CATL’s new product, Tesla’s first taste, what exactly is the M3P battery?

Since last year, some media have said that CATL 's M3P battery is in the process of research and development, and has a higher energy density than lithium iron phosphate batteries and is safer than ternary lithium batteries .

At the 2022 World Power Battery Conference, CATL officially announced that its newly developed M3P battery will be put into use next year as a battery system for pure electric vehicles with a range of about 700km. However, CATL did not reveal more information. It is rumored that the battery is a manganese iron phosphate lithium battery based on the basic structure of lithium iron phosphate batteries .

What is an M3P battery, or a manganese iron phosphate lithium battery , and how is it different from a lithium iron phosphate battery and a ternary lithium battery?

1.

First, let’s take a look at lithium iron phosphate batteries.

Lithium iron phosphate battery is a variant of lithium-ion battery , using lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material. It has attracted wide attention and application for its high safety, long cycle life and high energy density.

When lithium-ion batteries work, it involves the interaction between the positive electrode, the negative electrode and the electrolyte. For lithium iron phosphate batteries, when charging , lithium ions are separated from the lattice of lithium iron phosphate crystals that constitute the positive electrode material and transferred to the electrolyte. The reaction is roughly: LiFePO4 ═ Li+ + FePO4. At the same time, the lithium ions on the negative electrode are adsorbed into the layered structure of graphite that constitutes the negative electrode material. The reaction is: C + Li+ ═ LiC. The electrolyte is prepared by a mixture of organic solvents and lithium salts. The lithium ions in it are responsible for maintaining the charge balance between the positive and negative electrodes during the charging and discharging process. During discharge, the process is the opposite of charging. The lithium ions in the positive electrode material are re-embedded into the lithium iron phosphate lattice, and the lithium ions on the negative electrode are released into the electrolyte. The charge transfer between positive and negative charges generates current in the circuit, thereby realizing energy supply.

The structure of the lithium iron phosphate lattice belongs to the orthorhombic system, using an iron-oxygen hexahedron structure, in which iron ions (Fe2+) and lithium ions (Li+) are located at the vertices of the hexahedron, and phosphate ions (PO43-) are located at the center of the hexahedron. This crystal structure is called an olivine structure. The phosphate ions in the iron phosphate crystal structure form a stable lattice with the iron ions by sharing oxygen atoms. The lithium ions in the lattice conduct ions through tunnels in the lattice during the charge and discharge process. These tunnels are provided by the voids and pores in the hexahedron structure.

The lattice structure of lithium iron phosphate has an important influence on its electrochemical performance. The stability of the lattice structure helps to inhibit the precipitation of lithium ions and the side effects of the internal reaction of the battery, making the positive electrode of the lithium iron phosphate battery insensitive to temperature and charge. Therefore, the lithium iron phosphate battery has high thermal stability and high temperature resistance. It is less likely to have thermal runaway or explosion under high temperature and overcharge and overdischarge conditions, and has higher safety than other types of lithium-ion batteries. In addition, the stability of the lattice structure helps the battery maintain an intact lattice structure after full charge and discharge, allowing the lithium iron phosphate battery to have a longer cycle life than other types of lithium-ion batteries.

However, the disadvantages of lithium iron phosphate batteries are equally obvious. The olivine structure lattice does not leave much space for lithium ions, and the channels for the movement of lithium ions are relatively narrow and long. These two points lead to the low energy density of lithium iron phosphate batteries and weak high-power charging and discharging performance.

2.

After understanding lithium iron phosphate batteries, let’s learn about ternary lithium batteries.

Like lithium iron phosphate batteries, ternary lithium batteries are also rechargeable batteries that use lithium ions as charge carriers. Its positive electrode is composed of three different metal element oxides, namely ternary, which are used to store and release lithium ions. Common metal oxide combinations are nickel cobalt manganese oxide (NMC) or nickel cobalt aluminum oxide (NCA). The negative electrode material of ternary lithium batteries is also graphite.

Taking the ternary lithium battery with NMC as the positive electrode as an example, when charging, the external power supply provides current, and the nickel, manganese and cobalt oxides in the positive electrode material release the lithium ions in the oxide. The reaction is: Li1-x(Ni1/3Mn1/3Co1/3)O2 ? Li+?+ xLi1-x(Ni1/3Mn1/3Co1/3)O2, x represents the degree of insertion of lithium ions, 0 ≤ x ≤ 1. At the same time, the graphite material on the negative electrode absorbs and embeds lithium ions, and the reaction is: LiC6 ? Li+?+ C6. When discharging, the process is opposite to that of charging, and the lithium ions are reinserted into the lattice of the positive electrode material, and the lithium ions embedded in the negative electrode graphite return to the electrolyte.

The positive electrode material of ternary lithium batteries usually adopts a layered structure, in which nickel, manganese, cobalt or aluminum ions are located in the structure of the layered oxide, and lithium ions are arranged between the layered structures. The lattice structure usually belongs to the orthorhombic system, in which nickel, manganese, cobalt or aluminum ions occupy the lattice positions in the form of regular tetrahedrons, and oxygen ions are located in the gaps between the regular tetrahedrons.

This structure is exactly the opposite of the olivine structure characteristics mentioned above. The lithium ions arranged between the layers are more embedded than in the olivine structure, and the lithium ions have more space to move in the plane and move faster. Therefore, the energy density and fast charge and discharge performance of the ternary lithium battery are better than those of the lithium iron phosphate battery. However, the stability of the layered structure is poor, and it is easy for the layers to be dislocated and collapsed, so the long-term cycle life of the ternary lithium battery is low, and the safety is worse than that of the lithium iron phosphate battery. In addition, because the positive electrode material of the ternary lithium battery contains rare metals such as cobalt, its cost is relatively high.

3.

In the early stage of the development of electric vehicles in China , lithium iron phosphate batteries have been widely used in the products of major new energy vehicle companies due to their excellent safety, low cost and relatively high energy density (cell about 90Wh/kg) compared with traditional lead-acid batteries and other lithium-ion batteries at the time. They occupy the mainstream market of new energy vehicle power batteries and even exceeded 70% of the market share in 2015.

However, in 2016, the national new energy vehicle subsidy policy began to use battery energy density as a reference indicator. The higher the density, the more subsidies. At that time, the energy density of ternary lithium batteries could reach more than 200Wh/kg. Since then, ternary lithium batteries with higher energy density have begun to dominate the market, reaching 67% of the market share at one point.

Until June 2019, the national new energy policy subsidies declined, and the price of rare metals rose sharply, causing the cost of ternary lithium batteries to remain high. At the same time, after years of intensive cultivation, the energy density of lithium iron phosphate batteries has reached 160Wh/kg. Note that the energy density here refers to the battery energy density rather than the cell energy density. The weight of the management module, etc., must be included in the calculation of battery energy density. Therefore, major new energy manufacturers have refocused their attention on lithium iron phosphate batteries.

However, after years of development, the energy density of lithium iron phosphate batteries has reached its ceiling. If we want to continue to increase the energy density, we still need to start by optimizing the positive electrode material.

4.

So, is there a battery that can simultaneously meet the requirements of low cost, high safety and high energy density?

Let's review the calculation of battery energy density: the energy density of a battery is equal to the energy stored in the battery divided by the weight of the battery cell or battery pack, and the energy stored in the battery (Wh) is equal to the product of the battery's output voltage (V) and capacity (Ah). On the basis that lithium iron phosphate batteries can already achieve low cost and high safety, increasing the output voltage of the battery cell can increase the battery's energy density.

How to do it specifically? Some of the iron atoms can be replaced with manganese atoms, and this type of battery is called lithium manganese iron phosphate battery.

The positive electrode material of lithium manganese iron phosphate battery is composed of two crystals: lithium iron phosphate and lithium manganese oxide (LiMn2O4). Lithium iron phosphate crystals can ensure its excellent safety and cycle life, while in lithium manganese oxide, manganese ions have multiple oxidation states and can undergo reversible redox reactions during the battery charging and discharging process. The crystal structure of lithium manganese oxide belongs to the spinel structure, which is composed of octahedral manganese oxide crystals and octahedral lithium ions. This structure provides more embedding and release sites for lithium ions, thereby increasing the storage capacity of the battery. In addition, the oxidation state of lithium manganese oxide can provide a higher potential, that is, the standard electrode potential of the positive electrode is relatively high.