Lithium Batteries–Positive and Negative Electrode Materials

The positive and negative electrode materials of lithium batteries are directly related to the various properties of lithium batteries, such as battery capacity, life, charge and discharge magnification, etc.We often see professional lithium-ion battery terms such as lithium iron phosphate and ternary. These are based on the cathode material of lithium-ion batteries to distinguish the types of lithium-ion batteries.Relatively speaking, the positive and negative electrode materials of lithium-ion batteries have a relatively large impact on battery performance, which is an aspect that everyone is more concerned about.So, what are the common positive and negative electrode materials on the current market?What are the advantages and disadvantages of using them as lithium-ion batteries?

1. Cathode material

First of all, let’s take a look at the cathode material. The choice of cathode material is mainly based on the following factors.：

1) It has a high redox reaction potential, so that the lithium-ion battery can reach a higher output voltage.；
2) The high content of lithium and the high bulk density of the material make lithium-ion batteries have a high energy density.；

3) The structural stability in the chemical reaction process is good, so that the lithium-ion battery has a long cycle life；
4) The conductivity should be high, so that the lithium-ion battery has good charge and discharge magnification performance；
5) Good chemical stability and thermal stability, not easy to decompose and heat up, making lithium-ion batteries have good safety；
6) The price is cheap, making the cost of lithium-ion batteries low enough；
7) The manufacturing process is relatively simple, which is convenient for large-scale production；
8) Low pollution to the environment and easy to recycle.

At present, some key indicators such as the energy density, charge-discharge magnification, and safety of lithium-ion batteries are mainly subject to the cathode material.

Based on these factors, after engineering research and market-oriented inspection, the common cathode materials in the market are shown in the following table：

The commercial application of lithium cobalt acid was the earliest. The first generation of lithium-ion batteries for commercial application was the cobalt-acid lithium-ion battery INTRODUCED to the MARKET by SONY IN 1990, and then it was used on a large scale in consumer products.With the large-scale popularity of mobile phones, notebooks, and tablet computers, lithium cobalt oxide was once the material with the largest sales volume of cathode materials for lithium-ion batteries.However, its inherent disadvantage is that the mass specific capacity (not equivalent to the energy density) is low, and the theoretical limit is 274mAh/G. Due to the consideration of the structural stability of the positive electrode, it can only reach 50% of the theoretical value, which is 137mAh/G.At the same time, due to the relatively low reserves of cobalt on the earth, the cost of lithium cobalt oxide is too high, making it difficult to popularize on a large scale in the field of power batteries, so lithium cobalt oxide cathode materials will be gradually replaced by other materials.

Due to the shortcomings of stability, safety, material synthesis difficulties, etc., lithium nickel acid has fewer commercial applications and is rarely seen in the market. It will not be discussed here.

The commercial application of lithium manganese oxide is mainly in the field of power batteries, which is a more important branch of lithium-ion batteries.For example, Nissan’s leaf pure electric sedan uses AESC’s manganese-acid lithium-ion battery from Japan, and the early Chevrolet Volt also uses LG Chem’s manganese-acid lithium-ion battery from South Korea.The outstanding advantages of lithium manganese oxide are low cost and good low temperature performance. The disadvantage is that the specific capacity is low, the limit is 148mAh/g, and the high temperature performance is poor, and the cycle life is low.Therefore, there are obvious bottlenecks in the development of lithium manganese oxide. In recent years, the research direction has mainly been to modify lithium manganese oxide and change its shortcomings by doped with other elements.

Lithium iron phosphate materials have been hot in China for a while. On the one hand, they are driven by American scientific research institutions and enterprises in terms of technology, and on the other hand, they are driven by BYD’s domestic industrialization. In the past few years, domestic lithium-ion battery companies have basically focused on lithium iron phosphate materials in the field of power batteries.However, with the increasing requirements of countries around the world for the energy density of lithium-ion batteries, the theoretical limit of the specific capacity of lithium iron phosphate is 170mAh/g, but in fact it can only reach about 120mAh/g, which can no longer meet the current and future market demand.In addition, the general magnification performance of lithium iron phosphate and the shortcomings of poor low temperature characteristics also limit the application of lithium iron phosphate.Recently, BYD has developed a modified lithium iron phosphate material, which has increased the energy density a lot. The specific technical details have not been disclosed, and it is not known what materials are doped in it.In terms of product application areas, the power energy storage market should be an important market for lithium-ion iron phosphate batteries. Relatively speaking, this market is not particularly sensitive to energy density, but the urgent demand for long-life, low-cost, and high-safety batteries is precisely the advantage of lithium iron phosphate materials.

In recent years, Japanese and Korean companies have vigorously promoted the application of ternary materials. Nickel-cobalt-manganese ternary materials have gradually become the mainstream of the market. Domestic companies have also adopted follow-up strategies and gradually turned to ternary materials.The specific capacity of ternary materials is relatively high. The current products on the market can already reach 170~180mAh/g, which can increase the energy density of the battery cell to close to 200Wh/kg to meet the long-range requirements of electric vehicles.In addition, by changing the ratio of ternary materials (the values of x and y), good magnification performance can also be achieved, so as to meet the needs of PHEV and HEV models for large-magnification and small-capacity lithium-ion batteries. This is the reason why ternary materials are popular.As can be seen from the chemical formula, the nickel-cobalt-manganese ternary material combines some of the advantages of lithium cobalt oxide (LiCoO2) and lithium manganese oxide (LiMn2O4). At the same time, because of the doped nickel element, it can improve the energy density and magnification performance.

The nickel-cobalt-aluminum ternary material, strictly speaking, is actually a modified lithium nickel acid (LiNiO2) material, in which a certain proportion of cobalt and aluminum are doped (accounting for relatively little).The commercial application is mainly done by Japan’s Panasonic Corporation, and other lithium-ion battery companies have basically not studied this material.The reason for the comparison is because the famous Tesla uses Panasonic’s 18650 nickel-cobalt-aluminum ternary battery as the power battery system of electric vehicles, and has achieved a range of close to 500 kilometers, which shows that this cathode material still has its unique value.

2. POSITIVE MATERIAL

Relatively speaking, there are not as many research on anode materials for lithium-ion batteries as cathode materials, but anode materials still play a vital role in improving the performance of lithium-ion batteries. The selection of anode materials for lithium-ion batteries should mainly consider the following conditions：

1) It should be a layered or tunnel structure to facilitate the de-embedding of lithium ions；
2) There is no structural change when the lithium ion is de-embedded, and it has good charge and discharge reversibility and cycle life.；
3) Lithium ions should be embedded and prolapse as much as possible in it, so that the electrode has a high reversible capacity；
4) The potential of the redox reaction should be low, and it should be combined with the cathode material to make the battery have a higher output voltage.；
5) The specific capacity of the first irreversible discharge is smaller；
6) Good compatibility with electrolyte solvents；
7) Rich resources and low prices；
8) Good safety；
9) Environmentally friendly.

There are many kinds of anode materials for lithium-ion batteries. According to the chemical composition, they can be divided into metal anode materials (including alloys), inorganic non-metallic anode materials and metal oxide anode materials.

(1) Metal anode materials: Most of these materials have ultra-high lithium-embedded capacity.The earliest anode material studied was lithium metal.Due to the safety issues and poor cycle performance of batteries, lithium metal has not been widely used as a negative electrode material.In recent years, alloy anode materials have been extensively studied, such as tin-based alloys, aluminum-based alloys, magnesium-based alloys, antimony-based alloys, etc., which is a new direction.
(2) Inorganic non-metallic anode materials: The inorganic non-metallic materials used as the anode of lithium-ion batteries are mainly carbon materials, silicon materials and other non-metallic composite materials.
(3) Transition metal oxide materials: Such materials generally have the advantages of stable structure and long cycle life, such as lithium transition oxides (lithium titanate, etc.), tin-based composite oxides, etc.

As far as the current market is concerned, in terms of large-scale commercial applications, anode materials are still mainly carbon materials, and both graphite and non-graphite carbon materials are used.In the field of automobiles and power tools, lithium titanate also has certain applications as a negative electrode material, mainly because it has very excellent cycle life, safety and magnification performance, but it will reduce the energy density of the battery, so it is not the mainstream of the market.Other types of anode materials, except for SONY’s product launches in tin alloys, most of them are still based on scientific research and engineering development, and there are relatively few market-oriented applications.