Strong, persistent superficial oxidation-assisted chemical bonding of black phosphorus with multiwall carbon nanotubes for high-capacity ultradurable storage of lithium and sodium

We report a new composite of black phosphorus and multiwall carbon nanotubes (BP-CNT) prepared via a surface oxidation-assisted chemical bonding procedure. The controlled air exposure successfully changed the naturally hydrophobic BP powder to the desired hydrophilicity, which was found indispensable to stable bond formation between the BP and the functionalized CNTs during ball milling. The BP-CNT composites were further fabricated into anodes for both Li- and Na-ion batteries, using a sodium carboxyl methyl cellulose-poly(acrylic acid) (NaCMC-PAA) binary polymeric binder. The hydrophilicity of BP also played a very important role in forming strong bonds with the hydroxyl groups of NaCMC and the carboxylic acid groups of PAA. The plausible mechanisms of stable bond formation were comprehensively examined, and the results revealed two types of strong connections: P-O-C bonds and dehydration cross links. Consequently, the material delivered outstanding electrochemical performance in the anode, with a high discharge capacity of 1681 mA h g^-1 after 400 cycles at a current density of 0.2C (1C = 2596 mA g^-1) for Li-ion batteries. It also successfully delivered a first discharge capacity of 2073 and 850 mA h g^-1 at 0.2C and 2C for Na-ion batteries, respectively, with excellent capacity retentions at both rates after 200 cycles. These salient results, which originated from the modified hydrophilic BP, will give further impetus to explore BP-based composites for use as high-performance materials for advanced energy storage applications.