Search

Severe capacity degradation at high operating voltages and poor interphase stability at elevated temperature have thus far precluded the practical application of LiNi0.5Mn1.5O4(LNMO) as a cathode material for lithium-ion batteries. Addressing these challenges through a combination of experimental and theoretical methods in this work, we demonstrate how a fluorinated carbonate electrolyte enables both high-voltage and high temperature operation by mitigating the traditional interfacial reactions observed in electrolytes with conventional carbonate solvents. Computational studies confirm the exceptional oxidation stability of fluorinated carbonate electrolyte which reduces deprotonation at high voltage. The mitigated deprotonation will then minimize the formation of HF acid which corrodes the LNMO surface and leads to phase transformation and poor interphases. With fluorinated carbonate electrolyte at elevated temperature, it was found on LNMO’s subsurface a reduced amount of Mn3O4phase which can block Li+transfer and result in drastic cell failure. Leveraging this approach, LNMO/graphite full cells with a high loading of 3.0 mAh/cm2achieve excellent cycling stability, retaining ∼84 % of their initial capacity at room temperature (25 °C) after 200 cycles and ∼68 % after 100 cycles at 55 °C. This advanced electrolyte also shows promise for improving calendar life, retaining >30 % more capacity than the carbonate baseline after high temperature storage. These results indicate that electrolytes based on fluorinated carbonates are a promising strategy for overcoming the remaining challenges toward practical commercial application of LNMO.

The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties.

Among the several challenges to enable next-generation batteries is the development of an electrolyte that maintains a dendrite-free and high Coulombic efficiency lithium-metal anode over extended cell cycling. A new electrolyte solvation structure and transport mechanism is demonstrated in fluoromethane-based liquefied gas electrolytes with the introduction of additive amounts of tetrahydrofuran, which is shown to fully coordinate with the lithium cations and greatly enhance salt dissociation and transport. The resulting electrolyte shows a high conductivity and transference number (t+> 0.79), which leads to a dramatic improvement of the cycling performance of the lithium-metal anode. Systems using the enhanced liquefied gas electrolytes demonstrate a long-term high average Coulombic efficiency of 99.6%, 99.4%, and 98.1% (± 0.3%) at capacities of 0.5, 1, and 3 mAh·cm−2, respectively, with dendrite-free morphology and remarkable rate capability. Both the rate and cycling performance are well maintained from +20°C to −60°C.

A modified co-precipitation method is introduced to synthesize P2-type Na0.67Fe1/4Mn3/4O2 cathode for low-cost Na-ion batteries. The meso-structure of the obtained material is well controlled through regulated cooling after a high temperature calcination process. The material via slow-cooling consists of sphere-like secondary particles with a uniform dispersion while the quenched material exhibits a hexagonal plate-like primary particle without meso-structure. Meso-structure is found to have a distinct effect upon the electrochemical performance of P2-type layered cathode. The slow-cooled sample exhibits a larger capacity and improved cyclability compared with the quenched sample, which is attributed to the larger surface area, reduced surface contamination, and surprisingly higher transition metal redox activity. This work demonstrates that cooling rate plays the key role in controlling the formation of spherical meso-structure for sodium iron-manganese oxides with enhanced electrochemical performance.

Using molecular dynamics simulations, small-angle neutron scattering, and a variety of spectroscopic techniques, we evaluated the ion solvation and transport behaviors in aqueous electrolytes containing bis(trifluoromethanesulfonyl)imide. We discovered that, at high salt concentrations (from 10 to 21 mol/kg), a disproportion of cation solvation occurs, leading to a liquid structure of heterogeneous domains with a characteristic length scale of 1 to 2 nm. This unusual nano-heterogeneity effectively decouples cations from the Coulombic traps of anions and provides a 3D percolating lithium–water network, viawhich 40% of the lithium cations are liberated for fast ion transport even in concentration ranges traditionally considered too viscous. Due to such percolation networks, superconcentrated aqueous electrolytes are characterized by a high lithium-transference number (0.73), which is key to supporting an assortment of battery chemistries at high rate. The in-depth understanding of this transport mechanism establishes guiding principles to the tailored design of future superconcentrated electrolyte systems.

Density functional theory (DFT) was used to examine the O2reduction reaction on Fe–N4-containing graphitic carbon clusters (Fe–N4–G) modeled after recent experimentally identified active sites, Mössbauer spin-state predictions and electrochemical reaction behavior in alkaline media. A detailed analysis of the O2, O, H2O, OOH, and OH adsorbate interactions on the Fe–N4–G cluster with solvation and/or dispersion corrections are considered. The total and partial density of states for the α- and β-spin orbitals are compared for the adsorbate of interest, Fe atom and surrounding graphitic cluster. Relative free-energy diagrams are constructed, which allow us to compare DFT predictions to experimental results for O2reduction on systems containing embedded Fe–N4clusters. For all reaction steps, different DFT functionals are explored and the respective geometries, energetics, and spin-states for each adsorbate interaction are reported for six commonly used functionals including B3LYP, M06-2X, M06-L, PBE, TPSSh, and ωB97X-D. Functionals with high fractions of exact exchange were found to favor higher spin-states, as well as stronger binding of the adsorbates, making these methodologies less feasible for Fe–N4-containing electrocatalysts when compared to experimental data. Pure functionals with and without empirical correlation exhibit different ground spin-states and geometries, however the free energy diagrams yield similar conclusions at relevant overpotentials. The activation energy for the O–OH bond scission step, as well as OH desorption from the Fe–N4–G cluster are discussed since the barrier could prohibit a pure 4e–ORR. Finally, we discuss the energetically unfavorable steps for select overpotentials, which provides the experimentalist with a tuning knob for electrocatalytic design.

Challenges with the quantum chemistry based-screening of electrochemical stability of solvents and salts with potential applications in lithium batteries are discussed. Initial high throughput screening of carbonate and phosphate-based electrolyte solvents provided insight into first and second reduction and oxidation potentials and reorganization energies of these solvents. It has been found that it was important to include a lithium cation in the screening of semifluorinated solvents. Two reduction pathways has been found for lithium complexed with semifluorinated solvents and salts such as lithium bis(fluorosulfonyl)imide (LiFSI): the low rate defluorination reaction occurring at high potentials and fast solvent or anion reduction occurring at significantly lower potentials. A spontaneous deprotonation of carbonate solvents at the surface of the completely de-lithiated LiNi0.5Mn1.5O4 cathode has been found.

A modified co-precipitation method is introduced to synthesize P2-type Na0.67Fe1/4Mn3/4O2cathode for low-cost Na-ion batteries. The meso-structure of the obtained material is well controlled through regulated cooling after a high temperature calcination process. The material via slow-cooling consists of sphere-like secondary particles with a uniform dispersion while the quenched material exhibits a hexagonal plate-like primary particle without meso-structure. Meso-structure is found to have a distinct effect upon the electrochemical performance of P2-type layered cathode. The slow-cooled sample exhibits a larger capacity and improved cyclability compared with the quenched sample, which is attributed to the larger surface area, reduced surface contamination, and surprisingly higher transition metal redox activity. This work demonstrates that cooling rate plays the key role in controlling the formation of spherical meso-structure for sodium iron-manganese oxides with enhanced electrochemical performance.

Challenges with the quantum chemistry based-screening of electrochemical stability of solvents and salts with potential applications in lithium batteries are discussed. Initial high throughput screening of carbonate and phosphate-based electrolyte solvents provided insight into first and second reduction and oxidation potentials and reorganization energies of these solvents. It has been found that it was important to include a lithium cation in the screening of semifluorinated solvents. Two reduction pathways has been found for lithium complexed with semifluorinated solvents and salts such as lithium bis(fluorosulfonyl)imide (LiFSI): the low rate defluorination reaction occurring at high potentials and fast solvent or anion reduction occurring at significantly lower potentials. A spontaneous deprotonation of carbonate solvents at the surface of the completely de-lithiated LiNi0.5Mn1.5O4cathode has been found.