# Electrochemical and Chemical Reactivities of Titanium Oxide-Based Materials with a Chloroaluminate Ionic Liquid Electrolyte for Aluminum Batteries

Maxime Sorriaux, Seongkoo Kang, Kyle G Reeves, Ana Gabriela Porras

Gutierrez, Sandrine Leclerc, Vincent Sarou-Kanian, Franck Fayon, Christophe

Legein, Monique Body, Gwenaël Gouadec, et al.

► To cite this version:

Maxime Sorriaux, Seongkoo Kang, Kyle G Reeves, Ana Gabriela Porras Gutierrez, Sandrine Leclerc, et al.. Electrochemical and Chemical Reactivities of Titanium Oxide-Based Materials with a Chloroaluminate Ionic Liquid Electrolyte for Aluminum Batteries. ACS Applied Energy Materials, 2024, 7 (21), pp.9852-9862. 10.1021/acsaem.4c01840. http://onlinelibrary.wiley.com/doi/abs/10.1021/acsaem.4c01840

HAL Id: hal-04882226

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Distributed under a Creative Commons Attribution 4.0 International License
Electrochemical and chemical reactivities of
titanium oxide-based materials with chloroaluminate
ionic liquid electrolyte for aluminium batteries

Maxime Sorriaux,<sup>a,b</sup> Seongkoo Kang,<sup>a,b</sup> Kyle G. Reeves,<sup>a,b</sup> Ana Gabriela Porras Gutierrez,<sup>a,b</sup>

Sandrine Leclerc,<sup>a,b</sup> Vincent Sarou-Kanian,<sup>c</sup> Franck Fayon,<sup>c</sup> Christophe Legein,<sup>d</sup> Monique

Body,<sup>d</sup> Gwenaël Gouadec,<sup>e</sup> Olaf J. Borkiewicz,<sup>f</sup> Damien Dambournet<sup>a,b*</sup>

<sup>a</sup> Sorbonne Université, CNRS, Physicochimie des Electrolytes et Nanosystèmes Interfaciaux,
UMR CNRS 8234, 75005 Paris, France

<sup>b</sup> Réseau sur le Stockage Electrochimique de l’Energie, RS2E, FR CNRS 3459, 80039 Amiens
Cedex, France

<sup>c</sup> CNRS, UPR3079 CEMHTI, 1D avenue de la Recherche Scientifique, 45071 Orléans Cedex 2,
France

<sup>d</sup> Institut des Molécules et Matériaux du Mans, UMR 6283 CNRS, Le Mans Université, 72085
Le Mans, France

<sup>e</sup> Sorbonne Université, CNRS, de la Molécule aux Nano-objets : Réactivité, Interactions et Spectroscopies, MONARIS, UMR 8233, 75005 Paris, France

<sup>f</sup> X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne,
Illinois 60439, United States
**ABSTRACT.** By selecting three different types of electrode materials, we intended to better understand the Al³⁺ intercalation chemistry of titanium oxide-based frameworks with acidic chloroaluminate electrolyte. In agreement with previous reports, we confirmed that the native interstitial sites of anatase TiO₂ are less prone to accommodate Al³⁺ than Li⁺ or Na⁺ ions while introducing cationic vacancies largely increases the electrochemical storage capacity. Upon the first cycle, the highest reversible capacity, up to 277 mAh/g, was obtained for a hydrated layered structure featuring cationic vacancies. Total scattering data showed that the insertion of Al³⁺ ions induced a strong distortion of the framework. In addition, a combined ²⁷Al MAS NMR and DFT-calculations revealed that in oxy-hydroxylated vacant sites, the coordination mode of Al³⁺ ions depends on the arrangement of anions around vacancies inducing the occurrence of 4-, 5 and 6-fold coordination modes. Further cycling experiments revealed a progressive capacity fading for all electrode materials. Using cyclic voltammetry on the used electrolyte, we evidenced that a partial dissolution has occurred, which is more pronounced for the layered hydrate compound, and that solubilized species are electrochemically active, giving rise to specific signatures in both CVs and galvanostatic experiments. Raman spectroscopy enabled to characterize these species which derived from the Ti-Cl system. The solubilized species, however, eventually precipitated as shown by a purple deposit observed on the separator and tentatively assigned to TiCl₃, known to be insoluble in this medium. By providing further information on the Al³⁺ intercalation chemistry and a better understanding of the electrochemical and chemical reactivities of electrode materials, this work will enable progress to be made in the development of aluminium-ion batteries.

**Keywords.** Al³⁺-ion insertion chemistry, defects, anionic environments, dissolution, shuttle effect.
INTRODUCTION

Aluminium batteries could provide safe and sustainable high energy-density electrochemical energy storage devices.¹ Indeed, while lithium-ion batteries are among the state-of-the-art energy-storage systems developed to this date, the low availability of lithium resources raises concerns considering the evergrowing demand for energy storage solutions following the current mass-electrification driven by the automotive sector. The ongoing consensus is set on the development of complementary systems that do not use lithium as a charge carrier. Among these, aluminium appears as an interesting candidate.²,³ This is motivated by aluminium elemental availability, its developed recycling chemistry, and its polyvalent character. This high charge allows to exploit a three-electron exchange reaction, and provides aluminium a volumetric specific higher than that of lithium: about 8052 Ah/L for Al, against 2062 Ah/L for Li. However, the electrochemical intercalation of high-charge Al³⁺ cations implies stronger interactions with the anionic sublattice, as compared to Li⁺ ions.⁴ These coulombic interactions are evaluated by the polarizing power (Z/r²): 10.68 e/Å² for Al³⁺, against 1.73 e/Å² for Li⁺. Consequently, aluminium ions meet higher activation energies than lithium, in order to diffuse inside a host framework. These aforementioned features represent major bottlenecks of finding viable intercalation compounds. Hence, the understanding of the Al³⁺ intercalation (electro)chemistry is for now effectively limited.

Inorganic compounds that can reversibly insert Al³⁺ cations include oxide and sulfides, with in most cases, capacities that are well below those obtained for Li⁺ ions. This indirectly translates to a different intercalation mechanism. In the case of the Chevrel phase Mo₆S₈, it was reported that the Al-intercalation proceeded in a different manner than that of monovalent and divalent cations because of the smaller cation size and stronger electrostatic repulsion of Al³⁺.⁵ Vanadium oxide
has been also investigated as potential cathode material for Al-batteries. 6-12 Gu et al, 9 showed that the intercalation of Al³⁺ induced an amorphization of the upper surface region, which can be tentatively explained by the strong polarizing power of Al³⁺ inducing local distortion. Concerning titanium sulfides, their intercalation properties depend on the polymorph considered with higher capacities found for the layered form as compared to the spinel structure. In the layered TiS₂, Al³⁺ ions occupied an octahedral coordinated empty sites located between the sulfite layers. 13 In addition, anion-rich titanium polysulfide compounds were shown to exhibit anionic redox upon Al³⁺ insertion. 14

As popular intercalation compounds, the insertion of Al³⁺ into titanium dioxide has also been investigated. 15-18 In the anatase polymorph, we showed that the insertion of Al³⁺ is thermodynamically more stable in cationic vacancies than in native interstitial sites, which suggested that implementing defects could be a viable way to discover multivalent-ion battery electrodes. 15 This approach has been further used in defective rutile polymorph (Ti₀.95□₀.05O₁.79Cl₀.08(OH)₀.13) for aqueous Al-ion electrode material. 19

The driving force to insert Al³⁺ ions into vacant sites has been shown to depend on the local environment of the vacancy.²⁰ Although, a fluoride-rich environment was shown to be thermodynamically more favourable, the formation of strongly covalent Al-F bonds induced strong local distortions, which prevents the insertion of a second Al³⁺ ion in a paired-titanium-vacancy, hence limiting the storage capacity.²⁰

In a previous work,²¹ we showed that by revisiting the sol-gel chemistry of titanium oxide, different compounds with various structural features could be prepared. In addition to pure anatase TiO₂, the adjustment of the solvothermal temperature can yield to cationic defective anatase stabilized by charge compensation involving OH groups only. This represents an
opportunity to go further in the understanding of the effect of the local environment of vacant
sites on their insertion properties. Another appealing structural feature for promoting multivalent
ion intercalation is structural disorder.²² In this view, this revisited sol-gel process can be also
used to prepare a disordered titanate material, consisting of a layered lepidocrocite network
stabilized by structural water layers and featuring up to 25 % of cationic vacancies. Such a
compound is able to host different charge carriers spanning from monovalent to divalent
ions.²¹,²³,²⁴ Hence, using a sol-gel method, we can screen the insertion properties of various
compounds exhibiting different structural features.

A literature survey on the use of chloro-aluminate-based electrolyte highlighted the necessity
to consider the chemical compatibility between the electrolyte and electrode materials. As an
example of electrolyte reactivity, when iron(III) chloride was used as an electrode material, the
cell self-discharged due to the dissolution of the active material and further transport to the
aluminium negative.²⁵ A similar degradation mechanism was observed for VCl₃, but the authors
showed that the presence of fluorobenzene as an electrolyte additive can improve the capacity
retention.²⁶ This showed that dissolution issues can be mitigated, provided that the degradation
mechanism and the nature of the dissolved species are investigated. For example, Wen et al.²⁷
showed that V₂O₅ decomposed in different species such as VOCl₃, VO₂Cl, or AlCl₃VO₃⁻.
Therefore, it is crucial to investigate the different aspects of Al³⁺ electrochemistry to enable an
overall understanding of such a complex battery system.

In this work, we employed three polymorphs of titanium oxide to investigate how the presence
of cationic vacancies affected the electrochemical intercalation of Al³⁺. Local structural analysis
with computational studies evidenced a favourable electrochemical insertion of Al³⁺ around Ti
vacancies stabilized by hydroxyl groups, due to the formation of thermodynamically stable local
coordination modes of intercalated Al³⁺ around cationic vacancies. Furthermore, the long-term
cycling of titanium oxide cathode showed low structural stability due to the dissolution of Ti
species. This study provides an overall view point of the Al³⁺ electrochemistry of positive
electrode materials based on titanium oxide frameworks.
METHODS

**Synthesis.** The synthesis of the different titanium oxide-based materials followed the protocol previously published.²¹ Briefly, for each material, 0.81 mL of ultrapure water was added to a solution of 25.19 mL isopropanol (≥99.5%, Sigma-Aldrich) and 4 mL titanium (IV) isopropoxide (≥97%, Sigma-Aldrich) under stirring in a 45 mL Teflon-lined autoclave. The mixtures were sealed and heated at 90°C, 110°C, or 150°C for 12 h. The precipitates were washed several times with ethanol and separated by centrifugation. The samples were finally dried at 80 °C and ground.

**Electrochemical characterizations.** Galvanostatic discharge-charge cycling. Measurements were performed on a Bio-Logic VMP3 potentiostat, at 20 mA/g, in the voltage range 0.00 – 1.80 V vs Al³⁺/Al, at 30°C, in a 3-electrode Swagelok cell. Al metal foil (99.99%, Alfa Aesar) and Al metal wire (≥ 99.99%, Sigma-Aldrich) were used as the reference and counter electrode, respectively. The working electrode was prepared on Mo foil (Goodfellow) current collector by drop-coating the mixture of 80 wt% active material, 10 wt% conductive carbon (Super P, Timcal) and 10 wt% polytetrafluoroethylene (60 wt% dispersion in water, Sigma-Aldrich) suspended in ultrapure water. Typical mass loading of active material was 1-2 mg per cm². The electrolyte EMImCl-AlCl₃ (1:1.5 compostion) was prepared in an argon-filled glove box, by mixing 1-Ethyl-3-methylimidazolium chloride EMImCl (Solvionic) and AlCl₃ (99.99%, Anhydrous, Sigma-Aldrich). A volume of 100µL was added to perform electrochemical measurements. Borosilicate glass-fibre filter paper (Whatman grade GF/D) was used as a separator.

*Cyclic voltammetry.* Measurements were performed on a Bio-Logic BP-300 potentiostat, at a 50 mV/s scan rate, in the voltage range 0.00 – 1.80 V vs Al³⁺/Al, inside a glovebox, in an
electrochemical 3-electrode glass cell. Inside the cell, 2 mL of the 1:1.5 EMImCl-AlCl₃ electrolyte was introduced, paired with both Al metal wire reference and counter electrodes, and glassy carbon as the working electrode.

*Post-electrochemical treatments.* After performing the different electrochemical measurements, the used working electrodes were extracted from the electrochemical cells, inside a glovebox. To realise further analyses, these used electrodes were then washed by immersion in DMC (≥ 99 %, Anhydrous, Sigma-Aldrich), for one or two days.

**Pair Distribution Function (PDF) analysis.** High-energy X-ray data were collected at the 11-ID-B station at the Advanced Photon Source (Argonne National Laboratory). After corrections (background and Compton scattering), PDFs were extracted from the data using PDFgetX2 software.²⁸

**²⁷Al MAS NMR spectroscopy.** ²⁷Al ($I = 5/2$) MAS spectra were acquired at RT on a Bruker Avance NEO 850 MHz spectrometer (20.0 T, ²⁷Al Larmor frequency of 221.5 MHz) using a 1.3 mm ultrafast MAS probe. Quantitative ²⁷Al one-dimensional (1D) MAS spectra were recorded with an excitation pulse length of 0.55 µs, *i.e.*, a flip angle of $π/12$ (radio-frequency field $v_{RF} = 75$ kHz).²⁹ 20,000 up to 30,000 accumulations were recorded with a recycle delay of 0.5 s. All ²⁷Al spectra are referenced with respect to a 1 M solution of Al(NO₃)₃. Spectra were simulated using the DMFit software with the implemented Gaussian isotropic model.³⁰

**Raman Spectroscopy.** The Raman spectra collection was performed using a Bruker RFS100/S Raman FT-spectrometer (Bruker optics), coupled by optical fibre on a Bruker RamanScopeIII microscope and using a Leitz x40 LWD/IR objective. The Raman signal is analysed by interferometry, recorded with a 77K-cooled Ge detector, and the Rayleigh scattering is eliminated by a Notch filter. The excitation beam is produced by a near-infrared 1064nm
Nd:YAG laser (Coherent Compass 1064-500N, 500mW maximum power), and set so that the samples receive an effective laser power of 110mW. The samples were prepared in an argon-filled glovebox, and sealed in septum-capped Pyrex tubes. The spectra were obtained by combining 30000 scans, over the 50 cm⁻¹ to 3500 cm⁻¹ range. Notably, in this range, the studied samples display a fluorescence pattern. This fluorescence was first minimized by the near-infrared laser excitation used, and the remaining signal is removed by subtracting a baseline from the recorded spectra.

**Computational methods.** A layered lepidocrocite structure was computed using a simulation cell, as previously reported,²¹,²³,²⁴ of two titanate layers (4x4x1 supercell) and two layers of interlayer water with an overall composition of Ti₆₃□₁O₁₂₄(OH)₄·32H₂O where the □ represents a single Ti⁴⁺ vacancy whose charge is compensated by the four hydroxides. The relative positions of these four hydroxide ions can vary amongst the six possible positions of the oxygens that were previously coordinated to the now missing Ti⁴⁺. In total, this represents fifteen possible arrangements. To understand the impact of the distribution of these OH groups, on the insertion energy of Al³⁺ ion within the vacant site, all sixteen structures were explicitly computed. Systems were computed with a total neutral charge to simulate the state of the electrode following a discharge process. To determine the energy of formation of Al³⁺ ion within the cationic site, the structure was relaxed and the total energy of the system is then recomputed for this arrangement of atoms but without the intercalated ion. The energy of formation (eq.1) is computed as

$$E_f = E[X + TiO_2] - (E[X] + E[TiO_2]) \quad (\text{eq.1})$$

where the first term represents the combined system and the terms in parentheses represent the sum of energies of the ion X and titanate systems computed separately. Density functional theory
(DFT) calculations were performed using the CP2K code via the Quickstep algorithm.<sup>31,32</sup>
Electrons were described using the PBE exchange-correlation function with a plane-wave cut-off
of 400 Rydberg. DZVP-MOLOPT-SR-GTH basis sets were used along with Goedecker-Teter-
Hutter pseudopotentials.<sup>33,34</sup>

## RESULTS AND DISCUSSION

**Selection of electrode materials.** In this work, different positive electrode materials (Figure 1)
were selected to scan a wide array of insertion sites for Al³⁺ ions, aiming to better understand
their intercalation chemistry. Using sol-gel chemistry, we previously demonstrated that the
synthesis temperature influences the dimensionality of titanium oxide-based structure, ranging
from pure anatase TiO₂ to defective anatase, and a layered structure featuring crystal water
molecules and inorganic slabs with cationic vacancies.²¹ The allotropic form anatase TiO₂
(hereafter denoted A-TiO₂) exhibits interstitial sites capable of accommodating a large content of
Li⁺ or Na⁺ ions.³⁵⁻³⁷ These sites, however, are less prone to accommodate multivalent ions such
as Mg²⁺ and Al³⁺. Instead, cationic vacancies stabilized by monovalent anions are
thermodynamically more favourable insertion sites.¹⁵ Therefore, the second selected compound
is defective anatase (hereafter denoted dA-TiO₂), featuring titanium vacancies stabilized by
monovalent hydroxide groups, i.e., Ti₁₋ₓ□ₓO₂₋₄x(OH)₄x. Finally, the third compound exhibits a
layered structure derived from the lepidocrocite framework (hereafter denoted L-TiO₂),
consisting of defective inorganic slabs separated by layers of water molecule. The general
chemical composition of this compound is Ti₂₋ₓ□ₓO₄₋₄x(OH)₄x·nH₂O. Similarly to defective
TiO₂, the structure includes hydroxide groups, resulting in a significant number of vacancies in
the inorganic layers. X-ray diffraction analysis confirmed that anatase compounds (A-TiO₂ and
dA-TiO₂) crystallized in the *I*4₁/*amd* tetragonal space group, while the XRD pattern of the lepidocrocite layered structure presents diffuse scattering characteristic of amorphous compound (Figure S1). Thermal analyses (Figure S2) established the chemical composition as Ti₀.₈₃□₀.₁₇O₁.₃₂(OH)₀.₆₈ for dA-TiO₂ and Ti₁.₅□₀.₅O₂(OH)₂·0.55H₂O for L-TiO₂, highlighting a large content of cationic vacancies.²¹

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Structural models of the various compounds (A-TiO₂, dA-TiO₂, and L-TiO₂) showing their respective possible host sites for Al³⁺ ion insertion.</figcaption></figure>

**First electrochemical discharge-charge profiles.** The initial galvanostatic discharge-charge profiles of the three positive electrode materials are presented in **Figure 2**. Since the titanate electrodes are used as the positive electrode here, reduction occurs during the discharge phase of the cycle, corresponding to the Al³⁺ intercalation into the titanate materials. Considering that this intercalation occurs via a three-electron exchange reaction, the amount of reversibly intercalated aluminium can be determined. We used the specific charge capacity as the first discharge, slightly higher than the charge one, might include capacity originating from electrolyte decomposition and/or solid-electrolyte-interface formation. A-TiO₂ exhibits the lowest electrochemical properties, with a charge capacity of 59 mAh/g, corresponding to 0.06 Al³⁺ ion
per formula unit (theoretical capacity capacity: 335 mAh/g based on $Ti^{4+}/Ti^{3+}$ redox couple).

This low content is attributed to the unfavourable insertion of $Al^{3+}$ within the interstitial sites.¹⁵

The introduction of the defective sites within the anatase structure of the $dA-TiO_2$ positive electrode material increased the charge capacity to 152 mAh/g, corresponding to 0.165 $Al^{3+}$ ion per formula unit (theoretical capacity capacity: 307 mAh/g based on $Ti^{4+}/Ti^{3+}$ redox couple).

This value is close to the content of vacancies in the sample. Additionally, it is higher than the capacity obtained using hydroxyfluorinated anatase $Ti_{0.78}□_{0.22}O_{1.12}F_{0.40}(OH)_{0.48}$, which has charge capacity of 92 mAh/g. In this compound, the formation of covalent Al-F bonds induces a strong local distortion of the network, preventing further $Al^{3+}$ ion insertion.²⁰ Hence, it can be postulated that the insertion of two $Al^{3+}$ ions into paired-vacancy sites is more favourable in a hydroxylated environments than in hydroxyfluorinated one. Lastly, the layered structure of the $L-TiO_2$ material displays the highest charge capacity, up to 277 mAh/g, corresponding to 0.52 $Al^{3+}$ per formula unit (theoretical capacity capacity: 270 mAh/g based on $Ti^{4+}/Ti^{3+}$ redox couple).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** First galvanostatic discharge-charge profiles of the $A-TiO_2$ (yellow), $dA-TiO_2$ (green), and $L-TiO_2$ (blue) electrode materials at 20 mA/g.</figcaption></figure>
**Structural features of Al³⁺ ion intercalation in L-TiO₂.** The high reversible capacity obtained with L-TiO₂ motivated us to investigate the effect of Al³⁺ ion intercalation on its structural features. Due to the disordered nature of the pristine compound, high-energy X-ray total scattering experiments were conducted on a series of samples including the pristine, discharged and charged electrodes. Fourier transform of the total structure functions S(Q) was used to yield the pair distribution functions (PDF)³⁸ of the three samples. As a histogram of all atom-atom distances within the sample, the peak position reflects atomic connectivity. The range of interatomic distances r indicates the ordering length (scattering coherence domain), which measures approximately 1 nm (not shown) for the pristine compound, consistent with its amorphous nature. **Figure 3** gathers the PDF data obtained for the pristine, discharged and charged electrodes. Note that we observed a significant difference in terms of peak intensity specifically for the pristine electrode. For all the samples, the main peaks characteristic of Ti-O, edge-shared Ti···Ti, and corner-shared Ti···Ti bond distances are present. With the insertion of Al³⁺, these three peaks are both enhanced and broadened. For the first peak, the increase of intensity can be due to the presence of an additional of Al³⁺ ions. Indeed, this peak is centred at 1.88 Å, which is typical of bond length expected for Al-O(H). In addition, we observed a shoulder at around 2.1 Å, which can be ascribed to reduced Ti ions. Upon charged, the intensity of the first peak decreases and the shoulder is less visible demonstrating that it is related to the insertion of Al³⁺ ions. At higher r region, the broad peak assigned to O(H)···O(H) distances became sharper. Such a feature has been previously observed in oxy-hydroxyfluorinated anatase featuring vacancies²⁰, and can be related to a combined effect of more regular Ti octahedra and the polarizing power of Al³⁺ ions. Upon release of Al³⁺, these characteristic peaks diminish and narrow again. However, the initial atomic arrangement is not fully restored, indicating that some intercalated
aluminium remains inside the host structure, maintaining structural distortion due to the
polarizing power of Al³⁺ ions.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Pair distribution function data obtained from the high-energy X-ray total scattering measurements of L-TiO₂ material electrode samples at different states of charge.</figcaption></figure>

Owing to its sensitivity to the local atomic-scale structure, high magnetic-field (20 T) solid-state ²⁷Al MAS NMR was used to probe the environments of the aluminium atoms to obtain deeper insights into their intercalation mechanism. Indeed, an important feature of ²⁷Al solid-state NMR is the dependence of the isotropic chemical shift on local Al coordination through a large range of isotropic chemical shift: AlO₄ (~80 to 45 ppm), AlO₅ (~52 to 14 ppm), and AlO₆ (~20 to -20 ppm).³⁹,⁴⁰ Such broad range of chemical shift makes ²⁷Al NMR particularly useful to investigate Al³⁺ ion insertion mechanisms occurring in different types of materials.²⁰,⁴¹ In addition to L-TiO₂, we considered dA-TiO₂ as both compounds feature vacant sites that are oxyhydroxylated. A-TiO₂ was not considered here because of a low discharge capacity and expected single environment in interstitial sites. The one-dimensional ²⁷Al MAS NMR spectra of the central transitions of the discharged L-TiO₂ and dA-TiO₂ electrodes (Figure 4) are comparable for both materials, suggesting a similar distribution of local anionic environments for intercalated Al³⁺ ions. They present three main overlapping broad resonances whose line shapes are
characteristic of disordered Al sites. Their reconstruction was achieved with six resonances, considering distributions of isotropic chemical shifts ($\delta_{iso}$) and nuclear quadrupole coupling constant ($C_Q$) values, assigned to tetra-, penta-, and hexacoordinated aluminium ($^{[n]}$Al with n=4, 5, or 6, **Table S1**). This suggests that Al-intercalation sites exhibit a broad range of local anionic environments, including large radial and angular distortions, which we ascribe to substitutional disorder at both the titanium site (mixed occupancy by $Ti^{4+}$, $Ti^{3+}$, $Al^{3+}$ and vacancies) and the anionic site (mixed occupancy by $O^{2-}$ and $OH^{-}$), and possible additional $Al^{3+}$ ion occupation of interstitial sites. The relative intensities of the $^{[n]}$Al resonances indicate, as previously observed in hydroxyfluorinated defective anatase, the preferential formation of $^{[6]}$Al (~40%), versus $^{[5]}$Al and $^{[4]}$Al.$^{20}$ The reconstruction of the spectrum of the central transitions of the discharged $dA-TiO_2$ electrode also includes two contributions at ~100 & 102 ppm, assigned to $AlCl_4^-$ and $Al_2Cl_7^-$ coming from remaining electrolyte.$^{42,43}$

<figure><img src="image_5.png" /></figure>
**Figure 4.** Experimental (blue) and fitted (red dashed) one-dimensional $^{27}$Al MAS (60 kHz) NMR (20 T) spectra of the central transitions of discharged L-TiO$_2$ (bottom) and dA-TiO$_2$ (up) material electrode samples. Details about the fits are given as Supporting Information (**Table S1**). The central-contributions assigned to six-, five- and four-fold coordinated Al (in blue, green and brown, respectively) and chloroaluminate species (in red) and the center band of external transition pattern (in black) are shown below.

To better understand the intercalation mechanism of Al$^{3+}$ ions within the vacancy sites of the titanate materials, DFT calculations were performed on a simplified model of the lepidocrocite structure (i.e. a supercell $Ti_{63}□_{1}O_{124}(OH)_{4}·32H_{2}O$ where the □ represents a single Ti$^{4+}$ vacancy whose charge is compensated by four hydroxides). The computed formation energies were found to largely depend on the starting anionic environment of the vacancy. The latter is stabilized by the substitution of four oxides by four hydroxides in proximity to the vacancy. There are fifteen possible combinations for distributing the four OH groups on the six oxygen sites. For each combination, the relaxed geometry and corresponding energy were computed via DFT calculations, and plotted as a function of the position of the aluminium ion within the vacancy. Results are presented in **Figure 5**. The position of the missing titanium atom was set to zero, and the Al-displacement was defined as the distance perpendicular to the titanate layer. A positive displacement represents the position of an ion that is closer to the interlayer, and a negative displacement represents a position that is closer to the centre of the titanate layer. The coordination number adopted by the Al$^{3+}$ ion was determined for each configuration. As depicted in **Figure 5**, the distribution of OH groups around the vacancy significantly influences the formation energy and the coordination number of the Al$^{3+}$ ion. The coordination modes observed range from 4-fold to 6-fold, consistent with experimental findings from NMR spectroscopy.
Moreover, the most stable configuration corresponds to a 6-fold coordinated Al³⁺ ion, which aligns with the dominant species observed in <sup>27</sup>Al NMR data. For each of the three coordination modes adopted, the most stable structure was computed (Figure S3). The coordination number was observed to increase with the mean Al³⁺-anion distance, being 1.82 Å for <sup>[4]</sup>Al, 1.88 Å for <sup>[5]</sup>Al and 1.92 Å for <sup>[6]</sup>Al. The influence of the metal-ligand distances has been established as a critical factor in determining the coordination number.<sup>44,45</sup> Notably, closer proximity of OH groups leads to higher coordination numbers.

<figure><img src="image_6.png" /></figure>

**Figure 5.** Total energy for each of the fifteen possible anionic environments generated by four protons on six possible oxygen sites (Inset). Each energy is plotted as a function of the displacement of the aluminium ion in the vacancy. Based on an Al-O cut-off radius of 2.0 Å, the coordination mode of each aluminium ion was identified as either 4-fold (violet), 5-fold (blue) or 6-fold (green).
**Electrochemical cycling properties.** After the first galvanostatic cycle, prolonged cycling experiments revealed a reduction in recorded capacities. **Figure 6** shows up to the tenth discharge-charge cycle for the three different materials. Both the L-TiO₂ and dA-TiO₂ materials experienced drastic capacity fades, reaching 97.1% and 83.6%, respectively after ten cycles. In contrast, despite initially lower capacity in the first cycle, A-TiO₂ exhibited much less capacity fade, reaching only 30.5% by the tenth cycle. Furthermore, as the capacity gradually decreases, the discharge-charge profiles evolve for both L-TiO₂ and dA-TiO₂ (**Figure S4**). Pseudo-plateaus appear at around 1.05V and 0.50V during discharge, and around 0.40V and 1.15V during the charge. The occurrence of these redox events contrasts with the expected smooth voltage profile for insertion reaction in a disordered structure, suggesting that both L-TiO₂ and dA-TiO₂ electrodes experienced a degradation mechanism during the electrochemical treatment. Following the cycling experiment, purple precipitates, assigned to titanium chlorides as discussed after, were observed deposited on the separators for both L-TiO₂ and dA-TiO₂ (**Figure S5**). The deposition of this precipitate appeared more pronounced on the counter electrode side and in the case of the L-TiO₂ material. To better understand these degradation mechanisms, the electrochemical response at the counter electrode was monitored during the first galvanostatic discharge-charge cycle for each of the three electrode materials (**Figure S6**). The two anatase material systems display small flat signals with a slight polarization of around 0.6 mV, albeit being noisy for the dA-TiO₂ material. From the L-TiO₂ system, an intense and widely noisy signal was recorded, with a high polarization of around 1.5 V. This feature indicates that the electrochemical cell using the L-TiO₂ material suffers from severe side reactions.

Overall, despite clear evidence that the structural features of the different compounds investigated here affect the intercalation properties of the Al³⁺ ions, we observed that an
additional reactivity with the chloroaluminate electrolyte is at stake. This reactivity appears much more pronounced towards the defective materials and leads, during the prolonged electrochemical treatment, to the four main observations: i) a major capacity drop, ii) an evolution of the discharge-charge profile, iii) reactivity at the counter electrode side, and iv) the precipitation of a purple compound.

Such degradation might either be induced by the electrochemical treatment performed or result from the chemical solubility of the electrode material in the electrolyte. Indeed, the dissolution of electrode material by the acidic chloroaluminate ionic liquid has already been reported, for other oxide-based electrode materials.²⁷
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Ten cycles of 20 mA/g galvanostatic discharge-charge cycling, were performed on the L-TiO₂ (blue), dA-TiO₂ (green), and A-TiO₂ (yellow) materials. Only the first (dark), fifth (medium), and tenth (light) cycles are displayed. The grey arrows indicate the experimental progression direction.</figcaption></figure>
**Electrode dissolution evidence.** To investigate the degradation mechanisms described above, electrochemical characterizations were conducted on the used electrolyte (Figure 7). Initially, ten cycles of the galvanostatic experiment were performed on a L-TiO₂ material electrode in a glass-type cell. Then, the titanate electrode was replaced with a glassy carbon electrode to record cyclic voltammograms on the remaining system. (Figure 7 inset). After the galvanostatic treatment, the voltammogram of the used electrolyte revealed two distinct coupled redox peaks at (A/A') E₁/₂ = 1.02 V and (B/B') E₁/₂ = 0.5 V, along with one reduction peak (C) at E = 0.25 V vs. Al³+/Al. These peaks were not present in the pristine electrolyte, suggesting the presence of new redox-active species originating from the degradation of the titanate electrode material.

Similar potential peaks have been observed in studies involving redox reactions of chloro- or chloroaluminate-titanium complexes within various chloroaluminate ionic liquids.<sup>46-48</sup> These peaks can thus be attributed to the successive redox couples Ti<sup>IV</sup>/Ti<sup>III</sup> (A), Ti<sup>III</sup>/Ti<sup>II</sup> (B), and the reduction of Ti<sup>II</sup> to Ti<sup>0</sup> (C). The lower intensity of the oxidation peak C' suggested a possible instability of Ti<sup>0</sup> in this medium. It was indeed reported that Ti<sup>0</sup> can decompose to form TiCl<sub>3</sub> depending on the scan rates and duration time of the experiment.<sup>48</sup> In addition, both the oxidation of Ti<sup>II</sup> (B') and the reduction of Ti<sup>IV</sup> (A) produce Ti<sup>III</sup>. This Ti<sup>III</sup> can further precipitates as red-purple TiCl<sub>3</sub>, as observed experimentally here on the separator.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Voltammograms of the 1:1.5 electrolyte before (dotted black) and after (solid blue) the galvanostatic cycling of a L-TiO₂ electrode cell. The displayed voltammogram is the 50th cycles of 50 mV/s CV scans performed using glassy carbon as the working electrode. The inset summarises the electrochemical treatments applied.</figcaption></figure>

To further investigate the degradation mechanism of L-TiO₂, we performed a time-dependent cyclic voltammetry using a glass-type electrochemical cell, with a large volume of electrolyte, and a glassy carbon at the working electrode. At first, a pristine electrode of L-TiO₂ was immersed at the bottom of the cell, without being connected to the scanning electrochemical circuit (Figure 8 Inset). Each hour, multiple cyclic voltammetry scans were performed, and the obtained results are summarized in Figure 8. Within the first day of the experiment, the measured signal was observed to increase. As no current flows through the L-TiO₂ electrode, this increase is attributed to the dissolution of titanium complexes.
The maximum intensity recorded was reached after 26 hours of experiments, coinciding with the transformation of the yellowish electrolyte into a vivid red colour. Subsequently, the intensity of the CVs gradually decreased, accompanied by the appearance of a purple precipitate at the bottom of the cell, similar to observations in a previous cycling experiment.

This purple precipitate was identified as $TiCl_3$, formed from dissolved $Ti^{IV}$ complexes and documented to be insoluble in the chloroaluminate ionic liquid.<sup>48-50</sup> Furthermore, cyclic voltammetric scanning treatments generated $TiCl_3$ from $Ti^0$ and $Ti^{II}$ species, contributing to the red coloration observed in the ionic liquid. The formation of $TiCl_3$ ultimately led to a depletion of redox-active species near the scanning electrode, resulting in the recorded intensity loss, akin to the fading of previous electrochemical properties.
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Cyclic voltammetries from the L-TiO₂ material dissolution monitoring experiment, performed at a 50 mV/s scan rate. Data collected on the first day of the experiment is displayed in the top figure, whereas data collected afterwards is displayed in the bottom figure. The arrows</figcaption></figure>
indicate the evolution in time of the monitored profiles. Insets: schemes of the experimental setup.

**Structural features affecting the compatibility with chloroaluminate ionic liquid electrolyte.**

To complement our study on the degradation mechanism, we investigated two additional sets of parameters that can influence reactivity with the chloroaluminate electrolyte.

- *Influence of the State-of-charge.* To evaluate how electrochemical aluminium intercalation-deintercalation (i.e. state-of-charge) affects the dissolution mechanism of the electrode material, the dissolution monitoring experiment was conducted using electrodes at different states of charge: pristine, discharged, and charged L-TiO₂ electrodes (Figure 9). Similarly to the experimental conditions used above, for each experiment the electrode was immersed at the bottom of the cell, without being connected to the scanning electrochemical circuit. Initially, the voltammograms of the three titanate materials showed similar profiles during the first cycles, featuring the three characteristic redox peaks identified previously. However, over the course of the experiment, they exhibited distinct evolution patterns.

After a day of experimentation, the voltammogram profile of the discharged electrode appeared flattened, while that of the charged electrode expanded. The pristine electrode maintained a higher intensity compared to the charged electrode. This discrepancy in intensity is believed to stem from some electrode material dissolution occurring during previous galvanostatic treatment, which did not contribute to the current signal measured here. In contrast, the signal observed from the discharged electrode showed no evidence of such dissolution. This suggests that the mechanism of electrode material dissolution primarily occurs when the electrode is in a charged state.
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Cyclic voltammograms recorded using a glass electrochemical cell for the pristine (black), discharged (red), and charged (blue) L-TiO₂ electrodes. Only the cycles of maximum intensity, obtained after a day of experiment, are displayed.</figcaption></figure>

- *Solubility tests.* To investigate the chemical stability of the different titanate materials and the acidic chloroaluminate ionic liquid, each compound was immersed within a large excess of 1:1.5 EMImCl:AlCl₃ electrolyte (at a 10 g/L concentration) and stirred inside a glovebox. After one week, L-TiO₂ was completely dissolved, whereas solid A-TiO₂ and dA-TiO₂ materials remained visible in their respective vials after one month. Subsequently, Raman spectroscopy was employed to characterize the dissolution products within the electrolyte. **Figure 10** shows the collected Raman spectra, along with the spectrum recorded for the pristine electrolyte. The dissolution of L-TiO₂ led to a significant decrease in the peaks located at 311 cm⁻¹ and 431 cm⁻¹, characteristic of the dimers Al₂Cl₇⁻ species. Concomitantly, new peaks appeared at 267 cm⁻¹, 384
cm⁻¹, and 500 cm⁻¹. According to the literature, the most intense peak at 384 cm⁻¹ can be attributed to titanium complex species derived from TiCl₄.⁴⁶,⁵¹⁻⁵³ This peak likely exhibits broadening due to the presence of other species within the vicinity of titanium.⁴⁸,⁴⁹,⁵⁴,⁵⁵ We noted that this peak is slightly present in the case of dA-TiO₂, confirming that this compound undergoes some reactivity with the electrolyte, consistent with observations from electrochemical characterizations. Therefore, the presence of OH groups seems to trigger the instability issue. Finally, we further confirmed the stability of TiCl₃ in the chloroaluminate medium (Figure 10 bottom).

To sum up, we have evidenced that a drastic solubilization followed by shuttle effect occurred during Al³⁺ insertion in L-TiO₂. Such effects have been previously observed in other types of electrode materials. Different strategies can be used to mitigate these effects such as the use of electrolyte additives²⁵. Another strategy proposed by Yu et al,⁵⁶ to mitigate dissolution and shuttle effects observed in the case of conversion materials consisted by constructing an electrochemical cell comprising double reaction zones. As a perspective, such approach could be used in order to not only to target practical aspects by also to fully understand the chemical and electrochemical reactivities of Ti-based oxide materials. Another approach could be to tune the anionic sublattice to introduce for instance, chloride anions as it would combine electrolyte stability and possibly structural variation, provide that adequate synthesis method can be developed.
<figure><img src="image_11.png" /><figcaption>**Figure 10. (top)** Raman spectra from the dissolution tests of L-TiO₂ (blue), dA-TiO₂ (green), A-TiO₂ (yellow) in the 1:1.5 EMImCl:AlCl₃ ionic liquid (black). **(bottom)** Raman spectra from the dissolution test of TiCl₃ in the 1:1.5 EMImCl:AlCl₃ ionic liquid (purple) against the pristine ionic liquid (black). Asterisks indicate the contribution caused by dissolved titanium species.</figcaption></figure>
CONCLUSIONS

In this work, we investigated the electrochemical properties of three titanium-based oxide compounds aiming to better understand the Al³⁺ intercalation chemistry using acidic chloroaluminate electrolyte. We confirmed that only a few number of Al³⁺ ions can be inserted within the native interstitial sites of anatase TiO₂ and that introducing cationic vacancies largely increases the capacity. Remarkably, the hydroxylated environments of vacancies seem to be more favourable for insertion than the previously reported hydroxyfluorinated sublattice. A combined ²⁷Al MAS NMR and DFT-calculations revealed that in oxy-hydroxylated vacant sites, the coordination mode of Al³⁺ ions depends on the arrangement of anions around vacancies inducing the occurrence of 4-, 5 and 6-fold coordination modes.

Upon cycling experiments, all electrode materials display progressive capacity fading. Using cyclic voltammetry on the used electrolyte, we evidenced that a partial dissolution has occurred, which is more pronounced for the layered hydrate compound, and that solubilized species are electrochemically active involving subsequent reduction of TiIV/TiIII, TiII/TiII, and the reduction of TiII to Ti⁰. However, the instability of Ti⁰ eventually lead to TiCl₃, known to be insoluble in this medium, and experimentally evidenced by the appearance of a purple precipitate in the cell.

This work shows that a careful examination of the data should be performed to interpret electrochemical measurements in reactive electrolyte. Design of adaptable frameworks based on heteroanionic compounds such as oxide-chloride could be an interesting avenue to discover new electrode materials for Al³⁺ ion batteries.
## ASSOCIATED CONTENT

Supporting Information: XRD patterns and thermal analyses of the three samples; details about the fits of the $^{27}$Al NMR spectra; snapshots of DFT-calculations; 5$^{th}$ cycle of the 20 mA/g galvanostatic cycling of the three titanate electrode materials; photograph of a used separator, after 10 galavanostatic cycles performed using a L-TiO$_2$ material electrode; voltage profiles of the counter electrode during the first cycle of the 20 mA/g galvanostatic discharge-charge cycling experiment, for the three samples. The following files are available free of charge.

## AUTHOR INFORMATION

### Corresponding Author

* damien.dambournet@sorbonne-universite.fr

### Funding Sources

This work was supported by a French government grant managed by the Agence Nationale de la Recherche, project ReAlCharge, reference ANR-20-CE92-0035.

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENT

This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Financial support from the IR INFRANALYTICS FR2054 for conducting the research is gratefully acknowledged. We would
like to thank Assia Hessani from the MONARIS laboratory (PLASVO platform) for her
assistance with the Raman measurements.

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<figure><img src="image_12.png" /><figcaption>TOC</figcaption></figure>