Flexible solid-state supercapacitors provide a promising energy-storage alternative for the rapidly growing flexible and wearable electronic industry. Further improving device energy density and developing a cheap flexible current collector are two major challenges in pushing the technology forward. In this work, we synthesize a nitrogen-doped graphene/MnO2 nanosheet (NGMn) composite by a simple hydrothermal method. Nitrogen-doped graphene acts as a template to induce the growth of layered δ-MnO2 and improves the electronic conductivity of the composite. The NGMn composite exhibits a large specific capacitance of about 305 F g(-1) at a scan rate of 5 mV s(-1). We also create a cheap and highly conductive flexible current collector using Scotch tape. Flexible solid-state asymmetric supercapacitors are fabricated with NGMn cathode, activated carbon anode, and PVA-LiCl gel electrolyte. The device can achieve a high operation voltage of 1.8 V and exhibits a maximum energy density of 3.5 mWh cm(-3) at a power density of 0.019 W cm(-3). Moreover, it retains >90% of its initial capacitance after 1500 cycles. Because of its flexibility, high energy density, and good cycle life, NGMn-based flexible solid state asymmetric supercapacitors have great potential for application in next-generation portable and wearable electronics.

This paper studies the problem of global exponential stability and exponential convergence rate for a class of impulsive discrete-time neural networks with time-varying delays. Firstly, by means of the Lyapunov stability theory, some inequality analysis techniques and a discrete-time Halanay-type inequality technique, sufficient conditions for ensuring global exponential stability of discrete-time neural networks are derived, and the estimated exponential convergence rate is provided as well. The obtained results are then applied to derive global exponential stability criteria and exponential convergence rate of impulsive discrete-time neural networks with time-varying delays. Finally, numerical examples are provided to illustrate the effectiveness and usefulness of the obtained criteria. Index TermsImpulsive discrete-time neural networks, global exponential stablity, exponential convergence rate, Halanay inequality.

Sub-ion magnetic holes are rich in the terrestrial plasma sheet and magnetosheath. Here, we statistically investigate 60 sub-ion magnetic holes in the solar wind at 1 AU using the high-resolution data measured by the Magnetospheric Multiscale mission. We find that they are observed with a duration of 0.1-0.5 s, and the lengths of their cross-section are~0.1-0.6 ion gyroradius. These structures prefer to occur in the slow solar wind with a weak ambient magnetic field strength. They also prefer to occur in the marginally mirror stable or unstable environments. Electron vortices as well as an enhancement of the electron perpendicular temperature and electron fluxes at~90°pitch angle tend to be observed inside some magnetic holes with a large ambient magnetic field strength. By contrast, there are no clearly observational electron vortices as well as the electron fluxes at~90°pitch angle inside some magnetic holes with a weak ambient magnetic strength. The current density with a value of~10-50 nA/m 2 reveals that the corresponding maximum electron velocity is <10 km/s inside some magnetic holes, lower than the level of the observational electron velocity noise, which prevents the detection of the weak electron vortex. We suggest that electron vortices exist inside all the sub-ion magnetic holes in the solar wind. The generation of these sub-ion magnetic holes can be explained by the electron magnetohydrodynamics soliton and the electron vortex magnetic hole.

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