Why is Your Marine Lithium Battery Sooo Hot?—-Thermal Runaway

Maybe you think thermal runaway is very far from your marine lithium battery because you are in the sea or river which are in a quite temperature. Do not ignore some small points.

What is Thermal Runaway?

Thermal runaway refers to the chain reaction phenomenon caused by various triggers, which causes the battery to emit a lot of heat and harmful gases in a short period of time, and in severe cases it can even cause the battery to catch fire and explode. There are many reasons for thermal runaway, such as overheating, overcharge, internal short circuit, collision, etc. Battery thermal runaway often starts from the decomposition of the negative electrode SEI membrane in the battery cell, and then the diaphragm decomposes and melts, resulting in a reaction between the negative electrode and the electrolyte, followed by the decomposition of the positive electrode and the electrolyte, causing a large-scale internal short circuit, causing the electrolyte to burn, and then spread to other batteries, causing serious thermal runaway, causing the entire battery pack to spontaneously ignite.

Some people make an experiment about thermal runaway of a LiFePO4 battery and they give out some characteristics of the thermal runaway flue gas of lithium iron phosphate battery: the amount of flue gas is small, the release rate is low, and no oxygen is released. According to experiment data, we can know that:

1.The self-heating starting temperature of lithium iron phosphate is 144.7℃, and the starting temperature of thermal runaway is 205.4℃, which is the most difficult for thermal runaway to occur.

2.The maximum temperature of thermal runaway is 319.3℃, and the lowest thermal runaway calorific value per unit body weight is 185.1J/g, which corresponds to the smallest thermal hazard of the monomer thermal runaway, and the smaller the tendency of thermal runaway diffusion.

We can know that, when LiFePO4 battery thermal runaway, some changes will appear.

• Temperature change
• Voltage change
• Outlook change

Why we need to know thermal runaway of marine lithium battery?

• To protect equipment
• To protect environmentCharacteristics of thermal runaway flue gas of LFP battery
• To avoid horrible hazards

Dangers of lithium battery thermal runaway

• Cause the danger of fire or explosion

If batteries experience thermal runaway, there is a chance that they could catch fire. Firefighters need to be aware that batteries are a potential fire hazard because lithium ion batteries can produce high heat, smoke and ash.

• Cause the danger of human health

If the battery catches fire, toxic fumes can be released into the surrounding environment and can be inhaled by people, posing a risk to their health.

• Pollute surrounding environment

If the battery catches fire, it can lead to toxic fumes being released into the air. These fumes can be harmful to the surrounding environment.

Factors of lithium battery thermal runaway

• HEAT ABUSE

Thermal abuse does not occur spontaneously inside the battery. Often due to mechanical abuse and other reasons, the internal temperature of the battery rises to the threshold value, and the lithium battery will be heated locally, leading to thermal abuse, which further induces temperature out of control and causes the battery to spontaneously ignite.

• ELECTRICTY ABUSE

What we said electricity abuse is that we wrongly give our lithium marine battery electricity such as Over-charging, Over-discharging and giving mismatched chargers.

• over-charging

During the charging process, Li+ prolongs from the positive electrode compound and reaches the negative electrode lattice. The positive electrode is in a lithium-poor state with a high potential and the negative electrode is in a lithium-rich state with a low potential.In order to balance the charge, the same amount of e-protrudes from the negative electrode and is embedded in the positive electrode.During the discharge process, just push back.

If the lithium battery is overcharged (the lithium battery is fully charged and continues to be charged), there will be an excess of Li+ embedded in the negative electrode, and the positive electrode will cause the structure to collapse (heat generation + oxygen release) due to the excessive de-embedding of Li+. The release of oxygen will further cause the electrolyte to decompose, the internal pressure of the battery increases, and the risk of thermal runaway increases greatly.

Studies have shown that the temperature of the negative electrode is always higher than that of the positive electrode during over-discharge. When an internal short circuit occurs, heat accumulates in the winding, thereby increasing the risk of thermal runaway.

• over-discharge

Many friends should have encountered that the lithium-ion batteries of smart phones and laptops use about 3-7%, and the system will automatically shut down/enter standby.This is to prevent the battery from over discharging (over discharge, continue to discharge below the threshold voltage).

If the over-discharge occurs too violently, the battery with the lowest voltage will “reverse”. This weakest and helpless battery will be reverse charged by other series batteries. The voltage is negative, and the structure of the active substance collapses. It is equivalent to a resistor, abnormal heat generation, game Over.

• short circuit

The internal short circuit is well understood, that is, the positive and negative electrodes in the single lithium marine battery are shorted, which is equivalent to suicide. “Suicide” is also divided into two types: “rapid suicide” and “chronic suicide”. Puncture is actually a “rapid suicide” among internal short circuits. The most dangerous one is immediate retribution and directly destroys everything.

The internal short circuit of chronic suicide is to mention the “lithium dendrites” that we have only talked about more in the past two years. In fact, this problem has plagued the battery industry for decades. Regarding the reasons for its formation in lithium-ion batteries, we will talk about it in a few steps:

1. When a lithium-ion battery with a liquid electrolyte is charged and discharged for the first time, the electrode material and the electrolyte undergo a very complex reaction at the solid-liquid interface, forming a passivation layer covering the surface of the electrode material, of which the SEI film on the negative electrode has a greater impact on the battery.This layer of SEI film is insoluble in organic solvents.Because the SEI film is an electron insulator, e-cannot pass, and because the SEI film is a good ion conductor, Li+ can easily pass through the SEI film.

2. SEI has so much credit, the result is that there is something called “lithium dendrite” to destroy it.When lithium ions are deposited unevenly on the surface of the negative electrode, they will become lithium dendrites, and it will continue to grow unevenly, becoming longer, thicker, and sharper.

3. When the lithium dendrites grow to a certain extent, the dendrites will detach from the electrode and become “dead lithium” that loses electrochemical activity, so the battery capacity is reduced.

4. These free lithium dendrites will pierce the battery SEI diaphragm.As a result, the internal insulation state of the lithium battery is broken, the internal short circuit, spontaneous combustion, and even explosion.

The growth rate of lithium dendrites is positively correlated with the Li+offset speed, so fast charging is easier to generate lithium dendrites, and more battery cycles will have more lithium dendrites (old electric vehicles are more likely to spontaneously ignite)

• MECHANICAL ABUSE

Physical attacks on lithium batteries by the external environment become mechanical abuse

• Piercing marine battery

This is the deadliest physical attack. A conductor (such as a steel needle with a diameter of 3mm) is inserted into the power battery, and the positive and negative electrodes are directly shorted. The thermal runaway speed is super fast, and flames begin to spray out almost instantly.

The mechanism of puncture is an internal short circuit. This situation is like a submarine being physically penetrated underwater. There is no other possibility except for all the crew to be buried on the bottom of the sea.

• Squeezing marine battery
• Hitting marine battery

The above three conditions are all situations that may be encountered during navigation, of which ship bumps are the most common.If the mechanical properties of the battery itself are not good, or the module is not made strong, there are certain risks.Of course, this possibility is very low. Some new energy vehicles have been recalled before due to poor wiring harness fixation, and it is not the internal wiring harness of the battery pack.

Extrusion, collision, and fire are all hazards that may arise from navigation collisions, which may cause problems such as battery structure collapse, SEI membrane tearing, electrolyte leakage, and internal short circuits.

• Marine battery getting old

When our lithium marine battery is old, the ability of transferring Li⁺ will decrease, it is easily to form lithium dendrite. We have illustrated that lithium dendrite will pierce SEI film and cause short circuit. Thus, if your lithium marine battery can not provide enough energy, you can ask professional ones for help, or you can measure capacity or SOH of your lithium battery. Please give an eye to your lithium marine battery before your trip.

Is lithium battery very dangerous?

No. The damage which the thermal runaway of the battery caused will be devastating.  However, you do not need to over nervous. Lithium batteries are widely used in vehicles, medical equipment, and other communicated machines. In new energy cars, 60% of fire accidents are caused by the thermal runaway of the battery itself,  30% are charging accidents, and only about 3.6% are due to the impact in driving accidents. Yes, these numbers are horrible, but the fire rate is very very low, in 2020, it is 0.18 per ten thousand. With the vigorous development of lithium batteries in all walks of life, people pay more attention to the safety of lithium batteries. Nowadays, although lithium batteries still have some dangerous factors, they do not affect personal safety.

How to protect marine lithium battery from thermal runaway in theory?

• • To optimize the stability of the cathode material
    • Set up a safety valve
    • Install a thermistor
    • Improve the film-forming quality of SEI
    • Increase the melting point of the diaphragm
  • To use solid electrolytes instead of liquid electrolytes
  • To use composite collector instead of traditional collector

How to protect marine lithium battery from thermal runaway in daily life?

• • • keep your marine lithium battery in non-water environment
    • Do not wrongly connect positive and negative connectors
    • Keep your marine lithium battery in a room-temperature place
    • Do not put your marine lithium battery with flammable substances
    • Follow instructions to charge your lithium marine battery
    • Choose marine lithium battery which match your equipment
    • Match your marine battery with good BMS

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