Li rich layered cathode is one of the candidates for next generation cathode active material of Lithium ion batteries, because of large capacity more than 250 mAh/g. On the other hand, because of the higher potential than conventional cathode active materials, Li rich layered cathode has the problem of the capacity fading and gas emission. We have studied fluorinated electrolyte solvents for high voltage operation batteries with LiNi_0.5Mn_1.5O₄ cathode^{1) 2)}, and we have reported that fluorinated ether and fluorinated phosphate were effective to reduce gas emission in high voltage operation cells at elevated temperature. In this report, we examined fluorinated ether and sulfone as electrolyte solvents for Lithium ion batteries with Li rich layered cathode and SiO anode.

Experimental

Li(Li_xNi_yMn_1-x-y)O₂ was used as a cathode active material, and SiO was used as an active anode material. The discharge capacity as a function of cycle number was tested using stack type laminated cells with these active materials. Ethylene carbonate (EC), diethyl carbonate (DEC), DES (Diethyl sulfone) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (FE) were used as electrolyte solvents. EC/DEC (30/70 v/v) and EC/DES/FE (x/(30-x)/70 v/v/v (x=0,5,10)) were used as electrolyte solutions. The volume of the cells was measured by the Archimedes method before and after initial 3 charge discharge cycles between 4.5 V and 1.5 V. After that, cycle tests were performed between 4.5 V and 1.5 V at 45 ºC.

Results

Figure 1 shows discharge capacity retention as a function of cycle number for Li(Li_xNi_yMn_1-x-y)O₂/SiO cells with electrolyte solutions of EC/DEC (30/70 v/v) and EC/DES/FE (5/25/70 v/v/v). Capacity retention of EC/DES/FE after 200 cycles was larger than that of EC/DEC. DES and FE seem to have higher oxidation resistance than DEC and to have the effect of cycle life improvement. Figure 2 shows the volumes increase of cells after initial 3 charge-discharge cycles at 45 °C. The cause of volume increase is gas emission. The volume increase of the cell with EC/DEC (30/70 v/v) was larger than that with EC/DES/FE (x/(30-x)/50 v/v/v). Gas volume increased with amount of EC in electrolyte of EC/DES/FE. This result shows that the cause of gas emission at initial charge-discharge process seems to be due to the reaction between electrolyte solvents and cathode. EC is considered to be the cause of gas emission in initial charge discharge cycles.

References

1. T.Noguchi, M.Uehara, Y.Katoh, K.Utsugi, PRiMe 2012 Abstract, No.824 (2012)

2. T.Noguchi, Y.Katoh, K.Utsugi, 228th ECS Meeting Abstract, No.358(2015)