Research background:
The electric vehicle (EV) market requires batteries with high energy density and low cost. Among existing rechargeable battery technologies, lithium-ion batteries have the highest energy density, such as golf car lithium batteries and RV lithium batteries. In terms of cost, the price of commercial lithium-ion batteries is relatively low. However, to achieve large-scale marketization, the key is for the BEV to achieve a range of over 500 kilometers, and for the cost of the car to be less than $40000.
Unfortunately, lithium-ion batteries based on graphite anodes and transition metal lithium oxide anodes are almost impossible to achieve such high energy density. In order to improve energy density, it is necessary to develop new anode and cathode electrode materials. Due to its ultra-high capacity and lowest electrochemical potential, lithium metal batteries are considered the most promising anode material to replace graphite.
Lithium metal batteries such as Li-LMO and Li-S have overwhelming advantages over LIB in terms of energy density and cost, which brings huge opportunities for the future long-distance and low-cost electric vehicle industry.
Achievement introduction:
In terms of energy density and cost, lithium-metal batteries have significant advantages over existing lithium-ion batteries, providing huge opportunities for long-distance and low-cost electric vehicles. In this article, researchers discussed the requirements and challenges of high energy density lithium metal batteries in future applications of electric vehicle battery packs and systems and emphasized the latest developments in these fields.
In order for electric vehicles to successfully replace traditional internal combustion engine vehicles, driving distance and price are the most important considerations, while another important factor is battery life. As the energy density of lithium metal batteries increases, more energy can be stored in battery packs of the same size or weight. Due to the longer driving distance per charge, the cycle life requirements of lithium metal batteries may be reduced. For the application of electric vehicles, lithium metal batteries require a minimum cycle life of 500 to 1000 cycles.
The long cycle life of lithium metal batteries is the formation of a stable solid electrolyte intermediate phase (SEI) layer on the graphite anode, which can prevent further decomposition and peel of the electrolyte during the cycling process. However, unlike graphite, lithium metal anodes exhibit significant volume and morphological changes during repeated electroplating or stripping processes.
Operating temperature range:
The temperature has a significant impact on lithium metal batteries, and the electrochemical reaction kinetics and ion conductivity will vary with temperature. Under extreme conditions, it can cause safety issues such as fire, gas release, and explosion.
People have found that the performance of batteries rapidly decays below 0 ℃. This attenuation is believed to be due to lower ion conductivity, higher charge transfer resistance, and lower lithium ion diffusion at low temperatures. At low temperatures, lithium plating occurs on the anode electrode.
When working at high temperatures, lithium-ion batteries rapidly age, reflecting a rapid decline in battery capacity and performance. The side reactions between the cathode and electrolyte, as well as the changes in nonactive electrode components, are key to aging degradation at high temperatures.
Quick discharge and charging:
The high rate capability of batteries is crucial for the performance of electric vehicles. For any electric vehicle, a fast pulse discharge and continuous charging energy storage system for regenerative braking or "refueling" is required. One challenge of the high-speed operation of lithium metal batteries is the generation of heat, which increases the battery temperature and accelerates battery aging. On the other hand, the form and recyclability of lithium anodes depend on the current density of stripping and electroplating.
Changes in battery volume. Firstly, the matrix-free deposition and dissolution of lithium lead to periodic expansion and contraction of the lithium anode and the entire battery during charging and discharging. The change in lithium anode will cause about a 15% change in the volume of the entire battery. As the thickness of the cathode increases and there is excess lithium, this relative volume change will decrease. In order to apply lithium metal battery technology to electric vehicles, it is necessary to design the battery pack appropriately to adapt to this periodic change in battery volume.
Expansion of soft pack batteries in different electrolytes. Another issue is that the battery undergoes irreversible expansion after prolonged cycling. The fundamental reason is the growth of loose powdery lithium structures formed by the accumulation of SEI and "dead" lithium particles insulated by SEI, which is the result of irreversible continuous reactions between lithium and electrolyte.
Safety issues:
The fire safety of lithium metal batteries is a major consideration for electric vehicles. Lithium metal batteries with high energy density and high flammability are sensitive to various conditions, including vibration, collision, overcharging or discharging, external short circuits, high temperatures, and so on.
Improper use of conditions will destroy the stability of the structure, cause internal short circuits, trigger chain reactions, lead to Thermal runaway, and cause serious safety problems, such as smoke, gas injection, combustion, and even explosion. Spontaneous failures and internal short circuits of batteries can also lead to safety hazards. In addition, the loose SEI formed after repeated cycles and the crushed "dead" lithium can also cause safety issues.
High-safety performance lithium metal battery solutions for electric vehicles:
The challenges of lithium metal batteries mainly come from the structural changes and unstable SEI of lithium during the cycling process. The strategy of the solution mainly focuses on three directions, including electrolyte engineering, lithium interface engineering, and lithium Structural engineering.
Electrolyte engineering. Different electrolytes have different effects on the stability of lithium, greatly affecting the performance of SEI and the deposition morphology of lithium, resulting in significant differences in cyclic Coulomb efficiency and lithium metal anode expansion. In addition, the electrolyte determines the operating temperature range, high-rate performance, and battery safety of LMB.
Some lithium halide, Cs, and Rb salt additives can achieve dendrite-free lithium deposition under different working mechanisms, improving safety. In addition to conventional solvents, a liquefied electrolyte for metal lithium anodes has also been developed.
The manufacturing cost of all Solid-state batteries (ASSBs) may be higher than that of LMBs using liquid electrolytes. Therefore, the development of ASSB based on lithium metal is still in its early stages. If you want to know what a Solid-state battery company has, you can click the link to learn.
Interface engineering. Ideally, the SEI on lithium should be thin, uniform, dense, highly elastic, and highly ionic, but with poor conductivity. It can withstand significant volume changes and allow Li+ to pass quickly, while preventing electrolyte decomposition, resulting in efficient dendrite-free cycling of the lithium anode. SEI naturally formed by electrolyte decomposition is difficult to meet all these requirements. Therefore, applying a protective layer or artificial SEI on the lithium anode is an effective method.
Lithium Structural Engineering. In order to minimize the volume change of lithium metal negative electrodes, some three-dimensional collectors or stable main structures of pre-stored lithium have been introduced. Increase the active lithium surface of the collector and main structure, reduce local current density, thereby improving amplification performance, and suppressing the generation of lithium dendrites through ion flux homogenization.
Battery assembly strategy:
For the application of electric vehicles, lithium metal batteries are always assembled in the form of modules, including hundreds or thousands of large batteries. Therefore, a Battery management system is needed, which is crucial to the safe operation of the battery. For lithium metal batteries, researchers believe that the Battery management system should include at least three functions, including advanced pre-detection, Stress management, and thermal management.
Advanced pre-detection:
The battery health monitoring function currently used in electric vehicles is far from being able to detect impending battery failures. However, for battery packs made of metal lithium-based batteries, it is necessary to use non-destructive online monitoring technology to detect batteries that are about to fail. Impedance measurement has been proposed as an effective method, but more systematic research is still needed.
Stress management:
It has been found that the optimal pressure also depends on the electrolyte. Therefore, designing a battery pack structure with a Stress management system to monitor or control the pressure within the optimal range can not only improve the battery performance but also help the safe operation of the battery pack.
Thermal management:
Thermal runaway of a single battery and battery pack shall be managed to prevent potential safety hazards of the battery or battery pack during storage or operation. The main contents of research and development include temperature distribution, sensors, heat conduction, and Battery management system control.
Summary and Outlook:
High-energy lithium metal batteries are expected to be used in long-distance and low-cost electric vehicles, but they face serious challenges in achieving long lifespan, low volume change, high safety, and reliable operation at extreme temperatures.
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