Advanced Electrolytes for Fast‐Charging High‐Voltage Lithium‐Ion Batteries in Wide‐Temperature Range

Abstract

IntroductionLithium (Li)-ion batteries (LIBs) have now been the indispensable power sources for portable electronic devices, electric vehicles, stationary or grid applications, etc. [1] However, further efforts on extending the cycle life, rate capability, energy density and working temperature range and improving the safety of LIBs are still facing significant challenges for their large-scale applications. Focusing on the increase in energy density of a battery, the possible approach is to use the high capacity electrode (cathode or anode) material and the high voltage cathode material. Ni-rich layered oxides LiNi x Mn y Co 1−x−y O 2 (NMC) with Ni content ≥80% (e.g., NMC811) are regarded as one of the most potential candidates to usher in the new stage of ultra-high energy density LIBs due to their increased specific capacities at higher voltages and the low cost with less Co content. However, the practical applications of these Ni-rich NMC cathode materials are greatly hindered by the poor cathode-electrolyte interface (CEI) layer formed on such cathode surface in the state-of-the-art electrolytes comprised of lithium hexafluorophosphate (LiPF 6 ) in carbonate solvents, especially at voltages higher than 4.3 V versus Li/Li + , [2] causing continuous electrolyte oxidative decomposition and other related side reactions such as transition metal dissolution from the cathode surface, thus leading to poor cycling stability, especially at elevated temperatures and high operating voltages. [3] Therefore, advanced electrolytes with better oxidative protection to Ni-rich NMC cathode materials, especially under high voltages are critically important for enabling application of Ni-rich NMCs in LIBs.Significant efforts have been made to develop novel electrolytes for high voltage cathode materials, mainly through using high anodic solvents to substitute carbonate solvents, the increase of salt concentration and the utilization of film-forming additives. Fan et al. developed an all-fluorinated electrolyte of 1 m LiPF 6 in fluoroethylene carbonate/3,3,3-fluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether (FEC/FEMC/HFE, 2:6:2 by wt), which significantly enhanced the cycling stability of Li||NMC811 (2.7-4.4 V) and Li||LiCoPO 4 (3.5-5.0 V) in high voltages by effectively inhibiting electrolyte LiNi x Mn y Co 1−x−y O 2 (NMC) cathode materials with Ni ≥ 0.8 have attracted great interest for high energy-density lithium-ion batteries (LIBs) but their practical applications under high charge voltages (e.g., 4.4 V and above) still face significant challenges due to severe capacity fading by the unstable cathode/electrolyte interface. Here, an advanced electrolyte is developed that has a high oxidation potential over 4.9 V and enables NMC811-based LIBs to achieve excellent cycling stability in 2.5-4.4 V at room temperature and 60 °C, good rate capabilities under fast charging and discharging up to 3C rate (1C = 2.8 mA cm −2 ), and superior low-temperature discharge performance down to −30 °C with a ca...

Regulating Interfacial Chemistry in Lithium‐Ion Batteries by a Weakly Solvating Electrolyte**

“…29,30 Considering the high cost and viscosity of SCE, diluting SCE with non-polar solvents emerged in recent years as an alternative to mitigate these issues. [31][32][33] The diluted SCE is termed localized superconcentrated electrolytes (LSCE) because the local solvation structure of LSCE is very similar to that of SCE, and therefore they belong to the same methodology.…”

Challenges and Strategies for Fast Charge of Li‐Ion Batteries

“…For example, Zhang et al showed that at low temperatures (‐20 and −30 °C), Gr/NCA cells deliver higher capacity with a LiBF 4 electrolyte than with a LiPF 6 counterpart while the LiBF 4 electrolyte has lower ionic conductivity. Similarly, Yamada et al demonstrated that a Li/Gr cell with 3.6 M LiFSI‐DME electrolyte having a lower ionic conductivity (7.2 mS cm −1 ) outperforms that with 1.0 M LiPF 6 ‐EC‐DMC electrolyte (∼10 mS cm −1 ); Zhang et al reported that at 5 C, a Gr/NMC811 cell with 2.8 mAh cm −2 of NMC811 loading demonstrates higher capacity with a 1.4 m LiFSI DMC‐EC‐TTE electrolyte (2:0.2 : 3 by mol, 1.07 mS cm −1 ) than with a 1.0 m LiPF 6 EC‐EMC electrolyte (3 : 7 by wt+2 wt% VC, 6.07 mS cm −1 ). The above results reveal that the ionic conductivity of the electrolytes is not the only factor governing the fast‐charging capability.…”

Design aspects of electrolytes for fast charge of Li‐ion batteries

“…For the mixture of solvents, cyclic ethylene carbonate (EC) is considered to be an indispensable co-solvent for the formation of robust solid-electrolyte-interphase (SEI) on the graphite anode, and the other solvent can be a linear dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), or their mixture for reduced viscosity and low freezing temperature of the electrolytes. In some cases, small amounts of esters [7][8][9] or ethers 10,11 that have a low boiling point and acceptably chemical stability are added to enable operations at low temperatures or/and high current rates. However, such benefits often come with a trade-off in reversibility and lifetime.…”

“…Therefore, the solutions to the first question should be centered on the second and third steps as shown in Figure 1. While fast charge necessitates high ionic conductivity, several studies revealed that some electrolytes with LiBF 4 26,27 and LiFSI 11,28 have lower conductivity, however, the resultant batteries exhibit higher capacity and lower polarization at low temperatures and high rates compared with those using the conventional LiPF 6 ‐carbonate electrolytes. Although the mechanism for the above phenomena is yet unclear, these findings point out a fact that the ionic conductivity of the electrolytes is not the only factor governing the fast‐charging capability.…”