The practical specific capacity of conventional positive electrodes, such as LiCoO₂, remained less than the half of graphite negative-electrodes. Hence, increasing the specific capacity of positive-electrodes effectively improves the energy density of lithium-ion batteries (LIBs). A layer-structured LiNi_0.5Co_0.2Mn_0.3O₂ positive-electrode can deliver a high discharge capacity of ca. 200 mAh g^-1 by charging to around 4.6 V. However, the severe decomposition of conventional electrolyte solutions is a problem at such a high potential. The stability of electrolyte solutions against oxidation can be improved by increasing the concentration, and there are three major problems to be solved toward the practical use; 1) high costs due to an extensive use of lithium salts, 2) high viscosities which make it virtually impossible to inject the electrolyte solutions into cylindrical and laminated cells in the production processes of practical LIBs, and 3) low rate capabilities due to the low ionic conductivity of highly viscous electrolyte solutions. These problems can be settled by diluting the highly concentrated electrolyte solution with an appropriate diluent.

We previously reported that propylene carbonate (PC)- and γ-butyrolactone-based concentrated electrolyte solutions showed high stability against oxidation at LiNi_0.5Co_0.2Mn_0.3O₂ and 5-V class LiNi_0.5Mn_1.5O₄ [1-3]; irreversible capacities due to the oxidative decomposition of electrolyte solution decreased with increasing the electrolyte concentration, while the discharge capacities increased. In particular, in the nearly saturated 7.25 mol kg^-1 LiBF₄ /PC electrolyte solution, LiNi_0.5Co_0.2Mn_0.3O₂ retained a high discharge capacity of ca. 186 mAh g^-1 even after 50 charge/discharge cycles at a C/10 rate. The polarization on charge/discharge reactions remained small even at the very high concentration [2]. Considering the wide operating temperature range of LIBs, the electrolyte solution needs to be stable even at elevated temperatures. 7.25 mol kg^-1 LiBF₄ /PC allowed charge/discharge cycles of LiNi_0.5Mn_1.5O₄ between 3.5 and 5.0 V at an elevated temperature of 50 °C, whereas not in the nearly saturated 4.27 mol kg^-1 LiPF₆ /PC [4]. In electrolyte solutions of moderate concentrations, the dissolution of active materials become more of a problem at elevated temperatures. The highly concentrated LiBF₄/PC suppressed the dissolution of manganese ions from a LiNi_0.5Mn_1.5O₄ electrode more effectively than LiPF₆ /PC [4].

Highly concentrated LiBF₄ /PC can be diluted with a diluent to reduce the high viscosity and concentration. Fluorinated solvents have relatively low donor number and high stability against oxidation because they have electron-withdrawing fluorine atoms, and are suitable as diluents for concentrated LiBF₄ /PC systems. We explored various kinds of fluoroalkyl ethers [5,6] and esters as a diluent, of these, bis(2,2,2-trifluoroethyl) carbonate (TFEC) has a relatively low HOMO energy, and could hardly dissolve LiBF₄. Unfortunately, however, the miscibility between TFEC and concentrated LiBF₄ /PC was not high, and hence tris(2,2,2-trifluoroethyl) phosphate (TFEP) was introduced as a co-solvent of PC to prepare 2.03 mol kg^-1 LiBF₄ /PC + TFEP (1:2 by volume, PC/Li⁺ molar ratio = 1.35). This concentrated electrolyte solution was diluted with 50 vol.% TFEC to obtain 1.00 mol kg^-1 LiBF₄ /PC+TFEP+TFEC (1:2:3 by volume, PC/Li + molar ratio = 1.35). The initial discharge capacity reached 192 mhA g^-1 and 98.0% of it was retained even at the 50th cycle (Fig.1) [7]. Thus, the degradation of a LiNi_0.5Co_0.2Mn_0.3O₂ electrode was extremely suppressed even though the upper cut-off voltage was as high as 4.6 V. The initial Coulombic efficiency (84.2%) was low, while the average Coulombic efficiency from the 2nd (97.6%) to 50th cycle (99.1%) was high at 98.7%. Thus, the present electrolyte solution has a low viscosity (ca. 7 mPa s), moderate lithium salt concentration, and high stability against oxidation, and is therefore suitable for higher-voltage operating of LIBs.

This research has been supported by “Advanced Research Program for Energy and Environmental Technologies” from NEDO, Japan.

Reference

[1] T. Doi et al., Electrochim. Acta, 209 (2016) 219-224.

[2] T. Doi et al., J. Electrochem. Soc., 163 (2016) A2211-A2215.

[3] T. Doi et al., ChemElectroChem, 4 (2017) 2398-2403.

[4] T. Doi et al., ChemistrySelect, 2 (2017) 8824-8827.

[5] T. Doi et al., J. Electrochem. Soc., 164 (2017) A6412-A6416.

[6] T. Doi et al., Sustain. Energy Fuels, 2 (2018) 1197–1205.

[7] T. Doi et al., Curr. Opin. Electrochem., 9 (2018) 49-55.