Tuning the transport and thermodynamic properties of isovalently substituted olivines for electrochemical applications.

摘要

Lithium-ion batteries are a vital component to contemporary lifestyles which rely heavily on mobile devices. The key characteristic of these lithium-ion batteries is the reversible intercalation of lithium ions into anode and cathode materials. As the global demand for these devices grows a strain will be placed upon the supply of the currently used cathode and anode materials, pointing to the need for less expensive materials to be developed. Furthermore, new mobile applications such as hybrid electric vehicles demand the development of high power cathode and anode materials.;Olivine lithium iron phosphate's value as a cathode material in lithium ion batteries is well known. However, functional questions persist concerning its electronic conductivity and ionic diffusivity. In this work these questions are addressed by synthesizing olivines with controlled ratios of metal cations. The effect of this substitution on reaction mechanism was investigated via galvanostatic intermittent titration and high energy X-ray diffraction. It will also be shown that isovalent substitution of the iron site will stabilize a solid solution throughout galvanic cycling. This single phase stabilization allows for the determination of the lithium diffusion coefficient at various states of charge (SOC). The lithium diffusion coefficient was found to be directly related to the SOC and to range between 10-7 and 10 -8 cm2/s.;Lithium manganese phosphate is a desirable cathode material since its redox potential is 0.6 V higher than that of lithium iron phosphate. Both of these phosphates remain olivine after the electrochemical extraction of lithium. The delithiated iron olivine is stable up to 600°C, which is well above the operating temperature of most batteries. Above 600°C, the octahedrally coordinated iron irreversibly transitions to a tetrahedral coordination, rendering the material electrochemically inactive. The delithiated manganese olivine, on the other hand, decomposes to Mn2P2O 7 and O2 at 200°C, raising questions concerning the viability of LiMnPO4 as a cathode material. In this study it will be shown via variable temperature powder X-ray diffraction that olivine manganese phosphate can be stabilized with iron substitution. It was found that 10% of iron substitution raised the decomposition temperature to 350°C. The single crystal X-ray diffraction, magnetic, and electrochemical studies of the first reported copper substitution of the olivine structure is also discussed.