Fast kinetics in free-standing porous Cu₃P anode for Li-ion batteries

Abstract

Anode materials for Li-ion batteries based on a conversion mechanism show very high theoretical specific capacities. In particular, phosphide materials display volumetric and gravimetric capacities far beyond graphite, approaching those of silicon-based materials. However, the slow kinetics and large mechanical strain during the conversion process are challenging issues toward enabling phosphide-based anode materials.

Here, we synthesize a copper phosphide free-standing membrane and show the benefits of this approach, which provides lighter electrodes with faster kinetics during lithiation and delithiation. The Cu₃P membrane is both binder-free and carbon black-free, and its synthetic process involves a simple chemical vapor deposition of phosphorus on a composite polymer-copper membrane at a moderate temperature of 429 °C. Microscopic and other methods show the purity of the phase and of the porous structure within the membrane. Galvanostatic cycling reveals an initial discharge reaching the theoretical capacity, high reversible capacity of 320 mAh/g_electrode (including the current collector), and good kinetics for phosphide-based materials at C/5 rate.

Introduction

The current trend in research of Li-ion batteries (LIBs) is to reach high gravimetric energy density with abundant materials that exhibit safe behavior [1]. Graphite stands as the un-challenged anode material, and the commercial LIBs approach theoretical capacity. As the thermodynamic limit is near, further improvement could require a significant change in the type of materials used as anodes. Phosphides have long been considered very interesting materials to replace graphite due to their high gravimetric and volumetric energy density, together with a safe behavior with a voltage window of 0.5–1.5 V. Metal phosphides are also good candidates for use in Na-ion batteries [2].

In the past, phosphides of several transition metals were synthesized and used as anodes in LIBs [3]. The electrochemical mechanism is usually viewed as a conversion from the metal phosphide to the metal and lithium phosphide. Therefore, the gravimetric energy density is directed by the phosphorus-to-metal ratio, and higher ratios produce greater energy densities. In several cases, the binary phase diagram allows synthesis of phosphorus-rich phosphides, for instance iron phosphides [4]. In other cases, a metal-rich stable phosphide phase is obtained, such as Cu₃P. Copper is compatible with lithium as alloys of the two metals are not formed, and the processing of its foils as well as its use as current collector in LIBs are well-established [5]. Although the theoretical gravimetric energy densities of Cu₃P and graphite are comparable (377 vs. 372 mAh/g), the high volumetric energy density of Cu₃P makes it an attractive anode material (2778 vs. 830 mAh/L). Nonetheless, Cu₃P cycling was demonstrated at a low rate, and the capacity fades rapidly after a few cycles to 200 mAh/g, much below the theoretical value of 377 mAh/g [6].

Previous attempts were made to convert a copper foil to Cu₃P and use it as anode material. Villevieille et al. [7] grew a Cu₃P columnar structure on a copper foil through a membrane used as template. Other attempts were made to grow a three-dimensional (3D) network and reach higher current density, for instance porous copper was obtained from Cu foams, and a solid-state reaction enabled partial conversion to copper phosphide [8]. The porous Cu₃P/Cu showed good electrochemical performance, although the overall gravimetric and volumetric capacities were limited by the incomplete conversion to Cu₃P. Interestingly, the electrochemistry of Cu₃P is also relevant to black phosphorous since copper is crucial for the electrochemical performance of black phosphorus [9].

Here, in order to increase the volumetric density of the anode, we fabricate a copper phosphide free-standing membrane from a Cu/polymer composite mesh that is much thinner than the Cu mesh reported previously. The resultant free-standing membrane demonstrates the benefits of the approach, which supplies electrodes that are lighter and exhibit faster kinetics during electrochemical test. The Cu₃P membrane is free of binder and carbon black, prepared by a simple chemical vapor deposition of phosphorus on a composite polymer-copper membrane at a temperature of 429 °C. The purity of the phase and of the porous structure within the membrane are revealed by SEM, HR-SEM, EDS and XRD.