Lithium battery safety – influencing factors and measures to improve it

Effects of acupuncture, short circuit and extrusion on lithium battery safety

When the battery is externally short-circuited or subjected to needle sticking and extrusion, it is often easy to cause the diaphragm inside the battery to rupture, causing a large current to flow through the battery in a very short period of time. This will directly lead to a sharp rise in the internal temperature of the battery, which will trigger a series of violent reactions in a short period of time, and even cause lithium battery safety problems such as combustion and explosion.

For batteries with poor safety performance, this process is often completed within seconds. Due to the high specific energy of lithium-ion batteries, once a short circuit occurs, the battery can reach a high heating power, causing the battery to rise to an extremely high temperature in a short period of time. The aluminum explosion-proof valve at the battery seal after the danger is often melted.

When the battery is subjected to thermal shock, it can withstand a short period of thermal shock due to the relatively slow heat conduction rate of the battery. Compared with short circuit and needle extrusion, if the temperature of thermal shock is lower, it will have little effect on the battery.

If the temperature of the thermal shock is high, the SEI film on the surface of the negative electrode of the battery will decompose, and the highly lithium-intercalated negative electrode material will undergo an exothermic reaction with the electrolyte.

At this time, the battery enters a dangerous period, but whether the danger occurs depends on the rate of battery heat dissipation. If the heat dissipation rate is slow, it is easy to cause the internal temperature of the battery to rise further, the diaphragm melts, a short circuit occurs inside the battery, the temperature rises sharply, triggers the cathode material to participate in the reaction, and finally explosions and other dangerous accidents occur. This process is generally called thermal runaway.

Effect of overcharging on lithium battery safety

When the battery is overcharged, different electrode materials have different effects due to their different chemical properties. In the early stage of high-rate charging, most of the electric energy is stored through reversible chemical reactions, and the battery generates less power. However, due to an irreversible chemical reaction in the later stage of charging, the electrical energy becomes heat energy, which causes the battery temperature to rise rapidly and trigger a series of chemical reactions.

If the heat dissipation of the battery is good during overcharging, or the overcharging current is small, the temperature of the battery is low at this time, and only the decomposition of the electrolyte occurs after overcharging, and the battery is still safe at this time.

If the heat dissipation of the battery is poor at this time, or the battery temperature is high due to high-rate charging and chemical reactions are triggered, it often leads to lithium battery safety problems that are more serious than simple thermal shock.

Effect of different cathode materials on lithium battery safety

When the battery temperature rises rapidly, the lithium battery safety of different cathode materials varies. Among them, the lithium battery safety with lithium iron phosphate as the cathode material is the best, and the nickel-cobalt lithium manganese oxide battery is better than the lithium cobalt oxide battery. Since other parts of the battery are basically the same, the safety of the cathode material determines the lithium battery safety.

Thermal stability

When the battery is at a very high temperature, it will inevitably cause the highly active cathode material in an oxidized state to react with other parts of the battery, such as the electrolyte, separator, anode, and even the aluminum current collector. Since transition metal oxide cathode materials can release oxygen at high temperature, or provide combined oxygen for chemical reactions, this type of cathode material has high reactivity.

Since the thermal stability of nickel-cobalt lithium manganate ternary material is slightly better than that of lithium cobalt oxide, the safety of this type of battery is also slightly better than that of lithium cobalt oxide battery. Lithium iron phosphate is charged to generate iron phosphate.

Because the chemical properties of iron phosphate are very stable, it is difficult to decompose and release oxygen or provide combined oxygen for the reaction even at high temperatures. Therefore, the safety of lithium iron phosphate batteries is very good.

Effect of overcharge

When the battery is overcharged, in addition to the different lithium battery safety caused by different thermal stability, different cathode materials are also prone to lead to lithium battery safety problems caused by the different specific capacities of the materials themselves.

In contrast, lithium iron phosphate has a theoretical utilization rate of 100% of lithium ions, and the capacity of the anode can ensure that overcharging does not cause lithium precipitation at the anode, so such problems are not prone to occur.

Measures to improve lithium battery safety

Safe structure design

Because there are many potential safety hazards in lithium-ion batteries, some special measures must be taken to ensure safety when designing batteries. At present, more lithium ion battery structure designs are used to adopt heat-sealable diaphragms, series positive temperature coefficient resistors and explosion-proof caps.

Heat sealed separator

When a large current passes through the battery due to acupuncture or extrusion, causing the temperature of the battery to rise, the porous diaphragm inside the battery softens rapidly. Due to the tightness of the battery core, the separator is squeezed, and the porous structure adheres to each other to form an almost completely closed structure, which can no longer provide channels for ion transmission. At this time, the current flowing through the battery is quickly cut off.

The temperature of a battery with better safety performance rises rapidly after acupuncture, but begins to drop when it reaches the softening temperature of the separator, and the battery is no longer dangerous. However, if the separator does not form a good closed structure when the temperature rises, or if the uneven force causes shrinkage and deformation to cause an internal short circuit in the battery, the battery will easily rise to a very high temperature and be dangerous.

Positive temperature coefficient resistor

When the battery is overcharged or externally short-circuited, a large current flows through the positive temperature coefficient resistor in series with the battery, causing the resistor to heat up, and the resistance value increases to reduce the current flowing through the battery, thereby providing safety protection for the battery.

Safety valve

When the battery is overcharged, the electrolyte decomposes to generate gas, or the gas pressure inside the battery increases due to the vaporization of the electrolyte after heating, and the explosion-proof valve or explosion-proof membrane ruptures, releasing the internal pressure of the battery.

In addition, the explosion-proof valve in the cathode cap of the general cylindrical battery pack cuts off the connection with the electrode sheet while deforming, thereby preventing the battery from being dangerous.

Chemical method to improve lithium battery safety

Polymerizable additives

When the battery is overcharged, the additive undergoes electrochemical polymerization on the cathode of the battery to generate H+, which is then reduced to H2 on the anode of the battery, which significantly increases the internal pressure of the battery and forces the battery to open the explosion-proof valve before it enters a dangerous state. At the same time, pull off the lead wire at the battery seal to cut off the charging current.

If the generated polymer has better conductivity, it can cause a micro-short circuit inside the battery, and slowly release the electric energy stored in the battery after the battery is in danger, so the lithium battery safety can be further improved.

Voltage sensitive diaphragm

When the battery is overcharged, the diaphragm changes from an insulating state to a conductive state. At this time, the charging current passes directly through the battery without oxidation of the electrode material and electrolyte. After several years of research, separators with different oxidation potentials have been developed for different batteries.

Redox couple

In order to make Li-ion batteries have an oxygen cycle mechanism similar to aqueous secondary batteries, a redox couple can be added to the electrolyte. When the battery is overcharged, the additive is oxidized on the cathode and then diffused to the anode to be reduced, thus providing a kind of overcharge protection for the battery.

Flame retardant additive

Since the lithium-ion battery uses a carbonate-based organic electrolyte, it is easy to cause problems such as combustion. Therefore, in order to suppress the combustion of the electrolyte, a method of adding a flame retardant to the electrolyte can be used. When the flame retardant reaches a certain concentration, it can completely inhibit the combustion of the electrolyte, or use fluorinated esters with non-combustible properties as the solvent of the electrolyte.

Conclusion

Various lithium-ion batteries have been widely used due to their excellent performance, and lithium battery safety has also received extensive attention. In order to improve lithium battery safety, safety measures such as explosion-proof valves, heat-sealed diaphragms, and positive temperature coefficient resistors are generally used at present.

At the same time, various chemical methods to improve lithium battery safety are also being studied, and often the combination of various safety measures can better improve lithium battery safety.