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.
LiFePO4 has a theoretical specific charge capacity of 170 mAh/g and high stability upon cycling at various temperatures but suffers from low intrinsic electronic conductivity due to slow ion diffusion. There were inconsistent reports of bulk Li diffusion between experimental and computational studies and we found a possible explanation is the presence of anti-site defects, which has a lower diffusion barrier than diffusion along a channel. We have also studied the bonding of Li at the surface and found the undercoordinated Fe3+/Fe4+ redox couple at the surface gives a high barrier for charge transfer, but it can be stabilized by nitrogen or sulfur adsorption.
Oxygen loss can lead to high-capacity Li2MnO3-based lithium-rich layered cathodes and transition-metal substitutions of Mn can significantly affect the amount of oxygen loss and capacity during the first charge/discharge cycle. We have explained the effect of transition-metal substitutions on oxygen loss in lithium-rich layered oxides, Li[Li1/3M2/3]O2, from the electronic structure calculated with DFT+U. We find that the band gap of the cathode determines the oxygen binding energy because the unoccupied metal bands provide the empty energy levels for electrons from the removed O.
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.
NaxV2O2(PO4)2F was synthesized by our collaborators using environmentally friendly solvothermal methods. Experimental voltage profiles show two plateaus indicating three stable phases whereas density functional theory calculations identifies four stable phases of NaxV2O2(PO4)2F the Na concentrations of x = 0, 1, 2, 3 giving rise to three values of the Na chemical potential and three voltage plateaus for Na intercalation. Our calculations show that the third Na will only be removed at 5.3 V, owing to the stability of the V-O bonding state and a strong Coulomb attraction between the Na and the anions. This high voltage is outside of the (4.8 V) stability window of the electrolyte phase therefore cannot be observed experimentally. We find a possible approach to access the third Na per formula unit is to substitute O with Cl to form Na3V2Cl2(PO4)2F without significant reduction in the cell potential or energy density of the material as confirmed experimentally. Additionally, we show that Na3-xLixV2O2(PO4)2F can also be used as a cathode in a Li ion cell with an even higher rate capability.
Li/TiO2(B) is a high capacity anode material that is in a family of titanium oxide polymorphs. TiO2(B) though, has been successfully synthesized as different nano-architectures and differences in differential capacitance plots of nano-3D and 2D materials have been seen by our collaborators. Our studies revealed that two different diffusion mechanisms are possible in the monoclinic C2/m TiO2(B) host depending on the dimensionality of the system. For TiO2(B) nanoparticles, A2 sites near equatorial TiO octahedra are filled first, followed by A1 sites near axial TiO octahedra. No open-channel C site filling is observed in the voltage range studied. Conversely, TiO2(B) nanosheets incrementally fill C sites, followed by A2 and A1. DFT+U calculations suggest that the different lithiation mechanisms are related to the elongated geometry of the nanosheet along the a-axis that reduces Li Li interactions between C and A2 sites.
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.
G. K. P. Dathar, D. Sheppard, K. J. Stevenson and G. Henkelman,
Calculations of Li ion diffusion in olivine phospha
Chem. Mater. 23, 4032-4037 (2011).
K.-S. Park, P. Xiao, S.-Y. Kim, A. Dylla, Y.-M. Choi, G. Henkelman, K. J. Stevenson, and J. B. Goodenough, Enhanced charge-transfer kinetics by anion surface modification of LiFePO4, Chem. Mater. 24, 3212-3218 (2012). DOI
P. Xiao, Z. Q. Deng, A. Manthiram, and G. Henkelman, Calculations of oxygen stability in lithium rich cathodes, J. Phys. Chem. C 116, 23201-23204 (2012). DOI
M.-W. Xu, P. Xiao, S. Stauffer, J. Song, G. Henkelman, and J. B. Goodenough, Theoretical and experimental study of vanadium-based fluorophosphates cathodes for rechargeable batteries, Chem. Mater. 26, 3089-3097 (2014). DOI
A. G. Dylla, G. Henkelman, and K. J. Stevenson, Lithium insertion in nanostructured TiO2(B) architectures, Acc. Chem. Res. 46, 1104-1112 (2013). DOI
A. G. Dylla, P. Xiao, G. Henkelman, and K. J. Stevenson, Morphological dependence of lithium insertion in nanocrystalline TiO2(B) nanoparticles and nanosheets, J. Phys. Chem. Lett. 3, 2015-2019 (2012). DOI
S. Murugesan, O. A. Quintero, B. P. Chou, P. Xiao, K.-S. Park, J. W. Hall, R. A. Jones, G. Henkelman, J. B. Goodenough, and K. J. Stevenson, Wide electrochemical window ionic salt for use in electropositive metal electrodeposition and solid state Li-ion batteries, J. Mater. Chem. A 2, 2194-2201 (2014). DOI