Plasma Enhanced Atomic Layer Deposition of Nickel and Cobalt Phosphate for Li-Ion Battery Applications

Abstract

Ever since the introduction of iron phosphate as a Li-ion battery (LIB) electrode by Sony in 1991, phosphates have played an important role in various research fields. One advantage for the use of phosphates in for example LIB’s is the strong covalency in the metal-oxygen-phosphorus bonds allowing for a high structural and thermal stability. Next to this, the strong interaction between the metal atom and the phosphate anion allow for these materials to operate at a higher potential than their oxide counterparts[1]. Nevertheless, the energy density of an iron phosphate electrode is still limited, which is why the use of nickel and cobalt phosphate as electrode materials (operating at a much higher potential) is currently actively being investigated. Apart from their use as LIB electrodes, these phosphates have also shown to be of interest for a variety of other applications, such as e.g. electrocatalysis.With these applications in mind, the research towards the deposition of these materials also becomes increasingly important. The use of atomic layer deposition (ALD) has for example already allowed for the deposition of a series of metal phosphates, including iron[2] and cobalt phosphate[3]. An ALD process for the deposition of nickel phosphate is however still missing. In this work, a new plasma-enhanced ALD process (PE-ALD) was developed towards the deposition of a nickel phosphate. The process is based on earlier reported process, combining a TMP plasma (TMP*) with an oxygen plasma (O2*) and a metal precursor. When using nickelocene as the metal precursor in this work, saturated growth of an amorphous nickel phosphate was observed at a substrate temperature of 300°C, with a growth per cycle of 0.2 nm/cycle (figure 1). Using XPS, the nickel is thought to be built in with an oxidation state of 2+ while the position of the oxygen and phosphorus peak agree with the formation of a phosphate-like material.This material was then electrochemically characterised as a LIB electrode, together with cobalt phosphate (using a previously reported ALD process[3]). Both materials unfortunately did not result in the high energy (approximately at a potential of 5 V vs. Li+/Li) electrochemical activity that was expected, but did show activity at a lower voltage window (figure 2). The reason for this is expected to be due to the 2+ oxidation state of nickel (and cobalt) in the as-deposited material, while a 3+ oxidation state would be needed for the high energy redox peak to appear. Nevertheless, besides the observation of an electrochemically active nature (at a low voltage) for both materials, this work can help in the research towards enabling the higher energy variant of these phosphates (i.e. by further fine-tuning the proposed ALD processes) and/or other potential applications for these specific materials (such as electrocatalysis, for which the cobalt phosphate used in this work has already shown to result in beneficial behaviour).[1] Masquelier, C., & Croguennec, L. (2013). ChemInform Abstract: Polyanionic (Phosphates, Si...