UNIST team develops novel coated LMO cathode material with resistance to elevated temperatures for EVs
|Schematic view of fabrication process and a spinel particle surrounded by layered phase surface. Credit: ACS, Lee et al. Click to enlarge.|
While a large amount of research is targeting the development of new materials and chemistries for Li-ion batteries and “beyond Li-ion” solutions for electric vehicles, work is also proceeding apace on improving and optimizing materials currently in use for such an application. One such is LiMn2O4 (LMO); the material offers high power and a lower cost per kg, but cell life at elevated temperatures can be problematic. (Earlier post.)
In a paper in the ACS journal Nano Letters, a team from the Ulsan National Institute of Science and Technology (UNIST) in South Korea, led by Dr. Jaephil Cho, reports on a novel heterostructure LiMn2O4 material with an expitaxially grown layered surface phase. This layered surface phase provides an efficient path for ionic and electronic mobility for the host spinel, and also protects the spinel from being directly exposed to the highly active electrolyte. The heterostructure LiMn2O4 exhibited a discharge capacity of 123 mAh g–1 and retained 85% of its initial capacity at the elevated temperature (60 °C) after 100 cycles.
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, LiNi1−x−yCoxMnyO2, 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 LiNi0.5Mn0.5O2 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.
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