Lithium-ion battery electrolyte

Comprehensive analysis of lithium-ion battery electrolyte

ByRD CENTER

In lithium-ion batteries, although the electrolyte does not directly store energy, it profoundly influences energy density, rate capability, cycle life, and safety. It can be regarded as the “circulatory system” connecting the cathode and anode. The performance of the electrolyte directly determines whether a battery can operate stably, efficiently, and over the long term.

I. Why Must Lithium-Ion Batteries Use Non-Aqueous Electrolytes?

In traditional battery systems, aqueous electrolytes are widely used because of their strong solvating ability, mature chemistry, and low cost. However, aqueous systems are not feasible for lithium-ion batteries.

This is because:

  • Metallic lithium and lithiated graphite (LiC₆) are chemically unstable in water;

  • Water reacts rapidly with lithium, causing severe side reactions, gas evolution, and even safety hazards;

  • The electrochemical stability window of water is limited and cannot meet the relatively high operating voltage of lithium-ion batteries.

Therefore, lithium-ion batteries must use non-aqueous electrolyte systems, which are typically composed of organic solvents, lithium salts, and small amounts of functional additives.

II. Main Types of Lithium-Ion Battery Electrolytes

From the perspective of form and composition, lithium-ion battery electrolytes can be broadly classified into three categories:

  • Liquid electrolytes

  • Polymer electrolytes (solid or gel-type)

  • Room-temperature molten salt electrolytes (rarely used in practice)

Among them, liquid electrolytes remain the absolute mainstream in commercial lithium-ion batteries, while polymer electrolytes are considered an important future development direction. The following sections focus on these two systems.

III. Liquid Electrolytes: The Mainstream Commercial Solution

1. Basic Composition of Liquid Electrolytes

A typical organic electrolyte for lithium-ion batteries consists of three main components:

  • Lithium salts: provide mobile lithium ions;

  • Organic solvents: serve as the medium for lithium-ion transport;

  • Functional additives: improve SEI formation, rate performance, and safety.

A balance must be achieved among conductivity, stability, and safety.

2. Lithium Salts: The Core Determinant of Ionic Conductivity

An ideal lithium salt should possess:

  • High ionic conductivity;

  • Good thermal and electrochemical stability;

  • Good compatibility with electrode materials.

Lithium salts that have been widely studied or applied include:

Lithium Salt Characteristics and Application Status

LiClO₄

High conductivity, but strong oxidizing nature poses safety risks, making commercialization difficult

LiAsF₆

Excellent performance, but contains arsenic, with high toxicity and cost

LiBF₄

Good high- and low-temperature performance, but relatively low conductivity and high cost

LiPF₆

Best overall performance; the mainstream choice for commercial lithium-ion batteries

LiPF₆ offers high ionic conductivity and good electrode compatibility, but it also has drawbacks:

  • Moderate thermal stability;

  • Tendency to react with moisture to generate HF;

  • Complex synthesis process and relatively high cost.

As a result, modified lithium salts and composite lithium-salt systems based on LiPF₆ have long been a major focus of electrolyte research and development.

3. Organic Solvents: Affecting Low-Temperature Performance, Rate Capability, and Safety

Organic solvents constitute the main body of the electrolyte. Their viscosity, dielectric constant, melting point, and flash point directly determine the operating temperature range and safety level of the battery.

(1) Carbonate Solvents (Most Critical)

Carbonate solvents are the most widely used solvents in lithium-ion batteries and can be divided into:

  • Cyclic carbonates: EC, PC;

  • Linear carbonates: DMC, DEC, EMC, etc.

Among them:

  • EC (ethylene carbonate) can form a stable SEI film on graphite anodes and is an indispensable base solvent;

  • However, EC has a relatively high melting point and is usually mixed with low-viscosity solvents such as DMC, DEC, or EMC.

A reasonable solvent formulation allows a balance between low-temperature performance, safety, and cycle life.

(2) Ether Solvents

Ether solvents exhibit good low-temperature performance and lithium-salt solubility, and are often used in:

  • Primary lithium batteries;

  • Special high-rate or research-oriented systems.

However, due to their poor oxidative stability, their application in mainstream lithium-ion batteries is limited.

(3) Ester Solvents

Ester solvents have low melting points and can improve low-temperature performance, but:

  • Their cycle stability is generally limited;

  • Compatibility with mainstream systems is restricted.

They are usually employed as auxiliary solvents or in research-oriented additive systems.

(4) Novel and Modified Solvents

By fluorination or chlorination of carbonate solvents, it is possible to:

  • Increase flash point and safety;

  • Improve SEI film structure;

  • Reduce solvent flammability.

Examples such as FEC and Cl-EC have become important components in modern high-performance electrolytes.

IV. Electrolyte Additives: Small Amounts, Huge Impact

Adding only 1%–5% of functional additives to the electrolyte can significantly improve battery performance. These additives mainly include the following categories:

1. SEI-Forming Additives

  • Promote the formation of dense and stable SEI films;

  • Improve initial coulombic efficiency and cycle life;

  • Typical examples: VC, FEC, ES.

2. Conductivity-Enhancing Additives

  • Promote lithium-salt dissociation;

  • Increase lithium-ion transference number;

  • Commonly used in low-temperature or high-rate systems.

3. Overcharge Protection Additives

  • Absorb excess energy through redox or polymerization reactions during overcharge;

  • Enhance battery safety;

  • Typical substances: biphenyl, anisole, etc.

4. Flame-Retardant and Safety Additives

  • Increase electrolyte flash point;

  • Reduce the risk of thermal runaway;

  • Due to cost considerations, they are mainly used in high-end or special-purpose batteries.

5. Additives for Moisture and HF Control

  • Suppress LiPF₆ decomposition;

  • Protect the SEI film structure;

  • Improve the long-term stability of the electrolyte.

V. Polymer Electrolytes: A Path Toward Higher Safety

Polymer electrolytes are a class of ion conductors based on polymer matrices and feature:

  • Low leakage risk;

  • High design flexibility;

  • Enhanced safety.

Depending on their form, they mainly include:

  • All-solid polymer electrolytes;

  • Gel polymer electrolytes;

  • Porous polymer electrolytes.

At present, polymer electrolytes still suffer from relatively low ionic conductivity at room temperature. However, through:

  • Incorporation of nano-scale inorganic fillers;

  • Molecular structure design;

  • Multiphase interfacial engineering,

their performance is continuously improving, making them a key foundational material for all-solid-state batteries and next-generation lithium batteries.

VI. Conclusion: Electrolytes as an Art of Balancing Performance and Safety

There is no “universal solution” for lithium-ion battery electrolytes. Instead, they involve:

  • Continuous trade-offs among conductivity, stability, safety, and cost;

  • Highly customized designs tailored to specific application scenarios.

It is precisely the ongoing advancement of electrolyte technology that has enabled lithium-ion batteries to continuously push performance boundaries in consumer electronics, medical devices, power applications, and energy storage systems.

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