Comprehensive knowledge of fast charging lithium ion battery
- The principle of fast charging lithium ion battery
- Effects of fast charging lithium ion battery recession
- Multi-scale design to achieve fast charging
- Fast charging lithium ion battery strategy
- The influence of thermal management
- Battery security
In recent years, lithium-ion batteries have been widely used in electric vehicles. Compared with traditional fuel vehicles, mileage anxiety and long charging time are the main problems hindering the development of electric vehicles.
At present, the improvement of fast charging lithium ion battery is one of the key development goals of battery manufacturers and OEMs. Studies have shown that, on the one hand, low temperature and high-rate charging will cause rapid degradation of battery performance such as capacity and output power.
On the other hand, the large amount of heat generated by the battery during charging is difficult to dissipate evenly and effectively, which also causes accelerated performance degradation and safety issues.
The principle of fast charging lithium ion battery
When the temperature drops, both the charge rate and the charge cutoff voltage should be reduced to ensure safety. The risk of lithium precipitation also increases significantly as the temperature decreases. Although many studies have shown that lithium precipitation often occurs below 25°C, it is also prone to occur at high temperature, especially when the charging rate is high and the energy density is high.
In addition, fast charging lithium ion battery efficiency is also closely related to temperature. At 25°C and -25°C, the charging efficiency of 50 kW charging piles is 93% and 39%, respectively. This is mainly because the BMS will limit its rated power at low temperatures. Traditional lithium-ion batteries are mainly composed of oxide positive electrodes, graphite negative electrodes, electrolytes, separators, and metal current collectors.
The transport paths during charging are:
- Through the solid-state electrode.
- Through the positive/negative electrode/electrolyte interface.
- Through the electrolyte (including the solvation and desolvation processes of Li+).
Compared to the positive electrode, the negative electrode is the main concern during the charging process. Several studies have shown that the degradation of the cathode and the growth of the cathode CEI film have no effect on the fast charging lithium ion battery.
Factors affecting Li deposition and deposition structure include:
- Li-ion diffusion rate within the anode.
- The concentration gradient of the electrolyte at the anode interface.
- Salt deposition on the current collector.
- Side reactions at the electrode/electrolyte interface.
The reduction of the internal resistance of the negative electrode is very important to improve the fast charging lithium ion battery capability. In addition, too low or too high temperature will be detrimental to battery performance, and the increase in battery temperature during fast charging will help its own balance. Electrode thickness also has an impact on charging performance.
Thin electrodes are believed to perform better lithium ion transport, while thick electrodes may deposit lithium salts near the current collector during fast charging, resulting in uneven electrode utilization and increased local current density.
Effects of fast charging lithium ion battery recession
The heat distribution in pouch, cylindrical and prismatic case batteries is not uniform. In addition, the current density and heat generation rate are also different at different locations. For large size cells, the non-uniformity is further amplified. The positive current collector (aluminum) is more resistive than the negative current collector (copper), so the positive tab temperature will also be higher than the negative tab.
The uneven distribution of heat does not only exist in the battery cells. The design of the thermal management system has a great impact on the battery pack level. Over time, the different decay behaviors of the battery cells will also affect the pack’s heat production uniformity and cause greater impacts.
Many degradation mechanisms in Li-ion batteries are temperature dependent. The SEI film grows faster at high temperature and becomes loose and unstable. At low temperature, the diffusion rate and reaction rate of lithium ions slow down, and the possibility of lithium precipitation and lithium dendrite growth increases.
In addition, the increased polarization at low temperature leads to increased heat generation, reducing energy efficiency. In most cases, the growth of the SEI film at the anode/electrolyte interface is the main decay mechanism, and the SEI film increases the internal resistance of a battery and reduces the capacity.
Lithium precipitation refers to the process in which lithium ions are deposited as lithium metal on the surface of the negative electrode, rather than intercalated into negative electrode particles. Lithium precipitation may occur when the negative electrode potential falls below Li/Li+.
As more lithium is deposited under the SEI film, the SEI film is broken, and a new SEI film is formed on the lithium surface, and the concentration of lithium salt gradually decreases. Li metal begins to grow perpendicular to the surface of the pole piece, forming Li dendrites. If the dendrites pierce the separator and cause an internal short circuit, the battery will generate heat faster.
Non-desorption lithium characterization techniques are important for battery applications. The detections that can generally be used for the characterization of lithium evolution include SEM, TEM, NMR, and XRD, etc., but these methods all require the destruction of the battery or the use of special battery configurations.
Commonly used non-degradable lithium characterization methods include but are not limited to: decay rate, voltage platform for lithium re-insertion, model prediction and other methods.
Mechanical pulverization is another fast charging induced decay phenomenon that has been demonstrated in various electrode materials (graphite, NMC, LCO, NCA, Si, etc.).
Mechanical decay can be divided into the following parts:
- Electrode particle rupture.
- Separation of electrode particles from conductive carbon and binder.
- Separation of active material from current collector.
- Electrode delamination. These phenomena are mainly caused by the stress mismatch between components caused by the gradient distribution of lithium concentration during fast charging.
The effects of mechanical decay on battery performance can be divided into active material loss, active lithium loss, and impedance increase. First, the crack will lead to poor contact. Second, the crack will expose more fresh surface to react with the electrolyte, these reactions accelerate the growth of SEI, aggravate the increase of impedance and loss of active lithium, etc. Third, the consumption of electrolyte reduces the wettability of the electrode surface and hinders ion transport.
Multi-scale design to achieve fast charging
Select the appropriate electrolyte and electrode material so that it can exert high specific capacity and high rate performance. The potential of the traditional graphite anode is very close to the redox potential of lithium, which makes the battery show higher energy density, but at the same time, the possibility of lithium precipitation increases. Therefore, improving anode materials has become one of the important ways to improve the performance of lithium-ion batteries.
Designing suitable electrode structures at the nanoscale can also achieve high power and energy densities. Such as: 2D hollow structure, core-shell structure and yolk-core structure, etc. In addition to the selection, modification, and nanostructure design of anode materials, the electrode/electrolyte interface also greatly affects the performance of anode materials.
The growth of lithium dendrites can also be suppressed by optimizing the anode/electrolyte interface, such as amorphous carbon-coated graphite to form a uniform SEI film. The geometric parameters of electrode design also have an important impact on battery performance. Increasing the porosity and anode thickness can suppress lithium precipitation, but also reduce the energy density.
The capacity ratio of anode to cathode (N/P ratio) can significantly affect Li deposition. Commercial Li-ion batteries generally have an N/P greater than 1, and a larger N/P ratio helps reduce the mechanical stress, SEI formation, and loss of active lithium in the negative electrode.
The geometric parameters of the battery are also an important factor affecting fast charging lithium ion battery. Cell shape affects current density and temperature distribution, as do tab location, material, structure, and welding process, as well as uniform current density distribution, localized heat generation, and decay rates.
Pack design needs to consider more parameters. There are still many problems in the pack design of fast-charging batteries:
- The need for high consistency of battery cells.
- The need for a more advanced BMS.
- The need for a more advanced thermal management system.
Fast charging lithium ion battery strategy
Types of charging strategies:
- Standard charging: first charge with constant current to the cut-off voltage (CC stage), and then charge with constant voltage until the current is close to 0 (CV stage).
- Multi-stage constant current charging: It consists of two or more steps of constant current stage, followed by a constant voltage stage.
- Pulse charging: During the pulse charging process, the current exhibits periodic changes to reduce concentration polarization, reduce the risk of local potential becoming negative, and reduce the mechanical stress caused by uneven insertion and extraction of lithium.
- Boost charging: a larger average current is used in the initial stage, followed by CC-CV charging with an appropriate current.
- Variable current charging: As the battery declines, the current needs to be adjusted according to the change in internal resistance at the same voltage.
The influence of thermal management
Air cooling is low cost and relatively simple, but has poor thermal conductivity, poor cooling rate and temperature uniformity, and is not suitable for fast charging lithium ion battery systems. Liquid cooling is 3,500 times more efficient than air, but is costly, complex systems and there is a risk of leaks.
Efficient and uniform cooling is especially critical compared to standard charging conditions. The thermal conductivity of the interior of the battery is poorer relative to the surface, and the cooling system is connected to the surface of the battery. And it also affects the fast charging lithium ion battery.
These factors exacerbate the inhomogeneity of the temperature distribution inside and outside the battery. The charging pile can provide an external cooling system, this method helps to reduce the cost and weight of the on-board cooling system. The four common methods of preheating in low temperature environment are:
- Using resistance heating and cooperating with fan heat convection, but the efficiency is not high and the heating is uneven.
- Mutual pulse heating, the battery components, etc. Two groups of capacity, the charge is exchanged in the form of pulses between the two groups, and the resistance is used to generate heat, which is more efficient.
- AC heating, this method heats up faster.
- Design the battery configuration to achieve rapid preheating, such as in A thin nickel foil is inserted between the two single-sided negative electrodes of the battery.
The effect of fast charging lithium ion battery on thermal runaway
Studies have shown that the thermal runaway behavior of batteries will change after fast charging lithium ion battery, and thermal runaway is caused by a series of chain reactions.
The thermal runaway of the fast charging lithium ion battery can be divided into three stages. The first stage (60 °C < T < 110 °C)), the precipitated lithium reacts with the electrolyte to heat the battery, and the SEI film is continuously ruptured and regenerated, and the temperature is relatively low at this time.
In the second stage (thermal runaway triggering process), lithium is consumed in large quantities in the reaction with the electrolyte, causing the temperature to rise sharply, the diaphragm shrinks, and the positive and negative electrodes come into contact. In the third stage (thermal runaway to the highest temperature), the positive/negative electrode reacts with the electrolyte and the positive/negative electrode.
Thermal runaway caused by overcharge
The process can be divided into 4 stages:
Stage 1 (100% < SOC < 120%): The voltage exceeds the charge cut-off voltage and increases slowly, possibly causing some side reactions in the battery material.
Stage 2 (120% < SOC < 140%): Excessive extraction of lithium causes transition metal ions of the positive electrode, such as Mn2+, to begin to dissolve. The cell potential exceeds the electrochemical window of the electrolyte, and oxidation begins. Lithium precipitation occurs in the negative electrode, lithium reacts with the electrolyte to form a new SEI film, and the internal resistance of the battery increases.
Stage 3 (140% < SOC < 160%): The oxidative decomposition of the electrolyte generates more heat, and the battery pack expands with gas production. When the SOC is close to 160%, a large amount of Mn2+ is dissolved in the positive electrode, the structure of the positive electrode changes, and the battery voltage decreases after reaching the maximum value.
Stage 4 (140% < SOC < 160%): The oxidative decomposition of the electrolyte produces a large amount of gas, which leads to rupture of the battery pack, displacement of the diaphragm, etc., and a large-area short circuit occurs inside the battery, resulting in thermal runaway.
Two design approaches to help prevent battery overcharging:
- Increase the oxidation potential of the electrolyte from 4.4 V to 4.7 V, which will make the electrolyte more stable. This can be achieved by adding functional additives or redox shuttle additives.
- Delay battery rupture by optimizing the pressure relief design of the battery or using a separator with higher heat exchange stability.
So far, there is still no reliable method to detect battery degradation (such as lithium deposition and mechanical cracking). Many new electrode materials have good fast charging lithium ion battery capability, but they still need to be considered in terms of stability, decay mechanism, large-scale production and cost.
Existing modeling methods still have great limitations. Many studies on fast charging lithium ion battery protocols are based on empirical or experimental nature only. There are few special cases of low temperature fast charging lithium ion battery. There is a need to develop more advanced BMS with balanced cell.