What is the SEI layer in a lithium-ion battery?

The solid electrolyte interphase (SEI) is a thin passivation film that forms on the anode during the first charges. It lets lithium ions pass while blocking electrons, which stabilizes the cell and enables long life — at the cost of some irreversible capacity.

What do electrolyte additives like VC and FEC do?

Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are consumed during SEI formation to build a thinner, more stable film. They improve cycle life, keep internal resistance low, and are especially important for silicon-bearing anodes.

How Does Battery Electrolyte Work? Composition & SEI Explained

Q: What is battery electrolyte made of?

It depends on the chemistry. Lead-acid uses sulfuric acid; nickel-cadmium and nickel-metal-hydride use an alkaline potassium hydroxide solution; lithium-ion uses lithium salts such as LiPF6 dissolved in a mix of organic carbonate solvents, plus performance additives.

Q: Why is lithium-ion electrolyte flammable?

Lithium-ion electrolyte uses flammable organic solvents rather than water, because lithium reacts with water. If a cell overheats, the SEI can break down around 75–90 °C and trigger self-heating, which is why non-flammable electrolytes are an active research area.

How Does Battery Electrolyte Work? Composition, SEI & Additives | Xnergy Materials

Xnergy Materials

Abstract

The electrolyte is the quiet ingredient between the electrodes — the medium that lets ions move. Get it right and the cell lasts; get it wrong and nothing else matters.

Keywords: battery electrolyte · lithium-ion · SEI layer · LiPF₆ · carbonate solvents · VC · FEC · electrolyte additives

1.The ion highway

A battery has two separate pathways. Electrons travel the outside circuit and do the useful work; ions travel inside the cell, and the electrolyte is the medium that carries them. On charge, ions move one way between cathode and anode; on discharge, they move back. No electrolyte, no ion flow — and no current.

It comes in several forms: liquid, gel, solid polymer or ceramic, even molten salt. Whatever the format, it pairs with the separator, which blocks electrons while letting ions through.

Figure 1. Inside the cell, the electrolyte carries lithium ions between the electrodes while electrons take the external circuit. The direction of ion flow reverses between charge and discharge.

2.Electrolyte, by chemistry

Each battery family uses a different electrolyte, matched to its electrodes and operating conditions.

Lead-acid

Sulfuric acid

An aqueous acid whose density shifts with state of charge — denser when charged, near-water when discharged. Simple, cheap, highly corrosive.

NiCd / NiMH

Potassium hydroxide

An alkaline solution (caustic potash) shared by both nickel chemistries. Robust across hot and cold climates and frequent cycling.

Lithium-ion

Lithium salt in carbonates

A flammable organic solution — lithium salts dissolved in mixed carbonate solvents, tuned with additives. The same approach extends to Na-ion electrolytes.

3.Inside a lithium-ion electrolyte

Because lithium reacts with water, Li-ion uses an organic system instead. It's built from three ingredient classes — and the exact recipe is where cell makers compete. Browse the full range of Xnergy electrolytes, or start from a ready-to-use 1M LiPF₆ EC/DMC/DEC standard.

~10–15 wt%

Lithium salt

The ion source. LiPF₆ is the workhorse; LiFSI and others raise conductivity and thermal stability. See battery-grade LiPF₆ and the full salts range.

bulk

Carbonate solvents

Mixed cyclic and linear carbonates (EC, DMC, EMC, DEC) dissolve the salt and set the conductivity and temperature window. See solvents.

<5 wt%

Functional additives

Small doses of VC, FEC and others build a better SEI, cut gassing and extend life — the proprietary "secret sauce." See vinylene carbonate (VC) and ready-made formulations.

4.The SEI: the film that makes Li-ion possible

Lithium-ion electrolyte isn't perfectly stable, and that turns out to be essential. During the first charges, it reacts at the anode surface to form a thin passivation layer — the solid electrolyte interphase (SEI). Like a built-in separator, it lets lithium ions through but blocks electrons.

The SEI is what gives Li-ion its long life, but it isn't free: forming it consumes lithium and permanently trims a little capacity, and electrolyte oxidation at the cathode costs a bit more. The fix is chemistry — additives such as vinylene carbonate (VC) are sacrificed to build a thinner, tougher film that keeps resistance low as the cell ages. That's why our standard 1M LiPF₆ electrolyte already ships with 1% VC built in. For silicon-bearing anodes, where the surface keeps churning, FEC is often the deciding additive — a theme we cover in the silicon anode guide.

5.A safety and aging story

Two properties of the organic electrolyte shape how a cell behaves over its life. First, it's flammable — the trade-off for using a non-aqueous solvent. If a cell overheats, the SEI can start to break down around 75–90 °C, releasing energy that can self-heat toward thermal runaway if it isn't cooled. That's why non-flammable and ionic-liquid electrolytes are an active field of research.

Second, the electrolyte slowly dries out. As it's consumed and degrades over thousands of cycles, the cell loses the very medium that moves its ions — and once the liquid is effectively gone, the battery is finished. Electrolyte health is, in the end, one of the truest measures of a cell's state of health.

6.Anatomy of a Li-ion electrolyte

The components, what each one does, and common examples.

Table 1. Components of a lithium-ion electrolyte and their function.
Component	Role	Common examples
Lithium salt	Supplies the mobile Li⁺ ions; sets conductivity	LiPF₆, LiFSI, LiBF₄
Cyclic solvent	High permittivity to dissolve the salt	Ethylene carbonate (EC)
Linear solvent	Lowers viscosity; widens temperature range	DMC, EMC, DEC
SEI-forming additive	Builds a stable anode film; extends life	VC, FEC
Functional additives	Reduce gassing; improve high-temp cycling & safety	Various proprietary

7.Frequently asked

What is battery electrolyte made of?

It varies by chemistry. Lead-acid uses sulfuric acid; nickel-cadmium and nickel-metal-hydride use alkaline potassium hydroxide; lithium-ion dissolves a lithium salt like LiPF₆ in a blend of organic carbonate solvents, with performance additives on top.

What is the SEI layer?

The solid electrolyte interphase is a thin film that forms on the anode in the first charges. It passes lithium ions but blocks electrons, stabilizing the cell and enabling long life — while consuming a little capacity to form.

What do additives like VC and FEC do?

They're sacrificed during SEI formation to build a thinner, more durable film. That improves cycle life, keeps resistance low, and is critical for silicon anodes. Browse vinylene carbonate (VC) and ready-made formulations.

Why is lithium-ion electrolyte flammable?

Lithium reacts with water, so Li-ion uses flammable organic solvents instead. Above roughly 75–90 °C the SEI can break down and self-heat, which is why thermal management and non-flammable electrolyte research both matter.

8.References & further reading

Peled, E. (1979). The electrochemical behavior of alkali and alkaline earth metals in nonaqueous battery systems — the solid electrolyte interphase model. Journal of The Electrochemical Society, 126(12), 2047–2051.
Xu, K. (2004). Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chemical Reviews, 104(10), 4303–4417.
Xu, K. (2014). Electrolytes and interphases in Li-ion batteries and beyond. Chemical Reviews, 114(23), 11503–11618.
Battery University. BU-307: How does Electrolyte Work? Cadex Electronics. batteryuniversity.com

From the lab

Formulating an electrolyte? We can help.

Xnergy Materials supplies lithium salts, carbonate solvents, additives and ready-to-use electrolytes — backed by a team that can help tune a formulation to your cell.

Explore electrolytes Talk to our team