Combined X-ray powder diffraction and DFT calculation to elucidate molecular and crystal structure of fluorostyrenes

Structure and Structure-Related Electrochemical Properties of Li_xMn_2-xTi_xO₄ (0.2 £ x £1.5) Spinels

K. Petrov¹, R-M. Rojas², J-M. Rojo², J-M. Amarilla², M. G. Lazarraga², L. Pascual²

¹Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev street, bl 1, 1113 Sofia, Bulgaria

²Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Cantoblanco, 28049 Madrid, Spain

Series of lithium manganese titanium spinel-like oxides LiMn_2-xTi_xO₄ (0.2 < x £ 1.5) were synthesized and characterized by X-ray powder diffractometry, thermal analysis, EPR spectroscopy and electrochemical methods. Compounds whose composition falls within the interval 0.2 < x £ 1.0 crystallize in the space group Fd3m, whereas those having x > 1.0 crystallize in the space group F4₃32. In some earlier studies it was claimed that the 8a sites in LiMnTiO₄ have mixed occupancies (Li0.75Ti0.25) [1, 2]. Data from XRD Rietveld structural refinement revealed that, in the whole compositional interval studied, the tetrahedral sites of the spinel-like structure are shared by Li⁺ and Mn²⁺, the partial occupancy of the latter increasing from 0.017 to 0.495 with increasing x from 0.2 to 1.5. The end member LiMn_0.5Ti_1.5O_4,synthesized in argon atmosphere, is reported for the first time. It crystallizes (see Table 1 and Table 2) in the cubic space group F4₃32, a = 8.437 Å.

Table 1. Structural parameters of LiMn_0.5Ti_1.5O₄, space group P4₃32, a = 8.4369(1) Å

Mn2+	8c	0.0014(2)	0.0014(2)	0.0014(2)	0.65(5)	0.495(4)	Rwp=0.131
Li+	8c	0.0014(2)	0.0014(2)	0.0014(2)	0.65(5)	0.505(4)	Rp=0.095
Ti4+	12d	0.125*	0.3698(2)	0.8802(2)	0.66(3)	1.00	Rexp=0.113
Mn3+,Mn4+	4b	0.625	0.625	0.625	0.77(4)	0.010(4)	Χ2=1.38
Li+	4b	0.625	0.625	0.625	0.77(4)	0.990(4)	GOF=1.17
O2-(1)	8c	0.3892(4)	0.3892(4)	0.3892(4)	1.13(7)	1.0	RF=0.027
O2-(2)	24e	0.1032(5)	0.1266(5)	0.3909(5)	1.23(7)	1.0	RB=0.038

Table 2. Selected tetrahedral (L_A)8c, average tetrahedral <L_A> , octahedral (L_B)4b, octahedral (L_B)12d and average octahedral - <L_B> bond lengths for LiMn_2-xTi_xO₄ (1.2 < x < 1.5), space group P4₃32.

L_A– O(1)

(8c)

L_A– O(2)

(8c)

<L_A>

(8c)

L_B– O(2)

(4b)

L_B– O(1)

(12d)

L_B– O(2)

(12d)

L_B–O(2)

(12d

<L_B>)

1.2

2.054(4)

1.996(3)

´ 3

2.010

2.069(3)

2.010(3)

´ 2

1.936(3)

´ 2

1.985(3)

´ 2

1.977

1.3

2.048(2)

2.002(2)

´ 3

2.014

2.104(2)

2.022(2)

´ 2

1.909(2

´ 2

1.983(2)

´ 2

1.971

1.5

2.040(4)

2.010(4)

´ 3

2.018

2.139(2)

2.041(2)

´ 2

1.884(3)

´ 2

1.984(2)

´ 2

1.970

The refined positional parameters for LiMn_0.5Ti_1.5O₄are in a good agreement with those reported earlier for several iso-structural spinel-like oxides with a general formula Li_1-yM_y[(Li_yM_0.5-y) Ḿ_1.5]O₄, (M = Mg, Co, Zn; Ḿ = Ti; y » 0.5) and (M = Co, Zn; Ḿ = Ge; y » 0.5) [3, 4]. Refined site occupancies indicate clearly that Li and Mn atoms share the 8c tetrahedral sites with almost equal probability. Practically complete 1:3 ordering is observed in the octahedral (4b and 12d) sites, occupied by Li and Ti, respectively. The observed average metal-oxygen bond lengths are in excellent agreement with those calculated additively from the corresponding Shannon ionic radii. The values of the calculated BVS prove unambiguously that 4b and 12d sites are occupied exclusively by Li⁺ and Ti⁴⁺, correspondingly, supporting convincingly the validity of the refined structural model. The electrochemical performance in the 4 V region of the compounds used as cathode materials in lithium cells has been studied as a function of composition. A gradual loss of capacity with increasing x from 0.2 to 0.8, followed by an abrupt fall at x=1.0 was observed for the first charge. Charged samples undergo an irreversible phase transformation from spinel to defect rock-salt type structure, which explains the observed poor cycling behaviour after the first charge. Electrochemical data about charge/discharge capacities in the 3 V and 4V region have been used to estimate the Mn³⁺ content in the formula unit of the pristine cathode materials.

[1] B. Krutzsch and S. Kemmler-Sack, J. Less-Comm. Met., 124(1986)111-123.

[2] M. A. Arillo, M. L. López, M. T. Fernández, C. Pico, M. L. Veiga, A. Jiménez-López, E. Rodríguez-Castellon, J. Alloys and Compounds, 317-318(2001)160-163

[3] Hiroo Kawai, Mitsuharu Tabuchi, Mikito Nagata, Hisashi Tukamoto and Anthony R. West, J. Mater. Chem, 8(5)(1998)1273-1280

[4] V. S. Hernandez, L. M. T. Martinez, G. C. Mather and A. R. West, J. Mater. Chem., 6(1996)1533-1536.

Acknowledgements

The financial support of MECD, Spain, grant SAB: 2002-0002, is gratefully acknowledged by one of the authors (K. P.).