High Li Capacity Research Articles

Vanadium and Niobium MXenes-Bilayered V2 O5 Asymmetric Supercapacitors.

MXenes offer high metallic conductivity and redox capacitance that are attractive for high-power, high-energy storage devices. However, they operate limitedly under high anodic potentials due to irreversibleoxidation. Pairing them with oxides to design asymmetric supercapacitors may expand the voltage window and increase the energy storage capabilities. Hydrated lithium preintercalated bilayered V2 O5 ( δ-Lix V2 O5 ·nH2 O) is attractive for aqueous energy storage due to its high Li capacity at high potentials; however, its poor cyclability remains a challenge. To overcome its limitations and achieve a wide voltage window and excellent cyclability, it is combined with V2 C and Nb4 C3 MXenes. Asymmetric supercapacitors employing lithium intercalated V2 C (Li-V2 C) or tetramethylammonium intercalated Nb4 C3 (TMA-Nb4 C3 ) MXenes as the negative electrode, and a δ-Lix V2 O5 ·nH2 O composite with carbon nanotubes as the positive electrode in 5m LiCl electrolyte operate over wide voltage windows of 2 and 1.6V, respectively. The latter shows remarkably high cyclability-capacitance retention of ≈95% after 10 000 cycles. This work highlights the importance of selecting appropriate MXenes to achieve a wide voltage window and a long cycle life in combination with oxide anodes to demonstrate the potential of MXenes beyond Ti3 C2 in energy storage.

Computational Investigation of Electric Field Driven Lithium Intercalation Into Bilayer Graphene

Most of the anodes of the state-of-the-art lithium ion batteries are made of graphite due to its specific properties such as relatively low cost, abundance, high energy density, power density, very long cycle life and hierarchical structure1. Because of the relatively wide inter space between two adjacent graphene layers, lithium ions easily intercalate into the graphite anode material, thus avoiding the structure, shape and size variations of the electrode material during the charge–discharge process2. Besides the advantages of using graphite as anode material, quite a few drawbacks of graphite anodes such as safety issues, low specific capacity and rate capacity have led to more research on the performance enhancement of carbon-based material anodes3. In our study we focused on enhancement the specific capacity of graphite. To enhance the specific capacity of the graphite electrode, modifying carbon electrode surfaces with heteroatom doping is an effective strategy4. In this work, we investigated the Li intercalation into germanium (Ge) doped graphite electrodes via density functional theory (DFT) calculations. The van der Waals density functional (vdW-DF) has been used to describe weak interaction between graphene layers. Also DFT calculations have been carried out in combination with the effective screening medium (ESM) to compute the surface that is charged up and/or exposed to an electric field5. For the DFT calculations with ESM, bilayer graphene slab model which is periodic in the direction parallel to the surface but is not periodic in the perpendicular direction, has been adopted to represent the graphite electrode. Sandwich slab model between two semi-finite media such as vacuum, an electrode, or an electrolyte has been formed. Charged C6LiC6 slab model is shown in Figure 1. We investigate both charge density difference, ∆ρ, and the electrostatic potential difference, ∆V of our lithium intercalated bilayer graphene model under charge addition and removal. In addition, our results indicate high Li capacity of heteroatom doped bilayer graphene electrodes compared to undoped one for Li-ion batteries.

Promising anode material BN/VS2 heterostructure for the Li-ion battery: The first-principles study

Although monolayer VS2 shows metal property and well Li storage property when as the anode for the Li-ion battery (LIB), the structural degradation problem cannot be ignored. Our results show BN/VS2 heterostructure can not only improve the poor stability of monolayer VS2 to achieve excellent structural stability and higher mechanical stiffness, but also change the insulator electronic property of BN monolayer into metallic, which are very important for the application of LIB anodes. The average adsorption energies of Li atoms on the BN sheet side and the VS2 monolayer side of BN/VS2 heterostructure are enhanced when compared to that on BN and VS2 monolayers respectively. The Li capacity of BN/VS2 heterostructure is 590.4 mAh/g and larger than that of VS2 monolayer (466 mAh/g) and graphene/VS2 heterostructure (569 mAh/g). As to the Li diffusion in BN/VS2 heterostructure, energy diffusion barrier of Li+ ion is relatively low (0.25 eV). In view of large Ead, low Ebarrier, high Li capacity and conductivity, BN/VS2 heterostructure should be a more ideal anode material for LIB than monolayer VS2 and graphene.

The first-principle study on the performance of biaxial strained graphdiyne as the Li-ion battery anode

Good reversibility and high Li capacity is necessary to be balanced in the electrode materials, the poor cycling performance of Li-ion battery (LIB) is usually caused by the severe volume expansion of anodes during lithiation/delithiation process. The Li storage and diffusion performance of the strained monolayer and bilayer graphdiyne are researched by the density functional theory. The Li capacity of monolayer and bilayer graphdiyne can be improved to 1985 mAh/g under the external 6% biaxial strain, which is much larger than five times of the 372 mAh/g for graphite. The diffusion energy barrier and diffusion coefficient at 300 K of Li atoms on the monolayer and bilayer graphdiyne under the 6% strain is much respectively smaller and larger than that without strain. The volume expansion of the most stable AB-2 stacking bilayer structure after adsorbing Li atoms is only 7%. The remarkable performance of external strained monolayer and bilayer graphdiyne shows that they are promising LIB anodes.

The good performance of bilayer β-antimoneneas an anode material for the Li-ion battery study

The safety and poor reversibility of the lithium-ion batteries (LIBs) are usually induced by the large volume expansion existing during the lithiation process. Thus, the good reversibility and high Li capacity is necessary to be balanced in the electrode materials. The density functional theory is used to explore the adsorption and diffusion of Li ions on bilayer two-dimensional (2D) β-antimonene (β-Sb) as the anode material. The metallic property of Li adsorbed bilayer β-Sb can insure fast electron transport in the electrode during lithiation/delithiation process, which is important for the electrode materials of LIBs. The moderate adsorption energy and diffusion mobility of Li atoms on the bilayer β-Sb explores its good Li storage capacity. Importantly, the AA-stacking bilayer β-Sb has only 4% volume expansion in the Li adsorption process, which is much smaller than the 200% of Sb metal and indicates it is well-kept upon adsorption of Li even at the maximum concentration, suggesting it can avoid the rapid attenuation of electrode capacity in the charge-discharge cycle. Excellent performances of Li capacity and diffusion ability make AA-stacking bilayer β-Sb as the promising candidates for the anode material of LIBs.

Optimized Li storage performance of B, N doped graphyne as Li-ion battery anode materials

As the economy grows and the environment deteriorates, the renewable energy is urgently needed. The advanced energy storage technology in electronic equipment, electric vehicle, smart grid, etc. becomes more significant. For example, the rechargeable batteries, hydrogen storage media, supercapacitors, the new energy storage devices have received much attention today. The anodes of the lithium ion battery (LIB), as the main body of charging and discharging, should be most important. The ideal anode material for LIBs is required to possess a higher Li capacity and a lower volume expansion. Good reversibility and high Li capacity are balanced necessarily in the electrode material. The poor cycling performance of LIB is usually due to the severe volume expansion of anode in lithiation/delithiation process. In this paper, the Li storage performance of B and N doped graphyne is explored by using the density functional theory method. The Perdew-Burke-Ernzerhof functional of the generalized gradient approximation is chosen. The calculations indicate that the doping of B atoms can enhance the adsorption strength between the Li atom and the graphyne, which can greatly increase the Li storage capacity. The Li storage capacity of B doped graphyne can reach as high as 2061.62 mAh/g, which is 2.77 times that of pristine monolayer graphyne. Meanwhile, the B doping reduces the out-plane diffusion energy barrier of Li, but increases the in-plane diffusion energy barrier slightly by 0.1 eV. On the other hand, the doping of N atoms reduces the interaction between Li and graphyne, however, the Li capacity also increases to 1652.12 mAh/g because the number of the available Li adsorption sites increases. Moreover, the doping of N atoms greatly improves the diffusion performance of Li on graphyne. The in-plane diffusion energy barrier drops to 0.37 eV, and thus the charge-discharge performance of the N doping graphyne is well improved. Therefore, the doping of B and N atoms can remarkably improve the performance of graphyne as the LIB anodes. The remarkable performance of B and N doped graphdiyne shows that it will become a promising LIB anode in the future. The present research can provide a good theoretical basis and thus conduce to guiding the developing of good Li storage materials, and can also supply strong background for experimental researches.

Anomalously High Lithium Storage in Three-Dimensional Graphene-like Ordered Microporous Carbon Electrodes

Zeolite-templated carbon, having a three-dimensional graphene-like ordered microporous structure with high electrical conductivity, is a fascinating anode material for Li-ion batteries (LIBs). Herein, we report an extremely high Li capacity of 2950 mA h g–1 (equivalent to Li1.3/C), which is 7.9 times the maximum capacity of graphite, Li/C6. This is equivalent to the crowded packing of 20 Li+ per pore with 0.9 nm diameter. Approximately 59% of the capacity was reversible. According to the characterizations by electron energy loss spectroscopy, 7Li NMR, and 13C NMR, most of the Li species existed as Li+ within the carbon micropores. Contrary to the often-made assumption, only a small amount of solid–electrolyte interphase layers was detected at the external surface of the carbon particles but not inside the micropores. The anomalously high Li capacity is attributed to the extremely narrow pore environment, where Li+ would be difficult to be fully solvated. Tailoring of the carbon pores to a subnanometric ra...

Behavior of Lithium Metal Anodes under Various Capacity Utilization and High Current Density in Lithium Metal Batteries

Summary Lithium (Li) metal batteries (LMBs) have recently attracted extensive interest in the energy-storage field after silence from the public view for several decades. However, many challenges still need to be overcome before their practical application, especially those that are related to the interfacial instability of Li metal anodes. Here, we reveal for the first time that the thickness of the degradation layer on the metallic Li anode surface shows a linear relationship with Li areal capacity utilization up to 4.0 mAh cm −2 in a practical LMB system. The increase in Li capacity utilization in each cycle causes variations in the morphology and composition of the degradation layer on the Li anode. Under high Li capacity utilization, the current density for charge (i.e., Li deposition) is identified to be a key factor controlling the corrosion of the Li metal anode. These fundamental findings provide new perspectives for the development of rechargeable LMBs.

Theoretical Study of Sp2-sp3 Hybrid Carbon Networks for Energy Storage

Carbon nanostructures have attracted a lot of attentions in hydrogen storage for its light weight and high surfaces area. However, carbon nanotubes, graphene expanded structures suffer from the weak H2 physisorptions for practical use.[1] Light metal doping is thought to be a feasiable way to improve the H2 binding and those structures are predicted to have ultra-high hydrogen storage capacity.[2] Nevertheless, no experiment has yielded comparable capacity to the theoretical expectations. Sigal et al. [3] doubt about the practicability of hydrogen storage on metal-doped two-dimensional (2D) systems since even slight oxygen content in H2 atmosphere can block the metal sites which suppresses the H2 adsorption. The metal-decorated porous structures could be more practical by hindering the oxygen access to the metal sites. Three-dimensional (3D) porous structures such as carbide-derived carbons,[4] metal organic framework (MOF)[5] and covalent organic framework (COF) [6] are designed and exhibit promising capacity close to the DOE target. However, none of nanoporous structures could meet the requirement of hydrogen storage in both weight percentage and volumetric density up to now. We present here that the bundles of carbon nanotubes could undergo phase transition to a type of 3D sp2-sp3 hybrid carbon networks (HCN) by changing intertube van der Waals forces to strong sp3 covalent bonding. With well-aligned and size-tunable 1-D nanopores, we show high Li capacity, stable Li-ion intercalation voltage profile, low Li diffusion barriers, slight volumetric change upon Li-doping in HCN, making it a potential Li-ion battery anode material. [7] Moreover, the unique nanoporous strcuture is beneficial for the achoring Li atoms and induces improved H2 binding energy due to the polarized interaction between H2 and Li ion. A maximum hydrogen capacity of 6.28 wt % is reached with average binding energy of -0.19 eV in Li-doped HCN. Combined with an ultrahigh volumetric density of hydrogen (102 g/L), it is a promising candidate for the hydrogen storage medium.[8]

Facile synthesis of Mo0.91W0.09S2 ultrathin nanosheets/amorphous carbon composites for lithium-ion battery anode

We report a facile process to synthesize Mo0.91W0.09S2/amorphous carbon composites with mean thickness of only 5nm ultrathin nanosheets and their improved electrochemical performance as anode materials for lithium ion batteries (LIBs). Structural and morphological characterizations of the nanocomposite have been investigated by XRD, SEM and TEM. It was found the amorphous carbon to be well dispersed on the ultrathin nanosheets surface. Due to the increased interlayer distance and the synergistic effects between Mo0.91W0.09S2 nanosheets and amorphous carbon, the as-prepared Mo0.91W0.09S2/C electrode deliveres exhibit a high Li capacity, an excellent rate capability, and a stable cycling performance, which suggests the Mo0.91W0.09S2/C composites as promising electrode material for LIBs.

Silicene/germanene on MgX2 (X = Cl, Br, and I) for Li-ion battery applications.

Silicene is a promising electrode material for Li-ion batteries due to its high Li capacity and low Li diffusion barrier. Germanene is expected to show a similar performance due to its analogous structural and electronic properties. However, the performance of both the materials will be determined by the substrate, since freestanding configurations are unstable. We propose Si/MgX2 and Ge/MgX2 (X = Cl, Br, and I) as suitable hybrid structures, based on first-principles calculations. We find that Li will not cluster and that the Li capacity is very high (443 and 279 mA h g(-1) for silicene and germanene on MgCl2, respectively). Sandwich structures can be used to further enhance the performance. Low diffusion barriers of less than 0.3 eV are predicted for all the hybrid structures.

Ultrafast synthesis of MoS2 or WS2-reduced graphene oxide composites via hybrid microwave annealing for anode materials of lithium ion batteries

An ultrafast and simple strategy to synthesize metal sulfides (MoS2 and WS2) anchored on reduced graphene oxide (RGO) composites is reported as anode materials for lithium ion batteries (LIBs). Metal sulfide nanocrystals with homogeneous dispersion onto conducting RGO sheets are obtained in only 45 s by hybrid microwave annealing (HMA) method. The synthesized materials, especially MoS2/RGO composite, exhibit a high Li capacity, an excellent rate capability, and a stable cycling performance, comparable to the reported best MS2/carbon composite electrodes. The results highlight the effectiveness of HMA method to fabricate the metal sulfide/RGO composites with excellent electric properties.

Role of Surface Structure on Li-Ion Energy Storage Capacity of Two-Dimensional Transition-Metal Carbides

A combination of density functional theory (DFT) calculations and experiments is used to shed light on the relation between surface structure and Li-ion storage capacities of the following functionalized two-dimensional (2D) transition-metal carbides or MXenes: Sc2C, Ti2C, Ti3C2, V2C, Cr2C, and Nb2C. The Li-ion storage capacities are found to strongly depend on the nature of the surface functional groups, with O groups exhibiting the highest theoretical Li-ion storage capacities. MXene surfaces can be initially covered with OH groups, removable by high-temperature treatment or by reactions in the first lithiation cycle. This was verified by annealing f-Nb2C and f-Ti3C2 at 673 and 773 K in vacuum for 40 h and in situ X-ray adsorption spectroscopy (XAS) and Li capacity measurements for the first lithiation/delithiation cycle of f-Ti3C2. The high-temperature removal of water and OH was confirmed using X-ray diffraction and inelastic neutron scattering. The voltage profile and X-ray adsorption near edge structure of f-Ti3C2 revealed surface reactions in the first lithiation cycle. Moreover, lithiated oxygen terminated MXenes surfaces are able to adsorb additional Li beyond a monolayer, providing a mechanism to substantially increase capacity, as observed mainly in delaminated MXenes and confirmed by DFT calculations and XAS. The calculated Li diffusion barriers are low, indicative of the measured high-rate performance. We predict the not yet synthesized Cr2C to possess high Li capacity due to the low activation energy of water formation at high temperature, while the not yet synthesized Sc2C is predicted to potentially display low Li capacity due to higher reaction barriers for OH removal.

Theoretical Study of sp2-sp3 Hybridized Carbon Network for Li-ion Battery Anode

We discover through first-principles calculations a new type of nanoporous carbon network structure formed out of small diameter nanotubes that features unique sp2-sp3 bonding hybridizations and highly ordered 1-D channels for Li-ion diffusion. Unlike other graphitic materials that are primarily bonded by intertube/interlayer van der Waals forces, the predicted sp2-sp3 hybridized carbon networks (HCNs) are held together by strong sp3 covalent bonding at the junctions, with sp2-hybridized interconnects providing conducting π-electrons near the Fermi level. With well-aligned and size-tunable 1-D nanopores, we show that besides desirable high Li capacity, stable Li-ion intercalation voltage profile, and low diffusion barriers, the volumetric change of HCN between fully lithiated/delithiated phases is <1%, making it a very promising Li-ion battery anode candidate.

New Battery Cathode Materials: Synthesis, Characterization, and Electrochemical Performance of M1-xV3O8-yFz·nH2O (M = NH4, K)

The aqueous synthesis and electrochemical properties of the new nanocrystalline material M1-xV3O8-yFz·nH2O (M = NH4, K; x ≈ 0.0−0.2; y ≈ 0.0−0.2; z ≈ 0.1−0.2; n ≈ 1) are described. This material is easily prepared by precipitation from aqueous HF-acidified ammonium or potassium vanadate solutions. M1-xV3O8-yFz·nH2O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The proposed atomic structure is related to that of (NH4)2V6O16, with V6O16 layers separated by layers of hydrated ammonium cations. The M = NH4 material exhibits a high Li capacity with good reversibility within the first few charge−discharge cycles. The lithium capacity of an electrode composed of (NH4)0.9V3O7.9F0.1·nH2O/EPDM/carbon (88/4/8) is 409 (mA h)/g (C/80 rate), and the energy density is ∼1000 (W h)/kg (128-μm-thick cathode, 4−1.5 V versus Li metal anode). The HF concentration, starting vanadate salt, and temperature are important parameters in the synthesis of...

High Lithium Capacity MxV2O5Ay·nH2O for Rechargeable Batteries

The aqueous synthesis and electrochemical properties of nanocrystalline MxV2O5Ay·nH2O are described. It is easily and quickly prepared by precipitation from acidified vanadate solutions. MxV2O5Ay·nH2O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The atomic structure is related to that of xerogel-derived V2O5·nH2O. In MxV2O5Ay·nH2O, M is a cation from the starting vanadate salt and A is an anion from the mineral acid. This material exhibits high, reversible Li capacity and may be considered for use in a cathode in primary and secondary batteries. The lithium capacity of an electrode composed of MxV2O5Ay·nH2O/EPDM/carbon (88/4/8) is ∼380(mA h)/g (C/80 rate) and the energy density is ∼1000(W h)/kg (120-μm-thick cathode, 4–1.5 V, versus Li metal anode). Critical parameters identified in the synthesis of MxV2O5Ay·nH2O, with respect to achieving high Li-ion insertion capacity, are acid/vanadium ratio, starting vanadate salt, and temperature. Inclusion of carbon black in the synthesis yields a composite that maintains the high Li capacity, lowers the electrochemical-cell polarization, and preserves the lithium capacity at higher discharge rates. Li-ion coin cells, using pre-lithiated graphite anodes, exhibit electrochemical performance comparable to that of Li-metal coin cells.

Local Structure and Vibrational Spectroscopy of Disordered Carbons for Li Batteries: Neutron Scattering Studies

Compositions, local atomic structures, porosity, and vibrational dynamics of carbons obtained by pyrolyzing epoxy novolak resins at 650 and 1000°C are investigated using several neutron scattering techniques. The ultimate goal is to understand the origin of the very high capacity for Li uptake exhibited by these materials. Neutron radial distribution function analysis and incoherent inelastic scattering show that the structural motif is a planar hexagonal graphene fragment ∼10 Å on a side, with edge carbons terminated by single hydrogen atoms and random stacking between fragments. Small‐angle neutron scattering reveals substantial porosity on a length scale comparable to the size of the graphene fragments. Coupled with computer simulations, these results are consistent with two proposed mechanisms for unusually high Li capacity, one analogous to conventional intercalation but with Li on both sides of isolated graphene fragments, the other involving bonding of Li to H‐terminated edge carbons.