Locally Superconcentrated Electrolytes for Ultra-Fast-Charging Lithium Metal Batteries with High-Voltage Cathodes

Abstract

Electric vehicles capable of recharging in the same time it takes to refuel a gasoline-powered car require electrolytes that maximize areal ion flux to enable electrochemical reactions to proceed at the same rate that current is passed through the external circuit. While strategies for increasing ionic charge carrier concentration in electrolytes are well-established, enhancements are made at the expense of carrier mobility, placing a ceiling on areal ion flux below the requirement for fast-charge. Here, we explore locally superconcentrated electrolytes, which employ a noncoordinating diluent to reduce viscosity, for delivering an 80% change in state-of-charge in Li|NMC622 batteries in 5–15 min. We investigate the effects of concentration, viscosity, ionic conductivity, and solvation on lowering fast-charge overpotentials and extending cycle life. We identify divergent failure mechanisms that occur on different time scales when cycling the cells at different charge rates and depths of discharge, which has implications for future electrolyte designs.

Design Criteria of Dilute Ether Electrolytes toward Reversible and Fast Intercalation Chemistry of Graphite Anode in Li-Ion Batteries

“…Li metal plating, which causes resistance buildup and electrolyte consumption, is severe in LP57, while no dead Li is observed in 1 M LiFSI DOL. 51,52 The half cells (>3 mAh/cm 2 ) and full cells (∼2.4 mAh/cm 2 ) can be seen in Figure S36 and Figure 5d. 1 M LiFSI DOL contributes to a high capacity retention (∼84% after 200 cycles at C/3) and excellent CE.…”

Designing Electrolytes With Controlled Solvation Structure for Fast‐Charging Lithium‐Ion Batteries

“…As reported in our previous work, LHCEs without free DMC lead to anion‐derived SEI and CEI on the Gr and NMC811, which are thin yet robust to protect the batteries from continuous electrolyte/electrode side reactions. [ 21 ] In addition, the coordinated DMC has higher oxidative stability at high voltage compared to the free DMC due to the reduced electron density on the O in DMC when it coordinates to Li + , as shown in linear sweep voltammetry (LSV) testing (Figure S2, Supporting Information). Therefore, in order to minimize the potential side reactions of the LiFSI‐ x DMC‐TTE electrolytes on the electrode surfaces, the DMC amount needs to be controlled to a highly coordinated level so that a minimum free DMC exists, while LiFSI is at highly dissociated level to ensure a high ionic conductivity.…”

“…[14][15][16][17] By utilizing a noncoordinating hydrofluoroether (HFE) as a diluent to dilute a high concentration electrolyte (HCE), the LHCE maintained the high concentration of Li + and anion aggregation clusters as in the HCE, which effectively suppressed continuous electrolyte decomposition and formed more coordinated, robust, and salt-derived passivation layers on the cathode and anode surfaces, respectively. [14,[18][19][20][21] However, due to the unique compact solvation nature of LHCEs, the salt dissociation degree in LHCEs was still very low as it was in HCEs, which drastically impaired the ion transport properties of LHCEs with low carrier mobility. The reduced ionic characteristics of LHCE prevented it from practical high current density applications.…”

“…Baird et al reported a systematic study of formulating locally superconcentrated electrolytes (LSCEs) and investigated the effect that the coordination environment and ionic properties of their LSCEs had on the fast-charging capabilities in Li||NMC622 cells to facilitate 80% SOC within 5-15 min with good cycle life. [21] Nevertheless, the design principle for these fast-charging electrolytes was still not clear.…”

Tailored PVDF Graft Copolymers via ATRP as High-Performance NCM811 Cathode Binders

“…Nickel-rich Li­(Ni x Co y Mn 1– x – y O 2 ) ( x ≥ 0.6) (NCM) cathodes are regarded as the predominant cathode materials to meet the ever-increasing demand of high energy density next-generation Li-ion batteries, particularly in the development of electric vehicles. − However, undesired structural changes and thermal instability are often observed with increased Ni content, e.g. Ni 0.8 Co 0.1 Mn 0.1 (NCM811), and can be attributed to transition metal (TM) ion dissolution, , phase changes, gas release, and microcracks formed on the secondary particles during cycling. − As a result, the rapid capacity loss, poor capacity retention, as well as thermal decomposition-related safety issues have hindered the successful practical application of NCM811 cathodes. − Several effective strategies have since been proposed to resolve the issues involved with NCM811 cathodes, including metal doping, surface coating and treatments, − gradient structures, modification of liquid electrolytes, − and alternative design of solid electrolytes. − An easy and universal remedy is to stabilize the NCM811 cathodes with functional polymer binders. Poly­(vinylidene fluoride) (PVDF), because of its remarkable electrochemical stability, has been used in lithium-ion batteries for decades and often serves as a benchmark material.…”