Motivation

Silicon (Si) anodes represent one of the most promising alloying materials to upgrade the current Li-ion batteries (LIBs) to next-generation high-energy batteries due to its high theoretical capacity (3579 mAh g^-1 of Li₁₅Si₄ vs 372 mAh g^-1 of LiC₆), low electrochemical potential (~0.3 V vs Li/Li⁺), and natural abundance¹. However, the large volume change (~300%) from Si to Li₁₅Si₄ during lithiation/delithiation cycles induces cracks of Si particles and continuous formation of unfavorable solid electrolyte interface (SEI), which results in irreversible capacity loss, low Coulombic efficiency (CE) and poor cell cycle life². The highlighted strategies to tackle the volume expansion issue for Si anodes include downsizing the particles to nano-region or integrating the nanoSi with graphite (Gr) to form the nanoSi-Gr composite anodes³. Nevertheless, nano-sized Si particles will increase the manufacturing cost and reduce the cell calendar life, and the nanoSi-Gr composite anodes will also sacrifice the cell energy density. Therefore, from both the economic and energy utilization perspective, micro-sized Si (uSi) is the ideal material for high-energy batteries.

Electrolyte Challenges

For commercial carbonate-based electrolytes,  it is very challenging to achieve efficient electrochemical reversibility on alloying anodes due to the inevitable cracks of both alloy particles and organic SEI during the repeated charge/discharge cycles (Figure 1a). A more robust SEI is required to accommodate the large stress and volume change of the alloying anodes for long-term cycle performance (Figures b-c).

The key to realizing high reversibility on alloying anodes is to construct high-modulus SEI (e.g. LiF) that has a weak bond to the Li_xM phase (M-phobic) (Figure 1b), therefore reversible volume expansion/contraction of the Li_xM phase can be achieved inside the M-phobic SEI shell, allowing for prolonged cycle performance. Hence, the M-philic organic SEI (Figure 1a) from solvent reduction needs to be suppressed, and the M-phobic inorganic SEI from anion reduction should be promoted. Unless the reduction of the solvent can provide beneficial SEI components like Li₂O⁴. Our previous work indicates that the PF₆^- anion reduction is favorable over FSI^- or TFSI^- anions for effective LiF SEI construction on alloying anodes when using low-reduction solvents⁵. However, the LiPF₆ salt has been found incompatible with the widely used ether solvent like dimethoxyethane (DME) due to their easy polymerization⁶. The challenging question is, is there a way to make the low-reduction ether solvents (with high boiling point) compatible with LiPF₆ salt?

Strategy and Key Findings

Recently, molecular design has arisen as an efficient tool to realize advanced electrolyte development and ionic liquid electrolytes have been long pursued due to their environmental friendliness and high safety. To make the DME solvent compatible with LiPF₆ salt, we chemically introduced the “DME” fragment into the methylpyrrolidinium cation, forming the new ionic liquid, N-Methyl-N-(2-methoxyEthoxy)methyl Pyrrolidinium hexafluorophosphate (abbreviated as NMEP). This asymmetric molecular design resolved the long-lasting DME/LiPF₆ incompatible issues and brought several beneficial advantages for NMEP/LiPF₆ electrolytes to be used on alloying anodes (Figure 2).

(1) The asymmetric design with bulky methylpyrrolidinium cation on one side of the “DME” fragment stops the repeated unit of {Li⁺(DME)_n}(PF₆^-), making “DME” compatible with LiPF₆ salt;

(2) The ether-functionalization reduces the melting point of NMEP below room temperature, combined with the low-reduction of methylpyrrolidinium cation, the NMEP/LiPF₆ electrolytes will promote the formation of LiF SEI on alloying anodes same as mTHF/THF/LiPF₆ electrolytes;

(3) The ionic nature of NMEP improves the anodic stability of NMEP/LiPF₆ electrolytes, therefore high-energy cathodes like NMC811 can be paired with alloying anodes to achieve high energy density.

As a result, the developed NMEP/LiPF₆ electrolytes enabled uSi anodes (325 mesh, Sigma) to deliver a high capacity of >2800 mAh g^-1 for 400 cycles with a capacity retention of >80% and cycle CE of >99.9%. Other low-cost alloying anodes like Al, Sn, Bi also showed excellent cycle performance in the proposed electrolytes, validating its universal application. The uSi||NMC811 full cells were demonstrated for over 200 cycles with a capacity retention of >85%, showing great promise for high-energy batteries. However, the ionic liquid electrolytes are viscous and have issues at higher cycle rates. Inspired by the asymmetric cation design in the NMEP/LiPF₆ electrolytes, we further evaluated the compatibility between LiPF₆ salt and other asymmetric ether solvents. As shown in Figure 2, all three ether solvents showed good solubility for LiPF₆ salt (>2.0 M) without polymerization. We demonstrated that the EME/LiPF₆ electrolytes enable the uSi anodes (325 mesh, Sigma) to deliver >1100 mAh g^-1 capacity at 5C rate, and the uSi||NMC811 full cells for 200 cycles at C/3 rate with a high capacity retention of >90%, projecting high commercialization potential for uSi||NMC811 batteries.

Implications

As the development of electrolytes for uSi anodes is currently still at an early stage, the asymmetric design in the present work opens new doors for engineering advanced electrolytes using ether solvents and LiPF₆ salt, which has been thought impossible before. We believe that the formulation between the asymmetric ether solvents and LiPF₆ salt is expected to become the preferred industrial electrolyte for uSi anodes. Another important message of this work is that the solvation structure of the ether/LiPF₆ electrolytes is also vital for effective LiF SEI formation, which can be fine-tuned for state-of-the-art electrolytes.

More details of this work can be found in our article published in Nature Energy

Ai-Min Li, …et al. Chunsheng Wang* Asymmetric Electrolyte Design for High-Energy Lithium-Ion Batteries with Micro-sized Alloying Anodes. Nat. Energy (2024).

DOI: https://doi.org/10.1038/s41560-024-01619-2

Contributors: Ai-Min Li, Professor Chunsheng Wang

Reference

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