Accurate capacity estimation is crucial for the reliable and safe operation of lithium-ion batteries. In particular, exploiting the relaxation voltage curve features could enable battery capacity ...estimation without additional cycling information. Here, we report the study of three datasets comprising 130 commercial lithium-ion cells cycled under various conditions to evaluate the capacity estimation approach. One dataset is collected for model building from batteries with LiNi
Co
Al
O
-based positive electrodes. The other two datasets, used for validation, are obtained from batteries with LiNi
Co
Mn
O
-based positive electrodes and batteries with the blend of Li(NiCoMn)O
- Li(NiCoAl)O
positive electrodes. Base models that use machine learning methods are employed to estimate the battery capacity using features derived from the relaxation voltage profiles. The best model achieves a root-mean-square error of 1.1% for the dataset used for the model building. A transfer learning model is then developed by adding a featured linear transformation to the base model. This extended model achieves a root-mean-square error of less than 1.7% on the datasets used for the model validation, indicating the successful applicability of the capacity estimation approach utilizing cell voltage relaxation.
Lithium-ion batteries (LIBs), with relatively high energy density and power density, have been considered as a vital energy source in our daily life, especially in electric vehicles. However, energy ...density and safety related to thermal runaways are the main concerns for their further applications. In order to deeply understand the development of high energy density and safe LIBs, we comprehensively review the safety features of LIBs and the failure mechanisms of cathodes, anodes, separators and electrolyte. The corresponding solutions for designing safer components are systematically proposed. Additionally, the in situ or operando techniques, such as microscopy and spectrum analysis, the fiber Bragg grating sensor and the gas sensor, are summarized to monitor the internal conditions of LIBs in real time. The main purpose of this review is to provide some general guidelines for the design of safe and high energy density batteries from the views of both material and cell levels.
Graphic Abstract
Safety of lithium
-
ion batteries (LIBs) with high energy density
becomes more and more important in the future for EVs development. The safety issues of the LIBs are complicated, related to both materials and the cell level. To ensure the safety of LIBs, in-depth understanding of the safety features, precise design of the battery materials and real-time monitoring/detection of the cells should be systematically considered. Here, we specifically summarize the safety features of the LIBs from the aspects of their voltage and temperature tolerance, the failure mechanism of the LIB materials and corresponding improved methods. We further review the in situ or operando techniques to real-time monitor the internal conditions of LIBs.
•Charge transfer resistance calculation model considering battery state is derived.•Method to converter charge transfer resistance to any state is proposed.•State of health is estimated with charge ...transfer resistance.
State of health diagnosis and estimation of lithium-ion batteries is a key feature of an advanced battery management system. Electrochemical impedance spectroscopy is frequently used for the state of health diagnosis and estimation. In the paper, the charge transfer resistance of the battery is obtained by fitting the impedance spectroscopy with an equivalent impedance model to estimate state of health. And an analytical calculation model with temperature and state of charge as inputs is derived and verified considering their effect on the charge transfer resistance. With the calculation model, the charge transfer resistance at randomly selected state of charge and temperature is converted to the standard state to be comparable for the state of health estimation. It is indicated that state of health estimated with the converted and the direct fitted charge transfer resistance agree well. The battery state of health estimation method with the converted charge transfer resistance eliminates the need to specifically control the temperature and state of charge during the impedance spectrum measurement. It benefits the practical application of the state of health estimation with electrochemical impedance spectroscopy.
Strongly coupled magnetic resonance (SCMR), proposed by researchers at MIT in 2007, attracted the world's attention by virtue of its mid-range, non-radiative and high-efficiency power transfer. In ...this paper, current developments and research progress in the SCMR area are presented. Advantages of SCMR are analyzed by comparing it with the other wireless power transfer (WPT) technologies, and different analytic principles of SCMR are elaborated in depth and further compared. The hot research spots, including system architectures, frequency splitting phenomena, impedance matching and optimization designs are classified and elaborated. Finally, current research directions and development trends of SCMR are discussed.
18650-type cells with 2.5 Ah capacity are cycled at both 25 °C and 0 °C separately, and at 25 °C two charging protocols (constant current, and constant current-constant voltage charge) are used. ...Differential voltage analysis (dV/dQ) and alternating current (AC) impedance are mainly used to investigate battery degradation mechanisms quantitatively. The dV/dQ suggests that active cathode loss and loss of lithium inventory (LLI) are the dominating degradation factors. Significant microcracks are observed in the fatigued cathode particles from the scanning electron microscopy (SEM) images. Crystal structure parameters of selected fatigued batteries at fully charged state are determined by in situ high-resolution neutron powder diffraction. Obvious increases of ohmic resistance and solid electrolyte interphase (SEI) resistance occur when the battery capacity fade falls beneath 20%. Continuous charge transfer resistance and Warburg impedance coefficient (W.eff) increase are observed in the course of cycling. Correlation analysis is performed to bridge the gap between material loss as well as LLI and impedance increase. The increase of the charge transfer resistance is related to both active cathode loss and LLI, and a functional relationship is revealed between LLI and W.eff regardless of the used cycling protocols.
•18650-type Cells are cycled at 0 °C and 25 °C using two charging protocols.•Main degradation factors are loss of lithium inventory (LLI) and active cathode loss.•Neutron powder diffraction and post-mortem analysis are done for deep understanding.•Correlations between material loss and impedance parameters are revealed.•Warburg impedance coefficient could be correlated to LLI in the course of cycling.
► We use an equivalent circuit model to describe the characteristics of battery. ► A dual time-scale estimator is used to calculate pack average SOC and cell SOC. ► The estimator is based on the ...dynamic descriptions and extended Kalman filter. ► Three different test cases are designed to validate the proposed method. ► Test results indicate a good performance of the method for EV applications.
For the vehicular operation, due to the voltage and power/energy requirements, the battery systems are usually composed of up to hundreds of cells connected in series or parallel. To accommodate the operation conditions, the battery management system (BMS) should estimate State of Charge (SOC) to facilitate safe and efficient utilization of the battery. The performance difference among the cells makes a pure pack SOC estimation hardly provide sufficient information, which at last affects the computation of available energy and power and the safety of the battery system. So for a reliable and accurate management, the BMS should “know” the SOC of each individual cell. Several possible solutions on this issue have been reported in the recent years. This paper studies a method to determine online all individual cell SOCs of a series-connected battery pack. This method, with an equivalent circuit based “averaged cell” model, estimates the battery pack’s average SOC first, and then incorporates the performance divergences between the “averaged cell” and each individual cell to generate the SOC estimations for all cells. This method is developed based on extended Kalman filter (EKF), and to reduce the computation cost, a dual time-scale implementation is designed. The method is validated using results obtained from the measurements of a Li-ion battery pack under three different tests, and analysis indicates the good performance of the algorithm.
Battery impedance is essential to the management of lithium-ion batteries for electric vehicles (EVs), and impedance characterization can help to monitor and predict the battery states. Many studies ...have been undertaken to investigate impedance characterization and the factors that influence impedance. However, few studies regarding the influence of the internal temperature gradient, which is caused by heat generation during operation, have been presented. We have comprehensively studied the influence of the internal temperature gradient on impedance characterization and the modeling of battery impedance, and have proposed a discretization model to capture battery impedance characterization considering the temperature gradient. Several experiments, including experiments with artificial temperature gradients, are designed and implemented to study the influence of the internal temperature gradient on battery impedance. Based on the experimental results, the parameters of the non-linear impedance model are obtained, and the relationship between the parameters and temperature is further established. The experimental results show that the temperature gradient will influence battery impedance and the temperature distribution can be considered to be approximately linear. The verification results indicate that the proposed discretization model has a good performance and can be used to describe the actual characterization of the battery with an internal temperature gradient.
•Describe the aging mechanism of lithium-ion battery with electrochemical kinetics.•Establish the fading rate equation based on Eyring Equation.•The established equation is applicable to any reaction ...order.•Integrate the internal kinetics with external degradation characteristics.
Battery life prediction is one of the critical issues that restrict the development of electric vehicles. Among the typical battery life models, the mechanism model focusing on the internal physical or electrochemical processes has a stronger theoretical foundation and greater accuracy. The empirical formula, which relies on the simplified mechanism, has a concise model structure and more flexibility in vehicle applications. However, the internal aging mechanism rarely correlates with the external operating characteristics.
Based on the summary of the capacity fading mechanism and the reasoning of the internal kinetics of side reactions during the aging process, a lifetime model of the lithium-ion battery is established in this paper. The solutions to the vital parameters based on the external accelerated life testing results are also presented. The testing sample is a manganese oxide lithium-ion battery of 8 Ah. The validation results indicated that the life model established in this paper can describe the capacity fading law of the lithium-ion battery and the operability and accuracy for vehicle applications.
The lack of data samples is the main difficulty for the lifetime study of a lithium-ion battery, especially for a model-based evaluation system. To determine the mapping relationship between the ...battery fading law and the different external factors, the testing of batteries should be implemented to the greatest extent possible. As a result, performing a battery lifetime study has become a notably time-consuming undertaking.
Without reducing the number of testing items pre-specified within the test matrices of an accelerated life testing schedule, a grey model that can be used to predict the cycle numbers that result in the specific life ending index is established in this paper. No aging mechanism is required for this model, which is exclusively a data-driven method obtained from a small quantity of actual testing data. For higher accuracy, a specific smoothing method is introduced, and the error between the predicted value and the actual value is also modeled using the same method.
By the verification of a phosphate iron lithium-ion battery and a manganese oxide lithium-ion battery, this grey model demonstrated its ability to reduce the required number of cycles for the operational mode of various electric vehicles.
•Present a new method for accelerating battery life test.•Establish a residual grey model combining the aging law to describe the battery life trend.•Predict the battery cycle life with a small number of testing samples.
To reveal the impact of alternating current (AC) amplitude on impedance, this study investigates the electrochemical impedance with different AC amplitudes for a lithium-ion battery (NCA vs. ...graphite) and half cells under different states of charge (SOCs), at room and low temperatures. To determine the relationship of different polarization processes between the full cell and half cells, the symmetric cells and the distribution of relaxation times (DRT) are utilized for electrochemical impedance spectroscopy (EIS) analysis. The experimental results indicate that the medium- and low-frequency impedance arcs gradually shrink with the increase of the AC amplitude at low temperatures. DRT focuses on the anode solid electrolyte interphase (SEI), cathode electrolyte interphase (CEI), and charge transfer processes. It is proved that the impedance arc shrinkage is determined by the nonlinear relationship that can be described by the Butler-Volmer equation between current and resistances of the SEI and the charge transfer processes. When the AC amplitude increases to a certain extent, lithium plating also causes impedance arc shrinkage. Moreover, the impedance arc shrinkage of the full cell is mainly affected by the NCA cathode under low SOC. At medium and high SOCs, it is determined jointly by the NCA cathode and graphite anode.
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•The impedances of full and half cells at different AC amplitudes are studied.•The impedance arcs shrink with the increase of the AC amplitude at low temperatures.•The charge transfer and SEI resistances become smaller is the reason.•The Butler-Volmer equation can be used to describe the changes of polarization resistances.
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