Fast charging technology: Currently, lithium-ion batteries are advancing in three main directions. Firstly, there is a focus on achieving faster charging speeds. For smartphones, the charging magnification typically ranges from 1C, and the maximum charging magnification has reached 6C. This means that a mobile phone can be fully charged in as little as 16 minutes. Secondly, there is a push for higher energy density. The current system platform operates at 4.45V, which has already achieved successful commercialization. Researchers are now exploring platforms with voltages of 4.48V or even higher. This advancement in voltage is a popular research direction. Thirdly, there is an emphasis on longer cycle life. In the past, 3C lithium-ion batteries had a service life requirement of 500 cycles. However, major manufacturers have now increased this to 800 cycles.
Fast charging refers to a charging method that rapidly charges the battery to full or near-full capacity within a very short period. However, it is crucial to ensure that the lithium-ion battery can still meet the specified cycle life, safety performance, and electrical performance. Although ordinary commercial lithium-ion batteries can handle occasional charging and discharging at high magnifications, long-term high-power charge, and discharge will significantly reduce their cycle life. Fast-charging lithium-ion batteries require special materials selection and design considerations to meet customer requirements. Drawing on extensive experience in fast-charging battery design, this paper discusses the key design aspects of fast-charging lithium-ion batteries and the factors that influence their performance.
In fast charging technology, the diffusion process involves several steps, including Li+ migration from the positive electrode material, Li+ migration in the electrolyte, Li+ transport through the separator, Li+ insertion into the negative electrode, and Li+ diffusion within the negative electrode material. Improving the fast charging performance of lithium-ion batteries requires research and development in these areas.
The solid-phase diffusion coefficient within the anode material is generally small, which limits the high-current charge and discharge capacity of the battery and becomes the controlling factor in the electrode reaction. Li+ migrates from the positive electrode to the negative electrode driven by the electric field and concentration gradient. This migration involves liquid-phase diffusion, and the concentration of the electrolyte significantly affects the improvement of fast charging performance. The porosity of the separator determines the migration of Li+. If the porosity is low, high-current charging can lead to pore blockage. The thickness of the separator determines the distance of Li+ diffusion, with thinner separators allowing for shorter diffusion paths.
1.1 Negative Materials
Graphite is commonly used as a negative material due to its advantages of a two-dimensional layered structure and low-voltage platform. The spacing between layers (C-C spacing) can reach 0.340nm, allowing Li+ to be embedded in the graphite layers to form the interlayer compound LixC6. However, the layered structure of graphite requires Li+ to be embedded from the end of the graphite, increasing the diffusion path and slowing down the diffusion rate. Charging at high magnifications can lead to the deposition of lithium dendrites on the graphite surface, posing potential safety hazards. To improve the performance of graphite, surface coating modification is commonly employed. Soft and hard carbon materials, with slightly larger layer spacing than graphite, facilitate lithium ion diffusion. Typically, the surface of graphite is coated with a layer of soft and hard carbon to enhance the electrochemical properties. This surface modification creates a carbon layer with an amorphous structure, increasing the lithium-ion channels, improving lithium-ion diffusion, and enhancing the charging performance. In the design of fast-charging lithium-ion batteries, small particles, and soft and hard carbon-coated negative materials are commonly used.
High-concentration electrolytes exhibit excellent charging performance. Experimental studies have shown that a phosphate-based electrolyte composed of 5mol/L lithium bis(fluoro sulfonyl)imide (LiFSI) prepared in a trimethyl phosphate (TMP) solvent demonstrates good compatibility with graphite anode materials. It forms a stable solid electrolyte interface (SEI) layer rich in LiF, which effectively inhibits the growth of lithium branches in lithium metal batteries. Another study involves adding propionitrile or nitrile-assisted solvents to traditional vinyl carbonate ester-based electrolytes. This addition significantly enhances the electrolyte’s conductivity and promotes high-magnification charging capacity, particularly at low temperatures (-20 °C), enabling fast charging under such conditions. To achieve better fast charging, it is advisable to select an electrolyte with high concentration, high conductivity, and low viscosity.
The quality of the separator plays a crucial role in the interface structure and internal resistance of the battery, directly impacting its magnification, cycle life, and safety performance. To ensure desirable properties such as electronic insulation, low resistance, high ion conductivity, electrolyte corrosion resistance, and efficient infiltration, various aspects of the separator are investigated during selection, including thickness, porosity, air permeability, infiltration, pore size, puncture strength, and thermal stability. Among these factors, the thickness, porosity, and permeability have significant effects on the fast charging of lithium-ion batteries. Thin separators with high porosity and permeability result in minimal hindrance to the transmission of lithium ions from the positive to the negative electrode, thereby reducing polarization during the charging process. When designing fast-charging batteries, it is common to choose thin separators with high porosity.
Consumer lithium-ion batteries have four main internal structure types based on their production methods: ordinary structure, polar ear middle structure, multi-pole ear structure, and laminated structure. The ordinary structure consists of a single polar ear for both the positive and negative electrodes, located at one end of the separator and created through winding. The polar ear of the polar ear middle structure is situated in the middle of the separator and is typically processed using techniques such as laser cleaning, interval coating, and tape application. This structure exhibits low internal resistance and good magnification performance.
The multi-pole ear winding pole features multiple polar ears, the positions of which can vary in design. This structure also contributes to lower battery resistance and improved magnification performance. Laminated batteries are created by cutting polar pieces into specific shapes and folding them alternately between positive and negative electrodes. Each layer in this structure contains a polar ear, resulting in the best magnification performance.
2.1 Polar Ear Middle Structure
Experimental studies have examined the impact of the polar ear position on the performance of lithium-ion batteries. The position of the polar ear significantly affects the internal resistance and magnification of the battery. When the polar ear is located in the middle of the positive and negative electrodes, the battery demonstrates the lowest internal resistance and optimal magnification performance, closely approaching the performance of laminated batteries. In the normal structure, the polar ear is situated at one end of the separator, while in the polar ear’s middle structure, the polar ear is positioned in the middle of the battery pole.
For the same battery model, the internal resistance and DC resistance (DCR) vary significantly between the polar ear middle structure and the non-polar ear middle structure. The internal resistance of the normal structure reaches 30mΩ, whereas the polar ear structure exhibits only 17mΩ of internal resistance. The DCR of the normal structure under a 50% charge state is 56.xn--6m-fcc, whereas the polar ear middle structure records 47.xn--4m-fcc of DCR.
When considering battery magnification performance, there is little difference in small magnifications between the two structures, but a noticeable difference appears at large magnifications. The primary reason for this disparity is that the polar ear in the polar ear central structure is positioned in the middle of the separator. During the discharge process, electrons diffuse from the middle towards the two ends. As the magnification increases, the excessive number of electrons causes channel blockage, resulting in poor performance at high magnifications. Additionally, the polar plate generates heat, leading to diminished cyclic performance.
2.2 Multi-pole Ear Winding
Multi-pole ear winding involves shaping fixed polar ears in the carrier and subsequently welding the carrier to the polar ear, creating a multi-polar ear battery. Due to its increased number of polar ears and more uniform distribution, this structure exhibits improved magnification performance and reduced temperature rise during charge and discharge, making it suitable for high-power equipment. Currently, drones widely employ this structure.
However, the multi-pole ear winding structure incurs higher battery costs due to welding requirements and increased precision. The advantages of the multi-pole ear structure include further reduction of battery impedance, enhanced charge and discharge performance at high magnifications (supporting 5C~10C discharge), effective reduction of temperature rise during high magnification discharge (surface temperature rise below 20°C for 10C discharge), and improved battery cycle life due to lower operating temperatures.
The multi-pole ear winding battery exhibits significantly lower internal resistance and a higher percentage of constant current charging capacity in relation to the total capacity. Currently, many mobile phone manufacturers claim fast charging capabilities, but such claims often pertain only to the initial 30 minutes. The multi-pole ear winding technology can enhance the charging experience during the constant voltage stage. However, due to its numerous ears, welding requirements, and complexity of implementation, the energy density of the multi-pole ear structure is lower. Improving the energy density of multi-pole ear structure batteries will be a primary focus of this technology in the future.
2.3 Laminated Technology
In the laminated battery structure, each layer includes a polar ear. This structure currently boasts the highest fast-charging performance among all existing structures. However, due to limitations in automation, it is less commonly used in consumer electronics and finds greater application in the military industry and power batteries. It is expected that as automation capabilities improve, laminated technology will become mainstream in the future.
The fast charging performance of lithium-ion batteries is closely tied to battery design. Several factors significantly impact the fast charging performance, such as the coating volume of the polar sheet, compaction density, the thickness of copper or aluminum foil, the size of the polar ear, and the width of the polar sheet. The surface density and compaction density of the battery have an obvious effect on magnification, cycle performance, and other battery characteristics. Fast-chargeable lithium-ion batteries require a low surface density design, while high or low compaction density leads to performance variations. Excessive compaction density can crush the active material in the polar sheet, resulting in rapid capacity decline, while insufficient compaction density leads to inadequate contact between active substances, resulting in high battery impedance and poor fast charging performance.
Experiments have investigated the influence of different foil thicknesses on lithium-ion battery performance. Thicker foils exhibit better conductivity, lower resistance, and improved magnification compared to thinner foils. However, increased thickness inevitably leads to a decrease in battery energy density. The principle applies to polar ears as well, where larger cross-sectional areas result in lower resistance values. Currently, commonly used polar ear thicknesses are 0.08 or 0.1mm, with widths ranging from 4 to 6mm. Fast-charging lithium-ion batteries typically employ polar ears of size 0.1mm × 6mm. Additionally, copper-nickel-plated polar ears can reduce resistance.
Moreover, the length, width, and size of the battery also impact its fast charging performance.
4.1 Construction of Conductive Network
Typically, more conductive materials like carbon nanotubes (CNT) or carbon black (SP) are utilized as conductive additives to establish a conductive network, thereby further enhancing the fast charging performance of lithium-ion batteries. The amount of conductive agent must be appropriately balanced. Insufficient amounts may fail to establish an effective conductive network, while excessive amounts reduce the active substance content in the electrode and decrease energy density. Constructing a conductive network optimizes the electrical performance of electrode materials, which holds practical significance for achieving fast-charging lithium batteries.
4.2 Effect of binder and carbon coating carrier
As an auxiliary material in lithium-ion batteries, the binder’s quantity may be small, but it significantly impacts battery performance. Its primary role is to enhance battery resistance, performance, and lifespan. By utilizing different synthesis methods and adjusting the surface of Styrene-Butadiene Rubber (SBR), the electrolyte infiltration into SBR can be improved, thereby enhancing the battery’s low-temperature and magnification performance. Conductive coating modification of the lithium-ion battery collector fluid greatly improves the bond between the collector fluid and the active substance, reducing battery impedance and significantly enhancing high-power charge and discharge performance. Additionally, the use of modified carriers in practical applications can also improve the substantial difference in thickness between the front and tail of the polar film, further enhancing the life of lithium-ion batteries.
4.3 High-voltage overcharge
The charging process of a lithium-ion battery consists of two steps. The first step involves constant current charging until the battery reaches its maximum voltage. The second step is constant voltage charging at this voltage, where the current gradually decreases. When the current reaches the set value, charging terminates. The constant voltage charging stage is lengthy with limited charging capacity. The concept of high-voltage overcharging aims to reduce constant voltage charging and increase the ratio of constant current charging. The battery’s cutoff voltage during charging is increased by approximately 0.02 to 0.03V, allowing for an increase in constant current charging time by 1 to 2 minutes. The cutoff current for charging the same capacity battery is higher, significantly improving charging speed. However, it should be noted that the higher the battery’s charging cutoff voltage, the greater the impact on battery life. Different models need verification before implementation to prevent rapid battery degradation.
4.4 Multiple strings and combinations
To enhance charging efficiency in mobile phones, manufacturers have focused not only on battery improvements but also on battery combination research. Currently, OPPO mobile phones are leading the way in fast charging technology in China. OPPO’s SuperVOOC flash charging technology has improved charging speed by increasing the charging power (Power = Voltage × Current). For lithium-ion batteries, the maximum voltage they can withstand is approximately 4.4 to 4.5V. Even with high-voltage overcharging, the increase is only 0.02 to 0.03V. In addition to conversion efficiency, the general input voltage for mobile phones is 5V. Charging at high currents generates excessive heat in the device, accelerating component aging and adversely affecting the device’s service life, and potentially posing safety risks. OPPO adopts a dual-cell connection mode and a 5A/10V charging system. A single battery can withstand the current and voltage of 5V/5A, effectively improving the charging power of the mobile phone. However, it should be noted that this method must be used with fast-charging lithium batteries to prevent a significant impact on the battery’s cycle life.
When considering materials, the negative electrode, separator, and electrolyte play crucial roles in fast charging. In the design of fast-charging lithium-ion batteries, small particles, and soft and hard carbon-coated materials are commonly used for the negative electrode to enhance its discharging speed. For the separator, choosing a porous membrane with a thinner thickness can reduce the distance for lithium-ion transmission and improve the speed of ion passage. It is advisable to select an electrolyte with high conductivity, high concentration, and low viscosity to minimize polarization and internal resistance, thereby increasing the migration speed of lithium ions.
Regarding design, the coating quantity and compaction density are key factors. Lower coating quantity and compaction density are more favorable for fast charging performance and battery lifespan.
In terms of structure, the current mainstream options such as the polar ear middle structure, multi-pole ear winding, and lamination technology significantly enhance the fast charging capability and cycle life of the battery.
In other aspects, constructing conductive networks and utilizing stronger ion-conductivity binders can optimize the battery’s charging mode. Additionally, the combination of high-voltage overcharge technology and multiple strings can improve the charging speed of the battery.
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