Porous Li2MTi3O8 (M = Zn, Co) Flake Particles with Enhanced Lithium-Storage Properties

Compared with the Li₄Ti₅O₁₂ anodes, the spinel structure Li₂MTi₃O₈ (M = Zn, Co, Mg, Cu) has recently attracted increasing attention for LIBs (lithium batteries) because of its high specific lithium storage capacity (>200 mA h g^-1) and cycling stability.^{[1, 2]} In this paper, we reported a novel synthetic method of porous Li₂MTi₃O₈ (M = Zn, Co) flake particles (f-Li₂MTi₃O₈) by a one-step solution-combustion. In relation to the conventional synthesis methods, this route is easy to control with  less laborious procedures and no need for certain precursor. Moreover, the combustion synthesis in liquid phase ensures excellent homogeneity of the product with a high surface area.

As shown in Figure 1, both of the Li₂MTi₃O₈ samples prepared via the combustion method have flake structure and its secondary particles compose of nanometer-size primary particles with embedded nano and micro pores. These pores are in-site obtained due to concomitant formation of Li₂MTi₃O₈with vigorous evolution of large volume of gases during the combustion reaction.

To evaluate the cycleability of the f-Li₂MTi₃O₈ (M = Zn, Co) flake particles electrodes, a charging and discharging current density of 100 mA g^-1 was used. Both of the samples exhibited excellent cycle stability (Figure 2). For the f-Li₂ZnTi₃O₈ electrode, a capacity of 192 mA h g^-1 could be retained after 200 cycles with a capacity retention of 81%. In the case of the f-Li₂CoTi₃O₈ electrode, it delivered 201 mA h g^-1with a high capacity retention of 89% after 200 cycles.

To gain more electrochemistry properties information on the materials, the rate performance tests were carried out. Figure 3 shows the rate capabilities of  the synthesized f-Li₂ZnTi₃O₈ and f-Li₂CoTi₃O₈ electrodes at different current rates and their rate capacities at  200th cycle was summarized in Table 1. As observed from these data, both of the two samples show the considerable cycle stability even at high current rates.  And the f-Li₂CoTi₃O₈ electrodes showed considerably higher reversible capacities and more excellent capacity retention than the f-Li₂ZnTi₃O₈ electrodes irrespective of the rate used. Interestingly, more than 100 mA h g^-1 can be retained after 200 cycles even at the current rate of 2000 mA g^-1. The excellent rate performance may be attributed to the special morphology and structure.

These results indicate that the as-prepared anodes with spinel flake particles have a large reversible capacity and high rate performance, which can be attributed to the  3D porous flamework structures and their intrinsic characteristics. This special 3D porous flamework structure could provide a diffusion space for lithium-ion storage and extraction from the material. In addition, the porous flamework composed of several tens of nanometers greatly decreased the diffusion distance for lithium ions and electrons in the solid state. Thus, we believe that such a facile solution-combustion method could be applied in a wide range of fields and the f-Li₂MTi₃O₈(M = Zn, Co)  could be promising anode materials for high-rate lithium batteries.

Acknowledgment: High Technology Research and Development Program of China (2012AA110204), the Fujian Natural Science Foundation (2012J05028) and Electric Vehicle Project (I) of Xiamen (3502J20121002). Qian Xiao expresses her special thanks for National Found for Fostering Talents of Basic Science (J1210014). The authors also wish to express their thanks to Dr. Bo Liu for valuable suggestions.

Figure 1. SEM images of (a) f-Li₂ZnTi₃O₈ and (b) f-Li₂CoTi₃O₈.

Figure 2. Cycle performance for (a) f-Li₂ZnTi₃O₈ and (b) f-Li₂CoTi₃O₈ electodes for 200 cycles at low capacity rate of 100 mA g^-1.

Figure 3. Capacity versus cycle number plots for (a) f-Li₂ZnTi₃O₈ and (b) f-Li₂CoTi₃O₈electrodes for 200 cycles at different discharge current rates.

Table 1 The capacities for Li₂ZnTi₃O₈ and Li₂CoTi₃O₈ electrodes after 200 cycles at different discharge current rates.

 

Rate(mA g-1)	400	800	1600	2000
f-Li2ZnTi3O8(mA h g-1)	164	100	74	73
f-Li2CoTi3O8 (mA h g-1)	175	161	141	101

References

 

[1]. N.Reeves, D.Pasero,A.R.West,Journal of Solid State Chemistry. 2007,180, 1894.

[2].  L.Wang, L.J. Wu,  Z.H. Li, G.T. Lei, Q.Z. Xiao, P. Zhang, ElectrochimicaActa. 2011,56, 5343.