Hysteretic Phenomena between Charging and Discharging in LiNi_0.5Mn_1.5O_4-σ

Abstract

Clarification of the mechanism of hysteretic phenomena between charging and discharging in LiNi 0.5 Mn 1.5 O 4-σ under a closedcircuit voltage is crucial for achieving a high energy efficiency. We performed in situ X-ray diffraction to understand the relationship between the overpotential and the kinetically preferred state of LiNi 0.5 Mn 1.5 O 4-σ during charging and discharging. Very different crystal structures were observed for the cathode during charging and discharging, especially in the low state-of-charge single-phase reaction region. We consider that charging can be a lithium extraction process through the large lattice spacing, and lattice shrinkage occurs just after lithium diffusion, which enables a continuous reaction. Furthermore, we consider that discharging corresponds to the continuous insertion of lithium, requiring successive "forcible" lattice expansion for the reaction to occur. A kinetic reaction model for the cathode was proposed based on these results to explain the hysteresis for charging and discharging under a closed-circuit Spinel-type cathode materials containing Ni ions (LiNi 0.5 Mn 1.5 O 4−σ (σ ≥ 0): LNMO) are attractive candidates for lithiumion batteries because they exhibit a high theoretical energy density and high operation voltage, and they are cost-effective and nontoxic.1-4 Disordered LiNi 0.5 Mn 1.5 O 4−σ (d-LNMO) is known to have a superior rate performance, 5 and to react via a single phase, mixed/unresolved phases, and two structural phase processes with different lattice parameters according to the lithium concentrations. [6][7][8] Thermodynamically, a clear phase separation may be undesirable in this material because of the coherency strain, which causes defect formation. 9-11Kim et al. reported a large hysteresis for charging and discharging in d-LNMO and ordered LNMO (o-LNMO) under a closed-circuit voltage (CCV).5 This hysteresis depends on the deviation of the overpotential from the open-circuit voltage (OCV), especially during high rate operation, 5 which significantly reduces the energy efficiency. However, the hysteresis in d-LNMO is much less than that in o-LNMO. This hysteresis depends on the deviation of the overpotential from the open-circuit voltage (OCV). The deviation is high during discharging.Singer et al. employed X-ray diffraction (XRD) to investigate the hysteretic phenomena for the phase transition of d-LNMO.9 They directly observed the hysteretic phenomena in crystal structure at the single nanoparticle level. Their study certainly provides new avenues for understanding the fundamental reaction mechanism. Meanwhile, the data indicate that most of the phase transition proceeds during the last stage of charging and discharging, even though the operation rate is extremely small (1/4 C). They discussed the large discrepancy between the electrochemical data and XRD patterns due to the surface-dependent model based on the fact that electrochemistry is surface dependent and XRD patterns are volume-dependent. However, their explanation does not explain the...

Revisiting phase transformation mechanisms in LiNi0.5Mn1.5O4 high voltage cathodes with operando microdiffraction

“…The trends observed are qualitatively in agreement with many previous reports of in situ XRD on LMNO. 18,27,29,32,43 These changes are roughly symmetrical upon discharge, with a small hysteresis detected in the peak position.…”

“…18 However, several in situ diffraction studies on LMNO do not observe any untransformed phases, despite having a detection limit sufficient to observe them. 21,29,32 These contradictory results may be attributed to differences in the crystal morphology, crystal quality, crystallite size, pre-cycling, and charging rate of the LMNO used in each study, which lead to substantial differences in solid state reactivity. 46 One of the possible explanations is that some small fraction of LMNO crystallites experience strong heterogeneity in lithiation, with large barriers to phase transitions.…”

“…22,28 Significant variations in the competition of the solid-solution vs phase segregation mechanisms during both phase transitions have been reported by many groups. This has led to contradictory mechanisms being proposed including single-phase, 18,29 two-phase, 21,23,30 and even three phase 16,27 coexistence inside individual crystals. The relatively flat voltage curve of LMNO places all of these states very close in energy, which makes experimentally probing the phase diagram difficult.…”

Revisiting Phase Transformation Mechanisms in LiNi0.5Mn1.5O4 High Voltage Cathodes with Operando Microdiffraction

“…One major objective is to understand and maximize the solid-solution behavior between these three states during operation, since sharp phase segregation is well-known to slow down kinetics and accelerate degradation. , Significant variations in the competition of the solid-solution vs phase segregation mechanisms during both phase transitions have been reported by many groups. This has led to contradictory mechanisms being proposed including single-phase, , two-phase, ,, and even three phase , coexistence inside individual crystals. The relatively flat voltage curve of LMNO places all of these states very close in energy, which makes experimentally probing the phase diagram difficult.…”

“…Many groups have repeatedly noted the difficulty of extracting more than around 90% of the Li ions from spinel active materials based on electrochemical and Li stoichiometry measurements. ,,,, In an extreme case, Kan et al showed that a carefully annealed LMNO had dramatically lowered capacity, where less than 20% of the Li was capable of being extracted in the first cycle, and with the capacity gradually increasing on subsequent cycles . However, several in situ diffraction studies on LMNO do not observe any untransformed phases, despite having a detection limit sufficient to observe them. ,, These contradictory results may be attributed to differences in the crystal morphology, crystal quality, crystallite size, precycling, and charging rate of the LMNO used in each study, which lead to substantial differences in solid state reactivity …”