An Innovative and Environmentally Friendly Lithium Ion Battery Configuration for High Power Applications with Nano-Sized ZnFe2O4-C As Anode Active Material

Since their first commercialization in 1991, the lithium-ion battery (LIB) market has continuously grown. Only in the past few years, the development of new materials has led to remarkable enhancements of both energy and power density. Such advances have enabled the employment of LIB beyond the consumer electronics field, for example in the automotive sector. Unfortunately, the high power density demanded by such applications requires the electrode kinetics to be further improved. Several nano-sized materials have been proposed to facilitate Li insertion and, thus, allow a rapid battery cycling. However, while the scientific literature abounds of studies performed in half-cell, only few reports are available on full lithium ion batteries with power-oriented features. With this regard, Li₄Ti₅O₁₂/LiCoO₂ and Li₄Ti₅O₁₂/LiFePO₄ are the most common configurations. Promising results have been recently reported by combining an alternative Sn-C-based anode with, respectively, LiNi_0.5Mn_1.5O₄ and carbon-coated LiFePO₄ cathodes^1,2.

Differently from these previous works, the battery configuration here proposed employs a multi-walled carbon nanotube-LiFePO₄ composite cathode (LiFePO₄-CNT) and a highly performing carbon-coated ZnFe₂O₄ (ZnFe₂O₄-C) nanoparticle-based anode (see Figure 1a). Due to the combination of conversion and alloying mechanism, ZnFe₂O₄-C can reversibly deliver capacities higher than 1000 mAh g^-1 in a potential range of 3-0.01 V vs. Li/Li⁺, with remarkable rate capability³. Here, by electrochemically pre-doping the anode with defined amounts of lithium, we were able to tune the negative electrode potential window and, therefore, the overall lithium battery voltage. Our attention was principally focused on the investigation of the effect of the negative electrode insertion mechanism on the overall cell performance. As shown in Figure 1b, the pre-doping is an effective strategy for increasing the cell discharge voltage from 1.58 V (fully delithiated anode) to 2.12 V (pre-doped anode). This results on an enhancement of the battery specific energy of 37% (from 148 Wh kg^-1 to 202 Wh kg^-1, considering the overall active material weight on both electrodes). The negative electrode lithiation degree does not substantially affect the rate performances of the battery, which is always capable of delivering more than 50% of the initial capacity at the highest rate of 6 mA cm^-2 (corresponding to about 5C and 20C with respect to ZnFe₂O₄ and LiFePO₄, respectively). On the other hand, as displayed in Figure 1c, the pre-doping grants a Li reservoir which is beneficial in terms of cycling stability. In fact, the full cell employing the pre-doped anode can deliver 85% of the initial capacity even after 10000 cycles at 3 mA cm^-2 (ca. 10C for LiFePO₄).

In conclusion we demonstrate, as a proof of concept, the great potential of the ZnFe₂O₄-C/LiFePO₄-CNT combination. In addition to the fact that only electrodes were employed, which were prepared with environmentally friendly water-based binder (Na-carboxymethyl cellulose), more remarkably, this new lithium-ion full-cell provides advanced high power performance (up to 3.72 kW kg^-1) and excellent cycling stability⁴.

 

References

1. G. Derrien et al., Advanced Materials, 19, 2336–2340 (2007)

2. S. Brutti et al., Journal of Power Sources, 217, 72–76 (2012)

3. D. Bresser et al., Advanced Energy Materials, 3, 513–523 (2013)

4. A. Varzi et al., Advanced Energy Materials, in press (2014)