Brief analysis of the main parameters of lithium-ion batteries

Main parameter index

Lithium-ion batteries have the advantages of high energy density, high conversion efficiency, long cycle life, no memory effect, no charge and discharge delay, low self-discharge rate, wide operating temperature range and environmental friendliness, thus becoming an ideal carrier for electric energy. It is widely used in various fields.

In general, when we use lithium-ion batteries, we will pay attention to some technical indicators, as a major factor in measuring the performance of their performance. So, which indicators are we need to pay special attention when we need them?

Capacity

This is a parameter that everyone cares about. Smart phones have long been popular. When we use smart phones, the most worrying thing is that the battery is low, it needs to be recharged frequently, and sometimes it can't find a place to charge. Early function machine, under normal use, the full charge battery can stand for 3~5 days, and some products can even stand for more than 7 days. However, in the era of smart phones, the standby time is terrible. One of the most important reasons for this is that the power consumption of mobile phones is getting bigger and bigger, while the capacity of batteries is not growing in proportion.

The unit of capacity is generally "mAh" (mAh) or "Ah" (Ah), and there is a difference between rated capacity and actual capacity when in use. Rated capacity refers to the total amount of electricity that a fully charged lithium-ion battery can provide under laboratory conditions (preferred temperature and humidity) when discharged to a cut-off voltage at a specific discharge rate (C-rate). The actual capacity is generally not equal to the rated capacity, which is directly related to temperature, humidity, charge and discharge rate, and the like. In general, the actual capacity is smaller than the rated capacity, and sometimes even smaller than the rated capacity. For example, in the winter in the north, if the mobile phone is used outdoors, the battery capacity will drop rapidly.

2. Energy density

Energy density refers to the unit volume or unit weight of the battery, the amount of electricity that can be stored and released. There are two types of units: Wh/kg, Wh/L, which represent the weight ratio energy and volume ratio energy, respectively. The amount of electricity here is the integral of the capacity (Ah) and the operating voltage (V) mentioned above. When applied, the indicator of energy density is more instructive than capacity.

Based on current lithium-ion battery technology, the energy density level that can be achieved is about 100~200Wh/kg, which is still relatively low, and has become a bottleneck for lithium-ion battery applications in many applications. This problem also occurs in the field of electric vehicles. In the case where the size and weight are strictly limited, the energy density of the battery determines the single maximum mileage of the electric vehicle, so the unique term “mileage anxiety” appears, if the electric vehicle has to travel a single mileage of 500 kilometers (comparable to a conventional fuel vehicle), the energy density of the battery unit must reach 300 Wh/kg or more.

The increase in energy density of lithium-ion batteries is a slow process, far lower than the Moore's Law of the integrated circuit industry, which causes a difference in the performance of electronic products and the increase in energy density of batteries. expand.

3. Charge and discharge rate

This indicator affects the continuous current and peak current of a lithium-ion battery. The unit is usually C (short for C-rate), such as 1/10C, 1/5C, 1C, 5C, 10C, etc. Give an example to illustrate the specific meaning of the rate indicator. The rated capacity of a battery is 10Ah. If its rated charge-discharge rate is 1C, it means that this type of battery can be repeatedly charged and discharged with a current of 10A, the cutoff voltage to charge or discharge. If its maximum discharge rate is 10C@10s and the maximum charge rate is 5C@10s, the battery can be discharged with a current of 100A for 10 seconds and with a current of 50A for 10 seconds.

The continuous power and peak power of the lithium ion battery can be obtained by multiplying the current value corresponding to the charge and discharge rate by the operating voltage. The more detailed the definition of the charge and discharge rate indicator, the greater the guiding significance for use. In particular, lithium-ion batteries, which are power sources for electric vehicles, need to specify continuous and pulse rate indicators under different temperature conditions to ensure that lithium-ion batteries are used within a reasonable range.

4. Voltage

The voltage of the lithium-ion battery has some parameters such as open circuit voltage, working voltage, charge cut-off voltage, discharge cut-off voltage, etc. This article does not discuss them separately, but concentrates on an explanation.

Open circuit voltage, as the name implies, is that no external load or power supply is connected to the battery, and the potential difference between the positive and negative terminals of the battery is measured. This is the open circuit voltage of the battery.

The working voltage is the external load or power supply of the battery, which is in the working state. When there is current flowing, the potential difference between the positive and negative electrodes is measured. Generally, due to the internal resistance of the battery, the operating voltage in the discharging state is lower than the open circuit voltage, and the operating voltage during charging is higher than the open circuit voltage.

The charge/discharge cutoff voltage is the maximum and minimum operating voltage allowed by the battery. Exceeding this limit will cause irreversible damage to the battery, resulting in a decrease in battery performance and, in severe cases, even a fire or explosion.

The open circuit voltage and operating voltage of the battery have a certain correspondence with the capacity of the battery.

5. Life expectancy

The life of a lithium-ion battery will gradually decrease with use and storage, and it will have a more obvious performance. Still taking a smart phone as an example, after using the mobile phone for a period of time, it can be obvious that the mobile phone battery is “not durable”. It may only be charged once a day, and may need to be charged twice a day. This is the battery life is declining the embodiment.

The life of a lithium-ion battery is divided into two parameters: cycle life and calendar life. The cycle life is generally expressed in number of times characterizing the number of times the battery can be cycled for charge and discharge. Of course, there are conditions here. Generally, under the ideal temperature and humidity, the depth of charge and discharge (100% DOD or 80% DOD) is performed with the rated charge and discharge current, and the battery capacity is reduced to 80% of the rated capacity. The number of cycles experienced.

The definition of calendar life is more complicated. The battery can't be charged and discharged all the time. It has storage and shelving. It can't always be in ideal environmental conditions. It will experience various temperature and humidity conditions. The charge and discharge rate is also changing at times, so the actual the service life requires simulation and testing. Simply put, the calendar life is the time span of the battery under certain environmental conditions and end-of-life conditions (such as capacity decay to 80%) under ambient conditions. Calendar life is closely tied to specific usage requirements. It is usually necessary to specify specific usage conditions, environmental conditions, storage intervals, and so on.

Calendar life is more practical than cycle life, but because the calculation of calendar life is very complicated and takes too long, the average battery manufacturer only gives data on cycle life. If you need to get data on calendar life, you usually have to pay extra and wait a long time.

6. Internal resistance

The internal resistance of a lithium-ion battery refers to the resistance that the current flows through the inside of the battery during operation. It includes ohmic internal resistance and polarization internal resistance. The polarization internal resistance includes electrochemical polarization internal resistance and concentration difference internal resistance.

The ohmic internal resistance consists of the electrode material, the electrolyte, the diaphragm resistance, and the contact resistance of each part. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including the resistance caused by electrochemical polarization and concentration polarization.

The unit of internal resistance is generally milliohm (mΩ), and the battery with large internal resistance has large internal power consumption and severe heat generation during charging and discharging, which will cause accelerated aging and life decay of lithium ion battery, and also limit large magnification, charge and discharge applications. Therefore, the smaller the internal resistance is, the better the life and rate performance of the lithium ion battery will be.

7. Self-discharge

When the battery is placed, its capacity is decreasing. The rate at which the capacity drops is called the self-discharge rate, usually expressed as a percentage: %/month.

Self-discharge is what we don't want to see. A fully charged battery will have a much lower power for a few months, so we hope that the self-discharge rate of the lithium-ion battery will be as low as possible.

Special attention should be paid here. Once the self-discharge of a lithium-ion battery causes the battery to be over-discharged, the effect is usually irreversible. Even if it is recharged, the available capacity of the battery will be greatly lost, and the life will be rapidly attenuated. Therefore, long-term placement of unused lithium-ion batteries, you must remember to periodically charge, to avoid over-discharge due to self-discharge, performance is greatly affected.

8. Operating temperature range

Due to the characteristics of the internal chemical materials of lithium-ion batteries, lithium-ion batteries have a reasonable operating temperature range (common data is between -40 ° C and 60 ° C). If it is outside the reasonable range, it will have the performance of lithium-ion batteries causes a greater impact.

Lithium-ion batteries of different materials have different operating temperature ranges, some have good high-temperature performance, and some can adapt to low-temperature conditions. The operating voltage, capacity, charge and discharge rate and other parameters of a lithium-ion battery will change significantly with temperature. Long-term use of high temperature or low temperature will also accelerate the life of lithium-ion batteries. Therefore, efforts to create a suitable operating temperature range will maximize the performance of lithium-ion batteries.

In addition to the limited operating temperature, the storage temperature of lithium-ion batteries is also strictly restricted. Long-term high-temperature or low-temperature storage will have an irreversible impact on battery performance.

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