Materials for Rechargeable Ion Batteries

Current demands of commercial batteries are that they are safe, economical, stable over a range of temperatures and have high capacity and charging rates to be useful in electric vehicles and on-grid storage. Rechargeable ion batteries in general have potential to fulfill these demands, however the properties of electrodes and electrolytes need much more understanding because secondary batteries are fundamentally limited by the interaction of these components.

We study alternative materials for Li- and Na-ion battery electrodes and electrolytes. Using various transition state and electronic structure methods, we predict contributing factors to capacitance/voltage profiles, ion diffusion mechanisms and provide fundamental understanding to experimental phenomena.

Cathodes

While Li-ion batteries are superior to other secondary batteries due to high gravimetric capacity, Na-ion batteries gaining attention due to its natural abundance in the Earths crust and offer an alternative for on-grid storage applications where high gravimetric capacity density is not strictly required. Metal sulfur batteries have high energy density with a theoretical capacity of 1672 mAhg^-1 for Na-S battery, which involves two electron redox per sulfur. Cathode hosts for sulfur have been studied extensively because traditional hosts like graphene cause dissolution of metal sulfides away from the cathode hosts leading to poor battery performance. Polar, hollow hosts like cobalt chalcogenides were synthesized as cathode hosts for binding metal polysulfides where the performance of CoS₂, CoSe₂ and CoTe₂ were compared experimentally and computationally.

Since cobalt chalcogenides are half metal, typical DFT techniques failed to optimize the electronic structure to the correct ground state. Hence, meta-GGA method in VASP were incorporated for geometry optimzations and binding energies of different sodium polysulfides were calculated to compare the performance of three metal chalcogenide hosts. When compared to graphene surface,metal polysulfide binding energies were calculated to be significantly stronger on chalcogenides indicating improved performance. This improved performance is attributed to the apparent oxidation of the cobalt sites on the surface which resulted in the reduction of magnetic moment on the cobalt sites interacting directly with the sulfur atoms of metal polysulfides. Thus, polar hosts have significant improved performance in terms of cyclability and capacity retention of metal sulfur batteries.

Carbon coated nanoparticles of NASICON-structured cathode material, Na₃MnZr(PO₄)₃ were synthesized by our collaborators. In the Na₃MnZr(PO₄)₃ structure, earth abundant Mn and Zr ions are disordered on a single metal site. Experimentally, two Na⁺ ions per formula unit can be removed electrochemically from the Na₃MnZr(PO₄)₃ structure leading to a high discharge capacity of 105 mAhg^-1. Two voltage plateaus at 3.5 and 4.0 V were observed corresponding to the sequential oxidation of the Mn²⁺/Mn³⁺, Mn³⁺/Mn⁴⁺ redox couples. The desodiation mechanism was understood with DFT(+U) calculations. Our calculations show that the occupation of Na+ ions on the three distinct sites in the structure is sensitive to the local Mn/Zr coordination. Na sites coordinated by Zr⁴⁺ ions are preferentially removed during desodiation over sites coordinated by Mnn+ ions. The presence of Mn/Zr disorder precludes the formation of Na-Na orderings which are known to induce deleterious structural transformations in analogous NASICON materials. The lack of structural transformation leads to a volume change of less than 3% during desodiation and a high capacity retention of 91% after 500 cycles.

Anodes

Stable anode-free all-solid-state battery (AF-ASSB) with sulfide-based solid-electrolyte(SE) (argyrodite Li6PS5Cl, LPSCl) is achieved by tuning wetting of lithium metal on “empty” copper current-collector. Lithiophilic 1 μm Li2Te is synthesized by exposing the collector to tellurium vapor, followed by in-situ Li activation during the first charge by our collaborator. The Li2Te significantly reduces the electrodeposition/electrodissolution overpotentials and improves Coulombic efficiency (CE) which originate from homogeneous Li electrodeposition/electrodissolution. This can be shown in full cell configuration of AF-ASSB with NMC811 cathode which delivers an initial CE of 83% at 0.2C, with a cycling CE above 99%. By contrast, unmodified Cu current collector promotes inhomogeneous Li electrodeposition/electrodissolution, electrochemically inactive “dead metal”, dendrites that extend into SE, and thick non-uniform solid electrolyte interphase (SEI) interspersed with pores.

Further Density functional theory and mesoscale calculations were provided in our group to unveil the nucleation-growth behavior. The binding energies of Li atoms and Li clusters on the (110) fcc Li2Te surface were calculated and compared to (111) fcc Cu and (110) bcc Li. The binding energies were calculated in two configurations: (a) Li clusters and (b) individual Li atoms. A key comparisonis for the energy differences within each class of supports, in addition to a cross-comparison between the classes. Three cross comparisons should be made: The energy of clusters versus metal atoms on a (111) fcc Cu surface, the energy of clusters versus metal atoms on (110) bcc Li surface, and the energy of clusters versus metal atoms on (110) fcc Li2Te surface.

Solid Electrolytes

As well, alternative electrolytes to conventional organic solvent/inorganic salt mixtures would greatly reduce capacity loss and safety hazards associated with electrolyte decomposition, which has increased research into the viability of solid electrolyte.

A high-purity ionic crystal of PP₁₃PF₄ stable to water and air was successfully synthesized and characterized by our collaborators and showed a wide electrochemical window of 7.2 V, which we confirmed computationally. Ionic crystals showed enhanced Li-ion transport with an ionic conductivity of 2.4x10^-4 S/cm at elevated room temperatures. The calculated energy barrier for the Li-ion conductivity of only 0.4 eV matches well with the experimentally determined activation energy. Further, MD simulations indicated that the ionic crystal exhibits facile molecular motions which facilitate Li-ion transport.

References

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