Differentiating the Synergistic Interactions between Li+ Salts and Cyclic to Linear Carbonate Ratios to Enable Wide-Temperature Performance of Lithium-Ion Batteries

Lithium-ion batteries operate without significant degradation between a temperature range of +15 °C to +35 °C. Below this range, sluggish kinetics due to the resistive solid-electrolyte interphase (SEI) layer can impede the charge-transfer process, and above this range, accelerated decomposition of the electrolyte on the surface of the electrodes leads to continuous growth of thick, high-impedance SEI and cathode-electrolyte interphase (CEI) films. In traditional nonaqueous electrolytes, a Li+ salt is dissociated in the presence of a strongly coordinating solvent such as ethylene carbonate (EC), while a comparatively weaker coordinating solvent, for example, ethyl methyl carbonate (EMC), is employed to transfer the solvated Li+ through the bulk electrolyte between the electrodes. In the design of electrolytes, the selection of Li+ salt and solvents are fundamental to controlling the composition of SEI and CEI films, and thus the performance of the battery over a wide-temperature range. In this work, we identify the resultant interactions between the Li+ salt counter ions and the electrolyte cosolvents across a wide-temperature range of -40 °C to +60 °C. Specifically, we selected LiPF6, LiDFOB, and LiFSI to be tested with varied ratios of EC and EMC to identify the best combination for the improved cycling performance of Li1.02Ni0.8Mn0.1Co0.1O2 (NMC811) cathode and graphite anode full cells. Current literature asserts that EC assists in LiPF6 electrolytes due to the benefits of having a high dielectric constant and by the formation of an SEI with greater high-temperature stability and conductivity than SEI formed by EMC. This work demonstrates the opposite effect in LiDFOB electrolytes, where EC-free electrolytes deliver improved -40 °C discharge performance and greater stability during 60 °C cycling. We identify the ways in which dual-salt electrolytes can be employed to amalgamate the benefits of each Li+ salt, culminating in a tailored electrolyte design for wide-temperature cycling of lithium-ion batteries. In this work, electrochemical impedance spectroscopy (EIS) analysis indicates that electrolytes with EC > 20 v% exhibit impedance growth in the charge-transfer resistance (Rct) of the cell, regardless of the choice of Li+ salt. Furthermore, electrolytes with EC < 20 v% experience larger growth in the high-frequency region associated with SEI and CEI resistance. Impedance measurements performed at -40 °C identify LiDFOB as generating the lowest cell impedance in EMC, while the lowest amount of impedance growth after high-temperature cycling occurs for a dual-salt combination of 0.5 M LiDFOB + 0.5 M LiFSI in EMC. Symmetric cells of pouch cell electrodes cycled in single-salt EC-free electrolytes identify that the high impedance growth occurs on the cathode in LiPF6 and the anode in LiFSI electrolytes, while LiDFOB passivates both the cathode and anode, allowing for the retention of low-temperature performance after high-temperature cycling. These findings give rise to an improved understanding of the interactions of common nonaqueous electrolyte co-solvents with different Li+ salts. By identifying Li+ salt and solvent combinations favorable for wide-temperature performance, we suggest new electrolyte formulas designed to assist in extending the temperatures lithium-ion batteries can reliably operate within.