scope:

Introduction

A reliable secondary (rechargeable) battery of high energy and power density is needed for a variety of new existing technologies. This introductory chapter is divided into two parts. Section 1.2 ("Design Considerations') introduces several definitions and the general considerations that make electrochemical cells using Li⁺ as the working ion the most promising elemental component for a secondary battery of high energy and power density. Section 1.3 ("Insertion of Lithium into Structures Containing Polyanions') illustrates structural and chemical considerations that may be useful for the design of (search for) a competitive cathode material. It compares lithium insertion into two types of transition-metal oxides: those having a close-packed oxide-ion array and those forming framework structures in which the oxide ions are replaced by the polyanions (SO₄)^2- (PO₄), (AsO₄)^3- and (P₂O₇)^4-. Of particular interest are (1) tuning of the transitionmetal redox energies through the inductive effect, (2) the trade-off between improved Li⁺ ion diffusion and polaronic electron conduction in open frameworks, (3) the use of mixed phases to buffer aganist overdischarging as exemplified by LiTi₂(PO₄)₃ mixed with LixFe2(SO₄)₃, (4) the identification and explanation of a reversible capacity fade at higher current densities, and (5) the identification of new cathode materials operating on the Fe3+/ Fez+ redox couple: the olivine Li_xFePO₄ with a flat Vα=3.5 V vs lithium and an Li_3+xFe₂(PO₄)₃a ving the rhombohedral NASICON structure with a closedcircuit voltage centered at V = 2 .8 V  and a capacity of 95 mAh-g^-1 at a current desity of 1 mA-cm^-2.