Battery Cell Introduction

1.Classification of battery cells

1.1.Introduction to energy storage

Nickel battery: Nickel-metal hydride battery is a good performance battery. The positive active material of nickel-metal hydride battery is Ni(OH)2 (called NiO electrode), the negative active material is metal hydride, also called hydrogen storage alloy (the electrode is called hydrogen storage electrode), and the electrolyte is 6mol/L potassium hydroxide solution. The advantages of nickel battery are: high energy density, fast charge and discharge speed, light weight, long life, no environmental pollution; the disadvantages are slight memory effect, more management problems, and easy formation of monomer battery separator melting.

Liquid flow battery: Liquid flow energy storage battery is a type of device suitable for fixed large-scale energy storage (electricity storage). Compared with the currently commonly used lead-acid batteries, nickel-cadmium batteries and other secondary batteries, it has the advantages of independent design of power and energy storage capacity (energy storage medium is stored outside the battery), high efficiency, long life, deep discharge, and environmental friendliness. It is one of the preferred technologies for large-scale energy storage technology. The advantages of liquid flow batteries are: flexible layout, long cycle life, fast response, and no harmful emission; the disadvantage is that the energy density varies greatly.

 

Sodium-sulfur battery: Sodium-sulfur battery is a secondary battery with metallic sodium as the negative electrode, sulfur as the positive electrode, and a ceramic tube as the electrolyte membrane. Under a certain working temperature, sodium ions pass through the electrolyte membrane and undergo a reversible reaction with sulfur, resulting in the release and storage of energy. Advantages and disadvantages of sodium-ion batteries: specific energy up to 760Wh/kg, no self-discharge, discharge efficiency of almost 100%, and life span of 10 to 15 years; the disadvantage is that the high temperature of 350ºC melts sulfur and sodium.

 

Lead-acid battery: a battery whose electrodes are mainly made of lead and its oxides, and whose electrolyte is sulfuric acid solution. When the lead-acid battery is discharged, the main component of the positive electrode is lead dioxide, and the main component of the negative electrode is lead; when it is charged, the main components of the positive and negative electrodes are lead sulfate. The advantages of lead-acid batteries are: safe sealing, venting system, simple maintenance, long service life, stable quality, and high reliability; the disadvantages are that the lead pollution is large and the energy density is low (that is, too heavy).

As can be seen from the chart, there are many types of batteries, and the most widely used ones at this stage are lead-acid batteries and lithium batteries.

The comparative data of lithium-ion batteries, lead-acid batteries, sodium-sulfur batteries, and flow batteries are as follows

2. Introduction of lithium battery

At present, lithium-ion batteries can be divided into the following categories according to the positive electrode active materials: lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel cobalt manganese oxide (LiNiMnCoO2 or NMC), lithium nickel cobalt aluminum oxide (LiNiCoAlO2 or NCA), lithium iron phosphate (LiFePO4), and lithium titanate (Li4Ti5O12).

(1) Lithium Nickel Cobalt Aluminate (NCA)

NCA is actually similar to the ratio of NCM battery 811. The common ratio of nickel, cobalt and aluminum in NCA is 8:1.5:0.5, and the upper limit of theoretical density is 350wh/kg, which is more superior to NCM. However, in the production process, aluminum is an amphoteric metal and is not easy to precipitate. Correspondingly, the sealing requirements of production equipment are high, and the temperature and humidity requirements are more stringent than NCM. The humidity is required to be controlled below 10% (LiNiO2 in NCA is easy to absorb moisture), which increases the technical requirements and production costs accordingly. In particular, there are serious side reactions in the charging and discharging process that produce gas, which will cause the battery to swell and deform easily, and there will be safety hazards, so NCA will adopt a cylindrical battery shell that is more resistant to internal pressure. In addition, because of the presence of nickel, the battery performance is very good. The disadvantages of high-nickel NCM batteries are the same as high-nickel NCA. While increasing energy density, it also reduces thermal stability. Therefore, NCA production technology was basically controlled in Japan and South Korea in the early days, and domestic NCA started late.

(2) Lithium Nickel Cobalt Manganese Oxide ( NCM)

As the name suggests, NCM is a battery composed of three elements: nickel, cobalt and manganese. According to their different proportions, it is further subdivided into NC M111, NC M433, NC M523, NCM622, and NCM811 . The most famous ones are NCM523 and NCM811.

The advantages of high-nickel batteries are obvious, that is, they can reduce costs (less cobalt) while increasing battery energy density (more nickel). However, the disadvantages are also prominent, that is, there is less cobalt and more nickel, which leads to poor stability of the layered structure, easy decomposition of the electrolyte and side reactions between the electrolyte and the surface of the material, and the by-products produced lead to poor interface conductivity. The high -nickel, low-cobalt, and low-manganese solution uses a low-cost solution to increase energy density (increase battery life), but in return, the cycle life, safety, and thermal stability are reduced.

(3) Lithium cobalt oxide (LCO)

Lithium cobalt oxide is the first commercial lithium-ion battery cathode material, and it is still the most widely used cathode material for consumer batteries. Although this type of battery technology is relatively mature, with high power, high energy density, and high consistency, it has a low safety factor, poor thermal and electrical properties, and a relatively high cost. It is currently mainly used as a cathode material for lithium-ion batteries in the manufacture of mobile phones, laptops, and other portable electronic devices. Lithium cobalt oxide batteries have a stable structure, high specific capacity, and outstanding comprehensive performance, but their safety is poor and their cost is very high. They are mainly used for small and medium-sized batteries and are widely used in small electronic devices such as laptops, mobile phones, MP3/4, etc., with a nominal voltage of 3.7V.

(4) Lithium manganese oxide LMO

From the distribution of the power lithium battery market, the mainstream positive electrode materials are ternary, iron lithium and ternary & lithium manganese oxide composite system. The reason for this situation is that each material has its own advantages and disadvantages. The market attention of ternary and iron lithium is high because their advantages are highly concerned by the market, the energy density of ternary is high and the safety of iron lithium is good. But in fact, both of them also have their own shortcomings. The safety of ternary is poor and the cost is high; while the energy density of iron lithium is low, the low temperature performance is poor, and the voltage platform is low. The same is true for lithium manganese oxide, which has certain defects, such as low capacity and poor cycle performance. But lithium manganese oxide also has very outstanding advantages. In addition to low cost, the safety of lithium manganese oxide is also relatively good, but the material itself is not very stable and is prone to swelling. Decomposition produces gas, so it is mostly used in combination with other materials. Lithium manganese oxide can form a composite material system with ternary, which effectively makes up for the shortcomings of the poor safety performance of the pure ternary system. Lithium manganese oxide also has very excellent low temperature performance. At more than 20 degrees below zero, lithium manganese oxide batteries can still discharge sufficient electricity. In fact, due to the problems of lithium manganese oxide itself, even from the perspective of cost-effectiveness alone, lithium manganese oxide is one of the positive electrode materials of lithium-ion batteries that cannot be ignored .

        Lithium manganese oxide is an important positive electrode material for lithium-ion batteries. Currently, Xingheng and ATL are using it. Xingheng Power has been following the lithium manganese oxide route for many years, and its technical reserves and technical research and development strength in this area are among the first echelon in China. In order to solve the shortcomings of lithium manganese oxide cycle performance, breakthrough progress has been made in the synthesis of lithium manganese oxide and the selection of electrolyte.

(5) Lithium iron phosphate (LFP)

CATL, which is very popular, has become the industry leader with its ternary lithium battery. BYD, which is catching up, has been thriving in the new energy and energy storage track with lithium iron phosphate. In 2020, Yinlong New Energy switched to lithium titanate and became a standout in the industry. In 2022, energy storage can take lithium iron phosphate to a higher level. Payne and Penghui are soaring,In the early years, NCM and NCA have been popular in the market. The energy density of the NCA battery used by Tesla in the early days is more than twice that of BYD's LFP battery. However, the thermal stability of ternary batteries is poor, while lithium iron phosphate has strong thermal stability and long cycle life, so energy storage generally adopts lithium iron phosphate batteries. The strong thermal stability of the battery itself also means that the requirements for system BMS thermal management are low. Looking at the BMS industry chain, both software design and hardware chips can adopt cost-effective solutions. Even if the BMS management is relatively extensive, passive protection measures such as flame retardancy and loss reduction can ensure the safe use of lithium iron phosphate batteries during a longer life.

Whether it is the high-nickel, low-cobalt version of NCM NCM811 or NCA, the high price of cobalt makes the cost of ternary lithium power batteries much higher than that of lithium iron phosphate batteries (LFP). In the era of cost being king in the energy storage industry , LFP is undoubtedly the best choice.

(6) Lithium titanate

In the lithium battery industry, lithium titanate batteries have several outstanding superior properties: fast charging, better safety performance than ternary and lithium iron phosphate batteries, longer cycle life, and a wider operating temperature range than other lithium batteries. Therefore, lithium titanate batteries are rarely used because of their low energy density, but they are often used in electric buses. Buses have large areas and can use more lithium titanate batteries to provide power. As the saying goes, learn from each other's strengths and make up for each other's weaknesses .When lithium titanate batteries are charged and discharged, the insertion and deintercalation of lithium ions will not cause changes in the crystal structure of lithium titanate, so there is almost no effect on the structure of lithium titanate materials. For this reason, lithium titanate is also called "zero strain material". Compared with the current lithium iron phosphate batteries, lithium titanate batteries have outstanding advantages. The average cycle life of ordinary batteries is 3,000 times, while the number of complete charge and discharge cycles of lithium titanate batteries can reach more than 10,000 times .

(7) Comparative analysis of NCA, NMC, and LFP

· Battery Life

Battery life mainly depends on the number of battery cycles. The general cycle number of ternary batteries is 1,000 times, while that of lithium iron phosphate batteries can exceed 2,000. Therefore, in terms of battery life, lithium iron phosphate batteries are better.

· Energy density and driving range comparison

The ternary lithium battery generally referred to on the market is a nickel-cobalt-manganese lithium battery. Compared with lithium iron phosphate batteries, ternary lithium batteries have a higher energy density and a higher voltage, so battery packs of the same weight have a larger capacity, longer battery life, and greater output power.

· Space occupancy comparison

Similarly, in the same space, the battery of ternary lithium battery is easier to be larger, so within the limited space of the vehicle, while solving the weight problem, it also saves space for family cars.

· Relatively low cost

In this era of soaring battery materials, lithium iron phosphate batteries require less lithium than ternary lithium batteries. There is no precious metal cobalt.

· High security

The most famous test for lithium batteries is probably the needle puncture battery pack test. After being punctured, the ternary lithium battery immediately exploded, while the lithium iron phosphate battery only smoked and did not cause any fire.

· High and low temperature performance

Whether it is NCA or NCM, the high temperature performance of ternary batteries is relatively average, and the stability is poor. When the temperature reaches 250-350℃, it is easy to thermal runaway, and there is a high risk of spontaneous combustion during fast charging. Therefore, the requirements for heat dissipation performance of ternary lithium batteries are very stringent, which also has higher technical requirements for BMS battery management systems; while the high temperature performance of lithium iron phosphate is more stable, but the low temperature performance is poor. If the lithium iron phosphate battery is not equipped with a thermal management system, the endurance in winter will be very poor. However, the low temperature performance of ternary batteries is better than LFP, and it is more suitable for cold areas. Therefore, LFP electric vehicles are almost invisible in the Northeast.

2.Battery cell basic parameters

1. Voltage

Nominal voltage: The nominal voltage of a battery refers to the voltage displayed during normal operation. The nominal voltage of a secondary nickel-cadmium nickel-metal hydride battery is 1.2V; the nominal voltage of a secondary lithium battery is 3.6V; the nominal voltage of a lithium iron phosphate battery is 3.2V.

Open circuit voltage: refers to the potential difference between the positive and negative electrodes of the battery when the battery is not working, that is, when no current flows through the circuit . This is the open circuit voltage of the battery.

The working voltage, also known as the terminal voltage, refers to the potential difference between the positive and negative electrodes of the battery when the battery is connected to an external load or power source and current flows through the circuit . The working voltage is related to the circuit composition and the working state of the equipment and is a variable value. Generally speaking, due to the existence of the internal resistance of the battery, the working voltage in the discharge state is lower than the open circuit voltage, and the working voltage in the charging state is higher than the open circuit voltage.

The charge/discharge cut-off voltage refers to the maximum and minimum operating voltage allowed for the battery. Exceeding this limit will cause irreversible damage to the battery, resulting in reduced battery performance, and in severe cases even fire, explosion and other safety accidents.

2. Temperature

Due to the characteristics of the chemical materials inside lithium-ion batteries, lithium-ion batteries have a reasonable operating temperature range (common data is between -20℃ and 60℃). If they are used beyond the reasonable range, the performance of lithium-ion batteries will be greatly affected.

3. Internal resistance

Battery internal resistance: refers to the resistance encountered by the current flowing through the battery when the battery is working. It consists of two parts: ohmic internal resistance and polarization internal resistance. A large battery internal resistance will lead to a lower discharge voltage and a shorter discharge time. The size of the internal resistance is mainly affected by factors such as the battery material, manufacturing process, and battery structure. It is an important parameter for measuring battery performance. Note: Generally, the internal resistance in the charged state is used as the standard. The internal resistance of the battery must be measured with a special internal resistance meter, and cannot be measured with the ohm range of a multimeter.

Ohmic internal resistance is composed of electrode materials, electrolytes, diaphragm resistance and contact resistance of various parts. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reactions, including resistance caused by electrochemical polarization and concentration polarization. Batteries with large internal resistance have large internal power consumption and severe heat generation during charging and discharging, which will cause accelerated aging and life decay of lithium-ion batteries, and will also limit high-rate charging and discharging applications. Therefore, the smaller the internal resistance, the better the life and rate performance of lithium-ion batteries.

Polarization resistance is the resistance generated at the moment of current loading. It is the sum of various tendencies inside the battery that hinder the charged ions from reaching their destination. Polarization resistance can be divided into two parts: electrochemical polarization and concentration polarization. Electrochemical polarization is caused by the speed of electrochemical reaction in the electrolyte not being able to keep up with the speed of electron movement; concentration polarization is caused by the speed at which lithium ions are embedded in and out of the positive and negative electrode materials and move in the materials less than the speed at which lithium ions gather at the electrodes.

4. Energy density

Energy density, battery energy density, for a given electrochemical energy storage device, the ratio of the energy that can be charged to the mass or volume of the energy storage medium. The former is called "mass energy density" and the latter is called "volume energy density", the units are watt-hour/kilogram Wh/kg, watt-hour/liter Wh/L respectively. The amount of electricity here is the integral of the capacity (Ah) and the operating voltage (V) mentioned above. When applied, the energy density indicator is more instructive than the capacity.The improvement of lithium-ion battery energy density is a slow process, far slower than Moore's Law in the integrated circuit industry. This has resulted in a scissors gap between the performance improvement of electronic products and the improvement of battery energy density, which continues to expand over time.

5. Self-discharge

The self-discharge phenomenon is the phenomenon that if the battery is left unused, it will also lose power. When the battery is left unused, its capacity is constantly decreasing. The rate of capacity decrease is called the self-discharge rate, which is usually expressed as a percentage: %/month. Once the self-discharge of a lithium-ion battery causes the battery to be over-discharged, the impact is usually irreversible. Even if it is recharged, the available capacity of the battery will be greatly reduced, and the life will decay rapidly. Therefore, lithium-ion batteries that are not used for a long time must be charged regularly to avoid over-discharge due to self-discharge, which greatly affects the performance.The health status of a single battery cell, the self-discharge K value is an evaluation indicator. The definition of K value: OCV1 is measured at time t1, and OCV2 is measured at time t2. K= (OCV1-OCV2)/ (t2-t1)A better battery K value is generally less than 2mV/d or 0.08mV/h.

6. Capacity

The rated capacity of a battery refers to the minimum amount of electricity that should be discharged under certain discharge conditions as specified or guaranteed when the battery is designed and manufactured. The IEC standard stipulates that nickel-cadmium and nickel-metal hydride batteries should be in an environment of 20℃±5℃. For lithium-ion batteries, it is stipulated that the amount of electricity discharged when charging for 3 hours at room temperature, constant current (1C)-constant voltage (4.2V) controlled, and then discharged at 0.2C to 2.5V is its rated capacity. The actual capacity of a battery refers to the actual amount of electricity discharged by the battery under certain discharge conditions, which is mainly affected by the discharge rate and temperature (so strictly speaking, the battery capacity should indicate the charging and discharging conditions). The units of battery capacity are Ah, mAh (1Ah=1000mAh).

The actual capacity refers to the amount of electricity that the battery can provide under a certain discharge condition (a certain depth, a certain current density and a certain termination voltage). The actual capacity is generally not equal to the rated capacity, and it is directly related to temperature, humidity, charge and discharge rate, etc. In general, the actual capacity is slightly smaller than the rated capacity, and sometimes even much smaller than the rated capacity;Theoretical capacity refers to the amount of electricity produced when all active substances participate in the battery reaction. That is, the capacity under the most ideal conditions;Rated capacity refers to the capacity of the motor or electrical appliance indicated on the nameplate that can work continuously for a long time under rated working conditions. Usually it refers to the apparent power for transformers, the active power for motors, and the apparent power or reactive power for phase-changing equipment, with units of VA, kVA, MVA .Discharge residual capacity: When a rechargeable battery is discharged with a large current (such as 1C or above), the battery reaches the terminal voltage before the capacity is fully discharged due to the "bottleneck effect" of the internal diffusion rate caused by the excessive current. It can continue to discharge with a small current such as 0.2C, and the released capacity is called the residual capacity.

7. Magnification

The charge and discharge rate is a measure of charging speed. This indicator affects the continuous current and peak current of lithium-ion batteries when they are working. Its unit is generally C (abbreviation of C-rate), such as 0.1C, 0.5C, 1C, 5C, 10C, etc. For example, if the rated capacity of the battery is 20Ah, if its rated charge and discharge rate is 0.5C, it means that this battery can be repeatedly charged and discharged at a current of 20Ah*0.5C=10A until the cut-off voltage of charging or discharging is reached. If its maximum discharge rate is 10C@10s and the maximum charge rate is 5C@10s, then the battery can be discharged at a current of 200A for 10 seconds and charged at a current of 100A for 10 seconds.Generally, lithium batteries can be divided into three levels according to the rate. Below 3C is basically a capacity model, which does not support fast charging and discharging. 3-8C is a power type battery, which is mainly used in electric vehicles and industrial equipment. This can basically meet the needs of fast charging and discharging, but it is dwarfed by the rate type battery. However, the power type battery is the mainstream battery at present. Finally, there is the rate type battery, which is basically above 10C, which means that the discharge current can instantly reach 10 times the capacity. For example, a 2000mah 10C rate battery can reach a discharge current of 20A.

8. Lifespan

Life (Cycle Life, unit: times) The life of lithium-ion batteries is divided into two parameters: cycle life and calendar life. Cycle life is generally measured in times, which represents the number of times the battery can be charged and discharged. Of course, this is also conditional. Generally, it is calculated under ideal temperature and humidity, with rated charge and discharge current for deep charge and discharge (80% DOD), and the number of cycles experienced when the battery capacity decays to 20% of the rated capacity.
Depth of Discharge ( DoD) Depth of discharge is the percentage of battery discharge to the rated capacity of the battery. The depth of discharge of shallow cycle batteries should not exceed 25%, while deep cycle batteries can release 80% of the power. The battery starts discharging at the upper voltage limit and ends at the lower voltage limit. All discharged power is defined as 100%. The battery standard 80% DOD means discharging 80% of the power. For example, if the initial SOC is 100%, and the discharge stops at 20%, this is 80% DOD.

9. Discharge platform

Discharge platform: The discharge platform of a rechargeable battery usually refers to the voltage range in which the working voltage of the battery is relatively stable when the battery is discharged under a certain discharge system. Its value is related to the discharge current. The larger the current, the lower its value. The discharge platform of a lithium-ion battery is generally the discharge time when the voltage reaches 4.2V and the current is less than 0.01C at a constant voltage, and then the charging is stopped, and then the battery is left for 10 minutes, and discharged to 3.6V at any discharge current rate. It is an important criterion for measuring the quality of a battery.

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