Quasi-Gel Polymer Electrolyte Interfaced with Electrodes through Solvent-Swollen Poly(ethylene oxide) for High-Performance Lithium/Lithium-Ion Batteries.

Solid polymer electrolytes (SPEs) are regarded as a superior alternative to traditional liquid electrolytes of lithium-ion batteries (LIBs) due to their improved safety features. The practical implementation of SPEs faces challenges, such as low ionic conductivity at room temperature (RT) and inadequate interfacial contact, leading to high interfacial resistance across the electrode and electrolyte interfaces. In this study, we addressed these issues by designing a quasi-gel polymer electrolyte (QGPE), a blend of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(ethylene oxide) (PEO), and succinonitrile (SN), with the desired mechanical strength, ionic conductivity, and interfacial stability through a simple solution casting technique. The QGPE features a thin solvated PEO layer on its surface, which wets the electrode, reducing the interfacial resistance and ensuring a homogeneous Li-ion flux across the interface. The optimized QGPE exhibits a good lithium-ion conductivity of 1.14 × 10-3 S cm-1 with a superior lithium-ion transference number of 0.7 at 25 °C. The Li/QGPE/Li symmetric cell exhibits a highly reversible lithium plating/stripping process for over 1300 h with minimal voltage polarization of ∼20 mV. The Li/QGPE/LiFePO4 full cell demonstrates good rate capability and excellent long-term cycling performance at a 0.1 C rate at 25 °C, maintaining a specific discharge capacity of 148 mAh g-1 over 200 cycles. The effectiveness of QGPE for LIBs is proven using a graphite/QGPE/LiFePO4 4 × 4 cm pouch cell, showcasing outstanding flexibility and tolerance against intentional abuse.

Crystal Chemistry and Ionic Conductivity of the NASICON-Related Phases in the Li3-xNaxV2(PO4)3 System.

In the present work, we studied crystal phases in the Li3-xNaxV2(PO4)3 system over a wide range of x prepared by four synthesis methods: mechanochemically assisted solid-state synthesis, 'soft chemistry' sol-gel approach, chemical (CIE) and electrochemical (EIE) ion exchange starting from Li3V2(PO4)3 (anti-NASICON, P21/c S.G.), and Na3V2(PO4)3 (NASICON, C2/c S.G.). EIE was studied by operando and ex situ XRD in Li3V2(PO4)3 vs Na and Na3V2(PO4)3 vs Li electrochemical cells. It was shown that both mechanochemically assisted solid-state and sol-gel synthesis methods do not result in the single-phase Na3-xLixV2(PO4)3. In contrast, CIE and EIE lead to deep substitution degrees and proceed much easier in the NASICON framework (Na3V2(PO4)3), where more than 2/3 of Na+ ions per f.u. are replaced with Li+ resulting in Na0.6Li2.4V2(PO4)3 (R3̅ S.G.), while in the anti-NASICON framework (Li3V2(PO4)3), only 1/3 of Li+ ions are replaced with Na+ resulting in Li2NaV2(PO4)3 (Pbcn S.G.), which was shown to be a metastable phase, and after high-temperature treatment, it decomposes into two NASICON-type compounds. The ionic conductivity was analyzed both theoretically and experimentally, and the results show that in the NASICON framework, the migration of both Na+ and Li+ ions is realized, while in the anti-NASICON framework, the Li+ migration is preferable. The contribution of the electronic component to total conductivity was determined.

Concentration Gradient-Driven Aluminum Diffusion in a Single-Step Coprecipitation of a Compositionally Graded Precursor for LiNi0.8Co0.135Al0.065O2 with Mitigated Irreversibility of H2 ↔ H3 Phase Transition.

LiNi1-x-yCoxAlyO2 (NCA) possessing a nano-/micro hierarchical architecture delivers a high specific capacity of 200 mAh/g with an upper cutoff voltage of 4.4 V. However, the structural reconstruction due to the irreversibility of the H2 ↔ H3 phase transition at higher voltage increases the initial irreversible capacity loss and charge-transfer impedance and reduces the performance at higher C-rates. Structural and electrochemical stability can be achieved by reducing the nickel content and increasing the electrochemically inactive aluminum at the surface. Nonetheless, getting an aluminum concentration gradient in NCA-(OH)2 is difficult owing to the difference in the solubility constant and reaction kinetics of Al(OH)3 compared to that of NiCo-(OH)2. Hence, we have exploited the high diffusion of nano-Al(OH)3 driven by the concentration gradient of Al across the hierarchical hydroxide structure and synthesized LiNi0.8Co0.135Al0.065O2 (NCA) with reduced Ni and increased Al at the surface. The process of formation of a concentration gradient was analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, and cross-sectional elemental mapping. The concentration-graded NCA exhibited superior electrochemical performance compared to its pristine counterpart. The graded NCA shows excellent reversibility of the H2 ↔ H3 phase, leading to low impedance development, confirming the reduced surface reconstruction during the initial cycles. Therefore, the specific capacity of graded NCA is 65% higher than that of pristine NCA at 10 C. Both in half-cell and in full-cell configurations, the graded NCA exhibited superior first cycle reversibility and specific capacity. Specifically, in the full-cell configuration, the capacity retention of graded NCA is 91.5%, while that of pristine NCA is 83% after 150 cycles when cycled between 3 and 4.3 V. Further, the capacity loss reduces to 1% even after 500 cycles when the upper cutoff voltage is reduced to 4.2 V in the case of graded NCA.