Chunmei Ban

Nanostructured Fe3O4/SWNT Electrode: Binder‐Free and High‐Rate Li‐Ion Anode

Rechargeable Li-ion batteries are currently being explored for high-power applications such as electric vehicles. However, in order to deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes made from durable, nontoxic, and inexpensive materials with a high charge/discharge rate and a high reversible capacity. Transition metal oxides such as Fe3O4, Fe2O3, MoO3, and Co3O4 [1–5] are capable of Liþ insertion/ extraction in excess of 6 Liþ per formula unit, resulting in a significantly larger reversible capacity than commercially employed graphite. In contrast to the intercalation mechanism that occurs for graphite, the transitionmetal oxides are reduced in a conversion reaction to small metal clusters with the oxygen reacting with Liþ to form Li2O. [1,2,6] This usually leads to large volume expansion and destruction of the structure upon electrochemical cycling, especially at high rate. Hence, optimizing particle size and mixing the particles with various carbon additives have been employed to improve the reversible capacity and rate capability of metal oxide electrodes. Among the transition metal oxides, Fe3O4 is both nontoxic and abundant (inexpensive) and is thus considered one of the most promising electrode materials. However, a truly durable high-rate capability and a high capacity for metal oxide based electrodes including Fe3O4 have not yet been achieved. To achieve high-rate capability and high capacity using metal oxide nanoparticles mixed with carbon materials, there are three key issues that must be considered: i) the size of the nanoparticles must be optimized such that rapid Li-ion diffusion and reaction with metal oxide nanoparticles are achieved, ii) an optimized carbon matrix must be developed that ensures both electrical conductivity and good thermal conductivity (to improve heatdissipation), and iii) the conductive additive must maintain a flexible and strong matrix that accommodates large volume changes. In most conventional electrodes, metal oxide nanoparticles are directly mixed with a carbon additive and a binder to help maintain electrical conductivity, and the large volume expansion then results inmechanical degradation of the electrode when cycled at high rate. Here we employ the unique properties of highly crystalline and long single-walled carbon nanotubes (SWNTs) to simultaneously address all of the three key issues with a simple two-step process to synthesize Fe3O4 nanoparticles embedded uniformly in an interconnected ‘‘SWNT net.’’ Furthermore, no polymer binder is required to maintain electrical conductivity. The electrodes contain 95wt% active material with only 5wt% SWNTs as the conductive additive (typical electrodes contain 80wt% active material and 20wt% conductive and binder additives). Most importantly, by using these binder-free electrodes, we have demonstrated a high reversible capacity of 1000mAhg 1 ( 2000mAh cm ) at C rate as well as high-rate capability and stable capacities of 800mAhg 1 at 5C (both for over 100 deep charge/discharge cycles) and 600mAhg 1 at 10C. Raman spectroscopy suggests that this remarkable rate capability is achieved because the Fe3O4 nanoparticles are actually bound to the flexible nanotube net. We also believe that this fabrication method may be employed for other active materials to achieve a binder-free, high-rate, and durable electrode. The FeOOH nanorods, employed as a precursor in the electrode fabrication process, have a width of 50 nm, length of 250 nm, and thickness of 20 nm and are formed with a simple hydrothermal process. X-ray diffraction (XRD) spectra of the as-prepared nanorods and reference a-FeOOH phase (goethite, JCPDS 81-0463) are shown in Figure 1a. All of the reflection peaks can be indexed to the tetragonal a-FeOOH phase. Next we created Fe3O4 nanoparticles embedded in an interconnected SWNTnetwork using FeOOHnanostructures and SWNTs as precursors for a vacuum-filtration and subsequent annealing process. We found that annealing the FeOOH nanorods without SWNTs to 450 8C in an argon atmosphere leads to a mixture of a-Fe2O3 (hematite) and Fe3O4 (magnetite) as indicated by the XRD patterns in Figure 1b. The peaks marked with * are indexed to the Fe3O4 phase (JCPDS 88-0315) and the remainder of the diffraction peaks are indexed to a-Fe2O3, (JCPDS 33-0664). In contrast, annealing FeOOHnanorods mixed with 5wt% SWNTs at 450 8C in an argon atmosphere leads to the complete reduction of FeOOH to Fe3O4, as indicated in Figure 1c. It is therefore evident that the SWNTs actually facilitate the formation of Fe3O4 nanoparticles, enabling excellent Fe3O4 nanoparticle/SWNT electronic and mechanical contact, which is further confirmed by the Raman spectroscopy analysis discussed later. The elegant morphology of the Fe3O4 nanorods embedded uniformly in the SWNT net is clearly depicted in the scanning electronmicroscope (SEM) image of Figure 2a. Figure 2b displays

Phase evolution for conversion reaction electrodes in lithium-ion batteries.

The performance of battery materials is largely governed by structural and chemical evolutions during electrochemical reactions. Therefore, resolving spatially dependent reaction pathways could enlighten mechanistic understanding, and enable rational design for rechargeable battery materials. Here, we present a phase evolution panorama via spectroscopic and three-dimensional imaging at multiple states of charge for an anode material (that is, nickel oxide nanosheets) in lithium-ion batteries. We reconstruct the three-dimensional lithiation/delithiation fronts and find that, in a fully electrolyte immersion environment, phase conversion can nucleate from spatially distant locations on the same slab of material. In addition, the architecture of a lithiated nickel oxide is a bent porous metallic framework. Furthermore, anode-electrolyte interphase is found to be dynamically evolving upon charging and discharging. The present study has implications for resolving the inhomogeneity of the general electrochemically driven phase transition (for example, intercalation reactions) and for the origin of inhomogeneous charge distribution in large-format battery electrodes.

Reversible High‐Capacity Si Nanocomposite Anodes for Lithium‐ion Batteries Enabled by Molecular Layer Deposition

The molecular-layer deposition of a flexible coating onto Si electrodes produces high-capacity Si nanocomposite anodes. Using a reaction cascade based on inorganic trimethylaluminum and organic glycerol precursors, conventional nano-Si electrodes undergo surface modifications, resulting in anodes that can be cycled over 100 times with capacities of nearly 900 mA h g(-1) and Coulombic efficiencies in excess of 99%.

Atomic layer deposition of amorphous TiO2 on graphene as an anode for Li-ion batteries

Atomic layer deposition (ALD) was used to deposit TiO2 anode material on high surface area graphene (reduced graphene oxide) sheets for Li-ion batteries. An Al2O3 ALD ultrathin layer was used as an adhesion layer for conformal deposition of the TiO2 ALD films at 120 ° C onto the conducting graphene sheets. The TiO2 ALD films on the Al2O3 ALD adhesion layer were nearly amorphous and conformal to the graphene sheets. These nanoscale TiO2 coatings minimized the effect of the low diffusion coefficient of lithium ions in bulk TiO2. The TiO2 ALD films exhibited stable capacities of ~120 mAh g(-1) and ~100 mAh g(-1) at high cycling rates of 1 A g(-1) and 2 A g(-1), respectively. The TiO2 ALD films also displayed excellent cycling stability with ~95% of the initial capacity remaining after 500 cycles. These results illustrate that ALD can provide a useful method to deposit electrode materials on high surface area substrates for Li-ion batteries.

In Situ Transmission Electron Microscopy Probing of Native Oxide and Artificial Layers on Silicon Nanoparticles for Lithium Ion Batteries

Surface modification of silicon nanoparticles via molecular layer deposition (MLD) has been recently proved to be an effective way for dramatically enhancing the cyclic performance in lithium ion batteries. However, the fundamental mechanism of how this thin layer of coating functions is not known, which is complicated by the inevitable presence of native oxide of several nanometers on the silicon nanoparticle. Using in situ TEM, we probed in detail the structural and chemical evolution of both uncoated and coated silicon particles upon cyclic lithiation/delithation. We discovered that upon initial lithiation, the native oxide layer converts to crystalline Li2O islands, which essentially increases the impedance on the particle, resulting in ineffective lithiation/delithiation and therefore low Coulombic efficiency. In contrast, the alucone MLD-coated particles show extremely fast, thorough, and highly reversible lithiation behaviors, which are clarified to be associated with the mechanical flexibility and fast Li(+)/e(-) conductivity of the alucone coating. Surprisingly, the alucone MLD coating process chemically changes the silicon surface, essentially removing the native oxide layer, and therefore mitigates side reactions and detrimental effects of the native oxide. This study provides a vivid picture of how the MLD coating works to enhance the Coulombic efficiency, preserves capacity, and clarifies the role of the native oxide on silicon nanoparticles during cyclic lithiation and delithiation. More broadly, this work also demonstrates that the effect of the subtle chemical modification of the surface during the coating process may be of equal importance to the coating layer itself.

Lithiation of silica through partial reduction

We demonstrate the reversible lithiation of SiO2 up to 2/3 Li per Si, and propose a mechanism for it based on molecular dynamics and density functional theory simulations. Our calculations show that neither interstitial Li (no reduction), nor the formation of Li2O clusters and Si–Si bonds (full reduction) are energetically favorable. Rather, two Li effectively break a Si–O bond and become stabilized by oxygen, thus partially reducing the SiO2 anode: this leads to increased anode capacity when the reduction occurs at the Si/SiO2 interface. The resulting LixSiO2 (x<2/3) compounds have band-gaps in the range of 2.0–3.4 eV.

Structure and Reactivity of Alucone-Coated Films on Si and LixSiy Surfaces

Coating silicon particles with a suitable thin film has appeared as a possible solution to accommodate the swelling of silicon upon lithiation and its posterior cracking and pulverization during cycling of Li-ion batteries. In particular, aluminum alkoxide (alucone) films have been recently deposited over Si anodes, and the lithiation and electrochemical behavior of the system have been characterized. However, some questions remain regarding the lithium molecular migration mechanisms through the film and the electronic properties of the alucone film. Here we use density functional theory, ab initio molecular dynamics simulations, and Greens function theory to examine the film formation, lithiation, and reactivity in contact with an electrolyte solution. It is found that the film is composed of Al-O complexes with 3-O or 4-O coordination. During lithiation, Li atoms bind very strongly to the O atoms in the most energetically favorable sites. After the film is irreversibly saturated with Li atoms, it becomes electronically conductive. The ethylene carbonate molecules in liquid phase are found to be reduced at the surface of the Li-saturated alucone film following similar electron transfer mechanisms as found previously for lithiated silicon anodes. The theoretical results are in agreement with those from morphology and electrochemical analyses.

First-Principles Study of Lithium Borocarbide as a Cathode Material for Rechargeable Li-Ion Batteries

Computational simulations within density functional theory are performed to investigate the potential application of a lithium borocarbide (LiBC) compound as a unique material for lithium ion batteries. The graphene-like BC sheets are predicted to be Li(+) intercalation hosts with the Li ion capacity surprisingly surpassing that of graphite. Here, the layered LixBC structure is preserved with x ≥ 0.5, indicating that half of the Li ions in the LiBC compound are rechargeable. Furthermore, the intercalation potential (equilibrium lithium-insertion voltage of 2.3-2.4 V relative to lithium metal) is significantly higher than that in graphite, allowing Li0.5BC to function as a cathode material. The reversible electrochemical reaction, LiBC ⇌ Li0.5BC + 0.5Li, enables a specific energy density of 1088 W h/kg and a volumetric energy density of 2463 W h/L. The volume change is less than 3% during the charging and discharging process. This discovery could lead to the development of a unique high-capacity LiBC Li ion cathode material.

P-type doping of lithium peroxide with carbon sheets

The interaction of lithium peroxide (Li2O2) with carbon electrodes in Li-air batteries is studied with model systems of graphene-intercalated Li2O2, using density functional theory (DFT) methods. Although both the Li2O2 bulk and its stoichiometric surface structures (without single O atoms) are insulating, the incorporation of graphene sheets into the Li2O2 introduces hole states in the oxygen orbitals due to the electron transfer from the anti-bonding O2 orbitals to the graphene sheets. This indicates that carbon sheets not only provide conducting channels by themselves, but they also open new channels in Li2O2.

An artificial interphase enables reversible magnesium chemistry in carbonate electrolytes

Magnesium-based batteries possess potential advantages over their lithium counterparts. However, reversible Mg chemistry requires a thermodynamically stable electrolyte at low potential, which is usually achieved with corrosive components and at the expense of stability against oxidation. In lithium-ion batteries the conflict between the cathodic and anodic stabilities of the electrolytes is resolved by forming an anode interphase that shields the electrolyte from being reduced. This strategy cannot be applied to Mg batteries because divalent Mg2+ cannot penetrate such interphases. Here, we engineer an artificial Mg2+-conductive interphase on the Mg anode surface, which successfully decouples the anodic and cathodic requirements for electrolytes and demonstrate highly reversible Mg chemistry in oxidation-resistant electrolytes. The artificial interphase enables the reversible cycling of a Mg/V2O5 full-cell in the water-containing, carbonate-based electrolyte. This approach provides a new avenue not only for Mg but also for other multivalent-cation batteries facing the same problems, taking a step towards their use in energy-storage applications.Mg-based batteries possess potential advantages over their lithium counterparts; however, the use of reversible oxidation-resistant, carbonate-based electrolytes has been hindered because of their undesirable electrochemical reduction reactions. Now, by engineering a Mg2+-conductive artificial interphase on a Mg electrode surface, which prevents such reactivity, highly reversible Mg deposition/stripping in carbonate-based electrolytes has been demonstrated.