Nickel Metaphosphate as a Conversion Positive Electrode for Lithium‐Ion Batteries

Abstract Lithium storage schemes based on conversion chemistry have been used in a large variety of negative electrodes achieving capacities 2–3 times higher than graphite. However, to date, relatively few positive electrode examples have been reported. Here, we report a new conversion positive electrode, Ni(PO 3 ) 2 , and systematic studies on its working and degradation mechanisms. Crystalline Ni(PO 3 ) 2 undergoes an electrochemistry‐driven amorphization process in the first discharge to form a fine microstructure, consisting of Ni domains ∼2 nm wide that form a percolating electron‐conducting network, embedded in a glassy LiPO 3 matrix. P does not participate electrochemically, remaining as P 5+ in [PO 3 ] − throughout. The electrode does not recrystallise in the following first charge process, remaining amorphous over all subsequent cycles. The low ionicity of the Ni−[PO 3 ] bond and the high Li + conductivity of the LiPO 3 glass lead to high intrinsic electrochemical activity, allowing the micro‐sized Ni(PO 3 ) 2 to achieve its theoretical capacity of 247 mAh/g. The performance of the Ni(PO 3 ) 2 electrode ultimately degrades due to the growth of larger and more isolated Ni grains. While the theoretical capacity of Ni(PO 3 ) 2 is itself limited, this study sheds new light on the underlying chemical mechanisms of conversion positive electrodes, an important new class of electrode for solid‐state batteries.