What is LiFePO4 in lithium batteries??
Introduction to Lithium Iron Phosphate Materials
Lithium iron phosphate (molecular formula LiFePO₄, lithium iron phosphate, LFP, also known as lithium iron phosphate or ferrous lithium phosphate) is a cathode material used in lithium-ion batteries. Its characteristics are that it does not contain precious elements such as cobalt or nickel, the raw material price is low; and carbon, lithium, and iron are abundant in the earth's crust, which can meet the market demand of more than one million tons per year. As a cathode material, lithium iron phosphate has a moderate working voltage (3.2 V), high specific capacity (170 mA·h/g), high discharge power, fast charging capability, and long cycle life, with good stability in high-temperature and high-heat environments.
The lithium iron phosphate crystal belongs to one type of olivine structure. In mineralogy, it is named triphylite, derived from the Greek word roots tri and lylon. In ores, the color can be gray, reddish-brown gray, brown, or black, while actual products are black or gray-black. Certain natural mineral materials contain lithium iron phosphate, but the grade is low and does not reach the level of practical application. Lithium iron phosphate belongs to the composite phosphate category, and its general chemical formula should be LiMPO₄, where M can be any divalent metal, including Fe, Co, Mn, Ti, etc. Because the first company to commercialize LiMPO₄ produced lithium iron phosphate, people have become accustomed to treating lithium iron phosphate as the only composite phosphate cathode material. However, for compounds with the olivine structure, lithium iron phosphate is not the only one that can be used as a cathode material in lithium-ion batteries. According to current knowledge, there are also LiMnPO₄, LiMnFePO₄, LiVPO₄, LiCoPO₄, and many other materials.
The origin of lithium iron phosphate materials can be traced back to 1996, when Japan's telecommunications company NTT first discovered that AMPO₄ (A is an alkali metal, M is Co or Fe) with olivine structure, in the combination of LiFeCoPO₄, can be used as a lithium-ion battery cathode material. Subsequently, it was discovered by the Goodenough research group at the Massachusetts Institute of Technology in the United States, while studying framework compounds, that lithium iron phosphate material has the reversible property of lithium-ion (Li⁺) intercalation and deintercalation. On April 23, 1997, the University of Texas at Austin filed a patent titled "Cathode materials for rechargeable lithium secondary batteries" (WO1997010541), marking the beginning of the patent monopoly on lithium iron phosphate materials.
The simultaneous publication of olivine-structured phosphate (LiMPO₄) cathode materials by the United States and Japan attracted great attention, triggered extensive research, and rapidly advanced the industrialization process. Compared with traditional lithium-ion secondary battery cathode materials-spinel-structured lithium manganese oxide (LiMn₂O₄) and layered-structured lithium cobalt oxide (LiCoO₂)-LiMPO₄ has more widely available and cheaper raw materials with no environmental pollution. In particular, safety has been greatly improved, arousing great interest from researchers and industry.
According to research results in recent years, lithium iron phosphate material possesses a well-crystallized olivine structure, and its lithium-ion diffusion channels differ from those of traditional cathode materials. Traditional cathode materials have layered or spinel structures, allowing lithium ions to move rapidly between layers or in larger channels, thus endowing the materials with good discharge performance. In contrast, the lithium-ion diffusion channels in lithium iron phosphate materials are one-dimensional, meaning that within the crystal there is only a "tunnel" for lithium-ion diffusion, so the lithium-ion migration rate is relatively slow and the diffusion distance is short. Especially under high-rate discharge conditions, the internal lithium ions cannot migrate out in time, resulting in significant electrochemical polarization.
Batteries can be fabricated using pure lithium iron phosphate material to verify the above conclusions. Experiments have shown that the capacity utilization of pure lithium iron phosphate material is very low, and the battery experiences rapid capacity decay during cycling. Figure 2.1 shows the cycling performance of a lithium-ion coin cell made by the author using hydrothermally synthesized pure lithium iron phosphate (without carbon coating). It can be seen that after approximately 15 charge–discharge cycles, the battery capacity has decayed by more than 20%. Therefore, pure lithium iron phosphate material is not suitable for lithium-ion battery systems.
In 2000, Hydro-Québec (H-Q), Canada's national public utility, was the first to file patents on coating lithium iron phosphate with conductive materials, including the use of carbon coating on lithium iron phosphate materials. This enabled lithium iron phosphate to achieve high specific capacity and extended its cycle life to more than 2000 cycles. This marked the beginning of the industrialization process of lithium iron phosphate as a cathode material.
