As a researcher deeply engaged in the field of energy storage systems, I have observed that energy storage technology is a critical pillar for constructing new power systems, achieving high-quality energy development, and realizing the dual-carbon goals. The novel energy storage sector is experiencing rapid growth, demanding that lithium-ion battery systems undergo a comprehensive transformation toward high-quality development. In my work, I focus extensively on the insulation materials used in packaging DC 1500V lithium-ion battery energy storage systems, analyzing their performance and identifying improvement measures. I have also documented typical cases where insulation failure occurred due to damaged insulation components. In this article, I will summarize the current state and future directions of insulation materials for energy storage battery system packaging.
1. Introduction to 1500V Battery System Integration
Lithium-ion battery energy storage systems offer high energy density and fast response, yet their integration complexity introduces risks such as manufacturing defects, aging effects, and environmental factors that can cause corrosion of insulation components, current surges, and thermal shorts from external short circuits. As shown in the typical integration scheme, a 1500V battery cluster consists of 8 battery modules connected in series, each module comprising 4 submodules in series, and each submodule containing 13 cells. With each cell having a maximum voltage of 3.6V, the total string voltage reaches approximately 1500V. To maintain temperature uniformity, I recommend a single-row arrangement with up to 8 modules, and liquid cooling is preferred due to the increased number of cells per module.
2. Insulation Material Performance Analysis for Battery Cells
The battery cell, as the smallest unit of the system, is typically encased in an aluminum shell coated with a blue single-sided release protection film. This film is based on polyethylene terephthalate (PET) with a specialized flame-retardant adhesive. The electrical parameters of the materials used are summarized in Table 1.
| Material | Relative Permittivity εr | Conductivity γ (S/m) |
|---|---|---|
| Air | 1 | 1.0×10-14 |
| Insulation film (PET) | 3.2 | 1.0×10-16 |
| Aluminum shell | 1.0×108 | 3.593×107 |
| Terminal (Copper) | 1.0×108 | 5.998×107 |
The lithium-ion cell consists of an aluminum shell, positive and negative terminals, and an outer insulation film. During operation, the aluminum shell is nearly equipotential with the positive electrode. With a total positive potential of 5370V DC (simulated for a 1500V system string), I performed an electro-quasistatic field simulation on a single cell, including an air domain around it. The calculated electric field strengths at various locations are given in Table 2.
| Location | Maximum Electric Field (kV/mm) | Withstand Test Voltage (kV) |
|---|---|---|
| Large surface | 5.34 | 5.37 |
| Side edge | 5.55 | 5.37 |
| Bottom edge | 6.04 | 5.37 |
| Bottom corner | 6.75 | 5.37 |
The simulation reveals that the highest field strength occurs at the bottom corner of the cell, making it the weakest insulation point. For long-term reliable operation of a 1500V system, it is essential to enhance the insulation, flame retardancy, and thermal resistance of the protective film on each energy storage cell. The wrapping process must account for the field distribution, particularly at the bottom corner, which is also vulnerable to mechanical damage during manufacturing, potentially leading to insulation breakdown.
2.1 PET Protective Film Enhancement
Currently, the development of wrapping materials and processes specifically for 1500V battery systems is still in progress. Compared to 1000V systems, the higher voltage demands greater attention to insulation failure and consequent safety incidents. When encapsulating cells into modules, additional insulation materials such as foam spacers, aerogel thermal pads, polycarbonate (PC) sheets, and mica sheets are often used to indirectly enhance the insulation strength on the large surface areas. Between cell side edges, a millimeter-sized air gap exists between two layers of PET film. For the bottom edges and corners adjacent to the metal support structure, I propose using reinforced insulation, as illustrated in the concept of applying an extra insulating layer at the bottom corners, which can satisfy long-term stability requirements and is relatively easy to implement.
2.2 Composite PET Protective Film
PET protective films sometimes exhibit delamination over time, particularly at the side edges. Given the electric field distribution on the cell, resistance to delamination is a critical performance indicator. Researchers have developed composite films by adding phase-change layers and halogen-free flame-retardant adhesives to the PET substrate. These modifications help maintain thermal balance within the energy storage cell during operation, preventing overheating or overcooling and ensuring long-term adhesion.
2.3 Application of Polyimide Insulation Film
Polyimide (PI) insulation materials are known for their excellent comprehensive properties, making them promising for energy storage and aerospace applications. Often called “golden film,” PI offers high electrical strength (300 kV/mm), wide operating temperature range (up to 350°C), and dimensional stability. It is already used in ultra-high voltage reactor windings, wind turbine windings, and DC motor inter-turn insulation. Table 3 compares the heat resistance and breakdown voltage of 0.025mm thick PI and PET films.
| Material | Operating Temperature Range (°C) | Breakdown Voltage (kV) |
|---|---|---|
| PI film (0.025mm) | −269 to 250 | 37.0 |
| PET film | −70 to 150 | 6.5 |
The insulation margin of PI film is far superior to that of PET. However, research on PI for energy storage cell protection is scarce, and long-term field applications are lacking. Studies have investigated the DC corona resistance mechanism of PI films, and composite films with SnO fillers have been synthesized to achieve high dielectric constants (up to 456) and breakdown strengths of 146.9 MV/m. Yet, most current PI composites compromise some properties to enhance others. Research on long-term environmental tolerance, aging, and degradation under multiphysics conditions is ongoing, and I anticipate that cost-effective PI films will eventually be commercialized for insulation protection of energy storage cells.
3. Insulation Material Performance Analysis for Battery Management Unit
To improve installation and maintenance convenience, battery cells and the battery management unit (BMU) are integrated into a battery module. In such modules, voltage and temperature analog signals from each energy storage cell are collected via wiring harnesses connected to the BMU’s front-end analog processing chip. Current BMU designs comply with 1500V system safety standards (e.g., UL 60950). Internal isolation between the analog front-end chip and the computing chip provides digital isolation with a withstand voltage exceeding 6000V DC, which is higher than the battery’s insulation test voltage of 5370V DC. Voltage sampling accuracy is ≤±5 mV with a period ≤20 ms, and temperature accuracy is ≤±2°C with a period ≤100 ms.
The battery module is a novel primary-secondary integrated electromechanical product. The BMU, as a secondary assembly, is installed locally to collect, condition, and digitize analog signals. Compared to benign environments like protection relay cabinets, the module faces challenges such as high energy density, large spatial dimensions, complex electromagnetic fields, and varying temperature and humidity. Insulation issues related to the BMU require special attention.
The module casing is grounded (low-voltage side). The wiring harnesses are equipotential with the cells and thus have a potential difference with the casing, making them potential weak points in insulation. Common problems include:
- Wiring harnesses pressed against the casing or other low-voltage parts.
- Damaged wiring harnesses placed near the casing.
- Connectors with exposed metal placed near low-voltage parts.
- Wiring harness connectors positioned close to low-voltage surfaces.
3.1 Insulation Reinforcement for Collection Harness Sensors
Temperature sensors (Negative Temperature Coefficient thermistors, NTC) are installed near each energy storage cell. The resistance-temperature relationship of NTCs is nonlinear and suffers from poor consistency and aging. Voltage sensing is typically achieved by mechanical connection to the cell terminals via busbars, with analog signals transmitted through wiring harnesses to the BMU. Researchers often encapsulate the temperature sensor with electronic potting compound to improve aging resistance and insulation performance. However, there is no unified standard for potting compounds used in energy storage cells, and variations in batch quality, stability, and process parameters introduce insulation failure risks and safety hazards.
3.2 Insulation Protection for Wiring Harness
Tapes are commonly used for insulation protection of wiring harnesses. Important properties include temperature resistance, flame retardancy, noise reduction, compatibility with wire insulation, abrasion resistance, low fogging, and oil/corrosion resistance. Rough surfaces and sharp edges can cut tapes and wire insulation, leading to loss of insulation, bundling, or sealing functions, potentially causing accidents. The tape’s adhesive system (acrylic, rubber, synthetic rubber, polyacrylate, etc.) must be compatible with the wire insulation, especially for halogen-free cables. In the battery system container, tapes used on wiring harnesses must exhibit low fogging (low volatile content). When ambient temperature rises, volatile substances and odors from the tape can irritate occupants, reduce comfort, and increase accident probability.
During transportation and installation, the prefabricated energy storage container may experience mechanical shocks such as vibration, collision, and jolting. These shocks can cause internal component displacements, especially BMU wiring harnesses moving or their outer insulation being damaged, thereby reducing the system’s insulation withstand level and evolving into a safety hazard.
4. Simulation and Diagnostic Formulas
To quantitatively analyze electric field distribution, I often use the following relationship for a parallel-plate approximation:
$$ E = \frac{V}{d} $$
where \(E\) is the electric field strength, \(V\) the applied voltage, and \(d\) the insulation thickness. For more complex geometries, I employ finite element simulations. The breakdown condition can be expressed as:
$$ V_{bd} = E_{bd} \cdot d $$
where \(E_{bd}\) is the dielectric breakdown strength. For composite insulation, the effective permittivity of series layers is given by:
$$ \frac{1}{\varepsilon_{eff}} = \frac{f_1}{\varepsilon_1} + \frac{f_2}{\varepsilon_2} $$
where \(f_1, f_2\) are volume fractions of the two materials. The field enhancement factor at corners can be estimated using:
$$ \beta = \frac{E_{max}}{E_{avg}} = 1 + \frac{a}{r} $$
with \(a\) being a geometric constant and \(r\) the radius of curvature. These formulas guide my design of insulation reinforcements for energy storage cells.
5. Conclusion and Outlook
With the rapid development of electrochemical energy storage, massive numbers of prismatic aluminum-shell lithium-ion cells are packaged in prefabricated battery containers. The encapsulation process involves a wide variety of insulation materials, and effective quality control is challenging. The 1500V battery system, as a novel primary-secondary integrated product, lacks long-term operational data on various insulation materials under high energy density, large spatial scales, complex electromagnetic fields, and varying environments. Safety concerns remain a common focus for practitioners and researchers.
Facing intense market competition with declining battery prices, all participants in the battery system integration chain must not only reduce cost and increase efficiency but also proactively address potential insulation hazards from higher voltage levels. Strict control over the selection and evaluation of insulation materials is essential to safeguard safety and support the healthy development of the electrochemical energy storage industry. Future directions include:
- Establishing standardized selection and evaluation systems for insulation materials specifically used in energy storage cell packaging.
- Developing one-stop safety solutions for insulation materials in energy storage cell packaging, integrating manufacturing to better control quality and reduce costs.
- Implementing product verification through technical standards, including critical tests such as aging, to ensure that verification cycles serve as a robust quality moat.
In summary, the insulation materials used in 1500V energy storage battery systems require continuous improvement to meet the demands of higher voltage, reliability, and safety. My work has shown that enhancing the protective film on each energy storage cell, reinforcing wiring harness insulation, and adopting advanced materials like polyimide can significantly reduce the risk of insulation failure. I believe that with concerted research and standardization efforts, the industry can achieve high-quality development while maintaining the highest safety standards.