The structure of the mineral olivine, taken by the cathode LiFePO₄. Strong covalent bonds in PO₄ make it thermally stable, but Li⁺ diffusion is one-dimensional and electronic conductivity is low.
Japanese name: オリビン型構造
Olivine structure appears in 1 practice question across 1 topic, including Battery and energy materials.
Field: Battery and energy materials
LiFePO₄ is orthorhombic (space group Pnma), with Fe²⁺ and Li⁺ in octahedral sites and P in tetrahedral sites within slightly distorted hexagonal close-packed oxygen. PO₄ tetrahedra link the FeO₆ octahedra, and the resulting framework is very robust. On the other hand, Li⁺ can use only the pathways running along the crystal's b axis (the [010] direction), so its diffusion is one-dimensional. A weakness of one-dimensional diffusion is that a defect in which Fe enters a pathway (an antisite defect) blocks the entire channel, so that the Li in it can no longer be used.
Charge and discharge of LiFePO₄ proceed by a two-phase reaction, in which the two phases LiFePO₄ and FePO₄ coexist while their proportions change. At constant temperature and pressure, when 2 phases coexist in a 2-component system, the Gibbs phase rule leaves no degrees of freedom, so the potential stays constant even as the Li content changes. The result is a very flat voltage curve at about 3.4 V. A stable voltage is convenient for device design, but it also makes it difficult to estimate the state of charge (SOC) from the voltage. The volume change between the two phases is about 6–7%.
LiFePO₄ has very low electronic conductivity (about 10⁻⁹ S/cm or less), and Li⁺ diffusion is one-dimensional, so as is it cannot be charged and discharged at high current. The keys to its commercialization were shrinking the particles to tens to hundreds of nm to shorten the distances Li⁺ and electrons must travel, and coating the particle surfaces with a thin carbon film to provide paths for electrons. Making plate-like particles that are thin along the b axis shortens the pathways further. With these improvements, capacities close to the theoretical value of about 170 mAh/g can be obtained even at relatively high currents.
In LiFePO₄ the oxygen is strongly covalently bonded to P, so even fully charged FePO₄ hardly releases oxygen at high temperature, making the material less prone to thermal runaway than layered oxide cathodes. Fe and P are abundant and inexpensive, and the absence of Co and Ni is another advantage. On the other hand, its voltage is low at about 3.4 V and its density is small, so its energy per unit volume and mass is lower than that of layered cathodes. It is therefore widely used where safety, life and cost matter most, such as stationary storage batteries, electric buses and some electric vehicles. Goodenough and coworkers reported it as a cathode material in 1997.
Answer: B. LiCoO₂ is layered rock salt, LiMn₂O₄ is spinel and LiFePO₄ is olivine
LiCoO₂ has the layered rock salt structure (α-NaFeO₂ type), with Li layers and Co layers alternating within close-packed oxygen, and Li⁺ moves two-dimensionally between the layers. LiMn₂O₄ is a spinel, with three-dimensionally connected Li⁺ pathways within its Mn–O framework and an operating voltage of about 4 V. LiFePO₄ has the olivine structure, the same as the mineral olivine, in which Li⁺ moves along one-dimensional tunnels in a framework of PO₄ tetrahedra and FeO₆ octahedra, with an operating voltage of about 3.4 V. The other two choices shuffle the names of these three structures. Layered, spinel and olivine are the three basic structures used to classify cathode materials.
It is a lithium-ion battery that uses olivine-type LiFePO₄ as the cathode. Its energy density is somewhat lower than that of batteries with layered oxide cathodes, but it excels in safety, cycle life and cost.
Because the oxygen in PO₄ is strongly covalently bonded to P, which makes the Fe–O bond more ionic and lowers the energy of the Fe³⁺/Fe²⁺ redox couple (the inductive effect). As a result, a voltage of about 3.4 V is obtained, higher than with simple iron oxides.
Yes. Olivine is a mineral of composition (Mg,Fe)₂SiO₄, and LiFePO₄ takes the same type of structure, so it is called olivine type. The gemstone peridot is also olivine.
LiFePO₄ has the olivine structure, the same as the mineral olivine, in which Li⁺ moves along one-dimensional tunnels in a framework of PO₄ tetrahedra and FeO₆ octahedra, with an operating voltage of about 3.4 V.
Which correctly pairs lithium-ion battery cathode materials…Layered rock salt structure Polymer electrolyte fuel cell Shockley–Queisser limit Solid electrolyte Spinel structure Theoretical capacity Thermoelectric figure of merit