Effect of Using Fluorinated Phosphate Ester and Fluorinated Ether As Electrolyte Solvents for Lithium Ion Batteries with LiNi0.5Mn1.5-XTixO4 Cathode

Introduction

LiNi_0.5Mn_1.5O₄¹⁾ is one of the candidates for next generation cathode active material of Li ion batteries, because of large capacity of around 130 mAh/g and the high charge-discharge potential of around 4.65V versus Li. On the other hand, because of the higher potential than conventional cathode active materials, 5V class spinel has the problem of the capacity fading due to decomposition of electrolyte and Mn dissolution. We have studied the improvement of lifetime by Ti substitution for Mn of LiNi_0.5Mn_1.5O₄^{2) 3) 4) 5)}. To suppress electrolyte decomposition, it was necessary to develop electrolyte solvents which have higher resistance to oxidation. The decomposition reaction of electrolyte may cause gas generation, and it can cause swelling of cells. At room temperature, amount of gas was not large even in conventional electrolyte, but in the case of operation at elevated temperature large amount of gas generation was a problem. We have reported the decrease of gas emission by using fluorinated phosphate (FPE) as an electrolyte solvent⁶⁾. In this report, we evaluated the composition dependence of electrolytes with ethylene carbonate, FPE and fluorinated ether (FE). By results of HOMO value calculation, FE was expected to have higher resistance to oxidation than conventional solvents.

  

Experimental

LiNi_0.5Mn_1.5-xTi_xO₄ was used as the cathode active material, and graphite was used as the active anode material. The discharge capacity as a function of cycle number was tested using stack type laminated cells with these active materials.  1 M solution of LiPF₆ in 40:60(v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) or 0.8 M solution of LiPF₆in 20:(80-x):x (v/v) (x = 0 to 50) mixture of EC, FPE and FE were used as electrolytes. Cycle tests were performed at 45 ºC. The volume of the cells was measured by the Archimedes method.

 

Results

Figure 1 shows discharge capacity retention as a function of cycle number for LiNi_0.5Mn_1.5-xTi_xO₄/graphite cells with 0.8M-LiPF₆EC/FPE/FE=20/(80-x)/x (v/v) (x=0,10,20,30, 50) as electrolytes. Capacity retention after charge-discharge cycles increased with FE content in electrolyte. By evaluating impedance tests, the increase of resistance after cycles was suppressed with increase of FE content. Existence of FE is thought to have the effect of suppress of forming SEI during charge-discharge cycles. Figure 2 shows the increase rate of cell volumes after 300 charge-discharge cycles at 45 °C. The volume increase ratio of the cell with EC/DMC=40/60 was 59%, and those of EC/FPE/FE=20/80/0 and EC/FPE/FE=20/50/30 were less than 1%.  Amount of gas generation was found to be significantly reduced by using EC/FPE/FE. These results show that FE also had high oxidation resistance around 4.7V versus Li.

  

References

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