The secrets of lithium batteries: focus on Electrolyte

In the world of rechargeable batteries, lithium batteries stand out for their high performance and ability to power a wide range of devices, from simple toothbrushes to electric vehicles, smartphones, electric bicycles... At the heart of these batteries lies a component that is essential to their operation, but potentially dangerous: the electrolyte.

1 - Overview of lithium batteries

The operation of a lithium battery is based on the movement of lithium ions through the electrolyte. During charging, lithium ions are extracted from the cathode and pass through the electrolyte before being stored in the anode. Conversely, during discharge, the ions return to the cathode through the electrolyte, generating an electric current that powers the device. This process is illustrated in the following diagram, which highlights the central role of the electrolyte in the charge/discharge cycle of a lithium battery.

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2 - Electrolyte, an essential part of batteries

In a lithium battery, the electrolyte is a conductive medium that allows the passage of lithium ions between the cathode and anode, two key components of the battery. This substance can be liquid, gelled or solid. Liquid electrolytes are the most common, consisting mainly of lithium salts dissolved in organic solvents. They play an essential role in energy storage and release, making the electrolyte the lifeblood of lithium batteries.

The essential properties of electrolyte

The electrolytes used in lithium batteries must possess a set of properties that are essential for their operation, safety and durability. These are the essential properties of electrolytes:

1. High ionic conductivity: for optimum performance, the electrolyte must allow lithium ions to pass easily and quickly between cathode and anode. High ionic conductivity reduces the battery's internal resistance and improves its energy efficiency.
2. Chemical and electrochemical stability: it must remain stable over a wide temperature range, and not react chemically with other battery components such as electrodes and separator. This stability is essential to prevent battery degradation and extend its life.
3. Electrode compatibility: to avoid premature electrode degradation and maintain high performance, the electrolyte must be compatible with the anode and cathode materials. This compatibility helps prevent the formation of insulating or reactive layers on the electrodes, which can reduce battery capacity and output.
4. Wide electrochemical window: the electrolyte must be stable at high electrical potentials to enable the use of high-energy electrode materials. A wide electrochemical window minimizes the risk of electrolyte decomposition, which can lead to internal short circuits and battery failure.
5. Low viscosity: this promotes better mobility of lithium ions through the electrolyte, contributing to faster charging and discharging of the battery. It also improves battery performance at low temperatures.
6. Thermally stable: the electrolyte must retain its properties over a wide temperature range, from very low to very high. Thermal stability is essential for safety, as it reduces the risk of fire and explosion in the event of overheating.
7. Non-toxicity and eco-compatibility: ideally, electrolytes should not contain substances that are highly toxic or harmful to the environment. Non-toxicity and eco-compatibility are important for the safe manufacture, use and recycling of batteries.

Researchers and battery manufacturers are continually working to improve these properties to develop safer, more efficient and longer-lasting electrolytes capable of meeting the growing demands of modern battery technologies.

The different electrolytes

Electrolytes for lithium batteries fall into several categories, each with its own characteristics, advantages and disadvantages. Here's an overview of the different types of electrolyte used in lithium battery technologies:

Liquid electrolytes

These are the types of electrolyte most commonly used in lithium batteries. They consist of lithium salts dissolved in organic solvents. Liquid electrolytes offer excellent ionic conductivity, but present risks of flammability and leakage.

Gel electrolytes

Gel electrolytes are obtained by adding a gelling agent to liquid electrolytes, forming a semi-solid material. This type of electrolyte combines the high ionic conductivity of liquids with greater safety and flexibility. They are often used in lithium-polymer batteries.

Solid electrolytes

Solid electrolytes, such as ceramics or solid polymers, represent a promising alternative to liquid and gel electrolytes. They are non-flammable and have the potential to enable safer batteries with higher energy density.

Hybrid electrolytes

Hybrid electrolytes combine characteristics of solid and liquid electrolytes to exploit the advantages of each type. For example, a hybrid electrolyte may use a solid polymer impregnated with a liquid electrolyte, offering good ionic conductivity while improving safety.

Liquid ion electrolytes

Liquid ion electrolytes are a class of salts that are liquid at room temperature. Composed solely of ions, they have low volatility and high thermal and chemical stability. Their non-flammability makes them attractive for applications requiring a high level of safety.

Water-based electrolytes

Although less common due to lithium's reactivity with water, recent advances have enabled the development of stable aqueous electrolytes for lithium batteries. These electrolytes use strategies such as the addition of high-concentration lithium salts to form a passivation layer on the lithium electrode, enabling safer use.

3 - Risks associated with electrolytes

The electrolytes in lithium batteries present a number of major risks, mainly due to their chemical composition and physical properties.

Hazards

Flammability

The liquid electrolytes commonly used in lithium batteries contain volatile, flammable organic solvents. In the event of battery leakage or rupture, these solvents can ignite on contact with air, causing fires that are difficult to extinguish and can be particularly dangerous (rapid spread, toxic fumes).

Hazardous chemical reactions

If an electrolyte comes into contact with water or is exposed to air, it can react violently, releasing toxic gases. In addition, thermal decomposition of electrolytes can also produce hazardous gases, increasing the risk of explosion or combustion.

Toxicity

The chemical components of electrolytes, including some lithium salts and organic solvents, can be toxic if ingested, inhaled or come into contact with the skin. Exposure to these substances may cause irritation, organ damage or other adverse effects on human health.

Risk of explosion

Lithium batteries can explode if the electrolyte overheats, which can happen when the battery is short-circuited, overcharged or physically damaged. Overheating can lead to a chain reaction known as "thermal runaway", where the temperature and pressure inside the battery rise rapidly, potentially resulting in a violent explosion.

Damage and leakage

Over time, or if mishandled, electrolyte can degrade, reducing battery efficiency and increasing the risk of leakage. Electrolyte leakage can cause damage to surrounding electronics and present health and safety risks.

Risk prevention and management

To mitigate these risks, lithium battery manufacturers are implementing various design and manufacturing strategies, such as using less flammable solid or gel electrolytes, integrating battery management systems (BMS) to monitor and control battery condition, and developing more stable materials and advanced separator technologies.

In addition, strict safety standards and proper handling protocols are applied during the production, use and recycling of lithium batteries to reduce the risks associated with electrolytes.

Finally, other solutions are starting to come onto the market, such as sodium batteries, with lesser risks, particularly in terms of flammability.