Multilevel Gradient-Ordered Silicon Anode with Unprecedented Sodium Storage.

While cost-effective sodium-ion batteries (SIBs) with crystalline silicon anodes promise high theoretical capacities, they perform poorly because silicon stores sodium ineffectively (capacity <40 mAh g-1 ). To address this issue, herein an atomic-order structural-design tactic is adopted for obtaining unique multilevel gradient-ordered silicon (MGO-Si) by simple electrochemical reconstruction. In situ-formed short-range-, medium-range-, and long-range-ordered structures construct a stable MGO-Si, which contributes to favorable Na-Si interaction and fast ion diffusion channels. These characteristics afford a high reversible capacity (352.7 mAh g-1 at 50 mA g-1 ) and stable cycling performance (95.2% capacity retention after 4000 cycles), exhibiting record values among those reported for pure silicon electrodes. Sodium storage of MGO-Si involves an adsorption-intercalation mechanism, and a stepwise construction strategy of gradient-ordered structure further improves the specific capacity (339.5 mAh g-1 at 100 mA g-1 ). Reconstructed Si/C composites show a high reversible capacity of 449.5 mAh g-1 , significantly better than most carbonaceous anodes. The universality of this design principle is demonstrated for other inert or low-capacity materials (micro-Si, SiO2 , SiC, graphite, and TiO2 ), boosting their capacities by 1.5-6 times that of pristine materials, thereby providing new solutions to facilitate sodium storage capability for better-performing battery designs.

Effect of Different Nitrogen Configurations on Sodium Storage Properties of Carbon Anodes for Sodium Ion Batteries.

Nitrogen doping carbon materials are considered to be promising candidates for Na+ storage anodes. However, hitherto, the effects and mechanism of specific single N configuration (among pyrrolic N, quaternary N, and pyridinic N), on the sodium storage behaviors of carbon materials, are still puzzling, owing to the difficulties in accurately synthesizing a certain type of single N configuration dominated carbon materials (NCDCMs). Here, various NCDCMs have been successfully controlled and synthesized by small molecule polymerization methods, and their synthesis process has been also verified by NMR, MOLDI-TOF, TG-MS, etc. When serving as sodium ion battery anodes, the NCDCMs dominated by a high concentration of pyrrolic N (>80.3%) exhibits a satisfactory reversible capacity (434.5 mA h g-1 at 50 mA g-1 and 146.7 mA h g-1 at 2000 mA g-1, respectively). It is revealed that pyrrolic N has more suitable adsorption energy and larger interlayer spacing, by density functional theory calculations and electron orbital theory, respectively, which synergistically makes the material obtain excellent electrochemical performance. This research exhibits a more efficient way to reveal the differences in the sodium ions storage behavior of different nitrogen configurations doped carbon, and provides new insight for the precise design and synthesis of a certain type of heteroatom doping to achieve satisfactory electrochemical performance.

Elucidating the Mechanism of Fast Na Storage Kinetics in Ether Electrolytes for Hard Carbon Anodes.

The sodium storage performance of a hard carbon (HC) anode in ether electrolytes exhibits a higher initial Coulombic efficiency (ICE) and better rate performance compared to conventional ester electrolytes. However, the mechanism behind faster Na storage kinetics for HC in ether electrolytes remains unclear. Herein, a unique solvated Na+ and Na+ co-intercalation mechanism in ether electrolytes is reported using designed monodispersed HC nanospheres. In addition, a thin solid electrolyte interphase film with a high inorganic proportion formed in an ether electrolyte is visualized by cryo transmission electron microscopy and depth-profiling X-ray photoelectron spectroscopy, which facilitates Na+ transportation, and results in a high ICE. Furthermore, the fast solvated Na+ diffusion kinetics in ether electrolytes are also revealed via molecular dynamics simulation. Owing to the contribution of the ether electrolytes, an excellent rate performance (214 mAh g-1 at 10 A g-1 with an ultrahigh plateau capacity of 120 mAh g-1 ) and a high ICE (84.93% at 1 A g-1 ) are observed in a half cell; in a full cell, an attractive specific capacity of 110.3 mAh g-1 is achieved after 1000 cycles at 1 A g-1 .

Crystal Phase-Controlled Modulation of Binary Transition Metal Oxides for Highly Reversible Li-O2 Batteries.

Reducing charge-discharge overpotential of transition metal oxide catalysts can eventually enhance the cell efficiency and cycle life of Li-O2 batteries. Here, we propose that crystal phase engineering of transition metal oxides could be an effective way to achieve the above purpose. We establish controllable crystal phase modulation of the binary MnxCo1-xO by adopting a cation regulation strategy. Systematic studies reveal an unprecedented relevancy between charge overpotential and crystal phase of MnxCo1-xO catalysts, whereas a dramatically reduced charge overpotential (0.48 V) via a rational optimization of Mn/Co molar ratio = 8/2 is achieved. Further computational studies indicate that the different morphologies of Li2O2 should be related to different electronic conductivity and binding of Li2O2 on crystal facets of MnxCo1-xO catalysts, finally leading to different charge overpotential. We anticipate that this specific crystal phase engineering would offer good technical support for developing high-performance transition metal oxide catalysts for advanced Li-O2 batteries.

Rational Tuning of a Li4SiO4-Based Hybrid Interface with Unique Stepwise Prelithiation for Dendrite-Proof and High-Rate Lithium Anodes.

Lithium metal batteries (LMBs) are among the most promising candidates for high energy-density batteries. However, dendrite growth constitutes the biggest stumbling block to its development. Herein, Li4SiO4-dominating organic-inorganic hybrid layers are rationally designed by SiO2 surface modification and the stepwise prelithiation process. SiO2 nanoparticles construct a zigzagged porous structure, where a solid electrolyte interface (SEI) has grown and penetrated to form a conformal and compact hybrid surface. Such a first-of-this-kind structure enables enhanced Li dendrite prohibition and surface stability. The interfacial chemistry reveals a two-step prelithiation process that transfers SiO2 into well-defined Li4SiO4, the components of which exhibits the lowest diffusion barrier (0.12 eV atom-1) among other highlighted SEI species, such as LiF (0.175 eV atom-1) for the current artificial layer. Therefore, the decorated Li allows for an improved high-rate full-cell performance (LiFePO4/modified Li) with a much higher capacity of 65.7 mAh g-1 at 5C (1C = 170 mAh g-1) than its counterpart with bare Li (∼3 mAh g-1). Such a protocol provides insights into the surface architecture and SEI component optimization through prelithiation in the target of stable, dendrite-proof, homogenized Li+ solid-state migration and high electrochemical performance for LMBs.