Power and energy storage battery packs are assembled by series and parallel connection of cells, yet cells are not in direct contact with one another. Lithium-ion cells undergo volume expansion during charging and discharging, so buffer materials must be arranged between cells to absorb mechanical stress. At present, foam materials are widely adopted to satisfy both dynamic buffering and static supporting requirements.
01 Swelling Phenomenon of Lithium-ion Battery Modules
Lithium-ion batteries are vividly named "rocking chair batteries": driven by potential difference, lithium ions shuttle back and forth between cathodes and anodes to store and release electric energy.
Nevertheless, the intercalation and deintercalation of Li⁺ into and out of cathode and anode active materials cause volume expansion of active substances. Such swelling falls into two categories: reversible swelling and irreversible swelling. Irreversible swelling accumulates continuously as charge-discharge cycles increase.
There are three root causes for cell swelling: electrochemical swelling, gas generation swelling and temperature effects.
Epoxy boards are commonly used as buffer materials between lead-acid batteries for two-wheeled electric vehicles. However, epoxy boards feature high brittleness and zero resilience, failing to absorb cell expansion force. Therefore, they are unsuitable as buffer materials for lithium-ion battery packs despite their ultra-low cost.
Silicon-anode lithium-ion batteries deliver drastically higher energy density compared with carbon-anode counterparts, representing an effective approach to further boost battery energy density. Yet their excessive swelling rate (thickness rises by 15%–30% after 100 cycles) has long hindered large-scale commercial application.
02 Main Types of Buffer Materials
Cell swelling is an inherent characteristic throughout the full service life of batteries. Mitigating adverse impacts brought by swelling becomes critical to extending battery cycle life.
Failure to adequately buffer irreversible cell swelling will further accelerate capacity degradation.
Four core rules governing lithium-ion cell swelling:
Spatial distribution: Expansion force in the central zone is greater than that at edges.
Capacity correlation: Larger cell capacity corresponds to higher swelling rate (energy storage cells generally feature large capacities).
Cell chemistry: Ternary cells swell more severely than LFP cells. NCM811 cells achieve a swelling rate of 6%–8%, while LFP cells only reach 3%–5%. In general, cells with higher energy density exhibit higher swelling rates.
Cycle count: Irreversible swelling accumulates with increasing charge-discharge cycles, leading to greater overall cell expansion.
Buffer materials are therefore indispensable components for modules, serving three core functions: pressure distribution, displacement compensation, thermal insulation and flame retardancy.
(1) Silicone Foam
The base silicone resin has a density of 1.17 g/cm³; after foaming, the density drops to 0.16–0.20 g/cm³. It achieves UL94-V0 vertical flame retardancy and low compression set. Silicone foam is recommended for high-capacity energy storage modules with strong expansion force.
(2) EVA Foam
EVA foam is widely seen in daily products such as slipper soles. Users may notice its obvious drawback: the heel part undergoes permanent flattening after long-term use, indicating a high compression set of EVA. EVA foam is low-cost and mainly applied between prismatic cell modules.
(3) CR Foam
CR foam, full name Chloroprene Rubber Foam, complies with UL94 V-0 flame retardant certification and RoHS environmental standards. Features: low density, lightweight, low hardness, high compressibility, excellent thermal insulation and flame retardancy, superior resilience and long service life.
(4) PU Foam
PU refers to copolymers composed of countless micro-pores and polyurethane resin frameworks, including polyurea and modified polyurethane. PU delivers slightly inferior compression resilience and is mostly used between pouch cell modules. Features: porous structure, low density, high specific strength, decent rebound and air permeability, low hysteresis loss, high compression load ratio, good flame resistance, and outstanding resistance to thermal aging, humidity aging and dynamic fatigue.
(5) MPP Foam
MPP is a porous foamed polypropylene (PP) material manufactured via clean supercritical carbon dioxide technology to generate numerous micron-sized bubbles inside the substrate. MPP meets RoHS and REACH standards and is widely applied in the new energy industry. Features: lightweight, high strength, efficient impact energy absorption, excellent flame retardancy and heat resistance.
(6) Aerogel
The above materials are elastic substrates primarily designed to absorb cell expansion force. Aerogel is another widely adopted inter-cell material focused on thermal insulation and flame retardancy; it also possesses decent compressibility to provide moderate buffering performance. For instance, under a load of approximately 200 kgf on a 1P13S module, a 2.0 mm thick aerogel sheet compresses down to roughly 1.5 mm.
Aerogel thermal insulation sheets are composite materials made of silica aerogel and ceramic fiber substrates. They feature low density, high porosity, high temperature resistance and low thermal conductivity, which effectively block heat propagation between cells during thermal runaway.
