Energy storage technology is a cornerstone for building new power systems, achieving high-quality energy development, and realizing the dual‑carbon goals. According to incomplete statistics from the China Energy Research Society Energy Storage Committee and the Zhongguancun Energy Storage Industry Technology Alliance, by the end of 2023, the cumulative installed capacity of energy storage projects (including physical, electrochemical, and molten salt heat storage) in China reached 86.5 GW, of which new energy storage accounted for 35.2 GW. Among new energy storage technologies, lithium‑ion batteries dominate, accounting for 97.3%. In 2024, the new energy storage sector is expected to continue its rapid development, while the overall lithium‑ion battery system must shift toward high‑quality growth.
Raising the DC voltage level of the lithium‑ion energy storage battery system to 1500 V is a major cost‑reduction measure. Consequently, enhancing the insulation performance of the battery system to meet application requirements has become a research hotspot. This article focuses on the current status and progress of insulation materials used in 1500 V lithium‑ion battery systems.
Integration Method of 1500V Energy Storage Battery System
Lithium‑ion energy storage battery systems offer high energy density and fast response, but their integration is complex. Defects during manufacturing, battery aging, and environmental factors can lead to corrosion of insulation components, current surges, and thermal shocks from external short circuits. As shown in the typical integration scheme, a number of 280 Ah LiFePO₄ prismatic cells are first packaged into modules. Several modules are then connected in series to form a battery block, and blocks are combined into a battery cluster installed inside a container. For example, a 1500 V cluster consists of 8 battery blocks in series, each block contains 4 modules in series, and each module comprises 13 cells in series. With each cell having a maximum voltage of 3.6 V, the total string voltage reaches 1500 V DC. To ensure temperature uniformity, the cluster is arranged in a single row with typically 8 blocks, and liquid cooling is adopted.
Performance Analysis and Improvement of Cell Insulation Materials
The battery cell is the smallest unit. Its aluminum case is usually wrapped with a blue single‑sided release protective film based on PET (polyester) substrate. The material electrical parameters are listed in Table 1. The film thickness ranges from 0.015 mm to 0.20 mm, with good adhesion, flame retardancy, and electrical insulation.
| Material | Relative Permittivity εr | Conductivity γ (S·m⁻¹) |
|---|---|---|
| Air | 1 | 1.0 × 10⁻¹⁴ |
| PET insulation film | 3.2 | 1.0 × 10⁻¹⁶ |
| Aluminum case | 1.0 × 10⁸ | 3.593 × 10⁷ |
| Copper terminal | 1.0 × 10⁸ | 5.998 × 10⁷ |
The lithium‑ion cell consists of an aluminum case (approximately at positive potential), positive/negative terminals, and an outer insulation film. With a total positive potential of 5370 V DC (based on 13 cells in series × 3.6 V per cell plus margin) and a 3 mm chamfer on the case, a quasi‑electrostatic field simulation was performed with an air domain around the cell. The calculated electric field intensity at critical positions is given in Table 2.
| Position | Maximum Field Intensity (kV·mm⁻¹) | Withstand Test Voltage (kV) |
|---|---|---|
| Large face | 5.34 | 5.37 |
| Side edge | 5.55 | 5.37 |
| Bottom edge | 6.04 | 5.37 |
| Bottom corner | 6.75 | 5.37 |
Simulation results indicate that the bottom corner has the highest field intensity, making it the weakest insulation point. Therefore, for long‑term reliability of the 1500 V energy storage battery system, the protective film at the cell bottom corner must be enhanced, especially considering mechanical damage during assembly. The electric field distribution can be expressed by Laplace’s equation in the electrostatic domain:
$$
\nabla^2 \phi = 0
$$
where φ is the electric potential. The field intensity is then E = –∇φ. The insulation film must withstand the maximum field without breakdown. For a 0.025 mm thick PET film, the breakdown voltage is typically 6.5 kV, giving a breakdown strength of 260 kV·mm⁻¹. However, the simulation shows 6.75 kV·mm⁻¹ at the bottom corner, which is far below the PET breakdown strength, but safety margins must account for manufacturing defects, aging, and moisture.
PET Protective Film Reinforcement
Domestic exploration of cell insulation for 1500 V systems is ongoing. Compared with 1000 V systems, the higher voltage level demands more attention to insulation failure and safety accidents. When cells are assembled into modules, insulating materials such as foam spacers, aerogel thermal pads, polycarbonate (PC) sheets, and mica sheets are used to indirectly improve insulation on the large faces. A millimeter‑sized air gap exists between the side edges of two adjacent cells due to the double PET layers. For the bottom edge and corner near the metal support structure, reinforcement using additional insulation layers (e.g., polyimide film or thicker PET) can satisfy long‑term stable operation.
Composite PET Protective Film
PET protective films may experience delamination over time, especially at the side edges. Adding phase‑change layers or halogen‑free flame‑retardant adhesives helps maintain thermal balance and adhesion. The film’s dielectric strength can be modeled using the Weibull distribution for breakdown probability:
$$
P(E) = 1 – \exp\left[ -\left( \frac{E}{\alpha} \right)^\beta \right]
$$
where α is the scale parameter and β is the shape parameter. Higher β indicates lower scatter in breakdown field.
Application of Polyimide (PI) Insulation Film
Polyimide (PI) film, known for excellent comprehensive properties, is widely used in aerospace and high‑voltage insulation. PI film (0.025 mm thick) has a breakdown voltage of about 37 kV and operates from –269 °C to 250 °C, far exceeding PET. Table 3 compares PET and PI.
| Material | Operating Temperature Range (°C) | Breakdown Voltage (kV) |
|---|---|---|
| PI film (0.025 mm) | –269 to 250 | 37.0 |
| PET film | –70 to 150 | 6.5 |
Research on PI for energy storage battery protection is still limited. Wang et al. studied DC corona resistance of PI films with 15% SiC fillers, showing improved electrical‑mechanical properties. Li et al. synthesized SnO/PI composite films with dielectric constant up to 456 and breakdown strength of 146.9 MV·m⁻¹. However, practical application requires further research on long‑term environmental tolerance and aging mechanisms.
Performance Analysis and Improvement of Battery Management Unit Insulation Materials
To improve installation and maintenance convenience, cells and the battery management unit (BMU) are integrated into a battery block. The BMU must comply with UL 60950 for 1500 V systems. Internal analog front‑end chips communicate with the computing chip via digital isolation with withstand voltage above 6000 V DC, exceeding the battery withstand test voltage of 5370 V DC. Voltage sampling accuracy is ≤ ±5 mV with a period ≤ 20 ms; temperature accuracy ≤ ±2 °C with period ≤ 100 ms.
The battery block is a novel primary‑secondary fusion electromechanical product. The BMU, as a secondary component, is installed locally, facing challenges such as high energy density, large spatial scales, complex electromagnetic environments, and variable temperature/humidity. The block casing is grounded (low potential), while the voltage/temperature sensing harnesses are at the same potential as the cells, creating a potential difference with the casing. This makes the sensing harness a potential insulation weak point.
Common Insulation Issues in Sensing Harnesses
Common problems include: (1) harnesses pressed against the casing; (2) damaged insulation due to abrasion near low‑potential parts; (3) exposed metal at connector junctions near the casing; (4) connector positions too close to the casing.
Reinforcement of Sensing Harness Sensors
Temperature sensors (NTC thermistors) and voltage sensing wires require encapsulation with electronic potting compounds to improve aging resistance and insulation. However, no unified standard currently exists for potting compounds used in energy storage battery temperature sensors, leading to risks from batch variability and process parameters.
Insulation Protection of Harness Wires
Adhesive tapes are commonly used for harness insulation. Key properties include temperature resistance, flame retardancy, noise reduction, compatibility with wire insulation, abrasion resistance, low fogging, oil and corrosion resistance. When the harness is routed near sharp edges, the tape and wire outer jacket may abrade, reducing insulation. Low‑fogging tapes are essential to avoid volatile emissions in elevated temperatures inside the container.
During transportation and installation, the pre‑assembled energy storage container may experience mechanical shocks (vibration, collision, road bumps) that can shift the BMU wiring harnesses or damage their insulation, reducing system insulation withstand capability.
Conclusion and Outlook
With the rapid growth of electrochemical energy storage, massive numbers of lithium‑ion prismatic aluminum‑case cells are packaged inside prefabricated energy storage containers. Many types of insulation materials are used during cell packaging, but effective quality control over each material remains challenging. The 1500 V energy storage battery system, as a novel primary‑secondary fusion product, lacks long‑term operational data under high energy density, large spatial scales, complex electromagnetic environments, and varying temperature/humidity. Safety concerns are a shared focus for practitioners and researchers.
Facing fiercely competitive cost reduction in energy storage batteries, all participants in system integration must not only reduce costs but also proactively address potential insulation risks from higher voltage levels. Strict control over the qualification and evaluation of insulation materials used in energy storage battery packaging is essential to uphold safety bottom lines and support healthy high‑speed development of the electrochemical energy storage industry. Future work should consider:
- Promoting the establishment of dedicated standards for insulation material selection and evaluation in energy storage battery packaging.
- Developing one‑stop safety solutions for insulation materials to form integrated manufacturing advantages, better control product quality, and reduce costs.
- Incorporating critical verification steps (e.g., aging tests) into technical standards to ensure product quality through adequate validation cycles.