Comprehensive understanding of the specific application of phase change materials in thermal management of power lithium batteries and analysis of phase change material thermal solutions

        Phase transition is a materialA term in science that refers to the state of existence of a substance with identical physical and chemical properties and clear boundaries with other surrounding substances. Macroscopically, there are three types of phases of matter: meteorological, liquid phase, and solid phase.

Phase change materials have been applied in many fields due to their ability to absorb and release heat, resulting in a stable system temperature and achieving an approximate constant temperature effect. Cooling of high-power power electronic devices, cooling of solar energy systems, building materials, industrial waste heat utilization, household vehicle air conditioning systems, and thermal management systems for lithium batteries.

The two indicator parameters of phase change materials, latent heat of phase change and phase change temperature, basically define the environmental types that a material can be applied to. The greater the latent heat of phase change, the stronger the material's ability to maintain a constant ambient temperature.

Phase change materials used in thermal management applications generally need to meet several conditions. Firstly, the material has a high thermal density and a large latent heat; Secondly, it has high thermal conductivity and rapid heat absorption and release processes; Again, it has good stability, is not easily decomposed or reacts with surrounding materials, has a long service life, and will not have any adverse effects on the system; Finally, the price is low.

Types of phase change materials

Thermal conductive phase change materialIt can be divided into three types: inorganic phase change materials, organic phase change materials, and composite phase change materials.

Inorganic phase change materials

Mainly refers to inorganic hydrated salt phase change materials, which have high latent heat of phase change and high melting temperature. Main inorganic phase change materials: CaCl2 · 6H2O, Na2SO4 · 10H2O, CaBr2 · 6H2O, CH3COONa · 3H2O, etc. Although inorganic phase change materials have the advantages of high thermal conductivity and low cost, they have defects such as supercooling, phase separation, and strong corrosiveness.

Organic phase change materials

The combination of various organic compounds, different crystal forms, and different polymer branch structures brings different constant temperature ranges. This also creates a significant advantage of organic phase change materials, which can adjust the phase transition temperature by mixing different types of materials; Another advantage is that there is no supercooling phenomenon during solidification. Paraffin wax and various acid esters are both organic compoundsphase change materialOrganic phase change energy storage materials mainly include three categories: solid-liquid phase change, solid solid phase change, and composite phase change.

Requirements for Phase Change Materials in Lithium Battery Thermal Management Systems

1. Low phase transition temperature, requiring adaptation to the optimal operating temperature range of 15 ℃ -35 ℃ for lithium batteries;

2. The phase transition temperature of materials can be adjusted within a small range, and the optimal operating temperature range for different types of cells is not completely consistent;

3. It is best not to have liquid or gaseous phases before and after the material shaping and phase transition;

4. If the latent heat of the material is high, the system has strong constant temperature capability;

5. The material has good insulation properties, avoiding the risk of insulation leakage in high-voltage systems.

6. The low mass density of phase change materials reduces their impact on the energy density of battery packs.

There are not many phase change material systems that meet the above requirements, among which paraffin expanded graphite is currently the most researched and closest to specific applications in lithium batteries.

Application Case Analysis of Paraffin Expanded Graphite

phase change materialThe application of lithium battery thermal management system can be traced back to 2004, when it was first applied to the temperature control system of electric scooters. Subsequently, paraffin graphite composite materials and paraffin expanded graphite composite materials were gradually applied to the thermal management system of lithium batteries.

Paraffin expanded graphite composite material, paraffin as a phase change material, is responsible for heat absorption and storage, achieving temperature control function. Graphite has a micro porous structure. When the paraffin phase turns into a liquid state, graphite plays a perfect adsorption role, avoiding the material from entering a liquid state.