【Battery】30 minutes to become a half lithium battery expert

As more and more people need to understand the energy storage industry, and the battery is the core of energy storage,Csip Technology has compiled relevant information and organized an in-depth information to facilitate your study. I believe that after reading it, you will form a more professional knowledge framework for batteries and become a half expert, wish you happy learning ~

I. Battery family

 

 

* The name "lithium battery" originally referred to lithium metal batteries, a primary battery, but because it is easy to explode, it has long ceased to be used. Now the lithium battery is generally known as lithium-ion batteries.
Normally we use No. 7, No. 5 batteries, are dry batteries; early brick mobile phone "big brother" is the use of nickel-hydrogen batteries; small tram is usually used in the lead-acid batteries, a group of four square packed together; and our mobile phones, laptops, and even electric cars, most of the use of lithium-ion batteries. batteries.
Comparison of the characteristics of the main electrochemical batteries for energy storage

 

2.The battery terminology analysis

 

SOX: the full name is State Of X, the state of the battery description, H is the English Health, C is capacity, P is power, E is energy, a bit like the parameters of the engine, displacement, power, energy, running time, etc.. The general meaning is the same.

 

SOC: (State of Charge) refers to the battery's charge, the battery's power is like the water in a bucket, at a certain moment in the battery contains how much available power that is called the moment the battery's SOC. in the case of completely discharged SOC is 0, fully charged SOC is 1. available capacity / actual capacity.

 

DOD: (Depth of Discharge) refers to the depth of discharge of the battery, the DOD of the battery is 0 when it is fully discharged and 1 when it is fully discharged.

 

SOH: (State of health) refers to the current actual capacity of the battery / initial rated capacity of the battery, as the battery ages, SOH will continue to decrease. Generally measured by capacity and internal resistance
SOH is most commonly defined in the literature using the decay of battery capacity, and is given as follows

 

Where: Caged is the current capacity of the battery; Crated is the rated capacity of the battery.

3. lithium battery classification

By practical performance: power type (short run, short time high power output) and energy type (long run, high energy storage)

 

According to the appearance: cylindrical, square (steel/aluminum shell), soft pack (aluminum-plastic film)

 

By electrolyte material: liquid lithium-ion batteries (LIB) and polymer lithium-ion batteries (PLB)
Liquid lithium-ion batteries use a liquid electrolyte (currently the majority of batteries used for power applications). Polymer Li-ion batteries are replaced by a solid polymer electrolyte, which can be either 'dry' or 'gel', with most currently using polymer gel electrolytes. With regard to solid-state batteries, this strictly means that the electrodes and electrolyte are both solid.

 

By anode material: Lithium Iron Phosphate (LFP), Lithium Cobaltate (LCO), Lithium Manganate (LMO), (Binary batteries: Lithium Nickel Manganate/Lithium Nickel Cobaltate), (Ternary: Lithium Nickel Cobalt Manganate (NCM), Lithium Nickel Cobalt Aluminium Acid (NCA)).

 

By anode material: lithium titanate (LTO), graphene batteries, nano carbon fibre batteries.

 

 

18650 battery

18650 is a type of lithium-ion battery, similar to dry cell No. 7, No. 5, No. 1 and so on.

 

18650 is the originator of lithium-ion battery - Japan SONY company in order to save costs and set a standard lithium-ion battery model, where 18 means the diameter of 18mm, 65 means the length of 65mm, 0 means for the cylindrical battery. Common 18650 battery is divided into lithium-ion batteries, lithium iron phosphate batteries. Lithium-ion battery voltage is 3.7v nominal voltage, 4.2v charge cut-off voltage, capacity is usually 1200mAh-3350mAh, lithium iron phosphate battery nominal voltage is 3.2V, charge cut-off voltage is 3.6v, common capacity is 2200mAh-2600mAh.

 

Advantages of 18650.

Standardization: 18650 batteries are the earliest, most mature and stable lithium-ion batteries, Japanese manufacturers have accumulated a high level of consistency, and good replacement when problems occur with individual cells.
Safety: 18650 batteries are generally steel-cased, which is safer in terms of possible collision problems; and as the production process of 18650 batteries continues to improve, so does the safety. Steel-cased lithium batteries are prone to explosion problems, but nowadays 18650 cells are designed with a safety valve that not only releases excessive internal pressure, but also physically disconnects the battery from the external circuit, which is equivalent to physically isolating the cell to ensure the safety of the other cells in the pack.

 

However, other batteries also have certain advantages over this, for example, lithium polymer batteries have high relative safety, high energy density, low strength packaging film and will not explode, the worst being combustion. The ability to customise designs, but the corresponding higher R&D costs and reduced versatility.

 

Lithium iron phosphate (LFP) batteries

Lithium iron phosphate (molecular formula LiFePO4, Lithium Iron Phosphate , also known as lithium iron phosphate, lithium iron phosphorus, or LFP), is a lithium-ion battery cathode material, also known as lithium iron phosphorus battery, featuring no cobalt and other precious elements, low raw material prices and phosphorus, lithium and iron exist in the earth's rich resource content, there will be no supply problems. Its working voltage is moderate (3.2V), large capacity (170mAh/g), high discharge power, fast charging and long cycle life, and high stability in high temperature and high heat environments.

 

4. lithium battery voltage, capacity

The voltage of lithium-ion batteries varies with discharge current, ambient temperature and positive and negative electrode materials.

 

This graph shows the discharge curve of a Panasonic 2550mAh Li-ion battery using lithium cobaltate as the cathode material. The three curves from top to bottom represent the change in voltage and capacity when using three different discharge currents.

 

Firstly, the voltage is continuously changing during the charging and discharging process. Taking 490mA as an example, the open circuit voltage is 4.2V when the battery is fully charged. As the discharge progresses, the voltage (vertical) slowly decreases and the discharge capacity (horizontal) gradually increases until the voltage starts to drop steeply at 3.5V. Although the voltage changes throughout the discharge process, for simplicity, only the mean value of 3.7V in the gentle discharge section of the curve is labelled as the battery voltage. This voltage is also called the nominal voltage.

 

This voltage is measured at low current and room temperature and will decrease as the discharge current increases and the temperature decreases.

 

Another important factor affecting battery voltage is the positive and negative electrode materials. The Panasonic batteries mentioned above use lithium cobaltate and graphite as the positive and negative electrode materials respectively, which were the standard materials for the entire lithium battery industry in previous years. With the application of new materials in batteries, a number of 3.6V or 3.8V lithium batteries have emerged in the last two years, which use different cathode materials, both of which are capable of increasing energy density, i.e. storing more power per unit weight and volume, compared to lithium cobaltate batteries.

 

Battery capacity is divided into rated capacity and actual capacity.

 

1. The actual capacity is the amount of electricity actually released by the battery under certain discharge conditions. The actual capacity is always lower than the theoretical capacity.

2. Rated capacity refers to the minimum amount of power that should be discharged from a battery under certain discharge conditions when the battery is designed and manufactured.

Battery capacity is generally calculated in AH (ampere hours, amp-hours) and is generally marked as mAh (milli-ampere hours) on a single battery for convenience. If the battery is rated at 1300mAh, i.e. 130mA of current is discharged to the battery, then the battery can continue to operate for 10 hours (1300mAh/130mA = 10h). This is an analysis in an ideal state; it is not always possible to have a constant current at a certain value when digital devices are actually working.

The capacity of 18650 lithium batteries is generally between 1200mah and 3600mah.
Nowadays, the unit for measuring the capacity of mobile phone batteries is mAh. High school knowledge tells us that this is the unit of power and also needs to be multiplied by a voltage to be the unit of energy.
Battery capacity calculation method.

 

How battery energy is calculated.

 

Wait a minute, a mobile phone consumes energy, how can it be measured in units of power? This is because in portable electronic devices such as mobile phones, lithium-ion batteries use lithium cobaltate as the cathode in order to reduce the size of the battery, which has a very high compaction density. Because the positive electrode of mobile phone batteries is made of the same material, the voltage of the battery is relatively similar (theoretically 3.7V, depending on the process of different battery manufacturers), and because there is often only one single battery in a mobile phone (please ignore the recent release of a mobile phone with two 2000mAh batteries), the capacity alone can actually measure the amount of energy stored in the battery. size.

 

However, the battery in a computer is marked with both the capacity (charge) and the energy of the battery. This is because there is not just one single battery in the computer, there are many batteries connected in series and parallel to form a battery pack, so you cannot use the capacity of the battery to measure it. High school knowledge tells us that for the same type of battery, when the batteries are connected in parallel, the voltage of the battery pack will remain the same, but it will increase the capacity of the battery pack; when the batteries are connected in series, the capacity of the battery pack will remain the same, but it will increase the voltage of the battery pack.

 

Batteries in parallel

 

Batteries in series

Now take the laptop I have on hand as an example, its specifications say "57.4Wh/[email protected] (typical capacity)", which means that this is a battery pack formed by four lithium-ion batteries with a capacity of 3782.5mAh and a voltage of 3.8V connected in series and parallel. 7565mAh x 7.6V = 57.494Wh. So why not two 7565mAh batteries in series? Hey, 7565mAh is too large a battery capacity, which is more demanding to produce; secondly, 3782.5mAh is actually the battery used on mobile phones, and for factories, as long as one production line can produce batteries for mobile phones and computers at the same time, this can reduce costs.
Then I went to look at the battery specification parameters of another laptop, see the chart below.

 

At first glance, the capacity of this battery is just a little larger than that of a mobile phone battery, so how can it be used for a laptop? But one look at its voltage and it dawned on me. This is a series-parallel connection of 6 batteries with a capacity of 2200mAh and a voltage of 3.6V, the legendary 6-cell Li-ion battery (with 6 individual cells inside). The shape of this battery (long) and the 2200mAh capacity show that it is an 18650 battery (that is, a cylindrical battery with a cell diameter of 18mm and a height of 65mm).
So, to measure the battery's range, it is more reliable to look at the energy and do the math yourself.

 

5. Why choose lithium-ion batteries

 

Lightweight

The weight energy density of lithium-ion batteries is currently generally in the range of 200 to 260wh/g; lead-acid batteries are generally in the range of 50 to 70wh/g, and NiMH batteries are in the range of 40 to 70wh/kg. This means that with the same capacity, the other two batteries are 3 to 5 times heavier than lithium batteries. So in terms of light weight of energy storage devices, Li-ion batteries have the absolute advantage.
The volume capacity density of Li-ion batteries is usually about 1.5 times that of lead-acid batteries, and the energy density of NiMH batteries is only 60-80% of that of Li-ion batteries, so the volume of Li-ion batteries is also smaller for the same capacity.

Fast charging

As lithium ions are active and move faster inside the battery, the charging current is higher and the charging speed is faster, a lithium ion battery can be fully charged in about 3 hours; while NiMH batteries are slow to charge and take roughly 1 day to fill.
No memory effect
The memory effect is an effect of crystallisation of the battery contents due to use. In the case of nickel-based batteries, the crystals on the nickel plates become coarse over time, affecting the contact with the electrolyte and resulting in a loss of capacity. A few complete charges and discharges will refine the crystals and allow the capacity to be partially restored, this is called "activation".
Lithium batteries do not have a memory effect and only require 3-5 normal charge and discharge cycles to activate the battery and restore normal capacity.

Environmental protection

Under the environmental protection policy, in order to protect the environment can reduce pollution, in the production, use and recycling of lead-acid batteries, there is pollution caused by improper handling, while the lithium battery due to its own packaging and sealing work is more perfect, will be relatively environmentally friendly.

 

6.the safety of lithium batteries

 

Lithium batteries have the advantages of lightweight, fast charging, etc., so why are other secondary batteries such as lead-acid batteries still in circulation in the market?
In addition to the cost, different fields of application and other issues, there is another reason is safety.

 

Lithium is the most active metal in the world, and because its chemical properties are too lively, when exposed to air, lithium metal will react with oxygen in a violent oxidation reaction, and therefore prone to explosion and combustion. In addition, the lithium battery will also produce internal redox reactions during the charging and discharging process, the explosion and spontaneous combustion are mainly due to the accumulation of lithium batteries after the heat, too late to diffuse and release the result. Simply put, lithium batteries generate a lot of heat during charging and discharging, which leads to an increase in the internal temperature of the battery and uneven temperatures between individual cells, resulting in unstable battery performance.
Thermal runaway

 

The unsafe behaviour of lithium-ion batteries (including overcharging and discharging, rapid charging and discharging, short circuiting, mechanical abuse conditions and high temperature thermal shock) can easily trigger dangerous side reactions within the battery that generate heat and directly damage the passivation film on the negative and positive electrode surfaces.

 

There are various triggering causes for thermal runaway accidents in lithium-ion batteries, which can be classified into mechanical abuse trigger, electrical abuse trigger and thermal abuse trigger according to the characteristics of the trigger.

Mechanical abuse: refers to pins and needles, crushing and heavy impacts caused by car collisions etc.

Electrical abuse: generally caused by improper voltage management or malfunctioning electrical components, including short circuits, overcharging and overdischarging
Thermal abuse: caused by overheating due to improper temperature management.

 

These three triggers are interrelated, as shown above, mechanical abuse will generally cause the deformation or rupture of the battery diaphragm, resulting in a direct contact short circuit between the positive and negative electrodes inside the battery and electrical abuse; and under electrical abuse, Joule heat and other heat production increases, causing the battery temperature to rise and develop into thermal abuse, further triggering the chain of heat production side reactions inside the battery and eventually leading to the occurrence of thermal runaway of the battery.

 

 

Thermal runaway is caused by a battery whose heat generation rate is much higher than its heat dissipation rate, and the heat accumulates in large quantities and is not dissipated in time. In essence, thermal runaway is a positive energy feedback loop: an increase in temperature causes the system to get hotter, which in turn causes the system to get hotter.

 

The process of thermal runaway:

 

1.The battery heats up internally, the SEI film on the surface decomposes at high temperatures and the lithium ions embedded in the graphite react with the electrolyte and binder, further pushing the battery temperature up to 150°C, at which temperature a new violent exothermic reaction takes place.

 

2.When the battery temperature reaches above 200°C, the cathode material decomposes, releasing a large amount of heat and gas, and the battery begins to bulge and continues to heat up. 250-350°C the lithium-embedded negative electrode begins to react with the electrolyte.

 

3.The charge state cathode material starts to decompose violently, the electrolyte reacts with oxidation violently, releasing a lot of heat, generating high temperature and a lot of gas, and the battery burns and explodes.

 

Lithium cobalt-acid batteries, when fully charged, still have a large amount of lithium ions left in the positive electrode. In other words, the negative electrode can not accommodate more lithium ions attached to the positive electrode, but in the overcharge state, the excess lithium ions on the positive electrode will still swim to the negative electrode, because it can not be completely accommodated, the negative electrode will form lithium metal, because this lithium metal is a dendritic crystal, and therefore is called dendrites, dendrites too long easily pierce the diaphragm, resulting in internal short circuit. As the main component of the electrolyte is carbonate, it has a low ignition and boiling point, so that it can burn or even explode at higher temperatures.

 

 

 In the case of polymer lithium batteries, the electrolyte is colloidal and prone to even more violent combustion.

 

To address this, scientists have tried to replace the cathode material with a safer one. The material of lithium manganate has the advantage that it ensures that the lithium ions from the positive electrode can be fully embedded in the carbon pores of the negative electrode at full charge, instead of having some residue in the positive electrode as in the case of lithium cobaltate, avoiding to some extent the creation of dendrites. The solid structure of lithium manganate makes its oxidation performance much lower than that of lithium cobaltate, and even if there is an external short circuit (rather than an internal short circuit), the precipitation of lithium metal is largely prevented from causing combustion and explosion. Lithium iron phosphate is safer due to its higher thermal stability and lower oxidation capacity of the electrolyte.

 

Ageing

 

The ageing of lithium-ion batteries is manifested externally by capacity degradation and increased internal resistance, and internally by the loss of active materials and available lithium ions. The risk of lithium precipitation from the cathode is also more likely to occur when the cathode material ages and decays and the cathode capacity is insufficient. Under conditions such as over-discharge, the cathode to lithium potential can rise above 3V, which is higher than the dissolution potential of copper, resulting in the dissolution of the copper collector.

The dissolved copper ions will precipitate on the surface of the cathode and form copper dendrites. The copper dendrites can pass through the diaphragm and cause an internal short circuit, seriously affecting the safety performance of the battery.

 

In addition, the overcharge resistance of ageing batteries decreases to a certain extent, mainly due to the increase in internal resistance and the reduction of active material in the positive and negative electrodes, resulting in an increase in Joule heating during the overcharge process, which can trigger a side reaction at a lower overcharge level and lead to thermal runaway. In terms of thermal stability, the precipitation of lithium from the negative electrode can lead to a dramatic decrease in the thermal stability of the battery. In short, the safety performance of an ageing battery will be greatly reduced and the safety of the battery will be seriously compromised.

 

The most common solution is to equip the battery storage system with a battery management system (BMS), such as the 8000 18650 batteries used in the Tesla Model S. This is achieved through its battery management system, which monitors the various physical parameters of the battery in real time, evaluates the state of use of the battery, and carries out online diagnosis and early warning, and also enables discharge and pre-charge control, battery equalisation management and thermal management. management and thermal management.  

 

7. lithium battery applications At this stage, in the lithium battery energy storage system, the application is mostly lithium iron phosphate (LFP) batteries. Lithium iron phosphate batteries have a high operating voltage, high energy density, long cycle life, green and a series of unique advantages.

 

Ternary Lithium Battery is a lithium battery with lithium nickel cobalt manganate or lithium nickel cobalt aluminate as the positive electrode material, graphite as the negative electrode material and lithium salt based on lithium hexafluorophosphate as the electrolyte. Because the positive electrode material contains three metal elements, namely nickel, cobalt and manganese/aluminium, it is named "ternary". Ternary lithium batteries are mainly used in the field of power batteries.