Composition-structure-property-performance relationship inMn-substituted LiMn2O4

Conference · Fri Mar 09 04:00:00 EST 2001

OSTI ID:860296

Horne, Craig R; Richardson, Thomas J; Gee, B; Tucker, Mike; Grush, Melissa M; Bergmann, Uwe; Striebel, Kathryn A; Cramer, StephenP; Reimer, Jeffrey A; Cairns, Elton J

The spinel LiMn{sub 2}O{sub 4} has been extensively studied as a positive electrode active material in lithium rechargeable batteries. Partial substitution of Mn by another metal has also been the subject of recent study in an effort to improve the cycling performance. In general, the literature has shown that Mn substitution results in improved cycling stability at the expense of capacity (1,2). Resistance to the formation of tetragonal phase upon lithiation of the starting spinel (via a higher nominal Mn oxidation state in the substituted spinel) has been suggested as a mechanism for the improved performance. The degree of substitution is an important factor to optimize in order to minimize capacity loss and costs. The spectroscopic investigations on LiMn{sub 2}O{sub 4} described in the previous paper (LixMn2O4) confirmed that the cooperative Jahn-Teller effect (CJTE) from the [Mn{sup 3+}O{sub 6}] octahedra is the mechanism for the cubic to tetragonal phase transformation. The driving force for the CJTE is based upon the electronic structure, therefore changes in electronic structure should lead to changes in the phase behavior. The fact that the LiMn{sub 1.5}Ni{sub 0.5}O{sub 4} does not form tetragonal phase upon discharging (FUJI3, MUCK?), unlike the 100% Mn{sup 4+} spinel Li{sub 4}Mn{sub 5}O{sub 12} (THAC5), led to the hypothesis that an increased degree of covalency as a source for the behavior. An increased covalence would remove the driving force for the transformation, the increased electronic stability achieved in tetragonally-distorted [Mn{sup 3+}O{sub 6}] octahedra, due to a change in electron density and widening of the Mn 3d bands. The STH field is dependent upon the amount of unpaired spin density transferred between the magnetic (transition-metal) and diamagnetic ions through an intermittent oxygen ion, attributable to overlap and electron transfer effects. Therefore, the magnitude of the STH coupling constant reflects the degree of covalency (GESC, HUAN). In the case of LiMn{sub 2-y}Me{sub y}O{sub 4}, the STH coupling constant characterizes the amount of unpaired spin density transferred to the Li{sup +} from the Mn, Co, or Ni. Similarly, the La/Lb ratio of the Mn L-XES is sensitive to the amount of electron density at the Mn site as a higher ratio indicates that the Mn 3d{sub 5/2} level is more populated (GRUS1). An investigation into the effects of Mn-substitution on the electronic structure along with the ramifications to the phase behavior upon changing lithium content was carried out. To accomplish this, a set of LiMn{sub 2-y}Me{sub y}O{sub 4} with Me = Li, Co, or Ni over a range of y were synthesized, characterized, and subjected to changes in lithium content by various techniques.

• Research Organization: Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
• Sponsoring Organization: USDOE Director. Office of Science. Office of Basic EnergySciences. Chemical Sciences Division
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 860296
• Report Number(s): LBNL--47601; BnR: KC0302040
• Country of Publication: United States
• Language: English

Related Subjects

25 ENERGY STORAGE
CAPACITY
COUPLING CONSTANTS
COVALENCE
ELECTRODES
ELECTRON DENSITY
ELECTRON TRANSFER
ELECTRONIC STRUCTURE
HYPOTHESIS
JAHN-TELLER EFFECT
LITHIUM
OXYGEN IONS
PHASE TRANSFORMATIONS
SPIN
SPINELS
STABILITY
VALENCE