In the article "Why Lithium-Sulfur/Lithium-Air Batteries Lack Application Prospects in Power Batteries", Professor Ai Xinping of Wuhan University has acknowledged the feasibility of the near-term and medium-term goals and discussed in detail the reasons why lithium-sulfur/lithium-air batteries lack application prospects in power batteries; in the article "Where Exactly is the Way Out for Innovation of Core Lithium Battery Materials?", he also discussed solutions for next-generation lithium battery materials.
However, in reality, besides innovations in certain core materials, lithium battery development still faces many technical challenges. These include battery safety issues and the design technology of high-capacity electrodes.
1. Battery safety issues.
The three companies that undertook the development of the 300Wh/kg battery project in 2016 still could not meet the assessment requirements for battery safety. Whether the 300Wh/kg battery can be truly installed in vehicles in 2020 is not a performance issue, but a safety issue.
Among these factors, the heat release from the decomposition of the cathode is a significant contributing factor to battery thermal runaway. Taking ternary lithium batteries as an example, whether high-nickel or standard ternary, their thermal stability is much worse than that of lithium iron phosphate batteries. They not only release more heat but also have lower decomposition temperatures, which will exacerbate future battery safety issues. Therefore, addressing safety issues requires a comprehensive approach, addressing materials, individual cells, and the entire system.
Ai Xinping believes that from the perspective of the overall solution to safety, materials are the foundation, as the type of material determines the level of safety; monomers are the key, as their quality is determined by the monomers; and the system is the guarantee, ensuring that thermal runaway of lithium-ion monomers will not trigger other systems.
This discussion focuses on a few solutions at the individual unit level.
The first approach is to develop battery self-heating protection technology.
Lithium-ion batteries are not temperature-sensitive; however, high temperatures can trigger thermal runaway. If a temperature-sensitive material were present in the battery that could effectively block the transport of electrons and ions at high temperatures, the battery would automatically shut down its reaction under abusive conditions, preventing further temperature increases.
The simplest way is to use PTC materials in batteries to achieve temperature sensitivity. In fact, PTC materials are used in many fields, just not in batteries. The main characteristic of PTC materials is that they have excellent conductivity at room temperature; when a certain transition temperature is reached, the resistance rises sharply, changing from a conductor to an insulator, thus cutting off electron transport at the electrodes.
The study also found that some conductive polymers exhibit a PTC effect and are soluble. This material can be used to prepare very thin coatings. For example, the polymer P3OT has relatively high conductivity between 30 and 80 degrees Celsius, but changes by three orders of magnitude between 90 and 110 degrees Celsius. This coating is less than one micrometer (approximately 600 nanometers) thick, thus not affecting the battery's energy density. This material exhibits thermal shutdown properties at 120 degrees Celsius, significantly improving battery safety under conditions such as overcharging, hot boxes, and nail penetration.
In addition, thermally shut-off separators are also a feasible solution. Existing three-layer separators all have thermally shut-off functionality. For conventional separators, the closure temperature is determined by the melting point of PE, approximately 135 degrees Celsius; the melting temperature is determined by the melting point of PP, approximately 165 degrees Celsius. Because the closure temperature is too high, after thermal shutdown, thermal inertia can easily cause the battery temperature to continue rising to 165 degrees Celsius, leading to separator melting and battery short circuits. Therefore, the thermal protection effect of conventional separators is limited.
If a layer of plastic micropores is coated on the surface of the separator, the surface microsphere layer will melt when the temperature reaches the melting point of the microspheres. After the microspheres melt, they will block the pores of the separator. As a result, whichever electrode the microspheres face, the pores on that electrode surface will be blocked. The effect is very obvious. Because ion transport is cut off, the battery reaction stops, and the battery becomes safe.
The second approach to solving the safety issue is to develop all-solid-state batteries.
In fact, from the perspective of improving volumetric energy density, all-solid-state batteries are very promising. As battery density increases, volumetric energy density becomes increasingly important for passenger vehicles. Information from the 57th Japan Battery Conference indicates that research institutions in South Korea and Japan are conducting research on solid-state batteries, and major domestic battery companies such as ATL are also carrying out research in this area.
Compared to liquid electrolytes, solid-state batteries offer the main advantage of higher safety. Another feature is the ability to achieve internal series connection, which is beneficial for improving the energy density of modules and systems. However, they suffer from high interfacial stress and poor stability. The solid electrolyte must be in full contact with the active material particles; otherwise, lithium-ion transport cannot be achieved. However, any electrode material, whether graphite or ternary, undergoes volume changes during charging and discharging. Once these volume changes cause solid/solid separation, lithium-ion conduction is hindered, and battery performance rapidly declines.
Therefore, one of the key focuses of solid-state battery R&D is the selection of solid electrolytes. Currently, sulfides are considered suitable because they are relatively soft. Secondly, there's the construction and stabilization technology of the solid/solid interface. This involves some tricks; it's impossible to achieve the desired result using a pure solid electrolyte. The best approach is a hybrid of inorganic and polymeric materials. Thirdly, there's the development of production processes and specialized equipment. The production process for solid-state batteries will definitely be different from our current industry practices.
2. Design technology for high-capacity electrodes.
With increased energy density, electrode design becomes more critical. The proportion of active material in a battery is a significant factor affecting its specific energy. With the same positive and negative electrode materials and the same specific capacity, a battery with a lower proportion of active material will have a lower energy density. Therefore, to increase energy density, it's essential to maximize the amount of active material within the same weight of battery. More active material means less auxiliary material; copper foil and aluminum foil should be reduced. Most importantly, the electrodes should be made thicker, which reduces the amount of current collector and separator required.
However, lithium-ion electrodes cannot be made too thick. Thicker electrodes result in increased surface polarization, reducing electrode utilization in the thickness direction and causing problems such as lithium plating at the negative electrode and decomposition at the positive electrode during charging. From the perspective of improving energy density, thicker electrodes are preferable; however, polarization theory tells us that thinner electrodes are better—these two are completely contradictory. As energy density increases, for example, from 100 Wh/kg to 300 Wh/kg, it means that the current carried per unit weight of material increases accordingly. Therefore, maintaining power performance in future high-energy-density batteries will be very difficult, making high-capacity electrode design technology increasingly important.
There is a way to resolve this contradiction. The liquid phase current is larger closer to the separator; this current is the external current. Along the electrode thickness direction, the liquid phase current gradually decreases, while the solid phase current gradually increases. Therefore, the electrode porosity should be higher closer to the separator, and lower closer to the electrode's polar fluid. Thus, to ensure both high energy density and power performance, an electrode with a gradient porosity distribution must be designed. How to achieve this is a matter for others to figure out. With the application of new materials and the improvement of battery energy density, the design of gradient porosity electrodes is becoming increasingly important. As for the appropriate gradient, it cannot be determined through experimentation alone, as that is very difficult; a polarization model must be established.
Finally, here is a summary by Professor Ai Xinping of Wuhan University:
1) Lithium-ion batteries remain a key focus of power battery development. Solving the problems of low cycle coulombic efficiency of silicon anodes and voltage decay of lithium-rich manganese-based batteries is expected to lead to the development of advanced lithium-ion power batteries with specific energy exceeding 400Wh/kg.
2) In the long term, innovative lithium-ion batteries are more feasible than lithium-sulfur and lithium-air batteries. Developing high-capacity lithium-rich oxide cathodes based on anion charge compensation mechanisms could lead to the development of power batteries with a specific energy greater than 500 Wh/kg.
3) Safety determines the prospects of high-energy-density battery applications in vehicles. Developing self-heating control technology and all-solid-state batteries is a feasible solution, and we need to step up our efforts to overcome this challenge.
4) High-capacity electrodes are the foundation for achieving high specific energy of batteries. Based on the polarization end, developing gradient porosity electrodes is of great importance and significance for the development of high specific energy batteries.
