Factors Contributing to Path Hysteresis of Displacement and Conversion Reactions in Li Ion Batteries

We investigate the thermodynamic and kinetic attributes of electrode materials that are necessary to suppress path hysteresis during displacement and conversion reactions in Li ion batteries. We focus on compounds in the Li–Cu–Sb ternary composition space, as the displacement reaction between Li_1+ϵCu_1+δSb and Li₃Sb can be cycled reversibly. A first-principles analysis of migration barriers indicates that Cu, while not as mobile as Li in the discharged phase (Li₃Sb), nevertheless should exhibit mobilities similar to that of Li in common intercalation compounds. A calculation of phase stability in the ternary Li–Cu–Sb system predicts that the intermediate phases along the reversible charge/discharge path are stable in a large Cu chemical potential window. This ensures that intermediate phases are not bypassed upon Li extraction even when large thermodynamic driving forces are needed to reinsert Cu into the discharged electrode. Our study suggests that the suppression of path hysteresis during displacement reactions requires (i) a high mobility of the displaced metal and (ii) the thermodynamic stability of intermediate phases along the reversible path in a wide metal chemical potential window. Even in the absence of path hysteresis, displacement and conversion reactions suffer from polarization needed to set up thermodynamic driving forces for metal extrusion and reinsertion. This polarization can be estimated with a Clausius–Clapeyron analysis

Accordion Strain Accommodation Mechanism within the Epitaxially Constrained Electrode

The tremendous benefits of all-solid-state Li-ion batteries will only be reaped if the cycle-induced strain mismatch across the electrode/electrolyte interfaces can be managed at the atomic scale to ensure that structural coherency is maintained over the lifetime of the battery. We have discovered a unique strain accommodation mechanism within an epitaxially constrained high-performance bronze TiO₂ (TiO₂-B) electrode that relieves coherency stresses that arise upon Li insertion. In situ transmission electron microscopy observation reveals the formation of anatase shear bands within the TiO₂-B crystal that play the same role that interface dislocations do to relieve growth stresses. While first-principles calculations indicate that anatase is the favored crystal structure of TiO₂ in the lithiated state, its continued propagation is suppressed by the epitaxial constraints of the substrate. This discovery reveals an accordion-like mechanism relying on an otherwise undesirable structural transformation that can be exploited to manage the cyclic strain mismatch across the electrode/electrolyte interfaces that plague all solid-state batteries

Native Defects in Li₁₀GeP₂S₁₂ and Their Effect on Lithium Diffusion

Defects in crystals alter the intrinsic nature of pristine materials including their electronic/crystalline structure and charge-transport characteristics. The ionic transport properties of solid-state ionic conductors, in particular, are profoundly affected by their defect structure. Nevertheless, a fundamental understanding of the defect structure of one of the most extensively studied lithium superionic conductors, Li₁₀GeP₂S₁₂, remains elusive because of the complexity of the structure; the effects of defects on lithium diffusion and the potential to control defects by varying synthetic conditions also remain unknown. Herein, we report, for the first time, a comprehensive first-principles study on native defects in Li₁₀GeP₂S₁₂ and their effect on lithium diffusion. We provide the complete defect profile of Li₁₀GeP₂S₁₂ and identify major defects that are easily formed regardless of the chemical environment while the presence of path-blocking defects is sensitively dependent on the synthetic conditions. Moreover, using <i>ab initio</i> molecular dynamics simulation, it is demonstrated that the major defects in Li₁₀GeP₂S₁₂ significantly alter the diffusion process. The defects generally facilitate lithium diffusion in Li₁₀GeP₂S₁₂ by enhancing the charge carrier concentration and flattening the site energy landscape. This work delivers a comprehensive picture of the defect chemistry and structural insights for fast lithium diffusion of Li₁₀GeP₂S₁₂-type conductors

Ionic Conduction in Cubic Na₃TiP₃O₉N, a Secondary Na-Ion Battery Cathode with Extremely Low Volume Change

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na₃TiP₃O₉N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na₃TiP₃O₉N has an average voltage of 2.7 V vs Na⁺/Na and cycles with good reversibility through a mechanism that appears to be a single solid solution process without any intermediate plateaus. X-ray and neutron diffraction studies as well as first-principles calculations indicate that the volume change that occurs on Na-ion removal is only about 0.5%, a remarkably small volume change given the large ionic radius of Na⁺. Rietveld refinements indicate that the Na1 site is selectively depopulated during sodium removal. Furthermore, the refined displacement parameters support theoretical predictions that the lowest energy diffusion pathway incorporates the Na1 and Na3 sites while the Na2 site is relatively inaccessible. The measured room temperature ionic conductivity of Na₃TiP₃O₉N is substantial (4 × 10^–7 S/cm), though both the strong temperature dependence of Na-ion thermal parameters and the observed activation energy of 0.54 eV suggest that much higher ionic conductivities can be achieved with minimal heating. Excellent thermal stability is observed for both pristine Na₃TiP₃O₉N and desodiated Na₂TiP₃O₉N, suggesting that this phase can serve as a safe Na-ion battery electrode. Moreover, it is expected that further optimization of the general cubic framework of Na₃TiP₃O₉N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature