Structural, Electrochemical, and Thermal Properties of Nickel-Rich LiNi_xMn_yCo_zO₂ Materials

Abstract

Nickel-rich LiNi x Mn y Co z O 2 materials (x + y + z = 1, x ≥ 0.6) (NMC) are one of the most promising positive electrode candidates for lithium-ion cells due to their high specific capacity, ease of production, and moderate cost. Conventional NMC materials such as LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442), LiNi 0 . 5 Mn 0 . 3 Co 0 . 2 O 2 (NMC532), Li-Ni 0.6 Mn 0.2 Co 0.2 O 2 (NMC622), etc. have 20% of costly Co among the transition metal atoms. To lower the Co content while maintaining good electrochemical performance, three series of materials with different transition metal ratios, LiNi 0.6 Mn 0.4−x Co x O 2 (x = 0, 0.1, 0.2), LiNi 0.9−x Mn x Co 0.1 O 2 (x = 0.1, 0.2, 0.25), and LiNi 0.8 Mn 0.2−x Co x O 2 (x = 0, 0.1, 0.2), were studied. The materials were synthesized via a coprecipitation/solid state sintering method. Powder X-ray diffraction and electrochemical measurements using coin-type cells were made to characterize the materials. Accelerating rate calorimetry was used to study the reactivity of charged NMC positive electrode materials in the presence of electrolyte at elevated temperatures. NMC721, NMC631, and NMC6.5:2.5:1, which have 50% less Co content than current commercialized NMC622, exhibited excellent specific capacity and thermal stability and therefore deserve careful consideration as next generation materials.

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

“…Moreover, researchers reported that cation disorder can lower spacing between atomic layers in the lattice of the NMC-based cathodes, hinder Li + movement and reduce the amount of active Ni and Li [53]. In the case of higher Ni percentage cathodes, Mn 4+ can partially be replaced by Ni, which increases the valence of Ni and therefore lowers the possibility of Ni atoms migrating to 3a sites [8]. However, because the total amount of Ni also increases, Ni 2+ migration still occurs to a larger extent.…”

“…Accelerating rate calorimetry (ARC) is another method adopted by researchers to measure self-heating rates (SHRs) versus temperature [8,142]. For example, in the comparison of the SHR of 4 types of Ni-rich NMC-based cathodes (Fig.…”

“…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.…”

“…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. Here, studies based on microscopy [21], crystallography [22], mass spectroscopy [23], gas measurements [24] and calorimetry [8] have revealed that parasitic reactions [7], cation mixing (leading to restructured surface regions) [25], active material dissolution [26] and oxygen release [23,25] are primary factors responsible for cathode degradation at the atomic scale. In addition, scanning electron microscopy (SEM) [13] together with computational studies [27] has revealed that intergranular cracking can weaken connections between primary particles.…”

Surface/Interface Structure Degradation of Ni‐Rich Layered Oxide Cathodes toward Lithium‐Ion Batteries: Fundamental Mechanisms and Remedying Strategies

“…a) Contour plot of percent Ni in the Li layer as a function of Ni content and Co content. Reproduced with permission . Copyright 2018, American Chemical Society.…”