Comparison of Single Crystal and Polycrystalline LiNi0.5Mn0.3Co0.2O2Positive Electrode Materials for High Voltage Li-Ion Cells

Abstract

Single-crystal LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) with a grain size of 2–3 μm was compared to conventional polycrystalline un-coated NMC532 and polycrystalline Al₂O₃-coated materials in this work. Studies were made to determine how single crystal NMC532 material with large grain size could be synthesized. Ultra high precision coulometry (UHPC), in-situ gas measurements and isothermal microcalorimetry were used to make comparative studies of the three materials in Li-ion pouch cells. All the diagnostic measurements suggested that the single crystal material should yield Li-ion cells with longer lifetime. Long-term cycling tests verified these predictions and showed that cells with single crystal NMC532 exhibited much better capacity retention than cells with the polycrystalline materials at both 40°C and 55°C when tested to an upper cutoff potential of 4.4 V. The reasons for the superior performance of the single crystal cells were explored using thermogravimetric analysis/mass spectrometry experiments on the charged electrode materials. The single crystal materials were extremely resistant to oxygen loss below 100°C compared to the polycrystalline materials. The major drawback of the single crystal material is its slightly lower specific capacity compared to the polycrystalline materials. However, this may not be an issue for Li-ion cells designed for long lifetime applications.