Dominant Solvent-Separated Ion Pairs in Electrolytes Enable Superhigh Conductivity for Fast-Charging and Low-Temperature Lithium Ion Batteries

The low ionic conductivities of aprotic electrolytes hinder the development of extreme fast charging technologies and applications at low temperatures for lithium-ion batteries (LIBs). Herein, we present an electrolyte with LiFSI in acetone (DMK). In DMK electrolytes, the solvation number is three, and solvent-separated ion pairs (SSIPs) are the dominant structure, which is largely different from other linear aprotic electrolytes where salts primarily exist as contact ion pairs (CIPs). With incompact solvation structures due to the weak solvation ability of DMK with Li+, the ionic conductivity reaches 45 mS/cm at room temperature. The percentage of SSIPs increases as temperatures decrease in DMK electrolytes, which is totally different from the carbonate-based electrolytes but greatly beneficial to low-temperature ionic conductivity. With the appropriate addition of VC and FEC, DMK-based electrolytes still exhibit a superhigh ionic conductivity. Even at -40 °C, the ionic conductivity is greater than 10 mS/cm. With DMK-based electrolytes, LIBs with thick LiFePO4 electrodes can be cycled at high rates and at low temperatures.

DFT Study on the Oxidation Mechanism of Common Cyclic Carbonates in the Presence of BF4- Anions

Oxidative decomposition reactions of common cyclic carbonates in the presence of BF₄^- anions were investigated using density functional theory. A polarized continuum model was utilized to model solvent effects in the oxidation of ethylene carbonate (EC) and propylene carbonate (PC) clusters. We have found that the presence of BF₄^- significantly reduces EC and PC oxidation stability, from 7.11 to 6.17 and from 7.10 to 6.06 V (vs Li⁺/Li), respectively. The sequence of EC and PC oxidative decomposition paths and the oxidative products were affected by the BF₄^- anion. The decomposition products of the oxidized EC-BF₄^- contained CO₂, vinyl alcohol, and acetaldehyde, while the decomposition products of the oxidized PC-BF₄^- contained CO₂, acetone, and propanal, in agreement with the previous experimental studies. The oxidative decomposition reactions for PC-BF₄^- are compared with those for the isolated PC, PC_2, PC-ClO₄^-, and PC-PF₆^-.

Algorithmic Explorations of Unimolecular and Bimolecular Reaction Spaces

Algorithmic reaction exploration based on transition state searches has already made inroads into many niche applications, but its potential as a general-purpose tool is still largely unrealized. Computational cost and the absence of benchmark problems involving larger molecules remain obstacles to further progress. Here an ultra-low cost exploration algorithm is implemented and used to explore the reactivity of unimolecular and bimolecular reactants, comprising a total of 581 reactions involving 51 distinct reactants. The algorithm discovers all established reaction pathways, where such comparisons are possible, while also revealing a much richer reactivity landscape, including lower barrier reaction pathways and a strong dependence of reaction conformation in the apparent barriers of the reported reactions. The diversity of these benchmarks illustrate that reaction exploration algorithms are approaching general-purpose capability.

Understanding and Suppressing the Destructive Cobalt(II) Species in Graphite Interphase

Co²⁺ species dissolved from LiCoO₂ in lithium-ion batteries have been well-established to be responsible for the cell performance fading, especially when the cells are charged to high voltage or at elevated temperatures. The accepted underlying mechanism is the deposition of Co²⁺ on the graphite anode that destroys the interphase. In this work, we report that the dissolved Co²⁺ exists in the form of both Co⁰ and Co²⁺ on the graphite anode surface, while Co⁰ formed at lithium insertion potential can be reoxidized to Co²⁺ during charging. Moreover, Co⁰ shows a higher catalytic activity than Co²⁺ toward the reductive decomposition of carbonate electrolyte. An interphase of ∼4 nm was thus engineered from a film-forming additive 3-sulflone, which completely eliminates the destructive effect of the deposited Co species. The understanding of the destructive role of the dissolved Co²⁺ on the interphasial stability of the graphite electrode and an effective strategy to suppress such a failure mechanism provides fresh insight into the failure mechanism of manganese-based cathode chemistries, which serves as a better guideline for electrolyte design for future batteries.

Role of solvent-anion charge transfer in oxidative degradation of battery electrolytes

Electrochemical stability windows of electrolytes largely determine the limitations of operating regimes of lithium-ion batteries, but the degradation mechanisms are difficult to characterize and poorly understood. Using computational quantum chemistry to investigate the oxidative decomposition that govern voltage stability of multi-component organic electrolytes, we find that electrolyte decomposition is a process involving the solvent and the salt anion and requires explicit treatment of their coupling. We find that the ionization potential of the solvent-anion system is often lower than that of the isolated solvent or the anion. This mutual weakening effect is explained by the formation of the anion-solvent charge-transfer complex, which we study for 16 anion-solvent combinations. This understanding of the oxidation mechanism allows the formulation of a simple predictive model that explains experimentally observed trends in the onset voltages of degradation of electrolytes near the cathode. This model opens opportunities for rapid rational design of stable electrolytes for high-energy batteries.

Lithium Bis(oxalate)borate Reinforces the Interphase on Li-Metal Anodes

Li metal provides an ideal anode for the highest energy density batteries, but its reactivity with electrolytes brings poor cycling stability. Electrolyte additives have been employed to effectively improve the cycling stability, often with the underlying mechanism poorly understood. In this work, applying lithium bis(oxalate)borate (LiBOB) as a chemical source for a dense and protective interphase, we investigate this issue with combined techniques of electrochemical/physical characterizations and theoretical calculations. It was revealed that the solid electrolyte interphase (SEI) formed by Li and the carbonate electrolyte is unstable and responsible for the fast deterioration of the Li anode. When LiBOB is present in the electrolyte, a reinforced SEI was formed, enabling significant improvement in cycling stability due to the preferential reduction of the BOB anion over the carbonate molecules and the strong combination of its reduction products with the species from the electrolyte reduction. The effectiveness of such new SEI chemistry on the Li anode supports excellent performance of a Li/LiFePO₄ cell. This approach provides a pathway to rationally design an interphase on the Li anode so that high energy density batteries could be realized.

Tris(trimethylsilyl) Phosphite as an Efficient Electrolyte Additive To Improve the Surface Stability of Graphite Anodes

Tris(trimethylsilyl) phosphite (TMSP) has received considerable attention as a functional additive for various cathode materials in lithium-ion batteries, but the effect of TMSP on the surface stability of a graphite anode has not been studied. Herein, we demonstrate that TMSP serves as an effective solid electrolyte interphase (SEI)-forming additive for graphite anodes in lithium-ion batteries (LIBs). TMSP forms SEI layers by chemical reactions between TMSP and a reductively decomposed ethylene carbonate (EC) anion, which is strikingly different from the widely known mechanism of the SEI-forming additives. TMSP is stable under cathodic polarization, but it reacts chemically with radical anion intermediates derived from the electrochemical reduction of the carbonate solvents to generate a stable SEI layer. These TMSP-derived SEI layers improve the interfacial stability of the graphite anode, resulting in a retention of 96.8% and a high Coulombic efficiency of 95.2%. We suggest the use of TMSP as a functional additive that effectively stabilizes solid electrolyte interfaces of both the anode and cathode in lithium-ion batteries.

Do primary carriers of both positive charge and unpaired electron spin exist in irradiated propylene carbonate?

Magnetic field sensitive fluorescence from irradiated propylene carbonate solutions indicates the existence of previously unobserved radical cations formed from the solvent molecules.

Towards high throughput screening of electrochemical stability of battery electrolytes

High throughput screening of solvents and additives with potential applications in lithium batteries is reported. The initial test set is limited to carbonate and phosphate-based compounds and focused on their electrochemical properties. Solvent stability towards first and second reduction and oxidation is reported from density functional theory (DFT) calculations performed on isolated solvents surrounded by implicit solvent. The reorganization energy is estimated from the difference between vertical and adiabatic redox energies and found to be especially important for the accurate prediction of reduction stability. A majority of tested compounds had the second reduction potential higher than the first reduction potential indicating that the second reduction reaction might play an important role in the passivation layer formation. Similarly, the second oxidation potential was smaller for a significant subset of tested molecules than the first oxidation potential. A number of potential sources of errors introduced during screening of the electrolyte electrochemical properties were examined. The formation of lithium fluoride during reduction of semifluorinated solvents such as fluoroethylene carbonate and the H-transfer during oxidation of solvents were found to shift the electrochemical potential by 1.5-2 V and could shrink the electrochemical stability window by as much as 3.5 V when such reactions are included in the screening procedure. The initial oxidation reaction of ethylene carbonate and dimethyl carbonate at the surface of the completely de-lithiated LiNi0.5Mn1.5O4 high voltage spinel cathode was examined using DFT. Depending on the molecular orientation at the cathode surface, a carbonate molecule either exhibited deprotonation or was found bound to the transition metal via its carbonyl oxygen.

Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening

Computational screening techniques have been found to be an effective alternative to the trial and error of experimentation for discovery of new materials. With increased interest in development of advanced electrical energy storage systems, it is essential to find new electrolytes that function effectively. This Perspective reviews various methods for screening electrolytes and then describes a hierarchical computational scheme to screen multiple properties of advanced electrical energy storage electrolytes using high-throughput quantum chemical calculations. The approach effectively down-selects a large pool of candidates based on successive property evaluation. As an example, results of screening are presented for redox potentials, solvation energies, and structural changes of ∼1400 organic molecules for nonaqueous redox flow batteries. Importantly, on the basis of high-throughput screening, in silico design of suitable candidate molecules for synthesis and electrochemical testing can be achieved. We anticipate that the computational approach described in this Perspective coupled with experimentation will have a significant role to play in the discovery of materials for future energy needs.

Oxidative stability and reaction mechanism of lithium bis(oxalate)borate as a cathode film-forming additive for lithium ion batteries

The most possible oxidative decomposition reaction path of EC–BOB⁻cluster.