From new materials to air batteries, large-scale battery research is on the right track (I): Cathode materials

The 51st Battery Symposium, which brought together more than 2,400 battery industry professionals, has concluded. At this symposium, the number of presentations on cathode materials, which are key to increasing the capacity of lithium-ion rechargeable batteries, increased significantly. In addition, presentations on all-solid batteries and lithium-air batteries, which are post-lithium-ion rechargeable batteries, also increased. With an eye on the rapidly growing large-scale battery market, research on next-generation batteries is becoming increasingly active.

In the field of large-scale batteries for electric vehicles such as electric vehicles and stationary power storage systems, the global development opportunity is becoming more mature. Starting with the next-generation lithium-ion rechargeable battery materials that surpass the performance of existing lithium-ion rechargeable batteries, research and development is in full swing in order to give birth to revolutionary batteries with new reaction principles.

The 51st Battery Symposium, which was held against this background, saw an increase in presentations on cathode materials for lithium-ion rechargeable batteries, all-solid batteries, and lithium-air batteries. This is because the current material development goal is to increase the energy density of lithium-ion rechargeable batteries for large batteries to about twice the current level, that is, 200 to 300 Wh/kg around 2015 to 2020 (Figure 1).

In addition, basic research aimed at realizing post-lithium-ion rechargeable batteries such as all-solid batteries and lithium-air batteries has also become active in order to put them into use around 2020-2030. The

reason why the number of publications on positive electrode materials has increased is that the specific capacity of positive electrode materials is currently the smallest compared to that of negative electrode materials, and the development of new materials has become a top priority. Among negative electrode materials, there are already candidates such as tin and silicon that have been put into practical use, with a specific capacity of more than 1000mAh/g, which is more than twice the current capacity, while there are currently no materials that have exceeded 200mAh/g in practical use for positive electrode materials. Therefore, research on positive electrode materials is relatively active.

*Specific capacity = current capacity per unit weight of electrode or active material.

On the other hand, the increase in the number of publications related to post-lithium-ion rechargeable batteries, all-solid batteries and lithium-air batteries, is because in recent years, companies such as Toyota Motor have actively published, which has increased people's attention to this field and the number of researchers has begun to increase.

Figure 1: Research and development of materials to solve problems In order to achieve high performance of lithium-ion rechargeable batteries, new materials for positive electrodes, negative electrodes, electrolytes, and separators are being developed.

Figure 2: Exploring high voltage and high capacity cathode materials In this battery discussion meeting, in addition to solid solution materials and olivine materials, new materials such as organic compounds were also announced.

Cathode Materials

Expectations for solid solution and olivine materials

The cathode materials of existing lithium-ion rechargeable batteries include lithium cobalt oxide (LiCoO ₂ ), ternary type (LiNiMnCoO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), lithium iron phosphate (LiFePO ₄ ), etc. However, the theoretical capacity of these cathode materials is less than 200mAh/g. Therefore, it is necessary to find new materials that exceed 200mAh/g, or use 5V cathode materials that can increase the lithium potential of only about 4V to about 5V to increase the energy density (Figure 2) (Note 1). (Note 1) The energy density of each electrode is the product of specific capacity and voltage Among them, solid solution type (Li ₂ MnO ₃ -LiMO ₂ ) materials that can achieve a specific capacity of more than 250mAh/g and belong to the 5V cathode material are highly expected. At this battery discussion meeting, companies such as Nissan Motor, Tanaka Chemical Research Institute, Toda Industry, and Sanyo Electric presented their materials. Although this material has a layered structure, its capacity is greater than the theoretical value of layered materials. Therefore, efforts to explore the mechanism of achieving high capacity are becoming increasingly active. This material originally has a layered structure divided into a lithium layer and a transition metal layer such as manganese, but after the initial charge, the transition metal moves into the lithium layer to form a skeleton structure. Research shows that the realization of high capacity is due to the charge compensation effect of oxygen in addition to the redox reaction of metals such as manganese. However, when the charging voltage is increased to about 4.8V, which exceeds the theoretical capacity, the capacity decreases more when the charge and discharge cycle is repeated. It is estimated that this is because the positive electrode material undergoes structural changes under the charge compensation effect of oxygen. If this phenomenon can be explained and the cycle characteristics can be improved, it is expected to open the way for a new generation of positive electrode materials. Improving the characteristics of LiMnPO ₄ Although the specific capacity is not large, olivine-based positive electrode materials can achieve high voltage and are attracting attention in terms of safety and cost. In this material, phosphorus (P) is firmly bonded to oxygen, and it is difficult to release oxygen even at high temperatures. Therefore, it is not easy to cause thermal runaway* and has high safety. Currently, LiFePO ₄ has been put into practical use, but the problem is that its potential to lithium is only about 3.4V. * Thermal runaway = abnormal heat generated in the battery cell due to internal short circuits, etc., leading to fire, smoke, and rupture. The development of lithium manganese phosphate (LiMn-PO ₄ ), which has a potential to lithium of 4.1V, 0.7V higher than LiFe-PO _4, is underway. At this battery discussion meeting, Toyota Motor and Sumitomo Osaka Cement presented the synthesis of LiMnPO ₄ using the hydrothermal synthesis method* . * Hydrothermal synthesis method = a method of synthesizing compounds and growing crystals in a high-pressure water vapor environment.

Figure 3: Li 3 V 2 (PO 4 ) 3 with excellent rate performance GS Yuasa has developed a lithium-ion rechargeable battery using Li 3 V 2 (PO 4 ) 3 (a). Its voltage can be improved compared to LiFePO 4 (b). The image was created by our site based on data from GS Yuasa.

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Toyota Motors has improved the output characteristics of LiMnPO _{4 , which is generally considered to have lower output characteristics than LiFePO 4} , by improving lithium ion conductivity and electrical conductivity. The method is to improve lithium ion conductivity by reducing the particle size of the primary particles to 20nm using a hydrothermal synthesis method, and to improve electrical conductivity by coating the particle surface with a carbon layer using a ball mill*. With these improvements, not only did it achieve a specific capacity of about 150mAh/g at 1C discharge, but it also achieved a specific capacity of 120mAh/g at a high discharge rate of 5C. *Ball mill = a pulverizer that can apply mechanical energy to the material using hard steel balls in a rotating body after adding materials. Its function is to use mechanical energy to promote chemical reactions, that is, to perform mechanochemical treatment. (Note 2) The presentation was titled "Improving the electrochemical characteristics of LiMnPO ₄ by controlling the particle structure " [Lecture number: 2C24]. Sumitomo Osaka Cement presented a method for increasing electrical conductivity, which is a catalyst method that heats carbon and a carbonization catalyst together to form a composite (Note 3). When carbon and LiMnPO ₄ are mixed and heated separately, the carbon film on the particle surface is uneven, but the catalyst method can form a uniform carbon film of about 2nm on the particle surface. In the 0.1C charge and discharge test of the trial button battery, the discharge capacity was 140mAh/g. The company said that by using the catalyst method, the conductivity can be improved even with a small amount of carbon. (Note 3) The presentation was titled "Development of high-voltage olivine positive electrode materials using hydrothermal synthesis method" [Speech number: 2C21]. Vanadium phosphate debuts On the other hand, among olivine materials, GS Yuasa proposed the use of lithium vanadium phosphate (Li ₃ V ₂ (PO ₄ ) ₃ ) (Figure 3) (Note 4). This material is not only expected to be comparable to LiFePO4 in terms of safety, but also has a theoretical capacity of 197mAh/g, which is more than 25mAh/g higher than LiFePO4 and LiMnPO4. In addition, the potential to lithium can reach about 3.8V, which is about 0.4V higher than LiFePO4. (Note 4) The presentation was titled "Synthesis and electrochemical properties of vanadium phosphate using the liquid phase method" [Speech No.: 2A19]. GS Yuasa used the liquid phase method to synthesize the material and then coated the particle surface with a layer of carbon. As a result, the specific capacity at 0.1C discharge was 130mAh/g. In terms of discharge characteristics, a capacity retention rate of 98% was maintained at 2C discharge, which is suitable for high-output applications such as hybrid vehicles. GS Yuasa used a prototype 5Ah-class square cell to test the output characteristics at a state of charge (SOC) of 50%. The cell using Li ₃ V ₂ (PO ₄ ) ₃ showed higher characteristics than the cell using LiFePO _{4 regardless of the length of the discharge time. In terms of output density 10 seconds after discharge, the cell using Li 3} V ₂ (PO ₄ ) ₃ was 25% higher. Organic compounds aiming to achieve two-electron reaction While discussing various positive electrode materials, this battery discussion meeting also made a series of presentations on the field of "organic rechargeable batteries" (Figure 4). The positive electrode material of this battery uses organic compounds. The reaction mechanism is the same as that of ordinary lithium-ion rechargeable batteries. The characteristics of organic rechargeable batteries are that not only the theoretical capacity can be close to 1000mAh/g, but also no heavy metals are used, the weight is light, and the resource restrictions are relatively small. However, although the energy density per unit weight is high, the energy density per unit volume is low. Moreover, the potential to lithium is mostly only 2 to 3.5V. Therefore, to achieve the same energy density as the current lithium-ion rechargeable battery, it is necessary to find an organic compound with a specific capacity of 400 to 600mAh/g.

Figure 4: Organic compounds that can increase the specific capacity of positive electrode materials
Given that organic compounds are expected to increase the specific capacity of positive electrodes compared to existing materials, many candidate materials are under research. The image was created by this site based on data from Murata Manufacturing.

The representative of organic rechargeable batteries is the organic radical battery* developed by NEC in 2001. The redox reaction of this battery is fast, and it can be charged and discharged at a high speed. However, this battery is a single-electron reaction, so it is difficult to increase the capacity, and the energy density per unit cell is about 20 to 30Wh/kg.

*Organic radical battery = a rechargeable battery that uses the redox reaction of stable free radicals. It uses the redox reaction of substances called "free radicals" that have unpaired electrons in the outermost layer of the electron orbit.

In order to solve this problem, organic compounds that can achieve reactions with two or more electrons are being developed. In the meantime, the existence of substances that participate in the reaction with four electrons has been discovered, and expectations for higher capacity are increasing. In fact, according to patent applications, major companies such as Toyota Motor and Panasonic are participating in research.

Murata Manufacturing believes that organic compounds are "a promising field." The company also presented related technologies at the last battery discussion meeting, and this time it reported on four research projects. For example, as a result of collaborative research with Honda R&D Institute, the company presented the results of using rubrosamine as a positive electrode material (Note 5). Even after 20 cycles of charge and discharge, the specific capacity remained above 460 mAh/g. (Note 5) Murata Manufacturing, Kobe City Technical College, Inabata Fine Tech, and Honda Research Institute presented "Electrochemical properties of ruby ​​​​and ruby ​​​​derivatives and organic rechargeable batteries using these substances" [Lecture number: 3G24]. Although it is still in the material exploration stage, Murata Manufacturing intends to "fully apply it to automobiles around 2020." Therefore, the company hopes to provide such products for consumer products around 2015. Sharp publishes research results Sharp has attracted attention in the publication related to positive electrode materials. This is the first time the company has published a rechargeable battery at a conference in more than ten years. The company and a research group at Kyoto University published a method for suppressing the high-temperature degradation of _LiMn2O4 , a representative positive electrode _material for large batteries (Note 6). (Note 6) The topic was "Improving the Cyclic Characteristics of LiMn ₂ O ₄ -Based Lithium-Ion Rechargeable Battery Positive Electrode Materials" [Speech No.: 1C19]. LiMn ₂ O ₄ has a high potential of 4.0V for lithium, and although the material is cheap, there is a problem of capacity degradation at high temperatures. When the charge and discharge cycles are repeated, the structure becomes unstable, resulting in a decrease in discharge capacity. The research team suppressed the volume shrinkage/expansion caused by lithium detachment/insertion into LiMn ₂ O ₄ by generating a microcrystalline phase of about 10×150nm called "nanoinclusion" inside the crystal phase of LiMn _{2 O} 4. When a certain amount of nanoinclusions are present, the discharge capacity can be maintained at about 98% even after 100 cycles. In contrast, when the nanoinclusions are not present, it can only maintain about 70%. (To be continued, reporters: Hiroshi Karishi, Hidehiro Kume)

Figure 5: Balancing large capacity and longevity is a challenge Although there are candidates for negative electrode materials that can achieve large capacity, the expansion/contraction of the materials is large and there are life issues. At this battery discussion meeting, there were presentations on silicon alloy negative electrodes and materials such as Si 6 H 6 , Li 2 Ti 4 O 15 and MoO 2 .