Bluetooth earphones are the application of Bluetooth technology to earphones, allowing users to easily communicate in various ways without the hassle of wires. Since the emergence of Bluetooth earphones, they have been a great tool in people’s daily lives.
Bluetooth technology was invented by Swedish telecommunications company Ericsson in 1994 to replace wired connections between devices.
Around 2000, the first consumer oriented Bluetooth earphones were born. They are almost all single ear or ear hanging designs, mainly used for business calls, with poor sound quality, short battery life, and bulky size, and regarded as exclusive to “tech geeks” or business professionals
The 2010s was a crucial decade for Bluetooth earphones to transition from being a “calling tool” to a “mainstream audio device”.
With the upgrade of Bluetooth protocols (such as A2DP), Bluetooth earphones that support stereo transmission have emerged. People started using them to listen to music, but the early sound quality and stability were poor.
The cancellation of the 3.5mm headphone jack on the iPhone 7 in 2016 was a groundbreaking event. It has forced the market to seek wireless solutions, strengthen confidence in the Bluetooth earphone market. Android manufacturers quickly followed suit, and wireless technology became an irreversible trend. Apple AirPods (released in 2016) are a true revolutionary. Although it is not the first true wireless earphone, its innovative design and perfect marketing have completely defined the category of “true wireless Bluetooth earphones” and ignited the global market.
The rapid development of Bluetooth earphones is not only due to the maturity of Bluetooth technology, but also to the support of other key hardware, and micro battery technology is one of them.
The technical difficulty of producing small polymer lithium batteries (usually referring to batteries below 100mAh or even smaller) is indeed much higher than that of producing standard sized mobile phones or power batteries. This is mainly because the requirements for accuracy and consistency increase exponentially as the size becomes smaller.
This is a small polymer lithium battery for bluetooth earphone, the size is only 4 × 9 × 9mm, 30mAh capacity, 3.7v nominal voltage. Can be used in the small space of most Bluetooth earphones to provide long-lasting power.
Next, let’s analyze the production technology difficulties of micro polymer lithium batteries：
1.Electrode Preparation and Slitting: Extremely High Precision Requirements
Ultra-thinning: The electrode sheets (positive and negative electrodes) for micro-batteries need to be extremely thin, possibly only 20-50 micrometers thick (including the current collector and active material coating). The uniformity of the coating is extremely important; any slight unevenness in thickness can lead to inconsistent battery capacity or the risk of micro-short circuits.
Micro-slitting: Wide electrode sheets are slitted into electrode strips with millimeter-level widths (possibly only 4-10 mm). This process easily generates burrs and metal dust. Even a micrometer-sized burr can pierce the separator, leading to high self-discharge or even a short circuit. This places extremely high demands on the precision, sharpness, and cleanliness of the slitting tools.
2.Winding/Layering Process: Miniaturized “Minimally Invasive Surgery”
Alignment Precision: At millimeter-level widths, the positive electrode, separator, and negative electrode must be precisely aligned across multiple layers with errors controlled within tens of micrometers. Misalignment leads to edge burrs or wasted active material.
Tension and Control: Tension control during the winding process for extremely thin separators and electrodes must be extremely precise. Excessive tension can break the material or cause deformation, and also result in loose winding and poor internal contact.
Dust-Free and Cleanliness: The entire process must be carried out in a Class 1000 or even Class 100 cleanroom environment; even a speck of dust invisible to the naked eye can be a fatal flaw.
3.Electrolyte Injection and Sealing: Micro-volume Control and Perfect Sealing
Micro-volume Injection: The amount of electrolyte injected into a battery may only be 0.1 to 0.5 grams. Precisely and quickly injecting this extremely small amount of electrolyte and ensuring sufficient wetting is a major challenge. Insufficient injection affects performance, while excessive injection may lead to swelling or leakage during sealing.
Reliable Sealing: Sealing is the first line of defense for battery safety. The sealing edges of micro-batteries are typically very narrow (1-2 mm), requiring reliable heat-sealing of multiple layers of materials (aluminum-plastic film, tabs, sealant) within a very small area to ensure no leakage or air ingress during long-term use. The treatment of the tab sealing area is particularly critical.
4.Formation and Aging: Consistency Management Challenges
Consistency Screening: Due to the small total amount of active material in micro-batteries, minute differences during production are amplified. After formation (the formation of the SEI film during the first charge), the dispersion in voltage, internal resistance, and self-discharge is more pronounced than in large batteries. Precise aging tests are required to screen out “defective” products with excessive self-discharge and unstable voltage, leading to immense pressure on yield management.
Micro-current Control: Formation and testing equipment needs to stably control and measure microampere-level (μA) currents, placing extremely high demands on equipment precision.
5.Safety and Reliability: More Severe Challenges
Balancing Energy Density and Safety: Achieving high capacity within a small volume means denser packing of active materials, placing higher demands on the puncture resistance of the separator and the stability of the electrolyte.
Irregular Shapes: To fit into the limited space of products like TWS earphones, batteries are often made into irregular shapes (such as elongated, button-type, or with tabs), further increasing the complexity of electrode design and assembly processes.
Cycle Life: In such a small volume, even minor side reactions (such as electrolyte consumption or lithium dendrite growth) have a greater relative impact on the total capacity. Therefore, the stability requirements for the material system and process control are more stringent to ensure capacity retention after hundreds of cycles.
6.Tabs and Connection Process: Tiny Yet Critical How to securely and with low resistance weld the paper-thin tabs (usually nickel or aluminum strips) to the electrode sheet, and ensure they do not break or increase internal resistance excessively during subsequent sealing and bending, is also a process requiring precise control.
In summary: Producing high-quality micro-polymer lithium batteries is essentially a contest of “precision manufacturing” and “ultimate consistency.”
