Suppressing High‐Current‐Induced Phase Separation in Ni‐Rich Layered Oxides by Electrochemically Manipulating Dynamic Lithium Distribution

Abstract

cathode materials and improve their stability and energy density. [1,2] In particular, Ni-rich NCM (LiNi x Co y Mn 1−x−y O 2 , x > 0.5) is a promising cathode material with high reversible capacity and has been successfully implemented in commercial energy storage systems, such as mobile devices and electric vehicles. [1] For Ni-rich layered oxides, the whole reaction pathway has been described as proceeding through a series of isostructural hexagonal phases, conventionally labeled as H1, H2, and H3 phases depending on the depth of charge. [3,4] The fundamental difference between H1 and H2 is the Li-content-dependent in-plane structure in the Li layer. [3,5] Because of the structural similarities of these evolving phases during cycling, the solid solution reaction has been predicted to be a thermodynamically favorable reaction pathway by first-principle studies [6] and was also observed by operando X-ray diffraction (XRD) experiments at moderate cycling rates in Ni-rich layered oxides, such as LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) [7] and LiNi 0.8 Co 0.1 Mn 0.1 O 2 . [3,4] Homogeneous Li transport, characterized by solid solution behaviors, is considered to be advantageous for obtaining a long cycle lifetime. [8] However, in dynamic situations such as fast cycling, limited Li diffusivity can induce a heterogeneous Li distribution within Understanding the cycling rate-dependent kinetics is crucial for managing the performance of batteries in high-power applications. Although high cycling rates may induce reaction heterogeneity and affect battery lifetime and capacity utilization, such phase transformation dynamics are poorly understood and uncontrollable. In this study, synchrotron-based operando X-ray diffraction is performed to monitor the high-current-induced phase transformation kinetics of LiNi 0.6 Co 0.2 Mn 0.2 O 2 . The sluggish Li diffusion at high Li content induces different phase transformations during charging and discharging, with strong phase separation and homogeneous phase transformation during charging and discharging, respectively. Moreover, by exploiting the dependence of Li diffusivity on the Li content and electrochemically tuning the initial Li content and distribution, phase separation pathway can be redirected to solid solution kinetics at a high charging rate of 7 C. Finite element analysis further elucidates the effect of the Li-content-dependent diffusion kinetics on the phase transformation pathway. The findings suggest a new direction for optimizing fast-cycling protocols based on the intrinsic properties of the materials.

Alleviating Anisotropic Volume Variation at Comparable Li Utilization during Cycling of Ni-Rich, Co-Free Layered Oxide Cathode Materials

“…Closer observation of the peak profiles of reflections at the beginning of discharge (Figure b) shows there is some broadening at high states of charge, particularly as the Li content progresses from 0.138 < x (Li) < 0.25. This may be related to phase segregation within the cathode material into Li-rich and Li-poor phases at high states of delithiation and at high current densities as a result of different Li diffusivity between the two phases. − This behavior has been previously observed in various Ni-rich materials with Ni content as low as y = 0.6 and thus could be expected to occur in all of the materials presented here …”

“…70−72 This behavior has been previously observed in various Ni-rich materials with Ni content as low as y = 0.6 and thus could be expected to occur in all of the materials presented here. 73 To compare and quantify the evolution of reflections observed in the diffraction data, Figure 4 shows the refined unit cell parameters of the NiMn9010 and NiCo9010 materials during the initial cycle, with identical axes used on both graphs. The quantitative evolution of lattice parameters was determined using sequential Rietveld refinements.…”

Failure mechanism of LiNi0.6Co0.2Mn0.2O2 cathodes in aqueous/non-aqueous hybrid electrolyte

“…The basis for the estimation is that a theoretical specific capacity of 276.5 mA h g −1 for NCM622 (ref. 25 and 26) corresponds to 100% delithiation, and ≈58% (at a cut-off voltage of 2.8 V) and ≈63% (at a cut-off voltage of 2.9 V) of the Li + were extracted from NCM622, respectively, which was calculated using the specific charge capacities of 160 and 173 mA h g −1 based on the cathode active material mass (Fig. S3†).…”

An Innovative Insight into Performance Degradation of NCM111 Cathode Induced by Suspension of Operation

“…On the other hand, the bulk structure of NCM suffers from a series of phase transitions over repetitive Li-(de)­intercalation, − which is another key factor to trigger capacity decay. Upon initial charging, an H1–H2 transition between two hexagonal phases (space group R 3̅ m ) emerges. , Typically, although the H1–H2 transition could lead to a huge microstructural strain, this transition only occurs during the first charging process and is generally considered not to affect the subsequent cycles and have little impact on capacity decay.…”

“…On the other hand, the bulk structure of NCM suffers from a series of phase transitions over repetitive Li-(de)­intercalation, − which is another key factor to trigger capacity decay. Upon initial charging, an H1–H2 transition between two hexagonal phases (space group R 3̅ m ) emerges. , Typically, although the H1–H2 transition could lead to a huge microstructural strain, this transition only occurs during the first charging process and is generally considered not to affect the subsequent cycles and have little impact on capacity decay. When further charging occurs above 4.3 V, another H2–H3 phase transition occurs, − accompanied by large interlayer contraction and significant anisotropic volumetric change.…”