Understanding the Low-Voltage Behavior of Stoichiometric Over Lithiated Spinel Li1+xNi0.5Mn1.5O4: An Electrochemical Investigation

Abstract

Nickel manganese spinel LiNi0.5Mn1.5O4 is one of the most promising candidates for next-generation cobalt-free active materials for cathodes in lithium-ion batteries. Despite the relatively low specific capacity of 147 mAh g−1, its high operating voltage of 4.7 V leads to a high specific energy of 690 Wh kg−1. By extending the operating voltage range from 3.0–4.9 V down to 1.5 V it is possible to access a lithiation degree up to x = 2.5 and a theoretical specific capacity of 346 mAh g−1. However, this causes pronounced capacity fading. Typical voltage profiles show unexpected additional step at about 2.1 V, which cannot be explained by open circuit measurements. We applied several electrochemical methods to investigate the lithiation of highly-ordered, stoichiometric spinel at low-voltages. Mixed potential measurements provided a comprehensive explanation for the low-voltage behaviour and supports interpretation of diffusion coefficients, rate capability tests, discharge at different temperatures and impedance spectroscopy. We show that anodic and cathodic partial reactions within the electrode can explain the presence of the additional 2.1 V step. This is caused by the kinetically favoured formation of the phase Li2.5Ni0.5Mn1.5O4 and the simultaneous re-transformation to the thermodynamically stable phase Li2Ni0.5Mn1.5O4.