In this study, we focused on the investigation of the effects of homogeneity of Al3+ doping into LiNi0.80Co0.15Al0.05O2. Two types of LiNi0.80Co0.15Al0.05O2 were synthesized from co-precipitated precursor. One was an inhomogeneous Al3+ doped LiNi0.80Co0.15Al0.05O2 (Sample A) which was made from a precursor using Ni2+ and Co2+ sulfate solution mixture. Another was a homogeneous Al3+ doped LiNi0.80Co0.15Al0.05O2 (Sample B) made from Ni2+, Co2+, and Al3+sulfate solution mixture.
First, the effect of homogenous Al3+ doping on crystalline parameters was investigated. Crystalline strain in delithiated cathode was evaluated as a function of the Williamson-Hall plot slope. It has already revealed that the lattice strain will increase when electrode charged up to higher potential and increasing strain will have a detrimental impact on the life and safety of LIBs [1]. Thus, we compared the lattice strain of the couple types of cathode at delithiated state. The LiNi0.80Co0.15Al0.05O2 cells were charged at a potential of 4.3 V vs Li+/Li at 25 oC and then disassembled them in a dry Ar-filled grove box. As a result, we revealed that homogeneous Al3+doping in cathode can suppress the lattice strain at the higher SOC.
Second, we measured structural change of cathode materials at delithiated cathodes as a quantification of thermal stability by using a temperature programmed X-ray diffraction (TPXRD) analysis in the temperature range from RT to 500 oC under Ar atmosphere. Three types of crystal phase; layered rock-salt, spinel and rock-salt were observed in our measurement. The ratio of deteriorated phases; spinel and rock-salt in the delithiated LiNi0.80Co0.15Al0.05O2 with temperature are shown in Fig. 1. The deteriorated phases which transformed from the layered rock-salt phase were observed in both cathodes at high temperature. The deterioration of Sample A occured from 200 oC. In contrast, Sample B remained layered rock-salt phase up to 250 oC. From this result we concluded that Sample B had higher thermal stability than Sample A.
In order to demonstrate the deterioration mechanism, micro structure of both Sample A and Sample B after 500 cycle-test at 45 oC were investigated using cross-section SEM and TEM. The TEM image of Sample A and Sample B are shown in Fig. 2. The lattice image of Sample A is unobvious that may be induced from charge/discharge damage at high temperature. In contrast, that of Sample B is clearer than Sample A as shown. These results were consistent with the trend of electrochemical stability. In conclusion, it was confirmed that the deterioration during cycling at 45 oC was suppressed by homogeneous Al3+ distribution in LiNi0.80Co0.15Al0.05O2.
In this study, we applied the Williamson-Hall analysis to estimate the structural stability for different homogeneity of Al3+ doping in LiNi0.80Co0.15Al0.05O2. We showed the strain analysis as a particularly useful methode which explained the structural stability of delithiated LiNi0.80Co0.15Al0.05O2in quantitatively.
Further details shall be presented at the meeting.
Reference
[1] Rosa Robert, Petr Novák, Journal of The Electrochemical Society 162(9), A1823 (2015).