Interlayer expansion of bulk MoS2 via top-down organic pillaring enables tunable Li+ intercalation and controlled solvent co-intercalation

Mg rechargeable batteries (MgRBs) stand out as a promising candidate beyond lithium ion battery technologies due to high volumetric energy density, resource abundance, and the dendrite-free deposition behavior of Mg, which ensures safe operation. Many of the advantages of MgRBs originate from the divalent nature and small ionic size of Mg ions; however, these properties also render the cation too polarizing to diffuse easily in most ion- intercalation materials. To overcome this challenge, we recently adopted an interlayer expansion approach via insertion of external pillars to transform two representative two-dimensional layered materials (MoS2 and V2O5) into efficient Mg storage host materials. To boost the Mg storage performance of MoS2, we intercalated appropriate amount of poly(ethylene oxide (PEO) to expand MoS2 structure through an exfoliation and restacking process. [1,2] The Mg solid state diffusivity and capacity increased accordingly with increasing amount of PEO. PEO2L-MoS2 (two layers of PEO in between MoS2 layers) showed the best performance with Mg2+ diffusivity increase by two orders of magnitude and capacity increase of about 300% compared with bulk MoS2. The flexible PEO serves as a structure pillar, which can increase the interlayer spacing but not block the diffusion of Mg2+ ions. In the case of V2O5, we turned to insert small water molecules in the layered structure to take advantage of the shielding effect of H2O.[3] We prepared graphene decorated V2O5 aerogel nanowires, and demonstrated it to be an effective cathode material, with specific capacity of 330 mAh g-1 at low rate, stable cyclability and good rate performance. The performance enhancement compared with bulk V2O5 is due to the increased space and charge shielding effect of crystal water in the aerogel to enhance the Mg2+ insertion and the porous structure of aerogel allowing easy access of electrolyte into the active material. Moreover, various interlayer spacing and crystal water amount were also studied and we found that the as-prepared sample with the highest amount of water and interlayer spacing showed the best performance. The merit of the interlayer expansion lies at the enlarged space for enhanced Mg2+ ion mobility and weakened interaction between Mg2+ ions and the electrode frameworks. This approach could be further extended to a wide range of host materials for the storage of Mg2+ ion and other multivalent ions. 1. Yanliang Liang, Hyun Deog Yoo, Yifei Li, Jing Shuai, Hector A. Calderon, Francisco Carlos Robles Hernandez, Lars C. Grabow, and Yan Yao, Nano Lett. 2015, 15, 2194-2202. 2. Yifei Li, Yanliang Liang, Francisco C. Robles Hernandez, Hyun Deog Yoo, Qinyou An, Yan Yao, Nano Energy 2015, 15, 453-461. 3. Qinyou An, Yifei Li, Hyun Deog Yoo, Shuo Chen, Qiang Ru, Liqiang Mai, and Yan Yao, Nano Energy 2015, 18, 265-272.

Reversible intercalation of the charge carrying ions remains the most reliable mechanism of the operation of rechargeable energy storage devices. While Li-ion batteries currently dominate the market, as the number of applications requiring batteries, including large-scale storage technologies, grows, it becomes obvious that new solutions, utilizing more abundant elements than lithium, are needed. Na-ion batteries (SIBs), an electrical energy storage system that belongs to beyond lithium ion (BLI) battery family, is an attractive candidate as they operate due to reversible intercalation of more abundant Na+ ions. However, Na+ ion is larger and heavier than Li+ ion, which can affect diffusion and cause deteriorated performance compared to that of Li-ion batteries. In addition, Na-ion batteries operate at lower voltages than Li-ion batteries, and thus higher capacity cathode materials are needed to increase the energy density of SIBs. Layered transition metal oxides with relatively high working voltages and large interlayer spacing, enabling a large number of intercalation sites and well-defined two-dimensional pathways for ion diffusion, are considered as attractive candidates for applications as cathodes in BLI batteries. In this presentation, we will demonstrate how electrochemical activity of the bilayered vanadium oxide in Na-ion batteries can be controlled through the modification of its chemical composition and interlayer spacing achieved in a versatile synthesis approach, called chemical pre-intercalation. Chemical pre-intercalation is a sol-gel-based process developed in our laboratory to insert specific types and amounts of guest species, such as inorganic ions and organic molecules, into the crystal structure of host electrode material leading to unprecedented compositional diversity. Post sol-gel process treatments, such as aging, hydrothermal treatment and thermal treatment, can be used to further tune chemical composition and structure of the synthesized materials [1]. We use vanadium pentoxide as the host material due to the ability of vanadium to reduce from V5+ to V3+, which is accompanied by the transfer of two electrons resulting in high specific capacity needed for cathodes in BLI batteries. More specifically, we focus on a unique allotropic modification of vanadium pentoxide, so-called bilayered (or δ-) V2O5 which is characterized by unusually large for oxides interlayer spacing of >9 Å. We will discuss the importance of the synthesis parameters, such as an extended aging step and addition of chemically pre-intercalated species during hydrothermal treatment process, for the formation of bilayered V2O5. We have successfully used chemical pre-intercalation to synthesize ion-preintercalated δ-MxV2O5 (M = Li, Na, K, Mg, Ca) with tunable interlayer spacing ranging from 9.6 Å for δ-KxV2O5 to 13.4 Å for δ-MgxV2O5. Chemical pre-intercalation of organic molecules, such as cetrimonium bromide (CTAB), allowed to expand the interlayer spacing further up to >30 Å. We will report the maximum content of chemically preintercalated species achieved in our work and discuss strategies for tuning the degree of chemical preintercaltion and how it affects electrochemical performance. Bilayered δ-NaxV2O5 demonstrated a record high initial capacity of 365 mAh/g in Na-ion cells, however this capacity decayed quickly [1]. We will show that low-temperature thermal annealing improves electrochemical stability of chemically pre-intercalated bilayered V2O5, which was attributed to the increased crystallinity of the materials. Annealing was carried out at 2600C under vacuum to prevent phase transformations and preserve the bilayered crystal structure. We also showed that electrochemical stability of the bilayered vanadium oxide can be improved through chemical pre-intercalation of electrochemically inactive, or stabilizing, ions (Li+, K+, Mg2+, and Ca2+). While the highest initial capacity (~350-365 mAh g-1) in Na-ion cells was exhibited by δ-LixV2O5 and δ-NaxV2O5, the greatest capacity retention (68% after 50 cycles at C/10) was demonstrated by δ-MgxV2O5 with the largest interlayer spacing (13.40 Å) achieved in this study. The highest capacity retention at higher current rates (~50% when current rate was changed from C/10 to 1C) were exhibited by Li- and Mg-stabilized phases. We will discuss the role of chemically preintercalated ion size and charge in electrochemical performance of δ-MxV2O5 (M = Li, Na, K, Mg, Ca). We will also report our findings on the effect of chemical pre-intercalation of organic molecules on structure and electrochemical performance of the bilayered vanadium oxide in Na-ion batteries. 1. Clites, Byles, Pomerantseva, J. Mater. Chem. A 4 (2016) 7754.

The low‐voltage charging capacity and rate performance are two crucial but mutually restricted properties of K‐ion batteries. Herein, a sulfonamide and amide linkages‐triggered N doping high‐temperature removal strategy is proposed to obtain porous carbon with rich carbon vacancies, high sp2 hybridized C content (65.3%), and large interlayer spacing using coal tar pitch as carbon precursor. Carbon vacancies and sp2 hybridized C are proved to be not only active sites for low‐voltage potassium storage but also conducive to the fast transfer of ions and electrons, endowing reversible and fast potassium storage at low voltage. Moreover, abundant carbon vacancies and large carbon interlayer spacing alleviate volume variation, improving the cycling stability for 8500 cycles (about 3700 h). This work sheds new light on the design of low‐voltage, durable, and fast potassium storage carbon anodes, and promotes the development of practical carbon anodes for high energy density K‐ion batteries in the future.

Mixed anion compounds offer unique opportunities to tune the redox properties of intercalation compounds. Among them, layered iron oxychloride (FeOCl) has long been envisioned as an energy-dense and sustainable alternative to layered oxides for Li–ion batteries. However, achieving full capacity has thus far remained elusive owing to detrimental solvent cointercalation typically observed in dilute liquid electrolytes and ionic liquids. In this work, we alleviate this limitation by developing a suitable electrolyte engineering approach and demonstrate that 1 Li atom can be reversibly intercalated in FeOCl. We demonstrate that solvent cointercalation can be suppressed in high-concentration electrolytes using a highly dissociating salt, lithium bis(fluorosulfonyl)imide (LiFSI), dissolved in solvents with low Li-solvent binding energy, including dimethylcarbonate and acetonitrile, achieving a capacity of 235 mAh/g and a material specific energy of 590 Wh/kg in its lithiated form. Combining X-ray absorption measurements at iron, oxygen, and chloride K-edges, we observe that the intercalation proceeds with a transition from distorted high-spin Fe3+ to low-spin Fe2+, while operando X-ray diffraction shows three successive biphasic processes associated with changes in interlayer spacing. Entropic potential measurements coupled with temperature-dependent cycling reveal a reversible cationic ordering event when half of the interlayer Li+ sites are filled. This ordering is associated with high activation energy and a slow phase-front diffusion, which does not prevent good cycling ability at room temperature even at high C-rate. Our work calls for revisiting through an electrolyte engineering approach energy dense and sustainable mixed anion materials previously discarded for their instability.

Efficient exfoliation of bulk MoS2 to nanosheets by mixed-solvent refluxing method

Hard carbon derived from sepals of Palmyra palm fruit calyx as an anode for sodium-ion batteries

Exploring novel two-dimensional layered transitional metal dichalcogenides and elucidating their reaction mechanism are critical to designing promising anode materials for lithium-ion batteries (LIBs). Herein, a novel layered TaS2 nanosheet was obtained via a typical solid-phase reaction method followed by a simple ball-milling treatment, and first explored experimentally as an anode for LIBs. The TaS2 nanosheet anode delivered an excellent cycling stability, with 234.6 mAh g-1 after 500 cycles at 1 A g-1. The optimized performance could be attributed to the large interlayer spacing, high conductivity, and reduced size of the TaS2 nanosheet, which effectively alleviated the volume change during the reaction process and accelerated the Li+ or e- transport. Especially, the TaS2 nanosheet anode presented an unusual intercalation reaction mechanism, accompanied with a reversible phase transition from the 2H to the 1T phase during the first de-lithiation process, which is evidenced by the multiple ex situ characterizations, further revealing the enhanced electrochemical performance results from the 1T phase with the larger interlayer spacing and higher electrical conductivity. This work provides a novel insight into the intercalation reaction mechanism of TaS2, which shows potential in high-performance LIBs.

Does β-ET 2I 3 really have a definite interlayer spacing?

New insights into the effect of hard carbons microstructure on the diffusion of sodium ions into closed pores

HPM-2 is a new organosilicate layered material synthesized by the fluoride route using 2-ethyl-1,3,4-trimethylimidazolium. The layers are of a new structural type (denoted mtf) and are held together by strong hydrogen bonds giving rise to a 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) at around 16 ppm and by Coulombic interactions between silanolates and the organic cations residing in the interlayer space. Upon calcination HPM-2 transforms into the pure silica MTF zeolite by topotactic condensation, a process that is essentially completed at 400 °C. Attempts to apply known methods to derive new materials (swelling, delamination, interlayer expansion) are described. In the case of the interlayer expansion reaction, a nonmicroporous dense phase is obtained, which is likely due to an unfavorable disposition of silanols in close couples within each layer. Silanol condensation between the newly incorporated silicon species occurs across the main window producing narrower (6 member ring, 6MR)...

Boron nitride substrate-induced reversible hydrogen storage in bilayer solid matrix via interlayer spacing

Interlayer engineering of graphite anodes in alkali metal ion (M = Li, Na, and K) batteries is carried out based on the first-principles calculations. By increasing the interlayer spacing of graphite, the specific capacity of Li or Na does not increase while that of K increases continuously (from 279 mA h g-1 at the equilibrium interlayer spacing to 1396 mA h g-1 at the interlayer spacing of 20.0 Å). As the interlayer spacing increases, the electrostatic potential of graphite becomes smoother, and the ability to buffer the electrostatic potential fluctuation becomes poorer in M ions. These two effects jointly lead to minima of the diffusion barrier of M ions on graphite (0.01-0.05 eV), instead of strictly monotonous declines with the increasing interlayer spacing. To perform the interlayer engineering of anode candidates more efficiently, a set of high-throughput programs has been developed and can be easily applied to other systems. Our research has guiding significance for achieving the optimal effect in interlayer engineering experimentally.

Insight to defects regulation on sugarcane waste-derived hard carbon anode for sodium-ion batteries

The weak van der Waals interaction between adjacent layers of quasi-two-dimensional materials provides opportunities for inserting intercalants to induce novel properties distinct from the host materials. Here we report induced superconductivity in an intercalated SnSe2 crystal by using a new type of intercalants—organic cations from ionic liquids, [C2MIm]+ and [DEMB]+. The intercalation of both cations increases the interlayer spacing and leads to induced superconductivity with T c of 7.1 and 6.9 K and a large superconducting anisotropy. Angle-resolved photoemission spectroscopy and Hall measurements reveal the importance of electron doping by the cations in the induced superconductivity, and the interlayer expansion and electric polarization of the cations in the large anisotropy. Our work reports induced superconductivity in an intercalated material with new intercalants which contribute both charge carriers and interlayer expansion, and provides a new pathway to the manipulation of superconductivity in layered materials.

Carbonaceous materials hold great promise for K-ion batteries due to their low cost, adjustable interlayer spacing, and high electronic conductivity. Nevertheless, the narrow interlayer spacing significantly restricts their potassium storage ability. Herein, hierarchical N, S co-doped exfoliated holey graphene (NSEHG) with ultrahigh pyridinic/pyrrolic N (90.6at.%) and large interlayer spacing (0.423nm) is prepared through micro-explosion assisted thermal exfoliation of graphene oxide (GO). The underlying mechanism of the micro-explosive exfoliation of GO is revealed. The NSEHG electrode delivers a remarkable reversible capacity (621 mAhg-1 at 0.05 Ag-1), outstanding rate capability (155 mAhg-1 at 10 Ag-1), and robust cyclic stability (0.005% decay per cycle after 4400 cycles at 5 Ag-1), exceeding most of the previously reported graphene anodes in K-ion batteries. In addition, the NSEHG electrode exhibits encouraging performances as anodes for Li-/Na-ion batteries. Furthermore, the assembled activated carbon||NSEHG potassium-ion hybrid capacitor can deliver an impressive energy density of 141 Whkg-1 and stable cycling performance with 96.1% capacitance retention after 4000 cycles at 1 Ag-1. This work can offer helpful fundamental insights into design and scalable fabrication of high-performance graphene anodes for alkali metal ion batteries.