With the rapid progress of automobiles and electronics, the presence of independent manufacturers of lithium-ion batteries has increased significantly (Part 2)

The development of solid solution materials is becoming increasingly active

Although lithium-ion rechargeable batteries have been fully equipped in vehicles, automobile manufacturers still have high requirements for improving battery performance. At AABC 2012, battery manufacturers gave speeches on the new generation of positive electrode materials. Among them, Envia Systems, Samsung Yokohama Research Institute and GS Yuasa of the United States gave speeches on solid solution positive electrode materials that are expected to greatly increase capacity, and Daw Chemical of the United States and others presented posters.

Although the solid solution positive electrode material adopts a layered structure, its capacity exceeds the theoretical value of layered materials - 275mAh/g, so it has attracted much attention. However, this material has problems such as high charging voltage of more than 4.5V, large decrease in capacity as the number of charge and discharge cycles increases, and difficulty in releasing large currents. AABC 2012 also published research results to solve these problems.

Among them, Envia Corporation (Note 3) has attracted the most attention. The company has achieved a battery with an energy density of 400Wh/kg, which is about three times that of current lithium-ion rechargeable batteries for vehicles. The company gave a speech entitled "Advances in Materials towards the Realization of Lithium-Ion Cells with Higher Energy Density" and reported the experimental results of achieving high energy density by combining a solid solution positive electrode material and a Si-C negative electrode material composed of a silicon alloy and a carbon material (Figure 5).

Figure 5:
Envia Systems has produced a 45Ah laminated cell (a). It achieved an energy density of 392Wh/kg when discharged at 1/3C (b). In a test using a button-type cell, a capacity retention rate of 91% was achieved after 300 cycles (c). Figure created by this publication based on data from Envia Systems.

Note 3) Envia was founded in July 2007 and developed lithium-ion rechargeable batteries using solid solution cathode material technology from the Argonne National Laboratory in the United States. In January 2011, GM, Asahi Kasei, and Asahi Glass invested in the company.

Envia gave a presentation on lithium-ion rechargeable batteries using solid solution cathode materials at the AABC 2010 conference in 2010. At the time, it reported that a 20Ah laminated cell achieved an energy density of 250Wh/kg.

This time, the energy density was further improved by combining Si-C negative electrode material with the improved solid solution positive electrode material. Envia trial-produced a 45Ah laminated unit and released the test results.

At 80% depth of discharge (DOD), 1/20C discharge achieved an energy density of 430Wh/kg, and 1/3C charge and discharge achieved an energy density of 392Wh/kg. In terms of charge and discharge cycle characteristics, the test results using button-type units were disclosed. At 80% depth of discharge, 1/3C charge and discharge and cycled 300 times, a capacity retention rate of 91% was ensured.

Envia said that lithium-ion rechargeable batteries that use a combination of solid solution cathode materials and Si-C anode materials "will be put into practical use in 2014" (Sujeet Kumar, president and chief technology officer of the company). By then, the company aims to achieve an energy density of "400Wh/kg" (Kumar).

In terms of battery cost, not only did they reduce the cost of cathode materials, but they also developed a method for manufacturing Si-C anode materials at a low cost, so they could achieve $180/kWh (Kumar). This is basically the same as the low-priced cylindrical unit products supplied for notebook computers, etc.

87% capacity is still maintained after 400 charge and discharge cycles

Samsung Yokohama Research Institute presented a speech on the high performance of lithium-ion rechargeable batteries using solid solution cathode materials. The company presented a speech titled "High Performance Overlithiated Layer Oxide (OLO) Cathode Battery". Samsung Yokohama Research Institute calls solid solution materials lithium-excess layered materials (OLO).

OLO previously had problems with gas generation during initial charging, which caused the unit to swell, or severe capacity degradation after charge-discharge cycles when charged at high voltage. Samsung Yokohama Research Institute has reduced the initial amount of gas generation to 1/50 of the previous level by improving the synthesis method of the positive electrode material. In addition, by improving the negative electrode material graphite, the amount of oxygen generated in the unit has been greatly reduced.

In addition, the diaphragm and electrolyte were improved to improve the charge and discharge cycle characteristics. For example, the diaphragm will undergo oxidative decomposition at 4.35V or above, and this reaction was suppressed by setting a protective layer on the surface of the diaphragm (Figure 6). As for the electrolyte, in order to prevent it from decomposing at high voltage, an electrolyte that forms fluoride with carbonate and ether materials was used.

Figure 6: 87% capacity maintained after 200 cycles at 45°C
Samsung Yokohama Research Institute improved the electrode material, separator and electrolyte of the unit using solid solution positive electrode material (a). The trial laminated unit maintained 87% capacity after 400 cycles at room temperature (25°C) (b). After 200 cycles at 45°C, 87% capacity maintenance rate was ensured (c). The figure was produced by this magazine based on the data of Samsung Yokohama Research Institute.

The laminated cell produced using improved OLO, graphite, separator and electrolyte has an initial discharge capacity of more than 250mAh/g. Compared with ordinary layered cathode materials, the capacity is as high as about 100mAh/g.

In terms of charge and discharge cycle characteristics, 87% of the capacity was maintained after 400 cycles at a charge and discharge rate of 1C at room temperature (25°C). In addition, a more rigorous test was carried out using a button-type unit in a high temperature environment of 45°C. The results showed that in the charge and discharge cycle test, 87% of the capacity was maintained after 200 cycles at a charge and discharge rate of 1C. "It is worth looking forward to as a sufficiently durable lithium-ion rechargeable battery" (Samsung Yokohama Research Institute).

Achieve 170Wh/kg

In addition, GS Yuasa also introduced the new generation of positive electrode materials - solid solution positive electrode materials and lithium manganese phosphate (LiMnPO4, LMP) under the title "High-performance Lithium-ion Battery for Electrified Vehicle Applications".

As for the solid solution positive electrode material, the positive electrode material composed of Li1.2Co0.1Ni0.15O2 is used to ensure a discharge capacity of 250mAh/g. In addition, the first and second discharge curves are basically the same, and the high charge and discharge efficiency is also a major feature.

GS Yuasa reported the results of a trial production of a 0.8Ah square cell using this material (Figure 7). The energy density of 170Wh/kg was achieved by charging and discharging at 0.1C at room temperature (25°C).

Figure 7: Achieving an energy density of 170Wh/kg
GS Yuasa has trial-produced a square unit using a solid solution cathode material and achieved an energy density of 170Wh/kg. This figure was created by our magazine based on data from GS Yuasa.

LMP has a voltage 0.6V higher than LFP, so it is easy to achieve a higher voltage battery pack and the energy density can be increased by about 18%, so it is worth looking forward to.

However, a major problem with LMP is that its electrical conductivity is lower than that of LFP. GS Yuasa has improved its performance by coating LMP particles with carbon and building a "carbon network" connecting the LMP particles (Figure 8).

Figure 8: Improving properties through carbon network
GS Yuasa has developed a technology to form a carbon network to improve conductivity, a problem that exists when using LMP. The carbon network significantly increases capacity. This figure was created by our magazine based on data from GS Yuasa.

For example, a material that only has a capacity of about 65 mAh/g when coated with carbon alone can have a capacity of about 132 mAh/g after building a carbon network connecting the particles.