Introduction
The bipolar lithium-ion iron phosphate battery, which was suddenly unveiled at Toyota’s recent presentation of new technology, is said to have a 20% longer range and cost 40% less than the bZ4X’s ternary lithium-ion battery.
Compared to the ternary lithium-ion batteries used in bZ4X, which are currently the mainstream of Japanese-made electric vehicles, the new battery can increase the cruising range by 20%. In other words, it can store 20% more energy than ternary lithium-ion batteries. As anyone familiar with batteries knows, the weak point of lithium iron phosphate batteries has been their low performance.”
In this article, I would like to discuss lithium-ion iron phosphate batteries, which have been the focus of much attention in recent years. First, let’s look at the current mainstream lithium-ion battery, which is the base of the lithium-ion battery.
lithium-ion battery
Batteries generate electricity when electrons are emitted from an internal material and flow through a circuit from the cathode to the anode. High-performance, high-capacity batteries use materials that can store more electrons, and a variety of elements have been used as battery materials. Among these, the lithium element is a very promising battery material because it can store a very large number of electrons.
リチウムイオン電池における特許をめぐる戦いより
The idea of using lithium to make batteries has existed since the 1960s, but it was not widely commercialized until the 1990s. Various improvements have been made to lithium-ion batteries, mainly in the electrode materials and structure, and the current mainstream is called a three-dimensional lithium-ion battery, which has a three-dimensional electrode structure.
In a normal battery, a flat cathode, anode, and separator are wound together with electrolyte and stored in a cylindrical or rectangular outer can or laminated package.
In contrast, in the case of a three-dimensional battery, layers of cathode, anode, and solid electrolyte are formed on the surface of protrusions molded using Si material and other materials. The protrusions are several tens of micrometers in diameter and have a very high aspect ratio. The cathode and anode face each other across the solid electrolyte.
The three-dimensional electrode structure increases the specific surface area of the electrode material per unit area. This is believed to increase current capacity and improve energy density. Furthermore, compared to conventional battery structures, the distance between the cathode and anode is reduced and the ion diffusion rate is increased, so an increase in power density can be expected.
Challenges with existing lithium-ion batteries
Such three-dimensional lithium-ion batteries have not only advantages over two-dimensional lithium-ion batteries, but also some challenges. The main challenges are described below.
- Electrolyte diffusion: In a three-dimensional lithium-ion battery, electrode materials are arranged in three dimensions. Therefore, uniform diffusion of electrolyte throughout the electrodes is a challenge. If sufficient diffusion of electrolyte is not achieved, battery performance and life may be affected.
- Electrode volume change: In lithium-ion batteries, electrode materials expand and contract during charging and discharging as lithium ions are inserted and extracted. This can cause the electrode material in a three-dimensional lithium-ion battery to experience larger volume changes. This volume change affects the structural stability of the electrode and the lifetime of the electrode material.
- High inter-electrode resistance: In three-dimensional lithium-ion batteries, electrodes have a more complex structure and more interfaces exist between electrodes. This increases the resistance between electrodes and can adversely affect battery performance. High inter-electrode resistance causes problems such as reduced charge-discharge efficiency and increased heat generation.
The lithium-ion iron phosphate battery is said to be a battery that can solve these lithium-ion battery problems and dramatically improve performance.
Lithium Iron Phosphate Lithium Ion Batteries (LFP Batteries)
<Overview of lithium-ion iron phosphate batteries>
Lithium iron phosphate (LiFePO4) batteries are a type of rechargeable battery that uses lithium iron phosphate (LiFePO4) as the cathode material and uses lithium ions for charging and discharging the battery. LFP batteries are used in a wide range of applications. (Example: commercially available LFP battery BLUETTI)
LFP batteries have only been made by Chinese manufacturers such as CATL because their major patents were held by a consortium of Chinese universities and research institutes. Since these patents will expire in 2022, car makers such as Tesla, ford, and VW are beginning to shift to the use of LFP batteries.
The structure of the LFP is shown below in comparison with the three-dimensional lithium-ion battery, which is currently the mainstream.
As shown in the figure above, LFP batteries have a more complex crystal structure (olivine structure) than lithium-ion batteries, making them more stable. The olivine structure is a crystal structure with a hexagonal close-packed oxygen skeleton, and is called the olivine structure because natural olivine has this structure.
Such complex structures appear in the perovskite structures described in “Overview of Solar Cells, Challenges and Perovskite Solar Cells” and in the GAA (Gate All Around) structures at 2 nm scale described in “Application of AI to Semiconductor Design Processes and Semiconductor Chips for AI Applications” as well as in other recent breakthrough technologies. It can be said that structural control at the very fine molecular level is necessary, and that material control technology plays an important role.
<Advantages of Lithium Iron Phosphate (LFP) Batteries>
Advantages of such LFP batteries include the following
- High safety: LFP batteries have high thermal stability and low risk of ignition or explosion even under severe conditions such as overcharging or high temperatures. This makes them suitable for applications that require high safety, such as electric and hybrid vehicles.
- Long cycle life: LFP batteries have a high number of charge-discharge cycles, typically several thousand or more. Compared to other common lithium-ion batteries, LFP batteries have a superior cycle life.
- High energy density: The energy density of LFP batteries is somewhat lower than that of other lithium-ion batteries, but high enough to be suitable for many applications.
- Wide operating temperature range: LFP batteries can operate successfully in a wide temperature range from -20°C to 60°C and can be used under a variety of climatic conditions.
- Low environmental impact: LFP batteries have relatively low environmental impact because they do not use materials such as lead or cobalt, and iron phosphate is an inexpensive material with a stable supply.
<Issues for lithium-ion iron phosphate batteries>
Despite these various advantages of LFP batteries, the following issues remain
- Low energy density: LFP batteries have a slightly lower energy density than other common Li-ion batteries. This is due to the relatively stable crystal structure of iron phosphate, the cathode material of LFPs.
- High internal resistance: LFP batteries have relatively high internal resistance, which may limit their performance at high charge/discharge rates.
- Effect of self-discharge: LFP batteries generally have relatively high self-discharge, and their capacity tends to decrease over charge-discharge cycles.
- Material cost of iron phosphate: Although iron phosphate is less expensive than other common cathode materials (e.g., cobalt), material cost can still be an issue.
bipolar lithium-iron oxide battery
Toyota’s announcement of the “bipolar iron lithium-acid battery” approach has greatly improved on these issues.
<Bipolar Battery>
Bipolar batteries have a special structure in which the anode and cathode are located in the same area and the ions move directly through the electrolyte, whereas in conventional batteries, the anode and cathode are located separately and the ions move through the electrolyte. The bipolar battery has the anode and cathode in the same area, and the ions move directly through the electrolyte. This structure makes it possible to connect the battery inside the cell, whereas conventional cells were constructed and connected individually as shown in the figure below, and enables the battery to supply high current with low internal resistance, making it possible to use the battery in systems that require high output power, such as electric and hybrid vehicles. This makes it possible to connect the battery inside the cell.
Bipolar batteries generally require advanced technology and design, and their manufacturing costs can be high. While they help reduce internal resistance by shortening the distance that ions travel in the battery, they also present challenges such as reduced battery capacity and possible degradation.
<Bipolar lithium iron oxide batteries>
The bipolar lithium iron oxide battery is a lithium iron oxide battery with such a bipolar structure. In a bipolar lithium iron oxide battery, the anode and cathode are located in the same area, and this structure allows ions to move directly through the electrolyte, resulting in low internal resistance and the ability to supply a high current. In addition, the bipolar structure improves the utilization efficiency of the active materials in the electrodes, which has the advantage of improving the energy density and output characteristics of the battery.
Toyota plans to complete mass production of this type of battery by 2027, which will give a Corolla-size electric vehicle a range of about 400 km, at a price not much different from current hybrid vehicles, and with a service life of well over 1 million km.
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