Complex hydrides as room-temperature solid electrolytes for rechargeable batteries
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A central goal in current battery research is to increase the safety and energy density of Li-ion batteries. Electrolytes nowadays typically consist of lithium salts dissolved in organic solvents. Solid electrolytes could facilitate safer batteries with higher capacities, as they are compatible with Li-metal anodes, prevent Li dendrite formation, and eliminate risks associated with flammable organic solvents. Less than 10 years ago, LiBH4 was proposed as a solid-state electrolyte. It showed a high ionic conductivity, but only at elevated temperatures. Since then a range of other complex metal hydrides has been reported to show similar characteristics. Strategies have been developed to extend the high ionic conductivity of LiBH4 down to room temperature by partial anion substitution or nanoconfinement. The present paper reviews the recent developments in complex metal hydrides as solid electrolytes, discussing in detail LiBH4, strategies towards for fast room-temperature ionic conductors, alternative compounds, and first explorations of implementation of these electrolytes in all-solid-state batteries.
KeywordsSolid Electrolyte Metal Hydride High Ionic Conductivity TiS2 LiBH4
1 Introduction: the relevance of solid-state electrolytes
The electrolyte in present-day lithium-ion batteries, typically based on oxidic intercalation materials such as lithium cobalt oxide (LiCoO2) cathodes and graphite intercalation anodes (Li x C), usually consists of lithium salts dissolved in organic liquids. This enables fast Li+ transport between the electrodes, but is associated inevitably with potential safety, stability, and cycle-life issues. Increasing the energy density is a major incentive for battery development and research. A factor of 2–3 in energy density should be gained to render the current driving range (of about 160 km) of all electric vehicles competitive to that of petrol-driven cars. Energy density becomes even more critical when considering also medium-scale applications, such as local load levelling of solar energy and wind power [1, 2, 3, 4].
Solid electrolytes could offer compatibility with Li-metal anodes, prevent short-circuiting in sulphur-based batteries, and eliminate the safety risks associated with the flammability and volatility of organic solvents. However, it is a formidable challenge to identify a light-weight and low-cost solid material with sufficiently high ionic conductivity (at least 10−4–10−3 S cm−1 at room temperature) which at the same time possesses a large electrochemical stability window and has negligible electronic conductivity. For thin film batteries in microdevices, so-called LISICON materials have been developed, of which Li10GeP2S12  seems one of the most promising ones, but large-scale application is hindered by the high cost of electrolyte fabrication and components. Several other multi-metal oxynitrides and titanates are being considered, but at present no material satisfying all requirements has been identified, and hence the search for new types of solid electrolytes is a very active field of research.
2 Complex metal hydrides as solid-state electrolytes
Complex metal hydrides are solids with an ionic lattice composed of metal cations and complex hydride anions in which the atoms are predominantly covalently bound. This family includes boron hydrides such as LiBH4, Li2B12H12, NaBH4, and Mg(BH4)2, aluminium hydrides such as LiAlH4 and Na3AlH6, and a wide range of other compounds. Several hydrides, such as LiAlH4 and NaBH4, are in wide use as reductants in organic synthesis. Due to their high hydrogen content, they are also considered promising reversible hydrogen storage materials [18, 19, 20]. Despite early reports , investigation of their potential as solid-state electrolytes has only taken full flight with the pioneering work of the group of Orimo [22, 23].
In this section, we start by briefly reviewing the findings on LiBH4 as a fast-ionic conductor, and how the high conductivity behaviour was extended to room temperature by either partial anion replacement (Sect. 3) or the formation of a nanocomposite (Sect. 4). Subsequently, we will also discuss alternative complex metal hydrides with high ionic mobilities, and we will end with a brief conclusion and outlook. More details on complex metal hydrides as solid-state electrolytes can also be found in .
3 Fast room-temperature ionic conduction in LiBH4 by partial anion substitution
Most notably Orimo and co-workers showed that replacing a fraction of the BH4 − anions by halide anions (I−, Br− or Cl−) stabilises the hexagonal structure at lower temperatures and leads to high ionic mobilities persisting down to room temperature. They found the partial replacement of the [BH4]− anion with I− anions, especially effective. A single-phase solid solution was formed for a wide range of compositions up to 25–30 % of anion replacement. An ionic conductivity of 2 × 10−4 S cm−1 at room temperature was obtained, and a minimum activation energy of 0.39 eV at a replacement of 13 % of the [BH4]− ions. Anion substitution stabilised the high-temperature phase to room temperature, but, as the solid solutions were obtained by ball milling, grain boundaries and defects also seem to play an important role [22, 29].
In the high-temperature phase, the Li+ ions are arranged in layers in the hexagonal plane, with probably two nearly equivalent Li+ sites. As in principle all sites are filled, the conduction is most likely related to Frenkel-pair defects (Li+ vacancies combined with interstitial Li+ sites). Interstitial Li+ can, with a relatively low activation barrier and probably helped by rotation of the neighbouring BH4 − units, jump from one interstitial site to the next (see Fig. 2) [22, 24, 29, 30]. In the LiBH4:LiI solid solutions, the mobilities of Li+ and [BH4]− increase with increasing iodine content [30, 31]. Using mixed components was reported to make the system susceptible to phase segregation and reduced thermal and electrochemical stability [32, 33].
4 Fast room-temperature ionic conduction in LiBH4/SiO2 nanocomposites
Nanocomposites of light metal hydrides with high surface area carbon materials have been widely investigated for reversible hydrogen storage applications, as creating a large interfacial area between the hydrides and carbon scaffolds has been found to improve the kinetics and reversibility of hydrogen sorption and result in high hydrogen and [BH4]− mobilities [34, 35, 36]. NMR suggested also high Li+ mobilities in LiBH4/C nanocomposites [35, 37], but carbon materials are electronically conductive, hampering detailed structural studies and ionic mobility measurements and their use as solid electrolytes.
The origin of the high ionic mobility in these nanocomposites is not yet well understood. Confinement of LiBH4 in the oxide pores leads to limited stabilisation of the hexagonal high-temperature phase in the core of the pores. However, the high ionic mobility seems rather associated with the fraction of LiBH4 that is close to the SiO2 interface [39, 40]. In general high defect densities and low diffusion barriers are found at the interface between two solids, due to effects such as disorder, strain, and space charge regions. Also for other compounds, mostly studied as proton and oxygen conductors for fuel cells, the addition of oxide nanoparticles has been found to increase conductivity [41, 42, 43].
5 Other Li+-containing complex hydrides
Apart from LiBH4, several other complex metal hydrides have been discovered to display fast conductivity. An interesting family is the system LiBH4–LiNH2–LiI. For instance, Li2(BH4)(NH2) and Li4(BH−4)(NH2)3 show conductivities of 2 × 10−4 S cm−1 at 300 K. Also, lithium aluminium hydrides such as LiAlH4 and Li3AlH6 are potentially interesting. Reported conductivities until now are relatively low (10−7–10−5 S cm−1 at 120 °C) , and they are better considered as anode materials [11, 45]. Another interesting class reported is the rare earth and chloride-containing materials from the LiRE(BH4)3Cl family (with RE=Ce, La, Gd), for which conductivities of up to 2 × 10−4 S cm−1 are reported at room temperature . These examples highlight that the high ionic conductivities found in LiBH4-based materials are by no means an exception, but in fact that high ionic conductivities are quite common in complex metal hydrides compounds, and hence, it is likely that many more potentially interesting materials will be synthesised and reported in the coming years. Having said that, existing literature also makes clear that moving towards increasingly complex materials, and increasing the number of elements and hence possible phases in the systems, makes it more and more a challenge to obtain phase-pure compounds, and is often not accompanied by a high stability upon cycling or temperature treatment.
6 Na+ superionic conductors
7 Integration of these solid electrolytes into a battery
Optimising one component is not yielding a commercially viable battery, as compatibility with the other components in the system is crucial for real application. Most efforts for the construction of all-solid-state batteries based on complex metal hydride electrolytes have been focussed on LiBH4 [51, 52, 53, 54]. LiBH4 is stable in contact with metallic lithium, but its stability at the interface with the positive electrode, while facilitating fast and totally reversible exchange of Li+ ions with the electrode, is a challenge. First attempts were made with a LiCoO2 electrode, intrinsically unstable in contact with LiBH4 operated in the high-temperature phase (at 120 °C), but the stability and reversibility were much improved by applying a Li3PO4 coating between the LiCoO2 and LiBH4 . Also TiS2 was tested as a positive electrode , and the solid electrolyte was even combined with a metal hydride negative electrode . More details on complex metal hydrides for bulk-type all-solid-batteries can be found in .
8 Conclusions and outlook
It is clear that for future-generation, higher-capacity, lithium-ion batteries, solid-state electrolytes are a crucial component. Complex metal hydrides, and specifically LiBH4-based compounds, have recently been shown to be a promising class of fast-ionic conductors. Although intrinsically macrocrystalline LiBH4 displays high conductivities only at temperatures above 110 °C, more recent strategies such as partial anion substitution and nanoconfinement have been developed that lead also to high conductivities at room temperature. Yet, LiBH4 is not unique, and many other complex metal hydrides containing Li+ as well as Na+ and probably other cations also exhibit high ionic conductivities. It is evident that we still need a more fundamental understanding of the ionic mobilities in these compounds, and how they can be influenced by such strategies as anion replacement and nanoconfinement. Also crucial is a better understanding of their chemical and electrochemical stabilities, most notably pertaining to their interface interactions with the positive electrode. Nevertheless, it is clear that this rapidly expanding family of complex metal hydrides with high room-temperature conductivities provides for a rather intriguing area of fundamental scientific research that can ultimately lead to exciting practical applications for these materials in next-generation, all-solid-state, lithium-ion, and other rechargeable batteries.
The IEA HIA Task 32—Hydrogen-Based Energy Storage is acknowledged for providing the opportunities for collaboration and fruitful discussions.
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