Electrolyte engineering is critical for developing Li metal batteries. While recent works improved Li metal cyclability, a methodology for rational electrolyte design remains lacking. Herein, we propose a design strategy for electrolytes that enable anode-free Li metal batteries with single-solvent single-salt formations at standard concentrations. Rational incorporation of –CF2– units yields fluorinated 1,4-dimethoxylbutane as the electrolyte solvent. Paired with 1 M lithium bis(fluorosulfonyl)imide, this electrolyte possesses unique Li–F binding and high anion/solvent ratio in the solvation sheath, leading to excellent compatibility with both Li metal anodes (Coulombic efficiency ~ 99.52% and fast activation within five cycles) and high-voltage cathodes (~6 V stability). Fifty-μm-thick Li|NMC batteries retain 90% capacity after 420 cycles with an average Coulombic efficiency of 99.98%. Industrial anode-free pouch cells achieve ~325 Wh kg−1 single-cell energy density and 80% capacity retention after 100 cycles. Our design concept for electrolytes provides a promising path to high-energy, long-cycling Li metal batteries. The realization of the full potential of Li metal batteries requires high-performance electrolytes. Here Z. Bao and colleagues develop low-concentration electrolytes with a single-solvent and single-salt formulation, offering promise for high-energy and long-cycling batteries.

Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries

Coulombic efficiency (CE) has been widely used in battery research as a quantifiable indicator for the reversibility of batteries While CE helps to predict the lifespan of a lithium-ion battery, the prediction is not necessarily accurate in a rechargeable lithium metal battery Here, we discuss the fundamental definition of CE and unravel its true meaning in lithium-ion batteries and a few representative configurations of lithium metal batteries Through examining the similarities and differences of CE in lithium-ion batteries and lithium metal batteries, we establish a CE measuring protocol with the aim of developing high-energy long-lasting practical lithium metal batteries The understanding of CE and the CE protocol are broadly applicable in other rechargeable metal batteries including Zn, Mg and Na batteries Coulombic efficiency (CE) has been frequently used to assess the cyclability of newly developed materials for lithium metal batteries The authors argue that caution must be exercised during the assessment of CE, and propose a CE testing protocol for the development of lithium metal batteries

Understanding and applying coulombic efficiency in lithium metal batteries

It would be unwise to assume ‘conventional’ lithium-ion batteries are approaching the end of their era and so we discuss current strategies to improve the current and next generation systems, where a holistic approach will be needed to unlock higher energy density while also maintaining lifetime and safety. We end by briefly reviewing areas where fundamental science advances will be needed to enable revolutionary new battery systems.

/pdf/prospects-for-lithium-ion-batteries-and-beyond-a-2030-vision-1h0ut0fkyt.pdf

Prospects for lithium-ion batteries and beyond-a 2030 vision

Fast charging of energy-dense lithium-ion batteries

Anode-free lithium metal cells store 60% more energy per volume than conventional lithium-ion cells. Such high energy density can increase the range of electric vehicles by approximately 280 km or even enable electrified urban aviation. However, these cells tend to experience rapid capacity loss and short cycle life. Furthermore, safety issues concerning metallic lithium often remain unaddressed in the literature. Recently, we demonstrated long-lifetime anode-free cells using a dual-salt carbonate electrolyte. Here we characterize the degradation of anode-free cells with this lean (2.6 g Ah−1) liquid electrolyte. We observe deterioration of the pristine lithium morphology using scanning electron microscopy and X-ray tomography, and diagnose the cause as electrolyte degradation and depletion using nuclear magnetic resonance spectroscopy and ultrasonic transmission mapping. For the safety characterization tests, we measure the cell temperature during nail penetration. Finally, we use the insights gained in this work to develop an optimized electrolyte, extending the lifetime of anode-free cells to 200 cycles. Anode-free batteries have emerged as a promising storage means to offer high energy density but still suffer from long-term reversibility. The authors analyse the cell failure mechanisms and present an optimized electrolyte to extend the lifetime of anode-free pouch cells.

Diagnosing and correcting anode-free cell failure via electrolyte and morphological analysis

https://robots.iopscience.iop.org/article/10.1149/2.0661914jes/pdf

Hot Formation for Improved Low Temperature Cycling of Anode-Free Lithium Metal Batteries

Electrolyte systems based on binary mixtures of organic carbonate ester cosolvents have limitations in ionic transport and thus limit extreme fast charge (XFC) and high-rate cycling of energy dense...

Ester-Based Electrolytes for Fast Charging of Energy Dense Lithium-Ion Batteries

https://iopscience.iop.org/article/10.1149/1945-7111/abe1da/pdf

Study of Electrolyte and Electrode Composition Changes vs Time in Aged Li-Ion Cells

https://iopscience.iop.org/article/10.1149/1945-7111/abd833/pdf

Optimizing Electrolyte Additive Loadings in NMC532/Graphite Cells: Vinylene Carbonate and Ethylene Sulfate


 LiNi0.5Mn0.3Co0.2O2/graphite cells with two different electrolytes underwent charge-discharge cycling at 70ºC. The 70ºC condition reduced the time it took for cells to lose significant capacity. Studies of the changes to the electrolyte after cycling by gas chromatography/mass spectrometry (GC/MS) and by Nuclear Magnetic Resonance spectroscopy (NMR) suggest that the same processes which cause cell failure and electrolyte degradation at 40ºC and 55ºC occur at 70ºC, only at an accelerated rate. Transition metal dissolution from the positive electrode was tracked using X-ray fluorescence studies of the negative electrode after testing. Based on the confidence obtained that the same degradation processes were occurring; advanced graphites were screened in NMC811/graphite cells at 70ºC. Differences in cell lifetime were apparent in weeks at 70ºC while the same differences took much longer to observe at 40ºC. It is our opinion that elevated temperature testing of Li-ion cells at 70oC is a viable rapid screening technique for advanced electrolytes and advanced electrode materials.

High Temperature Testing of NMC/Graphite Cells for Rapid Cell Performance Screening and Studies of Electrolyte Degradation

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