edinburgh | may-16 | the winton programme for the physics of sustainability

Smart Villages Battery Technology and Recycling Workshop

S E Dutton

Dutton Group Research Activities

StoichiometryCrystal structure Electronic structure

Physical properties

FUNCTIONAL ENERGY MATERIALS (FEM)

BatteriesMagnetocalorics

PyrochloresHybrid photovoltaics

Multiferroics 

Sample preparation• Solid state synthesis

• Controlled atmosphere to tune O2 partial pressure– Flowing gas (O2, Ar, 5%H2/Ar)– In vacuo– Dynamic vacuum

Measurement• XRD – crystal structure analysis

• Neutron diffraction – crystal and magnetic structure

Magnetic and Electronic measurements

• SQUID• PPMS• Battery testing

Uses of rechargeable batteries

Construction of a rechargeable battery

Solid state electrolytes and all solid state batteries

New electrodes for Li-ion and Na-

ion batteries

Mg-ion batteries

F. Lalère, et al., J Power Sources 247, 975 (2014)

Mg-ion batteries - motivation• Divalent ions

– generate more charge per intercalated ion • Possibility of using Mg anodes

– allows for higher energy densities• Cost and abundance

– Scaleable technology

Mg-ion batteries

• Reversible Mg-ion battery with MgxMo6S8 as the cathode

• Capacity = 70 mAh/g

• Voltage = 1-1.3 V

Aurbach, Nature 407 (2000) 724

Mg-ion batteries - practicalities

• Chemistry of Mg2+ is very different to Li+

– Mg2+ is often used as a dopant in electrodes for Li-ion batteries• assumed to be immobile• Often form materials with mixed Mg and transition metal

sites – Inherently lower voltage (by 0.73 V vs. Li)– Higher charge to radius ratio gives slower diffusion

• Whole battery systems not optimised– Current electrolytes are not stable at higher voltages– SEI formed on charge which limits capacity

TargetsHigh Voltage High Capacity

Reversible Rate capability

TargetsHigh Voltage High Capacity

Reversible Rate capability

Materials selection criteriaOxide or polyanion groups

Mg-ions on a crystallographically distinct siteRedox active ions

Pathways for Mg-ion diffusionSuitable ratio of Mg to redox active ions

Analogues of electrodes in Li-ion

batteriesMake electrochemically

Mg-ion exchangeMake directly?

TargetsHigh Voltage High Capacity

Reversible Rate capability

Materials selection criteriaOxide or polyanion groups

Mg-ions on a crystallographically distinct siteRedox active ions

Pathways for Mg-ion diffusionSuitable ratio of Mg to redox active ions

Analogues of electrodes in Li-ion

batteriesMake electrochemically

Mg-ion exchangeMake directly?

Explore Mg-containing materials with no Li-analogue

Identify suitable targets from reported materials

Exploratory synthesis

TargetsHigh Voltage High Capacity

Reversible Rate capability

Materials selection criteriaOxide or polyanion groups

Mg-ions on a crystallographically distinct siteRedox active ions

Pathways for Mg-ion diffusionSuitable ratio of Mg to redox active ions

Analogues of Li-ion batteries

• Preparation can be difficult– Often made electrochemically by removing Li and

then cycling vs. Mg• Intrinsically lower capacity – One Li-ion is replaced by ½ Mg-ion

• Not optimised for Mg-ion transport

Explore Mg-containing Materials

• High operating voltage• Higher capacities• Versatile structures– Can vary the TM ion• Mn, Fe, Co, V, Ni

– Can vary the oxidation state of the TM• Alter voltage of materials

MgMnB2O5

Theoretical capacity = 296 mAh/gMn2+

Performance in a Mg-ion battery

vs Mg with TFSI in ACN3.5V cutoff

Performance in a Mg-ion battery

vs Mg with TFSI in ACN2.5V cutoff

What is the maximum amount of Li which can be removed?

• Test in a Li-ion cell

What about putting Li into the structure?

• Reaches full theoretical capacity• There may be some side reactions as not completely reversible

• Though could be Li just occupy different sites

Intercalation of 1.25 Li

MgMnB2O5 vs. Li – C/25

• Similar discharge capacity to C/100

• Better efficiency• 600 Wh/Kg is good

(LiCoO2 ~240Wh/Kg)

Le Bail refinements of cycled MgMnB2O5

• High capacity at high rates (C/2)

• Batteries operate over multiple cycles

Conclusions

• It is possible to remove Mg ions from MgMnB2O5

• Overpotential is reduced when cycling vs. Li– Need to optimise construction of Mg-ion batteries

• Can reversibly cycle ~1.25 Li in demagnesiated MgMnB2O5

– Reversible over multiple cycles– Can be carried out at high rates

Acknowledgements

• Hugh Glass• Evan Keyser• Zigeng Lui• Jeongjae Lee• Paul Bayley• Clare Grey• Dominic Wright