Li-Mn-0 spinels

The composition of lithium-manga-nese-oxide spinel electrodes that are of interest for lithium battery applications fall within the Li[Mn2]04 - Li4Mn5Ot2 -Li2[Mn4]0() tie-triangle of the Li-Mn-0 

By contrast, lithium extraction from the tetrahedral sites in Li[Mn2]04, i.e., for 0 xd inLiA[Mn2]04, takes place at 4 V, with retention of the cubic symmetry of the spinel structure [105, 114, 120]. It is difficult to extract all the lithium electrochemi-cally from Li[Mn2]04, at least at practical voltages, without causing decomposi- 

Prior to the evaluation of Li[Mn2]04 as a rechargeable cathode material, the ideal spinel framework [Mn2]04, (commonly referred to as A — Mn02, after Hunter) was chemically synthesized by acid digestion of Li[Mn2]04 [121]. The formation of A— Mn02 by chemical methods differs from the electrochemical reaction because it dissolves 25 percent of the Mn cations from the original spinel framework  

There are distinct differences in the electrochemical behavior of lithium cells constructed with /1-Mn02 electrodes prepared by acid treatment and those containing Li[Mn2]04 electrodes [120].Cells with A-Mn02 electrodes show an essentially featureless voltage profile at 4V on the initial discharge on subsequent cycling, the cells show a profile more consistent with that expected from an Li[Mn2]04 electrode. 

Although LiJMn2]04 appears to have the ideal structure for an insertion electrode in 4V lithium cells, the cells lose capacity slowly when operated over the high voltage range. Several reasons have 

---

Experience with Li4Mri50 2 showed that 3V Li-Mn-0 spinel structures can be stabilized (with respect to lithium inser-tion/extraction) by cation substitution to increase the manganese oxidation state in fully charged electrodes to Mn . This concept was recently extended to the spinel Li[Mn,5Ni(,5]04 in which all the manganese ions are tetravalent if the structure is synthesized with in the spinel... 

Figure 3. Charge and discharge curves of an LiA.i[Lio.iMn, y]04 cell cycled at voltages between 3.0 and 4.4V at a rate of 0.1 mAcm at 30 °C. To describe the composition of an Li-Mn-0 ternary phase a defect-spinel formulation was assumed.

Unlike cobalt and nickel, manganese does not form a stable, pure LiMn02 phase and forms the spinel structure instead, named after the mineral of spinel (Table 1.3) with the composition of Lio.5Mn02, conunonly expressed as LiMn204. Manganese-based cathode materials are attractive mainly due to their low cost compared to cobalt and nickel. A wide variety of the hthiated manganese oxides with multiple structures and compositions (Li Mn 0 ratios) exist and many of these compounds can reversibly intercalate lithium, but amongst these materials, spinel materials and the layered derivatives are the most mature, to the point of commercial availability. Spinel-stractured materials will be discussed separately in the next part of this chapter. 

Lithium manganese oxide spinel belongs to a different structure class (Table 1.3, Fig. 1.5) from the previously discussed layered LiM02 materials (M = Co or Ni). The molecular formula of lithium manganese oxide spinel, commonly expressed as LiMn204 is based on atomic ratios of 1 2 4 (Li Mn 0). 

According to XPS [98], the Li/Mn ratio on the surface of particles of Mn02+Li0H mixture activated for 1 min is twice as large as that after 10 min activation. But after 10 min of mechanical activation, the Li/Mn ratio is still 1.6 times larger than in Merck spinel (see Table 6.2), i.e. the surface layer of particles in activated mixtures is enriched by amorphous LiOH. On contrary, for 1 min activated mixture Mn02+Li2C03, Li/Mn ratio is equal to 0.8. The ratio decreases as the activation time increases. 

Figure 3 Ex situ XRD profiles of spinel-based electrodes after cycling or storage at ambient temperature. Dots Li Mn O reflections diamonds A1 current collector. (A) Discharged to 2.4 V (B) pristine LiMrr,0 (C) charged to 4.3 V (D) cycled 50 times at C/3 (E) stored for 300 h at OCV. Reproduced Irom [32] with permission of The Electrochemical Society Inc.

Figure 8. Formation energy versus Li concentration for three structures of Mn oxide (top) and Co oxide (bottom) ( ) 5-LiJM204-labeled spinel, (0) 7-LiJMn02-labeled layered, and (+) partially inverse spinel structure with 1/4M tetrahedral (ps-(LiJM)tet(LijM3)oct08 0 < x< 1 and 0 < y < 2) labeled 1/4 Mn tet. As the Li content is increased, the Li is added to the tetrahedral site first of p5-(LiJM)ter (LijM3)octOs, and then to the octahedral sites. For Mn, there also is the energy of (a) a structure with one-sixth of the Mn in tetrahedral sites at Xu =1/3 labeled 1/6 Mn tet with a triangle data point and (x) a structure with one-eighth of the Mn in tetrahedral sites at Xu =1/4 labeled 1/8 Mn tet.

---