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Closed pores are widely accepted as the critical structure for hard carbon negative electrodes in sodium-ion batteries. However, the lack of a clear definition and design principle of closed pores leads to the undesirable electrochemical performance of hard carbon negative electrodes. Herein, we reveal how the evolution of pore mouth sizes determines the solvation structure and thereby redefine the closed pores. The precise and uniform control of the pore mouth sizes is achieved by using carbon molecular sieves as a model material. We show when the pore mouth is inaccessible to N 2 but accessible to CO 2 molecular probes, only a portion of solvent shells is removed before entering the pores and contact ion pairs dominate inside pores. When the pore mouth is inaccessible to CO 2 molecular probes, namely smaller than 0.35 nm, solvent shells are mostly sieved and dominated anion aggregates produce a thin and inorganic NaF-rich solid electrolyte interphase inside pores. Closed pores are accordingly redefined, and initial coulombic efficiency, cycling and low-temperature performance are largely improved. Furthermore, we show that intrinsic defects inside the redefined closed pores are effectively shielded from the interfacial passivation and contribute to the increased low-potential plateau capacity. Closed pores govern sodium-ion storage performance of hard carbon negative electrodes. Here, authors link pore mouth size evolution of the closed pores to the solvation structure and propose design principles for optimizing both closed pores and intrinsic defects.

Sodium metal batteries have potentially high energy densities, but severe sodium-dendrite growth and side reactions prevent their practical applications, especially at high temperatures. Herein, we design an inorganic ionic conductor/gel polymer electrolyte composite, where uniformly cross-linked beta alumina nanowires are compactly coated by a poly(vinylidene fluoride-co-hexafluoropropylene)-based gel polymer electrolyte through their strong molecular interactions. These  beta alumina nanowires combined with the gel polymer layer create dense and homogeneous solid-liquid hybrid sodium-ion transportation channels through and along the nanowires, which promote uniform sodium deposition and formation of a stable and flat solid electrolyte interface on the sodium metal anode. Side reactions between the sodium metal and liquid electrolyte, as well as sodium dendrite formation, are successfully suppressed, especially at 60 °C. The sodium vanadium phosphate/sodium full cells with composite electrolyte exhibit 95.3% and 78.8% capacity retention after 1000 cycles at 1 C at 25 °C and 60 °C, respectively. Here the authors show a beta alumina nanowires/gel polymer composite electrolyte design. The dense and homogeneous solid-liquid hybrid sodium-ion transportation channels promote uniform sodium deposition and stripping and significantly improve the performance of a Na metal battery.

HighlightsThe composite gel electrolyte with low tortuosity ion-conducting arrays (GPE/ICAs) exhibiting high room-temperature ionic conductivity (1.08 mS cm−1) was successfully prepared by directionally growing ice crystals and in-situ polymerization.The stable and rapid Li+ migration through ICAs in the GPE is proved by 6Li solid-state nuclear magnetic resonance and synchrotron radiation X-ray diffraction combined with computer simulations.Li/LiFePO4 full cells using GPE/ICAs exhibit excellent cycle performance and high-capacity retention at wide temperature (0–60 °C), which has the potential towards all-weather practical solid-state batteries.The rapid improvement in the gel polymer electrolytes (GPEs) with high ionic conductivity brought it closer to practical applications in solid-state Li-metal batteries. The combination of solvent and polymer enables quasi-liquid fast ion transport in the GPEs. However, different ion transport capacity between solvent and polymer will cause local nonuniform Li+ distribution, leading to severe dendrite growth. In addition, the poor thermal stability of the solvent also limits the operating-temperature window of the electrolytes. Optimizing the ion transport environment and enhancing the thermal stability are two major challenges that hinder the application of GPEs. Here, a strategy by introducing ion-conducting arrays (ICA) is created by vertical-aligned montmorillonite into GPE. Rapid ion transport on the ICA was demonstrated by 6Li solid-state nuclear magnetic resonance and synchrotron X-ray diffraction, combined with computer simulations to visualize the transport process. Compared with conventional randomly dispersed fillers, ICA provides continuous interfaces to regulate the ion transport environment and enhances the tolerance of GPEs to extreme temperatures. Therefore, GPE/ICA exhibits high room-temperature ionic conductivity (1.08 mS cm−1) and long-term stable Li deposition/stripping cycles (> 1000 h). As a final proof, Li||GPE/ICA||LiFePO4 cells exhibit excellent cycle performance at wide temperature range (from 0 to 60 °C), which shows a promising path toward all-weather practical solid-state batteries.

Electrochemical capacitors are primarily limited by the low electric double-layer storage capacity and narrow operating window that avoids the formation of solid electrolyte interface layers. Herein, we demonstrate that electric double-layer adsorption of solvated Na + in carbon nanopores is achievable under a large offset potential of −2.95 V vs . potential of zero charge in diethylene-glycol-dimethyl-ether electrolyte, even accompanying with the as-formed solid electrolyte interface layers. The largely enlarged offset potential in situ drives the continued partial desolvations in carbon nanopores, which largely reduces the average solvation numbers from 2.1 to 0.6, leading to a high electric double-layer capacitance of 172 F g −1 , a high capacity of 508 C g −1 and high initial coulombic efficiency of 92.2% at 0.1 A g −1 (0.5 mA cm −2 ), together with high-rate capability and long-term cycling stability. Thereby, such increased electric double-layer charge storage enables the redesign and assembly of sodium-ion capacitor pouch cells that display a high specific density of 40 Wh kg −1 (on cell level) and 30,000 cycles at a fast (dis)charging rate of 51 C (20 mA cm −2 ). Such sodium-ion capacitors are assembled without any pretreatments that are beneficial for scale-up fabrications in industry. Electrochemical capacitors are limited by low capacity. Here, authors report that the increased offset potential from potential of zero charge electrochemically drives the continued partial desolvations in carbon nanopores, leading to largely increased electric double-layer capacitance and capacity.

The growth of sodium dendrites and the associated solid electrolyte interface (SEI) layer is a critical and fundamental issue influencing the safety and cycling lifespan of sodium batteries. In this work, we use in-situ 23Na magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) techniques, along with an innovative analytical approach, to provide space-resolved and quantitative insights into the formation and evolution of sodium metal microstructures (SMSs; that is, dendritic and mossy Na metal) during the deposition and stripping processes. Our results reveal that the growing SMSs give rise to a linear increase in the overpotential until a transition voltage of 0.15 V is reached, at which point violent electrochemical decomposition of the electrolyte is triggered, leading to the formation of mossy-type SMSs and rapid battery failure. In addition, we determined the existence of NaH in the SEI on sodium metal with ex-situ NMR results. The poor electronic conductivity of NaH is beneficial for the growth of a stable SEI on sodium metal.Magnetic resonance imaging and spectroscopy provide quantitative insights into the growth of sodium microstructures in batteries.

The ionic conductivity of composite solid-state electrolytes does not meet the application requirements of solid-state lithium (Li) metal batteries owing to the harsh space charge layer of different phases and low concentration of movable Li + . Herein, we propose a robust strategy for creating high-throughput Li + transport pathways by coupling the ceramic dielectric and electrolyte to overcome the low ionic conductivity challenge of composite solid-state electrolytes. A highly conductive and dielectric composite solid-state electrolyte is constructed by compositing the poly(vinylidene difluoride) matrix and the BaTiO 3 –Li 0.33 La 0.56 TiO 3– x nanowires with a side-by-side heterojunction structure (PVBL). The polarized dielectric BaTiO 3 greatly promotes the dissociation of Li salt to produce more movable Li + , which locally and spontaneously transfers across the interface to coupled Li 0.33 La 0.56 TiO 3– x for highly efficient transport. The BaTiO 3 –Li 0.33 La 0.56 TiO 3– x effectively restrains the formation of the space charge layer with poly(vinylidene difluoride). These coupling effects contribute to a quite high ionic conductivity (8.2 × 10 −4  S cm −1 ) and lithium transference number (0.57) of the PVBL at 25 °C. The PVBL also homogenizes the interfacial electric field with electrodes. The LiNi 0.8 Co 0.1 Mn 0.1 O 2 /PVBL/Li solid-state batteries stably cycle 1,500 times at a current density of 180 mA g − 1 , and pouch batteries also exhibit an excellent electrochemical and safety performance. The authors developed a highly conductive and dielectric composite solid-state electrolyte by coupling BaTiO 3 and Li 0.33 La 0.56 TiO 3– x nanowires with a side-by-side heterojunction structure in a polyvinylidene difluoride matrix, which simultaneously promotes the dissociation of lithium salts to produce more movable Li ions and efficiently transports the generated movable Li ions.

Even though organic molecules with well-designed functional groups can be programmed to have high electron density per unit mass, their poor electrical conductivity and low cycle stability limit their applications in batteries. Here we report a facile synthesis of π-conjugated quinoxaline-based heteroaromatic molecules (3Q) by condensation of cyclic carbonyl molecules with o-phenylenediamine. 3Q features a number of electron-deficient pyrazine sites, where multiple redox reactions take place. When hybridized with graphene and coupled with an ether-based electrolyte, an organic cathode based on 3Q molecules displays a discharge capacity of 395 mAh g −1 at 400 mA g −1 (1C) in the voltage range of 1.2–3.9 V and a nearly 70% capacity retention after 10,000 cycles at 8 A g −1 . It also exhibits a capacity of 222 mAh g −1 at 20C, which corresponds to 60% of the initial specific capacity. Our results offer evidence that heteroaromatic molecules with multiple redox sites are promising in developing high-energy-density, long-cycle-life organic rechargeable batteries. Organic compounds can be used as electrode materials for Li-ion batteries, but problems such as facile dissolution and low electrical conductivity hinder their application. Here the authors report π-conjugated quinoxaline-based heteroaromatic molecules with multiple redox sites to tackle the problems.

The composite solid-state electrolytes (CSEs) are one of the most promising electrolytes for advanced solid-state Li metal batteries. However, it is unclear for the effect of the induced electric field inside CSEs on the Li-ion transport. Herein, we design a compact CSE by imbedding the lithium niobate (LiNbO 3 ) with both high ionic conductivity and dielectric constant into poly(vinylidene fluoride) matrix (NPC). The LiNbO 3 significantly enhances the internal electric field of NPC along the LiNbO 3 particles and establishes uniform interfacial electric field between NPC and electrodes, which fundamentally facilitates the Li-ion transport, weakens the space-charge layer and inhibits the growth of Li dendrites. Continuous fast ion-conducting channels with high concentration of Li-ions are constructed inside NPC, which contributes to a quite high ionic conductivity (7.39×10 −4 S cm −1 , 25°C) and ultra-low activation energy (0.112 eV). The LiNi 0.8 Co 0.1 Mn 0.1 O 2 /NPC/Li solid-state batteries exhibit quite stable cycling performance at 25°C.

Magnetic resonance imaging provides a noninvasive method for in situ monitoring of electrochemical processes involved in charge/discharge cycling of batteries. Determining how the electrochemical processes become irreversible, ultimately resulting in degraded battery performance, will aid in developing new battery materials and designing better batteries. Here we introduce the use of an alternative in situ diagnostic tool to monitor the electrochemical processes. Utilizing a very large field-gradient in the fringe field of a magnet, stray-field-imaging (STRAFI) technique significantly improves the image resolution. These STRAFI images enable the real time monitoring of the electrodes at a micron level. It is demonstrated by two prototype half-cells, graphite∥Li and LiFePO 4 ∥Li, that the high-resolution 7 Li STRAFI profiles allow one to visualize in situ Li-ions transfer between the electrodes during charge/discharge cyclings as well as the formation and changes of irreversible microstructures of the Li components and particularly reveal a non-uniform Li-ion distribution in the graphite.

Even though organic molecules with well-designed functional groups can be programmed to have high electron density per unit mass, their poor electrical conductivity and low cycle stability limit their applications in batteries. Here we report a facile synthesis of π-conjugated quinoxaline-based heteroaromatic molecules (3Q) by condensation of cyclic carbonyl molecules with o-phenylenediamine. 3Q features a number of electron-deficient pyrazine sites, where multiple redox reactions take place. When hybridized with graphene and coupled with an ether-based electrolyte, an organic cathode based on 3Q molecules displays a discharge capacity of 395 mAh g-1 at 400 mA g-1 (1C) in the voltage range of 1.2-3.9 V and a nearly 70% capacity retention after 10,000 cycles at 8 A g-1 . It also exhibits a capacity of 222 mAh g-1 at 20C, which corresponds to 60% of the initial specific capacity. Our results offer evidence that heteroaromatic molecules with multiple redox sites are promising in developing high-energy-density, long-cycle-life organic rechargeable batteries.