Chris Funch Advancement Independent Proposal 4/5/17

Chris Funch Advancement Independent Proposal 4/5/17

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Separating the Effects of Mg2+ Content, Structural Changes, and Grain

Properties on the Ionic Conductivity of Solid Magnesium Electrolytes

Abstract

Solid state rechargeable batteries can provide increased energy density for powering small-

scale electronics.1,2 Metal anodes of Mg ion batteries have a theoretical capacity of 3830 mAh cm-

3 compared to the theoretical 2060 and 1140 for Li and Na respectively.1,3 However, use of metallic

anodes in liquid electrolyte batteries to date has been limited by chemical incompatibilities

between liquid electrolytes and electrodes.4,5 Solid electrolytes present one solution to that problem

but have low ionic conductivities (< 10-4 S cm-1 at room temp versus 10-3–10-1 for liquid

electrolytes) limiting power output.2,6 Aliovalent doping of lithium- and sodium-based solid

electrolytes can increase the ionic conductivity of those materials through the generation of mobile

ion vacancies or increased ion content.7–11 It is proposed to study the effects of aliovalent doping

on the ionic conductivity and activation energy of MgZr4(PO4)6 and (MgxHf1-x)4/(4-2x)Nb(PO4)3,

both with the sodium super ionic conductor structure. I hypothesize substitution of tetravalent

species (Zr4+ or Hf4+) via doping with trivalent ions (Sc3+, Y3+, and Al3+) of different ionic radius

will increase magnesium content, modify lattice parameters of the crystal phase, and influence

grain size or grain boundary composition, which correlate with conductivity. Solid solutions with

varying compositions of Sc3+, Y3+, and Al3+ will be synthesized for characterization of their

conductivity and structural properties via impedance spectroscopy, x-ray diffraction, and scanning

electron microscopy with energy dispersive spectroscopy. This will aid in the separation of the

parameters that can increase conductivity, guiding future research for further improvements.

Understanding how tuning the structure and mobile ion concentration can maximize conductivity

in magnesium ion batteries will help build the next generation of safe and cost-effective batteries.


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1.0 Introduction

Battery technology has become

increasingly necessary as its utilization to

supply stored energy from the mega to the

micro scale (e.g. grid-level storage, electric

vehicles (EVs), portable user electronics, the

Internet of Things (IoT), and microsensors) has

expanded. While transistors have been able to

decrease in size and power utilization by

several orders of magnitude in the past three

decades, battery technology has only improved

by a factor of ~2.5 (measuring energy density,

Figure 1).12 As such, batteries are currently the

limiting factor for further decreasing the size of

small scale electronics compared to EVs or grid storage where size is less of a restriction.

Currently, lithium-ion batteries (LIBs) have the most widespread use and are extensively

researched due to their high power and energy densities (gravimetric and volumetric) compared to

other conventional chemistries (Figure 1).13,14 In an effort to optimize a battery’s general

performance criteria (i.e. high specific energy, high specific power, low cost, long life, wide

stability window, toxicity, and safety) for

specific applications there is also ongoing

research into solid state LIBs.5,15 All-solid-

state batteries can offer improved safety and

increased capacity, primarily due to the

removal of both organic liquid electrolytes

and filler materials necessary to contain the

electrolyte or physically separate the

electrodes (Figure 2).2 They can also offer

improved thermal stability, reduced self-

discharge rates, and a wider operational

window under different environmental

conditions.1 Similar to liquid electrolyte

batteries, there have been numerous

investigations into solid-state batteries, but no

substantial progress has been realized due to

their reduced ionic conductivity and

subsequently lower power output.

Figure 2: Progress of battery energy density over the past few

decades. Overcoming challenges with lithium ion batteries could

limit its ability to keep up with demand. Magnesium ion batteries

could offer improved energy density and are even better on a

volumetric comparison. Adapted from ref 40.

Figure 1: (left) Schematic of rechargeable liquid LIB. Due to safety

concerns, the Li metal anode is often replaced by graphite, which

lowers the overall capacity. A separator is needed to keep the two

electrodes from contacting each other and shorting the battery.

Additional filler material is needed to encapsulate the liquid

electrolyte. Adapted from ref 14. (right) Schematic of an all solid-

state LIB. Here. A lithium phosphorous oxynitride (LiPON) is a

solid electrolyte. It serves the dual purpose as electrolyte and

separator between electrodes. Overall cell volume is reduced

relative to liquid counterpart, enabling more cells in same battery

volume.


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There is potential for magnesium ion batteries

(MIBs) to compete with LIBs on both a capacity and

cost standpoint for small scale devices (Figure 3).5,16

The gravimetric charge capacity of magnesium may

not be as good as lithium for devices requiring low

weight (e.g. EVs) but its divalent nature offers almost

twice the volumetric charge capacity of lithium (and

more than five times that of the more common graphite

anode), which is advantageous for smaller devices.

However, even the higher conductivity materials

investigated for MIBs are not conductive enough to

meet some of the power requirements for low power

(<1mW) devices at room temperature. Investigations

into doping and structure for LIBs, and similar

monovalent ions, has seen recent advances.17–20 While

some conclusions may be applied to MIB materials,

low ionic conductivity is still limiting MIB usage.

In order to realize the true potential of MIBs, a more fundamental understanding of the

factors influencing divalent ionic conductivity in the solid electrolyte is needed. I have three

hypotheses to improve this component: (1) If the conductivity is limited by the concentration of

mobile ions, then substitution of a tetravalent with a trivalent cation will increase the concentration

of mobile, divalent cations (due to the need for charge neutrality) and increase conductivity. (2) If

the conductivity is limited by the size of the migration pathway (i.e. “bottleneck”) then substitution

of a larger ionic radius cation will increase the lattice parameter, lowering the activation energy

for mobile ion hopping to occur in the bottleneck, and increasing the conductivity. (3) If the

increased composition of dopant atom, or subsequently Mg2+ ions, either increases the average

grain size or increases the conductivity of the grain boundaries, then the overall conductivity

should also increase. These hypotheses will be tested by the doping of aliovalent species (Sc3+,

Y3+, and Al3+) in two systems with the sodium super ionic conductor (NASICON) structure

(general composition NaZr2(SiO4)x(PO4)3-x) : (MgZr4(PO4)6 and (MgxHf1-x)4/(4-2x)Nb(PO4)3). Ionic

conductivity, structure, grain size and grain composition of these materials will be determined with

impedance spectroscopy (IS), x-ray diffraction (XRD), scanning electron microscopy (SEM), and

energy dispersive spectroscopy (EDS) techniques, respectively. The synthesis and characterization

of these systems will help separate the impact of each component on bulk conductivity and provide

an improved understanding of divalent ionic crystals and the key parameters influencing their ionic

conductivity.

2.0 Background: Magnesium ionic conductors

The design of higher conducting Mg2+ systems often mimic those of Li+ or Na+. This

includes systems with the lithium phosphorous oxy-nitride (LIPON)21 and NASICON

structures.19,22 Of interest to this proposal are the two systems with the NASICON structure.

In one of the earlier reports of a Mg ionic conductor, Ikeda et al. synthesized different

compositions of MgxZr(3y-2x)/4(PO4)y to best match the structure of NaZr2(PO4)3.19 They identified

the composition MgZr4(PO4)6 best matched the target structure with XRD. They do not elaborate

Figure 3: In addition to being ~200 time more

abundant than Li metal, Mg also offers two times the

volumetric capacity compared to Li, and five times that

of the graphite anode. It also has a similarly low

reduction potential, which is necessary to deliver higher

power output. Data adapted from refs 1 and 40.


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on how the other structures differ from the NaZr2(PO4)3 “target.” It is difficult to draw any

conclusions with their other compositions since their bulk structure, overall composition, and Mg2+

content are not the same. There has been no exploration of the impact of doping with aliovalent

ions on the structure or concentration of mobile ions for this system.

Tamura et al. investigated the (MgxHf1-x)4/(4-

2x)Nb(PO4)3 system.22 Their primary focus was also on

structure. For 0.05 ≤ x ≤ 0.3, the collected XRD patterns

showed the transition from a NASICON structure to a

different phase around x ≤ 0.1. This transition at x = 0.1

resulted in the distortion of Zr2P3O18 “lantern” units, which

are ordered in the NASICON structure (Figure 4). They

suggest the smaller ionic radius of Mg2+ compared to Hf4+

influences this distortion. They did not investigate any

compositions with additional dopants, besides their

substitution of Mg2+ for Hf4+. Additional dopants could

enable increased concentration of Mg ions to charge

balance the system, but this could also come at the expense

of further structural distortions.

Grain size and phase segregation at

boundaries is also something that has not been

reported for either of these systems. The

impact of dopants, and other synthesis

methods on grain morphology have been

investigated for similar monovalent systems

and show an increase in conductivity.23–26

While these solid electrolytes highlight

improvement in the production and

understanding of Mg2+ conductivity, there is

opportunity to further both these endeavors

through aliovalent doping of these materials.

These undoped systems still have greatly

reduced conductivities relative to monovalent

compounds with similar structures (Figure 5).

The decreased conductivity is often attributed

to the increased electrostatics of divalent

versus monovalent ions. However, it is not

clear what factors can counter that force most effectively to improve overall conductivity. This

work will separate the factors that could play a role in conductivity through purposeful selection

of dopants and a systematic investigation of each potential factor, enabling a greater understanding

of how to improve conductivity.

Figure 5: Conductivities of the two Mg systems in this proposal

along with two monovalent systems with the same bulk structure.

The increased electrostatics for the divalent species is one primary

cause for the reduced conductivity. While the monovalent systems

are in a viable range for battery usage (room temperature

conductivity above 10-4 S cm-1) improvement is still needed for

divalent conductors. Data adapted from refs 19, 22, 41, and 42.

Figure 4: Representation of the NASICON

Structure. Alternating octahedral and

tetrahedral bodies form the framework for

Mg2+ to migrate through. The migration

pathway is highlighted by the black arrows.

Adapted from ref 28.


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3.0 Project description

Goal: To understand role of Mg2+ content, structural changes, and grain properties in the ionic

conductivity and activation energy of Mg-based solid electrolytes.

Research Objectives:

Objective 1 – Determine role of Mg2+ composition on ionic conductivity of of Mg(x+2)/2Zr4-

xMx(PO4)6 and Mgx/2Hf1-xMxNb(PO4)3 where Mx is Sc3+. These systems will be referred to as

MgZPO-M and MgHNPO-M, respectively, with M being the dopant atom.

Objective 2 – Determine role of lattice parameter and migration channel dimensions on ionic

conductivity and activation energy of systems identified above where Mx is Sc3+, Y3+, or Al3+.

Objective 3 – Determine impact of grain size and phase segregation at grain boundaries on the

conductivity and activation energy of the systems identified above.

4.0 Approach

4.1 Objective 1

4.1.1 Background

At low temperatures, a pure stoichiometric ionic solid has ions in all of the

crystallographically equivalent sites. These “normal” sites serve as the lowest energy positions for

the ions. In a solid electrolyte, ionic conductivity is only possible due to the presence of defects

where an ion is either absent from its normal site or exists in an interstitial site or lattice disorder.

The difference between the highest normal-site energy and the lowest interstitial-site energy serves

as the activation energy for ion migration, similar to the electronic energy gap of an insulator or

semiconductor. Defects can be classified as intrinsic or extrinsic. Intrinsic defects exist in

thermodynamic equilibrium with the lattice. These types of defects include vacancy and interstitial

for the ions in the pure crystal. As the crystal is heated close to its melting point, the rates of ion

motion are high enough to establish an equilibrium between their normal sites and a defect site.

This type of lattice disorder is reversible. In general, cations are more likely to form interstitial

defects than their anion counterparts due to their smaller ionic radii. Extrinsic defects can form

from the introduction of aliovalent (doping) ions into a lattice, while grain boundaries and

dislocations are another type of extrinsic defect. Extrinsic defects are much more dependent on the

preparation and thermal history of the crystal and are not in an equilibrium state. These defects

generate irreversible disorder and are the type under investigation in this study.

In addition to defects there can also be other mobile species besides Mg2+ in solid

electrolytes. The transport, or transference number (ti), is the ratio of the conductivity of the

working ion to the sum of all ionic and electronic conductivities.

𝑡𝑖 ≡ 𝜎𝑖 𝜎⁄ (1)

Here, σ is the sum of all ionic conductivities and any electrical conductivity. Ideally, the value of

ti is unity, since there should be no electronic conductivity and only the working ion should be able

to move through the electrolyte. In liquid electrolytes, both cations and anions are mobile, which

results in significantly lower transference numbers. For instance, common organic electrolytes for

LIBs have values between 0.2 - 0.5, which means a solid electrolyte can be 5 - 2 times lower in

conductivity, respectively, and have equivalent conductivity of the working ion.17 Thus, it is

necessary to ensure Mg2+ is the only mobile ion in these systems. A DC bias was applied across a

sample of the MgZPO electrolyte and composition analysis (e.g. electron probe micro analysis)


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revealed a buildup of only Mg2+ at the cathode.19 Similarly, IS tests under different ambient air

conditions (e.g. O2 rich vs O2 free) eliminated O2- as a mobile ion in MgHNPO.22 Based these

investigations into additional mobile species (e.g. Zr4+, P4+, O2-, e-, or h+) in both parent systems it

is assumed that Mg2+ is the only mobile species.19,22,27

4.1.2 Experimental Method

There is no knowledge on the possible change in conductivity of MgZPO or MgHNPO

systems when doped with aliovalent cations. The intrinsic conductivity of both these materials has

been reported to be ~1x10-4 S cm-1 at 600 °C,19,22 but it is not known whether doping could increase

conductivity or reduce activation energy. The first objective is a preliminary demonstration on the

effects of doping on the conductivity of Mg ion conductors. This will be done by synthesizing both

previously reported compositions along with the doped compositions, listed in Table 1, and

measuring their conductivities over a range of temperatures.

A solution-assisted solid-state reaction (SASSR), described by Ma et al. will be used to

synthesize the compounds of interest.28 Briefly, corresponding amounts of Mg(NO3)2, ZrO(NO3)2,

HfCl4, NbCl5, Sc2O3, Y2O3, and Al(NO3)3 solids are dissolved in either deionized water or HNO3

and mixed into one solution. A calculated amount of NH4H2PO4 is then added while stirring. The

mixture is dried and then calcined at 800°C. The powder is then milled in ethanol with zirconia

balls before being pressed into a pellet and sintered at ~1300 °C. These are the same, or similar,

precursors used in the co-precipitation methods used for the original compounds and should

provide reasonable comparisons. While compounds with Sc3+, Y3+, and Al3+ will all be synthesized

in this proposal, only Sc3+ will be used in Objective 1 assuming it will not have a significant change

in structure due to similar ionic radii between Sc3+, Zr4+, and Hf4+ (Figure 7). This will isolate any

changes in conductivity due to the expected change in Mg2+ content.

Before conductivity measurements are made, initial XRD patterns for the parent systems

and Sc3+ doped systems will be compared to confirm they have the same bulk structure. Once

confirmed, temperature dependent IS will be used to measure the conductivity, and extract

activation energy, for the prepared samples. Ion blocking electrodes (e.g. platinum or gold) are

applied to both sides of an electrolyte sample (Figure 6). At a given temperature, an AC bias is

applied to the sample over a range of frequencies. An equivalent circuit for the electrolyte enables

the calculation of both the grain and grain boundary resistance. The resistance is inversely related

to the ionic conductivity:

𝑅𝑖 = 𝐿 𝜎𝑖𝐴⁄ (2)

Table 1: Composition ranges

proposed in this study. Dopants

(M) are either Sc3+, Y3+, or Al3+.

Dopant mol % not to exceed 40%

due to concerns of phase

segregation or structural

distortion based on literature.

Dopant

mol % Mg(x+2)/2Zr4-xMx(PO4)6 Mgx/2Hf1-xMxNb(PO4)3

0 MgZr4(PO4)6 HfNb(PO4)3

5 Mg1.025Zr3.95M0.05(PO4)6 Mg0.025Hf0.95M0.05Nb(PO4)3






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for an electrolyte with thickness L and area A between the

electrodes where σi is the ionic conductivity. The measurement

of the grain (or internal) resistance will be used to assess any

change in conductivity caused by doping with aliovalent ions.

4.1.3 Hypothesis

Ma et al. observed an increase in conductivity by

substituting Sc3+ for Zr4+ in Na3Zr2(SiO4)2(PO4) and attributed

this to a difference in the concentration of Na+, not a structural

change, since the ionic radius of Sc3+ and Zr4+ are similar.28 I

hypothesize that substitution with a trivalent cation in the

proposed magnesium ionic conductors will increase the

concentration of Mg2+ ions, increasing the material’s

conductivity. While the use of Y3+ and Al3+ will likely have a

more pronounced effect on the structure due to their larger and

smaller ionic radii respectively (Figure 7). Details on the

influence of structure is addressed in Objective 2. Tamura et

al. saw evidence for phase segregation above 10 mol %

composition of Mg in (Mg0.1Hf0.9)4/3.8Nb(PO4)3.22 Similarly,

Aono observed a second phase for a lithium super ionic

conductor (LISICON) based system with Sc3+ above 30 mol

%.29 Therefore, cation doping at 5, 10, 20, 30 and 40 mol %

will be initially tested (Table 1). Higher percentages could be

tested but it is

unclear how

much doping the structure can incorporate,

especially with the larger Y3+ dopant, before phase

segregation will occur. The extrinsic defects formed

by the substitution of Sc3+ will provide detail on

changes in conductivity due to an increase in Mg2+

content, while the IS data for Y3+ and Al3+ will be

used in Objective 2 with Sc3+ as the control without

structural changes.

4.1.4 Alternative Strategies

If a greater difference in ionic radius between

dopant and substitution atoms limits the amount that

can be incorporated the range of substitution will be

narrowed, with smaller composition intervals to

better distinguish changes in conductivity. Similar

substitutions (elements or radius ratio) suggest

possible phase separation between 30 and 50 mol

%.22,29,30

Figure 6: (top) Schematic of

electrochemical impedance spectroscopy

setup. Ion blocking (e.g. gold or

platinum) electrodes are applied to a

solid electrolyte film. Temperature

dependent measurements are made by

applying an AC bias across the

electrolyte over a range of frequencies.

(bottom) Data is evaluated based on the

equivalent circuit for this system.

Resistive and capacitive elements for the

bulk and grain boundary phases is

included.

Figure 7: Ionic radii and electronegativity of the dopant

atoms along with the other cations in the systems to be

investigated. In these systems the dopant (Sc3+, Y3+, or

Al3+) is expected to substitute for Zr4+ or Hf4+. Since Sc3+

has a radius similar to both Zr4+ and Hf4+ it is expected to

have little influence on structural changes. Alternatively,

Y3+ and Al3+ are expected to increase and decrease the

lattice parameters respectively. Ionic radii data from:

Shannon, R. D. Acta Cryst. 1976, A32 751-767.

Electronegativity data from:

https://en.wikipedia.org/wiki/Electronegativity


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4.2 Objective 2

4.2.1 Background

Two structurally similar phases have been reported in the parent compounds.19,22 Ma et al.

investigated a similar system, Na3+xScxZr2-x(SiO4)2(PO4), and noted a shift from the monoclinic to

rhombohedral phase with doping of Sc3+.28 They highlight the fact

these two phases are only a slight twist of the unit cell structure, which

makes distinguishing the two difficult.28 There is no structural

information on either of the doped systems proposed in this study.

Martinez-Juarez et al. have suggested the lattice parameter is

proportional to the bottleneck size for the LISICON LiMaMb(PO4)3

system where Ma and Mb were Ge4+, Ti4+, Sn4+, or Hf4+.31 Similar to

the NASICON and LISICON compounds, the bottleneck in the

MgZPO-M and MgHNPO-M systems is a trigonal face generated by

three oxygen anions, depicted in Figure 8.31 The position of, and

distance between, the oxygen atoms that make up these bottlenecks

are influenced by the dimensions of the MO6 octahedral. However, no

in-depth analysis of any magnesium ionic conductors has been

conducted and it is not clear whether the activation energy is directly

affected by the expansion, or the contraction, of the lattice parameter.

Conductivity can be expressed in terms of the activation energy by

𝜎 = 𝐴𝑒(−𝐸𝑎 𝑅𝑇⁄ ) (3)

where, A is a pre-exponential factor related to hopping distance, carrier concentration, and

entropy.32 In an Arrhenius plot for the log of conductivity versus the inverse temperature (1/T), a

linear fit will provide a slope of –Ea/R. A decrease in activation energy due to change in bottleneck

size would increase conductivity at lower temperatures compared to the parent systems. Neither

changes in activation energy or lattice parameter for doped Mg2+ conductors has been reported.


XRD will be used to further refine

the bulk structure, or structures, of these two

systems. Using Rietveld analysis, the

quantitative weight ratio between these two

phases will be determined for the different

compositions of the two materials (Figure

9). This will also enable a more qualitative

measure of the change in lattice parameter

for these systems if both phases are present.

Lattice parameters will be used in the

correlation of bottleneck size and activation

energy.

Similar to Objective 1, temperature

dependent IS data for the Y3+ and Al3+ systems will be collected and compared against that for the

Sc3+ system. This will highlight what changes, if any, are a result of modified lattice parameters

due to changes in ionic radius versus the increase in Mg2+ content.

Figure 9: XRD pattern for Sr2+ doped LiZr2(PO4)3 with Rietveld

refinement. Based on the theoretical Bragg peak positions the ratio of

mixed phases can be determined using a least squares method. Here, a

second phase appears while 86% of the bulk has the NASICON

structure. Adapted from ref 23.

Figure 8: Illustration of the

bottleneck for Mg2+ ion migration.

Large circles are oxygen, small

white circle is mobile Mg2+, and

small black circles are vacant sites

it can move two. Bottleneck is

highlighted by triangular face

between the three blue circles.

Adapted from ref 31.


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4.2.3 Hypothesis

Substitution of larger radius ions has been shown to increase lattice parameters, and

conductivity, in both NASICON and LISICON systems.29,30,33 Due to the larger ionic radius of

Y3+ versus Sc3+ (0.9 vs. 0.745 Å), I hypothesize doping with Y3+ will expand the bottleneck for

ion migration, generating delocalization between active and interstitial sites, and therefore

lowering activation energy and increasing conductivity. I expect the opposite for the case of Al3+

doping due to its smaller radius (0.535 Å). It is possible that no difference will be observed in the

lattice parameters or activation energy. However, this would also be an important finding, because

it would suggest that ion concentration is the primary factor for improving conductivity in the

NASICON structure.

4.2.4 Alternative Strategies

As seen by Aono, ionic radius does not always correlate with conductivity.29 Some dopant

substitutions have resulted in no lattice parameter change, often attributed to a secondary phase or

incorporation of the dopant into the grain boundaries.26,29,30 If lattice parameters are not changed,

other dopants could be used (e.g. La3+ or Fe3+) or the synthesis could be modified to include the

use of grain boundary “binders,” as has been demonstrated previously.34

4.3 Objective 3

4.3.1 Background

To strengthen the findings in Objectives 1 and 2 and directly separate the parameters

affecting conductivity, grain size and grain boundaries will be investigated. While IS will help to

separate the grain and grain boundary conductivity contribution, elucidating the factors that either

increase grain size or increase grain boundary conductivity is necessary. Other investigations have

suggested grain size increases with doping, and is not solely due to the thermal treatment of the

sample during synthesis.23 Nothing has been reported on the grain properties for the parent, or

doped, systems in this study.


Average grain size will be measured with SEM. While this would only be a representative

sample for that measurement, grain size is expected to be on the µm (0.3-10 µm) scale based on

similar monovalent systems.23,25,35 Relative density is also known to play a role in bulk

conductivity, which will be measured by the pycnometric method.23,24 This method uses the change

in volume of a gas, similar to the Archimedes method for a liquid, to measure the density of a

porous material. This is done to avoid the dissolution of the material, or any secondary phases. A

density close to the theoretical for a given structure indicates each grain is in good contact with its

nearest neighbor (e.g. fewer inter-grain pores), which increases conductivity due to less resistive

interfaces.23 A change in density can also indicate the formation of other phases, which may be

amorphous.

SEM or TEM imaging will be used to identify the presence of any amorphous grain

boundary regions, which has been done for similar monovalent systems.25,36 XRD and density data

will be used to identify the presence of possible secondary, crystalline phases (e.g. NbPO5, ZrO2,

AlPO4). EDS will be used to either corroborate the grain boundary structure from those data or

identify the overall composition of any non-crystalline phases. These methods will provide insight

into whether a certain dopant or dopant concentration modifies properties of the grains or grain

boundaries.


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4.3.3 Hypothesis

Previous investigations into solid electrolytes have indicated substitution of dopants can

impact grain size.35 If the increased composition of dopant atom, or subsequently Mg2+ ions, either

increases the average grain size or increases the conductivity of the grain boundaries, then the

overall conductivity should also increase. An increase in grain boundary size should be

advantageous since grain boundaries are usually less conductive; the greater number of grain

interfaces an ion encounters, the lower the overall conductivity. An increase in Mg2+ content within

the grain boundaries could also bring the grain boundary conductivity closer to the bulk

conductivity, which has been observed in a Li electrolyte with excess Li added.24

4.3.4 Alternative strategies

If grain size changes over the range of doping and grain size or grain boundary composition

indicates increased conductivity it may be necessary to modify synthesis conditions (e.g. sintering

temperature) to maintain a given grain size for Objective 1. Doing so would more accurately

represent the improvements caused by the addition of dopants only. Depending on the scale of

grain size, other quantitative measures could be used. Scherrer analysis could quantitatively

measure average grain size from XRD patterns rather than through SEM or TEM imaging if

crystallite size is small enough (≤ ~0.1 µm).37

It is possible that all or none of the factors identified will influence conductivity. To provide

additional insight on the collective effect of these factors use of a design of experiment (DoE) or

statistical analysis system such as JMP could prove useful to help quantify the impact of a dopant

on the final conductivity. This should not be needed if the scale of changes for these systems is

similar to that seen in monovalent systems and will only be used if the magnitude of changes is

small (e.g. less than a factor of two for conductivity).

5.0 Summary & Significance

In summary, the proposed work will determine the conductivity of various compositions

of MgZPO-M and MgHNPO-M over a range of temperatures. Structural and grain boundary

information on these compositions will be collected to elucidate the impact, if any, these have

beyond the increase in concentration of mobile species. There is a need to understand the effect of

doping on these newly developed magnesium ionic conductors and identify which factors have the

greatest influence on conductivity.

The separation of these factors on the conductivity of a divalent cation will be the greatest

contribution if this work. This will allow for greater understanding and development of existing

and future systems within this structure family. For example, other starting materials or dopants

could be identified to tune the lattice parameter, or bottleneck, in the right direction to optimize

this structure for the conduction of magnesium. The outcomes from this work could also drive the

investigation of doping into other structures identified to demonstrate divalent ion conductivity for

electrolytes or electrodes in MIBs such as the beta-alumina, MgPON, or transition metal

silicates.21,38,39

Ultimately, this will help drive further progress of MIBs. There is a current need for high

capacity, low power, cost-effective batteries for small-scale electronics. Highlighting the specific

features that increase conductivity will enhance the rate at which non-lithium batteries can enter

the market; enabling a long-lasting effect on the use of small electronic devices (e.g. IoT, sensors,

etc.) to provide value to the global community.


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Chris Funch Advancement Independent Proposal 4/5/17