Electrochemical Performance of Lithium Ion Battery Cells
Core indicators determining energy output, cycle stability, and application reliability of lithium ion battery cells
The electrochemical performance of lithium ion battery cells represents core indicators that measure their energy output, cycle stability, and application reliability. These performance metrics cover multiple dimensions including electromotive force, internal resistance, voltage characteristics, capacity, charge-discharge behavior, cycle life, self-discharge, and storage performance. These properties are interrelated and collectively determine the practical application value of lithium ion battery cells. Their accurate testing and analysis are critical aspects of battery research, production, and application.
For researchers, manufacturers, and end-users of lithium ion battery cells, understanding these electrochemical properties is essential for optimizing design, ensuring quality control, and selecting appropriate battery solutions for specific applications. This comprehensive overview explores each performance parameter in detail, explaining its significance, measurement methods, and practical implications for lithium ion battery cells.
Key Performance Dimensions
Electromotive Force
Internal Resistance
Voltage Characteristics
Capacity
Charge-Discharge Behavior
Cycle Life
Self-Discharge
Storage Performance
Electromotive Force
The electromotive force (EMF) of lithium ion battery cells is the inherent potential difference in a thermodynamically equilibrium state, determined by the chemical potential difference between positive and negative electrode materials and the electrolyte interface characteristics. It is independent of battery size and charge-discharge state.
For example, the EMF of a lithium iron phosphate/graphite system is approximately 3.2V, while that of a ternary material (NCM)/graphite system can reach 3.6-3.8V. The influence of temperature on electromotive force follows the Nernst equation, with its value typically changing by ±1-2mV for every 10℃ increase.
Testing of lithium ion battery cells requires a high-impedance (≥10¹²Ω) voltmeter, measuring the open circuit voltage after the battery has been stationary for 24 hours. This avoids electrode polarization interference and ensures that the data reflects the true thermodynamic equilibrium state.
EMF Comparison in Different Lithium Ion Battery Cells
Key Considerations for EMF Testing
- Use high-impedance voltmeters (≥10¹²Ω) to prevent current flow during measurement
- Ensure lithium ion battery cells are fully rested (minimum 24 hours) before measurement
- Maintain constant temperature during testing to avoid thermal effects on readings
- Perform multiple measurements and calculate averages for improved accuracy
- Calibrate equipment regularly to ensure measurement precision
Internal Resistance
The internal resistance of lithium ion battery cells is the total resistance that impedes current flow within the battery, consisting of ohmic internal resistance and polarization internal resistance. Understanding and measuring this parameter is crucial for evaluating the performance and suitability of lithium ion battery cells for specific applications.
Ohmic Internal Resistance
Originates from:
- Electrode substrate resistance
- Electrolyte ion conduction resistance
- Current collector contact resistance
Can be quickly obtained through Direct Current Resistance (DCR) testing
Polarization Internal Resistance
Consists of:
- Charge transfer resistance (electrochemical reaction resistance)
- Diffusion resistance (lithium ion migration lag)
Requires analysis using Electrochemical Impedance Spectroscopy (EIS) in the 0.01Hz-100kHz frequency range
Internal resistance directly affects the high-current discharge capability of lithium ion battery cells. For example, power lithium ion battery cells typically need to have their internal resistance controlled below 50mΩ to avoid voltage drops under high current conditions.
In EIS analysis, the Nyquist plot's semicircle diameter corresponds to charge transfer resistance, while the斜线部分反映扩散过程。 Engineers and researchers use this information to optimize electrode materials, electrolytes, and cell designs for improved performance of lithium ion battery cells.
Nyquist Plot Example
Voltage Characteristics
Voltage and voltage characteristics are direct reflections of the operating state of lithium ion battery cells. The operating voltage changes dynamically during charge and discharge processes. During charging, it gradually rises from the cut-off voltage (e.g., 4.2V), while during discharge, it drops from the open circuit voltage to the termination voltage (e.g., 2.5V).
Voltage characteristics are manifested through charge-discharge curves, including the length of the plateau voltage (reflecting energy output stability) and slope changes (reflecting the uniformity of lithium intercalation/deintercalation in materials).
Testing requires high-precision (±0.1mV) charge-discharge equipment to record voltage values at different State of Charge (SOC) levels, plotting voltage-SOC curves. The wider the plateau region, the more stable the energy output of the lithium ion battery cells.
Charge-Discharge Voltage Curves
Key Voltage Parameters for Common Lithium Ion Battery Cells
| Battery Chemistry | Nominal Voltage (V) | Charge Cut-off Voltage (V) | Discharge Cut-off Voltage (V) |
|---|---|---|---|
| LFP (LiFePO₄)/Graphite | 3.2 | 3.65 | 2.5 |
| NCM 523/Graphite | 3.6 | 4.2 | 2.75 |
| NCM 622/Graphite | 3.65 | 4.2 | 2.75 |
| NCM 811/Graphite | 3.7 | 4.3 | 2.75 |
| LCO/Graphite | 3.7 | 4.2 | 2.5 |
Capacity
Capacity characterizes the charge storage capability of lithium ion battery cells and is one of the most critical performance metrics for users and manufacturers alike. It is essential for determining the runtime and energy density of devices powered by lithium ion battery cells.
Rated Capacity
The designed capacity of lithium ion battery cells, specified by the manufacturer as the nominal capacity under standard test conditions.
Actual Capacity
The measured discharge capacity of lithium ion battery cells during testing, which may differ from the rated capacity due to various factors.
Specific Capacity
The capacity per unit mass or volume of active materials in lithium ion battery cells, a key indicator of material efficiency.
Capacity of lithium ion battery cells is affected by several factors:
-
Charge-Discharge Rate
At 1C rate, capacity is typically the rated value, but at 2C rate, it may drop to 80% for some lithium ion battery cells.
-
Temperature
At -20℃, capacity may be reduced to only 50% of the room temperature capacity for many lithium ion battery cells.
-
Cycle Count
Capacity gradually decreases with increasing number of charge-discharge cycles for all lithium ion battery cells.
Standard Capacity Testing Protocol
- Constant current charge at 0.5C to cut-off voltage
- Switch to constant voltage charge until current ≤0.05C
- Rest for 1 hour
- Constant current discharge at 0.5C to termination voltage
- Record discharge capacity as the actual capacity
Capacity vs. Discharge Rate
Capacity vs. Temperature
Charge-Discharge Characteristics
The charge-discharge characteristics of lithium ion battery cells include charge-discharge rate (C-rate), efficiency, and voltage response. These characteristics determine how effectively and quickly lithium ion battery cells can be charged and discharged, directly impacting their practical application.
Efficiency Metrics
Coulombic Efficiency
The ratio of discharge capacity to charge capacity, expressed as a percentage.
High-quality lithium ion battery cells can achieve over 99% efficiency in the initial cycles.
Coulombic Efficiency = (Discharge Capacity / Charge Capacity) × 100%
Energy Efficiency
The ratio of discharge energy to charge energy, expressed as a percentage.
Typically lower than coulombic efficiency for lithium ion battery cells due to internal resistance losses.
Energy Efficiency = (Discharge Energy / Charge Energy) × 100%
Efficiency vs. Cycle Number
Rate Characteristics
Testing of lithium ion battery cells must be conducted under different rates (0.2C to 5C) for charge-discharge cycles, recording voltage, current, and time at each stage. This allows calculation of efficiency and analysis of rate characteristics - the higher the capacity retention rate at high rates, the better the fast-charging performance of the lithium ion battery cells.
C-rate Definitions for Lithium Ion Battery Cells
Cycle Performance
Cycle performance reflects the rate of capacity degradation of lithium ion battery cells after multiple charge-discharge cycles. It is measured by the capacity retention rate after n cycles (n-cycle capacity / initial capacity × 100%). This parameter is crucial for determining the lifespan and total cost of ownership of lithium ion battery cells in various applications.
Capacity Retention vs. Cycle Number
Cycle Life Requirements
Different applications have varying cycle life requirements for lithium ion battery cells:
-
Consumer Electronics
Typically require ≥80% capacity retention after 500 cycles for lithium ion battery cells used in smartphones, laptops, and similar devices.
-
Electric Vehicles
Power lithium ion battery cells need to satisfy over 1000 cycles while maintaining acceptable capacity levels (often ≥70-80%).
-
Energy Storage Systems
Stationary energy storage applications often require 2000-5000 cycles or more from lithium ion battery cells, depending on the specific system design.
Testing Protocol
Cycle performance testing employs a fixed charge-discharge regime (e.g., 1C charge / 1C discharge). A 0.5C capacity calibration is performed every 100 cycles to create a capacity-cycle number curve and analyze degradation trends for lithium ion battery cells.
Capacity Fade Mechanisms
Active Material Loss
Detachment and degradation of active materials from electrodes
Electrolyte Decomposition
Chemical breakdown of electrolyte components over time
SEI Layer Growth
Thickening of the solid electrolyte interphase layer
Lithium Plating
Formation of metallic lithium deposits on electrode surfaces
These degradation mechanisms can be further investigated through post-cycling electrochemical characterization such as EIS and XRD analysis of lithium ion battery cells.
Self-discharge and Storage Performance
Self-discharge Characteristics
Self-discharge refers to the capacity loss of lithium ion battery cells in a stationary state, primarily caused by side reactions (such as slow reactions between electrolyte and electrodes) and internal short circuits.
The testing method involves: fully charging the lithium ion battery cells, allowing them to stand, and measuring the open circuit voltage or discharge capacity at regular intervals (e.g., every 24 hours). The self-discharge rate is then calculated, with ≤2% daily capacity loss considered excellent for lithium ion battery cells.
Factors Affecting Self-discharge
- Temperature (higher temperatures accelerate self-discharge)
- SOC level (higher SOC typically increases self-discharge rate)
- Battery age and cycle history
- Electrode material and electrolyte composition
- Manufacturing defects and impurities
Self-discharge Rate vs. Storage Time
Storage Performance
Storage performance evaluates the performance of lithium ion battery cells after long-term storage. Testing requires storing cells under different temperature (e.g., 25℃, 45℃) and humidity conditions for 3-12 months, then testing capacity recovery rate, internal resistance changes, and cycle performance to determine optimal storage conditions.
Capacity Retention After Storage
Optimal Storage Conditions for Lithium Ion Battery Cells
-
Temperature
20-25℃ (room temperature) is ideal for long-term storage of lithium ion battery cells
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State of Charge
50% SOC is generally recommended to minimize both self-discharge and degradation
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Humidity
Low humidity environment (30-50% relative humidity) to prevent corrosion
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Maintenance
Recharge to 50% SOC every 6-12 months for long-term storage of lithium ion battery cells
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Environment
Store lithium ion battery cells away from direct sunlight, heat sources, and metal objects
Conclusion
In summary, the electrochemical performance testing of lithium ion battery cells requires a multi-dimensional analysis combining thermodynamics (electromotive force), kinetics (internal resistance, charge-discharge characteristics), and long-term stability (cycle and storage performance).
By employing high-precision equipment and standardized procedures, researchers and manufacturers can obtain reliable data that provides a solid foundation for the optimal design and application selection of lithium ion battery cells. Understanding these performance characteristics is essential for advancing battery technology and maximizing the efficiency, safety, and longevity of lithium ion battery cells in various applications.