A Comparative Mechanistic Study on the Intercalation Reactions of Mg²⁺ and Li⁺ Ions into (Mg_0.5Ni_0.5)₃(PO₄)₂

Magnesium-ion batteries represent promising environmentally sustainable energy-storage systems with higher energy densities than their lithium counterparts. In this work, the charge storage mechanisms of the olivine-related compound (Mg_0.5Ni_0.5)₃(PO₄)₂ using Mg²⁺ and Li⁺ ions were investigated and compared for the first time when copper was chosen as the current collector. A comprehensive physicochemical and electrochemical characterization was performed on the pristine powder and electrodes at different states of charge. Although (Mg_0.5Ni_0.5)₃(PO₄)₂ is electrochemically active, it undergoes irreversible conversion reactions in both Mg and Li chemistries. The conversion reactions proceed with an ionic exchange between structural Ni²⁺ and Mg²⁺ or Li⁺ cations, which results in the formation of sarcopside-Mg₃(PO₄)₂, a Cu–Ni alloy and poorly crystalline Li₃PO₄, respectively. A capacity of 600 mA h g⁻¹ was achieved with a Li metal counter electrode in the Li cell since the conversion reaction could go to completion. A capacity of 92 mA h g⁻¹ was delivered in the Mg cell using an activated carbon counter electrode. These findings shed light on the fundamental mechanism of activity in olivine-related compounds, underlining the importance of performing systematic studies to unveil the complex interactions between both single-valent and multivalent ions with novel structures.