The next generation of lithium based batteries can be expected to be based on lithium alloy forming anode materials which can store up to ten times more charge than the currently used graphite anodes. This increase in the charge storage capability has motivated significant research towards the commercialization of anode materials such as Si, Sn and Al. These alloy forming anode materials are, however, known to exhibit significant capacity losses during cycling. These are typically ascribed to the volume expansion associated with the formation of the lithium alloys (the volume expansion is e.g. about 280 % for Li_3.75Si) resulting in electrode pulverization as well as continuous solid electrolyte interphase (SEI) layer formation [1-3]. While significant progress has been made to decrease the volume expansion problems by the use of e.g. nanoparticles, nanorods and thin films, and/or capacity limitations [1-3], capacity losses are still generally seen [4,5]. This and previously published data suggest that the phenomenon may be due to another effect, possibly as a result of lithium trapping in the electrodes [6-8].

In the present work it is demonstrated (based on e.g. elemental analyses of cycled Sn, Al and Si electrodes) that lithium trapping can account for the capacity losses seen when alloy forming anode materials are cycled versus lithium electrodes, see Figure 1. It is shown that small amounts of elemental lithium are trapped within the electrode material during the cycling as a result of a two-way diffusion process [8] causing the lithium to move into the bulk material even during the delithiation step. This phenomenon, which can be explained by the lithium concentration profiles in the electrodes, makes a complete delithiation process very time consuming. As a result of the lithium trapping effect, the lithium concentration in the electrode increases continuously during the cycling. The experimental results also show that a similar effect can be seen also for commonly used current collector metals such as Cu, Ni and Ti. The latter means that these metals are unsuitable as current collector materials for lithium alloy forming materials in the absence of a thin layer of boron doped diamond serving as a lithium diffusion barrier layer [8].

References

1 M. N. Obrovac and V. L. Chevrier, Chem. Rev., 2014, 114, 11444.

2 X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B. W. Sheldon and J. Wu, Adv. Energy Mater., 2014, 4, 1300882.

3 J. R. Szczech and S. Jin, Energy Environ. Sci., 2011, 4, 56.

4 G. Zheng, S. W. Lee, Z. Liang, H-W. Lee, K. Yan, H. Yao, H. Wang, W. Li, S. Chu and Y. Cui, Nat. Nanotechnol., 2014, 9, 618.

5 K. Yan, H-W. Lee, T. Gao, G. Zheng, H. Yao, H. Wang, Z. Lu, Y. Zhou, Z. Liang, Z. Liu, S. Chu and Y. Cui, Nano Letters, 2014, 14, 6016.

6 G. Oltean, C-W. Tai, K. Edström and L. Nyholm, J. Power Sources, 2014, 269, 266.

7 A. L. Michan, G. Divitini, A. J. Pell, M. Leskes, C. Ducati and C. P. Grey, J. Am. Chem. Soc., 2016, 138, 7918.

8 D. Rehnlund, F. Lindgren, S. Böhme, T. Nordh, Y. Zou, J. Pettersson, U. Bexell, M. Boman, K. Edström and L. Nyholm, Energy Environ. Sci., 10 (2017) 1350.