Dimensionally-Stable Lithium Insertion Material of LiCoMnO4 for Long-Life Lithium-Ion Batteries

Abstract

Lithium insertion materials of LiMO2 (M; transition metal ions) change in their lattice dimensions during lithium insertion and extraction reactions (charge and discharge) due to change in ionic radius of transition-metal ions. Dimensional instability of lithium insertion materials resulting from change in lattice dimension causes particle fracture which leads to a deterioration of lithium insertion electrodes. One of the approaches to achieve long cycleability of lithium insertion electrodes is to minimize change in lattice dimension during charge and discharge. The zero-strain lithium insertion material of the lithium titanium oxide (Li[Li1/3Ti5/3]O4; LTO) is one of the ideal electrode materials for long-life applications, because it shows no change in lattice dimension during charge and discharge. LiCo1/3Ni1/3Mn1/3O2, in which lattice volume is constant in the charge capacity below 180 mAh/g, exhibits excellent cycleability. In this paper, we report dimensionally-stable positive electrode material of LiCoMnO4, which is high operating voltage at ca. 5 V with small change in lattice dimension (< 1 %). Highly-crystallized LiCoMnO4 was prepared by a two-step solid-state reaction, i.e., crystallization at a temperature above 900oC and oxidation below 700oC. When a Li / LiCoMnO4 cell was cycled in the voltage range in 3.0 – 5.4 V, discharge capacity was ca.120 mAh g-1 for the initial 3 cycles, which is 83% of the theoretical capacity (145 mAh g-1). In order to examine the change in crystal structures of the LiCoMnO4, the ex-situ XRD examination was carried out. A cubic lattice parameter of LiCoMnO4 contracted during charge, 8.06 Å at initial state and 8.00 Å at fully charged state. The change in lattice parameter of LiCoMnO4 was calculated to be 0.7 %, which is smaller than other lithium insertion material having spinel structure (ca. 2 %). To confirm dimensional stability of a LiCoMnO4 electrode, dilatometry of a LTO/LiCoMnO4 cell by means of high-precision dilatometer was carried out. Figure shows the dilatometric signal together with the cell voltage as function of time. Since the LTO negative electrode is a zero-strain lithium insertion electrode, a change in thickness of the cell corresponds to that of LiCoMnO4 electrode. The LiCoMnO4 electrode contracted during charge and expanded during discharge and a change in electrode thickness was only 0.5 % based on initial electrode thickness, which agrees well with the change in lattice dimension of LiCoMnO4. From these results, we will describe crystal structure and electrochemistry of dimensionally-stable material of LiCoMnO4 and discuss its reaction mechanism with emphasis on dimensional change. Figure 1