New Generation Cathode Materials Realizing Both Safety and High Energy Density
Authors: Satou Yoshinori , Komine Shigeki , Yamamoto Nobuo , Shimonishi Yuta 
Published: February 2020

Introduction

A lithium ion battery is the most promising battery for the next generation electric vehicle (xEV). The cathode material is a critical element of the lithium ion battery, since it determines the performance. Generally, it has been known that cathode materials have trade-off relation between the energy density and the safety. We proposed in our previous study that the formation of oxygen 2p hole in a layer-type cathode material, LiNi0.5Mn0.5O2, plays an important role in the charge compensation during battery operation[1]. Furthermore, we have cultivated the understanding about the durability and the safety and arrived at a hypothesis that these performances strongly depend on the fixing level of oxygen 2p electron. The excess oxygen 2p hole would cause the crystal degradation or O2 gas generation typically at the surface where mass transfer occurs easily. On the basis of these, we came up with the “trade-on relation” materials possessing both high energy density and safety. We introduced a layer-type LiNi0.33Mn0.16Co0.33Ge0.17O2 as a typical example of the “trade-on relation” materials.

Experimental

Active materials, LiNi0.33Mn0.16Co0.33Ge0.17O2 (LNMCGO) and LiNi0.33Mn0.34Co0.33O2 (NMC), were obtained by using a liquid-phase synthesis technique. Electrochemical measurements were conducted using a 2032-type coin cell. Cathode-slurry was prepared by mixing the active material, carbon black, and polyvinylidenefluoride with a weight ratio of 80:10:10 in N-methlpyrrolidone solution. The slurry was spread onto an aluminum foil current collector and dried in a 120 °C vacuum oven. The cathode and Li metal anode were separated by a PE membrane. 1M LiPF6 in ethylene carbonate/diethyl carbonate (3:7 v/v) was used as the electrolyte. Cells were assembled in an argon-filled grove box. The charge-discharge operations were repeated twice at a rate of 0.02 C within the range of 2.6-4.5 V using constant current mode. The rest time was set to 10 minutes.

Results and Discussion

Figure 1 shows the 1st and 2nd cycle performances for LNMCGO and NMC during 2.6 - 4.5 V at room temperature. It is clearly indicated that the coulomb efficiency of LNMCGO is far superior to that of NMC and the difference between the discharge capacities of these materials at the 2nd cycle is almost none. It is considered that the stable high energy capacity bases on the charge compensation due to the oxygen 2p hole formation and the Ge-O strong bonding which limits the excessive over-formation of oxygen 2p hole typically at the surface.
We also comfirmed the suprior safety as a battery for LNMCGO by using the TPD-MS technique which could measure the starting temeperature of oxygen elimination reaction and the amount of oxygen on the charged active materials. We plan to talk about the details of the characterization and the battery performances of the “trade-on relation” materials, e.g. the rate performance and the durability, at PRiME 2016.

Conclusion

On the basis of theoretical understanding about the function of cathode materials, we came up with the “trade-on relation materials” possessing both high energy density and safety. We introduced a layer-type LiNi0.33Mn0.16Co0.33Ge0.17O2 as a typical example of the “trade-on relation” materials.

Reference

[1] Y. Satou, S. Komine and S. Itou, LIBD2015 (France), abstract

Figure 1