In the past decades, Li ion batteries are widely considered to be the most promising rechargeable batteries for the rapid development of mobile devices and electric vehicles. There arouses great interest in Na ion batteries, especially in the field of static grid storage due to their much lower production cost compared with Li ion batteries. However, the fundamental mechanism of Li and Na ion transport in nanoscale electrodes of batteries has been rarely experimentally explored. This insight can guide the development and optimization of high-performance electrode materials. In this work, single nanowire devices with multicontacts are designed to obtain detailed information during the electrochemical reactions. This unique platform is employed to in situ investigate and compare the transport properties of Li and Na ions at a single nanowire level. To give different confinement for ions and electrons during the electrochemical processes, two different configurations of nanowire electrode are proposed; one is to fully immerse the nanowire in the electrolyte, and the other is by using photoresist to cover the nanowire with only one end exposed. For both configurations, the conductivity of nanowire decreases after intercalation/deintercalation for both Li and Na ions, indicating that they share the similar electrochemical reaction mechanisms in layered electrodes. However, the conductivity degradation and structure destruction for Na ions is more severe than those of Li ions during the electrochemical processes, which mainly results from the much larger volume of Na ions and greater energy barrier encountered by the limited layered spaces. Moreover, the battery performances of coin cells are compared to further confirm this conclusion. The present work provides a unique platform for in situ electrochemical and electrical probing, which will push the fundamental and practical research of nanowire electrode materials for energy storage applications.

SEI films directly affect the dissolution and deposition of lithium during discharge and charge and thus are one of the key factors that determine the safety, power capability, morphology of lithium deposits, shelf life, and cycle life of LIBs. In particular, the breakdown of SEI can result in the nucleation and growth of lithium dendrites and cause the safety risk for the practical applications of LIBs in the electric vehicles and other high-energy-density electronic devices. [5,6] While, a stable and robust SEI film can effectively inhibit the lithium dendrite growth and thus prominently enhance the cycling performance and safety of the batteries. [1,7] Owing to directly growing on anode surfaces, the formation and stability of SEI films are greatly influenced by the volume variation of anodes during the charge-discharge cycling, especially at a high current density. [1,8] Although the importance of SEI has been widely recognized, the structure and kinetics of SEI are the less wellunderstood phenomena impacting battery technology. Various in situ techniques, for instances, in situ spectroscopic ellipsometry, [9] in situ nuclear magnetic resonance (NMR), [10] in situ X-ray diffraction, [11] in situ Fourier transform infrared (FTIR) spectroscopy, [12] and in situ electrochemical impedance spectroscopy, [13] have been employed to investigate the evolution of SEI films. Nevertheless, most of them are limited to the nonintuitive investigations. Several key questions on the SEI film formation, structure and failure are still elusive.Recently, in situ transmission electron microscopy (TEM) has been utilized as a power tool to explore the evolution of SEI films in LIBs. In particular, a liquid cell TEM technique can well mimic the chemical and electrochemical reactions in liquid media with controllable charge and discharge conditions of LIBs. Sacci et al. performed the first in situ TEM observations of SEI film formation on a gold electrode during cyclic voltammetry testing. [14] They found that SEI films form heterogeneously on the gold electrode and possess dendritic morphology. The formation and growth of SEI films on graphite/electrolyte interfaces were studied by Unocic et al. using in situ TEM. [15] By utilizing the liquid cell TEM, Zeng et al. monitored the structural evolution of SEI films in a cyclic voltammetry process and found that the growth of SEI films is limited by the electron transport and produces gaseous The solid electrolyte interphase (SEI) spontaneously formed on anode surfaces as a passivation layer plays a critical role in the lithium dissolution and deposition upon discharge/charge in lithium ion batteries and lithiummetal batteries. The formation kinetics and failure of the SEI films are the key factors determining the safety, power capability, and cycle life of lithium ion and lithium-metal batteries. Since SEI films evolve with the volumetric and interfacial changes of anodes, it is technically challenging in experimental study of SEI kinetics. Here operando observations are reported of SEI...

No abstract

An aprotic lithium-oxygen battery with an ultrahigh theoretical energy density has attracted significant attention as the next-generation electrochemical energy device demanded by all-electric vehicles and other high-energy devices. Extensive effort has recently been devoted to improving the performances of cathodes, anodes, and electrolytes. However, as an integrated system, the overall battery properties are not determined by the individual components but by the synergy of all components. Despite important progress in the development of cathodes, anodes, and electrolytes, the system-level design and assembly of lithium-oxygen batteries have not benefited from these recent advances. Here, we report a graphene-based quasi-solid-state lithium-oxygen battery consisting of a rationally designed 3D porous graphene cathode, redox mediator-modified gel polymer electrolyte, and porous graphene/Li anode. This integrated prototype battery simultaneously addresses the major challenges of lithium-oxygen batteries and achieves stable cycling at a large capacity, low charge overpotential and high rate in both coin-type and largescale pouch-type batteries. For the first time, these lithium-oxygen batteries as a whole device deliver gravimetric and volumetric energy densities higher than those of a commercial Li-ion polymer battery. This study represents important progress toward the practical implementation of full-performance lithium-oxygen batteries.