The-key-to-develop-fast-charging-batteries-anode, cathode-and-electrolyte

The key to develop fast charging batteries – anode, cathode and electrolyte

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    Fast charging means to charge the battery with a fast charging speed in a short period of time, and charge the battery to full power or close to full power. However, it is necessary to ensure that the lithium ion battery can reach the specified cycle life, safety and performance during fast charging. Therefore, the development of fast charging is limited by materials and technologies, among which the development of anode, cathode and electrolyte is the key to the development of fast charging batteries.

    Mileage anxiety of electric vehicle

    With the widespread use of electrochemical energy storage in portable electronics and electric vehicles (EVs), the demand for and reliance on lithium-ion batteries has become higher than ever. After decades of development, compared to traditional internal combustion engine vehicles, the main challenge to the widespread use of electric vehicles is “mileage anxiety”.

    Tesla, the global leader in the electric vehicle industry, is using a third-generation charging station (250 kW) that can charge 250 kilometers in 15 minutes, but it still cannot meet the demand for fast charging. In order to provide refueling time comparable to that of a internal combustion engine vehicle, it is usually necessary to travel 400 kilometers in 15 minutes. However, ultra-fast charging brings new challenges for battery materials that need to be further improved.

    Mileage-anxiety-of-electric-vehicles

    What is fast charging

    The fast charging function refers to the recharge mode of high-power DC charging for electric vehicles, which requires off-board DC charging piles to be used for charging. The electric vehicle equipped with fast charging function can meet the demand of high power and fast charging.

    Fast charging batteries are usually achieved by improving the rate capability of conventional rechargeable batteries at high current densities by first extracting lithium ions from the cathode materials during charging and then diffusing them into the liquid electrolyte through the cathode/electrolyte interface.

    The extracted lithium ions are then solvated by solvent molecules. The solvated lithium ions migrate through the separator to the anode side and are then desolvated at the anode/electrolyte interface. Finally, the desolvated lithium ions are inserted into the interior of the anode material. At the same time, the electrons produced by the cathode are transferred to the current collector and then moved to the anode through an external circuit.

    What factors are fast charging batteries limited by

    Fast charging active material

    Currently, the battery materials used for electric vehicles are mainly anode materials such as graphite, lithium or silicon-based materials, and as for cathode, there are usually lithium iron phosphate (LiFePO4) or ternary layered cathode materials, and non-aqueous electrolytes.

    Electrode polarization is the main cause of battery failure and influence on fast charging, and it’s affected by the diffusion rate of lithium ions in the active material, the transport of lithium ions in the electrolyte and the charge transfer dynamics at the electrode/electrolyte interface.

    In terms of electrode materials, lithium evolution in the anode and lithium ion diffusion in the cathode are the main rate-limiting steps. In general, there are some traditional strategies including bringing in the electrolyte with high ionic conductivity and weak solvation, and constructing a stable solid electrolyte interface (SEI)/cathode electrolyte interface (CEI).

    In order to achieve fast charging, it is necessary to have a low energy barrier to allow lithium ions to migrate into the active material and diffuse within it. The impedance of the battery can be used to determine the energy barrier.

    When the ion of poorer active materials and electron transport power react, it can generate high overpotentials, which can lead to side physical and chemical reactions and cause battery failure. Therefore, fast charging active materials require three basic characteristics: high lithium ion diffusion coefficient, excellent charge transfer kinetics performance, and controllable lithium ion transport.

    Overpotential

    Another factor affecting fast charging is overpotential. If the overpotential exceeds a certain critical value, a performance degradation may occur on both cathode and anode, resulting in shortened battery life. In general, ionic conductivity of materials is significantly lower than electronic conductivity.

    Therefore, overpotential is mainly caused by ion transport. At high current density, if the transfer rate of lithium ions is lower than the electron transfer rate, lithium ions will be deposited at the electrode interface, resulting in loss of battery capacity and even security risks.

    AC-impedance-diagram
    AC impedance diagram

    Anode material of fast charging battery

    An important limiting factor of fast charging batteries is the inability of ions/electrons to transfer quickly to the anode material. According to the energy storage mechanism, anode materials can be divided into intercalation type, conversion type, and alloy type. In addition, some anode materials have more than one charge storage mechanism, such as porous carbon, transition metal chalcogenides, and nanomaterials.

    During the lithium storage process, the lithium ion transfer and bulk diffusion in the anode material are driven by local electric fields and concentration gradients, respectively, which are key factors determining the fast charging ability.

    Graphite anode

    Graphite is an ordered graphene layer stacked in ABABA with appropriate interlayer spacing (0.335 nm), which allows for reversible lithium ion intercalation/deintercalation. However, graphite has slow lithium ion intercalation kinetics and low lithiation voltage (~0.1 V), which seriously hinders its practical application.

    Researchers have developed several strategies to improve the electrochemical performance and rate capacity of graphite electrodes, such as shortening the diffusion path, expanding the interlayer spacing of graphite, and modifying the interface. Lithium ions must be embedded from the edge sites and gradually diffuse into the interior of the particles, resulting in a longer diffusion path, lower diffusion rate of lithium ions, and poor rate capacity.

    By optimizing the morphology and structure of graphite, its fast charging ability can be effectively improved. Forming pores in graphite is an effective method to shorten the diffusion path of lithium ions and improve fast charging performance. This allows lithium ions to not only enter from the edge surface of the graphite, but also embed from the base surface, shortening the migration path and exhibiting better rate capacity than the original graphite.

    Modification-strategy-for-graphite-anode
    Modification strategy for graphite anode

    Lithium titanate anode

    As an anode material, Li4Ti5O12 (LTO) has good cycle stability, high rate capacity, safety and low temperature performance. Meanwhile, the lattice shrinkage of the strong Ti-O bond during the two-phase transition is only 0.77%, which stabilizes its structure and is also called “zero-strain” material.

    In contrast, the LTO anode does not have such shortcomings as the formation of lithium dendrites and unstable solid electrolyte interface (SEI) during charging, but the inherent low conductivity and slow Li+ diffusion kinetics limit the further improvement of fast charging performance. In addition, gas production remains a challenge for large-scale commercial applications.

    The modification strategy is:
    ①surface modification. Surface modification on LTO is a widely accepted method to increase magnification capacity.
    ②Element doping is another important strategy to improve the intrinsic electron/ion conductivity of Li+.
    ③Other strategies, such as the preparation of nanosized LTO, combined with pore structure or control morphology to improve the multiplier performance. Various nanoscale morphologies of LTO, such as nanotubes, nanowires and nanosheets, exhibit excellent magnification properties when used as anode materials. In addition, due to their high cost, LTO may be more suitable for practical applications in some special fields.

    Modification-strategy-for-lithium-titanate-anode
    Modification strategy for lithium titanate anode

    Silicon anode

    Silicon is a potential anode material for next-generation lithium-ion batteries due to its abundant resources, high specific capacity (4200 mAh/g) and relatively low discharge potential platform (0.4V). With the industrialization of silicon – based anode materials, the market demand increases gradually. During the lithiation process, lithium ions are inserted into Si particles and form a series of Si-Li phases, eventually resulting in the incorporation of 4.4 lithium ions into each Si atom but a volume expansion of about 420%.

    Large volume changes will produce large internal stresses, which will eventually lead to the fracture and comminution of Si particles, which is a great challenge in the design and manufacture of silicon anode. In addition, the inherent low conductivity of silicon (1.56×10-3 S/cm) also limits the improvement of its multiplier performance.

    The modification of silicon-based materials includes reduction of particle size, design of new microstructure and surface coating. At present, the market products mainly focus on silicon-based anode materials with specific capacity around 450 mAh/g. Tesla uses Si/C anode materials in Model 3, which promotes the rapid expansion of silicon-based anode market.

    There are two commercial routes for silicon-based materials: Si/C composites and SiOx anode materials. The capacity of the latest generation of Si/C anode materials is reported to be up to 1500 mAh/g, while SiOx greater than 1600 mAh/g. High capacity Si/C materials are more likely to be used primarily for cylindrical batteries in the future, as commercial and square aluminum-shell batteries are still very sensitive to swelling.

    Modification-strategy-for-silicon-anode
    Modification strategy for silicon anode

    Cathode material of fast charging battery

    From the perspective of the cathode, the internal particle stress caused by lithium ion diffusion at large current is generally amplified, which will increase heterogeneity and generate more stress throughout the battery cycle, leading to the destruction of material structure and deterioration of capacity.

    In order to improve the fast charging performance of cathode materials, the current strategies usually focus on the construction of high conductivity path and the short lithium ion diffusion path. Currently, LiFePO4, LiCoO2, and LiNixMnyCozO2 are the main commercial cathode materials.

    LiFePO4

    LiFePO4 has been considered one of the most promising cathode materials for electric vehicles since its discovery in 1997 due to its advantages of low cost, moderate voltage platform and high safety. It is worth noting that the cell volume is about 6.8% during charging and discharging.  The small expansion not only avoids the capacity decrease caused by the drastic volume change during the cycle, but also effectively compensates for the anode volume change during the lithium process.

    However, the low intrinsic electron conductivity is also a major drawback limiting its electrochemical performance and commercial application. Therefore, various methods have been proposed to overcome the shortcomings of LiFePO4:
    ①Coating modification is one of the main strategies to improve its electrochemical performance (such as capacity, cycle life and rate capacity)
    ②Doping is another important method to improve the intrinsic electron/ionic conductivity of LiFePO4. Replacing a small amount of Li+, Fe2+ or O22- with heterogeneous ions is expected to improve the capacity, cycle life and multiplier performance of LiFePO4 batteries to some extent.
    ③The lithium ion diffusion distance of LiFePO4 can be controlled by modifying the morphology, mainly by reducing the particle size and adjusting the directional growth of the crystal surface. Due to the shortened diffusion length, the magnification performance of nano-scale LiFePO4 is significantly improved, and the particles show better electrochemical performance than the particulates and large particles.

    Modification-strategy-for-LiFePO4-cathode
    Modification strategy for LiFePO4 cathode

    LiCoO2 cathode

    The theoretical capacity of layered rock salt structure LiCoO2 is 274 mAh/g, but the depth of LiCoO2 delithium to higher voltage is easy to induce lattice oxygen overflow, resulting in serious structural deterioration and rapid attenuation of capacity and cyclicality. The main disadvantages of LiCoO2 are poor thermal stability and low capacity.

    As shown in the figure, LiCoO2 undergoes gradual phase transitions from H1 to H2, M1, H3, M2, and O1, resulting in large anisotropic expansion and contraction along the C-axis and A-axis, respectively. As a result, LiCoO2 is structurally damaged due to its irreversible phase transition caused by high cut-off voltage, resulting in rapid capacity decay. In order to improve the cycling capacity and stability of LiCoO2, various methods such as element doping and surface modification have been widely used to improve the electrochemical performance of LiCoO2.

    Modification-strategy-for-LiCoO2-cathode
    Modification strategy for LiCoO2 cathode

    Multilayer cathode

    Multilayer cathode has attracted wide attention due to its good comprehensive performance, low cost and high energy density, which can overcome the disadvantages of monolayer materials. Compared to LFP, the multilayer positive electrode is more suitable for high rate cells due to its better conductivity, especially at low temperatures.

    Multilayered materials usually contain nickel, cobalt, manganese, or aluminum and have a hexagonal alpha-Nafeo2 (R3-m) structure and repeated O3 structure. Typically, Ni REDOX is used to achieve high capacity of the material, the presence of Co inhibits cationic mixing, while Mn or Al helps stabilize the structure, prompting the research community and industry to increase the proportion of Ni in pursuit of higher energy density.

    Unfortunately, increased Ni content leads to a number of problems such as reduced structural stability, microcracks, increased side reactions and gas production, resulting in reduced battery life and safety. Therefore, it is necessary to resolve the structural and chemical instability associated with increasing Ni content in order to construct highly stable Ni-rich cathode materials to improve their thermal stability and increase their practical capacity.

    Modification-strategy-for-multilayer-cathode
    Modification strategy for multilayer cathode

    Electrolyte of fast charging battery

    Cathode and anode materials of high performance and non-aqueous electrolyte are the internal factors to achieve high performance batteries. The electrolyte, known as the “blood” of the battery, acts as a bridge between the cathode and anode, performing the function of ion conduction within the battery. It can not only adjust the electrode/electrolyte interface, but also affect the performance of the battery, including capacity, internal resistance, rate charge and discharge performance, operating temperature and safety performance.

    In general, the diffusion coefficient of lithium ions in liquid electrolytes is higher than that in solid electrodes, so the desolvation of the solvated lithium ions at the electrode/electrolyte interface will be a more important factor in determining the fast charge ability of the battery.

    In most cases, increasing the ionic conductivity of the electrolyte is conducive to reducing the solvation and desolvation activation energy of lithium ions, which is conducive to achieving fast charging. The instability of the electrode/electrolyte interface is another major cause of electrolyte depletion, loss of recyclable lithium ions, and limited charge transfer between the electrode and electrolyte interface during fast charging.

    Modification-strategy-for-electrolyte
    Modification strategy for electrolyte

    A new technology more convenient than fast charging – swapping station

    At present, when fast charging technology and materials are still to be developed and improved, a new technology that is more convenient than fast charging has emerged, that is, swapping station. A swapping station is an energy station that meets the endurance by directly changing the battery instead of charging it, and realizes the separation of the car and the battery for replenishing energy.

    In the past, due to the small number of electric vehicles and the weak willingness of automobile enterprises to promote them, ev battery swapping mode did not achieve great development. However, after long-term research and accumulation, the reserve of electric swapping technology has been mature.

    Fast charging is restricted by factors such as limited site, inadequate marketization, imperfect construction and operation of supporting facilities, etc. In contrast, the battery swapping mode is favored by car users because it can realize the separation of the vehicle and battery.

    In the initial purchase cost of electric vehicles, the power battery accounts for about 40%, and the existence of the swapping station can realize the separation of vehicles and battery, greatly reducing the purchase cost of car owners.

    At present, charging piles are the main way of energy replenishing for electric vehicles. It takes 30 to 40 minutes to overcharge energy consumption through charging piles, which cannot meet the needs of operating vehicles with high efficiency requirements. And the power swapping mode only takes 3 to 5 minutes or less, which has much higher efficiency.

    In addition, the excessive power grid pressure during the charging peak is one of the factors restricting the large-scale overcharging of the fast charging station, and the power swapping mode can adjust the centralized slow charging time of the battery according to the demand, which can effectively reduce the power grid pressure, and is more popular among regional power grids.

    Moreover, centralized monitoring, maintenance and management of the battery in the power swapping station can effectively extend the service life of the power battery and improve the safety of the battery. The differences among fast charging mode, slow charging mode and battery swapping mode are as below.

    Data

    Fast chargingSlow charging

    Battery swapping

    Energy replenishment  time

    0.5-1hr6-10 hr

    Within 5min

    Energy replenishment location

    Public charging pilesPrivate residential

    Public swapping stations

    Energy replenishment method

    Individual /charging stationIndividual /charging station

    Battery replacement

    Standardization

    HighHigh

    Temporarily low

    Battery maintenance

    Strong current charging, significantly reducing battery lifeNormal current charging, little impact on battery life

    Professional maintenance, timely replace problematic batteries, longer battery life, higher safety

    Power grid impact

    Strong charging suddenness, causing fluctuations in the power grid,

    Increase grid load

    Commonly charge during low periods,

    effectively helps the current system in valley filling

    Reasonable planning of power resource supply, unified charging during low peak hours at night, can balance the power grid load, reduce the cost

    Floor area

    Public charging piles: 0.6-0.8㎡ per vehicle,

    Private charging piles: 10-12㎡ per vehicle

    0.2-0.4 ㎡ per vehicle

    Summary

    The development of fast charging materials is the key to realize fast charging of lithium ion batteries. This article reviews the current status of electrode and electrolyte materials used for fast charging, summarizes the current status of anode and cathode materials for fast charging lithium ion batteries, and the strategies to promote lithium ion diffusion kinetics or material structural stability, such as structural design, morphology modulation, surface/interface modification and so on.

    In addition to electrode and electrolyte materials design, battery engineering is also important for improving fast charge capability, cycle life, and safety. The fast charging capability can also be optimized by adjusting parameters such as electrode composition, thickness and porosity, as well as positive and negative electrode capacity ratio (N/P ratio). Electrode structure is an important factor affecting the high rate performance of the battery. It directly or indirectly affects the electrode resistance and the depth of charge and discharge.

    Although fast charging electrode materials have been well developed in both academia and industry, there are still many challenges that need to be further overcome. With further research and development of the materials, and with the appearance of swapping stations, it is believed that rechargeable batteries will make some new breakthroughs in advanced fast charging technology to better solve the “mileage anxiety” of electric vehicles.

    References

    Jianhui He, Jingke Meng, Yunhui Huang*, Challenges and recent progress in fast-charging lithium-ion battery materials, Journal of Power Sources, 2023.
    https://doi.org/10.1016/j.jpowsour.2023.232965

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