Zhenpeng Yao1 Qianqian Li1 Jinsong Wu1 Kai He1 Tim Fister2 Michael Thackeray2 Vinayak Dravid1 Christopher Wolverton1

1, Northwestern University, Evanston, Illinois, United States
2, Argonne National Laboratory, Lemont, Illinois, United States

Conversion-type materials like transition metal (TM) oxides and sulfides exhibit the possibility to take advantage of all the valence states of the TM and can thus achieve much higher capacity compared to the intercalation-type materials. However, the conversion-type electrodes, suffer from drawbacks such as huge volumetric expansion, inadequate reversibility, and large voltage hysteresis which have been hindering their practical applications. Thus, in order to push these materials and reactions towards real practical use, there is a large need to obtain a thorough understanding of the conversion reaction mechanisms. Different hypotheses have been suggested, but some critical issues like the origin of the large hysteresis during charge/discharge of these materials is still not well-understood. In this study, we design and utilize a novel computational mechanistic approach that provides, for the first time, a detailed explanation of the hysteresis and non-equilibrium reaction pathways associated with these conversion-type electrodes. We firstly apply this methodology to investigate various lithiation reaction pathways of typical conversion-type electrode materials as picked: Co3O4 and NiO by exploring the energetics of a large number of equilibrium and non-equilibrium structural configurations using first-principle calculations.1,2 The overall value of the voltages from our mechanistically-constructed, non-equilibrium pathway is in much better agreement with experimental lithiation than the calculated equilibrium voltage while the overall value of the latter reasonably agrees with experimental delithiation. We propose, therefore, that the charge and discharge processes proceed through very distinct equilibrium and non-equilibrium pathways which contribute significantly to the experimentally observed voltage hysteresis for Co3O4 and NiO. The methodology we have developed is not specific to these particular compounds, the application of this methodology has been extended to diverse conversion-type materials including (Co,Cu)3O4,3 CuS,4 MoS2,5 and CuBi.6 Our study thus provides new insights on the lithiation process of conversion-type electrodes and could help future experiments to overcome the current limitations of them promoting the development of more advanced LIBs.
(1) Yao, Z.; Kim, S.; Aykol, M.; Li, Q.; Wu, J.; He, J.; Wolverton, C. Chem. Mater. 2017, 29 (21), 9011.
(2) Li, Q.; Wu, J.; Yao, Z.; Thackeray, M. M.; Wolverton, C.; Dravid, V. P. Nano Energy 2017, 44, 15.
(3) Liu, H.; Li, Q.; Yao, Z.; Li, L.; Li, Y.; Wolverton, C.; Hersam, M. C.; Dravid, V. P. Adv. Mater. 2017, 1704851.
(4) He, K.; Yao, Z.; Hwang, S.; Li, N.; Sun, K.; Gan, H.; Du, Y.; Zhang, H.; Wolverton, C.; Su, D. Nano Lett. 2017, 17 (9), 5726.
(5) Li, Q.; Yao, Z.; Wu, J.; Mitra, S.; Hao, S.; Sahu, T. S.; Li, Y.; Wolverton, C.; Dravid, V. P. Nano Energy 2017, 38, 342.
(6) Amsler, M.; Yao, Z.; Wolverton, C. Chem. Mater. 2017, 29 (22), 9819.