3.2. Mechanism Comprehension
Electrochemical methods are in principle powerful tools to study redox systems. They could provide a description of redox processes by distinguishing reaction steps, giving the indication of the potential where they occur and of the number of electrons (and lithium equivalents) involved. Unfortunately, in real systems, undesired side reactions can take place, while kinetics problems can lead to incorrect evaluation of processes, convoluting them or completely hindering reactions. Therefore, the use of the simple electrochemical techniques is generally not enough to describe complex processes like conversion [13
In order to identify and decouple the possible competitive phenomena occurring upon lithium incorporation into electrodes, a possible strategy can exploit the synergic use of DFT and complementary experimental techniques [30
DFT could provide an estimation of the thermodynamic feasibility of conversion processes and estimate the potential vs. Li where reactions should occur [17
]. Indeed the mechanism can be modeled at an atomic level, and a description of the most convenient thermodynamic path can be obtained at several levels of detail [22
]. The occurrence of multistep reactions, with the formation of more or less stable intermediates is forecast, but it is even possible to describe the atom movements at the reactive surfaces [38
Still, the correspondence between thermodynamic evaluations and experimental electrochemical results can be questionable. In the case of MgH2
, the development of the electrochemical process in a potential range close to DFT estimation is certainly a good indication [17
]. Unfortunately, this is not the case of the other hydrides under consideration, for which the experimental reaction potential largely deviates from the computed one [22
]. Kinetics limitations are generally the origin of such differences [38
], and they can also subvert the order of the reactions in a multistep process, thus modifying the supposed path.
As a consequence, other experimental techniques must be combined with electrochemical tests and DFT in order to univocally describe complex processes like displacement reactions [30
]. The most useful is certainly XRD, either ex situ or in situ, with the only limitation that can be applied only to crystalline materials. Therefore, wherever possible and valuable, a comprehensive study has been carried on in order to precisely identify the reaction mechanism.
In the frame of this project, we carried out detailed multi-techniques studies for MgH2
] and for alanates [22
] aimed at identifying and decouple the different reaction mechanism upon the electrochemical incorporation and de-incorporation of lithium.
The combination of ex situ X-ray powder diffraction, transmission electron microscopy and DFT calculation proved that MgH2
converts to give Mg and LiH without the formation of any stable intermediate bulk phase like LiMgH3
, but with the possible occurrence of a metastable Lix
solid solution. In fact, upon lithium incorporation into the electrode, it was possible to observe that the MgH2
diffraction peaks’ position slightly shift, indicating a certain lattice contraction. Alloying of Li in hcp Mg can take place before the end of conversion, while at a deep state of discharge, there is the formation and lithium enrichment of a bcc Li-Mg solid solution. Our results were in excellent agreement with the available literature [13
The same set of techniques was exploited to study the lithium and sodium alanates’ conversion process [22
, potential profiles collected upon electrochemical lithium incorporation exhibited at least three plateaus, namely at 0.78, 0.27 and 0.15 V vs. Li. DFT predictions summarized in Table 4
, confirmed that, from a thermodynamic point of view, the HCR process occurs in a three-step path involving the formation of Li3
at 0.86 V vs. Li and its following conversion to LiH and Al at 0.74 V, while the final process could be Li alloying into Al.
The simple qualitative comparison between the calculated and experimental reaction potentials suggests that large overpotentials occurred, and therefore, the final confirmation of the true HCR process could be only provided by ex situ XRD on discharged electrodes. This analysis highlighted the formation of Li3AlH6 as the reaction intermediate and the final Li-Al alloying. Moreover, ex situ XRD further suggested the irreversibility of the HCR process and the inability to convert back to LiAlH4 from Li3AlH6.
Very detailed information about the sodium alanates family has been obtained by joining in situ X-ray powder diffraction and solid state NMR with electrochemical and computational studies [22
]. Several possible lithium incorporation redox reactions have been modeled by DFT, including the formation of LiNa2
and NaH before the complete reduction to LiH and metallic Na as reported in Table 4
. Calculations suggested that the most convenient thermodynamic path proceeds at least in four steps, with the first formation of LiNa2
, its conversion to Na3
, the reduction to NaH and its final displacement by Li to produce Na and LiH. At the same time, DFT evidenced how other reactions could occur at the same time as the effect of overvoltages. In particular, NaAlH4
could convert directly to Na3
or even to NaH. As a matter of fact, the four steps figured by calculation did not fit with the three steps highlighted in the experimental discharge potential profile, where reactions developed at much lower potential than expected (i.e., 0.41, 0.26 and 0.17 V vs. Li). The in situ XRD analyses allowed observing in real time the evolution of the involved phases and determining the effective conversion path. The discharge of electrodes based on NaAlH4
was observed separately. It was verified that NaAlH4
reduction to LiNa2
is competitive. LiNa2
firstly formed at higher potential, but Na3
appeared before NaAlH4
is fully converted. In situ XRD studies of the hexahydrides reduction in lithium cells demonstrated that there is no interconversion between one and the other, but both of them are reduced directly to metallic Na and LiH. Therefore, Na3
is a direct product of NaAlH4
reduction. NaH intermediate is never observed. At the discharge cutoff potential, NaAlH4
peaks had completely disappeared; Na and Al peaks were intense, but the process was not complete because hexahydrides conversion was not completed. Oxidation was also followed in situ, and it was surprisingly found that NaAlH4
was re-formed back. This result is remarkable in consideration that, as mentioned above, once metallic sodium is formed, its stripping should be thermodynamically more convenient than its reaction to re-produce NaH or any alanate.
Additional confirmations were obtained by solid state NMR experiments performed ex situ on electrodes at different states of charge [39
NMR analyses were carried out on both a pristine alanate electrode and on a carbon-milled NaAlH4
one. The above-mentioned conversion intermediates were all identified, but particularly interesting were the spectra of the recharged electrodes. The composite electrode, with its 70% of recharge efficiency, was actually composed of a mixture of alanates, with a dominance of NaAlH4
. The bare alanate-based electrode, with a recharge efficiency of only 30%, still contained a huge amount of metallic sodium in its recharged state. Clearly, the Na stripping suffers from very high overpotentials that allow sodium to participate in hydride reformation.
3.3. Materials Optimization Trials
At least two hydrides have been identified as valuable candidates for being applied as the negative electrode in lithium ion batteries, yet their poor cycle life is an important obstacle. The conversion processes entail massive structural reorganization and volumetric changes (for instance, 72% for NaAlH4
). These changes can lead to particle isolation and cracking as a result of electrode grinding and to a subsequent fading of the capacity after a few cycles. Attempts to address this issue through electrode engineering have been made for conversion reaction materials, using a number of strategies, such as forming nanocomposites with nanoporous carbon [12
Confinement strategies are commonly used in the design of hydrogen storage materials, like metal hydrides [40
]. Besides preserving conductivity, the presence of carbon is beneficial towards preventing grain growth and sintering by limiting the large volumetric changes encountered during lithium incorporation/de-incorporation [9
]. Along these lines, in order to improve the performances of hydrides in electrochemical cells, the possibility to confine MgH2
particles in mesoporous or nanometric carbon host matrices was investigated. Namely, nanocomposites based on (a) MgH2
on SuperP carbon and (b) NaAlH4
and a pyrolyzed resorcinol-formaldehyde carbon aerogel (CA) were prepared. All of the nanoconfined samples have been produced and manipulated in air-safe Ar-filled glove boxes to avoid moisture contact and degradation.
The nanocomposite Mg-carbon sample has been obtained by impregnating a 1 M solution of di-N-butyl-magnesium in heptane with Super-P carbon nanoparticles (SP-C) to a final nominal Mg:C molar ratio of 1:4. The solvent has been evaporated gently at room temperature. The impregnated powder has been transferred into quartz liner and sealed in a stainless-steel reactor. The reactor has been heated to 400 °C for three hours, and then, the final Mg@SP-C composite sample recuperated and stored in the glove box. The TEM micrographs of the Mg@SP-C composite material is shown in Figure 5
compared to the pristine SP sample.
The magnesium nanoparticles 2–5 nm in size nicely decorate the surface of the SP-C round-shaped nanoparticles (20–30 nm in size). The decoration is apparently homogeneous throughout the entire sample.
The hydrogenation of the Mg@SP-C sample to give the Mg@SP-C@H2 has been obtained in a Sievert apparatus at 120 °C under 20 bars of H2 for 24 h.
The XRD diffraction pattern is shown in the Figure 6
Apparently, the transformation of Mg to MgH2 is not complete, but longer hydrogenation steps only marginally improve the phase transition. Minor traces of MgO contamination were also detected.
This composite material shows very promising performance in lithium cells in terms of reversible specific capacity and cycling ability in galvanostatic tests, as reported in Figure 7
The final specific capacities are comparable with analogue performances demonstrated in the literature by Latroche and co-workers [42
]. In passing, it should be mentioned that the possible contribution of the SP-C carbon to the overall capacity is below 100 mAh·g−1
Turning to nanoconfined sodium alanate, a solvent-assisted infiltration method was chosen to highly disperse the hydride, as well as facilitate a close contact with the carbon material [43
]. The synthesized sample was a nanocomposite material with a homogeneous morphology consisting of alanate particles permeated into the carbon matrix [43
]. Nevertheless, the resulting nanoconfined material was highly reactive and therefore complex to handle: it easily burns upon exposure to air and starts to desorb hydrogen already below 100 °C. Furthermore, despite elemental analysis performed on carbon revealing just 1.75 wt% of residual oxygen, the chemical interaction between alanate and carbon is large. Both FTIR and solid state NMR analyses revealed rather an alanate oxidation, and thermal analysis demonstrated that only 54% of the total infiltrated hydride was preserved after confinement. Despite these issues, such a nanocomposite demonstrated improved cyclability in a lithium cell, being capable at the 10th discharge to deliver 37% of the capacity provided in the first cycle.
Apparently, confinement of both MgH2 and NaAlH4 is effective at reducing electrode pulverization upon cycling due to the huge volume variations occurring upon HCR.
Despite infiltration into a carbon matrix appearing to be a promising method to exploit the hydrides’ electrochemical potentialities, the optimization of the infiltration methods is necessary in order to maximize the magnesium hydrogenation and avoid the decomposition of the alanate during the synthesis. Further improvements might be obtained by optimizing the solvent-assisted infiltration method, for instance by improving the solvent purity, increasing carbon porosity and grinding under a reducing hydrogen atmosphere (by ball milling in closed anaerobic vials). Furthermore, the use of alternative melt infiltration techniques should be explored.
3.4. Electrochemical Cell Formulation Assessment
Another important aspect to consider in order to improve the electrochemical performances of hydrides regards their chemical reactivity towards the common solvents used in non-aqueous electrolytes.
The effect on the electrochemical performances of the use of a variety of electrolytes was tested with respect to MgH2
-based electrodes (MgH2
_B15D5 sample) in lithium cells. We screened (i) three liquid electrolyte systems, i.e., ethylene carbonate:dimethyl carbonate 1:1 v/v LiPF6
1 molal (LP30), ethylene carbonate:dimethyl carbonate 1:1 v/v Li triflate 1 molal (EC:DMC LiTf), 1,3-dioxolane:dimethoxyethane 1:1 v/v Li triflate 1 molal (DOL:DME LiTf), (ii) an ionic liquid added electrolyte, i.e., LP30:N-n-butyl-N-methylpyrrolidinium hexafluorophosphate ([Py14
) 7:3, and (iii) a polymer-based electrolyte constituted by LiTFSI dissolved in a polyethylene-oxide (MW 100,000) mixed with 5% of SiO2
LiTFSI). The electrochemical responses are shown in Figure 8
for liquid electrolytes, in Figure 9
for the ionic liquid and in Figure 10
for the polymeric system.
In liquid-based electrolytes (see Figure 8
), we observed either an effect of the salt and of the solvents on the electrochemical response. In particular, the use of ether-based solvents improves the starting capacity, but does not improve the cell efficiency or cycling ability, whereas the use of the triflate anion in carbonate-based electrolyte instead of hexafluorophosphate causes a significant increase of cell efficiency, thus leading to a residual reversible capacity of 200 mAh·g−1
after 10 cycles.
Ionic liquids have attracted much attention as electrolyte systems thanks to their high ionic conductivity, low toxicity, as well as their high thermal, chemical and electrochemical stability. By substituting 30% of LP30 with [Py14]PF6, improvements in the charge efficiency of MgH2
were obtained (see Figure 9
Analogously, the use of a PEO-based electrolyte apparently improves the electrode electrochemical response, thus boosting the reversible capacity in the first cycle to 933 mAh·g−1 and the cycle life.
In summary, a remarkable chance of the electrochemical responses has been observed in magnesium hydride-based electrodes by altering the electrolyte composition. Our evidence, although very limited, may suggest that the electrode/electrolyte interface is very far from being fully stable. Obviously, this analysis is only a first attempt, and much extended efforts must be accomplished in order to clarify the fundamental chemistry of the interfaces and optimize the electrochemical response of these complex electrode systems. However, this is beyond the scope of our project and of this general review.
3.5. Li-Ion-Hydride Proof of Concept
As the ultimate goal, the final grand target of this project has been to prove for the first time in the literature the concept of a lithium ion-hydride cell, i.e., a complete Li-ion cell where a hydride-conversion reaction (HCR) anode was coupled with a lithium insertion cathode material. A lab-scale prototype has been assembled using MgH2
_B15D5 as the negative electrode, commercial LiFePO4
(Custom Cell) as the positive electrode and LP30 as the electrolyte. The Li-ion cell has been assembled using a 2032 coin cell with a total capacity of ≈2 mAh. The mass ratio between the negative and positive electrodes has been optimized to N/P = 1:11, thus resulting in a balanced Li-ion configuration. An electrolyte volume of 25 μL·cm−2
has been used. The cell has been cycled with a current rate of 0.11 mA·cm−2
in the voltage range 3.6–0.5 V with a capacity limitation of 2000 mAh·g−1
with respect to the negative electrode mass. The voltage profile and the capacities upon cycling are shown in the Figure 11
The lithium ion cell suffers from the capacity fading intrinsic to MgH2 cycling. Nevertheless, the cell shows a very interesting voltage profile, with a flat charge process at 3 V, and discharge at 2.8 V. High average voltage means high energy, and low voltage hysteresis grants high energy efficiency. With such features, we strongly believe that the proposed lithium ion-hydride cell deserves much attention and will focus further the scientific community onto the optimization of hydrides for electrochemical energy storage.