First report of Pn-Li₂MnSiO₄ synthesized by ion exchange

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First report of Pn-Li

MnSiO

synthesized by

ion-exchange

H. Duncan, A. Kondamreddy, P. Mercier, Y. Le Page, Y. Abu-Lebdeh, M. Couillard, P.S. Whitfield and I.J. Davidson

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Introduction

• Li2MnSiO4 is a potential new cathode material for Li-ion

batteries with 4 V potential and 333 mAh g-1 theoretical capacity.

• Potentially interesting for mobile applications.

• However, Li2MnSiO4 suffers from severe capacity fading due to

the instability of the delithiated material.

•We took advantage of the rich polymorphism exhibited by the silicate family to synthesize a new polymorph by ion exchange that was projected to exhibit better structural characteristics.

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Introduction

•Li₂MnSiO₄ is reported in the

Pmn2₁, Pmnb and P2₁/n

structures.

•Na2MnSiO4 occurs in the Pn

structure, and readily obtained by a sol-gel synthesis.

• Na+ can be exchanged for Li+

by ion exchange methods.

• DFT methods (GGA+U) were used to predict the stability and the lattice parameter of the

polymorphs.

New poly morph Thermodynamic poly morph

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Experimental

Sol-gel method:

•Sodium and manganese acetate dissolved in water

•Tetraethylorthosilicate (TEOS) and ethanol added; pH adjusted to 3 with acetic acid

•Solution stirred and evaporated

•Powder crushed, pressed in pellets and sintered in Ar / 5% H2.

•Carbon-coated Na2MnSiO4 obtained by adding sucrose before

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Ion exchange

Three conditions were used to exchange Na+ for Li+:

• 5:1 LiBr:Na2MnSiO4 in hexanol, reflux (185°C) 8h

• 5:1 LiBr: Na2MnSiO4 in water, hydrothermal conditions (150°C)

8h.

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Ab-initio DFT Calculations

• Pn-Ag2ZnSiO4 used as starting point for atomic coordinates.

• Calculations performed with VASP using the exchange and correlation energies calculated under GGA.

• In contrast, “GGA+U” optimizations were performed for total-energy calculations required for enthalpy comparisons in the phase change and for calculation of average electrochemical delithiation potentials.

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Electrochemistry

• Carbon-coated material cast unto Al foil from a slurry

consisting of 80% active material, 5% graphite, 5% carbon black, 10% PVDF in NMP.

•Assembled in coin cells with Li foil as anode, Celgard 2500

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Results - DFT

• DFT methods (GGA+U) were used to predict the stability and the lattice parameter of the polymorphs.

• Pmn21 Li2MnSiO4 is more stable than the Pn polymorph by 4.1 kJ

mol-1.

•The delithiated Pn polymorph was predicted to be more stable

than the Pmn21 (thermodynamic) polymorph of Li2MnSiO4 by 19 kJ

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Results – Pn - Na

MnSiO

•Good agreement between experimental and calculated paremeters.

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Results – Pn -Li

MnSiO

• Pn structure confirmed by Rietveld refinement

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Results – Pn - Li

MnSiO

• Pn structure confirmed by Rietveld refinement

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Results – Pn-LiNaMnSiO

• Pn structure confirmed by Rietveld refinement

• Presence of a small unidentified impurity

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Results

• Exchange of Na+ for Li+ leads to

shift of peaks towards higher angles accompanied by the broadening of peaks.

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Results – molten salt exchange

• Exchange of Na+ for Li+ by molten

salts lead either to mixture of Pn and Pmn21 or total conversion to

Pmn21 polymorph. 20 30 40 50 60 70 80 -100 -50 0 50 100 150 200 250 300 350 LiI 500°C C ou n ts 2 (Cu K) Li₂MnSiO₄ Pmn2₁

Salt Temperature / °C Result

LiNO_{3 300} Pn - Pmn2₁

LiI:LiCl 64:36 400 Pn - Pmn21

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Results - Thermal stability

• Broad exothermic peak from 150°C – 420°C

• Comparable energy to 4.1 kJ mol-1

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Results

• The electrochemical activity was studied in coin cells.

•The Pn polymorph shows similar electrochemical activity to the

Pmn21 polymorph.

•Half-exchanged LiNaMnSiO4

shows lower but stable capacity. •Na2MnSiO4 can be cycled directly

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Results

• Lithium extraction voltage more positive in Pmn21

polymorph compared to Pn polymorph.

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Conclusion

• A novel polymorph of the cathode material Li2MnSiO4, projected

to be more stable than thermodynamic polymorph, was synthesized through an ion-exchange route.

•The structure was confirmed by comparison of simulated and experimental XRD patterns.

• The electrochemical activity of the novel polymorph is similar to that of the thermodynamic polymorph.

• The half-exchanged polymorph, LiNaMnSiO4, yields lower but

stable capacity.

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Acknowledgements

• Bussaraporn Patarachao for XRF measurements

•This work was funded by Natural Resources Canada’s Program of energy Research and Development (Electric Mobility

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Results – Pmn2

– Li

MnSiO

•Good agreement between experimental and calculated paremeters.

•Slight MnO impurity

20 30 40 50 60 70 80 90 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 Li₂MnSiO₄ 97.5% MnO 2.5% C ou n ts 2 (Cu K) Pmn2₁