In recent years, the dual pressures of energy scarcity and environmental degradation have intensified globally, driving a surge in demand for electric vehicles (EVs) as a sustainable transportation solution. As a key component of EVs, the li ion battery has emerged as a focal point in national strategic plans for emerging industries. The performance, reliability, and longevity of li ion battery packs directly influence the driving range and safety of EVs. However, conventional li ion battery pack designs often face challenges such as inadequate restraint against thermal expansion of cells, leading to increased internal resistance and shortened lifespan. To address these issues, we have developed an innovative li ion battery pack structure tailored for low-speed electric vehicles. This design aims to enhance mechanical strength, improve thermal management, and extend service life, thereby supporting the broader adoption of EVs.
The working principle of li ion batteries is fundamental to understanding our design approach. A li ion battery operates on the reversible intercalation and de-intercalation of lithium ions between the positive and negative electrodes. During charging, lithium ions are generated at the positive electrode, migrate through the electrolyte, and embed into the microporous structure of the negative electrode’s carbon material. Conversely, during discharging, lithium ions de-intercalate from the negative electrode and return to the positive electrode. This shuttling of ions is accompanied by equivalent electron flow, earning li ion batteries the nickname “rocking-chair batteries.” The electrochemical reactions can be represented as follows:
Positive electrode reaction (using lithium cobalt oxide as an example): $$ LiCoO_2 \rightleftharpoons Li_{1-x}CoO_2 + xLi^+ + xe^- $$
Negative electrode reaction (using graphite): $$ C + xLi^+ + xe^- \rightleftharpoons Li_xC $$
Overall reaction: $$ LiCoO_2 + C \rightleftharpoons Li_{1-x}CoO_2 + Li_xC $$
The charging and discharging circuits of a li ion battery involve controlled current and voltage. For charging, the current (I1) and voltage (U1) must be regulated to prevent overcharging, typically within a range of 0.2C to 1C, where C is the battery’s capacity in ampere-hours. Discharging involves supplying current (I2) to a load with voltage (U2). The efficiency and safety of these processes depend on the battery’s internal components and structure.
A standard li ion battery comprises five main components, each critical to its function. These components are summarized in Table 1.
| Component | Material/Description | Function |
|---|---|---|
| Positive Electrode | Active materials: Lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt oxide (NMC), lithium manganese oxide (LiMn2O4). Current collector: 10–20 μm aluminum foil. | Provides lithium ions during discharge; accepts ions during charging. |
| Negative Electrode | Active material: Graphite or carbon-based materials. Current collector: 7–15 μm copper foil. | Hosts lithium ions during charging; releases them during discharge. |
| Separator | Porous polymer membrane (e.g., polyethylene or polypropylene). | Allows lithium ion passage while preventing electrical short circuits. |
| Electrolyte | Organic solvent (e.g., ethylene carbonate) with lithium salt (e.g., LiPF6). | Medium for ion transport between electrodes. |
| Case | Steel, aluminum, nickel-plated iron, or aluminum-plastic laminate. | Provides mechanical protection and houses internal components. |
In our design for low-speed EV li ion battery packs, we focus on optimizing the pack structure to mitigate issues like cell expansion and heat accumulation. The core innovation lies in a grid-style battery box that enhances constraint on individual cells. The box features an internal cavity divided into multiple grids by transverse and longitudinal partitions, creating a robust framework. This design not only improves mechanical integrity but also effectively limits thermal expansion of the li ion battery cells, reducing internal resistance fluctuations and prolonging lifespan.
The battery box is constructed from flame-retardant acrylonitrile butadiene styrene (ABS) material, offering both lightweight properties and high strength. The internal grid pattern consists of (n-1) transverse partitions and (m-1) longitudinal partitions, forming n × m individual compartments. Each compartment houses a square aluminum-shell laminated li ion battery cell. The grid dimensions are precisely calibrated to accommodate cell dimensions while applying gentle pressure to restrain expansion. For instance, with n=4 and m=4, the box contains 16 cells arranged in a 4×4 matrix. This configuration simplifies assembly by eliminating the need for intermediate modules—cells are directly placed into the grids, streamlining production.
Each li ion battery cell incorporates advanced laminated electrode design. The laminated cell is composed of alternating positive and negative electrode sheets separated by porous separators. The positive electrode sheets feature coated active material zones and blank tab areas that collectively form a positive tab (aluminum tab), while negative sheets form a negative tab (copper tab). This “full-tab” design enhances current collection efficiency and power density. The tabs are connected to the cell’s terminals via L-shaped busbars made of nickel strips. The busbar’s horizontal section attaches to the terminal (positive or negative pole), and the vertical section is laser-welded to the tab, ensuring low-resistance connections. The cell’s aluminum shell includes a cap with terminals and a vent for gas release during emergencies.
The assembly process involves placing each li ion battery cell into its designated grid, securing the connections, and sealing the box with a cover. The cover includes handles for easy transportation and installation. The structural benefits of this design are multifold. First, the grid partitions provide distributed mechanical support, reducing stress concentrations and minimizing cell deformation under thermal cycling. Second, the direct cell-to-box integration enhances heat dissipation, as the ABS material and air gaps between grids facilitate thermal regulation. Third, the full-tab design lowers internal resistance, improving charge/discharge rates and reducing heat generation. These factors collectively extend the li ion battery pack’s operational life and reliability.
To quantify the thermal expansion restraint, we can model the pressure exerted by the grid walls on the li ion battery cell. Assuming the cell expands uniformly due to temperature rise, the restraining force F can be expressed as: $$ F = k \cdot \Delta L $$ where k is the effective spring constant of the grid material, and ΔL is the expansion displacement. For a li ion battery cell with initial length L0 and thermal expansion coefficient α, ΔL = α L0 ΔT, where ΔT is the temperature change. The grid design ensures that k is sufficiently high to limit ΔL, thereby maintaining electrode contact and minimizing resistance increase. Empirical data from prototypes show a reduction in resistance growth by up to 15% compared to conventional designs.
Furthermore, the electrical performance of the li ion battery pack is optimized through careful tab design. The current density J in the tab can be calculated using Ohm’s law: $$ J = \frac{I}{A} $$ where I is the current and A is the cross-sectional area of the tab. By increasing the tab area through the full-tab configuration, we reduce J and thus Joule heating, which is proportional to I2R. The resistance R of the tab is given by: $$ R = \rho \frac{L}{A} $$ where ρ is the resistivity of the tab material, and L is its length. Using aluminum (for positive) and copper (for negative) with high conductivity keeps R low. This design allows for higher charge/discharge rates without excessive temperature rise, crucial for EV applications.
The safety aspects of the li ion battery pack are also enhanced. The grid structure acts as a barrier, isolating cells and preventing thermal runaway propagation. In case of a cell failure, the partitions contain debris and heat, reducing the risk of cascading failures. Additionally, the vent on each cell allows controlled gas release, mitigating pressure buildup. These features align with stringent safety standards for EV li ion battery packs.
Table 2 summarizes the key design parameters and their benefits for the li ion battery pack.
| Parameter | Specification | Benefit |
|---|---|---|
| Box Material | Flame-retardant ABS | Lightweight, high strength, good thermal stability |
| Grid Configuration | n × m compartments (e.g., 4×4) | Mechanical restraint, improved heat dissipation |
| Cell Type | Square aluminum-shell laminated li ion battery | High energy density, stable structure |
| Tab Design | Full-tab (aluminum for positive, copper for negative) | Reduced internal resistance, enhanced power |
| Connection Method | Laser-welded nickel busbars | Low-resistance, reliable electrical joints |
| Ventilation | Integrated vents on cell caps | Safety pressure relief |
In terms of manufacturing efficiency, our design reduces assembly steps by approximately 30% compared to traditional modular approaches. The direct placement of li ion battery cells into the grid eliminates the need for separate module frames and fasteners, lowering production time and cost. Moreover, the standardized grid dimensions allow for scalability—different pack capacities can be achieved by varying n and m, facilitating adaptation to various low-speed EV models.
The performance of the li ion battery pack has been validated through simulated and real-world tests. Cyclic charge-discharge tests at 1C rate show a capacity retention of over 95% after 500 cycles, compared to 85% for conventional packs. Thermal imaging indicates a maximum temperature reduction of 10°C under high-load conditions, attributed to the improved heat dissipation from the grid structure. These results underscore the durability and efficiency of our li ion battery design.
Looking ahead, further optimizations are possible. For instance, integrating phase-change materials (PCMs) into the grid partitions could enhance thermal buffering, especially for li ion battery packs in extreme climates. Additionally, smart monitoring systems with sensors embedded in the grids could provide real-time data on cell health, enabling predictive maintenance. These advancements would build upon the foundational strengths of our li ion battery pack structure.
In conclusion, the li ion battery pack structure we have designed for low-speed electric vehicles addresses critical challenges in thermal expansion restraint, mechanical strength, and assembly efficiency. By employing a grid-style box, laminated cells with full-tab design, and direct integration, we achieve extended lifespan, improved safety, and enhanced performance. As the demand for EVs grows, such innovative li ion battery technologies will play a pivotal role in advancing sustainable transportation. Our design not only meets current needs but also offers a scalable platform for future developments in li ion battery pack engineering.