Elucidating the Nature of Secondary Phases in LiNi0.5Mn1.5O4 Cathode Materials using Correlative Raman-SEM Microscopy

LiNi_0.5Mn_1.5O₄ (LNMO) is a promising next-generation cathode material for lithium-ion batteries (LIBs) due to its high-energy and high-power density. However, its commercial adoption is hindered by the unstable LNMO/electrolyte interface due to high operating voltages and structural degradation arising from Jahn-Teller distortion and metal-ion dissolution resulting in poor cycling stability. Additionally, the high-temperature calcination beyond 700°C often results in secondary phases such as rock salt NiO, Li_1-xNi_xO, Ni₆MnO₈ or Li₂MnO₃, whose precise chemical compositions and their influence on electrochemical performance remain unclear. Traditional analytical techniques such as X-ray diffraction (XRD) or neutron diffraction face challenges in resolving these secondary phases due to low phase fractions and overlapping reflections with the LNMO phase. Here, we address these challenges using correlative Raman-Scanning electron microscopy (Raman-SEM) to characterize secondary phases in LNMO materials that were synthesized under various synthesis conditions and evaluate their impact on the electrochemical performance. Our results reveal the synthesis-dependent emergence of three distinct secondary phases in LNMO materials synthesized at 1000°C, a phenomenon that, to our knowledge, has not been previously reported. Specifically, LNMO synthesized at 900°C shows the coexistence of Ni₆MnO₈ and Li₂MnO₃ phases, while synthesized at 1000°C also exhibits a Mn₃O₄ phase. Furthermore, an increased amount of these secondary phases in LNMO led to a lower discharge capacity due to their electrochemical inactive nature. However, these phases do not affect the rate capability or the long-term cycling performance of the LNMO materials. These insights are crucial for advancing the development of LNMO cathode materials for next-generation LIBs.