5 research outputs found
Thermodynamic and Kinetic Properties of Materials for Next-Generation Rechargeable Batteries from First-Principles.
To break out of small electronics and into vehicles, rechargeable battery technology needs to overcome several obstacles. First, safety issues plague the usage of liquid electrolytes in traditional Li-ion battery systems. Secondly, alternative elements to Li would be more earth-abundant as well as potentially have higher capacities. With these factors in mind, we will discuss thermodynamic and kinetic properties from first-principles calculations surrounding three next-generation materials for rechargeable batteries: a solid electrolyte, Li3OCl, for use in Li-ion batteries. a magnesium battery electrode (MgTiS2), and a sodium battery electrode (NaCoO2).
First, we explore diffusion in Li3OX (X=Cl, Br), a superionic conductor with experimental conductivities on the order of 1 mS/cm. These compounds, which have an anti-perovskite crystal structure, have potential applications as solid electrolytes in Li-ion batteries to replace the currently-employed liquid electrolytes. We identify a low-barrier three-atom hop mechanism involving Li interstitial dumbbells. This hop mechanism is facile within the (001) crystallographic planes of the perovskite crystal structure and is evidence for the occurrence of concerted motion, similar to ionic transport in other solid electrolytes. Our first-principles analysis of phase stability predicts that antiperovskite Li3OCl (Li3OBr) is metastable relative to Li2O and LiCl (LiBr) at room temperature.
Second, we examine the thermodynamic and kinetic properties of MgTiS2 and compare it to its well-known analog, LiTiS2 in order to better understand the difficulties obtaining facile diffusion in Mg-ion batteries. We show that although thermodynamically, the two systems are incredibly similar, the extra electron that Mg has over Li hinders diffusion immensely.
Thirdly, we briefly investigate the spinel NaCoO2. Unlike any other spinel structure where intercalating species first occupy all tetrahedral sites then proceed to occupy octahedral sites, with sodium, both octahedral and tetrahedral sites are filled at various compositions leading to some unique thermodynamic and kinetic results.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111441/1/aemly_1.pd
Ab initio structure search and in situ 7Li NMR studies of discharge products in the Li-S battery system.
The high theoretical gravimetric capacity of the Li-S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li-S phase diagram using computational techniques and complement this with an in situ (7)Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li2S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li(+)-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.Fellowship support to KAS from the ConvEne IGERT Program of the National Science Foundation (DGE 0801627) is gratefully acknowledged. AJM acknowledges the support from the Winton Programme for the Physics of Sus-tainability. PDM and DSW thank the UK-EPSRC for financial support. This research made use of the shared experimental facilities of the Materials Research Laboratory (MRL), sup-ported by the MRSEC Program of the NSF under Award No. DMR 1121053. The MRL is a member of the NSF-funded Mate-rials Research Facilities Network (www.mrfn.org). CPG and ML thank the U.S. DOE Office of Vehicle Technologies (Con-tract No. DE-AC02-05CH11231) and the EU ERC (via an Ad-vanced Fellowship to CPG) for funding.This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/ja508982p
Mg Intercalation in Layered and Spinel Host Crystal Structures for Mg Batteries
We investigate electrochemical properties
of Mg in layered and spinel intercalation compounds from first-principles
using TiS<sub>2</sub> as a model system. Our calculations predict
that Mg<sub><i>x</i></sub>TiS<sub>2</sub> in both the layered
and spinel crystal structures exhibits sloping voltage profiles with
steps at stoichiometric compositions due to Mg-vacancy ordering. Mg
ions are predicted to occupy the octahedral sites in both layered
and spinel TiS<sub>2</sub> with diffusion mediated by hops between
octahedral sites that pass through adjacent tetrahedral sites. Predicted
migration barriers are substantially higher than typical Li-migration
barriers in intercalation compounds. The migration barriers are shown
to be very sensitive to lattice parameters of the host crystal structure.
We also discuss the possible role of rehybridization between the transition
metal and the anion in affecting migration barriers
Phase Stability and Transport Mechanisms in Antiperovskite Li<sub>3</sub>OCl and Li<sub>3</sub>OBr Superionic Conductors
We
investigate phase stability and ionic transport mechanisms in
two recently discovered superionic conductors, Li<sub>3</sub>OX (X
= Cl, Br), from first principles. These compounds, which have an antiperovskite
crystal structure, have potential applications as solid electrolytes
in Li-ion batteries. We identify a low-barrier three-atom hop mechanism
involving Li interstitial dumbbells. This hop mechanism is facile
within the (001) crystallographic planes of the perovskite crystal
structure and is evidence for the occurrence of concerted motion,
similar to ionic transport in other solid electrolytes. Our first-principles
analysis of phase stability predicts that antiperovskite Li<sub>3</sub>OCl (Li<sub>3</sub>OBr) is metastable relative to Li<sub>2</sub>O
and LiCl (LiBr) at room temperature. We also find that although the
band gap of Li<sub>3</sub>OCl exceeds 5 eV, the metastable antiperovskite
becomes susceptible to decomposition into Li<sub>2</sub>O<sub>2</sub>, LiCl and LiClO<sub>4</sub> above an applied voltage of 2.5 V, suggesting
that these compounds are most suited for low-voltage Li batteries
provided the formation of Li<sub>2</sub>O can be suppressed
Ab Initio Structure Search and in Situ <sup>7</sup>Li NMR Studies of Discharge Products in the Li–S Battery System
The
high theoretical gravimetric capacity of the Li–S battery
system makes it an attractive candidate for numerous energy storage
applications. In practice, cell performance is plagued by low practical
capacity and poor cycling. In an effort to explore the mechanism of
the discharge with the goal of better understanding performance, we
examine the Li–S phase diagram using computational techniques
and complement this with an in situ <sup>7</sup>Li NMR study of the
cell during discharge. Both the computational and experimental studies
are consistent with the suggestion that the only solid product formed
in the cell is Li<sub>2</sub>S, formed soon after cell discharge is
initiated. In situ NMR spectroscopy also allows the direct observation
of soluble Li<sup>+</sup>-species during cell discharge; species that
are known to be highly detrimental to capacity retention. We suggest
that during the first discharge plateau, S is reduced to soluble polysulfide
species concurrently with the formation of a solid component (Li<sub>2</sub>S) which forms near the beginning of the first plateau, in
the cell configuration studied here. The NMR data suggest that the
second plateau is defined by the reduction of the residual soluble
species to solid product (Li<sub>2</sub>S). A ternary diagram is presented
to rationalize the phases observed with NMR during the discharge pathway
and provide thermodynamic underpinnings for the shape of the discharge
profile as a function of cell composition