Microcrack generation and modification of Ni-rich cathodes for Li-ion batteries: A review
Introduction
In today's society, where fossil fuels are gradually exhausted, lithium-ion batteries (LIBs) have attracted extensive attention due to their high energy density, high operating voltage, long cycle life and no memory effects among most chemical power sources [[1], [2], [3], [4], [5], [6], [7]]. Replacing traditional fossil fuel with lithium-ion power battery has become the research direction of all countries and enterprises globally. Since Sony first commercialized LIBs in 1991, LIBs have been gradually used in modern portable applications such as electronic products and electrical vehicles (EVs). Their significant influence and contribution to modern society have been acknowledged by the 2019 Nobel Prize in chemistry. Although anode materials for LIBs have made considerable development [[8], [9], [10], [11], [12]], cathode materials as one of the key components still face formidable challenges [[13], [14], [15], [16], [17], [18]]. Thus, the main obstacles for the successful commercialization of LIBs are associated with the upper bounds of practical performance for cathode materials.
The cathode materials are generally lithium containing compounds which allow reversible lithium ion insertion/de-insertion, such as conventional layered LiCoO2 (≈160 mAh g−1), spinel LiMn2O4 (≈120 mAh g−1), olivine LiFePO4 (≈170 mAh g−1) [[19], [20], [21], [22]]. In the past decades, LiCoO2 was the most widely used cathode material for lithium-ion batteries owing to its advantages of high working voltage, simple synthesis and long-term cyclability. However, the reversible extraction/insertion amount of lithium ions in the structure is only 0.5 units, and the irreversible phase transition will occur at high state of charge (SOC), resulting in the actual capacity far less than its theoretical capacity (273.8 mAh g−1). In addition, Co is expensive due to scarcity and harmful to the environment, thus other alternative Co-less or even Co-free materials with high energy density at a cheaper cost have emerged as the times require [[23], [24], [25]].
Among these new generation of cathode materials, Ni-rich transition metal layered oxides are regarded as one of the most promising materials, but there are still many obstacles in its theoretical research and practical application [26,27]. Their discharge capacity increases in proportion as the increase of nickel content, along with a series of defects such as structure instability and deteriorated thermal property (Fig. 1b). The main reasons for the cell degradation of nickel-rich cathode materials are bulk structural degradation and interfacial side reactions [28]. And resolutions through bulk or interfacial modification have been proposed to deal with it. [29,30] In recent years, much scholars have found that the microcracks generating in the particles of Ni-rich cathode materials are also related to performance degradation to a great extent [[31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]].
At present, the cathode materials are mainly prepared by co-precipitation and high temperature solid-state method, i.e., the precursor is prepared by co-precipitation, then sintering with mixing lithium, and eventually the corresponding cathode materials are obtained. The synthesized materials are usually micron spherical secondary particles composed of several submicron primary particles. However, as depicted in Fig. 1a, with the increase of charging and discharging cycles of the secondary particles, especially under high voltage, the interface between the primary particles is easy to generate microcracks or pulverize, which improves the interface impedance and polarization. The spherical secondary particles have many internal pores and large area exposing to liquid electrolyte, which leads to more parasitic side reactions that form NiO-like impurity phase (Fig. 1c), severe oxygen release and eventually the deterioration of cycling performance and safety of the cell [42].
Microcracks can be divided into intergranular crack and intragranular crack, where the former occurs along grain boundaries of secondary particles and the latter occurs within the primary particles [31,34]. The formation and continuous propagation of microcracks can induce several malign effects. On the one hand, serious intergranular gaps or even cavities will appear in the secondary particles, which makes some primary particles isolated, and the intergranular gaps lack the effective contact of conductive matrix and electrolyte, thus the isolated particles are inert in electrochemical performance and be ineffective in Li+ extraction/insertion, leading to degradation of the material. On the other hand, when a large number of microcracks occur, the electrolyte will infiltrate into the material along the crack and cause a large area of side reaction, resulting in the formation of a new boundary film at the crack boundary in the particle, resulting in a sharp increase in impedance, which seriously hinders the diffusion of Li+ ion.
To date, several reviews have addressed the progress of degradation mechanism and optimization methods of Ni-rich cathodes, which mostly pay attention to all possible causes and electrochemical performance enhancement. This article exclusively focuses on microcrack and establish an in-depth understanding of its generation mechanism along with its harm to the cell. Besides, we thoroughly summarize the modification strategies of Ni-rich cathode materials for enabling structural stability and consequently maximizing the long-term cycling performance. More importantly, we provide perspectives toward designing the crack-free cathode materials for the nickel-rich LIBs.
Section snippets
Discovery of microcracks
As early as 2000, Dokko et al. [43] observed that LiNiO2 particles broke into several pieces during Li+ insertion through in situ optical microscopy (Fig. 2a). Since then, similar phenomenon has been discovered within Ni-rich cathode, as in the case of LiNi0.8Co0.15Al0.05O2 that be observed by focused ion beam technique [47]. Several microscopic examination methods, such as scanning transmission electron microscopy (STEM) and cross-sectional scanning electron microscope (SEM), were utilized to
Crack-free modification strategies
One of the main ways to solve the problem of poor cycle life and capacity retention of Ni-rich layered cathode materials is to design a material with minimum particle cracking and maintained cycling performance. In order to achieve this goal before commercial application of Ni-rich cathodes for LIBs, numerous strategies have been proposed and applied, including inactive ions doping, intergranular modification, concentration gradient, fabrication of single-crystal materials and dual modification.
Conclusions and outlook
Nickel-rich cathodes are the most promising materials regarding their discernible advantages among the cathode materials in LIBs. Nevertheless, they suffer from structure instability (microcracks formation and even particle fracture) and subsequent rapid capacity fading in their pristine state, particularly during long-term cycling with an upper cutoff voltage. At present, many researches that focus on these downsides have been done to solve them. Herein, the origins of microcracks and
Declaration of Competing Interest
The authors declare that they have no competing financial interest that would affect the work reported in this article.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51604081, 51774333 and 51974368), Hunan Provincial Natural Science Foundation of China (2020JJ2048) and the Fundamental Research Funds for the Central Universities of Central South University (No. 2021zzts0603).
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