I. The "Family Members" of Lithium-ion Batteries: Different Forms for Different Scenarios
ithium-ion batteries are not a single product; they can be divided into two main categories based on their core characteristics, adapting to different usage needs:
Based on the electrolyte form, they can be divided into liquid lithium-ion batteries (LIB) and solid-state lithium-ion batteries. Liquid lithium-ion batteries are currently the mainstream, offering high energy density and controllable costs, and are widely used in consumer electronics and electric vehicles; solid-state lithium-ion batteries are the future direction, using solid electrolytes instead of liquid ones, resulting in higher safety and greater energy density. Polymer lithium-ion batteries, in particular, can be made into thin, flexible, and other irregular shapes, perfectly suited for wearable devices such as smartwatches and wireless headphones.
Based on shape, there are three types: cylindrical, prismatic, and irregular. Cylindrical batteries have a stable structure and are commonly found in flashlights and some electric vehicles; prismatic batteries have high space utilization and can be flexibly adjusted in size, making them the preferred choice for mobile phones, computers, and most electric vehicles; irregular batteries, leveraging the properties of polymers, can be customized into ring-shaped, bean-shaped, and other forms, becoming key to the innovative shapes of wearable devices.
II. The Working Principle: The "Back and Forth Migration" Journey of Lithium Ions
The core principle of a lithium-ion battery is simple—lithium ions "move back and forth" between the positive and negative electrodes, realizing the conversion of chemical energy into electrical energy, hence it is also called a "rocking chair battery."
During discharge, lithium atoms at the negative electrode lose electrons and become lithium ions, moving towards the positive electrode through the electrolyte. The electrons then form a current through the external circuit, powering the device; during charging, the external power source forces the reaction to proceed in reverse, lithium ions are deintercalated from the positive electrode and return to the negative electrode for storage, completing the "storage" of electrical energy. This reversible "intercalation-deintercalation" process is the key to the repeated charging and discharging of lithium-ion batteries. However, as the number of cycles increases, the reversibility gradually weakens, which is the reason for battery aging.
III. Choosing Batteries Without Falling into Pitfalls: Understanding Key Performance Parameters
When choosing electronic products or electric vehicles, terms like "capacity," "range," and "charging speed" all correspond to the core performance parameters of lithium batteries. Understanding these indicators is sufficient:
- Battery Capacity: Measures the energy storage capacity and directly determines the range. The "4000mAh" or "70kWh" we usually see is the rated capacity; the actual capacity will be affected by the discharge rate and temperature.
- Discharge Rate: Determines the charging speed and instantaneous power output, expressed in "C-rate." For example, a 2C discharge rate means a battery that takes 1 hour to charge can be discharged in 30 minutes. The higher the discharge rate, the slightly lower the actual capacity.
- Voltage Parameters: Open-circuit voltage (voltage when not in operation) can quickly determine the battery status. The working voltage (voltage during operation) decreases with increasing discharge current, and low temperatures also cause the voltage to drop.
- Battery Life: For secondary batteries (rechargeable), the main factor is cycle life, which is the number of charge-discharge cycles before the capacity decays to 80% of the rated capacity. Consumer electronics batteries typically have 500-1000 cycles, while power batteries can reach over 2000 cycles.
IV. The Birth Journey: 5 Core Stages, Over 100 Rigorous Processes
The birth of a qualified lithium battery goes through five core stages: material preparation, coating, assembly, formation, and capacity testing. Every step must be meticulous:
1. Material Preparation: Mixing positive and negative electrode active materials, conductive agents, binders, etc., into a uniform slurry, controlling viscosity and environmental humidity to lay the foundation for performance.
2. Coating: Uniformly coating the slurry onto metal foils (positive electrode aluminum foil, negative electrode copper foil), then drying and rolling to form electrode sheets. Precision is controlled to the micron level to ensure uniform thickness and coating amount.
3. Assembly: Assembling the electrode sheets, separator, electrolyte, etc., into a battery cell. This includes processes such as cutting, welding tabs, winding/stacking, casing, electrolyte injection, and sealing. Every step must prevent short circuits and leaks. 4. Formation: The battery is activated through specific charging and discharging cycles to form a stable SEI film (a passivation film that protects the negative electrode), allowing the battery to "wake up" and stabilize its performance.
5. Capacity Sorting: The actual capacity of each battery is tested and sorted by capacity to ensure consistency within the same batch of batteries and prevent performance issues when used in series.