Why Do Thermal Greases for New Energy Batteries Require “Custom” Silicone Oils? Viscosity Directly Impacts Heat Dissipation Efficiency-Iota

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News > Dynamic industry > Why Do Thermal Greases for New Energy Batteries Require “Custom” Silicone Oils? Viscosity Directly Impacts Heat Dissipation Efficiency

Why Do Thermal Greases for New Energy Batteries Require “Custom” Silicone Oils? Viscosity Directly Impacts Heat Dissipation Efficiency

As the new energy vehicle and large-scale energy storage sectors accelerate, battery thermal management has become critical to safety and service life. As a core component of thermal interface materials (TIMs), silicone oil is not something that can be “added arbitrarily”—its viscosity selection directly determines the thermal resistance performance and long-term reliability of the thermal grease.

Thermal greases consist of high-thermal-conductivity fillers (e.g., alumina, boron nitride) and a base silicone oil. The role of silicone oil goes beyond merely wetting the filler and enabling processability; more importantly, it helps form a low-void, high-density thermal conduction pathway between battery cells (or chips) and heat sinks. However, lower viscosity isn’t always better:

Silicone oils with excessively low viscosity (<50 cSt) tend to migrate or bleed out at elevated temperatures, causing a “pump-out” effect.
Conversely, oils with very high viscosity (>10,000 cSt) exhibit poor flowability, making it difficult to achieve uniform spreading under assembly pressure—trapped air pockets then increase thermal resistance.

Recent experimental data show that, under an identical filler system (65 vol% boron nitride):

A thermal grease formulated with 1,000 cSt silicone oil achieves a steady-state thermal resistance as low as 0.18 °C·cm²/W.
The 50 cSt formulation, due to oil bleeding during aging, saw its thermal resistance rise to 0.32 °C·cm²/W after 72 hours at high temperature.
The 20,000 cSt variant started with a higher initial thermal resistance of 0.25 °C·cm²/W and suffered from poor coating uniformity.

“In new energy battery packs, space is extremely tight and heat flux density is high—TIM consistency and durability requirements far exceed those in consumer electronics,” explains a thermal management engineer at a leading battery manufacturer. “We’ve now included silicone oil viscosity in our supplier qualification criteria, typically preferring the 500–2,000 cSt range.”

Currently, several domestic silicone oil producers are collaborating with thermal material companies to develop “battery-specific” silicone oils, featuring narrow molecular weight distributions and low-volatility designs to enhance high-temperature stability. Some of these products have already passed 1,000 cycles of thermal shock testing from –40°C to 150°C with no significant phase separation observed.

With the growing adoption of 800V high-voltage platforms and liquid-cooled energy storage systems, TIMs are evolving from a supporting component into a critical safety barrier. Industry experts urge: material selection should not focus solely on thermal conductivity—greater attention must be paid to the rheological properties and long-term in-service performance of the base silicone oil. After all, every single degree Celsius of temperature reduction directly impacts the safety margin of the battery.

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