Yujian
Sun‡
ab,
Xilin
Xu‡
ab,
Luyu
Gan‡
a,
Sichen
Jiao
ab,
Shuangshuang
Han
c,
Yajun
Zhao
a,
Yan
Li
c,
Xiqian
Yu
*ab,
Jizhou
Li
*de,
Hong
Li
*ab and
Xuejie
Huang
ab
aBeijing Frontier Research Center on Clean Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail: xyu@iphy.ac.cn; hli@iphy.ac.cn
bCenter of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
cCollege of New Materials and Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102627, China
dDepartment of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong 999077, China. E-mail: lijz@ieee.org
eCUHK Shenzhen Research Institute, Shenzhen 518057, China
First published on 14th January 2025
Thermal safety remains a critical concern in the commercialization of lithium-ion batteries (LIBs), with extensive research dedicated to understanding the thermal behaviors of cathode materials. While a wealth of thermochemical test data is available in the literature, the variability in sample conditions and experimental testing parameters complicates the identification of fundamental relationships between the intrinsic properties and thermochemical reaction characteristics of materials. This study utilizes explainable machine learning (ML) methodologies to tackle this challenge by analyzing a comprehensive database derived from published differential scanning calorimeter (DSC) testing results. By employing meticulously curated, augmented, and filtered features that characterize material properties, sample conditions, and testing parameters, we leveraged ML models to predict and validate thermochemical reaction characteristics across the chemical compositional space of layered oxide cathode materials. Through the explainability, we elucidated multidimensional relationships between input features and thermochemical reaction characteristics, revealing that material properties predominantly dictate the initiation of the reaction, while external conditions exert a greater influence on the kinetics of heat release. This approach demonstrates the effectiveness of ML in decoding complex causal factors of cathode thermochemical reaction behaviors, thereby offering valuable insights for targeted thermal optimization in battery safety design.
Broader contextThermal safety concerns present a critical barrier to the widespread commercialization of high-energy density lithium-ion batteries. Despite extensive experimental studies and thermal testing data across diverse cathode materials, the heterogeneity in testing conditions and incomplete parameter documentation have impeded comprehensive analysis of the literature database. This study addresses this challenge through a systematic analysis of hundreds of thermal testing data points from the published literature, employing explainable machine learning to decode the complex relationships between thermal behavior characteristics and influential factors, including material properties, electrode conditions, and testing parameters. Our analysis reveals dynamic shifts in factor contributions throughout thermal reactions, suggesting the need for stage-specific thermal optimization strategies. These findings underscore the importance of standardized thermal testing protocols for establishing comprehensive databases conducive to future data-driven analyses. |
Recent advancements in machine learning (ML) have catalyzed the integration of data-driven methodologies within experimental science.11–16 These approaches leverage ML models to uncover underlying relationships from heterogeneous experimental datasets characterized by unevenly distributed features in high-dimensional spaces, thereby surmounting the constraints imposed by stringent variable control that is typical of traditional experiments.17 In the context of layered oxide cathode materials (Li1+xTM1−xO2, where TM represents 3d and 4d transition metal elements), which exhibit a broad compositional space, this paradigm has demonstrated to be highly effective in elucidating critical correlations between material composition and electrochemical performance, even in the presence of experimental noise.18,19 However, establishing high-fidelity experimental datasets from scratch is both labor-intensive and costly, leaving data scarcity to present a significant challenge for experimental data learning.20–24 Masalkovaitė et al. recently demonstrated the efficacy of transfer learning in predicting complex fractional heat output data from readily accessible mass ejection data and cell manufacturing specifications. The model's ability to generalize across diverse commercial cell types with minimal additional training data underscores its potential in addressing data scarcity challenges in battery thermal analysis.25 Despite the significance of this work, a more fundamental investigation linking intrinsic material physical and electrochemical properties to thermal stability remains conspicuously absent.
In this study, we systematically investigated experimental data of the cathode thermal behaviors in LIBs reported in the existing literature using a data-driven ML approach. We compiled nearly six hundred thermal stability data points for cathode materials, including lithium cobalt oxide (LiCoO2, denoted as LCO), nickel–cobalt–manganese ternary materials (LiNixCoyMnzO2, x + y + z = 1, denoted as NCM) and lithium-rich layered oxide (denoted as LR), extracted from DSC results in the literature. A comprehensive analysis was conducted on multiple causal features, including the material's intrinsic physical and chemical properties, electrode sample conditions, and DSC testing parameters. Our investigation primarily concentrated on the correlation between these features and thermal behaviors, characterized by critical metrics such as the onset and peak temperatures of heat release, along with the maximum heat release power. Utilizing the trained ML model, we identified the optimal composition of NCM materials concerning thermal stability by minimizing the influence of sample and testing conditions. Additionally, through the application of explainable ML techniques, we elucidated the dynamic evolution of feature contributions to thermochemical characteristics throughout the reaction process.
The initial step of feature selection entailed the removal of redundant features that displayed significant correlations, as these can give rise to multicollinearity, resulting in unstable regression outcomes, diminished generalization capabilities, and hindered interpretability of the model. Although Pearson coefficients are commonly utilized for multicollinearity detection, their assumption of continuous and normally distributed variables does not universally apply to our dataset. Consequently, we selected Spearman coefficients, which are more suitable for identifying general monotonic relationships across varied data distributions.26,27Fig. 1a presents the heatmap of Spearman correlation coefficients for all evaluated feature variables, with deep red or blue data points indicating strongly positively or negatively correlated feature pairs, respectively. For instance, the Spearman coefficient between “Ni content” and “radii-TM ave. (wt.)” is −0.98, signifying a robust negative monotonic relationship. This correlation is illustrated in Fig. 1b, where the data points are aligned along a monotonic line. This observation can be attributed to the smaller ionic radii of fully oxidized Ni4+ (0.48 Å) compared to Co4+ and Mn4+ (both 0.53 Å). Given that these three TM species dominate our dataset, the weighted average radii of TM are predominantly influenced by the stoichiometric ratio of Ni. In contrast, Fig. 1c demonstrates the uncorrelated relationship between Ni content and the cutoff voltage of the charged electrode sample for the DSC test. Following the principle of maintaining simpler and more fundamental features, we eliminated redundant variables based on a criterion of |Spearman coefficient| > 0.8. A detailed list of the features that have been eliminated is presented in the ESI.†
Further feature refinement was conducted using the null importance concept, which quantifies feature contribution to model performance when its relationship with the target is randomized.28 For each target variable, we evaluated the actual gain importance of features using a random forest (RF) model implemented in the LightGBM package, subsequently generating 100 null importance values for each feature through independent shuffling. Fig. 1d illustrates the null importance distribution of Ni content in predicting heat release onset temperature. The significant decrease in importance post-shuffling demonstrates the feature's genuine relevance. Conversely, Fig. 1e shows that the actual importance of the DSC scanning speed does not exceed its null importance, indicating its irrelevance to the exothermic onset temperature. To maintain a comprehensive feature set for subsequent analysis, we employed a conservative screening strategy that excludes only those features whose actual importance is below the first quartile (25%) of the null importance values. The null importance feature screening approach aims to mitigate any interference that may hinder the precision of feature contribution analysis and model interpretation. It is important to note that this process was implemented independently for each target output variable, which may lead to divergent feature sets across various predictions. For instance, while DSC scanning temperature was excluded for onset temperature inference, it was preserved for maximum heat release power prediction. Detailed feature screening results are provided in the ESI.†
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Fig. 2 Training and testing results of the selected features for the target thermochemical characteristic outputs of (a) onset temperature, (b) peak temperature, and (c) max power. |
Fig. 3a illustrates that compositions with balanced stoichiometric ratios of the three TM species exhibit elevated onset temperatures, indicating delayed thermal reactions. This aligns with the reported excellent thermal stability of LiNi1/3Co1/3Mn1/3O2.8 Conversely, mono-TM materials located at the triangle vertices (LiCoO2, LiMnO2, andLiNiO2) display relatively low onset temperatures, suggesting inferior thermal safety. Notably, the high-Ni region exhibits very low onset temperatures, reinforcing the prevailing understanding of the significant thermal safety challenges associated with high-Ni NCM ternary materials despite their elevated reversible capacity. It is worth noting that while an increased Mn content typically enhances structural and thermal stability, this trend does not apply in extreme cases such as LiMnO2, which has been reported to experience severe phase transition and oxygen release during cycling.29 Peak temperature predictions (Fig. 3b) mirror onset temperature trends, with Ni content exerting a more dominant influence. The composition corresponding to the highest peak temperature shifts towards lower Ni content, suggesting that high Ni content not only triggers early onset but also accelerates the thermal reaction process. Intriguingly, the predicted maximum heat release power map (Fig. 3c) diverges from the previous two. While the high Ni vertex remains the most thermally active, the high-Mn area exhibits the lowest power, indicating a suppression of reaction severity. We attribute this phenomenon to the spinel phase transition process occurring in Mn-rich materials during thermochemical reactions, which results in a delayed concentrated heat release.30 In summary, our analysis verifies the critical influence of Ni content on all three thermochemical characteristics, underscoring its significance in material thermal behaviors.
To validate our predictions, we juxtaposed our results against experimental data from two independent studies31,32 that were not incorporated into our training or testing datasets. We emulated the sample conditions and DSC testing parameters from these works, adjusting solely the nickel content to align with their experimental design. The charging cutoff voltages in these studies were 4.3 V and 4.4 V, respectively. Our predicted onset (Fig. 3d) and peak (Fig. 3e) temperatures exhibited a strong correlation with the observed values. However, significant discrepancies were noted in the maximum power predictions (Fig. 3f). We hypothesize that the significant discrepancies between the predicted and reported maximum power values may be attributed to the high sensitivity of heat release power to external factors, such as the use of a not hermetically sealed crucible during DSC measurements and the incorporation of unknown additives in the electrolytes. This hypothesis is substantiated by the feature significance analysis results discussed in the subsequent sections of this paper. Given the complexity of these external factors, which are often inadequately detailed in the literature, it is possible that we have overlooked certain experimental or sample preparation conditions that could influence the DSC test results. Nonetheless, we contend that under consistent sample and testing conditions, the observed variation trends in our predictions still yield valid insights into the thermal behavior of the materials, as evidenced in Fig. 3f.
The influences of features related to material properties are consistently observed across all three thermochemical characteristics, while the impacts of features associated with sample preparation and DSC testing conditions may vary. We compared the SHAP value distributions of several representative features, as illustrated in Fig. 4d–g. The most significant DSC testing parameter affecting onset and peak temperatures is the cutoff voltage to which the samples were charged. Higher cutoff voltages negatively impact both characteristic temperatures, aligning with the understanding that highly oxidized cathode materials are more reactive in thermal contexts. However, this influence diminishes for maximum power, as indicated by the SHAP value distribution showing no significant variation across different cutoff voltages (Fig. 4d), which we will address later.
Anomalous behavior is noted concerning the electrolyte content added during the DSC test. As shown in Fig. 4e, increased electrolyte content leads to earlier peak temperatures and higher power, likely due to the flammability of organic components in electrolytes. Conversely, the presence of electrolytes is anticipated to postpone the onset of thermochemical reactions, potentially explained by pre-reactions occurring between the electrolytes and cathode materials, which may safeguard the cathode from interacting with the electrolytes. Such moderate reactions may not produce prominent thermal release peaks in DSC results but can prolong the reaction and defer the heat release peak. Moreover, modifications in electrolyte composition significantly influence the thermal behavior of cathode materials. Fig. 4f indicates that electrolyte additives enhance battery thermal performance by increasing onset and peak temperatures while reducing heat release power, even though these additives primarily aim to improve electrochemical performance rather than thermal safety. We propose that improved thermal safety is a favorable byproduct of the enhanced stability of high-voltage interfaces achieved through the incorporation of electrolyte additives. Lastly, the type of crucible used in the DSC test is relevant; as shown in Fig. 4g, a sealed crucible is generally correlated with higher onset and peak temperatures, as well as increased instantaneous heat release power when compared to an open crucible.
It is important to note that SHAP values elucidate the relationships between input features and output characteristics; however, these correlations do not necessarily indicate causation. In the case of the crucible type, the observed correlation with maximum heat release power conflicts with the reported experimental phenomenon that thermal reactions proceed more gently under sealed systems,40 therefore lacking a clear physical explanation. This issue can be approached from a data-driven perspective. Sealed crucibles used in DSC tests typically prevent the volatilization of electrolytes, creating an underlying collinearity with electrolyte content. The presence of electrolytes significantly enhances reaction intensity, thus linking closed crucibles to higher heat release power. This underscores the necessity of feature screening prior to regression analysis. Nevertheless, we cannot assure that all influential factors have been comprehensively considered or adequately described in the existing literature. Therefore, we cautiously ascribe SHAP value distributions that exhibit no clear trends to experimental data noise rather than making arbitrary conclusions.
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Fig. 5 Variation trends of feature significance to thermochemical characteristics corresponding to different stages of the thermal runaway reaction. |
Upon a detailed examination of the red-colored features, we note that they are all related to material properties. Although the cutoff voltage is primarily regarded as a parameter in DSC testing, herein, it plays a crucial role in determining the actual lithium content within the delithiated material and influences the oxidation states of transition metal cations.41 In contrast, the green-colored features pertain to sample conditions and DSC testing parameters. Notwithstanding the constraints posed by data scarcity and fidelity, we successfully delineated a fundamental understanding of the thermochemical reaction process of the layered oxide cathode materials. Specifically, the initial phases of the thermochemical reaction are predominantly governed by intrinsic material properties, while external conditions progressively dictate the dynamics of the reaction as it evolves. This understanding highlights the critical importance of managing various factors at distinct stages of the thermochemical reaction process. The conclusion also implies the need for a multifaceted approach towards thermal safety and considerations for optimizing materials.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4eb00025k |
‡ These authors contributed equally to this work. |
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