High-nickel layered oxide cathodes have become one of the most promising cathode materials in LiB systems, and high-nickel layered materials are often used as cathodes along with graphite anodes. However, full cells using the above electrode materials do not yet guarantee sufficient cycling performance to meet the requirements of complex and demanding consumers. In general, the capacity decay of high-nickel ∥graphite full batteries is caused by various deteriorations occurring in the cathode and anode. For example, capacity decay occurs at the cathode due to crystal orientation, calcination during synthesis, long-range cation disorder, radial ordered arrangement of primary particles, structural reconstruction of rock salt phases with NiO-like surfaces, dissolution of transition metals (TMs), heterogeneity of state of charge (SOC), generation of microcracks, crushing of massive particles, etc. In addition, the growth of the solid electrolyte interface layer (SEI) depletes the available lithium stock (ALI) and dissolved TM on the anode surface, affecting the capacity decay of the full cell after a large number of cycles. In addition, the deposition of excess lithium metal on the surface of the anode and the formation of dendrites also pose the risk of internal short circuit and fire. For example, high temperatures (e.g., 45 °C and 65 °C) lead to a significant increase in internal resistance (Rb, Rct, RSEI) of a high-nickel cathode ∥ graphite full battery system compared to room temperature (RT, 25 °C), with poor cycling and rate performance. An interesting point is that when the high-nickel battery after the cycle is charged and discharged again in the lithium half-cell, the lowered ALI of the high-nickel battery in the whole cell can be largely restored. In addition, high temperatures and upper cut-off voltages increase the amount of TM dissolved in the cathode material, affecting significant capacity loss. A great deal of research on the high-temperature cycling performance of full cells based on high-nickel layer cathode materials is still ongoing, but there is a lack of research on how cathode materials deteriorate in whole battery systems from a crystallographic perspective.
Recently, the team of Professor Won-Sub Yoon of Sungkyunkwan University studied the high-temperature deterioration of high-nickel layered cathodes, which are widely used in electric vehicles. During the refresh process after a long cycle, the significant recovery of the lattice structure occurs mainly in the discharge zone. This pseudo-deterioration of the high-nickel cathode in the whole battery system is mainly due to the consumption of lithium ions that form a solid electrolyte interface layer on the graphite anode during cycling. In addition, irreversible deterioration that cannot be recovered by the refresh process is observed in both the charging and discharging regions. Structural analysis shows that long-term cycling at high temperatures will lead to the decay of charge-discharge mixing, and produce various deterioration phenomena, such as the formation of nickel oxide-like rock salt phase on the cathode surface, and increase the disorder of cations. These findings deepen the understanding of the deterioration behavior of high nickel cathodic in whole battery systems, and provide enlightenment for improving the high temperature cycle performance.
【Points】
Here, the authors focus on the charge-discharge attenuation of LiNi0.85Co0.12Al0.03O2(NCA) after 1000 cycles in a 45 °C full-cell system. Using X-ray photoelectron spectroscopy (XPS) and depth-profiled time-of-flight secondary ion mass spectrometry (TOF-SIMS), the authors investigated the compositional changes of the SEI layer grown on the surface of graphite anodes over long cycles. The study found that growing the SEI layer in the graphite anode consumes a large amount of lithium that would otherwise be re-injected into the cathode. Subsequently, synchrotron radiation-based characterization techniques, including high-resolution powder diffraction (HRPD), hard/soft X-ray absorption spectroscopy (XAS), and transmission X-ray microscopy (TXM), confirmed that lithium depletion in the anode led to the pseudodeterioration of the NCA cathode material. In other words, in a whole-cell system, the consumption of the ALI that forms the SEI layer on the graphite anode results in a recoverable structural change rather than a permanent deterioration of the NCA, hereinafter referred to as "pseudo-degradation". In fact, the NCA deterioration that occurs in a full battery system is largely recovered by the refresh process, especially in the discharge attenuation section. In addition, high temperature cycling creates permanent mixed attenuation at the NCA cathode, called "charge attenuation" and "discharge attenuation". The causes of charge attenuation and discharge attenuation are complex; This study shows that the formation of NiO salt facies on the surface and the increase of cation disorder in volume are some of the factors affecting charge attenuation and discharge attenuation, respectively. The authors believe that this study deepens the understanding of the capacity reduction of high-nickel layered materials in whole-battery systems during high-temperature cycling from a crystallographic perspective.
Figure 1: Electrochemical and structural behavior of long-term cycling NCAs. a) Capacity retention obtained after cycling (C/20, 2 cycles) at a rate of C/2 at 2.8-4.3 V. b) Corresponding charge and discharge voltage curve. c) Lattice parameters of NCA during the first discharge. NCA∥Graphite full cells for cycles 200, 400, and 1000 are superimposed on the lattice parameters of the 1st discharge process at 45°C and appear as differently colored triangles.
Figure 2.Optical and SEM images of NCA and graphite electrodes for the 2nd and 1000th cycles in a full cell at 45°C. Extract areas marked by black and white dotted squares from side optical images to obtain scanning EM data for NCA and graphite electrodes.
Figure 3.TOF-SIMS analysis of SEI layer in cycle 2nd and 1000th cycle states. Normalized depth curves and schematics of the 2nd and 1000th graphite electrode cycles at 45°C, full battery bag cycles.
图 4.a) Ni 2p1/2 和 2p2/3 的 XPS 谱,b) NI- 和 NiF3- 的 TOF-SIMS 3D 图,c) Co 3p 的 XPS 谱,d) CoF3- 的 TOF-SIMS 3D 图。
Figure 5: Structural recovery of NCA after the refresh process. a) Schematic diagram of the refresh process; b) Refinement analysis of XRD patterns for the 1000th and initial cycles of the NCA after refreshing. Each lattice parameter overlaps with the lattice parameters during the first discharge of the fresh NCA (green triangle).
Figure 6.Schematic of charge-discharge attenuation of NCA cathode material in a full-battery system.
【Conclusion】
In this work, the authors found that the high-nickel layer material has a false deterioration phenomenon during the high-temperature long-term cycle of the whole battery system, and the charge and discharge are attenuated at the same time. Through HRPD refinement, it was confirmed that the usable range of charge and discharge of NCA materials in the whole battery system gradually decreased, especially in the discharge part. XPS and TOF-SIMS results show that high-temperature cycling forms a thicker, denser SEI layer on the graphite surface compared to RT. This phenomenon that occurs at graphite anodes reduces the ALI that can be inserted into the cathode. Therefore, after a long cycle at high temperatures, a refresh process is performed to understand the true deterioration of the NCA cathode material. The discharge attenuation that occurred in the whole cell was significantly recovered by the refresh process, suggesting that a large part of the discharge attenuation in NCA is due to the degradation of the ALI due to the growth of the SEI layer in the graphite anode that consumes lithium. However, the charge attenuation shows only a slight recovery after the refresh process compared to the discharge attenuation. This suggests that the formation of NiO-like rock salt facies on the NCA surface observed in soft XAS is thought to be one of the causes of charge attenuation in the whole cell. In the case of discharge, a large amount of recovery is shown during the refresh process, but there is still a considerable part of the discharge attenuation that has not been restored, presumably due to changes in body structure, such as an increase in cation disorder. This means that when the NCA undergoes high-temperature cycling in a full-battery system, charge and discharge attenuation occur simultaneously. The authors' findings provide insight into the degradation mechanism of nickel-like oxide cathodes in whole battery systems at high temperatures, and highlight the importance of adopting strategic approaches to mitigate the effects of high temperatures on battery cycle performance.
https://doi.org/10.1002/aenm.202302209
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