Understanding the Capacity Loss in LiNi_0.5Mn_1.5O₄–Li₄Ti₅O₁₂ Lithium-Ion Cells at Ambient and Elevated Temperatures

The high-voltage spinel LiNi_0.5Mn_1.5O₄ (LNMO) is an attractive positive electrode because of its operating voltage around 4.7 V (vs Li/Li⁺) and high power capability. However, problems including electrolyte decomposition at high voltage and transition metal dissolution, especially at elevated temperatures, have limited its potential use in practical full cells. In this paper, a fundamental study for LNMO∥Li₄Ti₅O₁₂ (LTO) full cells has been performed to understand the effect of different capacity fading mechanisms contributing to overall cell failure. Electrochemical characterization of cells in different configurations (regular full cells, back-to-back pseudo-full cells, and 3-electrode full cells) combined with an intermittent current interruption technique have been performed. Capacity fade in the full cell configuration was mainly due to progressively limited lithiation of electrodes caused by a more severe degree of parasitic reactions at the LTO electrode, while the contributions from active mass loss from LNMO or increases in internal cell resistance were minor. A comparison of cell formats constructed with and without the possibility of cross-talk indicates that the parasitic reactions on LTO occur because of the transfer of reaction products from the LNMO side. The efficiency of LTO is more sensitive to temperature, causing a dramatic increase in the fading rate at 55 °C. These observations show how important the electrode interactions (cross-talk) can be for the overall cell behavior. Additionally, internal resistance measurements showed that the positive electrode was mainly responsible for the increase of resistance over cycling, especially at 55 °C. Surface characterization showed that LNMO surface layers were relatively thin when compared with the solid electrolyte interphase (SEI) on LTO. The SEI on LTO does not contribute significantly to overall internal resistance even though these films are relatively thick. X-ray absorption near-edge spectroscopy measurements showed that the Mn and Ni observed on the anode were not in the metallic state; the presence of elemental metals in the SEI is therefore not implicated in the observed fading mechanism through a simple reduction process of migrated metal cations