Design basis of lithium-ion batteries
Basic Design Principles
Battery design must be based on the specific needs of the electrical device and the characteristics of the battery itself. First, the technical parameters of each component, including electrodes, electrolyte, separator, and casing, must be clearly defined. Furthermore, the manufacturing process parameters need to be finely adjusted to ensure that the final battery pack meets the predetermined specifications and performance indicators (such as voltage, capacity, and volume). A reasonable and effective design is crucial for improving the battery's performance in actual use; therefore, pursuing the optimal solution throughout the design process is particularly important.
Design Requirements
In battery design, it is essential to fully understand the specific requirements of the target device for battery performance parameters and its operating environment. Generally, the following key factors should be considered:
(1) Battery operating voltage.
(2) Battery operating current, i.e., normal discharge current and peak current.
(3) Battery operating time, including continuous discharge time, service life, or cycle life.
(4) Battery operating environment, including ambient temperature, etc.
(5) Maximum permissible battery volume.
Lithium-ion batteries are widely used due to their excellent performance. However, in certain applications, they must meet a range of additional requirements, such as withstanding shocks and vibrations, adapting to extreme temperatures, and operating in low-pressure environments. When designing these batteries, in addition to considering these fundamental physical characteristics, it is necessary to comprehensively consider factors such as the selection of raw materials, key factors determining battery characteristics, overall performance, manufacturing processes, cost-benefit analysis, and operating temperature.
Key indicators for evaluating the performance of power batteries
Battery performance is generally evaluated through the following aspects:
1) Capacity
Battery capacity refers to the total amount of electricity a battery can provide under specific discharge conditions. This concept is usually expressed as the product of current and time, and is generally measured in ampere-hours (Ah). This parameter directly affects the battery's maximum operating current and the duration of continuous operation.
2) Discharge Characteristics and Internal Resistance
The battery's discharge characteristics reflect the stability of its output voltage, the voltage plateau, and its high-current discharge performance under specific conditions. These are important indicators for measuring the battery's load capacity. Furthermore, there are two types of resistance within the battery: ohmic internal resistance and polarization internal resistance. These two types of internal resistance have a particularly significant impact on overall discharge performance during high-current discharge.
3) Operating Temperature Range
To ensure stable operation of electrical equipment under various environmental conditions, batteries must maintain good performance within a specific temperature range.
4) Storage Performance
After a certain period of storage, battery performance may change due to various factors, leading to self-discharge, electrolyte leakage, short circuits, etc., thus affecting its performance.
5) Cycle Performance
Cycle performance refers to the number of charge-discharge cycles a secondary battery can withstand under specific charge-discharge rules until its performance drops to a predetermined level. It is one of the important indicators for evaluating the long-term reliability of a battery.
6) Safety Characteristics
Battery safety is mainly reflected in its safety level under abnormal usage conditions. These abnormal usage conditions include overcharging, short circuits, nail penetration tests, crush tests, hot box exposure, heavy object impacts, and vibrations. The battery's ability to withstand these extreme conditions is one of the key factors in determining its suitability for large-scale applications.
Equipment for the preparation and characterization of positive and negative electrode materials
preparation
Positive and negative electrode materials are key components of lithium-ion batteries, such as positive electrodes like LFePO4 or LiCoO2, and negative electrodes like graphite or silicon/carbon. Different materials require different preparation methods. This section uses ternary positive electrode materials as an example to introduce the equipment needed in the preparation process.
Characterization
The main materials of lithium-ion batteries, such as positive and negative electrode materials, are all electrochemical functional materials that significantly influence the electrochemical performance of lithium-ion batteries. There are relatively many technical indicators to consider during material production and development, as well as battery manufacturing and application, but they mainly fall into three categories: the first category is the material's crystal structure and microstructure; the second category is the material's physicochemical indicators, including particle size distribution, specific surface area, tap density, and elemental composition (including impurities); and the third category is the material's electrochemical performance characteristics, such as capacity, initial efficiency, and electrochemical impedance. The detection and characterization of various indicators of lithium-ion battery main materials are of great significance for material research and development, production control, and quality assurance.
Button lithium-ion battery manufacturing equipment
A coin cell, also known as a button cell or half-cell, typically uses lithium metal as the negative electrode and a positive electrode material as the positive electrode; or lithium metal as the positive electrode and a negative electrode material as the negative electrode. Its small size, low material consumption in electrode fabrication, and high flexibility make it an important device for the development and testing of active materials for lithium-ion batteries.
In the electrode fabrication stage, the materials and reagents required include positive and negative electrode materials, conductive agents, binders, and current collectors. First, the positive and negative electrode active materials, conductive agents, and binders are uniformly mixed in a specific ratio using manual grinding or mechanical homogenization. Then, the resulting slurry is coated onto the corresponding current collector. In laboratory coating, the coating method is determined based on the amount of slurry. For larger slurries, a small coating machine is used (Figure (a)); for smaller slurries, a film-forming device is used for manual coating (Figure (b)). The electrode is then dried in a drying oven and compacted by roller pressing. Slicing is the process of accurately cutting the electrode into circular pieces. Generally, the roller-pressed electrode is clamped between weighing paper and placed on a punching machine to quickly punch out small electrode pieces. The diameter of the small electrode pieces can be adjusted by the punching die size of the punching machine.
