Chinese scientists analyze new principles for the design of high-voltage lithium metal electrolytes

On November 17, 2020, the team of Fan Xiulin researchers at Zhejiang University and Tencent Youtu Lab published a new study entitled “Deciphering and modulating energetics of solvation structure enables aggressive high-voltage chemistry of Li metal batteries” in the journal Chem.

This achievement reports how to adjust the dynamic stability of the solvation structure to design a new high-voltage lithium metal electrolyte. The authors use experimental and theoretical calculations to confirm the relationship between diluent-anion interaction and the oxidation stability of the electrolyte, which provides a feasible scheme for the development of next-generation high-voltage electrolytes.

The first authors are Wu Zunchun, a master’s student and Dr. Li Ruhong of Zhejiang University, and the corresponding author of the paper is researcher Fan Xiulin of Zhejiang University. This paper is also strongly supported by Professor Chen Lixin and researcher Fan Liwu of Zhejiang University.

Lithium metal anodes have the highest theoretical specific capacity (3860 mAh g-1) and the lowest potential, so from the perspective of energy density, lithium metal batteries represent the ultimate in chemical energy storage. In recent years, high-concentration and locally high-concentration electrolyte systems have increased the plating/stripping coulombic efficiency of lithium metal to more than 99.1%, while stable cycling at 4.3 V high voltage cathode. Therefore, the research of LMBs has once again attracted the attention of industry and academia. Although these works have achieved very good results in improving the morphology and coulombic efficiency of lithium anode (LMA) deposition, and improving battery cycle performance, there is still a lack of in-depth research on the antioxidant mechanism on the positive electrode side. In addition, the formation conditions of pseudo-high concentration electrolytes are also poorly elaborated. The above key issues limit the further development of LMB high-voltage electrolytes.

In this work, Fan Xiulin’s team analyzed and elaborated the root cause of the oxidation stability of the electrolyte based on the local high-concentration electrolyte system of LiFSI/DME. By profiling the feasibility of multiple non-solvents as diluents, it was found that the diluent-anion interaction is key to avoiding phase separation. For DME systems, the electrostatic potential of the diluent needs to be > 25 kcal/mol. Under the premise of ensuring the homogeneity of the electrolyte, a weak diluent-anion interaction can improve the stability of the solvation structure, thereby improving the oxidation resistance of the electrolyte. In other words, diluents can affect the oxidation resistance of the electrolyte system by affecting the stability of the solvated structure. Based on the above analysis, the research group screened 2H,3H-decafluoropentane (HFC) as a new diluent, and after being formulated with DME into a new ether-based LHCE, in addition to stabilizing the negative cycle, the new electrolyte system >can stably cycle more than 180 cycles (capacity retention rate) in LMBs under practical conditions (20μm Li, NMC/LCO positive electrode load 3.7~4 mAh/cm2, N/P=1~1.08). >90%)。

Figure 1: Potential non-covalent interactions between electrolyte components in pseudo-highly concentrated electrolytes (LHCEs).

Diluents are screened based on the interactions between electrolyte components. The interaction force between the diluent and the lithium ion solvation structure will have a certain impact on the stability of the electrolyte system, and the greater the diluent-anion interaction force, the easier it is for DME to be decoupled from the coordination structure of Li+. To verify this mechanism, the research team selected HFCs with weak interaction forces from a series of inert non-solvents and compared them with the currently reported fluoroether diluents, taking tris(trifluoroethoxy)methane (TFEO) as an example.

Figure 2: Electrochemical performance of different electrolytes

Figure 3: Enkinetic studies of decoordination behavior in HFC-LHCE and TFEO-LHCE

Static and dynamic stability analysis of solvated structures. In Raman spectroscopy and molecular dynamics simulations, both locally concentrated electrolytes have a similar coordination structure of Li+-solvent-anion. However, when an electric field similar to an electric double layer is applied, the coordination stability of the anion and solvent shows some difference in HFC-LHCE and TFEO-LHCE. Through the two-dimensional potential energy surface scanning analysis of the decoordination process, it is found that the decoordination energy barrier of DME and FSI anions in HFC-LHCE is higher than that of TFEO-LHCE.

Figure 4: Characterization of CEI on the cathode of NMC811

Figure 5: Electrochemical performance of lithium-metal full battery

Experimental verification of oxidation stability improvement. Since the DME decoordination process in HFC-LHCE is inhibited, the solvent decomposition is inhibited, and a thinner CEI interface layer with higher inorganic content is formed, which reduces the interface impedance and enhances the protection of the cathode, which further prolongs the cycle life of lithium metal batteries.

Figure 6: Parameters describing the electrochemical performance of LHCE at the positive and negative electrodes

In summary, the stability of the negative cycle in LMB depends on the SEI formed by the decomposition of the electrolyte, while the stability of the positive cycle depends on the stability of the solvation structure. The Crystal Orbital Hamiltonian Layout (ICOHP) can reflect the tightness of the Li+ coordination. The lower this value, the more stable the coordination structure of lithium ions, which in turn reduces the DME content in the decoordinated state and improves the oxidation stability of the electrolyte. Therefore, for subsequent screening of ideal diluents, the following principles should be followed: 1) the polarity is weak enough not to dissolve any lithium salts; 2) The electrostatic potential is strong enough to be miscible with the electrolyte; 3) The effect with the lithium ion solvation structure is weak enough (especially with anions), and the solvation structure is not affected as much as possible when an applied electric field is applied. (Source: Science Network)

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