The commercialization of lithium ion batteries for the electric vehicles (EVs) requires a cathode material with high energy and power, high thermal stability, low cost, and other criteria such as excellent cycle life and low ion and electronic transport resistance. To meet the above requirement, many research groups have extensively and intensively investigated many possible cathode materials such as LiCoO₂, LiNi_1−x−yCo_xMn_yO₂, LiMn₂O₄, and LiFePO₄ for the applications in EVs’ batteries. Among many cathode candidates, the LiMn₂O₄ has been considered as one of the most promising cathode materials to be used for EVs due to its advantages of low cost, abundance, environmental affinity, and low safety hazard, which are the key factors for a large scale EV battery.

However, it still suffers from a fast capacity fading at 60 °C because the manganese on the surface of the LiMn₂O₄ dissolves in the liquid electrolyte solution containing acidic species. To solve this problem, many researchers have attempted to have the LiMn₂O₄ surface coated by inorganic materials such as Al₂O₃, AlPO₄, AlF₃, ZrO₂, and SiO₂. The coating materials acted as a protective layer that prevents the host LiMn₂O₄ from being exposed directly to the electrolyte. However, the coating layer also acted as a resistance layer when its thickness is too large, which results in deteriorating electrochemical performance. For this reason, the electrochemically and electrically active materials have been considered as potential coating candidates for the LiMn₂O₄ cathode.

—Lee et al.

In prior work, Cho’s group found that an electrically conductive carbon coated on the surface of the LiMn₂O₄ was very effective for improving electrochemical performances and stability at the room temperature. However, when the carbon coated LiMn₂O₄ was tested at the high temperature of 60 °C under the same experimental condition, its electrochemical performances deteriorated rapidly.

Attempts by other researchers using other electrochemically active surface coating materials also have not fared well at the elevated temperature. Thus, said the UNIST team, there is a need for a new approach to designing an optimized coating layer for the LiMn₂O4 cathode that can be operated in harsh and high temperature EV environments.

In addition to morphology, thickness, and electronic and electrochemical properties, the coating layer needs to satisfy an additional requirement such as a chemical affinity between a host material and a surface layer. This condition may provide a better sustainable coating layer on its host cathode material without forming any crystal defect so that it can be operated in a harsh environment for a longer time. For this reason, the Mn-rich layered phase has been considered as one of the potential coating candidates for the LiMn₂O₄ cathode. The Mn rich layered phase that consists of Mn₄⁺ such as LiNi_0.5Mn_0.5O₂ was additionally considered to eliminate the possibility of the Mn₂⁺ dissolution in electrolyte. After many experimental trials and errors, herein, we reported a new class of the heterostructured spinel cathode.

—Lee et al.

The researchers used a simple method of spray drying coating solution and further heat treatment; the Mn-rich layered phase (<10 nm thick) was epitaxially grown on the surface of the spinel LiMn₂O₄ host cathode without forming any defect. The developed coated material showed excellent electrochemical performances at various temperature ranges relative to the bare spinel LiMn₂O₄.

(a) Charge and (b) discharge capacity retention of bare LMO (BLMO) and epitaxially grown layer coated LMO (EGLMO) as a function of various C rates from 0.5 C (65 mA g−1) to 10 C (1300 mA g−1) between 3.0 and 4.3 V at 24 °C. Discharge profiles of (c) BLMO and (d) EGLMO as a function of temperature at 0.1 C rate. Credit: ACS, Lee et al. Click to enlarge.

Resources

• Min-Joon Lee, Sanghan Lee, Pilgun Oh, Youngsik Kim, and Jaephil Cho (2014) “High Performance LiMn₂O₄ Cathode Materials Grown with Epitaxial Layered Nanostructure for Li-Ion Batteries,” Nano Letters doi: 10.1021/nl404430e