Suppression of Mn-Ion-Dissolution of LiNi0.5 Mn1.5 O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures

In Situ Fabrication of Quasi-Solid Polymer Electrolytes for Lithium Metal Battery via Photopolymerization-Induced Microphase Separation

The photopolymerization-induced microphase separation (photo-PIMS) process involving a reactive polymer block was implemented to fabricate nanostructured quasi-solid polymer electrolytes (QSPEs) for use in lithium metal batteries (LMBs). This innovative one-pot fabrication enhances interfacial properties in LMBs by enabling in situ nanostructuring of QSPE directly onto the electrodes. This process also allows for customization of QSPE structural dimensions by tweaking the architecture and molar mass of poly[(oligo ethylene glycol) methyl ether methacrylate-co-styrene] (P(OEGMA-co-S)) macromolecular chain transfer agent. Bicontinuous nanoscale domains of soft P(OEGMA-co-S)/propylene carbonate/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) phase and hard poly[isobornyl acrylate-co-(oligo ethylene glycol)diacrylate] phase furnished the QSPEs with respectively high ionic conductivity (0.34 mS cm-1 at 30 °C) and interesting level of mechanical strength (106-107 Pa at 30 °C). The as-prepared QSPE showed decent electrochemical properties and an electrochemical stability window of about 4.2 V vs Li+/Li. This electrolyte enables the Li||SPE||Li symmetric cell to cycle over 350 h at 0.1 mA cm-2 without evidence of dendrite formation. By means of galvanostatic cycling studies on a prototype lithium metal battery with LiNi0.8Mn0.1Co0.1O2 or LiFePO4 positive electrodes, we further demonstrate that the in situ nanostructured QSPE exhibited better performances than the corresponding stacked battery.

Solution NMR of Battery Electrolytes: Assessing and Mitigating Spectral Broadening Caused by Transition Metal Dissolution

NMR spectroscopy is a powerful tool that is commonly used to assess the degradation of lithium-ion battery electrolyte solutions. However, dissolution of paramagnetic Ni²⁺ and Mn²⁺ ions from cathode materials may affect the NMR spectra of the electrolyte solution, with the unpaired electron spins in these paramagnetic solutes inducing rapid nuclear relaxation and spectral broadening (and often peak shifts). This work establishes how dissolved Ni²⁺ and Mn²⁺ in LiPF₆ electrolyte solutions affect ¹H, ¹⁹F, and ³¹P NMR spectra of pristine and degraded electrolyte solutions, including whether the peaks from degradation species are at risk of being lost and whether the spectral broadening can be mitigated. Mn²⁺ is shown to cause far greater peak broadening than Ni²⁺, with the effect of Mn²⁺ observable at just 10 μM. Generally, ¹⁹F peaks from PF₆ ^- degradation species are most affected by the presence of the paramagnetic metals, followed by ³¹P and ¹H peaks. Surprisingly, when NMR solvents are added to acquire the spectra, the degree of broadening is heavily solvent-dependent, following the trend of solvent donor number (increased broadening with lower solvent donicity). Severe spectral broadening is shown to occur whether Mn is introduced via the salt Mn(TFSI)₂ or is dissolved from LiMn₂O₄. We show that the weak ¹⁹F and ³¹P peaks in spectra of electrolyte samples containing micromolar levels of dissolved Mn²⁺ are broadened to an extent that they are no longer visible, but this broadening can be minimized by diluting electrolyte samples with a suitably coordinating NMR solvent. Li₃PO₄ addition to the sample is also shown to return ¹⁹F and ³¹P spectral resolution by precipitating Mn²⁺ out of electrolyte samples, although this method consumes any HF in the electrolyte solution.

All-Climate High-Voltage Commercial Lithium-Ion Batteries Based on Propylene Carbonate Electrolytes

Propylene carbonate (PC)-based electrolytes have many attractive advantages over the commercially used ethylene carbonate (EC)-based electrolytes like a wider operating temperature and higher oxidation stability. Therefore, PC-based electrolytes become the potential candidate for lithium-ion batteries with higher energy density, longer lifespan, and better low- and high-temperature performance. In spite of the superiority, PC is incompatible with the graphite anode because PC fails to passivate the graphite anode, leading to severe decomposition and gas evolution, which seriously restrict the development of the PC-based electrolytes. Nevertheless, it is recently found that the usage of diethyl carbonate (DEC) as a cosolvent will greatly improve the anodic tolerance of PC to realize the reversible lithiation/delithiation of the graphite anode in the PC-based electrolyte. It is because DEC induces anions into the solvation shell of Li⁺ to form an anion-induced ion-solvent-coordinated (AI-ISC) structure with higher reduction stability. In this work, we fabricated 4.4 V pouch cells to assess in detail the practical viability of the PC-based electrolyte in a commercial battery system. In comparison to conventionally used EC-based cells, the pouch cells with the PC-based electrolyte exhibit more excellent high-voltage tolerance and electrochemical performance at all temperature ranges (-40 to 85 °C), demonstrating the wide application prospect of the PC-based electrolyte.

Nanosized and metastable molybdenum oxides as negative electrode materials for durable high-energy aqueous Li-ion batteries

This study describes a high-energy and durable aqueous battery system with metastable and nanosized Mo-based oxides used as high-capacity negative electrodes. A wider electrochemical window is achieved with concentrated aqueous electrolytes through which highly reversible Li storage without the decomposition of water molecules is achieved for the Mo-based oxides. A full cell with an Mn-based oxide shows good capacity retention over 2,000 cycles. X-ray absorption spectroscopy reveals that the solid-state redox reaction of Mo ions reversibly proceeds in aqueous electrolytes for the metastable Mo oxide. This study opens a way to develop high-energy, durable, and safe batteries on the basis of metastable and nanosized oxides with aqueous electrolyte solutions.

The development of inherently safe energy devices is a key challenge, and aqueous Li-ion batteries draw large attention for this purpose. Due to the narrow electrochemical stable potential window of aqueous electrolytes, the energy density and the selection of negative electrode materials are significantly limited. For achieving durable and high-energy aqueous Li-ion batteries, the development of negative electrode materials exhibiting a large capacity and low potential without triggering decomposition of water is crucial. Herein, a type of a negative electrode material (i.e., Li_xNb_2/7Mo_3/7O₂) is proposed for high-energy aqueous Li-ion batteries. Li_xNb_2/7Mo_3/7O₂ delivers a large capacity of ∼170 mA ⋅ h ⋅ g⁻¹ with a low operating potential range of 1.9 to 2.8 versus Li/Li⁺ in 21 m lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) aqueous electrolyte. A full cell consisting of Li_1.05Mn_1.95O₄/Li_9/7Nb_2/7Mo_3/7O₂ presents high energy density of 107 W ⋅ h ⋅ kg⁻¹ as the maximum value in 21 m LiTFSA aqueous electrolyte, and 73% in capacity retention is achieved after 2,000 cycles. Furthermore, hard X-ray photoelectron spectroscopy study reveals that a protective surface layer is formed at the surface of the negative electrode, by which the high-energy and durable aqueous batteries are realized with Li_xNb_2/7Mo_3/7O₂. This work combines a high capacity with a safe negative electrode material through delivering the Mo-based oxide with unique nanosized and metastable characters.

Nanostructured LiMnO2 with Li3PO4 Integrated at the Atomic Scale for High-Energy Electrode Materials with Reversible Anionic Redox

Nanostructured LiMnO₂ integrated with Li₃PO₄ was successfully synthesized by the mechanical milling route and examined as a new series of positive electrode materials for rechargeable lithium batteries. Although uniform mixing at the atomic scale between LiMnO₂ and Li₃PO₄ was not anticipated because of the noncompatibility of crystal structures for both phases, our study reveals that phosphorus ions with excess lithium ions dissolve into nanosize crystalline LiMnO₂ as first evidenced by elemental mapping using STEM-EELS combined with total X-ray scattering, solid-state NMR spectroscopy, and a theoretical ab initio study. The integrated phase features a low-crystallinity metastable phase with a unique nanostructure; the phosphorus ion located at the tetrahedral site shares faces with adjacent lithium ions at slightly distorted octahedral sites. This phase delivers a large reversible capacity of ∼320 mA h g^-1 as a high-energy positive electrode material in Li cells. The large reversible capacity originated from the contribution from the anionic redox of oxygen coupled with the cationic redox of Mn ions, as evidenced by operando soft XAS spectroscopy, and the superior reversibility of the anionic redox and the suppression of oxygen loss were also found by online electrochemical mass spectroscopy. The improved reversibility of the anionic redox originates from the presence of phosphorus ions associated with the suppression of oxygen dimerization, as supported by a theoretical study. From these results, the mechanistic foundations of nanostructured high-capacity positive electrode materials were established, and further chemical and physical optimization may lead to the development of next-generation electrochemical devices.

Electrolyte Therapy for Improving the Performance of LiNi0.5Mn1.5O4 Cathodes Assembled Lithium-Ion Batteries

High voltage spinel manganese oxide LiNi₀.₅Mn_1.5O₄ (LNMO) cathodes are promising for practical applications owing to several strengths including high working voltages, excellent operating safety, low costs, and so on. However, LNMO-based lithium-ion batteries (LIBs) fade rapidly mainly owing to unqualified electrolytes, hence becoming a big obstacle toward practical applications. To tackle this roadblock, substantial progress has been made thus far, and yet challenges still remain, while rare reviews have systematically discussed the status quo and future development of electrolyte optimization coupling with LNMO cathodes. Here, we discuss cycling degradation mechanisms at the cathode/electrolyte interface and ideal requirements of electrolytes for LNMO cathode-equipped LIBs, as well as review the recent advance of electrolyte optimization for LNMO cathode-equipped LIBs in detail. And then, the perspectives regarding the future research opportunities in developing state-of-the-art electrolytes are also presented. The authors hope to shed light on the rational optimization of advanced organic electrolytes in order to boost the large-scale practical applications of high voltage LNMO cathode-based LIBs.

Enhanced Cycling Performance of Ni-Rich Positive Electrodes (NMC) in Li-Ion Batteries by Reducing Electrolyte Free-Solvent Activity

The interfacial (electro)chemical reactions between electrode and electrolyte dictate the cycling stability of Li-ion batteries. Previous experimental and computational results have shown that replacing Mn and Co with Ni in layered LiNi_xMn_yCo_1-x-yO₂ (NMC) positive electrodes promotes the dehydrogenation of carbonate-based electrolytes on the oxide surface, which generates protic species to decompose LiPF₆ in the electrolyte. In this study, we utilized this understanding to stabilize LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) by decreasing free-solvent activity in the electrolyte through controlling salt concentration and salt dissociativity. Infrared spectroscopy revealed that highly concentrated electrolytes with low free-solvent activity had no dehydrogenation of ethylene carbonate, which could be attributed to slow kinetics of dissociative adsorption of Li⁺-coordinated solvents on oxide surfaces. The increased stability of the concentrated electrolyte against solvent dehydrogenation gave rise to high capacity retention of NMC811 with capacities greater than 150 mA h g^-1 (77% retention) after 500 cycles without oxide-coating and Ni-concentration gradients or electrolyte additives.