Disassembly Method for Failed Lithium-ion Batteries
Aging failure of lithium-ion batteries is a common issue, with the decline in battery performance primarily attributed to chemical degradation reactions at the material and electrode levels (Figure 1).
- Electrode degradation includes the membrane on the electrode surface, pore blockage, and electrode cracks or adhesive failures.
- Material degradation involves membrane formation on particle surfaces, particle cracking, particle detachment, structural transformation on particle surfaces, dissolution, and migration of metal elements, among others.
Material degradation, for instance, can lead to capacity decay and increased resistance at the battery level.
Therefore, a comprehensive understanding of the degradation mechanisms occurring within the battery is crucial for analyzing failure mechanisms and extending battery life.
This article summarizes the methods for disassembling aged lithium-ion batteries and the physical-chemical analytical techniques used to analyze disassembled battery materials.
Figure 1 Overview of Aging and Failure Mechanisms in Lithium-Ion Battery Electrode and Material Degradation, Along with Common Analytical Methods.
1. Battery Disassembly Method
The analysis process of disassembling an aged and failed battery is illustrated in Figure 2, and it includes the following main steps:
(1) Pre-inspection of the battery.
(2) Discharge to the cut-off voltage or a specific state of charge (SOC).
(3) Transfer to a controlled environment, such as a dry room.
(4) Disassemble and open the battery.
(5) Separate various components, such as the positive electrode, negative electrode, separator, electrolyte, etc.
(6) Perform physical and chemical analysis on each component.
Figure 2 Disassembly Analysis Process of Aged and Failed Battery
1.1 Pre-inspection and Non-destructive Testing Before Disassembly of Lithium-Ion Battery
Before disassembling the cells, non-destructive testing methods can provide preliminary insights into battery degradation mechanisms. Common testing methods include:
(1) Capacity Testing:
The aging state of the battery is typically characterized by the State of Health (SOH), which is the ratio of the discharge capacity at time t to the discharge capacity at t=0.
Since the discharge capacity depends on temperature, Depth of Discharge (DOD), and discharge current, operational conditions are periodically checked to monitor SOH, such as at 25°C, DOD 100%, and discharge rate of 1C.
(2) Differential Capacity Analysis (ICA):
Differential capacity, represented by the dQ/dV-V curve, can convert voltage plateaus and inflection points in the voltage curve into dQ/dV peaks. Monitoring changes in dQ/dV peaks (peak intensity and shift) during the aging process provides information on active material loss/electrical contact loss, battery chemical changes, insufficient discharge/charge, lithium plating, and more.
(3) Electrochemical Impedance Spectroscopy (EIS):
During the aging process, the battery impedance typically increases, leading to slower kinetics, which is a partial cause of capacity decay.
The increase in impedance is due to internal physical and chemical processes in the battery. For example, an increase in resistance, possibly primarily caused by the Solid Electrolyte Interphase (SEI) on the anode surface. However, battery impedance is influenced by many factors and requires modeling and analysis using an equivalent circuit.
(4) Visual Inspection, Photo Documentation, and Weighing:
These are also routine operations for analyzing aging lithium-ion batteries. These inspections can reveal external deformations or leakage issues with the battery, which may also be factors influencing aging behavior or leading to battery failure.
(5) Non-destructive Testing Inside the Battery:
This includes X-ray analysis, X-ray computed tomography (CT) scans, and neutron tomography, among others.
CT can reveal many internal details of the battery, such as deformations inside the battery after aging, as shown in Figures 3 and 4.
Figure 3 Non-destructive Characterization Examples of Lithium-Ion Batteries
a) X-ray transmission image of a jellyroll battery.
b) Frontal CT scan near the positive terminal of an 18650 battery.
Figure 4 Axial CT Scan of a Deformed Jellyroll 18650 Battery
1.2 Disassembly of Lithium-Ion Batteries at Fixed SOC and Controlled Environment
Before disassembly, the battery must be charged or discharged to a specified state of charge (SOC).
From a safety perspective, it is advisable to perform a deep discharge (until the end-of-discharge voltage is 0 V) to reduce the risk of thermal runaway in case of a short circuit during disassembly. However, deep discharge may cause undesirable material changes.
Therefore, in most cases, the battery is discharged to SOC = 0% before disassembly. Sometimes, a small amount of charging may be considered for disassembling the battery, depending on research requirements.
Battery disassembly is generally conducted in a controlled environment to minimize the impact of air and moisture, such as in a dry room or a glovebox.
1.3 Lithium-Ion Battery Disassembly Procedure and Component Separation
During the battery disassembly process, it is necessary to avoid external and internal short circuits.
After disassembly, the positive electrode, negative electrode, separator, and electrolyte are separated. The specific disassembly process is not detailed here.
1.4 Post-Processing of Disassembled Battery Samples
After the various components of the battery are separated, the samples are washed with a typical electrolyte solvent such as DMC.
This step helps remove any residual crystals of LiPF6 or non-volatile solvents, reducing the corrosion of the electrolyte. However, the cleaning process may also impact subsequent test results.
For instance, washing may lead to the loss of specific SEI components, and DMC rinsing can remove insulation materials deposited on the graphite surface after aging.
It is generally recommended to wash the samples twice for approximately 1-2 minutes each with pure solvent to eliminate trace amounts of Li salts.
Additionally, all disassembly analyses should be washed in the same manner to obtain comparable results.
ICP-OES analysis can be performed using the active material scraped from the electrode, and this mechanical treatment does not alter the chemical composition.
XRD can also be applied to electrode or scraped powder materials. However, the particle orientation present in the electrode is lost in the scraped powder, potentially leading to differences in peak intensity.
To investigate cracks in the active material, cross-sections of the entire lithium-ion battery can be prepared (as shown in Figure 4).
After cutting the battery, the electrolyte is removed, and sample preparation is carried out using epoxy resin and metallographic polishing steps.
In comparison to CT imaging, cross-sectional analysis of the battery can utilize optical microscopes, focused ion beams (FIB), and scanning electron microscopes, providing significantly higher resolution for specific parts of the battery.
2. Post-Disassembly Physicochemical Analysis of Battery Materials
Figure 5 illustrates the main analysis scheme for batteries along with corresponding physicochemical analysis methods.
Test samples can be sourced from the anode, cathode, separator, current collector, or electrolyte. Solid samples can be taken from various parts, including the electrode surface, interior, and cross-section.
Figure 5 Lithium-Ion Battery Internal Components and Physicochemical Characterization Methods
Figure 6 presents specific analysis methods, including:
(1) Optical Microscopy (Figure 6a).
(2) Scanning Electron Microscopy (SEM, Figure 6b).
(3) Transmission Electron Microscopy (TEM, Figure 6c).
(4) Energy-Dispersive X-ray Spectroscopy (EDX, Figure 6d), often used in conjunction with SEM to obtain information about the sample’s chemical composition.
(5) X-ray Photoelectron Spectroscopy (XPS, Figure 6e), which allows the analysis and determination of oxidation states for all elements (excluding H and He) and their chemical environments.
XPS is surface-sensitive, capable of characterizing chemical changes on particle surfaces. XPS can be combined with ion sputtering for depth profiling.
(6) Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, Figure 6f) is used to determine the elemental composition of the electrode.
(7) Glow Discharge Optical Emission Spectroscopy (GD-OES, Figure 6g), a depth-profiling technique that provides elemental analysis of the sample by sputtering and detecting visible light emitted by sputtered particles excited in the plasma.
Unlike XPS and SIMS methods, GD-OES depth profiling is not limited to near the surface of particles but can analyze from the electrode surface to the current collector. Thus, GD-OES provides comprehensive information from the electrode surface to the electrode volume.
(8) Fourier Transform Infrared Spectroscopy (FTIR, Figure 6h): This method involves the interaction of the sample with infrared radiation, collecting high-resolution data within a selected spectral range.
By applying Fourier transform to the signal, actual spectra are created to analyze the chemical characteristics of the sample. However, FTIR cannot perform quantitative analysis of compounds.
(9) Secondary Ion Mass Spectrometry (SIMS, Figure 6i): Characterizes the elemental and molecular composition of material surfaces, a surface-sensitive technique that helps determine the nature of electrochemical passivation layers or coatings on current collectors and electrode materials.
(10) Nuclear Magnetic Resonance (NMR, Figure 6j): It is capable of characterizing materials and compounds in both solid-state and solvent dilution.
It also can provide not only chemical and structural information but also information about ion transport properties, migration rates, electron, magnetism, and thermodynamic and kinetic characteristics
(11) X-ray Diffraction (XRD, Figure 6k): This technique is commonly used for structural analysis of active materials in electrodes.
(12) Chromatographic Analysis (Figure 6l): The basic principle is to separate components in a mixture and subsequently detect them, used for electrolyte and gas analysis.
Figure 6: Schematic Diagram of Particles Detected in Different Analytical Methods
3. Electrochemical Analysis of Recombined Electrodes
3.1 Reassembly for Lithium-Half Cells
Failed electrodes can undergo electrochemical analysis through reassembly into lithium-half cells. For double-sided coated electrodes, one side of the coating must be removed. Electrodes obtained from fresh cells and those extracted from aged cells are reassembled using the same method for research. Electrochemical tests can yield the residual (or remaining) capacity of the electrode and measure reversible capacity.
For the negative electrode/lithium cell, the first electrochemical test should involve lithium removal from the negative electrode. In contrast, for the positive electrode/lithium cell, the initial test should discharge to embed lithium into the positive electrode lithiation. The corresponding capacity represents the remaining capacity of the electrode. To obtain reversible capacity, the negative electrode in the half-cell undergoes lithiation again, while the positive electrode undergoes delithiation.
3.2 Reassembly of Full Cells Using Reference Electrodes
Construct a complete cell using an anode, cathode, and additional reference electrode (RE) to obtain the potentials of the anode and cathode during the charging and discharging processes.
In summary, each physicochemical analysis method can only observe specific aspects of lithium-ion degradation.
Figure 7 outlines the functionalities of physicochemical analysis methods for materials after the disassembly of lithium-ion batteries. In terms of detecting specific aging mechanisms, the table shows that the methods highlighted in green have good capabilities, those in orange have limited capabilities, and those in red have no capability.
It is evident from Figure 7 that different analysis methods have broad capabilities, but no single method can cover all aging mechanisms. Therefore, it is recommended to use various complementary analysis methods to comprehensively understand the aging mechanisms of lithium-ion batteries.
Figure 7 Overview of Detection and Analysis Method Capacities
Waldmann, Thomas, Iturrondobeitia, Amaia, Kasper, Michael,et al. Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques[J]. Journal of the Electrochemical Society, 2016, 163(10):A2149-A2164.