The Past, Present and Future Prospects of Lithium-ion Batteries (Cathode Materials)

The rapid growth of the global electric vehicle industry has not only set off a wave of innovation and change in traditional car manufacturers but also brought a massive wave of new business opportunities to the battery power market. Estimates based on market research data say that global electric vehicle sales will rise from the 2.9 million units of 2020 to about 47.3 million in 2032. The penetration rate is predicted to increase from 4.2% to 44.6%, and the development of the battery power industry will reach a CAGR of 19% over the next 10 years. The ideal battery for electric vehicles must have a high energy density, long endurance, fast charging speed, high safety, and commercially-acceptable price/cost, etc. Carbon emission will also be a focus of consideration for the next generation of battery technology.

Vehicle battery technology mainly includes the hydrogen fuel cell and the lithium power battery. The former would be difficult to popularize in the short-term because of its need for supporting facilities, including the construction of hydrogen refueling stations and a safety monitoring system, and the high cost of hydrogen transportation and storage. On the other hand, lithium batteries are continuously improving their energy density, and the manufacturing costs continue to decline. As such, it has become the mainstream technology of the battery power market. Lithium power batteries can be divided into ternary lithium batteries and lithium iron phosphate batteries according to the cathode materials. The ternary lithium battery uses lithium cobalt oxide (LiCoO₂) for the cathode material. Due to the presence of highly active elements such as nickel and cobalt, its energy density is higher than that of lithium iron phosphate (LiFePO₄) materials. Therefore, it has always been the first choice for electric vehicle applications, with a market share of over 60%.

However, in terms of long-term development, the application of ternary lithium batteries comes with hidden safety concerns, and the rare elements used, such as cobalt and nickel, may face resource shortage issues and problems with rising raw material costs in the future. In contrast, lithium iron phosphate battery components are nontoxic and non-polluting. This greatly reduces the batteries’ impact on the environment. Also, since iron ore is plentiful, the raw material prices tend to fluctuate very little. In addition, the material also has a longer service life as it can be charged and discharged thousands of times or more. Furthermore, although the energy density of lithium iron phosphate batteries is comparatively low, their thermal stability and safety are relatively good. Therefore, it is expected that they will soon surpass ternary lithium batteries in the market and become the mainstream battery technology of the electrical vehicle industry.

For this issue of the “New Technology Channel | Collaboration Column “, MA-tek has specially invited Professor Jenq-Gong Duh, a top scholar in the field of battery materials research, to write a comprehensive overview of the development and trends of lithium power batteries and share the progress being made in the academic research of this important scientific and technological field with our readers.

Director of R&D Center & Marketing Division, Chris Chen, 2022/06/20

Application of Energy Storage Materials ─ The Past, Present and Future Prospects of Lithium-ion Batteries

(Cathode Materials)

Professor Jenq-Gong Duh

Ph.D. students：Zhe-ya Wu

Department of Materials Science and Engineering, National Tsing Hua University

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The lithium-ion battery is indispensible to today’s society, whether it’s for 3C products or electric vehicles. Lithium-ion batteries originated in the 1970s, when the scientist M. Stanley Whittingham discovered that lithium ions could be stored in TiS₂ layered materials [1] and that the Intercalation of lithium ions in materials could be used for efficient energy storage. The formula for the reaction is as follows： xLi + TiS₂→ Li_xTiS₂. Figure 1 shows the charge-discharge curves of the first lithium-ion battery. At that time, when TiS₂ was used for the charge and discharge material, the voltage was 2V. Thus began the research and development of lithium-ion batteries.

Figure 1.　The Charge-Discharge Curves of TiS2 at Different Current Densities [1]

In the 1980s, the scientist John B. Goodenough developed the LiCoO₂ layered ceramic material. Using it as the cathode material and lithium metal as the anode material enabled the operating voltage of Li-ion batteries to be increased to 4V. In 1985, the scientist Akira Yoshino created a lithium-ion battery that used a LiCoO₂ cathode and a carbon material anode. This greatly improved the safety of lithium-ion batteries. In 1991, Sony officially released the first commercial lithium-ion battery. Subsequently, other scientists have proposed various feasible materials for application in lithium-ion batteries, including LiFePO₄ and NMC (nickel cobalt manganese metal oxide) for cathodes and artificial graphite, MCMB (Mesophase Carbon Microbeads) and Si for anodes.

Lithium ion batteries are mainly composed of four parts：the cathode material, the anode material, the electrolyte and the separator (Figure 2). The cathode and anode materials determine the operating voltage and energy density of the entire battery. The electrolytes are mainly composed of organic components (such as EC, DEC, DMC) and lithium salts (LiPF6, LiClO4) and are responsible for conducting lithium ions through the battery. Finally, the separator is made of polymer and is meant to prevent short-circuiting caused by direct contact between the positive and negative electrodes. Lithium ions are released from the cathode during charging and conducted via the electrolyte into the anode. Conversely, during discharge, lithium ions are extracted from the anode and returned to the cathode.

Figure 2　Lithium-Ion Battery

After charging and discharging, the impedance in the battery will increase due to the forming of SEI (Solid Electrolyte Interface) on the surface, causing the battery capacity to become lower and lower after use. The application of various materials are also facing cost –related pressure. Therefore, at present, a primary development goal is to propose effective methods to improve the cycle life and energy density of lithium-ion batteries.

The most widely used cathode materials can be divided into three main types (Figure 3). These include the olivine structure material (LiFePO₄), the layered material (LiCoO₂, NMC811) and the spinel structure (LiMn₂O₄).

Lithium Iron Phosphate (LiFePO4) – Widely used in electric buses and energy storage containers, its charge and discharge cycle performance is stable, and it is very safe. However, due mainly to a lower theoretical capacitance (~170 mAh/g) and lower operating voltage (~3.4V), its energy density and electrical conductivity are lower than that of layered materials. Therefore, it is considered to be less suitable for use in electric vehicles. Spinel Structure (LiMn2O4) – It has a high operating voltage and good electrical and ionic conductivity, but its theoretical capacity is low at about 147 mAh/g. Furthermore, under high voltage, manganese ions will dissolve out of the poisonous electrolyte, cracking the electrolyte and causing performance degradation. Layered Materials (NMC811) – It has not only an excellent theoretical capacitance (~250 mAh/g) and operating voltage (~4V) but also a high energy density. However, it still needs to be improved in terms of stability and safety.

Figure 3.　The Three Main Structures and Characteristics of Lithium Battery Cathode Materials

The first commercial lithium-ion battery, LiCoO₂, exhibited good orderliness, structural stability and high ionic and electrical conductivity. However, during the charge and discharge process, due to the overlapping of the p orbital and energy of Co²⁺ with that of O^2-, O^2- was reduced and produced oxygen during discharge, causing material disintegration and battery expansion. This meant it could only provide half of the theoretical capacitance. Using nickel instead of cobalt as the main material can help prevent the generation of oxygen, thus maintaining the battery’s operating voltage and reducing the cost. For decades, many types of LiNiO₂ have been proposed as cathode materials. An examination of the literature reveals that there are four main factors behind the failure of high-nickel ternary cathode materials：

1．Ni²⁺ (0.69Å) and Li⁺ (0.76Å) have similar ionic radii. A portion of the nickel will exist in the form of Ni²⁺ and occupy the Li⁺ positions (Figure 4a)[2, 3]：

The Li/Ni shuffling effect occurs during material synthesis (Figure 4a). It is more serious in cycling tests. Cation shuffling leads to structural disintegration and changes in electrochemical performance. Makimura’s [3] team performed electrochemical tests for different ratios of lithium content and found that the lack of lithium causes instability in cycling tests (Figure 4b). Additionally, when nickel ions diffuse to the positions of the lithium ions, they block the paths that the lithium ions take during the charging and discharging process. This causes the phenomenon where polarization after charging and discharging gets bigger and bigger.

Figure 4.　(a) Schematic Diagram of Lithium and Nickel Shuffling

(b) Cycling Tests of NCA Powders with Different Lithium Contents at 0.1C [3]

2．Oxygen vacancies on the surface of the cathode make the material’s surface unstable：

There have been experiments and discussions focused on different sintering atmospheres [4]. Figure 5a is a diagram of the mechanism. You can observe the local oxygen vacancies at different oxygen partial pressures. These cause changes in electrochemical performance. The higher the oxygen partial pressure, the better the performance. The best electrochemical performance is obtained when sintering under pure oxygen.

Figure 5.　(a) Schematic Diagram of NMC Oxygen Vacancies

(b) Cycling Test Results from Sintering NNC Powders Under Different Oxygen Partial Pressures at 1C [4]

3．After the surface of the material is exposed to the carbon dioxide and moisture in the air, lithium carbonate precipitates on the surface, and the surface structure is converted into nickel oxide (NaCl structure)[5-7], causing the lithium ions to produce a great resistance as they move in and out：

An electrochemical performance test was conducted on NMC811 powder stored in the atmosphere for 30 days [5]. The powder was washed with water, and electrochemical performance was measured (Figure 6). It can be observed that the electrochemical performance of W2 was more stable than that of the other samples. It is mainly the new post-annealing process after the washing process that inhibits the formation of precipitants on the surface of the material structure.

Figure 6.　(a) Influence of the Water Washing Process and Annealing Temperature on the Charge-Discharge Cyclic

(b) High Current Charge-Discharge Performance of Different Samples in CC-CV Mode [5]

4．While charging and discharging, the material changes in volume, resulting in the formation of many tiny cracks [8], which causes the surface structure to form more nickel oxide structures： Electron microscope observations of the powder before and after charging and discharging (Figure 7) revealed that, in addition to the generation of many small cracks on the surface, which caused changes in electrochemical behavior, the polarization phenomenon also became more obvious during charging and discharging.

Figure 7.　SEM Images and Schematic Diagrams of the Material Before and After Charging and Discharging

Improving the powder surface structure and reducing lithium-nickel shuffling are essential to optimizing electrochemical performance. In recent years, many solutions have been proposed in various studies. They can be divided into the following four types：

1．Doping (Mg [9, 10], Mo [11], Zr [12], Ti [13])：

Doping with magnesium ions [9, 10] and testing at 0.5C results in an 81% retention after 350 cycles (Figure 8a). This enabled the development of a high-nickel cathode ternary material with low lithium-nickel shuffling (Figure 8b).

Figure 8.　(a) Cyclic Charge-Discharge Performance of NMC Powder Before and After Doping with Magnesium Ions

(b) XRD Analysis of NMC Powder Doped with Magnesium Ions [9, 10]

It was found that doping with zirconium ions [12] led to the expansion of the 003 interplanar spacing (Figure 9a), which reduced the energy barrier for lithium ions migrating in and out. This resulted in faster charge-discharge capabilities.

Figure 9.　(a) XRD Analysis of NMC Powder Doped with Zirconium Ions

(b) Cyclic Charge-Discharge Performance of NMC Powder After Doping with Zirconium Ions [12]

 2．Surface Coating (Oxides： Al₂O₃ [14], Li₂MnO₃ [15, 16], Li₂TiO₃ [17, 18] and SiO₂ [19-21]), Fluorides： AlF₃[22, 23], Phosphate Compounds： LiMnPO₄ [24])：

By using the hydrothermal method to develop a surface coating for NMC622 powder, the electrochemical behavior and stability after coating with Al2O3 and LiAlO2 were explored [14]. Figure 10(a) is a transmission electron microscope image of the surface coated with Al2O3. Figure 10(b) is a transmission electron microscope image of the surface oated with LiAlO2. It can be observed that the thickness of the coating is about 1 nanometer. The LiAlO2 material itself has a certain level of ability to conduct lithium ions, and its structural stability during charge and discharge and performance in rapid charge and discharge are both better than that of the Al2O3 coating (Figure 10(c) ~ (e)).

Figure 10.　(a) TEM Image of NMC622 Powder Coated with Al2O3; (b) TEM Image of NMC622 Powder Coated with LiAlO2

(c) 0.2C Charge and Discharge Results (Potential Window 2.7~4.7V vs Li/Li+); (d) Charge and Discharge Results at Different Current Densities (Potential Window 2.7~4.7V vs Li/Li+)

(e) 0.2C Charge and Discharge Results (Potential Window 2.7~4.5V vs Li/Li+)[14]

Modify the surface of the NCM532 powder via the gel-melting method [22, 23] then coat again with an appropriate amount of AlF₃. This can effectively improve the material’s charge and discharge capacity. However, if too much is added, the performance will be greatly degraded (Figure 11　(a)~(b)).

Figure 11 (a) High Current Charge and Discharge Capabilities with Different Levels of Coating

(b) 0.1C Charge-Discharge Curve [22, 23]

3．Surface Treatment Technology：

Surface surgery [25] means to oxidize the nickel on the surface of the material using Na₂S₂O₈ to form a complete and ordered layered structure. When the Ni²⁺ on the surface is oxidized and becomes Ni³⁺, cation shuffling was reduced, and the surface structure was reorganized to form an ordered β-NiOOH layer (Figure 12(a)~(d)). This further enhanced the smoothness of the lithium ion migration in and out. Additionally, after modification, the maintanance rate was 80% after 100 charge-discharge cycles (Figure 12(e)).

Figure 12.　Transmission Electron Microscope Image (a) Before the Precursory Oxidation Treatment; (b) After the Precursory Oxidation; (c) Unoxidized NCA Powder; (d) Oxidized NCA Powder

(e) 0.5C Charge-Discharge Cycle Results [25]

4．Concentration Gradient Powder：

In addition to surface coating, powders with concentration gradients have been developed (Figure 13a)[26]. This can be done by adjusting the flow of each element in the co-precipitation method. With the nickel, which reacts easily with the atmosphere, placed in the center, gradually increase the concentration of cobalt and manganese moving outward (Figure 13b). Doing this can help increase the stability and capacitance of the material’s surface (Figure 13c) to optimize performance. It can maintain about 200mAh/g of electric capacity after 100 cycles.

Figure 13.　(a) Schematic Diagram of Concentration Gradiant Powder

(b) Electronic Micro Carbon Analyzer Element Analysis Results

(c) 0.2C Charge-Discharge Cycle Results [26]

In summary, cobalt doping in high-nickel systems can increase stability. Co³⁺ can prevent Ni²⁺ from diffusing into the Li layer, thus reducing Li and Ni shuffling [27]. The presence of Co can also enhance electrical conductivity and facilitate high speed charging and discharging. In addition, a small amount of Mn doping in the material can improve the thermal stability in the delithiated state. For high-nickel cathode materials, the presence of manganese ions can suppress irreversible side reactions between the electrolyte and the electrode’s surface, thereby stabilizing the surface structure [28]. However, too much manganese can drastically reduce the capacitance. Only a small amount of Mn⁴⁺ is needed to achieve the effect of stabilizing the electrochemical performance.

In recent years, our laboratory (The Department of Material Science and Engineering, Tsing Hua University-Professor Jenq-Gong Du’s Research Team) has successfully developed a high-nickel ternary cathode material. The stability of the ternary cathode was improved through doping and surface modification technology. For this process technology, first, NMC811 is synthesized via the co-precipitation method [29], and boron doping is used to control the size of the c-axis of the crystal to improve the structural stability during charging and discharging. The cyan-blue nickel oxalate powder is synthesized via the co-precipitation method. Nickel oxide is obtained after the first stage of sintering. Then, after grinding with lithium hydroxide and sintering with oxygen, it is possible to obtain high purity NMC811 powder. For more information on the above-mentioned research and findings, please see the literature [29, 30].

There is still a lot of room for improvement in lithium-ion battery cathode layered materials. First, the cobalt content of the material should be further reduced. This would reduce the cost of materials and thus reduce the price of electric vehicles. Furthermore, although the capacitance and the energy density of the layered cathode material will be slightly improved by reducing the cobalt content and increasing the nickel content, the charge-discharge stability and thermal stability of the material will decrease accordingly. Battery life will also decrease. Therefore, surface engineering technology is required to improve the material’s charge-discharge cycle life. In addition, material changes during battery decay can be directly observed in situ observations. This observation can assist in further designing the composition and structure of the material to achieve the optimization of performance in order to pursue lithium-ion batteries with higher stability and safety for application in the future electric vehicle market.

Reference：

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The batteries account for about 30-40% of the overall manufacturing cost of electric vehicles, making the battery the core component with the highest proportion of the cost. Therefore, how to reduce the cost of batteries is the key to the success of the electric vehicle industry. The structure of the lithium battery is composed mainly of four parts： the cathode material, the anode material, the electrolyte and the separator. Among these parts, the cathode material accounts for about 28% of the overall manufacturing cost of the battery. Therefore, a cathode material with good performance will be the most important factor in improving the cost performance of lithium batteries.

Early on in the selection of lithium batteries, most chose the higher energy density ternary lithium battery with lithium cobalt oxide (LiCoO₂; NMC811) as the cathode material. In more recent years, with the expansion of the battery industry, upstream raw materials such as lithium, nickel, and cobalt are facing bottlenecks and supply shortages. As a result, car manufacturers in various countries are turning to lithium iron phosphate (LiFePO₄; LFP) batteries. For example, electric car manufacturer Tesla announced in October of 2021 that, in the future, its general car models will use lithium iron phosphate batteries. In addition, according to industry data, the number of lithium iron phosphate batteries in the Chinese market is growing faster than that of ternary lithium batteries. Since Lithium iron phosphate batteries do not require the use of rare metals, are low cost and safe, and have high cost-effective performance, they are the ideal choice for making affordable EVs.

In truth, the original disadvantage of lithium iron phosphate batteries was that their charging and discharging efficiency was too low. However, with the development of nanomaterial technology, there is no longer a significant difference between their charge and discharge efficiency and that of ternary lithium batteries. Also, the problem in the past where the remaining power in a lithium iron phosphate battery could not be accurately estimated has also been addressed through advances in power management technology. According to estimates made by market research firms, the number of lithium iron phosphate battery shipments will reach 65% of the battery market by 2025, surpassing ternary lithium batteries to become the mainstream technology of the future.

Regarding the structure of the anode materials in lithium batteries, it is mainly necessary to have enough space so that lithium ions can be intercalated or accepted in other forms, thus avoiding precipitation and accumulation of lithium ions on the anode. At present, graphite is a very important anode material as its theoretical capacity is 372 mAh∕g, and its material density is low, which helps reduce the weight of the battery itself. In recent years, research on silicon anodes has also become very popular in academia because silicon has a theoretical capacitance value more than 10 times higher than that of graphite. This means it may offer the opportunity to deliver more energy with less material. It will certainly be helpful to reducing both the size and weight of batteries. However, the volume expansion rate of silicon is as high as 400% after lithium ion intercalation. This can easily cause the active materials to break, which leads to the rapid decline of capacitance and seriously affects the cycle life of the battery.

To ameliorate this problem, many recent studies have attempted to modify silicon into nano-scale porous structures. The idea is to use the space reserved by the pores to buffer the swelling effect. In addition, some scholars have tried mixing silicon and carbon materials to reduce the volume change. These issues are gradually improving with the development of silicon anode technology. Future lithium batteries will have higher energy densities and provide electric vehicles with greater power and endurance.

The upstream, midstream and downstream of the lithium battery industry chain are focused on raw materials, battery cells and battery modules respectively. Upstream raw materials mainly include cathode materials, such as lithium, cobalt and other metal minerals, and anode materials, such as graphite and carbon silicon compounds. Lithium in particular is the core element in both ternary lithium and lithium iron phosphate batteries. There are currently only a handful of suppliers in the world, the two main companies being ALB in the United States and SQM in Chile. After obtaining the important raw materials for lithium batteries, midstream manufacturers will acquire other necessary compounds according to their own patented formulas and conduct battery cell manufacturing and testing. The global supply of battery cells is currently controlled mainly by China and South Korea. Four of the world’s top 10 battery cell manufacturers are from China. They have a market share of 39%. They are followed by 3 Korean companies with a market share of about 37%. The development of Japanese companies in this area is lagging behind with a market share of only 19%. As many large Japanese car manufacturers, such as Toyota and Honda, etc. chose after evaluating multiple battery technology strategies to focus on the development of hydrogen fuel cells, they have invested relatively little in the research and development of lithium batteries and so missed the opportunity to lead the market.

At present, the main suppliers of global battery cells are China’s CATL and BYD, South Korea’s LG Chem, and Japan’s Panasonic Corporation, etc. As for downstream suppliers, the most important thing is to package multiple battery cells to form battery modules, which are provided to terminal depots for use. The packaging requirements for batteries are very strict, and they need to meet various conditions such as impact resistance, fire resistance and a lightweight design. Currently, aside from Tesla, the most technologically competitive companies include China’s two major electric vehicle manufacturers, CATL and BYD. In addition, the battery management system (BMS) is another major focus of downstream industry development. The chip is usually designed and developed by the major car manufacturers themselves so that they can truly master the core technologies to ensure the efficient operation of their products.

Taiwan’s technological ventures in the battery industry started late. It is also facing a gap between midstream and terminal consumption. The industry as a whole has yet to take shape. At present, only two upstream raw material suppliers, CoreMax and Mechema, are capable of supplying lithium battery cathode materials to international manufacturers. However, domestic companies still have the opportunity to catch up in several promising technology areas such as next generation batteries, lithium battery recycling and power management systems. Investment and development in these areas may lead to key positions in the industry in the future. According to market observations, next generation batteries such as solid-state batteries and fuel cells, etc. all have the characteristics of being environmentally friendly and safe and having high energy densities. Therefore, they are likely to become the mainstream battery technologies of the future. As such, their related power management systems will also become a focus of development. In addition, with the substantial growth of the electric vehicle market, there will be a great number of batteries that will be depreciating and needing to be replaced. So recycling is bound to become a major problem. How to convert retired batteries into energy storage systems or how to recycle their rare materials are also key directions of recent industrial research and development.

This article provided a comprehensive introduction to lithium battery technology. It also clearly described the research and development of the related cathode and anode materials to help readers quickly learn and understand this market’s most promising, forward-looking technology. Professor Jenq-Gong Du obtained his PhD from Purdue University in 1983. Since then, he has been teaching at the Department of Material Science and Engineering, Tsing Hua University. During this period, he served successively as the Vice President for Student Affairs of Tsing Hua University and the Convener of the Materials Science Department of the National Science Council as well as the Chairman of the Taiwan Coating Technology Association, etc. He has won several awards, including the Outstanding Teacher Award of Tsing Hua University and the Outstanding Research Award of the National Science Council. He has made great contributions to the development of our domestic academia.

Professor Du has devoted himself to the academic research of electronic packaging, thin film materials, plasma technology, and various energy materials for many years. His team has also published numerous important research findings in internationally renowned journals, with a total of more than 460 articles. He has also acquired more than 25 technical patents and had many other excellent academic achievements. MA-tek is very honored to join hands with Professor Du to carry out industry-university cooperation this year by providing the complete analysis services required by his team in the research of electronic packaging materials. MA-tek has a comprehensive collection of testing equipment and professional testing experience. We are able to fully meet the various analysis and testing needs of electronic materials, processes and packaging.