Regulating Li-Ion Migration in Solid-State Electrolytes for Li Metal Anodes
Authors: Zhao Chen-Zi , Cheng Xin-Bing , Zhang Rui , Zhang Qiang 
Published: May 2020

Lithium (Li) metal batteries are strongly considered one of the most promising candidates for next-generation rechargeable batteries due to their high theoretical specific capacity and low reduction potential. However, Li metal anodes are suffering from limited cycle life, low cycling efficiency, and severe safety concerns resulting from sharp Li dendrites and electrolyte degradation.

The uncontrollable Li dendrite growth caused by the uneven distribution of Li ions is an extremely complicated issue coupled with the transport of anions, cations and electrons, as well as the chemical and electrochemical reactions at the anode-electrolyte interphase. In terms of the ion migration, space-charge theory indicates that the electrolyte is divided into a quasi-neutral region and a space-charge region. The quasi-neutral region approaches the cathode side, at which the ion transfer is governed by diffusion. In contrast, when ions travel close to the anode side, the ion transport is mainly driven by the electric field, leaving a space-charge region that accounts for ramified Li metal growth. Consequently, it is rewarding to design electrolytes that can regulate the ion distribution (including both Li cations and the counter anions) to inhibit Li dendrite growths.

Immobilizing anion has been theoretically proved to influence cation distribution, where a portion of immobilized anions (even as small as 10%) contribute to stable Li electrodeposition. Here, we develop an anion-immobilized solid-state composite electrolyte for Li metal anodes. Garnet-type Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramic particles are well dispersed in a polymer matrix to synthesize a polyethylene oxide (PEO)–lithium bis(trifluoromethylsulphonyl)imide (LiTFSI)–LLZTO (PLL) solid electrolyte. In contrast to routine electrolytes with mobile anions, the PLL solid electrolyte contributes to effective immobilization of anions for uniform ion distribution and dendrite-free Li deposition. Chemical and mechanical interactions between ceramic particles and the polymer matrix result in reduced PEO crystallinity and pinned TFSI−anions, enabling relatively fast Li+conduction. We further demonstrate the application of this flexible membrane in all–solid-state LiFePO4 (LFP) || Li and LiNi0.5Co0.2Mn0.3O2(NCM) || Li batteries.

Moreover, ion migration in batteries can be analogized to the fluid transportation in packed towers in the field of chemical industry. Redistributors are used to intensify the heat and mass transfer processes in a packed tower. Solid-state electrolyte LLZTO, with abundant 3D ion conduction channels, can act as a redistributor to regulate Li-ion distribution. We propose a concept of ion redistributors to eliminate dendrites by redistributing Li ions with LLZTO coated polypropylene (PP) separators in Li metal batteries. The standard deviation of ion concentration beneath the LLZTO composite separator is 13 times less than that beneath the routine PP separator. This approach enables a high specific capacityfor LiFePO4|| Li pouch cells and prolonged cycle life span of 800 hours for Li || Li pouch cells. This strategy is facile and efficient in regulating Li-ion deposition by separator modifications and is a universal method to protect alkali metal anodes in rechargeable batteries.

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