New Insights into the Compositional Dependence of Li-Ion Transport in Polymer–Ceramic Composite Electrolytes

Composite electrolytes are widely studied for their potential in realizing improved ionic conductivity and electrochemical stability. Understanding the complex mechanisms of ion transport within composites is critical for effectively designing high-performance solid electrolytes. This study examines the compositional dependence of the three determining factors for ionic conductivity, including ion mobility, ion transport pathways, and active ion concentration. The results show that with increase in the fraction of ceramic Li₇La₃Zr₂O₁₂ (LLZO) phase in the LLZO–poly­(ethylene oxide) composites, ion mobility decreases, ion transport pathways transit from polymer to ceramic routes, and the active ion concentration increases. These changes in ion mobility, transport pathways, and concentration collectively explain the observed trend of ionic conductivity in composite electrolytes. Liquid additives alter ion transport pathways and increase ion mobility, thus enhancing ionic conductivity significantly. It is also found that a higher content of LLZO leads to improved electrochemical stability of composite electrolytes. This study provides insight into the recurring observations of compositional dependence of ionic conductivity in current composite electrolytes and pinpoints the intrinsic limitations of composite electrolytes in achieving fast ion conduction

Understanding the Conduction Mechanism of the Protonic Conductor CsH₂PO₄ by Solid-State NMR Spectroscopy

Local dynamics and hydrogen bonding in CsH₂PO₄ have been investigated by ¹H, ²H, and ³¹P solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by ¹H and ²H NMR, and their chemical shifts (¹H) and quadrupolar coupling constants (²H) are consistent with one strong and one slightly weaker H-bonding environment. Two different protonic motions are detected by variable-temperature ¹H MAS NMR and <i>T</i>₁ spin–lattice relaxation time measurements in the paraelectric phase, which we assign to librational and long-range translational motions. An activation energy of 0.70 ± 0.07 eV is extracted for the latter motion; that of the librational motion is much lower. ³¹P NMR line shapes are measured under MAS and static conditions, and spin–lattice relaxation time measurements have been performed as a function of temperature. Although the ³¹P line shape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the ³¹P CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the <i>T</i>₁ measurements along with considerable line narrowing of both the ¹H and the ³¹P NMR signals

Efficient Co-Nanocrystal-Based Catalyst for Hydrogen Generation from Borohydride

Sodium borohydride (NaBH₄) has been proposed as a potential hydrogen storage material for fuel cells, and the development of highly active and robust catalysts for hydrolyzing NaBH₄ is the key for the practical usage of NaBH₄ for fuel cells. Herein we report Co-nanocrystal assembled hollow nanoparticles (Co-HNP) as an active and robust catalyst for the hydrolysis of NaBH₄. A hydrogen generation rate of 10.8 L·min^–1·g^–1 at 25 °C was achieved by using the Co-HNP catalyst with a low activation energy of 23.7 kJ·mol^–1, which is among the best performance of reported noble and non-noble catalysts for hydrolyzing NaBH₄. Co-HNP also showed good stability in the long term cycling tests. The mechanism of the catalytic hydrolysis of NaBH₄ on Co-HNP was studied by using ¹H and ¹¹B solid-state NMR, which provided unambiguous experimental evidence of the Co–H formation. The systematically designed NMR experiments unveiled the key role of Co-HNP in the activation of borohydride and the subsequent transfer of H^– to water for generating H₂ gas and helped to distinguish various hypotheses proposed for catalytic H₂ generation reactions. The porous hollow nanostructure of the Co-HNP catalyst provides large surface area and facilitates mass transfer. The facile preparation and outstanding performance of Co-HNP enables it as a very competitive catalyst for hydrogen production

Composite Polymer Electrolytes with Li₇La₃Zr₂O₁₂ Garnet-Type Nanowires as Ceramic Fillers: Mechanism of Conductivity Enhancement and Role of Doping and Morphology

Composite polymer solid electrolytes (CPEs) containing ceramic fillers embedded inside a polymer-salt matrix show great improvements in Li⁺ ionic conductivity compared to the polymer electrolyte alone. Lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) with a garnet-type crystal structure is a promising solid Li⁺ conductor. We show that by incorporating only 5 wt % of the ceramic filler comprising undoped, cubic-phase LLZO nanowires prepared by electrospinning, the room temperature ionic conductivity of a polyacrylonitrile-LiClO₄-based composite is increased 3 orders of magnitude to 1.31 × 10^–4 S/cm. Al-doped and Ta-doped LLZO nanowires are also synthesized and utilized as fillers, but the conductivity enhancement is similar as for the undoped LLZO nanowires. Solid-state nuclear magnetic resonance (NMR) studies show that LLZO NWs partially modify the PAN polymer matrix and create preferential pathways for Li⁺ conduction through the modified polymer regions. CPEs with LLZO nanoparticles and Al₂O₃ nanowire fillers are also studied to elucidate the role of filler type (active vs passive), LLZO composition (undoped vs doped), and morphology (nanowire vs nanoparticle) on the CPE conductivity. It is demonstrated that both intrinsic Li⁺ conductivity and nanowire morphology are needed for optimal performance when using 5 wt % of the ceramic filler in the CPE

Operando EPR for Simultaneous Monitoring of Anionic and Cationic Redox Processes in Li-Rich Metal Oxide Cathodes

Anionic redox chemistry offers a transformative approach for significantly increasing specific energy capacities of cathodes for rechargeable Li-ion batteries. This study employs operando electron paramagnetic resonance (EPR) to simultaneously monitor the evolution of both transition metal and oxygen redox reactions, as well as their intertwined couplings in Li₂MnO₃, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. Reversible O^2–/O₂^<i>n</i>– redox takes place above 3.0 V, which is clearly distinguished from transition metal redox in the operando EPR on Li₂MnO₃ cathodes. O^2–/O₂^<i>n</i>– redox is also observed in Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes, albeit its overlapping potential ranges with Ni redox. This study further reveals the stabilization of the reversible O redox by Mn and e^– hole delocalization within the Mn–O complex. The interactions within the cation–anion pairs are essential for preventing O₂^<i>n</i>– from recombination into gaseous O₂ and prove to activate Mn for its increasing participation in redox reactions. Operando EPR helps to establish a fundamental understanding of reversible anionic redox chemistry. The gained insights will support the search for structural factors that promote desirable O redox reactions

Lithiation and Delithiation Dynamics of Different Li Sites in Li-Rich Battery Cathodes Studied by <i>Operando</i> Nuclear Magnetic Resonance

Li in Li-rich cathodes mostly resides at octahedral sites in both Li layers (Li_Li) and transition metal layers (Li_TM). Extraction and insertion of Li_Li and Li_TM are strongly influenced by surrounding transition metals. pjMATPASS and <i>operando</i> Li nuclear magnetic resonance are combined to achieve both high spectral and temporal resolution for quantitative real time monitoring of lithiation and delithiation at Li_Li and Li_TM sites in Li₂MnO₃, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. The results have revealed that Li_TM are preferentially extracted for the first 20% of charge and then Li_Li and Li_TM are removed at the same rate. No preferential insertion or extraction of Li_Li and Li_TM is observed beyond the first charge. Ni and Co promote faster and more complete removal of Li_TM. The recovery of the removed Li is <60% for Li_TM and >80% for Li_Li upon first discharge. The study sheds light on the activity of Li_Li and Li_TM during electrochemical processes as well as their respective contributions to cathode capacity

Li Distribution Heterogeneity in Solid Electrolyte Li₁₀GeP₂S₁₂ upon Electrochemical Cycling Probed by ⁷Li MRI

All-solid-state rechargeable batteries embody the promise for high energy density, increased stability, and improved safety. However, their success is impeded by high resistance for mass and charge transfer at electrode–electrolyte interfaces. Li deficiency has been proposed as a major culprit for interfacial resistance, yet experimental evidence is elusive due to the challenges associated with noninvasively probing the Li distribution in solid electrolytes. In this Letter, three-dimensional ⁷Li magnetic resonance imaging (MRI) is employed to examine Li distribution homogeneity in solid electrolyte Li₁₀GeP₂S₁₂ within symmetric Li/Li₁₀GeP₂S₁₂/Li batteries. ⁷Li MRI and the derived histograms reveal Li depletion from the electrode–electrolyte interfaces and increased heterogeneity of Li distribution upon electrochemical cycling. Significant Li loss at interfaces is mitigated via facile modification with a poly­(ethylene oxide)/bis­(trifluoromethane)­sulfonimide Li salt thin film. This study demonstrates a powerful tool for noninvasively monitoring the Li distribution at the interfaces and in the bulk of all-solid-state batteries as well as a convenient strategy for improving interfacial stability

Li Distribution Heterogeneity in Solid Electrolyte Li₁₀GeP₂S₁₂ upon Electrochemical Cycling Probed by ⁷Li MRI

All-solid-state rechargeable batteries embody the promise for high energy density, increased stability, and improved safety. However, their success is impeded by high resistance for mass and charge transfer at electrode–electrolyte interfaces. Li deficiency has been proposed as a major culprit for interfacial resistance, yet experimental evidence is elusive due to the challenges associated with noninvasively probing the Li distribution in solid electrolytes. In this Letter, three-dimensional ⁷Li magnetic resonance imaging (MRI) is employed to examine Li distribution homogeneity in solid electrolyte Li₁₀GeP₂S₁₂ within symmetric Li/Li₁₀GeP₂S₁₂/Li batteries. ⁷Li MRI and the derived histograms reveal Li depletion from the electrode–electrolyte interfaces and increased heterogeneity of Li distribution upon electrochemical cycling. Significant Li loss at interfaces is mitigated via facile modification with a poly­(ethylene oxide)/bis­(trifluoromethane)­sulfonimide Li salt thin film. This study demonstrates a powerful tool for noninvasively monitoring the Li distribution at the interfaces and in the bulk of all-solid-state batteries as well as a convenient strategy for improving interfacial stability

Local Structure and Dynamics in the Na Ion Battery Positive Electrode Material Na₃V₂(PO₄)₂F₃

Na₃V₂(PO₄)₂F₃ is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long- and short-range structural changes and ionic and electronic mobility of Na₃V₂(PO₄)₂F₃ as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution ²³Na and ³¹P solid-state nuclear magnetic resonance (NMR). The ²³Na NMR spectra and XRD refinements show that the Na ions are removed nonselectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na₃V₂(PO₄)₂F₃ electrode upon charge have been observed, where <i>a</i> (= <i>b</i>) continues to increase and <i>c</i> decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the ²³Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t_2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1–Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The ³¹P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V⁴⁺) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na₃V₂(PO₄)₂F₃ when used in a NIB

Sidorenkite (Na₃MnPO₄CO₃): A New Intercalation Cathode Material for Na-Ion Batteries

Na-ion batteries represent an effective energy storage technology with slightly lower energy and power densities but potentially lower material costs than Li-ion batteries. Here, we report a new polyanionic intercalation cathode material of an unusual chemical class: sidorenkite (Na₃MnPO₄CO₃). This carbonophosphate compound shows a high discharge capacity (∼125 mAh/g) and specific energy (374 Wh/kg). <i>In situ</i> X-ray diffraction measurement suggests that sidorenkite undergoes a solid solution type reversible topotactic structural evolution upon electrochemical cycling. <i>Ex situ</i> solid state NMR investigation reveals that more than one Na per formula unit can be deintercalated from the structure, indicating a rarely observed two-electron intercalation reaction in which both Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox couples are electrochemically active