Lithium iron phosphate

Key takeaways

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Reference excerpt

Lithium iron phosphate (LFP) is an inorganic compound with the formula LiFePO4. It is a gray, green, or black solid that is insoluble in water. Lithium iron phosphate is used as a positive electrode material (cathode) of lithium iron phosphate batteries, a type of Li-ion battery. The negative electrode (anode) material for these batteries is typically graphite. This battery chemistry is used in power tools, electric vehicles, solar energy installations and large grid-scale energy storage. Lithium iron phosphate exists naturally in the form of the mineral triphylite, but as found naturally, this material has insufficient purity for use in batteries. Thus, battery-grade LFP powder must be synthesized.

History Arumugam Manthiram and John B. Goodenough first identified the polyanion class of cathode materials for lithium ion batteries. LiFePO4 was then identified as a battery cathode material in this class in 1996 by Padhi, Nanjundaswamy, and Goodenough. This work demonstrated reversible extraction of lithium from LiFePO4 and insertion of lithium into FePO4. Subsequent studies demonstrated the synthesis of nanosized and carbon-coated LFP particles, which are correlated with increased capacity and cycle stability. See the Commercialization and Intellectual Property section for the history of LFP's commercialization. As of 2021, China produced over 90% of the world's LFP.

Synthesis and production LiFePO4 may be synthesized by a variety of methods, including: solid-state synthesis, emulsion drying, sol-gel process, solution coprecipitation, thin-film deposition, mechanochemical activation, microwave-assisted synthesis (in which microwave energy is applied to accelerate chemical reactions, resulting in reduced synthesis time and energy consumption), hydrothermal synthesis, (ultrasonic) spray pyrolysis, and molten state synthesis. Of these, solid-state synthesis (ceramic method) is the most commonly employed in industrial level production. In a typical synthesis, iron(III) oxalate is reacted with ammonium dihydrogen phosphate ((NH4)(H2PO4)), diammonium hydrogen phosphate ((NH4)2(HPO4)), or phosphoric acid (H3PO4) to synthesize iron(III) phosphate. Iron(III) phosphate is mixed with lithium carbonate (or another lithium source, such as lithium hydroxide), and a carbon source (such as glucose, sucrose or starch, see Carbon coating below), and calcined at 700–800°C. Other lithium, iron, and phosphate precursors can also be used, including iron(II) sulfate and iron(III) oxalate. The hydrothermal synthesis routes is also industrially relevant, with main advantages being its low cost and ability to perform carbon-coating in one step.

Role of particle size LFP nanoparticles were found to have increased capacity and rate performance compared to larger particles. The particle size was found to have an important influence on the electrode resistance and discharge capacity. Nanosized LFP particles were also found to undergo single-phase Li extraction, which could have important implications for power density.

Carbon coating Coating LFP particles with a thin layer of carbon can be achieved during synthesis via carbothermic reduction. In addition to forming a conductive coating on the surface of otherwise electronically insulating LFP particles, the reductive synthesis environment also suppresses the formation of Fe(III) which can impede Li diffusion within the crystalline lattice. Carbon coating thicknesses of 1-10 nm are typical for increasing the conductivity of particles but allowing for Li intercalation.

Metal substitution and coating Coating LFP with inorganic oxides can also improve the electrochemical performance of LFP. Substituting other metals for the iron or lithium in LiFePO4 can also improve performance.

Physical and chemical properties

Structure of LiFePO4 In LiFePO4, lithium has a +1 charge, iron has a +2 charge (ferrous), and phosphate ion carries a −3 charge, balancing the charges. The iron atom and 6 oxygen atoms form an octahedral coordination sphere, described as FeO6, with the Fe ion at the center. The phosphate groups, PO4, are tetrahedral. The three-dimensional framework is formed by the FeO6 octahedra sharing O corners. LiFePO4's corner-sharing FeO6 octahedra are separated by the oxygen atoms of the PO3−4 tetrahedra and cannot form a continuous FeO6 network, reducing electronic conductivity. Lithium ions are octahedrally coordinated by O, and were shown to migrate within one-dimensional channels in the framework in a zigzag manner via neutron diffraction. A nearly close-packed hexagonal array of oxide centers provides relatively little free volume for Li+ ions to migrate within. For this reason, the ionic conductivity of Li+ is relatively low at ambient temperature. In crystallography, the crystal structure belongs to the Pmna space group of the orthorhombic crystal system. The lattice constants are approximately a = 10.33 Å, b = 6.01 Å, and c = 4.69 Å, giving a unit cell volume of 291.4 Å3.

Structure of FePO4 During charge, the lithium ions are extracted concomitant with oxidation of Fe(II) ions to Fe(III):

LiFeIIPO4 → FeIIIPO4 + Li+ + e− Extraction of lithium from LiFePO4 produces FePO4 with a similar structure. FePO4 adopts a Pmna space group with a unit cell volume of 272.4 Å3, an approximately 7% decrease compared to that of its lithiated precursor; extraction of lithium ions reduces the lattice volume. Upon discharge, the lithium ions are reinserted to the host lattice, and Fe(III) is reduced to Fe(II): FeIIIPO4 + Li+ + e− → LiFeIIPO4 This reversible reaction gives rise to a theoretical gravimetric (specific) capacity of 170 mAh/g, and energy density 540 Wh/kg.

Mechanism of (de)lithiation The phase diagram of Li1-xFePO4 was determined by Dodd, Yazami and Fultz, as well as by Delacourt et al. The phase separation between the lithiated LiFePO4 phase and delithiated FePO4 phase results in a wide compositional miscibility gap, and a flat voltage curve when LiFePO4 is used in a battery. The details of the phase separation mechanism have been investigated extensively. At high current rates on discharge, phase separation is suppressed, which could provide insight into the material's high power density.

Applications

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Illustrations

Worked examples

Example 1 — a first encounter with Lithium iron phosphate

Start with the simplest possible case. Write down what Lithium iron phosphate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lithium iron phosphate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lithium iron phosphate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lithium iron phosphate

In research: Lithium iron phosphate appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lithium iron phosphate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lithium iron phosphate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Iron(II) compounds, Lithium compounds, Phosphates, so understanding it makes those chapters shorter.
In everyday life: Look for Lithium iron phosphate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Lithium iron phosphate in 20 minutes

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Frequently asked questions

What is Lithium iron phosphate in simple terms?

Lithium iron phosphate (LFP) is an inorganic compound with the formula LiFePO4. It is a gray, green, or black solid that is insoluble in water.

Why does Lithium iron phosphate matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lithium iron phosphate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lithium iron phosphate.