Investigating the Reversibility of Structural Modifications of Li_xNi_yMn_zCo_1–y–zO₂ Cathode Materials during Initial Charge/Discharge, at Multiple Length Scales

Abstract

1In this work, we investigate the structural modifications occurring at the bulk, sub-2 surface, and surface scales of Li x Ni y Mn z Co 1-y-z O 2 (NMC; y, z = 0.8, 0.1 and 0.4, 0.3, 3 respectively) cathode materials during the initial charge/discharge. Various analytical tools, 4such as X-ray diffraction, selected-area electron diffraction, electron energy-loss 5 spectroscopy, and high-resolution electron microscopy, are used to examine the structural 6properties of the NMC cathode materials at the three different scales. Cut-off voltages of 4.3 7 and 4.8 V are applied during the electrochemical tests as the normal and extreme conditions, 8respectively. The high-Ni-content NMC cathode materials exhibit unusual behaviors, which 9is deviated from the general redox reactions during the charge or discharge. The transition 10 metal (TM) ions in the high-Ni-content NMC cathode materials, which are mostly Ni ions, 11 are reduced at 4.8 V, even though TMs are usually oxidized to maintain charge neutrality 12 upon the removal of Li. It was found that any changes in the crystallographic and electronic 13 structures are mostly reversible down to the sub-surface scale, despite the unexpected 14 reduction of Ni ions. However, after the discharge, traces of the phase transitions remain at 15 the edges of the NMC cathode materials at the scale of a few nanometers (i.e., surface scale). 16This study demonstrates that the structural modifications in NMC cathode materials are 17 induced by charge as well as discharge, at multiple length scales. These changes are nearly 18 reversible after the first cycle, except at the edges of the samples, which should be avoided 19 because these highly localized changes can initiate battery degradation.

Degradation Mechanisms and Mitigation Strategies of Nickel-Rich NMC-Based Lithium-Ion Batteries

“…Lattice Expansion/Contraction NMC oxide possesses an R3m structure (rhombohedral symmetry) with a Li-layer on the 3a site, an NMC layer on the 3b site and an oxygen layer on the 6c site [7] and can usually be indicated in the splitting of (110) and (108) peaks and of (006) and (102) peaks. For example, the expansion and contraction of the c-axis (14.21 Å) of NMC811 lattices can be observed by using in situ XRD measurements in the cell voltage windows of 3.0-4.0 V and 4.0-4.4 V, in which (003) peaks can decrease from 18.96° (3.0 V) and subsequently rise to 19.09° (4.4 V) [7,10,43]. This can also be observed for the movement of (104), (015) and (108) peaks.…”

“…However, although increasing Ni percentages can allow for enhanced Ni-rich NMC-based battery performances as compared with other NMC combinations in terms of capacity, capacity fading becomes severely pronounced. To address this, significant efforts have been made to elucidate the mechanisms of capacity fading and associated growth of impedance [7,8], as well as to find remedies [9][10][11][12]. And in recent decades, numerous studies and reviews have been published worldwide [13][14][15] in which various testing protocols [16], research methodologies, models [17][18][19] and physical characterizations [9,20] have been used to interpret the behavior of NMC-based cathodes with or without modifications.…”

Kinetic Stability of Bulk LiNiO2 and Surface Degradation by Oxygen Evolution in LiNiO2‐Based Cathode Materials

“…2019, 9,1802586 conventional sol-gel method as described in the Experimental Section. LiNiO 2 powders were synthesized using Adv.…”

“…[6,9,10,15] At the surface of layered oxides, carbonaceous molecular species and fluorine-based Li salts in the electrolyte can react with the cathode powder surface, which can induce undesirable interface phase formation or accelerate the oxide phase transition at surface. In the past two decades, LiCoO 2 has dominated the LIB market as the predominant commercial cathode material.…”

Determination of the mechanism and extent of surface degradation in Ni-based cathode materials after repeated electrochemical cycling

“…References of the ∆E and L 3 /L 2 indices are also provided in Figure 2(d), which are acquired from the sub-surface of initially charged and discharged NMC433 particles. 22 We chose the discharged state after the first electrochemical cycle as the reference rather than the pristine state. According to the chargedischarge curves [ Figure 1(a)], there is a considerable irreversible capacity loss during the first charge/discharge process, which is attributed to the formation of electrochemically inactive domains.…”