Let you know about lithium-ion battery electrode materials

Let you know about lithium-ion battery electrode materials.1 accurate article make it clear!

Electrode materials for lithium-ion batteries

Introduction

We often see professional lithium-ion battery terms such as lithium iron phosphate and ternary, which are used to distinguish the types of lithium-ion batteries based on the cathode material of the lithium-ion battery. Relatively speaking, the positive and negative electrode materials of lithium-ion batteries have a greater impact on battery performance, which is an aspect that everyone is more concerned about. So, what are the common positive and negative electrode materials currently on the market? What are the advantages and disadvantages of using them as lithium-ion batteries?

Cathode material

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

Has a higher redox reaction potential, enabling lithium-ion batteries to achieve higher output voltages;

The high lithium content and high material packing density make lithium-ion batteries have high energy density;

The structural stability during the chemical reaction process is better, so that the lithium-ion battery has a long cycle life;

The conductivity should be high so that the lithium-ion battery has good charge and discharge rate performance;

The chemical stability and thermal stability are better, and it is not easy to decompose and generate heat, making lithium-ion batteries have good safety;

Cheap price, making the cost of lithium-ion batteries low enough;

The manufacturing process is relatively simple and convenient for mass production;

It has low environmental pollution and is easy to recycle.

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

Based on these factors, after engineering research and market testing, the common cathode materials currently on the market are as follows:

Analysis of eight major parameters of lithium-ion batteries and positive and negative electrode materials

The commercial application of lithium cobalt oxide was the earliest. The first generation of commercially applied lithium-ion battery was the lithium cobalt oxide battery introduced to the market by SONY in 1990. Subsequently, it was widely used in consumer products. With the large-scale popularization of mobile phones, notebooks, and tablets, lithium cobalt oxide was once the material with the largest sales volume among lithium-ion battery cathode materials.

However, its inherent disadvantage is that the mass specific capacity (not equivalent to energy density) is low. The theoretical limit is 274mAh/g. Due to the stability of the positive electrode structure, it can actually only reach 50% of the theoretical value, that 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 also high, making it difficult to popularize it on a large scale in the field of power batteries. Therefore, the lithium cobalt oxide cathode material will be gradually replaced by other materials.

Due to shortcomings in stability, safety, difficulty in material synthesis, etc., lithium nickelate has few commercial applications and is rarely seen on the market, so it will not be discussed here.

The commercial application of lithium manganate is mainly in the field of power batteries, which is a relatively important branch of lithium-ion batteries. For example, Nissan’s leaf pure electric sedan uses lithium manganate batteries from Japan’s AESC, and the early Chevrolet Volt also used lithium manganate batteries from South Korea’s LG Chem.

The outstanding advantages of lithium manganate are low cost and good low-temperature performance. The disadvantages are low specific capacity with a limit of 148mAh/g, poor high-temperature performance and low cycle life. Therefore, there is an obvious bottleneck in the development of lithium manganate. In recent years, the research direction has mainly been to modify lithium manganate and change its shortcomings by doping other elements.

Lithium iron phosphate materials have been popular in China for a while. On the one hand, they are driven by the technology of American scientific research institutions and enterprises, and on the other hand, they are driven by BYD’s domestic industrialization. In the past few years, domestic lithium-ion battery companies basically used lithium iron phosphate materials in the field of power batteries. However, as countries around the world have increasingly higher requirements 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 is no longer able to meet current and future market demand.

The selection of appropriate electrode materials is crucial for optimizing the performance of lithium-ion batteries
The selection of appropriate electrode materials is crucial for optimizing the performance of lithium-ion batteries

In addition, the shortcomings of lithium iron phosphate such as average rate performance and 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 mixed in it. In terms of product application fields, the power energy storage market should be an important market for lithium iron phosphate batteries. Relatively speaking, this market is not particularly sensitive to energy density, but there is an urgent need for long-life, low-cost, and high-safety batteries. Demand is exactly the advantage of lithium iron phosphate materials.

Japanese and Korean companies have vigorously promoted the application of ternary materials in recent years. Nickel, cobalt and manganese ternary materials have gradually become the mainstream of the market. Domestic companies have also adopted a follow-up strategy and gradually turned to ternary materials. The specific capacity of ternary materials is high, and the products currently on the market can already reach 170~180mAh/g, which can increase the energy density of the battery cell to close to 200Wh/kg, meeting the long cruising range requirements of electric vehicles.

In addition, by changing the ratio of ternary materials (values ​​of x, y), good rate performance can also be achieved, thereby meeting the needs of PHEV and HEV models for high-rate and small-capacity lithium-ion batteries. This is also the case with ternary materials. The reason for its popularity. 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 manganate (LiMn2O4). At the same time, because it is doped with nickel, it can improve energy density and rate performance.

Strictly speaking, the nickel-cobalt-aluminum ternary material is actually a modified lithium nickel oxide (LiNiO2) material, which is doped with a certain proportion of cobalt and aluminum elements (a relatively small proportion). In terms of commercial application, Japan’s Panasonic is mainly doing it, and other lithium-ion battery companies have basically not researched this material. The reason for comparison. It’s because the famous Tesla uses Panasonic’s 18650 nickel-cobalt-aluminum ternary cells as the power battery system of its electric vehicles, and has achieved a cruising range of nearly 500 kilometers. It shows that this cathode material still has its unique value.

The above are only the more common lithium-ion battery cathode material, and do not represent all technical routes. In fact, both universities and research institutes, as well as enterprises, are working hard to research new lithium-ion battery cathode materials. It is hoped that key indicators such as energy density and lifespan will be improved to a higher level.

Of course, if we want to reach the energy density target of 250Wh/kg or even 300Wh/kg in 2020, none of the currently commercially applied cathode materials will be able to achieve it. Then the cathode materials require relatively large technological changes, such as solid solution materials that change the layered structure into a spinel structure, and organic compound cathode materials, which are currently popular research directions.

Anode material

Relatively speaking, there are not as many studies on lithium ion battery anode material 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 factors: conditions:

It should be a layered or tunnel structure to facilitate the deintercalation of lithium ions;

There is no structural change when lithium ions are deintercalated, and it has good charge and discharge reversibility and cycle life;

Lithium ions should be inserted and extracted as much as possible so that the electrode has a high reversible capacity;

The potential of the redox reaction should be low, and combined with the positive electrode material, the battery should have a higher output voltage;

The first irreversible discharge specific capacity is small;

Good compatibility with electrolyte solvent;

Rich resources and low prices;

Good safety;

Environmentally friendly.

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

Metal anode materials: Most of these materials have ultra-high lithium insertion capacity. The earliest anode material studied was metallic lithium. Due to battery safety issues and poor cycle performance, metallic lithium has not been widely used as an anode material. In recent years, alloy-based anode materials have been widely studied, such as tin-based alloys, aluminum-based alloys, magnesium-based alloys, antimony-based alloys, etc., which is a new direction.

Inorganic non-metallic negative electrode materials: Inorganic non-metallic materials used as negative electrodes for lithium-ion batteries are mainly carbon materials, silicon materials and other non-metallic composite materials.

Transition metal oxide materials: These 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, carbon materials are still the main anode materials, and both graphite and non-graphite carbon materials are used. In the fields of automobiles and power tools, lithium titanate also has certain applications as anode materials. It mainly has excellent cycle life, safety and rate performance, but it will reduce the energy density of the battery, so it is not mainstream in the market. For other types of negative electrode materials, except for SONY’s tin alloy products, most of them are still focused on scientific research and engineering development, with relatively few market applications.

Analysis of eight major parameters of lithium-ion batteries and positive and negative electrode materials

In terms of future development trends, if cycle performance can be effectively solved, silicon-based materials may replace carbon materials as the main anode material for the next generation of lithium-ion batteries. Tin alloy, silicon alloy and other alloy anode materials are also a very popular direction and will be industrialized. In addition, iron oxides with higher safety and energy density may replace lithium titanate (LTO) and be widely used in some fields with long life and high safety requirements.

In the following content, we will briefly discuss the two key indicators related to energy of lithium-ion batteries: energy density and charge and discharge rate.

Energy density refers to the amount of energy that can be stored per unit volume or weight. Of course, the higher the indicator, the better. Everything that is concentrated is the essence. The charge-discharge rate refers to the speed at which energy is stored and released, preferably in seconds. It can be fully charged or released in an instant, and it can come and go as soon as you call it.

Of course, these are ideals. In fact, they are subject to various practical factors. It is impossible for us to obtain unlimited energy, nor to achieve instantaneous transfer of energy. How to continuously break through these limitations and reach higher levels is a difficult problem that we need to solve.

The development of stable electrode materials is a key focus in the advancement of electrochemical sensors
The development of stable electrode materials is a key focus in the advancement of electrochemical sensors