Graphite, as the most common anode for commercial Li-ion batteries, has been reported to have a very low capacity when used as a Na-ion battery anode. It is well known that electrochemical insertion of Na þ into graphite is significantly hindered by the insufficient interlayer spacing. Here we report expanded graphite as a Na-ion battery anode. Prepared through a process of oxidation and partial reduction on graphite, expanded graphite has an enlarged interlayer lattice distance of 4.3 Å yet retains an analogous long-range-ordered layered structure to graphite. In situ transmission electron microscopy has demonstrated that the Na-ion can be reversibly inserted into and extracted from expanded graphite. Galvanostatic studies show that expanded graphite can deliver a high reversible capacity of 284 mAh g À 1 at a current density of 20 mA g À 1 , maintain a capacity of 184 mAh g À 1 at 100 mA g À 1 , and retain 73.92% of its capacity after 2,000 cycles.

Black phosphorus (BP) has attracted significant interest as a monolayer or few-layer material with extraordinary electrical and optoelectronic properties. However, degradation in air and other environments is an unresolved issue that may limit future applications. In particular the role of different ambient species has remained controversial. Here, we report systematic experiments combined with ab-initio calculations that address the effects of oxygen and water in the degradation of BP. Our results show that BP rapidly degrades whenever oxygen is present, but is unaffected by deaerated (i.e., O 2 depleted) water. This behavior is rationalized by oxidation involving a facile dissociative chemisorption of O 2 , whereas H 2 O molecules are weakly physisorbed and do not dissociate on the BP surface. Oxidation (by O 2 ) turns the hydrophobic pristine BP surface progressively hydrophilic. Our results have implications on the development of encapsulation strategies for BP, and open new avenues for exploration of phenomena in aqueous solutions including solution-gating, electrochemistry, and solutionphase approaches for exfoliation, dispersion, and delivery of BP.

Converting greenhouse gas carbon dioxide (CO) to value-added chemicals is an appealing approach to tackle CO emission challenges. The chemical transformation of CO requires suitable catalysts that can lower the activation energy barrier, thus minimizing the energy penalty associated with the CO reduction reaction. First-row transition metals are potential candidates as catalysts for electrochemical CO reduction; however, their high oxygen affinity makes them easy to be oxidized, which could, in turn, strongly affect the catalytic properties of metal-based catalysts. In this work, we propose a strategy to synthesize Ag-Sn electrocatalysts with a core-shell nanostructure that contains a bimetallic core responsible for high electronic conductivity and an ultrathin partially oxidized shell for catalytic CO conversion. This concept was demonstrated by a series of Ag-Sn bimetallic electrocatalysts. At an optimal SnO shell thickness of ∼1.7 nm, the catalyst exhibited a high formate Faradaic efficiency of ∼80% and a formate partial current density of ∼16 mA cm at -0.8 V vs RHE, a remarkable performance in comparison to state-of-the-art formate-selective CO reduction catalysts. Density-functional theory calculations showed that oxygen vacancies on the SnO (101) surface are stable at highly negative potentials and crucial for CO activation. In addition, the adsorption energy of CO at these oxygen-vacant sites can be used as the descriptor for catalytic performance because of its linear correlation to OCHO* and COOH*, two critical intermediates for the HCOOH and CO formation pathways, respectively. The volcano-like relationship between catalytic activity toward formate as a function of the bulk Sn concentration arises from the competing effects of favorable stabilization of OCHO* by lattice expansion and the electron conductivity loss due to the increased thickness of the SnO layer.

Behaviors of functional interfaces are crucial factors in the performance and safety of energy storage and conversion devices. Indeed, solid electrode-solid electrolyte interfacial impedance is now considered the main limiting factor in all-solid-state batteries rather than low ionic conductivity of the solid electrolyte. Here, we present a new approach to conducting in situ scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) in order to uncover the unique interfacial phenomena related to lithium ion transport and its corresponding charge transfer. Our approach allowed quantitative spectroscopic characterization of a galvanostatically biased electrochemical system under in situ conditions. Using a LiCoO2/LiPON/Si thin film battery, an unexpected structurally disordered interfacial layer between LiCoO2 cathode and LiPON electrolyte was discovered to be inherent to this interface without cycling. During in situ charging, spectroscopic characterization revealed that this interfacial layer evolved to form highly oxidized Co ions species along with lithium oxide and lithium peroxide species. These findings suggest that the mechanism of interfacial impedance at the LiCoO2/LiPON interface is caused by chemical changes rather than space charge effects. Insights gained from this technique will shed light on important challenges of interfaces in all-solid-state energy storage and conversion systems and facilitate improved engineering of devices operated far from equilibrium.

The modulation of electron density is an effective option for efficient alternative electrocatalysts. Here, p‐n junctions are constructed in 3D free‐standing FeNi‐LDH/CoP/carbon cloth (CC) electrode (LDH=layered double hydroxide). The positively charged FeNi‐LDH in the space‐charge region can significantly boost oxygen evolution reaction. Therefore, the j at 1.485 V (vs. RHE) of FeNi‐LDH/CoP/CC achieves ca. 10‐fold and ca. 100‐fold increases compared to those of FeNi‐LDH/CC and CoP/CC, respectively. Density functional theory calculation reveals OH− has a stronger trend to adsorb on the surface of FeNi‐LDH side in the p‐n junction compared to individual FeNi‐LDH further verifying the synergistic effect in the p‐n junction. Additionally, it represents excellent activity toward water splitting. The utilization of heterojunctions would open up an entirely new possibility to purposefully regulate the electronic structure of active sites and promote their catalytic activities.