Aqueous batteries with zinc metal anodes are promising alternatives to Li-ion batteries for grid storage because of their abundance and benefits in cost, safety, and nontoxicity. However, short cyclability due to zinc dendrite growth remains a major obstacle. Here, we report a cross-linked polyacrylonitrile (PAN)-based cation exchange membrane that is low cost and mechanically robust. Li 2 S 3 reacts with PAN, simultaneously leading to cross-linking and formation of sulfur-containing functional groups. Hydrolysis of the membrane results in the formation of a membrane that achieves preferred cation transport and homogeneous ionic flux distribution. The separator is thin (30 μm-thick), almost 9 times stronger than hydrated Nafion, and made of low-cost materials. The membrane separator enables exceptionally long cyclability (>350 cycles) of Zn/Zn symmetric cells with low polarization and effective dendrite suppression. Our work demonstrates that the design of new separators is a fruitful pathway to enhancing the cyclability of aqueous batteries.

Conversion-type transition-metal phosphide anode materials with high theoretical capacity usually suffer from low-rate capability and severe capacity decay, which are mainly caused by their inferior electronic conductivities and large volumetric variations together with the poor reversibility of discharge product (Li 3 P), impeding their practical applications. Herein, guided by density functional theory calculations, these obstacles are simultaneously mitigated by confining amorphous FeP nanoparticles into ultrathin 3D interconnected P-doped porous carbon nanosheets (denoted as FeP@CNs) via a facile approach, forming an intriguing 3D flake-CNs-like configuration. As an anode for lithium-ion batteries (LIBs), the resulting FeP@CNs electrode not only reaches a high reversible capacity (837 mA h g −1 after 300 cycles at 0.2 A g −1 ) and an exceptional rate capability (403 mA h g −1 at 16 A g −1 ) but also exhibits extraordinary durability (2500 cycles, 563 mA h g −1 at 4 A g −1 , 98% capacity retention). By combining DFT calculations, in situ transmission electron microscopy, and a suite of ex situ microscopic and spectroscopic techniques, we show that the superior performances of FeP@CNs anode originate from its prominent structural and compositional merits, which render fast electron/ion-transport kinetics and abundant active sites (amorphous FeP nanoparticles and structural defects in P-doped CNs) for charge storage, promote the reversibility of conversion reactions, and buffer the volume variations while preventing pulverization/aggregation of FeP during cycling, thus enabling a high rate and highly durable lithium storage. Furthermore, a full cell composed of the prelithiated FeP@CNs anode and commercial LiFePO 4 cathode exhibits impressive rate performance while maintaining superior cycling stability. This work fundamentally and experimentally presents a facile and effective structural engineering strategy for markedly improving the performance of conversion-type anodes for advanced LIBs.

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Increasing electrode thickness and loading can help Li-ion batteries achieve higher energy densities, but the resulting decay in electrochemical performance at elevated rates remains a significant challenge. In order to design an optimal thick electrode, understanding how performance loss occurs is necessary. While it is known that both ionic and electronic conductivity contribute to rate performance, we observed a stronger correlation between electronic conductivity and electrochemical performance of electrodes at a loading of >25 mg/cm 2 under C/3 to 1C, rates most relevant to electric vehicle applications. To illustrate this effect, we explore the mud-cracking phenomenon during electrode fabrication to obtain narrow, vertical channels which reduce electrode tortuosity, and therefore decrease the liquid phase ionic resistance in thick electrodes. Variation in crack densities enables us to systematically investigate the effects of ionic and electronic conductivity on electrochemical performance in electrodes with identical overall porosity and composition. Rate and cycling performances of mud-cracked thick electrodes have stronger correlations with electronic conductivity than ionic conductivity. These findings shed new light on the relative importance of electronic versus ionic conductivities, arguing for the need to further optimize electronic conduction in thick electrodes when they are cycled in conditions relevant to electric vehicle applications.

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