Although all-solid-state batteries have begun to deliver to small-capacity consumer electronics, even for high-capacity EV production, but in mass production technology There are still some problems in the aspect.
The demand for next-generation high-energy-density secondary batteries is growing. Currently, all-solid-state batteries developed for consumer electronics have begun, but a variety of materials Technology development is also accelerating in parallel at the same time. (Figure 1)
Figure 1: The problem with the silicon-based negative has been resolved,Waiting for innovation in electrolytes (liquid)
summarizes recent trends in secondary battery technology development. Although all-solid-state batteries have begun to be delivered to small-capacity consumer electronics, even for large-capacity EVs, there are still some problems with large-scale production technology. In the negative electrode material, the silicon-based material will gradually replace the original carbon-based material. In many cases, the specific gravity of the silicon base is 50% or less, but 90 to 100% of the technology has appeared.
However, only the replacement of the negative electrode material is not sufficient to greatly improve the performance of the battery. Some positive electrode materials with high potential and high capacity density have been developed, but the existing electrolyte solution (mainly LiPF6) decomposes at high potential and cannot be practically applied. A major technological innovation is needed in the electrolyte (liquid) solution. Signs such as the use of a mixture of ELiiy Power on ionic liquids are emerging.
The most notable of these are two directions: (1) commercialization of all-solid-state batteries for electric vehicles; (2) use of next-generation materials in negative electrodes (Figure 2).
Figure 2: Overseas manufacturers have begun mass production of next-generation secondary batteries
Overseas manufacturers in all solid-state, semi-solid batteries And the practical application and large-scale mass production of silicon-based anodes. For example, in terms of all-solid-state batteries, Chinese manufacturers have proposed mass production plans, and 100% Si anodes that achieve high energy density and high sustained durability. Case of material.
For the all-solid-state battery for EV mentioned in (1), the funds of 10 billion yen from all over the world are concentrated to certain overseas risks. Company (mostly in the US). Recently, Chinese companies have also participated, such as China's diaphragm manufacturer, Qingqing Tao Energy, announced that it will build a new solid-state production line, reaching 0.1Gwh by the end of 2018, and reaching 0.7GWh by 2020. Currently, the weight energy density of batteries to be mass produced is initially 300 Wh / kg. However, 400Wh/kg has been implemented in the laboratory and will gradually achieve this goal in mass production.
silicon negative electrode becomes the main material
direction(2) The next-generation negative electrode material mentioned in the above refers specifically to silicon (Si) and its oxide (SiOx). Current carbon-based materials,For example, the theoretical capacity density of graphite is 372 mAh/g, whereas Si is 10 times higher, reaching 4200 mAh/g. Silicon is an important candidate for anode materials in terms of increasing battery energy density. However, the main problem at present is that when silicon ions absorb lithium ions, the volume will expand by more than 4 times. It is generally impossible to withstand expansion and contraction at this scale. In the carbon-based negative electrode of a lithium ion secondary battery, in order to increase the energy density, a silicon-based material has been added for many years, but since the dose is not high, expansion or shrinkage does not occur. The weight of the silicon-based material is less than 10% or less by weight.
The proportion of silicon-based materials continues to increase, and it is replacing carbon-based materials as the main feature of negative materials.
Some radical actions by overseas vendors are equally noteworthy. Sila nanotechnology, British Xexon, Amprius, etc. have achieved 80% to 100%. and,They claim that they have solved the problem of expansion and contraction of silicon materials.
Absorb expansion through space and by-products
In the solution of Sila Nanotechnology, solve The solution is to produce porous silicon. Since it is porous at the time of generation, there is room at the beginning, and the problem of volume expansion of the negative electrode material is minimized. On the other hand, Amprius generates a sword-like silicon nanowire on a copper substrate and uses it as a negative electrode material, using the space between the silicon nanowires as a buffer for expansion.
In Japan, it seems that silicon-based anodes generally refer to silicon monoxide (SiO), such as Hitachi Maxwell, which drives local silicon-based anode materials, and is already commercialized. A lithium-ion battery with a silicon oxide weight of 50%, as of 2016, SiO2 is a negative electrode.
When the anode of SiO 2 is used for the first charge, in the subsequent charge and discharge cycles, 3/4 of the SiO releases oxygen atoms and exchanges Li ions and electrons as Si instead of oxide (Fig. 3). On the one hand, the remaining 1/4 of SiO 2 forms a stable compound Li 4 SiO 4 with Li and does not directly contribute to charging and discharging. Although it is disadvantageous in terms of high capacity, it is considered that the by-product causes expansion pressure. Relax and prevent collapse of the negative electrode.
Figure 3: Li pre-doping technology is mainly concentrated on the silicon-based negative electrode
In many Si-based anodes, a part of Li ions in the battery reacts with the anode material at the time of initial charge and solidifies, so there is a problem that the capacity is greatly reduced. As a countermeasure, there is pre-doping of Li ions. JSR and other companies Li ion pre-doping technology has been developed.
Under the expectation of the expansion of Si-based anode materials, it is called "Li pre-doping" The technology appears in the spotlight. This technology solves the problem that 1/4 of the lithium ion group is stable after the initial charge of the SiO anode material, and can not continue to be used for charge and discharge. >In 2018, JSR announced the development of a technology for pre-doping Li by roll-to-roll. Due to these directions, silicon-based anodes have become one of the main roles of lithium-ion secondary battery materials in the future.
limited performance improvement using only negative electrode materials
However, only Si-based negative electrodes are used,The weight energy density of the battery will not be greatly improved. In the ideal case, the increase is as high as 30%, and the upper limit is actually increased by 10% to 20%.
Even if 100% silicon-based material is applied to the negative electrode, as described above, the space or compound buffer accounts for about 3/4 of the volume of the negative electrode. Thus, the current> capacity density of the negative electrode is at most about 1000 mAh / g.
Although not lower, it is about 3 times that of graphite, but the weight or volume ratio of the negative electrode seen in the entire cell of the battery is up to 50%. . Even if the current capacity density of 50% of the components is tripled, the weight energy density of the entire battery will only increase by about 30%.
In fact, many negative electrode materials other than active materials such as conductive additives and adhesives are applied to the negative electrode material. The end result is that the energy density of the Si-based anode is only increased by 10-20%.
New generation of cathode materials is full of challenges
To overcome this problem, Must Increase the capacity density of the positive electrode and increase the potential. The company has developed several 5V candidate materials (such as the lithium cobalt pyrophosphate L2>i2CoP2O disclosed by FDK and other companies mentioned in Part 1 and many more companies. An example of the expected lithium cobalt cobalt LiCoPO4F). However, the practical application of these next-generation cathode materials has the obstacle of decomposition of LiPF6 in the electrolyte.
Therefore, the next-generation electrolyte solute, such as LiBF 4, which will not decompose under the 5V cathode material, has been studied as a substitute for LIPF6. However, LiBF 4 has a problem that it is unstable with respect to the graphite of the negative electrode and is easily decomposed. In a study by Professor Yamada Yukio of the University of Tokyo and others,If it contains a high concentration of LiBF 4 containing electrolyte, it will not decompose, but it will take some time to verify.
There are many reasons why recent solid-state batteries have attracted attention, such as no leakage; but they can be applied to the next generation of 5V cathode materials. Electrolytes are also one of the reasons for choosing solid electrolytes. The FDK is one of the examples.
The fourth solid electrolyte momentum is on the rise
Existing solid electrolyte candidate material system Research and development, especially sulfide-based, oxide-based and polymer-based, have entered the landing zone. It is particularly noteworthy that the ionic material polymer sheet has high Li ion conductivity even at room temperature, but the details are not clear.
Among them, there is a fourth material system that significantly improves traditional issues. "Composite hydride system" (Figure 4). From the laboratory of Prof. Yan Mao, he is the deputy director of the Advanced Materials and Materials Science Institute and a professor at the Institute of Metal Materials of Northeastern University.
(a) various clusters and irregular closo complex borohydrides
(b) The relationship between temperature and ionic conductivity of each cluster
Figure 4: As the irregularity increases, the phase transition temperature drops sharply
shows the ionic conductance of the recently improved clozo complex borohydride An overview of the rate. In the past, clusters such as BH 4 have high ionic conductivity at high temperatures, but phase transitions with decreasing temperature lead to a rapid decrease in ionic conductivity. Northeastern University's lab has successfully increased the size of clusters. At the same time, the element replacement improves the irregularity and greatly reduces the phase transition temperature. In particular, the material mixed with the two clusters has high ionic conductivity at least up to room temperature (Figure: 折茂实验室, (b) The red dotted line is added by the Nikkei.)
Initially, the labyrinth laboratory studied the application of solid electrolytes.Mainly lithium borohydride (LiBH 4 ) as a complex hydride. In the high temperature region of about 120 to 300 ° C, the Li ion conductivity of the material is higher than 2 × 10 -3 S / cm, which is as high as other candidate materials. In addition, it is stable to lithium metal and may be suitable for lithium-sulfur (Li-S) all solid state batteries.
But there is a big problem. When the temperature is 110 ° C, the material undergoes a phase change and the ionic conductivity decreases by almost three orders of magnitude. If it is assumed that a high temperature of 110 ° C or higher is a prerequisite, the practical application is unrealistic.
Keep high conductivity even at room temperature
Lab assistant professor Xianglun has solved this problem to a large extent. First, he focused on Li2B12H12, which has a larger cluster size in the composite hydride (Figure 4). "Large clusters are more likely to conduct lithium ions" (gold). however,The material itself undergoes a phase change at 360 ° C at very high temperatures.
Then, in Li2B12H12, one boron (B) constituting the LiCB11H12 cluster was replaced by a carbon atom (C), and the phase transition temperature was lowered to 120 °C. Further, for LiCB9H10 in which boron was decreased by 2, the phase transition temperature was lowered to 90 °C. "Alternatives of elements and the like are intended to distort the material and increase the disorder, which increases the freedom of movement of the molecular clusters and makes phase transformation difficult," Professor Jin said.
However, it is still up to 90 °C. Professor Jin tried to increase the irregularity and reduce the phase transition temperature by mixing solid solution with LiCB11H12 and LiCB9H10. As a result, phase transition does not occur at least 20 ° C or higher, and the state of high ion conductivity of Li ions is maintained over a wide temperature range. The conductivity of lithium ions is 6.7 x 10-3 S/cm at 25 ° C, and 8.5 x 10-2 S/cm at 110 ° C, which corresponds to a sulfide-based material.“In our current analysis, we expect no phase change in a fairly low temperature range that can be considered,” Professor Jin said.
However, there are still problems in the actual application. It is to ensure the stability of the positive electrode material. "In terms of bulk materials assumed in theory, even a 5V cathode material should be no problem, but in reality a complex phenomenon occurs at the interface of the cathode material" (gold). At present, researchers are said to be trying to improve by coating on a positive electrode material.
Sodium-ion rechargeable battery put into practical use in 2025
With next-generation lithium ion The development of the battery is not very different. The development of non-lithium-ion battery technology has made progress (Figure 5).
(a)Na Ion secondary batteries are laminated in cells
(b)NAS Summary of A and Honda's F-ion secondary battery
Figure 5:Development progress of Na ion secondary battery and F ion secondary battery
Na ion secondary battery developed by Nippon Electric Glass Co., Ltd. in 500 cycles After that, it has a capacity retention rate of 88%, which is a practical level that has reached consumer products. If the research and development is successful, the laminated battery like the AIST will be mass-produced for electric vehicles in 2025 (a). The development of F-ion batteries by NASA and Honda is a major breakthrough because electrolyte materials with high conductivity at room temperature can be found. The F ion is an anion that behaves as opposed to Li ions during charging and discharging. (b)
For example, a Na-ion secondary battery jointly developed by Nippon Electric Glass, Industrial Technology Research Institute, and Nagaoka University of Technology. In the past year, a palm-sized laminated battery has been developed from a coin-type battery. The cycle life is also 500 cycles, and the capacity retention rate is 88%, which is the level of use at the consumer product level.Nippon Electric Glass said: "The battery production plan for large-scale production of electric vehicles in 2025 is progressing."
Honda pushes 5000Wh/L secondary rechargeable battery
Honda Technology Industrial (Honda) developed a fluoride ion secondary battery in cooperation with NASA and other companies in December 2018. This is a breakthrough to develop electrolytes with high ionic conductivity even at room temperature. . The F ion secondary battery has a volumetric energy density of 5000 Wh/L, which is 8 times or more that of a lithium ion secondary battery. If it can be put into practical use, the social impact is very high.