Reinvestigation on the state-of-the-art nonaqueous carbonate electrolytes for 5 V Li-ion battery applications

Abstract

The charging voltage limits of mixed-carbonate solvents for Li-ion batteries were systematically investigated from 4.9 to 5.3 V in half-cells using Cr-doped spinel cathode material LiNi_0.45Cr_0.05Mn_1.5O₄. The stability of conventional carbonate electrolytes is strongly related to the stability and properties of the cathode materials in the de-lithiated state. This is the first time report that the conventional electrolytes based on mixtures of EC and linear carbonate (DMC, EMC and DEC) can be cycled up to 5.2 V on LiNi_0.45Cr_0.05Mn_1.5O₄ for long-term cycling, where their performances are similar. The discharge capacity increases with the charging cutoff voltage and reaches the highest discharge capacity at 5.2 V. The capacity retention is about 87% after 500 cycles at 1C rate for all three carbonate mixtures in half-cells when cycled between 3.0 V and 5.2 V. When cycled to 5.3 V, EC-DMC still shows good cycling performance but EC-EMC and EC-DEC show faster capacity fading. EC-DMC and EC-EMC have much better rate capability than EC-DEC. The first-cycle irreversible capacity loss increases with the cutoff voltage. The “inactive” conductive carbon is also partly associated with the low first-cycle Coulombic efficiency at high voltages due to electrolyte decomposition and possible PF₆^- anion irreversible intercalation.

Introduction

The state-of-the-art (SOA) Li-ion batteries have achieved great success in portable electronics and power tools. They are now starting to enter the electric vehicle (EV) and grid energy storage markets. For EV applications, the SOA Li-ion battery technology can only meet the requirements for short-range applications due to their limited energy density. To further improve the energy density of a Li-ion battery, cathode and anode materials with higher specific capacities and cathode materials with a higher voltage plateau are required. Some 5 V cathode materials have shown very promising results to enhance the energy density of Li-ion batteries, which include Li₃V₂(PO₄)₃ (charged to 4.8 V) [1], [2], LiNi_0.5Mn_1.5O₄ and its doped derivatives (4.9 V) [3], [4], [5], [6], [7], [8], [9], [10], LiCoPO₄ (5.0 V) [11], [12], [13], Li₂CoPO₄F (5.5 V) [14], [15], and others. However, the high voltage stability of the electrolytes has always been one of the main barriers to the application of these high operating voltage materials.

Previous literature indicates much confusion on the upper voltage limits of the SOA Li-ion battery electrolytes based on organic carbonate solvents. For example, it has been reported that the carbonate electrolytes are not stable at 4.3–4.5 V vs. Li/Li⁺ on LiCoO₂ and LiNi_xMn_yCo_zO₂ (where x + y + z = 1) electrodes [16], [17], [18], [19], [20]. However, some of the carbonate electrolytes exhibit very good battery performance when used for LiNi_0.5Mn_1.5O₄ materials in the voltage range up to 4.9 V [8], [9], [21], [22]. Therefore, it is necessary to re-evaluate the carbonate-based nonaqueous electrolytes for high voltage Li-ion batteries to clarify the issue. In this work we report our recent findings on the oxidation potential limits of carbonate-based electrolytes when used with Cr-doped LiNi_0.5Mn_1.5O₄ cathode material (LiCr_0.05Ni_0.45Mn_1.5O₄). The electrochemical stability and performance of this battery system will also be reported.