Thesis
Reference
Development of a New closo-Borate Solid Electrolyte and Its Implementation in All-Solid-State Batteries
DUCHENE, Léo
Abstract
All-solid-state batteries promise to simultaneously yield higher energy and power density as well as improved safety as compared to state-of-the-art lithium-ion technologies based on organic liquid electrolytes. A competitive all-solid-state battery requires a solid-state electrolyte with high ionic conductivity near room temperature and high thermal and electrochemical stability. Meeting these requirements simultaneously represents a major challenge. Closo-borates represent a promising, yet underexplored, class of solid electrolytes.
Specifically, their high-ionic conductivity, wide electrochemical stability window, and advantageous processing properties make them serious candidates to address the major challenges faced by all-solid-state batteries. This thesis presents the development and characterization of a new solid electrolyte within this family, namely Na4(B12H12)(B10H10). It demonstrates that this material possesses the appropriate combination of properties to develop a 3 V all-solid-state battery and discusses aspects related to the assembly and successful cycling of such a device.
DUCHENE, Léo. Development of a New closo-Borate Solid Electrolyte and Its
Implementation in All-Solid-State Batteries. Thèse de doctorat : Univ. Genève, 2019, no.
Sc. 5384
DOI : 10.13097/archive-ouverte/unige:126129 URN : urn:nbn:ch:unige-1261295
Available at:
http://archive-ouverte.unige.ch/unige:126129
Disclaimer: layout of this document may differ from the published version.
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Section de chimie et biochimie
Département de chimie physique Professeur H. Hagemann
EMPA – SWISS FEDERAL
LABORATORIES FOR MATERIALS SCIENCE AND TECHNOLOGY
Laboratory Materials for Energy Conversion Dr. Arndt Remhof
Development of a New closo-Borate Solid Electrolyte and Its Implementation in All-Solid-State Batteries
THÈSE
présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention chimie
par
Léo DUCHÊNE
de
Yvoire (France)
Thèse No 5384
GENÈVE
Atelier d'Impression ReproMail 2019
Celui-ci est pour toi
All-solid-state batteries promise to simultaneously yield higher energy density, improved safety, and lower cost as compared to state-of-the-art lithium-ion technologies based on organic liquid electrolytes. A compe- titive all-solid-state battery requires a solid-state electrolyte with high ionic conductivity near room temperature combined with high ther- mal and electrochemical stability. Meeting these three requirements simultaneously represents a major challenge.
In this thesis we evaluate a new type of solid electrolyte for all- solid-state batteries, in the family ofcloso-borates. We present a new material within this family, namely Na4(B12H12)(B10H10) and evaluate the main materials properties relevant to its use as a solid electrolyte, notably ionic conductivity and electrochemical stability window. We show that Na4(B12H12)(B10H10) satisfies the necessary requirements of high ionic conductivity and large electrochemical stability window to be used in an all-solid-state sodium ion battery with a voltage of 3 V.
Both ionic conductivity and electrochemical stability are related to the nature of the closo-borate anions. The rotational dynamics of these cage-like anions plays a major role to explain the high sodium-ion con- ductivity. This active role of the anion backbones gives rise to a unique temperature dependence of the ionic conductivity which shows a pro- nounced non-Arhhenius behavior. In addition, the high stability of the anions is key to the high stability of Na4(B12H12)(B10H10) compared to other solid electrolytes, notably sulfide and thiophosphates, showing similar ionic conductivity but lacking electrochemical stability.
We then focus on device integration challenges to build a 3 V battery using a sodium metal anode and a NaCrO2cathode. This second part demonstrates the importance of interface engineering for the stable cycling of all-solid-state batteries. We show that the creation of an intimate contact between the solid electrolyte and the cathode is key to a high cycling stability of the battery. Such a contact can be created with Na4(B12H12)(B10H10) by dissolving it into methanol and drying it onto the cathode particles, prior to cell assembly. The stable battery cycling at 3 V also serves as a confirmation for the electrochemical stability window of the electrolyte.
Finally, we further extend the work on solution processing to im- plement the solid electrolyte with conventional electrodes fabricated according to processes established in industry. Using isopropanol as a solvent, Na4(B12H12)(B10H10) can be directly crystallized in its highly conducting phase and thus infiltrated inside a porous electrode. The cycling results thus obtained show promises for the rapid develop-
v
readiness level.
In summary, this works focuses not only on intrinsic materials pro- perty but addresses practical challenges related to device integration of a representative solid electrolyte in the newcloso-borate family.
R É S U M É D E L A T H È S E
Par rapport à la technologie actuelle des accumulateurs lithium-ion, les batteries sodium-ion tout-solide promettent de stocker une plus grande densité d’énergie, d’atteindre une plus grande densité de puissance, tout en maintenant un haut niveau de sécurité ainsi qu’une réduction des coûts. Une telle technologie tout-solide demande de développer un électrolyte solide combinant une haute conductivité ionique à température ambiante, combinée à une grande stabilité thermique, chimique et électrochimique. Satisfaire à ces différents critères représente un défi technique et scientifique majeur.
Dans cette thèse, nous évaluons une nouvelle famille d’électrolyte pour des batteries tout-solide, lescloso-borates. Nous présentons un nouveau matériau au sein de cette famille, le Na4(B12H12)(B10H10) et en évaluons les principales propriétés liées à son intégration dans une batterie, notamment la conductivité ionique et la stabilité élec- trochimique. Nous montrons que le Na4(B12H12)(B10H10) satisfait aux critères de haute conductivité ionique et de grande stabilité électrochi- mique pour être utilisé dans une batterie tout-solide ayant un voltage de 3 V. Cette conductivité ionique ainsi que la stabilité électrochimique sont toutes deux liées à la nature des anions closo-borates. La dyna- mique de rotation de ces anions ayant une structure de cage joue un rôle majeur pour expliquer la haute conductivité des ions Na+. Cette participation active des anions donne lieu à une dépendance unique de la conductivité ionique en fonction de la température qui dévie fortement d’une simple loi d’Arrhenius. En outre, la grande stabilité des anions est la clé d’une haute stabilité électrochimique du Na4(B12H12)(B10H10) par rapport à d’autres électrolytes solides, en particulier les électrolytes sulfurés, qui possèdent une conductivité ionique similaire mais sont plus souvent moins stables.
La thèse se concentre ensuite sur les problématiques liées à la fa- brication d’une batterie de 3 V, utilisant une anode de sodium et une cathode de NaCrO2. Cette deuxième partie démontre l’importance de l’ingénierie des interfaces pour la stabilité de cyclage des batte- ries tout-solide. Nous montrons que la création d’un contact intime entre l’électrolyte solide et la cathode est essentiel à la stabilité de la batterie. Un tel contact peut être créé avec le Na4(B12H12)(B10H10) en
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la batterie à un voltage de 3 V sert aussi à confirmer la mesure de stabilité électrochimique de l’électrolyte.
Pour finir, nous poursuivons le travail sur le traitement en solu- tion pour implémenter l’électrolyte solide avec des électrodes conven- tionnelles fabriquées à partir de méthodes établies en industrie. En utilisant l’isopropanol comme solvant, Na4(B12H12)(B10H10) peut être cristallisé directement dans sa phase à haute conductivité ionique et infiltré dans une électrode poreuse. Les tests de cyclage ainsi obte- nus sont très prometteurs pour un développement rapide de ce type d’électrolyte solide pouvant être traité en solution.
En résumé, ce travail se concentre non seulement sur des propriétés intrinsèques au matériau mais essaye de répondre aux problématiques liés à l’intégration dans une batterie pour un électrolyte représentatif de la nouvelle famille descloso-borates.
vii
a s f i r s t au t h o r
[1] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hage- mann, and C. Battaglia. “A highly stable sodium solid-state electrolyte based on a dodeca/deca-borate equimolar mixture.”
In: Chemical Communications 53.30 (2017), pp. 4195–4198. doi:
10.1039/c7cc00794a.
[2] L. Duchêne, R.-S. Kühnel, E. Stilp, E. C. Reyes, A. Remhof, H.
Hagemann, and C. Battaglia. “A stable3V all-solid-state sodium- ion battery based on a closo-borate electrolyte.” In: Energy &
Environmental Science10.12(2017), pp.2609–2615.doi:10.1039/
c7ee02420g.
[3] L. Duchêne, S. Lunghammer, T. Burankova, W.-C. Liao, J. P.
Embs, C. Copéret, H. M. R. Wilkening, A. Remhof, H. Hage- mann, and C. Battaglia. “Ionic Conduction Mechanism in the Na2(B12H12)0.5(B10H10)0.5 closo-Borate Solid-State Electrolyte: In- terplay of Disorder and Ion-Ion Interactions.” In:Chemistry of Ma- terials31.9(2019), pp.3449–3460.doi:10.1021/acs.chemmater.
9b00610.
[4] L. Duchêne, A. Remhof, H. Hagemann, and C. Battaglia. “Status and prospects of hydroborate electrolytes for all-solid-state bat- teries.” In:Energy Storage Materials(2019).doi:10.1016/j.ensm.
2019.08.032.
[5] L. Duchêne, D. H. Kim, R. Moury, A. Remhof, H. Hagemann, Y. S. Jung, and C. Battaglia. “Crystallization ofcloso-borate elec- trolytes from solution enabling infiltration into slurry-casted porous electrodes for all-solid-state batteries.” Submitted.2019. a s c o-au t h o r
[6] Y. Yan, R.-S. Kühnel, A. Remhof, L. Duchêne, E. C. Reyes, D.
Rentsch, Z. Łodziana, and C. Battaglia. “A Lithium Amide- Borohydride Solid-State Electrolyte with Lithium-Ion Conducti- vities Comparable to Liquid Electrolytes.” In:Advanced Energy Materials7.19(2017), p.1700294.doi:10.1002/aenm.201700294. [7] T. Burankova, L. Duchêne, Z. Łodziana, B. Frick, Y. Yan, R.-S.
Kühnel, H. Hagemann, A. Remhof, and J. P. Embs. “Reorientati- onal Hydrogen Dynamics in Complex Hydrides with Enhanced Li+ Conduction.” In:The Journal of Physical Chemistry C121.33 (2017), pp.17693–17702.doi:10.1021/acs.jpcc.7b05651.
ix
Gigante, and H. Hagemann. “Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid- state electrolyte.” In:Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials75.3(2019), pp.406–413. doi:10.1107/S2052520619004670.
[9] R. Asakura, L. Duchêne, R.-S. Kühnel, A. Remhof, H. Hage- mann, and C. Battaglia. “Electrochemical Oxidative Stability of Hydroborate-Based Solid-State Electrolytes.” In: ACS Applied Energy Materials 2.9(2019), pp.6924–6930.doi:10.1021/acsaem.
9b01487.
[10] A. Gigante, L. Duchêne, R. Moury, M. Pupier, A. Remhof, and H. Hagemann. “Direct solution-based synthesis of the Na4(B12H12)(B10H10) solid electrolyte.” In:ChemSusChem(2019).
doi:10.1002/cssc.201902152.
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This work was carried out in the Laboratory Materials for Energy Con- version of Dr. Corsin Battaglia at Empa, the Swiss Federal Laboratories for Materials Science & Technology, under the academic supervision of Prof. Hans Hagemann in the Department of Physical Chemistry at the Université de Genève and was co-supervised my Dr. Arndt Remhof at Empa. First and foremost, I would like to thank them for giving me the opportunity to do this work. If I think back to my first visit at Empa I could not have imagined how fulfilling this experience has been, and it would not have been possible without them.
My supervisor at Empa, Dr. Arndt Remhof deserves all my grati- tude for his infallible willingness to help. I have found your super- vision during this work to be a perfect balance between guidance and freedom which allowed me to learn and grow as a scientist. I particularly appreciated our many discussions, which luckily were not always about work, and I am hoping to share anotherfeuerzangenbowle soon!
Thank you to my academic supervisor Prof. Hans Hagemann. If our meetings were more rare, they always were very important for me and I know I could always count on your support and help during this work. I also always felt well integrated at the University in Geneva.
I am very grateful to Dr. Corsin Battaglia for the many precious advice and discussions. You have helped me to see the value of this work in the bigger picture which always inspired me to do more.
I would like to thank all the team members at Empa, past and present, for their help and daily good mood. Everyone participated in creating the perfect atmosphere at work, and outside of it. Special thanks to Francesco for all the ’paper’ discussions, David for the constant flow of good work music, Marie-Claude for all the good coffees.
Thank you to all the collaborators of the Sinergia project in which this work was included. Our different collaborations and meeting were very stimulating and fun, a rare combination.
Thank you to Prof. Yoon Seok Jung for welcoming me for three months in is laboratory at Hanyang University. It was a very enriching experience and I had great pleasure working there with his him and his team, in particular with Dong Hyeon Kim who deserves my gratitude for his help and time.
Thank you to Prof H. Martin R. Wilkening for the great collaboration and for allowing me to visit his laboratory in TU Graz and perform experiments there. I am very grateful to Sarah Lunghammer for per-
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precious help in analyzing these data. The work and discussions I could have with both of them were most helpful to understand the more fundamental aspects of ionic conduction discussed in this work.
Thank you to Prof. Christophe Copéret at ETH Zürich for allowing me to perform experiments in his laboratory. Thank you to Dr. Wei Chih Liao for performing these experiments which turned out to be essential for this thesis.
I would of course like to thank Prof. Maximilian Fichtner and Prof.
H. Martin R. Wilkening for accepting to review and evaluate this thesis.
Pour finir, je ne pourrais jamais assez remercier mes amis, ma famille et surtout mes parents pour leur soutien. Rien de tout cela n’aurait été possible sans eux.
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i i n t r o d u c t i o n
1 l i t h i u m-i o n b at t e r i e s - s tat e-o f-t h e-a r t a n d l i-
m i tat i o n s 3
2 t o wa r d s a l l-s o l i d-s tat e b at t e r i e s 9
2.1 Hydro-closo-borate solid electrolytes . . . 14
2.2 Aim and scope of this thesis . . . 16
ii m at e r i a l s a n d m e t h o d s 3 e x p e r i m e n ta l m e t h o d s 25 3.1 Sample Preparation . . . 25
3.1.1 Mechanical milling . . . 25
3.2 Sample Characterization . . . 25
3.2.1 Thermal characterization . . . 25
3.2.2 X-ray diffraction . . . 26
3.2.3 Infrared and Raman spectroscopy . . . 27
3.2.4 Impedance spectroscopy . . . 28
3.2.5 Nuclear Magnetic Resonance spectroscopy . . . 33
3.3 Battery testing . . . 37
3.3.1 Voltammetry methods . . . 37
3.3.2 Galvanostatic methods . . . 39
4 s t u d y o f r e l a x at i o n m e c h a n i s m s i n i o n i c c o n d u c- t o r s 43 4.1 Conductivity relaxation . . . 43
4.1.1 The electric modulus formalism . . . 44
4.1.2 The conductivity formalism . . . 45
4.2 Models for ionic conduction . . . 47
4.2.1 Debye relaxation . . . 47
4.2.2 Non-Debye relaxation . . . 49
4.2.3 The Coupling Model . . . 50
4.2.4 Microscopic interpretations and relation to the conductivity formalism . . . 53
4.3 NMR Spin lattice relaxation . . . 54
4.3.1 Relation and differences between NMR and con- ductivity relaxation . . . 57
iii p u b l i c at i o n s 5 s u m m a r y o f t h e pa p e r s 65 5.1 Article I & II . . . 65
5.2 Article III & IV . . . 68
5.3 Article V . . . 70
6 a r t i c l e i 73
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7 a r t i c l e i i 83
8 a r t i c l e i i i 105
9 a r t i c l e i v 113
10 a r t i c l e v 141
iv c o n c l u s i o n & p e r s p e c t i v e s v a p p e n d i x
a a na ly t i c a l a p p r o x i m at i o n t o t h e c o u p l i n g m o d e l 165 a.1 Model derivation . . . 165 a.2 Igor code . . . 166
b r é s u m é 169
I N T R O D U C T I O N
1
L I T H I U M - I O N B AT T E R I E S - S TAT E - O F - T H E - A R T A N D L I M I TAT I O N S
Lithium-ion batteries (LIB), like other battery systems use reversi- ble electrochemical reactions to convert chemical energy into usable electrical energy, or to store electrical energy into chemical energy.
Such electrochemical energy conversion is particularly attractive as is does not rely on any thermal cycle and can thus by highly energy efficient.[1]
In a lithium-ion battery, the reactions are based on the transfer of Li+ between a cathode, typically a layered transition metal oxide, and an anode, typically graphite. The lithium ion diffuse through an electrolyte which blocks the transfer of electrons that have to travel through an external load. The use of lithium has two major advantages. First, lithium is light such that lithium-ion batteries can deliver high specific capacity (number of charges transferred per unit mass). Second, it is a strongly electropositive metal such that the standard potential of Li/Li+at 25◦C lies at−3.04 V vs. the standard hydrogen electrode potential. In turn, lithium-ion batteries can operate with a high voltage which combined to the high charge capacity enable the superior energy density of these devices. State-of-the-art LIBs can reach gravimetric energy density up to 250 W h kg−1 on the cell level, compared to 20 W h kg−1 to 40 W h kg−1 for lead-acid batteries which yet still dominate the market of rechargeable batteries with their use as car starter batteries.[2].
The high energy density of LIBs has enabled the widespread de- velopment of portable electronics. In addition, lithium-ion batteries are envisioned to play a major role in the growing electromobility market.[3] In2016already42% (in W h) of LIBs were sold for the auto- mobile industry and this share will continue to increase in the coming years.[4] While batteries for portable electronics are relatively small, in the range of 10 W h to 100 W h, much larger batteries are required for an electric vehicle to have an acceptable driving range. As shown in Fig.1.1a, battery modules with up to 100 kW h are needed for plug-in electric cars.[5] Even if the absolute number of batteries needed is sig- nificantly smaller than for portable applications, the resulting energy storage demand is significantly higher. This has several consequences
3
for the development of future battery systems. First, the increased size of the batteries poses certain safety concern due to the flammable nature of the organic liquid electrolytes used in conventional LIBs which can be costly to manage, e. g. with complex thermal manage- ment systems.[6] These safety concerns become particularly relevant for fast charging applications, i. e. in high power devices where heat generation can be important. Second, certain elements used in LIBs will quickly become critical. This is already the case of the cobalt used in the cathode whose known reserves could be quickly depleted at the current growth rate of the demand (seeFig.1.1b). While lithium is more abundant, it is not evenly distributed around the earth which can also cause significant price fluctuations.[5] Finally, the need for batteries in electric that can store more energy (longer driving range) but are also lighter and more compact drives the development of higher energy density devices. These high energy batteries should also offer fast charging/high power capabilities, while again maintaining a high level of safety.
(a)
(b)
Figure1.1: (a) Estimated number and size in W h of the batteries for diffe- rent application along with the associated total energy-storage demand in GW h. (b) Estimated production compared to the reser- ves of lithium and cobalt following the current (2016) production rate or assuming a2% annual growth. Figures reprinted by per- mission from ref. [5], Springer Nature, Nature Review Materials, 2018.
These challenges have driven the research towards post lithium-ion systems.[7] Among them, all-solid-state batteries (ASSB) are actively investigated and appear on the road-map for several companies as a mean to answer some of the challenges faced by conventional LIBs.[8] For example,Fig.1.2shows that ASSBs are foreseen by both Umicore and Volkswagen, among others, to enable an increase in volumetric and gravimetric energy density of future battery systems.[9,10] This increase could be obtained along with a significant gain in safety due to the solid nature of the electrolyte.
In parallel alternative chemistries are also considered to answer challenges associated to materials availability and cost. Sodium-ion batteries (NIB) have potential for cost reduction without drastically changing the well-known chemistry and working principle of LIBs.
They rely on much more abundant sodium which is available wor- ldwide, and many of the best performing sodium cathodes do not contain cobalt.[11] Last but not least, sodium does not form alloys with aluminum at low potential. The current collector on the anode side could be replaced by aluminum that is lighter, more abundant and cheaper than the copper used in LIBs.
While NIBs will inevitably exhibit lower energy density than LIBs due to the higher mass of sodium and its higher redox potential (-2.71 V vs. SHE), they have potential for stationary and/or lower energy applications.
(a)
(b)
Figure1.2: (a) Umicore and (b) Volkswagen road-maps for the development of future battery systems with increased gravimetric and volume- tric energy density. From ref. [9,10]
Currently the focus of industry remains largely on improving furt- her the current Li-ion technology, e. g. by decreasing the cobalt content in cathode materials, or by replacing the graphite anode by higher capacity silicon, as shown in Fig. 1.2a. However, LIBs in their cur- rent format will soon reach their practical limits in terms of energy density and materials availability such that both ASSBs and NIBs are considered as long-term solutions and are very active research fields. This thesis focuses in particular on all-solid-state batteries. The
advantages and challenges facing this technology are detailed in the following section, with the perspective of lithium based all-solid-state batteries since LIBs already are a benchmark technology. However the principles similarly apply for the development of a sodium based all-solid-state battery which we have worked on in this thesis.
2
T O WA R D S A L L - S O L I D - S TAT E B AT T E R I E S
Current lithium-ion battery technology already displays an impressive combination of high energy density, long cycle life and reasonable cost which represents a benchmark for the development of future battery systems. In this regard, an all-solid-state battery technology is mostly associated with three main advantages compared to current LIB technology:
• Improved safety
• Increased specific energy density
• Increased specific power density
The improved safety naturally comes from the solid nature of the electrolyte which prevents leakages of the electrolyte from the battery.
Several solid electrolytes are oxide based ceramics which are inherently non-flammable, thermally stable and do not lead to any significant gas evolution that can undermine the safety of an electrochemical device.
However, sulfide and thiophosphate based electrolytes which gained significant interest due to their high ionic conductivity,[12] are usually chemically and electrochemically unstable and can decompose under the release of toxic H2S in the presence of water.[13–15] Absolute safety may thus not be inherent to the use of solid electrolyte and should still be carefully evaluated.[16]
The evaluation of the energy density is not trivial for all-solid-state batteries. In fact, the mere replacement of the liquid electrolyte and separator by a solid electrolyte while keeping the same electrode com- position will generally tend to decrease the total gravimetric energy density of the cell because most solid electrolyte are significantly den- ser than liquids (seeFig.2.1a). This high density can still be beneficial for the volumetric energy density.
The specific energy density of a battery can be defined as follow:
Ecell[W h kg−1] = Ucell
1 Cneg +C1
pos +minact (2.1)
whereUcell is the nominal cell voltage,Cneg andCposare the capacities in A h kg−1 (more commonly reported as mA h g−1) of the negative
9
and positive electrodes respectively, and minact is the total specific mass of inactive materials in kg(Ah)−1. If the use of a solid electrolyte tends to increaseminact on the cell level, it can reduce it on a module or system level. Indeed, solid-state cells can be assembled in a more compact stacked design compared to conventional lithium-ion cells.
In addition, thermal management systems used for conventional LIBs may be reduced as solid electrolyte can potentially be safely operated over a larger temperature range.[6,16]
The dominant prospect is that ASSBs can allow for the use of li- thium metal as anode in place of conventional graphite. Lithium metal possesses a high theoretical specific capacity of 3862 mA h g−1 com- pared to 372 mA h g−1 for graphite which could lead to a substantial gain of both gravimetric and volumetric energy density.[4, 8] The use of a metal anode also slightly increases the overall cell potential compared to a cell with a graphite anode. Similarly for NIBs, a solid electrolyte could allow to use a sodium metal anode that has a capacity of 1165 mA h g−1 compared to commonly used hard carbons with a capacity around 300 mA h g−1.
(a)
(b)
Figure2.1: (a) Comparison of the density of conventional carbonate ba- sed liquid electrolyte with ionic liquids and different repre- sentative members of solid electrolytes families: Li7P3S11 (LPS), Li1.5Al0.5Ti1.5(PO4)3 (LATP), and Li7La3Zr2O12 (garnet). (b) Ex- pected energy density of ASSBs using a LPS electrolyte (20 µm), nickel-manganese-cobalt oxide cathode (NMC-811), and graphite (Gr) or Lithium anode (Li). In thecoated particlecase, the loading of the solid electrolyte is reduced from30vol% to15vol%. Figures reprinted by permission from ref. [4], Springer Nature, Journal of Solid State Elecrochemistry,2017.
With an organic liquid electrolyte the plating of lithium metal upon charging of the battery tend to be highly nonuniform with a needle- like (dendrite) or highly porous (mossy) lithium deposition.[17] This can result in a rapid failure of the battery as the liquid electrolyte continuously reacts with such high surface area lithium or because of short-circuits if the dendrites penetrate through the separator and reach the cathode. The latter mechanism also poses a serious safety concerns as it can lead to a fire of the battery.[18]
The commercialization of a few solid-state batteries using a lithium metal anode drives the hope for the development of high energy den- sity solid-state devices. These commercial examples are limited to thin film microbatteries using a LiPON (lithium phosphorous oxynitride) solid electrolyte or lithium metal-polymer batteries.[19] Both cannot
yet compete with LIBs in terms of energy density and the later also has to operate at an elevated temperature of 80◦C. The challenge remains to demonstrate a true lithium metal battery with high energy and power density, using a solid electrolyte and operating at room temperature. These challenges are mainly related to the stability of the lithium/solid electrolyte interface. First, it is not yet clear whet- her solid electrolytes, even hard ceramics, can prevent the formation of mossy of dendritic lithium.[20] In addition, the low potential of lithium metal requires a high reduction stability of the electrolyte.
While lithium containing binary compounds such as LiF are stable in contact with lithium, they are usually poor ionic conductors. On the contrary, many highly conducting compounds are unstable in contact with lithium.[14] The challenge is then to develop a stable passivating interface.[21]
Lastly, the energy density of LIBs can also be increased by using higher voltage cathodes to increaseUcell. Then, similar interface chal- lenges are to be solved. In this case, a good resistance of the solid electrolyte towards oxidation is required.
High power density in a battery is the ability to store and deliver a high energy under fast charge and discharge rates. When increasing the charge or discharge current, the internal resistance of the battery results in an overpotential, also called polarization, such that the voltage profile will deviate from equilibrium as shown in Fig. 2.2. In turn, the battery will require a higher potential to charge while delivering a lower discharge voltage, decreasing its energy efficiency.
Figure2.2: Effect of increased current on the discharge profile of a battery.
Reproduced by permission from ref. [1], Springer Nature,2009.
Such polarization of an electrochemical cell is given by the sum of contributions from the ohmic, charge transfer and the so-called concentration polarization:
ηtotal =ηohm+ηct+ηconc (2.2)
In particular, ηohm andηconc are related to electrolyte properties. The former is inversely proportional to the electrolyte conductivity while ηconc arises from a concentration gradient of charged mobile species in the electrolyte. It appears when the both cation and anion are mobile in the electrolyte while the electrodes are selective to the insertion of only one ion (e. g. Li+).[22]. It is expressed as:
ηconc ∝ 2RT
F (1−t+)ln(a+/are f) (2.3) wheret+ is the transference number of the cation in the electrolyte, that is the fraction of current carried by the cations. In a standard liquid electrolyte tLi+ < 0.5 and ηconc can become important under high charge or discharge currents. In solid electrolytes, one type of ions typically diffuses within a more rigid lattice that does not exhibit long range diffusion (except salt-in-polymer electrolytes which also contain mobile anions). In that case, the transference number for that ion is near unity and ηconc becomes negligible. Therefore, for an identical cation conductivity, the polarization will be lower when using a solid electrolyte instead of a liquid electrolyte with mixed
cation/anion conductivity. All-solid-state cells can thus exhibit lower
polarization and retain more capacity at high charge and discharge rates given that a solid electrolyte with sufficient ionic conductivity is used.[23] This reasoning assumes no change of the charge transfer resistance ηct at the different interfaces in the cell which in practice may change significantly when replacing the liquid-solid by solid-solid interfaces in conventional vs. solid-state cells. Again, the engineering of interfaces is critical for the development of ASSBs.[24]
In summary, whether or not all-solid-state battery technology can meet the aforementioned expectations will mainly depend on the properties of the solid electrolyte. Notably a solid electrolyte requires (see alsoFig.2.3):
• High selective cation conductivity of at least 1 mS cm−1 and ideally >10 mS cm−1at room temperature to cycle cells at reaso- nable rates and meet expectations of increased power density.
This high ionic conductivity should be combined with a low elec- tronic conductivity to prevent the self-discharge of the battery.
• Large electrochemical stability window including stability to- wards alkali metals and high voltage cathodes to increase the energy density.
• High thermal and chemical stability to maintain a high level of safety.
• Processability and favorable mechanical properties to maintain a low cost of cell fabrication and reduce interface resistances.
Simultaneously meeting all of these criteria represents a major chal- lenge.[25]
Figure2.3: Schematic cross-section of an all-solid-state battery with a metal anode. The challenges associated with the solid electrolyte and its interplay with the difference battery parts are also depicted.
FromArticle V.
2.1 h y d r o-closo-b o r at e s o l i d e l e c t r o ly t e s
None of the solid electrolyte materials class considered today can yet simultaneously meet the aforementioned requirements necessary to build a competitive battery.
Fig.2.4shows the performance of candidate families of solid electro- lytes. Among those, three have recently attracted most of the interest to develop an all-solid-state battery technology.
1. Oxides, often in the form of sintered ceramics are characterized by a high ionic conductivity on the order of 1×10−4S cm−1 to 1×10−3S cm−1, good stability including in particular good electrochemical stability towards both oxidation and reduction.
However they are hard materials which are difficult to contact without high temperature sintering and that face problems of
mechanical stability upon cycling. Device integration is thus currently difficult and costly.
2. Sulfides and thiophosphate have more recently emerged as one of the most promising family of solid electrolyte for both lithium and sodium batteries as they combine high ionic conductivity on the order of 1×10−3S cm−1 to 1×10−2S cm−1and more fa- vorable mechanical properties, compared to ceramic materials.
However, they mostly suffer from a low chemical and electro- chemical stability which also renders their implementation in a device challenging.
3. Finally, polymer electrolytes have achieved the highest level of device integration and are used in one of the rare example of commercialized solid-state lithium metal battery system.[19] Ho- wever their low conductivity at room temperature only allows the cells to be cycled at60-80◦C. In addition, they are not stable at high voltage which forces to use cathodes with a working po- tential<4 V (e. g. LiFePO4) and limits the maximum achievable energy density.
We do not discuss here halides and thin film electrolytes which are not as widely studied or are meant for niche applications.
Figure2.4: Radar plots summarizing the key metrics of the most common class of materials studied for their use as solid electrolyte in all- solid-state batteries. Reproduced by permission from ref. [26], Springer Nature, Nature Review Materials,2017.
Among the other families inFig.2.4hydrides are presented with some promising properties in terms of reduction stability as well as processability and device integration. This reference considers early
hydride solid electrolyte, e. g. LiBH4, LiNH2 and mixtures thereof, as for example Li(BH4)1–x(NH2)x with 0.5 ≤ x ≥ 0.75, which was developed at Empa and is reported in ref. [27] where I am also a co-author.
Within the hydride family, this thesis rather focuses on a class of hydroborate (general IUPAC denomination of compounds contai- ning boron-hydrogen based anions[28]) materials, the so-called metal closo-borates MyBxHx, and their implementation as solid electrolyte for all-solid-state batteries. Before the start of this work, preliminary studies had shown the promise of these compounds for high ionic conductivity. Both Na2B12H12and Na2B10H10 were shown to exhibit high Na+ conductivity triggered by a structural phase transition at elevated temperature, above 260◦C and 100◦C respectively.[29, 30] However these temperatures are not practical in view of room tempe- rature battery applications. Still, previous work on anion substitution in related LiBH4 or ball milling of Na2B12H12had demonstrated that these approaches could be successful in stabilizing high ionic con- ductivity at room temperature.[31,32] In addition,closo-borates have long been known for their high chemical and thermal stability due to the cage-like structure of the [BxHx]2– anions shown inFig.2.5which give them an aromatic character.[33,34] Finally, they are light material with a density generally near or even lower than that of an organic liquid electrolyte (∼1 g cm−3). However, at the start of this work,closo- borates remained a largely under-explored class of materials in terms of their implementation in all-solid-state batteries.
Figure2.5: Structure of the B12H122– and B10H102– anions.
2.2 a i m a n d s c o p e o f t h i s t h e s i s
The aim of this thesis is to bridge the gap between the attractive materials properties of closo-borates and their implementation into state-of-the art solid-state batteries. While the devices studied here are all-solid-state sodium-metal or sodium-ion batteries, this thesis does not aim to address the specific challenges of sodium-based batteries
but rather focuses on the development of an all-solid-state battery technology. In this regard, most of the work reported here could also apply for a lithium ion conductor sharing similar properties. The thesis is organized as follow:
Part IIdescribes the different experimental methods used to pre- pare and characterize the materials studied in this thesis. For certain methods a more detailed theoretical background is given. This is the case for impedance and nuclear magnetic resonance spectroscopy which have been used most extensively for this work.
Then,Part IIIcontains the main results of this thesis. It is divided into different articles published or currently under review in peer- reviewed journals. A summary of the papers as well as comments on my exact contribution are given at the beginning of this part. Briefly the papers discuss the following:
• Article Idescribes the preparation of a newcloso-borate electro- lyte, namely Na2(B12H12)0.5(B10H10)0.5. The materials structure, ionic conductivity, and electrochemical properties of the ma- terial are evaluated in view of its use as a solid electrolyte.
For the rest of this thesis this compound will be referred to as Na4(B12H12)(B10H10) to simplify the notation.
• Article II focuses on more fundamental aspects of the ionic conduction mechanism of Na4(B12H12)(B10H10).
• Article IIIdescribes the implementation of Na4(B12H12)(B10H10) in a3V class all-solid-state sodium battery with a sodium metal anode and NaCrO2 cathode, cycling at 60◦C. This work focuses notably on the interface engineering for stable cycling.
• Article IV describes the development of a solution process to crystallize Na4(B12H12)(B10H10) directly into porous composite electrolyte fabricated by conventional slurry-based methods as used in industry.
• Article Vis an invited review paper. It presents the recent deve- lopments on hydroborates electrolytes in view of battery appli- cation. Many of the recent results have been obtained in parallel to this thesis and this review puts this work into perspective and also identifies the research gaps that remain to be solved for this class of materials.
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[14] W. D. Richards, L. J. Miara, Y. Wang, J. C. Kim, and G. Ceder.
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[15] Y. Tian, T. Shi, W. D. Richards, J. Li, J. C. Kim, S. Bo, and G.
Ceder. “Compatibility issues between electrodes and electrolytes in solid-state batteries.” In:Energy & Environmental Science10.5 (2017), pp.1150–1166.doi:10.1039/C7EE00534B.
[16] T. Inoue and K. Mukai. “Are All-Solid-State Lithium-Ion Batte- ries Really Safe?–Verification by Differential Scanning Calorime- try with an All-Inclusive Microcell.” In:ACS Applied Materials
& Interfaces 9.2 (2017), pp. 1507–1515. doi: 10 . 1021 / acsami . 6b13224.
[17] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, and J.-G. Zhang. “Lithium metal anodes for rechargeable batteries.”
In:Energy Environ. Sci. 7.2 (2014), pp.513–537. doi: 10.1039/
C3EE40795K.
[18] X. Feng, M. Ouyang, X. Liu, L. Lu, Y. Xia, and X. He. “Thermal runaway mechanism of lithium ion battery for electric vehicles:
A review.” In: Energy Storage Materials 10 (2018), pp.246–267. doi:10.1016/j.ensm.2017.05.013.
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[20] Y. Ren, Y. Shen, Y. Lin, and C.-W. Nan. “Direct observation of lithium dendrites inside garnet-type lithium-ion solid electro- lyte.” In:Electrochemistry Communications 57(2015), pp.27–30. doi:10.1016/j.elecom.2015.05.001.
[21] Y. Zhu, X. He, and Y. Mo. “Origin of Outstanding Stability in the Lithium Solid Electrolyte Materials: Insights from Thermo- dynamic Analyses Based on First-Principles Calculations.” In:
ACS Applied Materials and Interfaces7.42(2015), pp.23685–23693. doi:10.1021/acsami.5b07517.
[22] D.-H. Kim, S. Hwang, J.-J. Cho, S. Yu, S. Kim, J. Jeon, K. H.
Ahn, C. Lee, H.-K. Song, and H. Lee. “Toward Fast Operation of Lithium Batteries: Ion Activity as the Factor To Determine the Concentration Polarization.” In:ACS Energy Letters(2019), pp.1265–1270.doi:10.1021/acsenergylett.9b00724.
[23] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, and R. Kanno. “High-power all-solid-state batteries using sulfide superionic conductors.” In:Nature Energy 1.4(2016), p.16030.doi:10.1038/nenergy.2016.30.
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M AT E R I A L S A N D M E T H O D S
3
E X P E R I M E N TA L M E T H O D S
In this chapter, the major preparation and characterization techniques used in this thesis are discussed. When needed, a more detailed theoretical background is given. Technical details about the type of instrument and experimental parameters are not given here but are detailed in each article inPart iii.
3.1 s a m p l e p r e pa r at i o n 3.1.1 Mechanical milling
Mechanical milling is a sample preparation technique used to grind and/or mix materials as well as to induce solid-state chemical reacti- ons.
We used the so-called high energy ball milling technique for which a powder sample is placed in a jar with balls. The high energy comes in a shaker mill configuration that causes strong impact and friction of the ball on the jar and sample. It causes extreme plastic deformation and brittle fracture of the sample that decreases particle size. Cer- tain changes of the material such as chemical reactions or structural transition can also be initiated by the local temperature and pressure increase in the jar.
Ball-milling was used extensively in this thesis to induce anion mix- ing and prepare Na4(B12H12)(B10H10) from Na2B12H12and Na2B10H10. In this case the milling step was used as an activation step followed by a heat treatment which was necessary to obtain the face-centered cubic phase of Na4(B12H12)(B10H10) with a high degree of crystallinity.
3.2 s a m p l e c h a r a c t e r i z at i o n 3.2.1 Thermal characterization
Differential scanning calorimetry (DSC) measures the differential heat flow between a sample and a reference as their temperature is varied at a constant rate. The reference typically consists of an identical, empty sample container. Different processes will yield signature heat profiles in a DSC scan.
25
First order transitions, which include structural phases changes, involve a latent heat (enthalpy) that can be released (exothermic tran- sition) or absorbed (endothermic transition) at a specific temperature.
In DSC this is characterized by a sharp peak in the heat flow whose area correspond to the latent heat of the transition. The onset of the peak is usually used to define the temperature of the transition. Note that by convention, a positive heat flow will refer to an endothermic process.
Second order transitions, such as glass transitions, involve a change of the specific heat capacity of the material. In DSC this is characterized by a step increase or decrease of the heat flow to the sample.
In this thesis DSC was used qualitatively to identify the presence or absence of a phase transition in potential solid electrolyte materials.
The identification of the transition temperature allowed to better un- derstand changes that were observed with other techniques such as X-ray diffraction or impedance spectroscopy. Because the instrument was not calibrated with regards to a material with known thermal properties, the heat flows are used qualitatively and reported with the raw signal unit, i. e. in µV mg−1.
InArticle I, DSC was used to confirm the successful suppression of the first order transitions of Na2B12H12and Na2B10H10 in the equi- molar Na4(B12H12)(B10H10) mixture which in turn was essential to achieve high ionic conductivity at room temperature.
InArticle II, a glass like transition could be identified near−30◦C and related to a change in the thermally activated ionic conductivity of Na4(B12H12)(B10H10) and to an increase of its thermal expansion coefficient.
3.2.2 X-ray diffraction
X-ray diffraction (XRD) is a technique used to study the atomic struc- ture of matter. The regular arrangement of atoms within crystalline matter is the equivalent to an optical grid where the atoms and more specifically their electron clouds are the individual scatterers. Con- structive interference occurs at certain angles depending on the crystal structure, i.e. its interatomic distances, type of atoms and on the symmetries of the studied materials.
The angle depends on distances between atomic planes in a crystal such that each diffraction peak corresponds to a specific lattice plane characterized by its Miller indices. In addition, the symmetry of the crystal only allows for certain planes to scatter light constructively.
Finally, the intensity of the diffracted beam is dependent on the type of atom that scatters the light and heavier atoms scatter light more strongly.
The diffraction pattern thus simultaneously carries information on the arrangement and nature of the atoms and is a fingerprint for a material. Many other information about the crystal size, strain in the material, etc, can also be extracted from the shape of the peaks.
In this thesis, XRD was extensively used in all the articles as a fingerprinting technique to identify the successful preparation of different battery materials, e. g. the Na4(B12H12)(B10H10) electrolyte or NaCrO2 cathode.
InArticle IXRD was used to show that Na4(B12H12)(B10H10) has the same crystal symmetry at 30◦C than Na2B10H10 at 180◦C, a key to its high ionic conductivity.
In addition, inArticle II the temperature dependent lattice para- meter of Na4(B12H12)(B10H10) was extracted from a fit of the XRD pattern at each temperature. The fits were performed with the LeBail method where only the peak position is adjusted based on the known space group symmetry of the material to get the unit cell parameters.
Parameters linked to the peak shapes and intensity are freely adjusted.
This analysis allowed to relate structural evolution of the electrolyte to changes in the ionic conductivity behavior.
3.2.3 Infrared and Raman spectroscopy
The characteristic frequencies at which molecules vibrate typically falls in the mid-infrared range (3-30 µm). The infrared absorption spectra of a sample in this spectral range will thus be characterized by absorption bands caused by the vibration of specific bonds. The absorption wavelengths not only depends on the bond nature and composition but is also linked to the symmetry of the molecule that vibrates.[1] Infrared spectroscopy is thus often used as a fingerprinting technique to characterize the composition of a compound. It can also help to better understand the local environment experienced by a molecule and which can influence its symmetry and thus IR spectrum.
In Raman spectroscopy, the vibrational modes are excited by a mo- nochromatic light in the visible range which gets inelastically scattered by interacting with molecular vibration. The difference in energy be- tween the incident and scattered beam corresponds to that of the vibrational mode. Raman and infrared spectroscopy are complemen- tary techniques which give similar information about the structure and symmetry of molecules.
The B-H bond in hydroborate anions can be probed by such vibra- tional spectroscopy. InArticle Iit was used to confirm the presence of both [B12H12]2– and [B10H10]2– anions in Na4(B12H12)(B10H10) after ball milling and heat treatment. Because these anions have a different geometry, i. e. Ih and D4dpoint group symmetries respectively, they
can be distinguished. In addition, the symmetry of the anions found at room temperature for Na4(B12H12)(B10H10) was the same than for the respective precursor phases at elevated temperature, i. e. in their cubic fast ion conducting phases where the anions show fast dynamics and ideal symmetry (identical to the anion in solution).
InArticle II, Raman spectroscopy on Na4(B12H12)(B10H10) showed no noticeable change of the anion symmetry down to cryogenic tempe- ratures of 77 K. This is an indication that the anion dynamics observed at room temperature and related to the fast cation dynamics is still present at 77 K as the anion still experience a very symmetric environ- ment.
3.2.4 Impedance spectroscopy
Impedance Spectroscopy (IS) is a technique used to probe the electrical response of a material that is subjected to an oscillating electric field.
The response of the material can be probed over a large range of frequencies, typically in the range of a few mHz to ∼10 GHz. Most of the processes involving the dynamics of ions in battery materials occur within a timescale corresponding to such frequencies. IS is thus a very commonly used technique for the study of both intrinsic materials properties, such as the quantification of the ionic conductivity of electrolytes, both liquid and solid, or for the study of full devices to understand processes of ionic diffusion through interfaces and their evolution during the battery lifetime (e. g. battery aging).
3.2.4.1 Data acquisition
In practice, the sample to study is subjected to an oscillating (sinus- oidal) ac voltage signal of fixed amplitude, i. e. V(t) = V0sin(ωt). V0 is kept small, in the 10 mV to 50 mV range, so that the response of the material remains linear. As a result of the applied field, the moving charges in the sample will generate a response current that oscillates with the same frequency but is shifted in phase, i. e. I(t) = I0sin(ωt−φ). The impedance Z(t) of the sample is then obtained as the ratio of V(t)/I(t). Because of the sinusoidal nature of the per- turbation and the response, is it more convenient to use a complex representation of V, I and Z, i. e.V =V0eiωt, I = I0ei(ωt−φ)which gives Z = VI = Z0eiφ = Z0cosφ+iZ0sinφ. The impedance is a complex number Z=Z0+iZ00 whereZ0 =Z0cosφandZ00 = Z0sinφ
3.2.4.2 Data analysis
There exists different ways to analyze impedance data, the choice of which depends on the goal of the analysis. Often, the measured complex impedance is represented in a so called Nyquist plot of Z00 vs.Z0 that can be fitted by the use of equivalent electrical circuits. For
example, in the case of a simple, ideal ionic conductor, the impedance response can be best approximated by that of a parallel RC circuit, with a resistance related to the ion generated current, in parallel with a capacitor related to the dielectric response of the material. The impedance of a RC circuit is written as:
1 Z = 1
ZR + 1 ZC
= 1
R+iωC (3.1)
For the real and imaginary impedance this gives:
Z0 = 1/R
(1/R)2+ (ωC)2 Z00 = −ωC
(1/R)2+ (ωC)2 (3.2)
For such a circuit, the Nyquist plot will have the typical shape of a semi-circle as shown inFig.3.1a, while the frequency dependence of the real and imaginary part ofZare shown inFig.3.1b. On the Nyquist plot, the highest frequencies are on the left (near Z0 = 0) while the lowest frequencies are on the right. A first characteristic value of the system can be found at the intersection of the Nyquist curve with the X- axis (Z00 =0), whereZ0 = R. This value is related to the direct current conductivity of the material, σdc = (C0R)−1 whereC0is a geometrical factor. For a typical measurement, the material under investigation is usually a disc of area A and thickness t and the factor C0 = At. The conductivity then has the unit ofΩ−1m−1 or equivalently S m−1 (S cm−1 is more commonly used). This method was used inArticle I to determine the ionic conductivity of Na4(B12H12)(B10H10)
Then, the frequency at whichZ00 reaches its maximum, is the cha- racteristic frequency of the circuit,ω =1/RC. In the time domain, it corresponds to a characteristic relaxation time τ=RCof the system.
It is the characteristic relaxation time of the current in the sample after a step function voltage has been applied. Further details on the study of relaxation function and the link to the analysis of impedance data will be given inChapter4
Finally, it is common to observe a linear tail following the semicircles towards the lowest frequencies. This tail is caused by the accumulation of ions at the electrodes through which the perturbation signal is applied. These electrode are often chosen to block the diffusion of ions causing this polarization effect at sufficiently low frequencies. It can be modeled in an equivalent circuit by a so-called Warburg element in series with the RC elements.
