Upgrading Carbonate Electrolytes for Ultra‐stable Practical Lithium Metal Batteries

Inorganic Composition Modulation of Solid Electrolyte Interphase for Fast Charging Lithium Metal Batteries

The solid electrolyte interphase (SEI) with lithium fluoride (LiF) is critical to the performance of lithium metal batteries (LMBs) due to its high stability and mechanical properties. However, the low Li ion conductivity of LiF impedes the rapid diffusion of Li ions in the SEI, which leads to localized Li ion oversaturation dendritic deposition and hinders the practical applications of LMBs at high-current regions (>3 C). To address this issue, a fluorophosphated SEI rich with fast ion-diffusing inorganic grain boundaries (LiF/Li3P) is introduced. By utilizing a sol electrolyte that contains highly dispersed porous LiF nanoparticles modified with phosphorus-containing functional groups, a fluorophosphated SEI is constructed and the presence of electrochemically active Li within these fast ion-diffusing grain boundaries (GBs-Li) that are non-nucleated is demonstrated, ensuring the stability of the Li || NCM811 cell for over 1000 cycles at fast-charging rates of 5 C (11 mA cm-2). Additionally, a practical, long cycling, and intrinsically safe LMB pouch cell with high energy density (400 Wh kg-1) is fabricated. The work reveals how SEI components and structure design can enable fast-charging LMBs.

A nonsolvolytic fluorine/LiNO3-containing electrolyte for stabilizing dynamic interfaces in Li||LiMn2O4 batteries

Mn-based high voltage cathodes, e.g., spinel LiMn2O4, are considered among the most promising materials for cost-effective, next generation energy storage. When paired with a Li metal anode, secondary batteries based on Li||LiMn2O4 in principle offer a straightforward, scalable approach for achieving cost-effective and high energy density storage demanded in applications. In practice, however, such batteries fail to live up to their promise. Rapid capacity fading caused by irreversible Mn dissolution at the cathode coupled with mossy/dendritic Li deposition at the anode limit their useful life. In this study, we report on the design of electrolytes based on a binary blend of two widely available salts, LiNO3 and LiTFSI, in ethylene carbonate (EC), which simultaneously overcome failure modes at both the cathode and anode of Li||LiMn2O4 batteries. The electrolyte design is motivated by a recent finding that compared with their linear counterparts (e.g., dimethyl carbonate), cyclic carbonates like EC dissolve considerably larger amount of LiNO3, which markedly improves anode reversibility. On the other hand, it is known that nonsolvolytic fluorine-containing Li salts like LiTFSI, lowers the electrolyte's susceptibility to solvolysis, which generates HF species responsible for Mn leaching at the cathode. In particular, we report instead that fluorine groups in the TFSI salt, promote formation of a favorable, fluorine-rich interphase on the Li metal anode. Electrochemical measurements show that the electrolytes enable remarkably improved charge-discharge cycling stability (>1000 charge-discharge cycles) of Li||LiMn2O4 batteries. In-depth atomic-resolution electron microscopy and X-ray/synchrotron diffraction experiments reveal the fundamental source of the improvements. The measurements show that crystallographic degradation of Mn-based cathodes (e.g., surface Mn leaching and bulk defect generation) upon cycling in conventional electrolytes is dramatically lowered in the LiNO3 + LiTFSI/EC electrolyte system. It is shown further that the reduction of Mn dissolution not only improves the cathode stability but improves the reversibility of the Li metal anode via a unique re-deposition mechanism in which Li and Mn co-deposit on the anode. Taken together, our findings show that the LiNO3 + LiTFSI/EC electrolyte system holds promise for accelerating progress towards practical Li||LiMn2O4 batteries because it stabilizes the dynamic interfaces required for long-term stability at both the Li anode and the LiMn2O4 cathode.

Nanocomposite of Conducting Polymer and Li Metal for Rechargeable High Energy Density Batteries

The structure and electrochemical performance of lithium (Li) metal degrade quickly owing to its hostless nature and high reactivity, hindering its practical application in rechargeable high energy density batteries. In order to enhance the electrochemical reversibility of metallic Li, we designed a Li/Li₂S-poly(acrylonitrile) (LSPAN) composite foil via a facile mechanical kneading approach using metallic Li and sulfurized poly(acrylonitrile) as the raw materials. The uniformly dispersed Li₂S-poly(acrylonitrile) (Li₂S-PAN) in a metallic Li matrix buffered the volume change on cycling, and its high Li ion conductivity enabled fast Li ion diffusion behavior of the composite electrode. As expected, the LSPAN electrode showed reduced voltage polarization, enhanced rate capability, and prolonged cycle life compared with the pure Li electrode. It exhibited stable cycling for 600 h with a symmetric cell configuration at 1 mA cm^-2 and 1 mA h cm^-2, far outperforming the pure metallic Li counterpart (400 h). Also, the LiCoO₂||LSPAN full cells with a cathode mass loading of ∼16 mg cm^-2 worked stably for 100 cycles at 0.5 C with a high capacity retention of 96.5%, while the LiCoO₂||Li full cells quickly failed within only 50 cycles.