Li-Feng Cui

Mn3O4−Graphene Hybrid as a High-Capacity Anode Material for Lithium Ion Batteries

We developed two-step solution-phase reactions to form hybrid materials of Mn(3)O(4) nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Selective growth of Mn(3)O(4) nanoparticles on RGO sheets, in contrast to free particle growth in solution, allowed for the electrically insulating Mn(3)O(4) nanoparticles to be wired up to a current collector through the underlying conducting graphene network. The Mn(3)O(4) nanoparticles formed on RGO show a high specific capacity up to ∼900 mAh/g, near their theoretical capacity, with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn(3)O(4) nanoparticles grown atop. The Mn(3)O(4)/RGO hybrid could be a promising candidate material for a high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for the design and synthesis of battery electrodes based on highly insulating materials.

Highly conductive paper for energy-storage devices

Paper, invented more than 2,000 years ago and widely used today in our everyday lives, is explored in this study as a platform for energy-storage devices by integration with 1D nanomaterials. Here, we show that commercially available paper can be made highly conductive with a sheet resistance as low as 1 ohm per square (Ω/sq) by using simple solution processes to achieve conformal coating of single-walled carbon nanotube (CNT) and silver nanowire films. Compared with plastics, paper substrates can dramatically improve film adhesion, greatly simplify the coating process, and significantly lower the cost. Supercapacitors based on CNT-conductive paper show excellent performance. When only CNT mass is considered, a specific capacitance of 200 F/g, a specific energy of 30–47 Watt-hour/kilogram (Wh/kg), a specific power of 200,000 W/kg, and a stable cycling life over 40,000 cycles are achieved. These values are much better than those of devices on other flat substrates, such as plastics. Even in a case in which the weight of all of the dead components is considered, a specific energy of 7.5 Wh/kg is achieved. In addition, this conductive paper can be used as an excellent lightweight current collector in lithium-ion batteries to replace the existing metallic counterparts. This work suggests that our conductive paper can be a highly scalable and low-cost solution for high-performance energy storage devices.

Carbon−Silicon Core−Shell Nanowires as High Capacity Electrode for Lithium Ion Batteries

We introduce a novel design of carbon-silicon core-shell nanowires for high power and long life lithium battery electrodes. Amorphous silicon was coated onto carbon nanofibers to form a core-shell structure and the resulted core-shell nanowires showed great performance as anode material. Since carbon has a much smaller capacity compared to silicon, the carbon core experiences less structural stress or damage during lithium cycling and can function as a mechanical support and an efficient electron conducting pathway. These nanowires have a high charge storage capacity of approximately 2000 mAh/g and good cycling life. They also have a high Coulmbic efficiency of 90% for the first cycle and 98-99.6% for the following cycles. A full cell composed of LiCoO(2) cathode and carbon-silicon core-shell nanowire anode is also demonstrated. Significantly, using these core-shell nanowires we have obtained high mass loading and an area capacity of approximately 4 mAh/cm(2), which is comparable to commercial battery values.

LiMn1−xFexPO4 Nanorods Grown on Graphene Sheets for Ultrahigh‐Rate‐Performance Lithium Ion Batteries

Olivine-type lithium transition-metal phosphates LiMPO4 (M=Fe, Mn, Co, or Ni) have been intensively investigated as promising cathode materials for rechargeable lithium ion batteries (LIBs) owing to their high capacity, excellent cycle life, thermal stability, environmental benignity, and low cost. However, the inherently low ionic and electrical conductivities of LiMPO4 seriously limit Li + insertion and extraction and charge transport rates in these materials. In recent years, these obstacles have been overcome for LiFePO4 by reducing the size of LiFePO4 particles to the nanoscale and applying conductive surface coatings such as carbon, which leads to commercially viable LiFePO4 cathode materials. Compared to LiFePO4, LiMnPO4 is an attractive cathode material owing to its higher Li intercalation potential of 4.1 V versus Li/Li (3.4 V for LiFePO4), providing about 20% higher energy density than LiFePO4 for LIBs. [14–19] Importantly, the 4.1 V intercalation potential of LiMnPO4 is compatible with most of the currently used liquid electrolytes. However, the electrical conductivity of LiMnPO4 is lower than the already insulating LiFePO4 by five orders of magnitude, making it challenging to achieve high capacity at high rates for LiMnPO4 using methods developed for LiFePO4. [14–19] Doping LiMnPO4 with Fe has been pursued to enhance conductivity and stability of the material in its charged form. Recently, Martha et al. have obtained improved capacity and rate performance for carbon-coated LiMn0.8Fe0.2PO4 nanoparticles synthesized by a high-temperature solid-state reaction. Graphene is an ideal substrate for growing and anchoring insulating materials for energy storage applications because of its high conductivity, light weight, high mechanical strength, and structural flexibility. The electrochemical performance of various electrode materials can be significantly boosted by rendering them conducting with graphene sheets. Recent work has shown improved specific capacities and rate capabilities of simple oxide nanomaterials (Mn3O4, Co3O4, and Fe3O4) grown on graphene as LIB anode materials. However, it remains a challenge to grow nanocrystals on graphene sheets in solution for materials with more sophisticated compositions and structures, such as LiMn1 xFexPO4, which is a promising but extremely insulating cathode material for LIBs. Herein we present a two-step approach for synthesis of LiMn1 xFexPO4 nanorods on reduced graphene oxide sheets stably suspended in solution. Fe-doped Mn3O4 nanoparticles were first selectively grown onto graphene oxide by controlled hydrolysis. The oxide nanoparticle precursors then reacted solvothermally with Li and phosphate ions and were transformed into LiMn1 xFexPO4 on the surface of reduced graphene oxide sheets. With a total content of 26 wt% conductive carbon, the resulting hybrid of nanorods and graphene showed high specific capacity and unprecedentedly high power rate for LiMn1 xFexPO4 type of cathode materials. Stable capacities of 132 mAhg 1 and 107 mAhg 1 were obtained at high discharge rates of 20C and 50C, which is 85% and 70% of the capacity at C/2 (155 mAhg ), respectively. This affords LIBs with both high energy and high power densities. This is also the first synthesis of LiMn0.75Fe0.25PO4 nanorods that have an ideal crystal shape and morphology for fast Li diffusion along the radial [010] direction of the nanorods. Figure 1 shows our two-step solution-phase reaction for the synthesis LiMn0.75Fe0.25PO4 nanorods on reduced graphene oxide (for experimental details, see the Supporting Information). The first step was to selectively grow oxide nanoparticles at 80 8C on mildly oxidized graphene oxide (mGO) stably suspended in a solution. Controlling the hydrolysis rate of Mn(OAc)2 and Fe(NO3)3 by adjusting the H2O/N,N-dimethylformamide (DMF) solvent ratio and the reaction temperature afforded selective and uniform coating of circa 10 nm nanoparticles of Fe-doped Mn3O4 (Supporting Information, Figure S1a; X-ray diffraction data in Figure S1b) on the mGO sheets without free growth of nanoparticles in solution. Importantly, our mGO was made by a modified Hummers method (Supporting Information), with which a sixfold lower concentration of KMnO4 oxidizer was used to afford milder oxidation of graphite. The resulting mGO sheets contained a lower oxygen content than Hummers GO (ca. 15% vs. ca. 30% measured by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy) and showed higher electrical conductivity when chemically reduced than [*] H. Wang, Y. Liang, H. Sanchez Casalongue, Y. Li, G. Hong, Prof. H. Dai Department of Chemistry Stanford University, Stanford, CA 94305 (USA) E-mail: hdai@stanford.edu Y. Yang, L. Cui, Prof. Y. Cui Department of Materials Science and Engineering Stanford University, Stanford, CA 94305 (USA) E-mail: yicui@stanford.edu [] These authors contributed equally to this work.

Inorganic Glue Enabling High Performance of Silicon Particles as Lithium Ion Battery Anode

Silicon, as an alloy-type anode material, has recently attracted lots of attention because of its highest known Li þ storage capacity (4200 mAh/g). But lithium insertion into and extraction from silicon are accompanied by a huge volume change, up to 300%, which induces a strong strain on silicon and causes pulverization and rapid capacity fading due to the loss of the electrical contact between part of silicon and current collector. Silicon nanostructures such as nanowires and nanotubes can overcome the pulverization problem, however these nano-engineered silicon anodes usually involve very expensive processes and have difficulty being applied in commercial lithium ion batteries. In this study, we report a novel method using amorphous silicon as inorganic glue replacing conventional polymer binder. This inorganic glue method can solve the loss of contact issue in conventional silicon particle anode and enables successful cycling of various sizes of silicon particles, both nano-particles and micron particles. With a limited capacity of 800 mAh/g, relatively large silicon micron-particles can be stably cycled over 200 cycles. The very cheap production of these silicon particle anodes makes our method promising and competitive in lithium ion battery industry.