Reversible extraction of lithium from
LiFePO4
(triphylite) and insertion of lithium into
FePO4
at 3.5 V vs. lithium at 0.05 mA/cm2 shows this material to be an excellent candidate for the cathode of a low‐power, rechargeable lithium battery that is inexpensive, nontoxic, and environmentally benign. Electrochemical extraction was limited to ∼0.6 Li/formula unit; but even with this restriction the specific capacity is 100 to 110 mAh/g. Complete extraction of lithium was performed chemically; it gave a new phase,
FePO4
, isostructural with heterosite,
Fe0.65Mn0.35PO4
. The
FePO4
framework of the ordered olivine
LiFePO4
is retained with minor displacive adjustments. Nevertheless the insertion/extraction reaction proceeds via a two‐phase process, and a reversible loss in capacity with increasing current density appears to be associated with a diffusion‐limited transfer of lithium across the two‐phase interface. Electrochemical extraction of lithium from isostructural
LiMPO4
(M = Mn, Co, or Ni) with an
LiClO4
electrolyte was not possible; but successful extraction of lithium from
LiFe1−xMnxPO4
was accomplished with maximum oxidation of the
Mn3+/Mn2+
occurring at x = 0.5. The
Fe3+/Fe2+
couple was oxidized first at 3.5 V followed by oxidation of the
Mn3+/Mn2+
couple at 4.1 V vs. lithium. The
Fe3+‐O‐Mn2+
interactions appear to destabilize the
Mn2+
level and stabilize the
Fe3+
level so as to make the
Mn3+/Mn2+
energy accessible.
State-of-the-art lithium (Li)-ion batteries are approaching their specific energy limits yet are challenged by the ever-increasing demand of today's energy storage and power applications, especially for electric vehicles. Li metal is considered an ultimate anode material for future high-energy rechargeable batteries when combined with existing or emerging high-capacity cathode materials. However, much current research focuses on the battery materials level, and there have been very few accounts of cell design principles. Here we discuss crucial conditions needed to achieve a specific energy higher than 350 Wh kg −1 , up to 500 Wh kg −1 , for rechargeable Li metal batteries using high-nickel-content lithium nickel manganese cobalt oxides as cathode materials. We also provide an analysis of key factors such as cathode loading, electrolyte amount and Li foil thickness that impact the cell-level cycle life. Furthermore, we identify several important strategies to reduce electrolyte-Li reaction, protect Li surfaces and stabilize anode architectures for long-cycling high-specific-energy cells.
To understand the role of structure on the position of the octahedral Fe37Fe2 redox couple in compounds having the same polyanions, four iron phosphates: Li3Fe2(P04)3, LiFeP2O7, Fe4(P207)3, and LiFePO4 were investigated. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The Fe3t/Fe2 redox couple in the above compounds lies at 2.8, 2.9, 3.1, and 3.5 eV, respectively, below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P-O bond lengths as well as to changes in the crystafline electric field at the iron sites.
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