As lithium battery technology advances, these batteries are now prevalent in consumer electronics, electric bikes, mobile devices, electric vehicles (EVs), and energy storage solutions. Safety concerns around lithium batteries have grown, prompting this article to explore and compare their safety features.

How Lithium Batteries Function

Before delving into their safety, let’s understand how lithium batteries work. The charging process involves lithium ions produced at the positive electrode, jumping into the electrolyte, traversing the diaphragm’s circuitous hole, and reaching the negative electrode, combining with negative electrons. During discharge, the electron flows from the negative to the positive electrode, while lithium-ion Li+ moves through the diaphragm’s curved hole, joining the positive electrode, creating what we commonly term battery capacity.

Lithium Battery Charging Process

The battery’s positive electrode generates lithium ions, which traverse the electrolyte via the diaphragm’s circuitous hole, eventually forming LiC compounds at the negative electrode.

Lithium Battery Discharge Process

During discharge, electrons flow from the negative to the positive electrode through the external circuit. Lithium-ion Li+ enters the electrolyte from the negative electrode, climbs through the curved hole on the diaphragm, swims into the positive electrode, and combines with running electrons. The resulting battery capacity represents the discharge capacity.

Battery Protection Board Simplified:

The battery protection board is like a guardian for rechargeable batteries (often called lithium batteries). Lithium batteries must be shielded from charging, discharging, over-discharging, overcurrent, short-circuit, and high temperatures. They always come with a protective plate and current fuse. The figure above illustrates the battery board protection circuit with PTC (positive temperature coefficient thermistor) and NTC (Negative temperature coefficient thermistor). As the ambient temperature rises, resistance drops, allowing timely responses from electrical or charging equipment to control internal interruptions, ensuring safe charging and discharging. U1 is a circuit protection chip, and U2 consists of two reverse connected MOSFET switches. Under normal conditions, U1’s CO and DO output high voltage, disconnecting both MOSFETs for unrestricted charging and discharging.

Characteristics Of Lithium Batteries

Long service life

The long-life lead-acid batteries have a cycle life of about 300 to 500 times, whereas lithium iron phosphate power batteries exceed 2000 cycles. Lead-acid batteries of similar quality last 1 to 1.5 years, but lithium-iron phosphate batteries under the same conditions have a theoretical lifespan of 7 to 8 years, more than four times that of lead-acid batteries. They can handle large current discharges and charge/discharge quickly at a high current of 2C. When charged at 1.5C with a suitable charger, the battery can reach full charge within 40 minutes, with a starting current of up to 2C, a capability not found in lead-acid batteries.

No memory effect

Frequent full charges causing capacity to drop below the rated value quickly is known as the memory effect, present in NiMH and nickel-cadmium batteries but absent in lithium batteries. Lithium batteries can be charged and used at any time without requiring a full discharge beforehand.

High energy density

Mainstream lithium iron phosphate batteries currently have an energy density below 200Wh/kg, while ternary lithium batteries range between 200-300Wh/kg. Lithium-ion materials in these batteries offer a significantly higher energy density, approximately three times that of lead-acid batteries, allowing them to store more energy and have a greater capacity in a similar-sized battery.

Lightweight

Lithium iron phosphate batteries of the same specifications and capacity are two-thirds the volume of lead-acid batteries and only one-third of their weight.

Environmental protection

Lithium iron phosphate batteries are believed to be free of heavy or rare metals (unlike nickel-hydrogen batteries), non-toxic (SGS certified), environmentally friendly, compliant with European RoHS regulations, and hold a green battery certificate, ensuring their eco-friendliness. This information helps determine the environmental safety of lithium batteries.

Risks of lithium batteries

Lithium batteries boast high energy density, with ternary lithium batteries exceeding 200Wh/kg. However, higher energy density compromises thermal stability, potentially leading to thermal runaway reactions escalating from combustion to detonation. The main risks associated with lithium batteries include overcharging, over-discharging, and the resulting chemical side effects that can impact battery performance and longevity. In extreme cases, these reactions can generate gas, increase internal pressure, and pose safety hazards like explosions and fires.

Design defects

Design flaws often stem from prioritizing high energy density over safety considerations. Take the NCM811 battery as an example, where an increased nickel content boosts energy density but compromises the roles of cobalt and manganese in enhancing stability and safety. Furthermore, inadequate diaphragm design or cost-cutting measures like compressing foil thickness can pose safety risks in lithium batteries.

Production process flaws

In lithium battery production, process defects or insufficient process control leading to excessive impurities or moisture can escalate battery side effects, raise internal pressure, and potentially cause explosions or spontaneous combustion.

Main safety issues of lithium-ion batteries

The safety of lithium batteries is primarily characterized by combustion and possible explosion due to thermal runaway. External factors like overcharging, fire, impact, puncture, or short circuits can also trigger safety incidents. Statistics show that a significant percentage of lithium-ion battery fire accidents in electric vehicles result from short circuits, charging/discharging issues, unexpected equipment starts, or other factors. Electrical failures and spontaneous combustion are the main causes of fires in electric vehicles, with concerns revolving around overcharging, battery core malfunctions, and circuit short circuits as root causes of fires and safety risks associated with lithium batteries.

Which battery is the safest?

Are lithium batteries truly safe? Lithium iron phosphate batteries exhibit superior safety compared to ternary lithium batteries due to their excellent heat resistance, with a thermal runaway temperature surpassing 800℃. Battery fires primarily result from short circuits, impacts, overcharging, and other factors, emphasizing the importance of lithium battery safety. The heat generated by over-potential and ohm polarization during current flow can lead to battery overheating, explosions, and fires. Ternary lithium batteries ignite at a mere 200℃, whereas lithium iron phosphate batteries require temperatures exceeding 800℃ to ignite, reducing the risk of spontaneous combustion. In high-temperature regions, ternary lithium batteries pose a higher risk of spontaneous combustion compared to lithium iron phosphate batteries.

Are you anticipating the arrival of solid-state batteries?

Transitioning from “liquid” to “solid” offers enhanced safety features for lithium batteries. Solid-state batteries also promise increased energy density, longer lifespan, reduced weight, and simplified packaging, presenting numerous advantages over conventional lithium batteries. Despite these benefits, solid-state battery technology remains in the experimental phase.

Addressing production process complexities, environmental requirements, material selection challenges, and cost control issues are crucial steps towards achieving large-scale production. Factors like extremely dry production environments with dew points below -40 degrees Celsius and intricate solid electrolyte processing contribute to the technical hurdles. The reliability of solid-state batteries increases with delayed mass production, with projections indicating semi-solid state battery mass production by 2024/2025 and full solid-state battery availability around 2030.