Correlation between Chemical and Morphological Heterogeneities in LiNi_0.5Mn_1.5O₄ Spinel Composite Electrodes for Lithium-Ion Batteries Determined by Micro-X-ray Fluorescence Analysis

Abstract

The high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) spinel is a promising material for high-energy battery applications, despite problems of capacity fade. This is due in part to transition metal leaching that produces chemical and morphological inhomogeneities. Using fast micro-X-ray fluorescence spectroscopy to scan the sample at medium spatial resolution (500 nm) over millimeter ranges, effects of cycling rate and state-of-charge on the elemental distribution (Ni and Mn) for LiNi 0.5 Mn 1.5 O 4 /carbon composite electrodes in LNMO/Li cells are visualized. Charge distribution is imaged by mapping the Ni oxidation state by acquisition of a stack of elemental maps in the vicinity of the Ni K edge. Our results show significant effects on morphology and elemental distribution, such as formation of elemental hot-spots and material erosion, becoming more pronounced at higher cycling rates. In nickel hot-spots, we observed hampered oxidation of nickel during charging. ■ INTRODUCTIONThe high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) spinel is a promising material for high-energy battery applications because of its high operation potential of ∼4.7 V versus Li + /Li 0 . However, this high potential lies outside the thermodynamic window of stability for conventional electrolyte solutions used in Li-ion batteries, and therefore presents a critical hurdle toward application. 1 As one result, oxidation of the electrolyte by the active material produces passivating products, causing reduced cycle life and capacity fade of the material. 2−4 Furthermore, Mn-containing phases such as the LiMn 2 O 4 spinel and also the LiNi 0.5 Mn 1.5 O 4 spinel are known to be susceptible to leaching of Mn 2+ and Ni 2+ to the electrolyte, 5 possibly as a consequence of the parasitic reactions with the electrolyte, 6,7 thereby causing elemental inhomogeneities in the electrodes. Formation of hydrofluoric acid, caused by trace amounts of water in the electrolyte, is regarded as one origin for this cathode corrosion. 8,9 The effect on capacity fade is especially pronounced in LNMO/graphite full cells, because of the detrimental contribution of the deposited 3d metals on the anode impacting the solid-electrolyte-interphase (SEI) growth on its surface. 2,10−13 The interplay between Ni and Mn cations leads to complex ordering effects, formation of secondary rock-salt structures, variable Mn 3+ content, and oxygen vacancies that all significantly influence electrochemical performance and are determined by the synthesis conditions. 6,14−16 Detailed investigations of the composition−structure relationship in LNMO spinel showed systematic deviations from the theoretical stoichiometry involving an excess of Mn. To compensate for the formation of excess Mn 3+ , formation of a rock-salt-type structure with a lower Mn/Ni ratio was observed. 17 A systematic study by Choi and Manthiram 5 on manganese dissolution into the electrolyte revealed a correlation with the Mn oxidation state, with samples containing initially more Mn 3+ being prone to the disproportionation reaction...

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