. These results demonstrate that the use of LLZTO ceramic electrolytes enables operation of the Li-air battery in real air at medium temperatures, leading to a novel type of Li-air fuel cell battery for energy storage.