Structural Changes and Thermal Decomposition of Ni Based Cathode Materials for Li-Ion Batteries Studied By in-Situ XRD

The three-component system Li[Ni_xCo_yMn_z]O₂ has outstanding electrochemical properties due to advantage of the high capacity of LiNiO₂, thermal stability and low cost of manganese in LiMnO₂ and layered characteristics of LiCoO₂. While capacity of Li[Ni_xCo_yMn_z]O₂ is similar to capacity of LiCoO₂, it is a promising alternative in terms of performance, safety and cost. Various material combinations are being developed to achieve the higher capacity by increasing Ni content while retaining its advantages. Although higher Ni content contributes to better capacity in Li[Ni_xCo_yMn_z]O₂, but on the other hand, it tends to decrease the thermal stability of electrode material. Safety is one of the most important considerations in EV and HEV.

 First experimental condition is same state of charge with different Ni contents. Interesting phase transition behavior during heating is observed in Li_0.66Ni_0.5Co_0.2Mn_0.3O₂ cathode material. Li_0.66Ni_0.5Co_0.2Mn_0.3O₂ without electrolyte converts from layered (R-3m) structure to disordered LiM₂O₄-type spinel (Fd-3m) around 415 ºC. Upon further heating, M₃O₄-type spinel (Fd-3m) appears and co-exists with LiM₂O₄-type spinel (Fd-3m) from 497 ºC and remains in this structure up to 600 ºC. In the absence of electrolyte, no peaks of MO-type rock salt phase (Fm-3m) appear until 600 ºC whereas in the presence of electrolyte, further phase transition from M₃O₄-type spinel (Fd-3m) to MO-type rock salt phase (Fm-3m) takes place above 415 ºC.

 The electrolyte accelerates the thermal decomposition of charged cathode materials. The presence of electrolyte alters the paths of structural changes and lowers the onset temperatures of thermal decomposition reactions. In case of Li_0.66Ni_0.5Co_0.2Mn_0.3O₂with electrolyte, more dramatic structural changes are observed in comparison with the one in the absence of electrolyte. The sample with electrolyte shows the structural decomposition to MO-type rock salt phase at 415 ºC, in the early stage of heating.

 Thermal behavior of Li_0.33Ni_0.6Co_0.2Mn_0.2O₂ cathode material is similar to the one of Li_0.66Ni_0.5Co_0.2Mn_0.3O₂. Li_0.33Ni_0.6Co_0.2Mn_0.2O₂ without electrolyte converts from layered (R-3m) structure to disordered LiM₂O₄-type spinel (Fd-3m) around 374 ºC. Upon further heating, M₃O₄-type spinel (Fd-3m) co-exists with LiM₂O₄-type spinel (Fd-3m) from 497 ºC and maintains this structure up to 600 ºC. Presence of electrolyte initiates additional phase transitions in this cathode material. M₃O₄-type spinel (Fd-3m) starts to convert into MO-type rock salt phase (Fm-3m) at 374 ºC. Above 579 ºC, pure MO-type rock salt phase (Fm-3m) is observed.

 Second condition is same Ni contents in electrode material with different state of charge. The thermal behavior of Li_0.66Ni_0.5Co_0.2Mn_0.3O₂ cathode material is mentioned above. Li_0.33Ni_0.6Co_0.2Mn_0.2O₂ without electrolyte converts from layered (R-3m) structure to disordered LiM₂O₄-type spinel (Fd-3m) around 340 ºC. Upon further heating, M₃O₄-type spinel (Fd-3m) co-exists with LiM₂O₄-type spinel (Fd-3m) from 449 ºC and maintains this structure up to 600 ºC. Presence of electrolyte initiates additional phase transitions in this cathode material. M₃O₄-type spinel (Fd-3m) starts to convert into MO-type rock salt phase (Fm-3m) at 436 ºC. Above 560 ºC, pure MO-type rock salt phase (Fm-3m) is observed.

 In the presence of electrolyte, thermal induced phase transition of Li_0.33Ni_0.6Co_0.2Mn_0.2O₂ shows conversion from LiM₂O₄-type spinel (Fd-3m) through M₃O₄-type spinel (Fd-3m) to MO-type rock salt phase (Fm-3m) step by step. On the other hand, thermal decomposition behavior of Li_0.66Ni_0.6Co_0.2Mn_0.2O₂ shows phase transition to M₃O₄-type spinel (Fd-3m) and MO-type rock salt phase (Fm-3m) at the same time. This result of different thermal behavior of Li_0.66Ni_0.6Co_0.2Mn_0.2O₂ and Li_0.33Ni_0.6Co_0.2Mn_0.2O₂contributes towards better understanding for thermal stability of Ni-based cathode materials.

 Working on other samples and more detailed discussion will be presented at the time of meeting.

Fig1. In-situ XRD patterns of the (a) 33% (b) 66% SOC LiNi_0.6Co_0.2Mn_0.2O₂ in the presence of electrolyte heated from 25℃ to 600℃