Investigation of Olivine and Nasicon Structured Phosphates As Potential Cathode Materials for Magnesium Ion Batteries
Authors: Romio Martina , Cupid Damian Marlon , Kahr Jürgen , Surace Yuri , Jahn Marcus , Abrahams Isaac 
Published: February 2020

The depletion of fossil fuel reserves as well as climate change motivate the development of new environmentally-friendly technologies for energy generation, storage, and consumption. Robust energy storage devices must be developed to meet the growing demand for portable electronics (i.e. laptops and mobile phones), electric vehicles and energy storage systems. Although lithium-ion batteries have mostly driven the revolution in modern electronics for the last three decades, new chemistries for “next-generation” batteries are currently under research and development. These new technologies are tasked with delivering better performance (capacity and current capability) at low cost while being environmentally friendly. In order to fill the gap between the current state of the art and future energy storage needs, magnesium is considered to be a possible alternative to lithium due to its natural abundancy and low toxicity. Furthermore, due to the divalent nature of the Mg2+ cations, a high theoretical volumetric capacity of 3833 mA h cm-3 can be attained if pure magnesium is used as the anode material.1 Additionally, since magnesium deposition takes place without dendrite formation on the Mg-metal anode surface, the Mg-cell is expected to be considerably safer than comparable cells based on lithium.
Despite these positive attributes, the development of magnesium secondary batteries is hindered by the slow solid-state diffusion of Mg2+ ions into the cathode lattice. In this work, this issue is explored by synthesising and characterising two different phosphate-based systems to investigate their suitability as cathode materials for Mg-ion batteries, with phosphates chosen due to their structural stability and safety.2,3 The NASICON and olivine phosphates are known for their high conductivity in lithium-based systems and were selected here as potential hosts for Mg2+intercalation.
NASICON compounds are usually reported as cathode or solid electrolyte materials for Li and Na-ion batteries, but there is a lack of reproducible data for Mg ion technologies. Therefore, the NASICON-structured Mg0.5Ti2(PO4)3 compound was selected for synthesis and further electrochemical characterisation. Additionally, the olivine-related (Mg0.5Ni0.5)3(PO4)2 was investigated as a novel cathode active material for the first time. This compound was chosen because of its open crystal structure, which is potentially favourable for magnesium ion diffusion. In parallel, V2O5 was used as a reference cathode active material in order to validate the electrochemical set-up, which is non-trivial for Mg chemistry.
Mg0.5Ti2(PO4)3 and (Mg0.5Ni0.5)3(PO4)2 were obtained by the solid-state method and their crystal structures and phase purities were determined by Rietveld analysis of X-ray powder diffraction data. Scanning electron microscopy (SEM) was performed to analyse the particle morphologies, with particle sizes and porosities characterised by dynamic light scattering (DLS) and nitrogen adsorption (BET) techniques, respectively. Both cyclic voltammetry and galvanostatic cycling with potential limitation were carried out in order to investigate the insertion-extraction mechanism of Mg2+ ions. Ex-situ powder X-ray diffraction and SEM confirmed the crystal structures and particle morphologies, respectively, of the cathode active materials after cycling, while X-ray photon electron spectroscopy (XPS) was used to monitor changes in the oxidation state of the transition metal cation.
Our results show that cells based on Mg metal anodes utilising Mg(ClO4)2 in acetonitrile as electrolyte suffer from the creation of a passivation layer leading to poor performance. In order to address this problem, activated carbon was used to replace metallic magnesium as the anode material, with the electrolyte acting as the main source for magnesium ions. The use of a carbon-based anode is seen to eliminate the passivation layer problem and improve capacity and cell cyclability.

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