Examining Failures in Lithium-ion Batteries

Millions of people depend on lithium-ion batteries. Lithium-ion is found in mobile phones, laptops, hybrid cars, and electric vehicles. The technology has faced extreme growth due to its high energy density, charging ability, and lightweight characteristics.

 

Voltage Effects

Lithium-ion batteries can experience overvoltage and undervoltage effects. As noted in Figure 1, the operating voltage and temperature of the battery must be maintained at the point marked with the green box. If it is not, the cells can be damaged.

 

Figure 1. Operating window of a lithium-ion cell. Image used courtesy of Simon Mugo

 

Overvoltage

Overvoltage is when the charging voltage of the lithium-ion battery cell is increased beyond the predetermined upper limit, typically 4.2 V. The excessive current flow into the lithium-ion cell causes overheating and lithium plating, which leads to battery failure.

When the current is in excess, the excessive joules will initiate more heat into the cell, causing overheating. The overheating leads to increased cell temperature hence failure.

Excessive current stops the quick accommodation of lithium-ion between the layers of intercalation of the anode made of carbon. Instead, the lithium ions build up on the anode surface and are deposited as metallic lithium. This leads to reduced lithium ions, significant loss of cell capacity, and further initiation of short circuits within the cell structure.

Low temperature also causes lithium plating due to non-uniformities occurring within the cell elements originating from the manufacturing defects or misuse of the cell.

 

Over-discharge/Undervoltage

Over-discharge is when voltage is drained from the battery cell to below two volts. Undervoltage is a condition that originates from storing the battery for a long time without use until the voltage goes below 2 V per cell. These two conditions lead to a breakdown in the anodes and cathodes.

The dissolution of the anode current collector into the battery electrolyte occurs, causing the battery cell self-discharge rate to go up while trying to increase the battery cell to above 2 V. The copper ion dissolved in the electrolytes is a dangerous ingredient for cell short circuits.

Long storage of cells at below 2 V also leads to cathode corrosion which eventually breaks down after a long time of oxygen being generated in the lithium manganese oxide and lithium cobalt oxide cathodes, initiating permanent cell capacity loss. 

 

State of Charge

To overcome the problems of overcharging, undercharging, and over-discharging, the battery cells should be subjected to a state of charge operation. The state of charge recommended for the lithium-ion battery is illustrated in Figure 2.

 

Figure 2. State of charge of the battery. Image used courtesy of Simon Mugo

 

If the lithium-ion battery cells are operated outside the conditions listed in Figure 2, expect reduced cell life.

 

Temperature Effects

Heat has been classified as one of the major battery life reducers. Both in excess or below the desired minimum limit is a battery killer. Therefore, Lithium-Ion cells should be subjected to a perfect temperature control mechanism.

 

Operation Under Low Temperatures

It is normal for chemical reaction rates to decrease as the temperature drops. Reduction of the operating temperatures reduces the rate at which the cell’s active chemicals get transformed. The effect is a reduction in the current carrying capacity for both the charging and discharging process. In simple order, the power handling capacity of the cell gets reduced. Another effect of low temperature is slowing the rate at which the Lithium ions are inserted in the batteries intercalation spaces. This induces an irreversible cell capacity loss due to lowered lithium plating and power.

 

Operation Under High Temperature

Subjecting the battery cell to high temperatures brings another set of problems different from what the low temperature induces. Under such high temperatures, a condition known as the Arrhenius effect drains higher power from the battery by increasing the rate of reaction within the battery. Besides, higher currents initiate high heat generation and dissipation.

 

Thermal Runaway

Several stages occur in thermal runaway buildup, leading to battery cell damage.

 

Stage 1: Thin Passivating SIE Layer Breakdown

The breakdown of this layer located on the anode happens due to physical penetration or overheating of the cell. Initially, the overheating can originate from the supply of current excess, extreme ambient temperature, and the overcharging of the battery cell. The splitting of the SEI layer occurs at a very low temperature of about 75°C. The breakdown of this layer causes the electrolyte and the anode to react similarly to what happened during the formation, but this occurs at a high rate and in an uncontrolled manner.

 

Stage 2: Organic Solvent Breakdown

As the temperature keeps building up, the anode reaction keeps generating heat which causes the organic solvent used in the battery electrolyte to break down, generating flammable hydrocarbon gases. The stage happens when the battery temperature hits 110°C. The generated gas creates an accumulation of pressure inside the battery cell. Luckily, the gases generated do not include oxygen and cannot burn. Most batteries are designed with a vent hole that releases these gases into the atmosphere, regulating this pressure and eradicating the possibility of cell rupture.

 

Stage 3: Melting of the Polymer Separator

This stage occurs when the battery temperature hits 135°C. Here, the polymer separator melts and introduces a short circuit between the electrodes.

 

Stage 4: Metal Oxide Cathode Breakdown

Metal oxide cathode breakdown is caused by the heat generated during the electrolytic breakdown. In this stage, the material of the metal oxide cathode releases oxygen gas which triggers the burning down of the battery electrolyte and other flammable gases in the cell. The process of cathode breakdown also generates a lot of heat, increasing the pressure and temperature of the cell. The process of cathode breakdown happens at around 200°C for batteries made of lithium cobalt oxide, but the temperature might be higher if other chemistries are involved.

 

Non-uniform Effects

The non-uniform flow of current originating from localized defects occurring between the anode and separator surface also contributes to Lithium plating effects. Below are examples of some of the defects.

 

Manufacturing Defects

Some of the manufacturing defects include:

• Local electrolyte drying
• Mechanical component deformation
• Uneven anode coating 
• Separator pore deformation or blockage
• Current collector delamination
• Active chemical contamination
• Some of the abuse defects occurring in the battery cells include:
• Copper deposition, an effect occurring due to prolonged battery charging
• Physical battery cell damage

 

Cycle Life

 

Figure 3. Graph of cell cycle life and temperature. Image used courtesy of Simon Mugo

 

Effects of the temperature and voltage failure on the battery cell tend to be immediate, but the effects it subjects on the cycle life are much less. Operating the cell outside the operating window is recommended, causing the cells to lose an irreversible capacity.

Figure 3 indicates that starting from the 15°C cycle, the life is progressively reduced when worked through low temperatures. When operated above 50°C also causes a reduction in cycle life, but when at 70°C, the thread changes to thermal runaway. The battery should have thermal management systems to keep cells operating at the set sweet spot every moment, reducing the wear and tear on the battery cell.

 

Takeaways of Lithium-ion Battery Failure

• Lithium-Ion battery cell failures can originate from voltage, temperature, non-uniformity effects, and many others.
• Voltage effects can occur either due to overvoltage or undervoltage effects.
• Overvoltage effects happen when there is an increase in the charging voltage of the cell beyond the predetermined upper limit of 4.2 V per cell.
• Overvoltage leads to more current being supplied to the cell, which initiates overheating and lithium plating.
• Undervoltage occurs when the cell falls below the minimum expected voltage of 2.0 V due to being stored for a long time without being charged, affecting the anode and cathodes of the cells.
• Temperature effects can harm the cell in low or high temperatures. 
• High temperatures lead to high heat dissipation and generation, which is bad for the battery's cells.
• Thermal runaway is another temperature effect on the battery cell, which builds up to damage the battery cell.
• Non-uniformity defects occur due to manufacturing and cell abuse defects.

Featured image used courtesy of Adobe Stock