I. Battery Family

The term “lithium battery” originally referred to lithium‑metal batteries, which are primary (non‑rechargeable) cells; however, due to their high risk of explosion, they have long since fallen out of use. Today, what are commonly called “lithium batteries” are almost always lithium‑ion batteries.
       The AA and AAA batteries we use every day are both primary (dry) batteries; in the early days, bulky “brick” mobile phones relied on nickel‑metal hydride batteries; electric scooters typically use lead‑acid batteries, which come packaged in square clusters of four; and in our smartphones, laptops, and even electric vehicles, lithium‑ion batteries are now the norm.
 

Comparison of the Characteristics of Major Electrochemical Energy Storage Batteries

II. Glossary of Battery Terms

SOX: Its full name is “State of X,” referring to the battery’s state. H stands for Health, C for Capacity, P for Power, and E for Energy—somewhat analogous to engine parameters like displacement, power, energy, and runtime. The general meaning is consistent.
SOC: (State of Charge) Refers to the battery’s state of charge. The battery’s charge is analogous to the water in a bucket; at any given moment, the amount of usable energy stored in the battery is defined as its SOC. When fully discharged, the SOC is 0; when fully charged, the SOC is 1. It is expressed as the ratio of available capacity to the battery’s total capacity.
DOD: (Depth of Discharge) Refers to the depth of discharge of a battery. When fully charged, the DOD is 0; when fully discharged, the DOD is 1. Therefore, under normal conditions, the DOD of a battery is a value between 0 and 1. The relationship between DOD and SOC is: DOD + SOC = 1.
SOH: (State of Health) Refers to the ratio of the battery’s current actual capacity to its initial rated capacity. As the battery ages, SOH continuously decreases. It is typically assessed based on capacity and internal resistance.

The majority of studies define SOH in terms of battery capacity degradation, with the following definition of SOH:

In the equation: Caged denotes the battery’s current capacity; Cra te d is the battery’s rated capacity.

III. Lithium Battery Classification
       Classified by practical performance: Power‑type (sprinting, short‑duration high‑power output) and endurance‑type (long-distance running, high energy storage)
       By appearance: Cylindrical, square (steel/aluminum casing), soft-pack (aluminum-plastic film)
       By electrolyte material: Liquid lithium-ion batteries (LIBs) and polymer lithium-ion batteries (PLBs).
       Liquid lithium-ion batteries use a liquid electrolyte (currently, most power‑type batteries are of this type). Polymer lithium-ion batteries, on the other hand, replace the liquid electrolyte with a solid polymer electrolyte; such polymers may be in a “dry” state or a “gel” state, with polymer gel electrolytes being the predominant choice today. As for all‑solid‑state batteries, in the strict sense they refer to systems in which both the electrodes and the electrolyte are solid.
       By cathode material: Lithium iron phosphate batteries (LFP), lithium cobalt oxide batteries (LCO), lithium manganese oxide batteries (LMO), (binary cells: lithium nickel manganese oxide/lithium nickel cobalt oxide), (ternary: lithium nickel cobalt manganese oxide batteries ( NCM ), nickel-cobalt-aluminum oxide lithium batteries (NCA).
       By negative electrode material: Lithium titanate batteries (LTO), graphene batteries, and nano-carbon-fiber batteries.

18650 battery
      18650 It is a model of lithium-ion battery, similar to AA, AAA, C, and other common dry-cell batteries.
       The 18650 is the progenitor of lithium-ion batteries—a standardized cell format established by Japan’s Sony Corporation back in the day to cut costs. The “18” denotes a diameter of 18 mm, the “65” indicates a length of 65 mm, and the “0” signifies a cylindrical shape. Common 18650 cells are categorized into lithium‑ion and lithium‑iron‑phosphate types. Lithium‑ion batteries have a nominal voltage of 3.7 V, a charging termination voltage of 4.2 V, and typical capacities ranging from 1,200 mAh to 3,350 mAh. Lithium‑iron‑phosphate batteries, on the other hand, feature a nominal voltage of 3.2 V, a charging cutoff of 3.6 V, and common capacities between 2,200 mAh and 2,600 mAh.
       Advantages of the 18650:
       Standardization: The 18650 battery is the earliest, most mature, and most stable lithium-ion battery. Japanese manufacturers have achieved a high level of consistency, making it easy to replace individual cells when issues arise.
       Safety: 18650 batteries typically feature steel casings, offering superior protection against potential impacts. Moreover, as manufacturing processes for 18650 cells continue to advance, their safety performance has steadily improved. While steel‑cased lithium batteries can occasionally experience explosions, modern 18650 cells are equipped with safety valves that not only relieve excessive internal pressure but also physically disconnect the cell from external circuits, effectively isolating it and safeguarding the other cells in the battery pack.
       However, other battery technologies also offer distinct advantages. For instance, lithium‑polymer batteries boast superior safety, high energy density, and a low‑strength packaging film that prevents explosions—though they may still burn under extreme conditions. They can be custom‑designed, but this comes at the cost of higher R&D expenses and reduced versatility.

Lithium iron phosphate battery (LFP)
       Lithium iron phosphate (chemical formula LiFePO4, also known as lithium iron phosphate or lithium‑iron‑phosphate, abbreviated LFP) is a cathode material for lithium‑ion batteries. It is also referred to as a lithium‑iron‑phosphate battery. Its key feature is that it contains no costly elements such as cobalt; the raw materials are inexpensive, and phosphorus, lithium, and iron are abundant in Earth’s resources, eliminating supply‑chain concerns. It also boasts a moderate operating voltage.       (3.2 V), high specific capacity (170 mAh/g), high discharge power, fast charging capability, and long cycle life, with excellent stability under high-temperature and thermal‑stress conditions.

IV. Lithium battery voltage and capacity

       The voltage of a lithium-ion battery varies with the discharge current, ambient temperature, and the specific materials used for the positive and negative electrodes.

       This figure shows the discharge curves of a Panasonic 2550 mAh lithium-ion battery with lithium cobalt oxide as the cathode material. From top to bottom, the three curves represent the voltage and capacity variations under three different discharge currents.
       First, during charging and discharging, the voltage continuously varies. Taking 490 mA as an example, when the battery is fully charged, its open-circuit voltage is 4.2 V. As discharge proceeds, the voltage (on the vertical axis) gradually decreases, while the amount of charge discharged (on the horizontal axis) steadily increases—until, at 3.5 V, the voltage begins to drop sharply. Although the voltage changes throughout the entire discharge process, for simplicity, we designate the average value of the relatively flat portion of the curve, 3.7 V, as the battery’s nominal voltage. This voltage is also referred to as the rated voltage.
       This voltage was measured under low-current conditions at room temperature and decreases as the discharge current increases and the temperature drops.
       Another key factor influencing battery voltage is the choice of cathode and anode materials. The Panasonic batteries mentioned above use lithium cobalt oxide and graphite, respectively, which were the standard materials across the lithium‑battery industry in previous years. With the introduction of new materials, lithium batteries with nominal voltages of 3.6 V or 3.8 V have emerged in recent years, employing alternative cathode materials. Compared to lithium‑cobalt‑oxide batteries, these newer designs can achieve higher energy densities, enabling greater energy storage per unit weight and volume.

       Battery capacity is categorized into rated capacity and actual capacity.
       1. Actual capacity refers to the amount of electric charge a battery can deliver under specified discharge conditions. The actual capacity is always lower than the theoretical capacity.
       2. The rated capacity refers to the minimum amount of electric charge that a battery is required to deliver under specified discharge conditions, as defined during its design and manufacture.
       Battery capacity is typically expressed in Ah (ampere-hours), while individual cells are usually labeled in mAh (milliampere-hours) for convenience. If the battery’s rated capacity is 1300 mAh, then discharging it at a current of 130 mA will allow it to operate continuously for 10 hours (1300 mAh / 130 mA = 10 h). This is an analysis under ideal conditions; in actual operation, the current drawn by digital devices cannot remain constant at a fixed value.
       The capacity of a 18650 lithium battery typically ranges from 1,200 mAh to 3,600 mAh.
       Today, the unit used to measure a smartphone battery’s capacity is mAh. As we learned in high school, this is a unit of electric charge; to express energy, you must multiply it by the voltage.

  Battery capacity calculation method:

Battery energy calculation method:

       Wait a minute—phones clearly consume energy, so why are they measured in units of electric charge?! The reason is that, in portable electronic devices like smartphones, to minimize battery size, lithium-ion batteries typically use cathodes made of… Lithium cobalt oxide , this cathode material possesses very high Compaction density . Precisely because the positive electrodes of all smartphone batteries use the same material, their voltages are fairly similar (theoretical value: 3.7 V, though this can vary depending on each manufacturer’s production process). Moreover, smartphones typically contain only a single cell. Single-cell battery (Please disregard a certain smartphone recently released by a particular manufacturer, which contains two 2,000 mAh batteries.) In fact, battery capacity alone is sufficient to measure the amount of energy a battery can store.
       However, batteries in computers typically display both the battery’s capacity (charge level) and its energy. This is because a computer doesn’t contain just a single cell; it houses many cells. Battery series and parallel connections , formed Battery pack , then battery capacity can no longer be used as a measure. High school physics tells us: For batteries of the same model, when connected in parallel, the battery pack’s voltage remains unchanged, but its capacity increases; when connected in series, the pack’s capacity stays the same, while its voltage rises.

V. Why Choose Lithium-Ion Batteries

      Lightweight
The gravimetric energy density of lithium-ion batteries currently ranges from 200 to 260 Wh/kg, whereas lead-acid batteries typically offer 50 to 70 Wh/kg, and nickel-metal hydride batteries fall within 40 to 70 Wh/kg. This means that, for the same capacity, the other two types of batteries weigh three to five times as much as a lithium‑ion battery. Consequently, lithium‑ion batteries hold a clear advantage when it comes to reducing the weight of energy‑storage systems.
Lithium‑ion batteries typically have a volumetric energy density about 1.5 times that of lead‑acid batteries, while nickel‑metal hydride batteries offer only 60–80% of the energy density of lithium‑ion batteries. Consequently, for a given capacity, lithium‑ion batteries also occupy less volume.

      Fast charging
Because lithium‑ion batteries have highly reactive chemistry and ions move rapidly within the cell, they support higher charging currents and charge more quickly—typically fully charged in about three hours. In contrast, nickel‑metal hydride batteries charge much more slowly, taking roughly a full day to reach a full charge.
      No memory effect
The memory effect is a phenomenon in which repeated cycling causes crystallization of the battery’s internal materials. For nickel‑based batteries, prolonged storage can lead to the growth of coarse crystals on the nickel electrodes, reducing their contact with the electrolyte and resulting in capacity loss. Performing several full charge–discharge cycles can refine these crystals and partially restore the battery’s capacity—this process is known as “activation.”
Lithium batteries, on the other hand, do not suffer from the memory effect; they can be activated and restore their full capacity after 3 to 5 normal charge–discharge cycles.

      Environmental protection
Under environmental protection policies, ensuring a reduction in pollution requires careful management throughout the production, use, and recycling of lead‑acid batteries; improper handling in these processes can still result in environmental contamination. In contrast, lithium batteries, with their more robust packaging and sealing, tend to be relatively more environmentally friendly.