Revisiting Phase Transformation Mechanisms in LiNi_0.5Mn_1.5O₄ High Voltage Cathodes with Operando Microdiffraction

Abstract

Understanding the phase transition mechanisms of active materials inside Li-ion batteries is critical for rechargeability and optimizing the power/energy density of devices. In this work, high-energy microfocused X-ray diffraction is used to measure in operando the state-of-charge heterogeneities inside a high-voltage spinel (LiMn 1.5 Ni 0.5 O 4 , LMNO) cathode. The structure of an active material which resists complete delithiation is studied to move toward unlocking the full storage capacity of ion-conductive spinels. High-precision diffraction also reveals nonlinear coupling between strain and lithiation state inside the cathode at high voltages, which suggests the phase diagram of this material is more complex than previously assumed. X-ray diffraction depthprofiling shows that large lithiation heterogeneities through the cross-section of the electrode are formed even at low currents and that decoupling these gradients are necessary to study the phase transitions in detail.

Investigating structural properties and reaction mechanism of non-stoichiometric spinel LNMOviasolid state NMR

“…51,52 However, it may contradict studies which claim that incomplete delithiation (only up to 90%) can be obtained with a full charge. 43 The complete delithiation of the spinel structure evinced by ex situ 7 Li here is a strong asset of the LNMO spinel which results in better use of lithium compared to other cathode materials such as NMC.…”

“…Lattice contraction of the Li 0 phase has been observed previously, but its origins remain unclear. 14,18,41,43 Upon discharge the above described changes are reversed, resulting in pattern congruent to pristine. This accentuates the good reversibility of the structural changes of the LNMO spinel phase, in accordance with ndings from ex situ NPD, see Fig.…”

In Situ and Operando X-ray Scattering Methods in Electrochemistry and Electrocatalysis

“…Higher flux allows for PXRD measurements down to the ms scale, which permits the same measurements described above, but over cycling rates relevant to battery operation. Spatially resolved PXRD allows for linescan or low-resolution imaging experiments where lithiation heterogeneity across electrodes can be evaluated . Measurements scanning the plane of the electrode to explore gradients at the mm and cm level require minimal focusing, while focusing toward the 1 μm level allows reactions gradients through the thickness of conventional electrode films to be evaluated.…”

Chemical Limits on X-ray Nanobeam Studies in Water

“…While X-rays typically offer lower spatial resolution than electrons do, their penetrating power seems to allow application to complex environments and operating conditions . This is part of the motivation for recently investing in modern nanofocus instruments at most of the world’s synchrotrons, − enabling operando experiments with a variety of nanobeam techniques. − In particular, BCDI has been increasingly applied to problems in heterogeneous catalysis, electrocatalysis, − and complex electrochemical energy conversion systems. − The BCDI method requires extremely high flux densities when applied to small and industrially relevant nanoparticles, and the recently constructed fourth-generation synchrotron sources are expected to enable this by providing brighter and higher-quality X-ray beams. , …”