Research Background
Lithium-ion batteries (LIBs) have become indispensable energy storage devices for portable electronic products, electric vehicles and smart grids. Despite their widespread application and great success, battery aging remains a critical challenge for manufacturers and users. To further improve the cycling performance and realize lifespan prediction of lithium-ion batteries, an in-depth understanding of battery aging mechanisms and rapid evaluation of long-term cycling performance are of vital importance. However, battery aging stems from the coupling of complex internal chemical and electrochemical reactions in batteries, presenting a complex one-to-many correspondence with aging mechanisms. In addition, aging mechanisms evolve dynamically with the degradation of battery performance, which greatly increases the difficulty of relevant research. Current studies on battery aging mechanisms mainly focus on qualitative analysis, lacking quantitative investigation of aging mechanisms and evaluation of the dynamic evolution process of aging mechanisms in practical battery systems. In this study, mass spectrometry titration (MST) and nuclear magnetic resonance (NMR) techniques were adopted to quantify the composition and evolution of inactive lithium in practical battery systems during long-term cycling, revealing the rapid aging mechanism of batteries under high-temperature conditions. The research findings of this work facilitate the understanding of battery aging and provide a valuable theoretical basis for battery performance optimization and lifespan prediction.
Research Brief Introduction
From the perspective of active lithium loss analysis, this study systematically conducts a quantitative analysis on the high-temperature aging mechanism of lithium iron phosphate//graphite batteries. Combined with multiple characterization techniques including mass spectrometry titration, nuclear magnetic resonance, cryo-electron microscopy and neutron imaging, six types of inactive lithium components and their contents were quantitatively determined, and their evolution laws during long-term cycling were clarified. The failure mechanism of lithium-ion batteries under high-temperature aging was further revealed in depth.
Key Research Highlights
Highlight 1: Revelation of the three-stage electrochemical aging process induced by high temperature
The electrochemical performance of lithium iron phosphate//graphite batteries cycled for 2000 times at 25 ℃, 45 ℃ and 65 ℃ indicates that batteries aged at 25 ℃ and 45 ℃ undergo an activation stage and a linear attenuation stage, while an additional accelerated attenuation stage is observed for batteries aged at 65 ℃. In the linear attenuation stage, the average capacity attenuation per cycle at 25 ℃, 45 ℃ and 65 ℃ is 0.0077 mAh, 0.0155 mAh and 0.0266 mAh respectively. Taking the average capacity attenuation at 25 ℃ as the benchmark, the attenuation rates of batteries tested at 45 ℃ and 65 ℃ are 2.01 times and 3.45 times higher, respectively. In the Stage III accelerated attenuation stage, the average capacity attenuation per cycle of batteries at 65 ℃ reaches 0.0655 mAh, with an attenuation rate 8.51 times that of the Stage II batteries at 25 ℃. Thermodynamic analysis demonstrates a strong linear correlation between battery capacity loss and active lithium loss (y=x, R²>0.999). This indicates that active lithium loss is the dominant mechanism responsible for the capacity degradation of lithium iron phosphate//graphite batteries under temperature-accelerated aging conditions.
Highlight 2: Quantitative analysis of six types of inactive lithium components
Mass spectrometry titration (MST) was used to quantitatively analyze the content and evolution of inactive lithium species in batteries, including Li/LixC6 (x<1), ROCO2Li, Li2C2, LiH and RLi (CH3Li, C2H3Li, C2H5Li and C3H5Li). Elevated temperatures not only increase the total amount of inactive lithium but also accelerate the growth of LixC6 and LiH. Nuclear magnetic resonance was employed to quantitatively detect anions in batteries at different aging states and evaluate the inorganic solid electrolyte interphase (SEI) derived from the decomposition of electrolyte salt anions. In the activation stage (the first 100 cycles), SEI formation is mainly dominated by the reduction of PF6⁻ rather than FSI⁻. In the accelerated aging stage of batteries aged at 65 ℃, anion loss is primarily caused by FSI⁻ consumption, and massive PF6⁻ loss induces the formation of unstable SEI, which further aggravates FSI⁻ depletion and accelerates battery degradation. All inactive lithium losses were classified into three categories: inorganic SEI, organic SEI and LixC6. Inorganic SEI is the dominant factor leading to battery capacity loss. High temperature promotes the growth of all inactive lithium components, and the rapid proliferation of inorganic components induces irreversible LixC6 loss and organic SEI growth, thereby triggering the rapid aging of batteries.
Highlight 3: Qualitative analysis of inactive lithium and kinetic aging analysis
Neutron imaging results show that the residual liquid electrolyte in battery gas pockets decreases with increasing temperature, indicating that electrolyte consumption intensifies at elevated temperatures. The neutron transmittance of electrode regions decreases with the rise of test temperature, demonstrating that more SEI is formed under high-temperature conditions. The SEI generated in high-temperature aged batteries aggregates into 300–400 nm clustered structures with a reduced Young's modulus distribution, indicating poor mechanical stability of high-temperature formed SEI. Electrochemical impedance analysis of full cells and symmetric cells reveals that the kinetic variations of lithium iron phosphate//graphite batteries are mainly attributed to the kinetic changes of the anode. Charge transfer impedance, especially the negative electrode charge transfer impedance (Rct), serves as the kinetic limiting step of lithium iron phosphate//graphite batteries. Batteries cycled at a high temperature of 65 ℃ exhibit increased generation of parasitic reaction byproducts, leading to rapid deterioration of kinetic performance.
Highlight 4: Evolution laws of different inactive lithium components
Systematic analysis reveals that the SEI evolution patterns are similar at all three temperatures. Specifically, LiF derived from anion decomposition acts as the primary SEI component in the initial aging stage, while the proportion of ROCO2Li increases continuously with aging progression. Distinct differences are observed in the evolution of irreversible LixC6. At 25 ℃, due to the kinetic activation process, the proportion of irreversible LixC6 is the highest in the early aging stage and gradually decreases with cycling. In contrast, the proportion of irreversible LixC6 keeps increasing during cycling at 65 ℃. The irreversible formation of LixC6 is strongly affected by battery kinetic performance, which can be adopted as an indicator to evaluate the dynamic characteristics of the aging process. Furthermore, the accelerated aging period observed in 65 ℃ samples corresponds to the rapid consumption of FSI⁻.
