Lithium-ion cell knowledge comprehensive explanation -Tritek Battery R&D Director

With the development of the global new energy industry, lithium battery has become a high-demand industry, and power lithium battery has become a concentrated area of demand growth of the lithium battery industry.

So do you fully understand lithium-ion cell technology knowledge? This is a comprehensive guide to this summary from Tritek’s R&D Director.

We also prepared Ebike Glossary Of Terms for you to quick search.

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Basic knowledge of cell

Battery development history

Battery is a device that converts chemical energy into electrical energy, providing direct electrical energy to the outside through chemical reactions in the battery.

battery development history

Battery Category

Batteries are mainly divided into three types: chemical battery, physical battery and biobattery. Here is a mind map of battery classification.

battery category

The development direction of power battery

the development direction of power battery

Ni-Cd Battery→Ni-Zn Battery→Lead-Acid Battery→Ni-MH Battery→Lithium-ion battery→Fuel Battery

Ni-Cd Battery: Serious pollution, has been basically eliminated.

Ni-Zn Battery: Short life, performance is not suitable for vehicle batteries.

Lead-Acid Battery: Firstly used as a vehicle battery, is still in use now, low cost, but with low specific energy, high self-discharge rate, low cycle life, not environmental-friendly.

Ni-MH Battery: Most commonly used in HEV(hybrid electric vehicle), which performs better but costs more and contains the polluting metal nickel.

Lithium-ion battery: Industrialization hot spot of automotive battery at the moment, good performance, environmental.

Triteks’ LEV products such as E-bike battery, Custom E-bike battery, E-motorcycle battery, Cargo-bike battery all apply lithium-ion battery.  

image 5  

Fuel Battery: Not yet industrialized, good performance, environmentally-friendly, but the price is high.

Lithium battery – Development history

With carbon as the negative electrode and lithium compound as the positive electrode; In the process of charging and discharging, lithium ions travel back and forth between the positive and negative electrodes, which is how lithium-ion batteries get their name.

lithium battery development history

 

Lithium battery – How it works

In the process of charge and discharge of lithium-ion battery, lithium-ion is in a state of motion from positive to negative to positive. It’s like a rocking chair, with lithium ions moving back and forth between the two ends of the battery. This electrochemical energy storage system is known as the ”rocking chair battery”.

Lithium battery – How it works – Charging process&Discharging process

1.6.1 Lithium ions are embedded in the layered structure of the positive electrode material before charging.

before charging  

1.6.2 After starting charging, the positive electrode material loses electrons and lithium ions escape from the positive electrode material. after charging

1.6.3 Lithium ions arrive at the negative graphite material through electrolyte and separator. charging process 1  

1.6.4 Lithium ions are embedded in the graphite layer, while electrons pass through the outer circuit to the negative electrode, forming a relatively stable lithium-embedded graphite. charging process 2  

1.6.5 During continued charging, the positive electrode material continues to lose electrons and lithium ions continue to be unembedded until the charging is completed.

1.6.6 Electrons leave from the negative electrode material and flow to the positive electrode through the outer circuit. Lithium ions that have lost electrons also escape from the graphite layers.

discharge process 1

1.6.7 Lithium ions removed from the negative electrode return to the positive electrode material through the electrolyte and separator, and combine with the electrons arriving at the positive electrode through the external circuit to form a relatively stable lithium-embedded positive electrode material.

discharge process 2  

Lithium Battery – Structural composition

lithium battery structural composition

Lithium Battery – Positive electrode material

Positive electrode materialLiCoO2Li(NiCoMn)O2LiMn2O4LiFePO4
Crystal StructureStratiformStratiformSpinelOlivine
Theoretical specific capacity/mAh·g-1274278148170
Actual specific capacity/mAh·g-1140-155130-22090-120130-150
The range of operating voltage/V3.0-4.33.0-4.353.5-4.32.5-3.8
Platform voltage/V3.6-3.73.6-3.73.7-3.83.2-3.3
Material processing propertyGoodHigherMediumBad
Cycle life/time>500>500>500>2000
Safety performanceBadBetterGoodExcellent
PriceHighHigherLowMedium
High Temperature PerformanceGeneralGeneralBadGood
Toxicity/Environmental conservationToxic cobaltToxic cobaltNon-toxicNon-toxic

Lithium Battery – Classification

Lithium batteries can be classified according to shape, shell and craft.

lithium battery classification 1
lithium battery classification classified according to shell
lithium battery classification craft

Tritek chooses coiled type cylindrical steel shell structure, mainly 18650 and 21700 batteries.

Lithium Battery – Performance parameter

  • Battery Capacity

The discharge capacity (Ah) of the battery at room temperature at 1I1(A) current to reach the termination voltage.

Formula: C=It, i.e. battery capacity (Ah) = current (A)x discharge time (h).

Battery capacity refers to the amount of power a battery can get or the amount of power it can store.

The capacity is determined by the active substance of the electrode and is mainly affected by the discharge rate and temperature (so strictly speaking, the battery capacity should specify the charging and discharging conditions).

Click to learn How To Choose the Right Battery Capacity for Improved E-Bike Performance

the relationship between capacity and discharge current

The relationship between capacity and discharge temperature:

the relationship between capacity and discharge temperature
  • Voltage

It refers to the potential difference(PD) between the positive and negative electrodes of the battery (the voltage is affected by the battery power, temperature and other conditions).

Lear more about —> How Battery Voltage Affects E-Bike’s Performance

How to Choose the Right Voltage for Electric Motorcycle Batteries

  • Open-circuit voltage(OCV)

The voltage of the battery when it is not connected to an external circuit or external load. The open circuit voltage is related to the remaining energy of the battery, and the power display is based on this principle.

  • Closed-circuit voltage(CCV)

It refers to the potential difference(PD) between the positive and negative electrodes of the battery in the working state, i.e., when there is current flowing through the circuit.

  • Stage of Charge(SOC)

Is the remaining battery power ratio, equal to the remaining battery power/total battery power, SOC=0% indicates that the battery is completely drained, SOC=100% indicates that the battery is fully charged.

SOC calculate by Battery Management System (BMS).

soc curve  

  • Depth of Discharge(DOD)

It refers to the discharge depth of the battery, i.e., the percentage of the battery discharge to the rated battery capacity, contrary to SOC, DOD=100% indicates that the battery is out of power, and DOD=0% indicates that the battery is fully charged. The relationship between DOD and SOC is as follows: DOD+SOC = 1.

dod  

  • Internal Resistant

Refers to the resistance of current flowing through the battery when the battery is working. Generally divided into AC(alternating current) internal resistance and DC(direct current) internal resistance. Normally, a battery with a small internal resistance has a strong discharge capacity, while a battery with a large internal resistance has a weak discharge capacity. Large internal resistance of the battery will generate a large amount of joule heat, which will cause the internal temperature of the battery to rise, resulting in the reduction of the working voltage of the battery discharge, the discharge time shortened, and the battery performance and service life are seriously affected.

  • Temperature rise

Heat generated when an electric current passes through a conductor Q=I2Rt  

Internal resistance of cell

Ohmic resistance

The active substance of the positive and negative electrodes, the current collector, and the electronic resistance of the lead

Obey Ohm’s law, is fixed, make the voltage drop is proportional to the current

Ionic resistance inside an electrolyte, separator, or electrode

Contact resistance between the active substance of the positive and negative electrodes and the current collector

Polarization resistance

Active polarization resistance – caused by an electrochemical reaction on the electrode surface

The polarization resistance increases linearly with the logarithm of current density (discharge rate)

Concentration difference polarization resistance – the difference in the concentration of reactants and products on the electrode surface and in the volume phase, as a result of material transfer.

internal resistant

  • Discharge platform time

Discharge platform time refers to the discharge time to a certain voltage when the battery is fully charged. Discharge platform is a feature of the battery discharge curve. The unit is minute.

  • Charge constant current ratio

The ratio of constant current charge to the total charge of constant current and constant voltage. The higher the constant current ratio, the better the battery performance. The unit of constant current ratio is the percentage (%).

  • Cycle life

Battery cycle life refers to the number of charging and discharging times experienced by a battery under a certain charging and discharging system,when the battery capacity drops to a certain specified value.

Understand Lithium Battery Lifespan: How Many Charging Cycles Can You Expect?

  • Charge-discharge rate

Charge and discharge rate refers to the current required by the battery to discharge the rated capacity within the specified time. 1C is equal to the rated capacity of the battery and is usually represented by the letter C.

  • Self-Discharge rate

Self-discharge rate, also known as charge retention, refers to the ratio of the reduced capacity to the initial capacity when the battery is fully charged and stored for a certain time under certain conditions in an open circuit state. The unit of self-discharge rate is the percentage (%).

The smaller the self-discharge, the better, and the larger the charge retention, the better. Batteries with large self-discharge tend to show a rapid voltage drop after storage for a while. It is mainly affected by the manufacturing craft, materials, storage conditions and other factors.

  • Charging efficiency

A measure of the extent to which a battery can store chemical energy by converting the electrical energy consumed during charging. It is mainly affected by battery craft, formula, ambient temperature, charging rate and other factors. In general, the higher the charging rate, the lower the charging efficiency. The lower the temperature, the lower the charging efficiency.

  • Discharging efficiency

Under certain discharge conditions, the ratio of the actual quantity released to the rated capacity of the battery from discharging to the endpoint voltage. It is mainly affected by discharge rate, ambient temperature, internal resistance and other factors. In general, the higher the discharge rate, the lower the discharge efficiency. The lower the temperature, the lower the discharge efficiency.

  • Energy

Formula: Energy (Wh) = working voltage (V)× working current (A)× working time (h)= voltage × capacity

  • Specific energy(Energy density)

The energy given by a unit mass or unit volume of a battery is called mass-specific energy or volume-specific energy, also known as energy density.

Usually expressed in terms of volume energy density (Wh/L) or mass energy density (Wh/kg). If a lithium battery weighs 143g, its rated voltage is 3.2V, its capacity is 6500mAh, and its energy density is 145Wh/kg (3.2*6500/143).

Energy densityLead-Acid BatteryNi-Cd BatteryNi-MH BatteryLi-ion battery
Wh/kg30-5050-6060-70110-220
Wh/L50-80130-150190-200350-400

 

1.4 Lithium Battery – Charging and discharging curve

  • Charging curve

charging curve
 

  • Discharging curve

discharging curve
 

Lithium Battery – High and low temperature performance

lithium battery charge and discharge rate performance

 

Lithium Battery – High and low temperature performance

lithium battery high and low temperature performance

Lithium Battery – Cycle performance at different charging rates

samsung ngm inr21700 50e

 

Lithium Battery – Cycle performance at different discharging rates

lithium battery cycle performance at different discharging rates

 

Lithium Battery – Effect of DOD on cycle life

lithium battery effect of dod on cycle life

Lithium Battery – Effect of temperature on cycle life

lithium battery effect of temperature on cycle life

 

Lithium Battery – Room temperature rate discharge, discharge efficiency, temperature rise

Room temperature rate discharge 

1. Charge: constant current constant voltage, current 1.25A (0.5C), upper voltage 4.2V, cut-off current 0.05A (0.02C);

2. Set aside: 10min;

3. Discharge: with different sizes of current constant discharge, the lower limit voltage 2.75V;

4. Discharge efficiency = discharge capacity at each rate /0.5A (0.2C) discharge capacity;

lithium battery room temperature ratio discharge, discharge efficiency, temperature rise
 

0.2C capacity/Ah

Discharge current

0.5A

1.25A

2.5A

5.0A

7.5A

10.0A

Discharge rate

0.2C

0.5C

1C

2C

3C

4C

 

 

2.573

Discharge capacity/Ah

2.573

2.483

2.473

2.465

2.498

2.516

Discharge efficiency/%

100.0%

96.5%

96.1%

95.8%

97.1%

97.8%

Maximum temperature/°C

27.3

29.6

33.6

44.1

55.8

67.5

Temperature rise/°C

0.9

3.5

7.3

17.8

29.5

41.1

 

Lithium Battery – Room temperature rate charge, charge efficiency, temperature rise

Room temperature rate charge

1. Discharge: current 0.52A (0.2C), constant current to 2.75V;

2. Set aside: 10min;

3. Charging: with different sizes of current constant current constant voltage charging, upper voltage 4.2V;

4. Charging efficiency = constant current charging capacity at each rate /2.5A (1C) discharge capacity;

room temperature ratio charge
 

Initial capacity

Discharge current

1.25A

2.5A

5.0A

7.5A

Discharge rate

0.5C

1C

2C

3C

 

 

2.481

Constant current charging capacity/Ah

2.285

2.161

1.960

1.771

Constant current charging efficiency/%

92.1%

87.1%

79.0%

71.4%

Maximum charging temperature

29.9

37.1

50.9

63.4

Temperature rise/°C

2.5

10.0

24.0

36.9

 

Lithium battery – Multiplier charge/discharge temperature rise principle

The heat generated by the charge and discharge process of lithium battery mainly consists of three parts

Form: heat of polarization, ohmic heat, heat of reaction, heat of reaction is endothermic reaction

Be responsible for;

The higher the charge and discharge ratio (current) is, the higher the internal resistance of polarization is, and the higher the heat production is.

Lithium Battery – Match battery PACK

 

“Eight consistent” match battery pack principles: consistent capacity, consistent internal resistance, consistent constant current ratio, consistent platform time, consistent self-discharge, consistent voltage, consistent carried charge, consistent cycle.

Material knowledge of cell

Material knowledge of cell – Positive electrode

Classification of positive electrode: lithium iron phosphate, Ternary NCM/NCA, lithium manganese, lithium cobalt oxides, Lithium ferromanganese phosphate.

Material knowledge of cell – Negative electrode

Classification of negative active substances: artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, hard carbon, carbon fiber.

Material knowledge of cell – Conductive agent

The conductive agent is to ensure that the electrode has good charge-discharge performance. A certain amount of conductive material is usually added when the electrode plate making, so as to improve the charge-discharge efficiency of the electrode.

Type

Grain size

nm

Electrical resistivityΩ·m

Specific surface area

m2/g

Feature

Super P

40

≤0.01

62

Small grain of conductive carbon black, which can be used in both positive and negative electrodes, have no lithium storage function at all and only conduct electricity.

KS-6

flake-like

≤0.0001

21±3

Large grain graphite powder, feather shape, has lithium storage function.

VGCF

150

≤0.0001

13

High efficiency superconducting carbon black, branched chain, high purity, especially good conductivity for lithium batteries.

CNTs

7-15

≤0.0001

190-260

Form of conductive network, good conductive performance, good thermal conductivity to reduce the polarization of the battery, improve the high and low temperature performance of the battery, high dispersion requirements.

GNs

flake-like

≤0.000001

5±0.5

Surface contact conductive agent, conductive performance is the best, the dispersion requirements are too high.

Material knowledge of cell – Adhesion agent

Type PVDFPoly(vinylidene fluoride) CMC Carboxymethyl Cellulose SBRPolymerized Styrene Butadiene Rubber
Structure pvdf cmc sbr
Composition White powder Vinylidene fluoride homopolymer White powder Carboxymethyl substituent cellulose White emulsion Butadiene and ABS
Function Positive electrode adhesion agent Thickening agent, settling agent, stabilizing agent Negative electrode adhesion agent
Supplier Solvay S.A./ARKEMA/KUREHA Group Ashland/CPKelco/Dow/DAICEL JSR/Zeon/Sojitz

    Material knowledge of cell – Foil

    Current collector refers to the collection of current generated by the active substance of the battery to form a larger current output, which is in full contact with the active substance, small internal resistance, with good conductivity.

    Aluminum foil – positive electrode potential is high, oxide film is very dense which can prevent current collector oxidation. The copper foil oxide film is looser, in order to prevent oxidation, low potential is better, and it is difficult for Li and Cu to form lithium-embedded alloy at low potential.

    aluminum foil

    lithium-embedded

    Copper foil-copper surface oxide film belongs to semiconductor, electron conduction, the oxide film is too thick, high resistance; AL will undergo LiAl alloying at the low potential of the negative electrode, which means that Al will be lithium-embedded at the negative electrode.

    copper foil

    Material knowledge of cell – Lug

    The lug is a metal conductor that leads from the positive and negative electrode of the battery. Generally speaking, the lugs of the positive and negative electrode of the battery are the contact points when charging and discharging.

    Project Positive electrode lug Negative electrode lug
    Material Aluminium strip Nickel strip
    Feature Good electrical conductivity Good electrical conductivity and corrosion resistance
    Purity ≥99.5% ≥99.9%
    Weldability Easy to weld Easy to weld

    Material knowledge of cell – Separator

    Material: single layer PE (polyethylene) or three layers of composite PP(polypropylene) +PE+PP

    separator

    Function:

        1. Separate the positive and negative electrode of the battery to prevent short circuit;
        2. Adsorb battery electrolyte to ensure high ionic conductivity;
        3. Some also prevent the transfer of substances harmful to the battery reaction between the electrodes;
        4. Ensures that the battery reaction stops when an abnormal occurs, improving the safety performance of the battery.

      Material knowledge of cell – Electrolyte

      The electrolyte plays a role of conducting ions between the positive and negative electrodes of the battery, and is the bridge connecting the positive and negative electrode materials. Only operate in a dry environment (such as a glove box with moisture less than 20ppm).

      electrolyte

          1. Lithium salt: LiPF6
          2. Solvent: EC、DMC、EMC
          3. Additive: Film-forming agent、anti-overcharge、flame retardant、stabilizing agent etc.

        Material knowledge of cell – Steel shell

        The main function of the battery steel shell is to provide a good electrochemical environment.

        steel shell

        Performance characteristics of battery steel shell:

            1. Material processing performance is good, high precision, high strength;
            2. The surface hardness of the battery is high, and it has certain bearing property.
            3. Nickel plating: The battery has good corrosion resistance.
            4. The bottom generally cannot be laser spot welding.

          Material knowledge of cell – Cap

          The main function of the cap is to provide battery sealing function, provide the role of safety valve, to play the role of positive conductive terminal.

          battery cell cap

          Technical requirements for cap:

              1. The material of the sealing ring is polyethylene propylene, 135℃ nondeformable;
              2. Smooth surface, uniform wall thickness, non-destructive appearance;
              3. The breaking pressure of cap is 1.2±0.1Mpa, and the opening pressure is 1.8±0.1Mpa;
              4. Small gasket does not deform and melt at 350℃ etc.;
              5. Caps are generally not soldering at high temperature.

            Material knowledge of cell – Safety device

            Some manufacturers’ cylindrical lithium iron phosphate battery built-in gas release vent and safety valve(CID) to improve its safety protection function, “double protection” effectively solves the safety problems caused by high temperature in extreme conditions such as overcharge, short circuit, collision, etc.

            cell design for e-bike battery

            Gas release vent device: When chemical reactions occur in the battery, the gas generated in the battery accumulates in the gas release vent through the vent hole and upper and lower openings, which helps the gas in the battery to disperse, ensure the air-pressure balance in the battery cell, and avoid potential safety risks.

            Safety valve(CID): When the internal pressure reaches 1.2±0.1Mpa, the positive lug and cap are disconnected, and the chemical reaction in the battery is suspended. When the internal pressure reaches 1.8±0.1Mpa, the safety valve will be opened and the gas discharged to avoid the risk of explosion.

            Craft knowledge of cell

            Craft knowledge of cell – Craft process

            Below is the craft process of cell:

            the craft process of cell

            Structure knowledge of cell

            Structure knowledge of cell

            Below is mind map of structure knowledge:

            structure knowledge of cell

            structure knowledge of cell

            Structure knowledge of cell – Square lamination

            structure knowledge of cell square lamination

            Structure knowledge of cell – Square Z shape lamination

            structure knowledge of cell square z shape lamination

            Structure knowledge of cell – Square filmed lamination

            structure knowledge of cell square filmed lamination

            Structure knowledge of cell – Square multi lug coiling

            multi lug

            Structure knowledge of cell – Cylindrical single& multi lug coiling

            cylindrical single& multi lug coiling

            Structure knowledge of cell – Cylindrical full lug coiling

            cylindrical full lug coiling

            Structure knowledge of cell – Summary

            Cell structure Methods of fabrication Manufacturing characteristics
            Z shape lamination Manipulator positioning laminate More edge, difficult to control burr, drop powder; Hard to bind tightly; Low efficiency;
            Coiling lamination Diaphragm winding, pole piece single piece More edge, difficult to control burr, drop powder; Complex machine structure; It is difficult to control alignment accuracy and high efficiency.
            Square coiling Continuous winding Less edge, easy to control burr, less powder, high manufacturing efficiency; Good consistency; Good mass flow design; But batteries are prone to deformation;
            Cylindrical coiling Continuous winding Less edge, easy to control burr, less powder, high manufacturing efficiency; Good consistency; Uniformity of the effusion; Difficult to design mass flow.

            Project Coiling Lamination
            Internal resistance Higher internal resistance because there are usually fewer lugs (except full lugs) Low internal resistance. It is equivalent to multiple small lug plates in parallel, reducing the internal resistance
            Rate It is difficult to complete the discharge fully with a large current because of fewer lugs. It is easier to complete large current discharge in a short time with multi lug plate parallel
            Energy density Monomer difference is not obvious In general, lamination is superior and PACK convenient
            Process control The process is simple and easy to automate The process is tedious, the rolled yield is low, and it is not easy to automate

            Summary:The advantage of roiling is that the process is easy to control, and the advantage of lamination is that the battery energy density is slightly higher and PACK convenient.

            Project Multi lug Full lug
            Technological difficulty The overall process of the craft is difficult:Whether use metal mold or laser cutting, can not avoid the production of a large number of dust and metal chips, is a great threat for the battery safety performance (square)Need to weld lug, low efficiency, lack of safety (cylinder)

            The overall process difficulty of the full lug craft is small:

            1. The craft difficulty of coating,  pressing, slitting, winding and other processes is less than multi lug craft;

            2. High automation degree

            Coiling This structure will cause tension fluctuations during coiling, resulting in electrode core folding and other problems. This structure will be beneficial to reduce the tension fluctuation during coiling and the coiling problem will be less
            Space utilization When the shell length/width is small, the relative utilization rate is higher The longer the length, the higher the utilization rate
            Rate performance General rate High-rate
            Equipment investment More Less

            Summary:

            Multi lug deficiency points more, especially a certain security risks; But when the capacity is small, it has the advantage of energy density.

            The full lug project is better overall, especially in the capacity battery, the full lug project should be the first choice.


            For more articles on lithium-ion cells:

            21700 vs 18650 battery, battle on LEV

            Which 18650 battery is best for an e-bike?

            Tritek is a professional lithium battery power solution company founded in Shenzhen. Tritek offers a wide range of power solutions for LEV lithium-ion batteries for both commercial and domestic usage.

            The experts at Tritek have 12 years experience in the design, R&D, and sales of LEV lithium-ion batteries. The lithium-ion batteries produced at Tritek are compliance with global certification standards for LEV batteries, such as EN15194:2017, UN38.3, CE, FCC, CB, UL, etc. Tritek has already set up customer service centers in Spain in 2022 and in Germany in 2023, also planning to set up in the United States to enhance the consumer experience.

            You can get a customized lithium battery pack for your light electric vehicles,such as E-bike, E-motorcycle, Cargo-bike, etc.If you’d like to customized lithium battery for you LEV, feel free to CONTACT US.

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