# Influence of the synthesis route of lithium adsorbent Li/Al LDHs on physicochemical properties and extraction properties

Maël Ferrand, Yves Barre, Alban Gossard, Guillaume Munier-Gondiant, Eric de Vito, Hussam Aldoori, Clio Bosia, Agnes Grandjean

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To cite this version:

Maël Ferrand, Yves Barre, Alban Gossard, Guillaume Munier-Gondiant, Eric de Vito, et al. In-fluence of the synthesis route of lithium adsorbent Li/Al LDHs on physicochemical properties and extraction properties. Water Resources Research, 2025, 34, pp.100317. 10.1016/j.wri.2025.100317. cea-05412266

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# Influence of the synthesis route of lithium adsorbent Li/Al LDHs on physicochemical properties and extraction properties

Maël Ferrand<sup>a</sup>, Yves Barré<sup>a</sup>, Alban Gossard<sup>a,*</sup> ID, Guillaume Munier-Gondiant<sup>b</sup>, Eric de Vito<sup>b</sup>, Hussam Aldoori<sup>c</sup>, Clio Bosia<sup>c</sup>, Agnès Grandjean<sup>d</sup>

## ARTICLE INFO

### Keywords:

- Lithium extraction

- DLE process

- Li/Al LDH adsorbent

- Adsorbent properties

- Extraction properties

## ABSTRACT

Lithium can be extracted from brines using a process commonly called Direct Lithium Extraction (DLE) using a lithium selective adsorbent as solid support. For that purpose, Li/Al layered double hydroxide (Li/Al LDH) are efficient materials for trapping $Li^+$ in the vacant octahedral sites and chloride ions in their interlayer space. This article presents two distinct synthesis routes of Li/Al LDH materials, hydrothermal and coprecipitation, their characterizations and their Li extraction properties in synthetic brine. Indeed, the synthesis route impacts the physicochemical properties of the adsorbent and hence the $Li^+$ extraction performances. A link between the structural and morphological properties of the adsorbents and the extraction properties was established. The material obtained by the coprecipitation method appears more effective for extracting lithium, as it is made up of grains of small particles with a greater pore volume and a larger exchange surface. This morphological property leads directly to a fast extraction kinetics, which is an essential point for a viable DLE process. This article also shows a close link between the LiCl content in the materials and the maximum achievable extraction capacity. The two active materials studied here have in fact the same maximum capacities due to an identical LiCl content in their crystalline structure, meaning that all sorption sites can be reached under optimal extraction conditions. It is therefore crucial to understand the influence of synthesis method on the extraction performance to optimize the process.

## 1. Introduction

Lithium plays a key role in the manufacturing of Li-ion batteries, and its demand is expected to increase significantly in the coming years due to the global transition from fossil fuels to electric power, particularly for the use in electric vehicles [1,2]. Nowadays, lithium is mainly sourced from hard rock mining and from solar evaporation ponds, using energy and water consuming processes [3,4]. To meet rising demand, a larger production will be necessary and alternative sources of Li beyond conventional sources have to be considered. To address this need, research is increasingly focused on new environment friendly extraction methods which can allow to tap into local unconventional sources of lithium. These sources contain lithium and other salts in varying amounts [5-7]. Among these

<footnote><sup>a</sup> CEA, DES, ISEC, DMRC, Univ. Montpellier, Marcoule, France</footnote>

<footnote><sup>b</sup> CEA, Liten, DTNM, LCAE, Univ. Grenoble Alpes, Grenoble, France</footnote>

<footnote><sup>c</sup> Lithium de France, Haguenau, France</footnote>

<footnote><sup>d</sup> CEA, DES, ISEC, DPME, Univ. Montpellier, Marcoule, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: alban.gossard@cea.fr (A. Gossard).</footnote>
resources, geothermal brines are particularly promising due to their widespread availability and potential for generating geothermal energy, making the extraction process more economically viable [8]. However, the extraction of lithium from such highly saline media is complex and requires efficient Direct Lithium Extraction (DLE) technologies [9]. In the most advanced DLE processes, the hot brine extracted from the underground containing various dissolved salts including lithium is passed through a fixed-bed column containing a solid adsorbent material. The lithium ions are selectively captured while the remaining solution leaves the column with minimal compositional alteration and can be re-injected into the reservoir with a limited impact on the environment [10,11]. The adsorbed lithium is then be recovered through an elution step, producing a more purified lithium eluate. Three families of adsorbents have been widely studied for selective extract Li: lithium manganese oxides (LMO), lithium titanium oxides (LTO) and lithium-aluminium layered double hydroxides (Li/Al LDH) [12]. Among these, layered double hydroxides (Li/Al LDH) structure ((LiCl)xAl2(OH)6nH2O) is a base for DLE process due to its various crucial properties [13,14]. This adsorbent is able to accommodate lithium chloride (Li⁺ in the octahedral voids of aluminium hydroxide sheets and Cl⁻ in the inter-sheet spaces) in its crystal structure and present a high selectivity for Li⁺ because of the adapted size of the octahedral voids of aluminium hydroxide sheets [15]. While Li/Al LDH exhibit promising properties in term of selectivity, sorption kinetic or cycling, their extraction performance is directly influenced by brine properties including ionic force, pH, impurities and temperature as well as the synthesis methods of the sorbent itself [16]. The lithium adsorption mechanism of Li/Al LDHs is driven by the ionic strength of the solution containing Li. In case of a highly saline solution (such as brines) containing lithium at a trace level, the adsorbent is able to adsorb Li, whereas it releases lithium when use in a weakly saline aqueous solution [16]. Moreover, it has been shown that the adsorption capacity is favoured in brine containing an increasing Mg/Li ratio [17]. On the contrary, the presence of anions other than chlorides, such as sulphates, reduces performances [18]. In the case of excessive elution, the interlayer space of the crystalline structure may collapse, resulting in a gibbsite-type crystal structure that decreases the extraction performances [17]. To avoid this phenomenon, the material can be doped, for example by inserting specific anions in the interlayer to keep it more stable during elution [19] or the structure can be regenerated in a concentrated LiCl solution to restore good extraction performance [20]. As lithium adsorption by Li/Al LDHs is an endothermic reaction, increasing temperature promotes adsorption efficiency [14,16].

A well-known synthesis route for lithium extraction application is the co-precipitation route [14,21]. A less detailed hydrothermal route is also described, but not for the extraction of Li [22–24]. Both synthesis routes lead to materials presenting different micro-structure. Understanding the physicochemical properties of Li/Al LDH is therefore crucial to improve the synthesis route and hence the overall DLE process. For this aim, this article investigates the protocol and characterization of two different synthesis methods of Li/Al LDH materials, and assesses their extraction performance in a synthetic brine containing lithium chloride alongside other salts.

## 2. Materials and methods

### 2.1. Materials and synthesis

The Li/Al LDH were synthesized by two distinct methods inspired from literature: a hydrothermal method [23–26] and a co-precipitation method [14,26,27]. Various synthesis factors and conditions were studied and tested to produce the two powders presented here.

- Hydrothermal synthesis route

Li/Al LDH were synthesized from a commercial aluminium hydroxide Al(OH)3 (Sigma-Aldrich, CAS: 21645-51-2), and lithium chloride LiCl salt (Thermo Scientific, CAS: 7447-41-8, ≥99 % purity). The Li/Al LDH powder was synthesized by contacting these two precursors in a hydrothermal reactor (Parr Instrument Company, 120 mL) in presence of milli Q water with a molar ratio Li/Al = 3.3. The reactor was placed at 90 °C for 24 h. The powder named BH was recovered by 0.1 µm Büchner filtration without washing and dried overnight at 80 °C. As the powder is obtained with a large excess of Li, an activation/washing step was required to activate the material. The powder was placed under agitation in contact with a Milli Q water solution with a solid/liquid ratio of 5 g/L at 25 °C for 48 h. As before, a 0.1 µm Büchner filtration was performed to recover the activated powder named BH1.

- Coprecipitation synthesis route

Li/Al LDH were synthesized by a coprecipitation process using a Li/Al solution obtained by dissolving aluminium chloride hexahydrate AlCl3·6H2O (Thermo Scientific, CAS: 7784-13-6, ≥99 % purity) and lithium chloride LiCl (Thermo Scientific, CAS: 7447-41-8, ≥99 % purity) in Milli Q water. Co-precipitation took place by adding the solution in a sodium hydroxide NaOH (Merck, CAS: 1310-73-2, ≥99 % purity) solution. The Li/Al LDH powder was obtained by adding dropwise a constant flow rate controlled by a peristaltic pump a solution of molar ratio Li/Al = 0.5 into a NaOH solution of initial pH 13.4 under magnetic stirring. The pH of the reaction medium was followed until total consumption of the Li/Al solution, then readjusting the pH by successive addition of NaOH solution until pH stabilizes between 6 and 7 at the end of the reaction. The powder named CP was recovered by 0.1 µm Büchner filtration, then a residual washing was then carried out with Milli Q water.

### 2.2. Materials characterization

X-ray diffraction (XRD) spectra were recorded on an X'Pert PRO-PANalytical with a copper Kα-1 radiation (λ = 1.5406 Å, 60 kV, 40
mA 2Θ = from 5 to 70°, step = 0.013°). Scanning electron micrographs were gathered using a Merlin ZEISS at 5 kV and 500 pA. The samples were previously metallized by a carbon deposit. Nitrogen adsorption/desorption were performed on an Anton Paar Nova 800 device. Thermal weight losses of different Li/Al-LDHs were recorded on a Thermal gravimetric analyser (TGA) Netzsch STA 409 PC with N₂ atmosphere and a heating rate of 10 °C/min until 1000 °C. Quantification of elements in solution was determined by inductively coupled plasma optical emission spectrometry analysis (ICP-OES, ThermoFischer Scientific, iCAP 6000 Series). Samples were previously dissolved in HF before being analysed by ICP-OES. X-ray photoelectron spectroscopy was performed on a PHI 5000 Versa Probe II using an aluminium source under conditions 100 µm, 25 W, 15 kV.

## 2.3. Batch extraction experiments

Extraction experiments were carried out in synthetic brine containing lithium chloride LiCl (Thermo Scientific, CAS: 7447-41-8, ≥99 % purity), magnesium chloride hexahydrate MgCl₂·6H₂O (Merck, CAS: 7791-18-6, ≥99 % purity), sodium chloride NaCl (Merck, CAS: 7647-14-5, ≥99 % purity), potassium chloride KCl (Merck, CAS: 7447-40-7, ≥99 %) and calcium chloride bihydrate CaCl₂·2H₂O (Thermo Scientific, CAS: 10035-04-8, ≥99 % purity).

To mimic at best the real application conditions and improve extraction performances, all extraction tests were carried out at 80 °C under stirring by contacting materials with a synthetic brine whose composition is given in Table 1. Note here that extraction tests have also been performed at ambient temperature, giving only a very slight adsorption. This brine is made up entirely of chloride salts, with magnesium chloride as the main salt.

Extraction kinetics tests were performed to obtain equilibrium times for each material by contacting powders and brine for different time intervals (from 15 min to 14 days) with a solid/liquid ratio (S/L) ratio of 50 g/L. Then, S/L ratio were varied from 10 g/L to 50 g/L to reach the maximum adsorption capacity of the materials at the previously established equilibrium times. Concentration of Li⁺ was measured by ICP-OES from which the adsorption capacity of the adsorbent was calculated using the following equation (1):

$$Q = (C_i - C_f) \frac{V}{m} \qquad (1)$$

With Cᵢ and Cᵣ (mmol/L) the initial and final concentrations of Li⁺ in solution, V (L) the volume of brine and m (g) the mass of the adsorbent.

When adsorption in controlled by the diffusion of the adsorbate from the solid surface to internal reaction sites, this step is generally the rate-limiting factor. Weber and Morris have used an empirical model based on the dependence of adsorption on the square root of time as described by equation (2) [28].

$$Q_t = K_{ID} \times t^{1/2} \qquad (2)$$

With Qᵦ the amount of lithium adsorbed at time t (mmol/g) and Kᵦ₃ the intraparticle diffusion rate constant (mmol/g/min¹/²) depending on the microstructure of the solid, the temperature and the concentration of lithium.

# 3. Results and discussion

## 3.1. Materials characterization

Diffractograms to identify the phases present in the powders have been plotted (Fig. 1) showing diffractograms of the commercial Al(OH)₃, BH, BH1 and CP powders. The Al(OH)₃ powder is a pure phase of bayerite with its main peak at 2Θ = 18°. The latter totally disappears for BH powder in favour of a new intense peak at around 2Θ = 12°, assigned to the Li/Al LDH phase (LiCl)ₓ·Al₂(OH)₆·nH₂O. The d-spacing of this peak corresponds to the Li/Al LDH interlayer space, which is dilated from the Al(OH)₃ interlayer space by the intercalation of chlorides [29,30]. BH1 powder still have the peak at 2Θ = 12° corresponding to Li/Al LDH phase, also a slight contribution from Al(OH)₃ peak can be seen at 2Θ = 18°, which can corresponds to the previous bayerite structure, but also a gibbsite structure [31]. The presence of peak corresponding to Al(OH)₃ is explained by the washing step performed at the end of the synthesis, during which a part of the intercalated LiCl in the initial structure was removed and the interlayer space collapses to give back Al(OH)₃ interlayer [17,32]. For the CP powder, there are only peaks corresponding to a pure Li/Al LDH phase [14,27]. In addition, the diffractograms of Al(OH)₃, BH and BH1 appear to be well defined with very fine peaks compared to the diffractogram of CP. A larger width at mid-peak height indicates smaller crystallite sizes, so CP powder is composed of smaller crystallite than Al(OH)₃, BH and BH1 [33,34].

The phase identifications are summarized (Table 2). After phases identification, SEM images were taken to see the morphology and grain sizes of the powders. SEM images of the different materials, shown (Fig. 2) revealed that Al(OH)₃, BH and BH1grains are identical in terms of grain sizes and have a striated/lamellar morphology [26]. These large and dense grains of around 10 µm therefore do not

<table><caption>**Table 1**

Synthetic brine composition obtained from [14].</caption><thead><tr><th></th><th>LiCl</th><th>MgCl<sub>2</sub>·6H<sub>2</sub>O</th><th>NaCl</th><th>KCl</th><th>CaCl<sub>2</sub>·2H<sub>2</sub>O</th><th>pH</th></tr></thead><tbody><tr><th>Concentration (mmol/L)</th><td>63</td><td>5550</td><td>68</td><td>15</td><td>1</td><td>4.4</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Diffractograms of Al(OH)₃, BH, BH1 and CP. The G index = gibbsite phase (01-080-6432) and L = Li/Al LDHs phase (01-087-1768).</figcaption></figure>

evolve after the hydrothermal Li/Al LDH synthesis. The CP grains present a different petal-like morphology with aggregates of tens of microns of small particles, with inter-grain porosity [14,35]. Moreover, SEM results indicate that CP grains are composed of smaller particles than BH and BH1 grains and therefore may develop a higher porous volume.

In order to verify porous volumes, porosimeter analyses were carried out and are shown (Fig. 3) which presents nitrogen adsorption-desorption isotherms of the different materials. The isotherms for Al(OH)₃, BH and BH1 powders are similar and almost overlap with a very low pore volume of around 0.02 cm³/g. However, the CP isotherm is different with a higher pore volume of around 0.3 cm³/g, which is in accordance with the SEM observations. The type and shape of the isotherms give an indication of the micro-structure. Al(OH)₃, BH and BH1 powders are of type II curves according to the IUPAC classification [36], corresponding to non-porous or only macroporous powders, while the CP isotherm is a type IV a) which is typical for mesoporous materials. In addition, hysteresis type 3 relates to sheet/platelet particles, which is consistent for the reported Li/Al LDHs structure [14].

Thermogravimetric analysis was conducted to estimate the water content of the different powders. The TGA curves of Al(OH)₃, BH, BH1 and CP powder and their first derivative (DTG) illustrated (Fig. 4), show different mass loss stages for the four samples. Below 200 °C, mass loss is attributed to physiosorbed water on and between particles. Above 200 °C until 450 °C, there is a large mass loss, occurring in one step for BH and in two stages for Al(OH)₃, BH1 and CP, corresponding to the loss of interlayer water. Finally, mass loss above 450 °C is attributed to a possible phase transition. This loss can occur in one step if the water binding energies are identical, or in two steps if the binding energies are different [37–39]. This quantity of interlayer water affects especially the size of the interlayer space and also interaction energy of Li/Al LDH structure, leading to different Li⁺ extraction performances [35,37]. The total amount of structure water, estimated from TGA data, indicates that the two powders BH1 and CP present amounts of interlayer water corresponding to 2.2.

XPS characterizations of the powders were carried out to confirm that only Li ions contained inside the Li/Al LDH crystal structure were taken into the account in the mass balance calculation. In fact, chloride salts including LiCl can deposit on the surface of Al(OH)₃ grains [40,41]. The same phenomenon could happen for similar surfaces such as Li/Al LDH and thus distort the mass balance and also the link with vacant sites. From the orbitals investigated (C 1s, O 1s, Li 1s, Cl 2p, Al 2p), only Li 1s and Cl 2p are presented (Fig. 5). Li 1s and Cl 2p orbital spectra of pure LiCl are presented (Fig. S1) for reference. Li 1s and Cl 2p XPS spectra obtained for LiCl present peaks at binding energies of 56.35 eV and 198.7 eV, respectively, typically found for pure Li-Cl. Peaks at similar binding energies, 56.25 eV and 198.7 eV, for Cl 2p₃/₂ and Li 1s, respectively, are detected for BH. These peaks are thus attributed to Li-Cl bonds. For Li 1s, a smaller peak, at a binding energy of 55.1 eV, is also detected. This peak is attributed to Li bonded with OH bonds on the surface. For Cl 2p, the Cl 2p₃/₂ peak at 198.7 eV is the only contribution. BH has undergone no washing, and only the first 5 nm of the sample were probed. It can thus be assumed that for BH sample, the Li and Cl peaks detected are related to LiCl deposited on the surface. On the other hand, BH1 has undergone a washing process to remove surface LiCl. The corresponding spectra presents a Li 1s peak at 55.3 eV and a main Cl 2p₃/₂ peak at 197.7 eV. A smaller Cl 2p₃/₂ peak at 198.7 eV is also detected. The Li 1s peak is attributed to Li inserted in the octahedral sites of the hydroxide lamella. The main Cl 2p peak is attributed to Cl inserted in the interlayer space of Li/Al-LDH, bonded with

<table><caption>**Table 2**

Summary of structural and morphology properties of powders.</caption><thead><tr><th></th><th>BH1</th><th>CP</th></tr></thead><tbody><tr><td>Crystalline structure</td><td>Li/Al LDH + Al(OH)<sub>3</sub></td><td>Li/Al LDH</td></tr><tr><td>Grains morphology</td><td>Striated/lamellar<br>Large and dense grains ≈ 10 µm</td><td>Petal-like<br>Aggregates ≈ 10 µm of small particles</td></tr><tr><td>Adsorbed volume (cm<sup>3</sup>/g)</td><td>0.02</td><td>0.3</td></tr><tr><td>Q<sub>max</sub> (mmol/g)</td><td>1.2</td><td>1.2</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Scanning electron microscopy of Al(OH)₃, BH, BH1 and CP.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Nitrogen adsorption-desorption of Al(OH)₃, BH, BH1 and CP.</figcaption></figure>

hydroxyl groups [19] whereas the smaller Cl 2p peak, at a higher binding energy, could be attributed to LiCl that deposited on the surface after desorption by the washing solution. CP (c) spectrum also presents one contribution for Li 1s, corresponding to bulk Li, and two contributions for Cl 2p, at binding energies of 197.7 eV and 198.6 eV, corresponding to Cl embedded in the bulk and deposited surface LiCl, respectively.

These results show that LiCl is present both in the structure and on the surface for BH powder. For BH1 and CP sample, despite it is very difficult to quantify the small amount of LiCl on the surface of the material the signals from the surface LiCl peaks are very weak. We can assume that all the Li and Cl ions for BH1 et CP are in the structure and the LiCl content in the surface can be neglected, Thus the estimated mass balance can be used with a relatively good accuracy to estimate the Li<sub>content</sub> in the structure for these 2 samples (BH1 and CP). The Li content (Li<sub>content</sub>) in the powders was finally calculated to estimate the adsorption capacity. The Li/Al LDH crystal structure with all sorption sites occupied corresponds to the formula ((LiCl).Al<sub>2</sub>(OH)<sub>6</sub>.nH<sub>2</sub>O). Based on this chemical formula and total amount of structure water calculated from thermogravimetric analysis, the theoretical maximum adsorption capacity (Q<sub>theoretical max</sub>) is calculated using equation (3).
<figure><img src="image_7.png" /><figcaption>Fig. 4. TGA thermograms and associated derivatives DTG of Al(OH)₃, BH, BH1 and CP.</figcaption></figure>

$$Q_{\text{theoretical max}} = \frac{M_{\text{Li}}}{M_{\text{LiClAl}_2(\text{OH})_6 \cdot n \text{H}_2\text{O}}} \quad \text{with } n = 2.2 \qquad (3)$$

Materials are considered active if vacant sorption sites are available in their structure [42], which means that $Li_{content}$ must be less than a $Q_{theoretical max}$ of 4.4 mmol/g. The different powders $Li_{content}$ calculated from HF dissolution and quantitatively composition analysis. The $Li_{content}$ for CP estimated by the HF dissolution method is 3.2 mmol/g which means that the powder is active due to the composition of the Li/Al coprecipitation solution. Indeed, all the aluminium supplied was coprecipitated, while leaving a small proportion of Li in the synthesis supernatant as wanted. The $Li_{content}$ of BH powder is 7 mmol/g, which means that BH does not appear to be activated. In fact, hydrothermal synthesis with a Li/Al precursor ratio of 3.3 filled the entire structure as expected, but there is $Li^+$ also sorbed on the surface, since the $Li_{content}$ of BH is greater than $Q_{theoretical max}$. This is in accordance with the XPS analysis. BH is therefore not able to extract lithium from brine due to the excess of $Li^+$ in its structure and was excluded from subsequent extraction tests. For BH1 powder, the calculated $Li_{content}$ is 3.2 mmol/g which means that the washing/activation step releases $Li^+$ resulting in a lower $Li_{content}$ leading to the same $Li_{content}$ than CP. This washing/activation step has been studied and various tests have been carried out to lead to the same order of magnitude as CP. The maximum adsorption capacity ($Q_{max}$) of BH1 and CP was then calculated using the formula (4).

$$Q_{\text{max}} = Q_{\text{theoretical max}} - Li_{\text{content}} \qquad (4)$$

The two synthesis routes lead to two materials (CP and BH1) having different structural and morphological properties as described (Table 2) while they have the same maximum adsorption capacity. Thus, the following extraction tests will help to understand the impact of the structure and morphology on extraction properties, such as kinetic and adsorbent concentration variation extraction tests.

## 3.2. Extraction experiments results

Extraction tests have been performed to assess the influence of morphological and microstructural differences between the two materials on the extraction kinetics. Then, extraction test with different adsorbent concentrations variation tests was set up by varying S/L ratios to have an excess of $Li^+$ compared to the calculated $Q_{max}$ and to ensure all the available sorption sites of the crystalline structure are filled at equilibrium.

### 3.2.1. Influence of material microstructure, morphology and crystalline structure on extraction kinetics

The extraction capacity $Q_t$ as a function of time and in addition, according to the Weber and Morris model [28], the normalized
<figure><img src="image_8.png" /><figcaption>Fig. 5. Li 1s a) and Cl 2p b) spectra for BH, BH1 and CP.</figcaption></figure>

extraction capacity used to illustrate and identify the various steps of the sorption mechanism up to kinetic equilibrium, i.e. the ratio between the extraction capacity and the equilibrium extraction capacity, as a function of the square root of time are shown (Fig. 6). BH1 and CP behave differently, indeed, BH1 leads to extraction kinetics reaching equilibrium after 7 days, while CP reaches equilibrium in 24 h, with extraction capacities at equilibrium respectively of 1 mmol/g and 0.8 mmol/g (Fig. 5).

As the BH powder is composed of large and dense grains developing a very low porous volume, the mass transfer is reduced, whereas CP grains are aggregates of small particles developing a larger porous volume with higher mass transfer rate. These differences ensure CP powder to extract Li⁺ much faster than BH1 powder. It is important to note that the brine contained no trace of aluminum after extraction tests, indicating the chemically stability of the material in these conditions, which is a crucial aspect for Li/Al LDHs application efficiency for many cycles. Following the Weber and Morris model, different diffusion mechanisms can be observed with the various linear steps. The adsorption process is divided in three stages for CP and four stages for BH1. For the CP particles, the first step (I) corresponds to the diffusion of LiCl in the liquid phase and through the boundary layer at the particles surface. The second one (II) is related to the LiCl diffusion in the solid phase, i.e. into the interlayer of the CP particles, and is then consequently slower (lower slope). Finally, an equilibrium step (III) is reached [43]. For BH1, the stage (I) is slightly slower than for CP and, only 50 % of the
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Extraction capacity Qt (mmol/g) depending on contact time (hours a) and Qt/Qeq depending on the square root of contact time b).</figcaption></figure>

equilibrium capacity is reached in 1 h, whereas 80 % of the equilibrium capacity is reached in 30 min for CP. These results are consistent with the porosimeter analyses indicating that CP has a greater pore volume and therefore a larger specific exchange surface area. The representation as a function of the square root of time highlights the existence of a supplementary stage for the LiCl diffusion in the solid phase of BH1 (stages (II a) and (II b)). Indeed, as BH1 is composed of a mix of Li/Al LDH and Al(OH)₃ phases as revealed by XRD analysis, the space between the sheets corresponding to the Al(OH)₃ phase must first be expanded for the arrival of LiCl to allow diffusion. Thus, the additional kinetic step present for BH1 and absent for CP may correspond to the enlargement and diffusion of LiCl into the interlayer space of the Al(OH)₃ phases [44]. Finally, an equilibrium step (III) is reached.

To confirm this hypothesis, diffractograms of BH1 and CP before and after kinetic tests having reached the kinetic equilibrium (1 week for BH1 and 24 h for CP) are given in Fig. 7. For BH1, the Al(OH)₃ peak at 2Θ = 18° has completely disappeared in favour of the Li/Al LDH phase peak at 2Θ = 12° after Li⁺ extraction, which is in agreement with the transformation of the Al(OH)₃ phase into the Li/Al LDH phase and thus with a supplementary diffusion step reducing the extraction rate of BH1. In contrast, the CP diffractogram (Fig. 6) after extraction is similar to the one before extraction.

To conclude about kinetics, the BH1 and CP powders with different morphology and porous volume lead to different kinetic properties in a given brine, the CP powder is therefore more efficient from a kinetic point of view than the BH1 powder as relatively fast kinetics are essential for developing a high-performance DLE material.

However, the extraction capacities measured during the kinetic tests do not correspond to the Qmax previously calculated. In fact, the capacities for BH1 and CP of 1 mmol/g and 0.8 mmol/g respectively under the condition of S/L = 50 g/L mean that a large proportion of the lithium supplied by the brine is consumed and intercalated into materials. Hence it is an important part to study the influence of the adsorbent concentration in the brine to fill the crystalline structure and therefore reach and determine their maximum capacity.

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Diffractograms of BH1 a) and CP b) before and after extraction test.</figcaption></figure>
### 3.2.2. Relation between maximum extraction capacity and Li content in the materials

The extraction capacity Q as a function of the S/L ratio and extraction efficiency are shown (Fig. 8). For both materials, the extraction capacity increases with the decrease of the S/L ratio. Indeed, the decrease of the S/L ratio increases the amount of available $Li^+$ in the solution compared to the $Q_{max}$, leading to a higher filling possibility of the materials sorption sites.

The maximum capacities $Q_{max}$ of BH1 and CP are similar around $1.4 \pm 0.1$ mmol/g, reached for an S/L ratio = 20 g/L. It has been previously demonstrated that the theoretical $Q_{max}$ is 1.2 mmol/g, which is equivalent to these experimental $Q_{max}$ considering both measurement uncertainties and uncertainties on the $Li_{content}$ value due to the low presence of surface LiCl. This proves that these extraction tests at S/L = 20 g/L led to a complete filling of the available sorption sites in the materials and therefore indicates that, at the thermodynamic equilibrium determined after the kinetic experiments, the sorption sites are accessible for the lithium under favourable conditions (temperature, excess of lithium) for both powders despite their microstructure differences. Moreover, despite the structural and morphological differences between the two materials, their identical Li content enables them to capture the same amount of $Li^+$ during the extraction tests if there is a large excess of $Li^+$ compared to the $Q_{max}$. The extraction yield, illustrating the thermodynamic equilibrium between powders and brines showing that at S/L ≥ 30 g/L, extraction yields are slightly better for BH1 powder than CP, illustrating a greater thermodynamic affinity in the direction of LiCl intercalation into the BH1 structure under these conditions. Conversely, at a ratio <30 g/L, extraction yields are better for CP powder. Insertion competition between $Li^+$ and $Mg^{2+}$ ions is known to exist, and it's possible that the crystallinity of the two materials is different, which may influence the selectivity of the ion competition to a greater or lesser effect [26,45].

A survey of the literature is useful to estimate and compare the extraction capacities obtained in this study. Although the $Li_{content}$ of the powders is not reported, Table 3 gives the maximum extraction capacities of Li/Al LDH materials in different brines composition with $Li^+$ and also different extraction conditions. Some data concerning other lithium adsorbent are also given in Table S1. The extraction capacities in the literature range from 0.5 mg/g to 1 mmol/g, which is the same order of magnitude of the maximum extraction capacities of the BH1 and CP powders presented in this work.

Finally, reducing the initial $Li_{content}$ with an appropriate activation will induce an increase of the materials $Q_{max}$ to optimize extraction performance, as long as the structural and microstructural properties are maintained [20].

## 4. Conclusions

This work shows that the morphological and structural properties of Li/Al LDH materials is a crucial aspect to obtain good extraction properties for a DLE process. It has notably been established a structure-activity relationship between the structural morphology characteristics of the materials and the lithium extraction performance. For that, a hydrothermal synthesis route has been used to be compared with a more classical synthesis way by co-precipitation. The physicochemical properties are consequently closely linked to the synthesis route used to obtain the Li/Al LDH powders. Indeed, the morphology and the porous volume is critical for a fast lithium extraction kinetic. The smaller petal-shaped particles with inter-grain porosity of the pure Li/Al LDH material synthesized by co-precipitation lead to a higher specific surface area. This enables thermodynamic equilibrium to be reached after 24 h, whereas the materials obtained from hydrothermal synthesis reached equilibrium after 1 week due to its large and dense grains, and the presence of $Al(OH)_3$ phase. Indeed, an additional kinetic step is present due to the transformation of the aluminium hydroxide phase into Li/Al LDH. Moreover, BH1 powder needs to pass through the washing/activation stage prior to use. The materials obtained by coprecipitation is thus more effective for extracting lithium from brines. These morphological properties lead directly to rapid extraction kinetics, an essential point in the development of a viable material for a DLE process. This article also shows that the LiCl content in the materials and the maximum achievable extraction capacity are closely linked. Indeed, the two active materials in fact have the same maximum capacities in optimal extraction tests (large excess of $Li^+$ compared to $Q_{max}$) due to the identical Li contents in their initial crystalline structure. The intrinsic properties of materials and their influence on extraction properties have been demonstrated in this paper. From now, to evaluate the practical potential of these materials, the influence of the brine composition and the influence of competitive ion effects on the extraction performances should be studied on these materials having different morphological and structural properties. Moreover, an in-depth investigation of stability and cycling would be crucial to perform and evaluate their long-term application.

### CRediT authorship contribution statement

**Maël Ferrand**: Methodology, Writing – original draft, Conceptualization, Visualization, Investigation, Writing – review & editing.
**Yves Barré**: Writing – review & editing, Investigation, Validation, Writing – original draft.
**Alban Gossard**: Validation, Writing – review & editing.
**Guillaume Munier-Gondiant**: Methodology, Investigation, Writing – review & editing.
**Eric de Vito**: Formal analysis, Writing – review & editing.
**Hussam Aldoori**: Writing – review & editing, Validation, Supervision.
**Clio Bosia**: Funding acquisition, Writing – review & editing.
**Agnès Grandjean**: Validation, Funding acquisition, Writing – review & editing.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
<figure><img src="image_11.png" /><figcaption>Fig. 8. Sorption capacity Q (mmol/g) and extraction efficiency (%) as function of S/L ratio (g/L).</figcaption></figure>

<table><caption>**Table 3**

Comparative Tab. of maximum capacities of some Li/Al LDH powder in the literature compared with BH1 and CP powders.</caption><thead><tr><th></th><th>BH1</th><th>CP</th><th>[13]</th><th>[43]</th><th>[25]</th><th>[46]</th></tr></thead><tbody><tr><td>Type of brine (complex: &gt;2 salts and simplified:=2 salts)</td><td>Complex</td><td>Complex</td><td>Complex</td><td>Simplified</td><td>Complex</td><td>Simplified</td></tr><tr><td>Li<sup>+</sup> concentration (mmol/g)</td><td>60</td><td>60</td><td>5</td><td>From 50 to 200</td><td>350</td><td>45</td></tr><tr><td>Maximum capacity (mmol/g)</td><td>1.4</td><td>1.4</td><td>1</td><td>0.5</td><td>0.5</td><td>0.7</td></tr></tbody></table>

## Acknowledgments

The authors warmly thank the Lithium de France company for their financial support. The authors are also grateful to Benoit Meilleray for its help for the HF dissolution operations.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.wri.2025.100317.

## Data availability

Data will be made available on request.

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