Battery design requirements
The battery is designed to meet the requirements of the object (user or equipment). Therefore, before proceeding with battery design, you must first understand the object’s requirements for battery performance indicators and usage conditions in detail, which generally include the following aspects: the working voltage of the battery and the required voltage accuracy; the working current of the battery, that is, the normal discharge current and peak current; the working time of the battery, including the continuous discharge time, service life or cycle life; the working environment of the battery, including the state and ambient temperature when the battery is working; the maximum allowable volume and weight of the battery.
Now take the production of square lithium-ion batteries as an example to illustrate and determine the design requirements of AA-type lithium-ion batteries assembled with selected battery materials: the ohmic resistance of the battery in the discharged state is not more than 40Ω; when the battery is discharged at 1C, it depends on different cathode materials, for example, the specific capacity of LiCoO2 is not less than 135mA·h/g; the 2C discharge capacity of the battery is not less than 96% of the 1C discharge capacity; during the first 30 1C charge and discharge cycles, the capacity above 3.6V is not less than 80% of the total battery capacity; during the first 100 cycles of 1C charge and discharge, the average capacity decay of the battery each time is not more than 0.06%; when the battery is charged, it will not explode when placed in an electric furnace at 135°C.
The battery assembled in accordance with the structural design and assembly process of the AA-type lithium ion battery, after experimental testing, if the result meets the above requirements, it indicates that the structural design is reasonable and the assembly process is perfect, and when researching the battery performance of different cathode materials, the battery can be assembled according to this structure design and process; if the result does not meet the above requirements, it means that the structural design is not reasonable or the process is not perfect, and repeated optimization is required until the experimental results meet the above requirements.
Because of its excellent performance, lithium-ion batteries are increasingly used in various fields, especially in special occasions and devices. Therefore, there are sometimes special requirements for battery design, such as vibration, collision, heavy impact, thermal shock, overcharge, short circuit, etc.
At the same time, it is also necessary to consider: the source of electrode materials; battery performance; factors affecting battery characteristics; battery technology; economic indicators; environmental issues and other factors.
Battery performance design
After clarifying the design task and making relevant preparations, the battery design can be carried out. According to the requirements of battery users, there are two ideas for battery design: one is to provide power supplies of rated capacity for electrical equipment and instruments; the other is to develop new batteries or special-shaped batteries with excellent performance for a given power supply's overall dimensions.
The battery design mainly includes parameter calculation and process formulation. The specific steps are as follows.
(1) Determine the number of single cells, the operating voltage and operating current density of the single cells in the assembled battery. Determine the total working voltage, working current and other indicators of the battery pack according to the requirements, select the battery series, and refer to the "volt-ampere curve" of the series (empirical data or obtained through experiments) to determine the working voltage and working current density of the single battery.
Number of single cells = total battery working voltage/single battery working voltage
(2) Calculate the total area of electrodes and the number of electrodes According to the required working current and the selected working current density, calculate the total electrode area (subject to the control electrode).
Total electrode product = working current (mA) / working current density (mA/cm2)
According to the requirements of the maximum size of the battery shape, select the appropriate electrode size, and calculate the number of electrodes.
Number of electrodes = total area of electrodes / plate area
(3) Calculate the battery capacity. Calculate the rated capacity according to the required working current and working time.
Rated capacity = working current × working time
(4) Determine the design capacity
Design capacity = rated capacity × design factor
The design factor is set to ensure the reliability and service life of the battery, generally 1.1~1.2.
(5) Calculate the amount of positive and negative active materials of the battery.
①Calculate the active material consumption of the control electrode, and calculate the material consumption of the control electrode in the single battery according to the electrochemical equivalent, design capacity and active material utilization rate of the active material of the control electrode.
Electrode active material consumption = design capacity × active material electrochemical equivalent / active material utilization rate
②Calculate the amount of active material of the non-control electrode. The amount of non-control electrode active material in the single battery should be determined according to the amount of control electrode active material. In order to ensure the battery has better performance, it should generally be excessive, usually with a coefficient of 1~2. Lithium-ion batteries usually use excess carbon material in the negative electrode, and the coefficient is 1.1.
(6) Calculate the average thickness of the positive and negative plates according to the capacity requirements to determine the amount of active material in the single battery. When the electrode substance is a single substance, then:
Single battery material consumption = electrode sheet material consumption / single battery pole plate number
The average thickness of electrode active material = the amount of material per electrode/[material density × electrode plate area × (1-porosity)] + current collector thickness
If the electrode active material is not a single substance but a mixture, the amount and density of the substance should be replaced by the amount and density of the mixed substance.
(7) Selection of diaphragm material and determination of thickness and number of layers. The main function of the separator is to separate the positive and negative poles of the battery to prevent the two poles from contacting and short-circuiting. In addition, it should also have the function of allowing electrolyte ions to pass through. The material of the diaphragm is non-conductive, and its physical and chemical properties have a great influence on the performance of the battery. The separators often used in lithium-ion batteries include polypropylene and polyethylene microporous membranes. Celgard's series of separators have been used in lithium-ion batteries. The number and thickness of the separator should be determined according to the performance of the separator itself and the performance requirements of the specific design battery.
(8) Determine the concentration and dosage of electrolyte The concentration and dosage of electrolyte are determined according to the characteristics of the selected battery system, combined with the specific design battery usage conditions (such as working current, working temperature, etc.) or based on empirical data.
The electrolyte systems of commonly used lithium-ion batteries include: 1mol/L LiPF6/PC-DEC (1:1), PC-DMC (1:1) and PC-MEC (1:1) or 1mol/L LiPF6/EC-DEC (1:1), EC-DMC ( 1:1) and EC-EMC (1:1).
Note: PC, propylene carbonate; EC, ethylene carbonate; DEC, diethyl carbonate; DMC, dimethyl carbonate; EMC, ethyl methyl carbonate.
(9) Determine the assembly ratio of the battery and the size of the single battery container. The assembly ratio of the battery is determined according to the selected battery characteristics and the electrode thickness of the designed battery. The general control is 80%~90%.
After the battery is selected according to the requirements of the electrical appliance for the battery, the width, length and wall thickness of the battery container are determined according to the physical and mechanical properties of the battery shell material. Especially with the thinning and lighter weight of electronic products, the space for batteries is getting smaller and smaller, which requires the selection of advanced electrode materials to prepare batteries with higher specific capacity.