Quantified Evidence of Homogeneous Al3+ Doping on LiNi0.80Co0.15Al0.05O2: Effects on the Electrochemical and the Thermal Stability
Authors: Masaki Ryuta , Sugihara Takahiko , Kajiyama Akihisa , Matsumoto Kazutoshi , Hirayama Masaaki , Kanno Ryoji 
Published: February 2020

Aluminum doped high Ni cathode active materials such as LiNi0.80Co0.15Al0.05O2
are well known as promised candidate for high energy density LIB. Two challenges for LiNi0.80Co0.15Al0.05O2
improvements, to realize both thermal stability at delithiated state and suppression of microstructure deterioration during repeated charge and discharge reaction, have been investigated by homogeneous substitution of transition metal ions with Al3+. We have also shown that micro strain obtained from the Williamson-Hall analysis at higher SOC was an effective factor to quantify the structural stability of cathode active materials and played important role in LIB’s life.

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).

Figure 1