# Unveiling the Reactivity of Li1+xAlxTi2-x(PO4)3 with Lithium Salts to Reduce Its Sintering Temperature

Morgan Guilleux, Christel Gervais, Cristina Coelho Diogo, Christel

Laberty-Robert, Arnaud Perez

► To cite this version:

Morgan Guilleux, Christel Gervais, Cristina Coelho Diogo, Christel Laberty-Robert, Arnaud Perez. Unveiling the Reactivity of Li1+xAlxTi2-x(PO4)3 with Lithium Salts to Reduce Its Sintering Temperature. ACS Applied Energy Materials, 2025, 10.1021/acsaem.4c02668. [https://doi.org/10.1021/acsaem.4c02668](https://doi.org/10.1021/acsaem.4c02668). [arXiv:2009.09268](https://arxiv.org/abs/2009.09268)

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# Unveiling the Reactivity of $Li_{1+x}Al_xTi_{2-x}(PO_4)_3$ with Lithium Salts to Reduce its Sintering Temperature

Morgan Guilleux¹, Christel Gervais¹, Cristina Coelho Diogo², Christel Laberty-Robert¹,³, Arnaud J. Perez¹,³,*

¹ Sorbonne Université, CNRS, Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP), F-75005, Paris, France

² Sorbonne Université, CNRS, Fédération de Chimie des Matériaux de Paris-Centre (FCMAT), FR 2482, F-75005, Paris, France

³ Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR 3459, F-80039, Amiens, France

* Corresponding author: arnaud.perez@sorbonne-universite.fr
**Abstract**

NaSICON-type materials, such as Li₁.₃Al₀.₃Ti₁.₇(PO₄)₃ (LATP), are considered promising solid electrolytes due to their good total ionic conductivity of 10⁻⁴ S.cm⁻¹ at room temperature and their stability at high potentials (4.1 V vs Li/Li⁺). However, decreasing their densification temperature is crucial for their integration into all-solid-state batteries (ASSBs). The minimum required heat treatment temperature for densification of LATP is 900 °C, which is incompatible with its integration in composite electrode of ASSBs due to reactivity with the positive electrode material (cathode). To lower this temperature, lithium salts are often proposed as sintering aids to promote liquid-phase sintering. However, the systematic formation of impurities, such as LiTiOPO₄ and Li₄P₂O₇, suggests that chemical reactivity plays a significant role in LATP densification. In this work, the chemical reactivity mechanism of lithium salts with LATP during densification and sintering was investigated. Various characterization techniques, including *in situ* and *ex situ* X-ray diffraction, TGA-DTA-MS, DSC, *ex situ* Raman and solid-state NMR spectroscopy (⁷Li, ²⁷Al, ³¹P), were employed to elucidate the mechanism. The formation of intermediate decomposition products Li₃PO₄ and TiO₂ is identified for the first time *via* the reactivity of the lithium salt with LATP prior to the melting temperature of the salt. These intermediates subsequently react with LATP at higher temperature, resulting in the formation of final impurities LiTiOPO₄ and Li₄P₂O₇. This unified mechanism provides important insights on the enhanced densification of LATP at lower temperatures with the use of Li salt sintering aids.

**Keywords**

Solid electrolyte, ionic conductor, ceramic, sintering aid, LATP, chemical reactivity.
# 1. Introduction

All solid-state batteries (ASSB) are considered as a promising candidate energy storage technology for the transportation field. By switching the liquid electrolyte from conventional lithium-ion battery (LIB) to a solid electrolyte in ASSB, the use of flammable solvents is limited and a Li-metal anode can be used.¹ Thus, it is expected that ASSB technology will improve user's safety and increase energy densities.²⁻⁴

The solid electrolyte, present in the separator and the composite positive electrode (cathode), is a key component of this system. This material must have specific characteristics, the main one being its ability to conduct lithium-ion. To compete with liquid electrolytes, solid electrolytes must have a total ionic conductivity greater than $10^{-3}$ S.cm⁻¹ at room temperature. However, most lithium ionic conductors that reach this value of conductivity are sulphide-based materials and exhibit a poor stability in contact with moisture.⁵,⁶

Oxide-based solid electrolytes, on the other hand, show a relatively good ionic conductivity at room temperature, good moisture stability and a wide electrochemical stability window. One of the most promising materials is $Li_{1+x}Al_xTi_{2-x}(PO_4)_3$ with $x = 0.3$ (LATP₀.₃), which belongs to the NaSICON structure type. It shows a bulk ionic conductivity superior to $10^{-3}$ S.cm⁻¹ at room temperature⁷,⁸ and a total ionic conductivity above $10^{-4}$ S.cm⁻¹ as a ceramic densified by a high-temperature sintering process.⁸⁻¹⁰ Usually, a sintering temperature higher than 1000 °C must be applied to the LATP to obtain relative densities above 95% and ionic conductivities above $10^{-4}$ S.cm⁻¹.¹¹,¹²

However, the application of high temperatures for co-sintering in the case of the composite cathode is problematic, since it triggers chemical reactivity depending on the active material used. Indeed, Yu et al. studied the high-temperature stability between LATP ($x = 0.4$) and various cathode materials (e.g., LCO, LNO, NMC, LNMO or LFP).¹³ From this study, they demonstrated a chemical reactivity by the formation of impurities from 700 °C with NMC and from 500 °C in the case of LFP. In other terms, it is necessary to find a way to prevent or limit the reactivity of LATP with the active material.

A strategy to limit this phenomenon is to succeed in densifying at lower temperature by using sintering aids. Several studies have investigated the use of lithium salts such as
Li₃BO₃,¹⁴,¹⁵ Li₃PO₄,¹⁴,¹⁶ LiBO₂,¹⁷ LiF,¹⁸⁻²⁰ LiNO₃,²¹ and LiBF₄²² as sintering aids. Rumpel *et al.* succeeded in decreasing the sintering temperature of LATP from 950 °C to 800 °C by adding 5% vol of Li₃PO₄¹⁶ A study of Dai et al, achieved 98% relative density at 800 °C for LATP with 5% wt of LiBF₄.²² One of the hypotheses that explains the densification at lower temperatures is that upon melting, the salt, which has a lower melting temperature than the sintering temperature of the ceramic, enables liquid phase sintering. This resultant liquid phase improves the rearrangement of particles and triggers grain growth at lower temperature. However, Waetzig *et al.* investigated the effect of several lithium salts (e.g. Li₃BO₃, Li₃PO₄, Li₂CO₃, LiCl, LiOH, LiBO₂) on the densification of LATP by optical dilatometry.²³ They observed that there is no correlation between the shrinkage temperature of the LATP pellet and the melting temperature of the associated lithium salt.²³ In other words, densification at lower temperatures with the assistance of lithium salts cannot be solely attributed to its melt. Moreover, the addition of lithium salt led to the formation of impurities like LiTiOPO₄ and Li₄P₂O₇ after heat treatment, whatever the nature of the salt. The formation of these impurities could play a role in the densification of LATP.

The objective of this study is therefore to investigate in details the reactivity mechanism
between the solid electrolyte LATP₀.₃ and various lithium salts. To achieve this goal, the
mechanism was examined across multiple LATP₀.₃/salt systems, incorporating lithium
salts in proportion exceeding those generally used in literature to maximize chemical
reactivity. The lithium salts selected for this study were used in several literature reports
before. Their variety in chemical composition present us with the opportunity to detect
different potential reactivity pathways. For a comprehensive investigation, several
characterization methods were used, including *ex-situ* X-ray diffraction, high-temperature
X-ray diffraction, Raman spectroscopy, TGA-DTA, DSC and solid-state NMR (³¹P, ⁷Li,
²⁷Al).
## 2. Methods

### 2.1. Synthesis of LATP<sub>0.3</sub>

The solid electrolyte Li<sub>1.3</sub>Al<sub>0.3</sub>Ti<sub>1.7</sub>(PO<sub>4</sub>)<sub>3</sub> (LATP<sub>0.3</sub>) was synthesized by solid state reaction from Li<sub>2</sub>CO<sub>3</sub> (Sigma-Aldrich), Al<sub>2</sub>O<sub>3</sub> (Sigma-Aldrich), TiO<sub>2</sub> anatase (Sigma-Aldrich, 99.9%) and NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> (Sigma-Aldrich, >98%). All starting material were mixed in stoichiometric proportions by ball-milling in a Retsch PM-100 planetary mill, with 1/2/10 mass ratios of sample powder/ isopropanol/ Ø5mm Zirconia beads. The milling process was carried out at a speed of 450 rpm for 2 h (10 min grinding cycle with 10 min pause). The solvent was then removed under air atmosphere for 12 h and the dried powder mixture was placed in an alumina crucible. The crucible with the powder was calcined at 900 °C for 2 h, with a heating ramp of 5 °C/min in a muffle furnace. The resultant white product was ground in an agate mortar into a powder.

To limit aggregates and decrease the particles size, the LATP<sub>0.3</sub> powder was further ball milled using a Retsch PM-100 planetary mill, with 1/10 mass ratios of sample powder/ Zirconia beads Ø5mm, for a duration of 60 min at 450 rpm.

### 2.2. Preparation of samples with lithium salts

High-temperature stability and high-temperature reactivity mechanism were studied from mixtures of LATP<sub>0.3</sub> and lithium salts such as LiNO<sub>3</sub> (Sigma-Aldrich, 99%), LiBF<sub>4</sub> (Sigma-Aldrich, 99.9%), LiBr (Sigma-Aldrich, 99.9%), LiCl (Sigma-Aldrich, 99.9%), Li<sub>2</sub>CO<sub>3</sub> (Sigma-Aldrich), Li<sub>3</sub>BO<sub>3</sub> prepared from Li<sub>2</sub>CO<sub>3</sub> + H<sub>3</sub>BO<sub>3</sub> Sigma-Aldrich (>99.5%),<sup>15</sup> and Li<sub>3</sub>PO<sub>4</sub> (Sigma-Aldrich). Salts were dried under vacuum and kept in a glovebox. Each mixture of LATP<sub>0.3</sub>/lithium salts was ground with an agate mortar in a glovebox to limit water adsorption to a minimum before transferring the sample for further heat treatment and characterizations. All samples were transformed into Ø13mm pellets using a uniaxial press (3.5 tons). A pellet of pure LATP<sub>0.3</sub> was also prepared as a control test.

### 2.3. Ex-situ X-ray diffraction (XRD)

The high-temperature stability was studied by heating each sample at 900 °C for 2 h with a heating ramp of 5 °C/min in a muffle furnace. The pellets collected after the heat treatment were ground using an agate mortar and characterized by powder X-ray
diffraction (XRD) for phase identification. The analyses were carried out on a Bruker D8 Advance diffractometer in Bragg-Brentano geometry, equipped with a LynxEye detector and a Cu emitter anode ($\lambda(K_{\alpha1}) = 1.54056$ Å, $\lambda(K_{\alpha2}) = 1.54439$ Å). The acquisitions were carried out in the range $2\theta = (10-80)°$, with 0.02° steps, a time of 0.2 s/step and a fixed slit of 0.18 cm, unless otherwise specified. Identification of species in diffraction patterns was done using the Diffrac Eva software (Bruker AXS).

## 2.4. High-temperature X-ray diffraction (HTXRD)

The high-temperature reactivity mechanism of LATP<sub>0.3</sub> + LiBr (1:1), undoped LiTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> (LTP) + LiBr (1:1) and LATP<sub>0.3</sub> (control test) pellets were studied using High-temperature X-ray Diffraction (HTXRD). The equipment used was a Bruker D8 Advance diffractometer in Bragg-Brentano geometry, equipped with a LynxEye detector and a Cu emitter anode ($\lambda K_{\alpha1} = 1.54056$ Å, $\lambda K_{\alpha2} = 1.54439$ Å). Pellets samples were placed on a ceramic support and calibration of the sample height as a function of the temperature were made to limit any shift in $\theta$. A flux of dry air was flushed through the sample chamber with a rate of 50 mL/min during the experiment. During analysis, samples were heated from 30 °C to 900 °C with a heating rate of 10 °C/min and cooled from 900 °C to 30 °C with the same heating rate. The XRD patterns were acquired in the range $2\theta = (10-60)°$ with 0.02° steps and a time of 0.1 s/step each 10 °C from 30 °C to 900 °C (short acquisition); and in the range $2\theta = (10-80)°$ with 0.02° steps and a time of 1 s/step each 10 °C from 30 °C to 900 °C and each 100 °C from 900 °C to 100 °C (long acquisition). All analysis was carried out with a fixed slit of 0.18 cm. The XRD patterns obtained every 100 °C were analyzed by Rietveld refinements using the FullProf program.

## 2.5. Thermogravimetric analyses (TGA)

Thermogravimetric analyses (TGA) were performed using a NETZSCH STA 409PC/PG to determine mass loss during heat treatment. The LATP<sub>0.3</sub> + LiBr (1:1) mixture, placed in an alumina crucible, were heated at 900 °C with a heating ramp of 10 °C under an air gas flow. A sample of LATP<sub>0.3</sub> was analyzed according to the same program as a control test.
## 2.6. Differential thermal analysis (DTA)

Differential thermal analyses (DTA) were performed using a NETZSCH STA 449 F3. The powders were placed in alumina crucibles and heated from 25 °C to 900 °C with a heating ramp of 5 °C/min under an air gas flow of 25 ml/min.

## 2.7. Wavelength dispersive X-ray fluorescence spectroscopy (WDXRF)

Pellets of LATP<sub>0.3</sub> + LiBr (1:1) were rapidly cooled (quenched) from various heat treatment temperatures, including 350, 500, 600 and 900 °C in a muffle furnace. Each pellet was then ground using an agate mortar and recompacted at 8 tons during 2 min using a uniaxial press for analysis. The pellets were analyzed using a S8 Tiger spectrometer (Bruker) in 4 kW, with a Rhodium tube as an X-ray source.

## 2.8. Chemical detection of released gases

The Fluorescein is a qualitative chemical test conducted to demonstrate the release of Br<sub>2</sub> (bromine). For this test, LATP + LiBr powder (1:1) was placed in an alumina crucible and then heat-treated in a tube furnace up to 600 °C with a heating ramp of 10 °C/min, under a flow of air. A fluorescein (Fluorescein sodium salt, Sigma Aldrich)-impregnated paper, previously solubilized in distilled water, was placed at the furnace outlet to check for the release of Br<sub>2</sub>.

## 2.9. Raman Spectroscopy

The LabRam HR800 (Horiba Jobin Yvon) is characterized by a focal length of 800 mm. For our experiments we use a 600 lines/mm grating (allowing the recording in one acquisition of the whole spectral range of our interest) and the 514 nm emission of an air-cooled Ar<sup>+</sup> Laser from Coherent (I-90C-6). The detection is performed through a Peltier-cooled CCD detector. The Rayleigh filtering is achieved using ultra narrow band Notch BragGrate Filters. Laser power at the sample is modulated in order to respect sample integrity and avoid thermal transformations. For our samples, power is generally adjusted below 900 µW at the sample. The spectrometer is coupled to an Olympus microscope, sample observation and analysis are performed using Olympus conventional 10x and long working distance 50x objectives, respectively. Acquisitions were managed with LabSpec software (Jobin Yvon-Horiba).
## 2.10. Solid-State NMR ($^{31}$P, $^{7}$Li, $^{27}$Al) and $^{27}$Al 2DMQMAS

Pellets of LATP$_{0.3}$ + LiBr (1:1) were rapidly cooled (quenched) at various heat treatment temperatures, including 200, 400, 600 and 800 °C in a muffle furnace. Each pellet was then ground using an agate mortar before analysis. Solid-state MAS NMR spectra were acquired on a Bruker AVANCE III 700 spectrometer (16.4 T) operating at v$_{0}$ = 283.29, 271.98 and 182.35 MHz for $^{31}$P, $^{7}$Li and $^{27}$Al respectively, using a commercial Bruker triple-resonance MAS probe. Powder samples were transferred to 3.2 mm ZrO$_{2}$ rotors in a glovebox and spun at a MAS rate of 20 kHz. $^{31}$P, $^{7}$Li and $^{27}$Al chemical shifts were referenced to H$_{3}$PO$_{4}$, LiCl and Al(NO$_{3}$)$_{3}$, respectively. Two-dimensional $^{27}$Al multiple-quantum magic-angle spinning (MQMAS) NMR spectra were recorded by using an amplitude modulated Z-filter sequence (p1-t1 (MQ evolution)-p2-τ-p3-t2(acquisition) sequence). The optimized pulse lengths of the 3Q excitation (p1) and reconversion (p2) were p1 = 4 µs and p2 = 2 µs whereas the soft π/2 Z-filter (p3) was set to 15 µs.
# 3. Results

## 3.1. Reactivity of LATP with different Li salts: formation of the same impurities at high temperature

Several lithium salts were chosen to investigate their chemical reactivity at high temperatures with $Li_{1.3}Al_{0.3}Ti_{1.7}(PO_4)_3$ (noted as LATP$_{0.3}$). The lithium salts used, listed in Table 1, cover a large range in terms of chemical nature and melting temperature and have all been proposed previously as sintering aids for LATP.

<table><caption>Table 1: The different lithium salts used to study the chemical reactivity with LATP$_{0.3}$ and their melting temperature (°C)</caption><thead><tr><td>Compounds</td><td>Melting temperature (°C)</td><td>References</td></tr></thead><tbody><tr><td>LiNO<sub>3</sub></td><td>253</td><td>24</td></tr><tr><td>LiBF<sub>4</sub></td><td>309</td><td>25</td></tr><tr><td>LiBr</td><td>551</td><td>Our work</td></tr><tr><td>LiCl</td><td>605</td><td>24</td></tr><tr><td>Li<sub>3</sub>BO<sub>3</sub></td><td>706</td><td>26,27</td></tr><tr><td>Li<sub>2</sub>CO<sub>3</sub></td><td>723</td><td>24</td></tr><tr><td>LiF</td><td>848</td><td>28</td></tr><tr><td>Li<sub>3</sub>PO<sub>4</sub></td><td>1205</td><td>29</td></tr></tbody></table>

The chemical reactivity was studied by conducting heat treatments on pellets made from mixtures of LATP$_{0.3}$ and lithium salts. For appropriate comparison between lithium salts, we fixed a 1:1 molar ratio between $Li_{1.3}Al_{0.3}Ti_{1.7}(PO_4)_3$ and the Li atoms from the salt. For example, the molar ratio was 1:1 for a mixture of LATP$_{0.3}$ and LiBr, 1:0.5 for a mixture of LATP$_{0.3}$ and Li$_{2}$CO$_{3}$, and 1:0.33 for a mixture of LATP$_{0.3}$ and Li$_{3}$PO$_{4}$. This molar ratio is larger than those used in the literature$^{23}$ in order to maximize the chemical reactivity and help us to observe the formation of reaction products. A pellet composed of pure LATP$_{0.3}$ was also studied as a control test. Figure 1a displays the XRD patterns obtained for each sample after a 2 h heat treatment at 900 °C in a muffle furnace.

<table><thead><tr><td>Compounds</td><td>Melting temperature (°C)</td><td>References</td></tr></thead><tbody><tr><td>LiNO<sub>3</sub></td><td>253</td><td>24</td></tr><tr><td>LiBF<sub>4</sub></td><td>309</td><td>25</td></tr><tr><td>LiBr</td><td>551</td><td>Our work</td></tr><tr><td>LiCl</td><td>605</td><td>24</td></tr><tr><td>Li<sub>3</sub>BO<sub>3</sub></td><td>706</td><td>26,27</td></tr><tr><td>Li<sub>2</sub>CO<sub>3</sub></td><td>723</td><td>24</td></tr><tr><td>LiF</td><td>848</td><td>28</td></tr><tr><td>Li<sub>3</sub>PO<sub>4</sub></td><td>1205</td><td>29</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Figure 1: (a) X-ray diffraction data of LATP<sub>0.3</sub> and mixtures of LATP<sub>0.3</sub> with several lithium salts heat treated at 900 °C during 2 h under air atmosphere. LiTiOPO<sub>4</sub> and Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> impurities are systematically present upon Li salt addition, as highlighted by the orange and grey areas respectively. The data is presented in log scale to highlight impurity peaks. (b) Ternary phase diagram of TiO<sub>2</sub>-P<sub>2</sub>O<sub>5</sub>-Li<sub>2</sub>O. The blue dashed line represents the addition of Li<sub>2</sub>O to LiTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>. The red line highlights the assumed equilibrium between LiTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> and Li<sub>3</sub>PO<sub>4</sub> in the study of Hupfer et al,<sup>30</sup> and the green line represents the actual equilibrium line between Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> and LiTiOPO<sub>4</sub>.</figcaption></figure>

a

b
The heat treatment applied to samples containing sintering aids results in the disappearance of the initial lithium salts and the formation of the same impurities: LiTiOPO₄ (Orthorhombic, Pnma) and Li₄P₂O₇ (Triclinic, P-1). This result is consistent with literature, as the formation of LiTiOPO₄ is systematically observed when significant amount of lithium salts is added to LATP₀.₃.¹⁶,²⁰,²¹,³¹,³² In contrast, reports of Li₄P₂O₇ are not systematic but has been observed for several systems.¹⁷,¹⁸,²¹,³¹ These impurities are absent when applying the same heat treatment to pure LATP₀.₃, confirming that they are due to the reactivity with the lithium salts. The absence of impurities related to the chemical nature of different lithium salts suggests that the reactivity follows a similar path whatever the nature of the salt.

Overall, this reactivity can be summarized by equation (1), where LATP₀.₃ is replaced by the un-doped LiTi₂(PO₄)₃ (LTP) parent compound to simplify the reaction, and Li₂O is used as a typical Li salt:

$$ (1) 2 \text{LiTi}_2(\text{PO}_4)_3 + 3 \text{Li}_2\text{O} \rightarrow 4 \text{LiTiOPO}_4 + \text{Li}_4\text{P}_2\text{O}_7 $$

However, this simple mechanism is unsatisfying for three reasons: i) it does not explain the whereabouts of the salt anions with different chemical natures, ii) nor that of the aluminium dopant and, iii) the Li₂O-TiO₂-P₂O₅ ternary phase diagram originally reported, as shown in Figure 1b, indicates that there is no thermodynamic equilibrium between LiTiOPO₄ and Li₄P₂O₇. This points out that the reactivity mechanism to form these impurities is more complex than it seems at first sight.

## 3.2. LATP-LiBr as a model system to understand the chemical reactivity

To shed light on the reactivity mechanism of LATP with lithium salts, we studied in detail how LiBr reacts with LATP₀.₃ (1:1 molar ratio). We used *in situ* X-ray diffraction to identify the formation and consumption of chemical species as a function of temperature. The Br⁻ anion of LiBr has the advantage to be relatively heavy, making it easier to study *via* different analytical techniques compared to other anions. Figure 2a presents an isointensity map of the diffraction patterns measured during the heat treatment between 30 and 900 °C in air.
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2: (a) High-temperature X-ray diffraction data of a pellet of LATP<sub>0.3</sub> and LiBr (1:1) from 30 to 900 °C under an air flow. Temperatures at which species are forming/disappearing are marked by dashed lines and described in the main text. (b) Evolution of relative mass percentages for each species determined by Rietveld refinements, as a function of temperature. The relative mass of Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> at 900 °C appears to be zero from the Rietveld quantification method, but it is in fact present under its molten state.</figcaption></figure>

Several steps are observed:

I. At room temperature, LiBr is not visible because of its strong hydration in contact with air, which means that only the diffraction peaks of LATP<sub>0.3</sub> (R-3c) and AlPO<sub>4</sub> (C222<sub>1</sub>) impurity are observed.

II. Hydrated LiBr.H<sub>2</sub>O (Cmcm) first crystallizes from 50 to 100 °C. Then, LiBr.H<sub>2</sub>O dehydrates at 110 °C to form anhydrous LiBr with a cubic structure (Fm-3m).

III. From 400 °C, LiBr disappears and Li<sub>3</sub>PO<sub>4</sub> (Pnma) forms. This temperature is lower than the melting temperature of LiBr (553 °C), suggesting that it reacts before melting. A diffraction peak at 25 ° indicates the formation of TiO<sub>2</sub> anatase (I4<sub>1</sub>/amd).

IV. From 710 °C, AlPO<sub>4</sub> and TiO<sub>2</sub> disappear while LiTiOPO<sub>4</sub> (P-1) forms.

V. At 770 °C, Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> (P12<sub>1</sub>/n1) forms.

VI. At 810 °C, Li<sub>3</sub>PO<sub>4</sub> and Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> disappear again and LiTiOPO<sub>4</sub> undergoes a phase transition from the P-1 to the Pnma polymorph.
Rietveld refinement was performed on selected diffractograms measured every 100 °C for a longer time in order to quantify the amount of each phase. The data is presented in Figure 2b and enables a better visualization of the evolution of each phase. It appears clearly that part of the LATP phase is consumed to form the final impurities. However, new intermediate species Li₃PO₄ and TiO₂ have been identified that participate to the reaction between LATP and LiBr.

### 3.3. The fate of bromine.

To understand the disappearance of LiBr before reaching its melting temperature, a thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) was carried out on a LATP₀.₃ + LiBr (1:1) mixture. The mass losses measured are presented in Figure 3a. Below 150 °C, mass variations are correlated with the detection of two H₂O⁺ emissions (m/z = 18) related to the dehydration of LiBr. From 350 °C to 550 °C, an important mass loss (14.3 %) is observed with the detection of Br⁺ and Br₂⁺ fragments (m/z = 79; 81 and 160). This measured mass loss value (14.3 wt%) is close to the complete degassing of the bromine initially present in the sample (17 wt%).

a

<figure><img src="image_5.png" /></figure>

b

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

Rietveld refinement was performed on selected diffractograms measured every 100 °C for a longer time in order to quantify the amount of each phase. The data is presented in Figure 2b and enables a better visualization of the evolution of each phase. It appears clearly that part of the LATP phase is consumed to form the final impurities. However, new intermediate species Li₃PO₄ and TiO₂ have been identified that participate to the reaction between LATP and LiBr.
Figure 3: (a) TGA (black), DTG (brown) curves of a mixture of LATP<sub>0.3</sub> and LiBr (1:1) heated from 30 to 700 °C under air, and corresponding MS signals. The blue areas correspond to the dehydration of LiBr with release of water molecules (m/z = 18), whereas the purple area corresponds to the release of bromine species (m/z = 79, 81, 180). (b) DSC curves of a mixture of LATP<sub>0.3</sub> and LiBr (1:1) in air (top) and pure LiBr in Ar (bottom) to prevent hydration before the measurement.

The nature of the gas released was further confirmed as Br<sub>2</sub> by the fluorescein test (see figure S1 in SI) and the amount of remaining bromine in the sample was measured by Wavelength Dispersive X-ray Fluorescence Spectroscopy (WDXRF) on ex situ samples heated at 350, 600 and 900 °C. Results are summarized in Table 2, wherein the mass content of each atom (Ti, O, Al, Br) was normalized to the phosphorus mass content. Only the Br/P ratio is modified during the various heat treatments. According to WDXRF measurements, all the bromine is lost after heat treatment at 600 °C, confirming the results of the MS analysis.

<table><caption>Table 2: WDXRF data of LATP<sub>0.3</sub> and LiBr (1:1) samples quenched at various temperatures including 350, 600, 900 °C in air. The starting ratios are calculated from the 1:1 molar ratio of LATP<sub>0.3</sub> and LiBr.</caption><thead><tr><th>Temperature (°C)</th><th>Ti/P</th><th>O/P</th><th>Al/P</th><th>Br/P</th></tr></thead><tbody><tr><td>Starting ratios</td><td>0.87</td><td>2.06</td><td>0.08</td><td>0.93</td></tr><tr><td>350</td><td>1.06 (±0.15)</td><td>2.56 (±0.09)</td><td>0.11 (±0.03)</td><td>0.94 (±0.02)</td></tr><tr><td>600</td><td>0.88 (±0.02)</td><td>2.45 (±0.08)</td><td>0.08 (±0.03)</td><td>0.007 (±0.008)</td></tr><tr><td>900</td><td>0.91 (±0.04)</td><td>2.33 (±0.19)</td><td>0.08 (±0.01)</td><td>0 (±0.000)</td></tr></tbody></table>

The formation of bromine results from the oxidation of LiBr and is summarized by equation (2):

$$ (2) 4 \text{LiBr} + \text{O}_2 \rightarrow 2 \text{Li}_2\text{O} + 2 \text{Br}_2 $$

The role of oxygen in the reaction was confirmed by repeating the same reaction in a sealed quartz tube under vacuum, for which no reactivity between LATP and LiBr was observed (see Figure S2 in SI). Furthermore, participation of oxygen reduces the expected mass loss from 17 % to 15.2 %, which is closer to the measured value of 14.3 %. It also explains that we observe an exothermic peak at 550 °C by DSC, instead of the endothermic peak expected for the fusion of LiBr. Indeed, the formation of two molecules

The nature of the gas released was further confirmed as Br<sub>2</sub> by the fluorescein test (see figure S1 in SI) and the amount of remaining bromine in the sample was measured by Wavelength Dispersive X-ray Fluorescence Spectroscopy (WDXRF) on ex situ samples heated at 350, 600 and 900 °C. Results are summarized in Table 2, wherein the mass content of each atom (Ti, O, Al, Br) was normalized to the phosphorus mass content. Only the Br/P ratio is modified during the various heat treatments. According to WDXRF measurements, all the bromine is lost after heat treatment at 600 °C, confirming the results of the MS analysis.
of Br₂ for one molecule of O₂ consumed contributes to the large negative enthalpy of the reaction. From *in situ* XRD data (Figure 2a), LiBr completely disappears around 410 °C whereas TGA and DSC results show that it starts to react as early as 350 °C and reach its maximum close to the melting temperature of LiBr (550 °C). This difference is nested in the different experiment kinetics (see Figure S3 in SI).

### 3.4. Formation of intermediate reaction products: Li₃PO₄ and TiO₂.

The formation of Li₂O could not be observed directly with the characterization methods used. It is expected to immediately react with LATP₀.₃ to form other species following equation (3).

$$ (3) \mathrm{LiTi}_{2}(\mathrm{PO}_{4})_{3} + 4 \mathrm{Li}_{2}\mathrm{O} \rightarrow 2 \mathrm{TiO}_{2} + 3 \mathrm{Li}_{3}\mathrm{PO}_{4} $$

The formation of Li₃PO₄ is clear from the *in situ* XRD data, starting at 400 °C. However, the formation of TiO₂ is more ambiguous as only one peak at 25° is observed, superimposed to peaks of LATP. To gather additional information, Raman spectroscopy was used on LATP₀.₃ + LiBr (1:1) samples heat treated at 200, 400, 600 and 800 °C in air. All the Raman spectra are shown in Figure 4. LiBr is not visible in Raman due to its strong ionic bonding character, therefore the spectrum of the mixture without heat treatment shows only the characteristic bands of LATP₀.₃.³³⁻³⁶ Heat treatment at 200 °C does not induce any change in the LATP₀.₃ bands, while at 400 °C and 600 °C, a slight variation in the intensity of the band at 139 cm⁻¹ is observed. At 800 °C, the characteristic bands of LATP are greatly modified, and we can observe the impurities LiTiOPO₄ and Li₄P₂O₇ at 781 and 1026 cm⁻¹, respectively.³⁷,³⁸
<figure><img src="image_7.png" /><figcaption>Figure 4: Raman spectra of (a) pure LATP₀.₃, pure TiO₂ anatase and (b) mixture of LATP₀.₃ and LiBr (1:1) at room temperature and quenched from 200 °C, 400 °C, 600 °C and 800 °C.</figcaption></figure>

In the literature, the band at 139 cm⁻¹ corresponds to translational vibrations of Ti⁴⁺ from LATP₀.₃,³⁸ but it may also be superimposed with the band at 141 cm⁻¹, associated with the Ti-O bond from TiO₂ anatase.³⁹ To distinguish between the two, the variations in the integration ratio of the band at 139-141 cm⁻¹ (associated with LATP₀.₃ and TiO₂) relative to the band at 312 cm⁻¹ (associated with P-O vibrational modes of LATP₀.₃) were

a

b
monitored and summarized in Table 3. These results confirm an increase in the intensity of the band at 139-141 cm⁻¹ relative to the band at 312 cm⁻¹ between 400 and 600 °C, highlighting the formation of TiO₂. In this temperature range, Li₃PO₄ is typically formed but remains difficult to observe in the spectra of Figure 4b, due to an overlap of its bands with those of LATP₀.₃.

<table><caption>Table 3: Average integration values for the Ti-O band at ~ 139-141 cm⁻¹ (associated with LATP₀.₃ and TiO₂) versus the band at 312 cm⁻¹ associated with P-O vibrational modes of LATP₀.₃ as a function of the heat treatment temperatures.</caption><thead><tr><td>Temperature (°C)</td><td>Mean of Ti-O/ P-O</td></tr></thead><tbody><tr><td>25</td><td>0.26 (±0.03)</td></tr><tr><td>200</td><td>0.17 (±0.03)</td></tr><tr><td>400</td><td>0.96 (±0.80)</td></tr><tr><td>600</td><td>1.00 (±0.20)</td></tr></tbody></table>

These results show an increase in the intensity of the range at 139-141 cm⁻¹ relative to the band at 312 cm⁻¹ between 400 and 600 °C, highlighting the formation of TiO₂ at these temperatures. The significant standard deviation determined for the value at 400 °C is explained by the non-homogeneity of the reaction progress in the sample as shown in Figure S4 in SI. This experiment proves that TiO₂ is formed at 400 °C already, and this formation correlates well with the formation of Li₃PO₄ according to equation (3).

### 3.5. Generalization of the mechanism to other salts.

Repeating HTXRD experiments with salts of different nature systematically showed the decomposition of the salt before its melting temperature, and the formation of Li₃PO₄ (see Figure S5 in SI). This suggests that the reaction described by equation (2) can be generalized to many types of salts. All mixtures exhibit an experimental mass loss within ±1% of that expected from the decomposition reaction of the salt as presented in Figure 5. According to these results, all the lithium salts studied except Li₃PO₄ present gas release, confirming their decomposition at elevated temperatures to form Li₂O or directly Li₃PO₄. In the case of Li₃PO₄, only 1.5% mass loss is measured, which can be explained by the release of adsorbed moisture from the LATP/Li₃PO₄ mixture.

monitored and summarized in Table 3. These results confirm an increase in the intensity of the band at 139-141 cm⁻¹ relative to the band at 312 cm⁻¹ between 400 and 600 °C, highlighting the formation of TiO₂. In this temperature range, Li₃PO₄ is typically formed but remains difficult to observe in the spectra of Figure 4b, due to an overlap of its bands with those of LATP₀.₃.

<table><caption>Table 3: Average integration values for the Ti-O band at ~ 139-141 cm⁻¹ (associated with LATP₀.₃ and TiO₂) versus the band at 312 cm⁻¹ associated with P-O vibrational modes of LATP₀.₃ as a function of the heat treatment temperatures.</caption><thead><tr><td>Temperature (°C)</td><td>Mean of Ti-O/ P-O</td></tr></thead><tbody><tr><td>25</td><td>0.26 (±0.03)</td></tr><tr><td>200</td><td>0.17 (±0.03)</td></tr><tr><td>400</td><td>0.96 (±0.80)</td></tr><tr><td>600</td><td>1.00 (±0.20)</td></tr></tbody></table>

These results show an increase in the intensity of the range at 139-141 cm⁻¹ relative to the band at 312 cm⁻¹ between 400 and 600 °C, highlighting the formation of TiO₂ at these temperatures. The significant standard deviation determined for the value at 400 °C is explained by the non-homogeneity of the reaction progress in the sample as shown in Figure S4 in SI. This experiment proves that TiO₂ is formed at 400 °C already, and this formation correlates well with the formation of Li₃PO₄ according to equation (3).

### 3.5. Generalization of the mechanism to other salts.

Repeating HTXRD experiments with salts of different nature systematically showed the decomposition of the salt before its melting temperature, and the formation of Li₃PO₄ (see Figure S5 in SI). This suggests that the reaction described by equation (2) can be generalized to many types of salts. All mixtures exhibit an experimental mass loss within ±1% of that expected from the decomposition reaction of the salt as presented in Figure 5. According to these results, all the lithium salts studied except Li₃PO₄ present gas release, confirming their decomposition at elevated temperatures to form Li₂O or directly Li₃PO₄. In the case of Li₃PO₄, only 1.5% mass loss is measured, which can be explained by the release of adsorbed moisture from the LATP/Li₃PO₄ mixture.
### 3.7. What is happening to Aluminium ?

So far, we took the liberty to consider the formula $LiTi_2(PO_4)_3$ instead of the Al-doped $Li_{1.3}Al_{0.3}Ti_{1.7}(PO_4)_3$ composition to explain the reactivity mechanism. This assumption is perfectly valid as the same results are observed with $LiTi_2(PO_4):LiBr$ mixtures (see Figure S7 in SI). Let's now focus on the whereabouts of Al, present initially in $LATP_{0.3}$ and as the $AlPO_4$ impurity. No other known phases containing Al were identified from XRD and Raman experiments. Being present in small amounts in the material, we understand that its reactivity has remained elusive so far.

A first hypothesis, raised by Kwatek et al.,¹⁸,⁴¹ is the increase of the level of substitution of $Ti^{4+}$ by $Al^{3+}/Li^+$ in $LATP_{0.3}$ via the decomposition of the salt. This substitution should be marked by a modification of lattice parameters of $LATP_{0.3}$, as the substitution of $Ti^{4+}$ by $Al^{3+}$ and $Li^+$ decreases these parameter values.⁴²⁻⁴⁴ In our case, a small variation is observed for the c value, from 20.825(2) Å to 20.87(2) Å, based on the thermodiffraction results from Figure 2, when comparing the lattice parameters of $LATP_{0.3}$ before and after the thermal treatment. However, this variation is also observed for a pure $LATP_{0.3}$ sample (Figure S8 in SI). The XRD results alone are insufficient to conclude, as lattice parameter values are averaged and vary significantly in the literature for the same level of $Al^{3+}$ substitution.⁴³ Instead, the $Al^{3+}$ substitution level can be measured using Magic Angle Spin Nuclear Magnetic Resonance (MAS NMR) spectroscopy, as some studies have investigated several substitution levels for $Li_{1+x}Al_xTi_{2-x}(PO_4)_3$ ($x$ = 0-0.7).⁴⁴,⁴⁵ $ ^{31}P$, $ ^7Li$, $ ^{27}Al$ MAS NMR spectroscopy experiments were carried out on pellets composed of mixtures of $LATP_{0.3}$ and $LiBr$ (1:1), quenched in air at different temperatures including 200, 400, 600 and 800 °C.

First, $^{31}P$ (spin $l$ = 1/2) and $ ^7Li$ (spin $l$ = 3/2) MAS NMR spectra of samples are presented in Figure 6a and Figure 6b respectively. The $^{31}P$ MAS NMR spectra of a sample kept at room temperature and heated at 200 °C shows an asymmetric broad signal centered at -27 ppm. It can be assigned to the $P(OTi)_{4-n}(OAl)_n$ phosphate groups (with $n$ = 0-4) of $LATP$ and is observed in all spectra.⁴¹,⁴⁶ This signal may be superimposed to the signal of $AlPO_4$. At 400 °C and 600 °C, new signals are observed at 10.8 ppm, -3.5 ppm and -5.6 ppm. The individual spectra of known impurities $Li_3PO_4$, $LiTiOPO_4$ and $Li_4P_2O_7$
are also presented in Figure 6d, and the signal at 10.8 ppm indicates the presence of Li₃PO₄ whereas the two smaller signals correspond to Li₄P₂O₇ (2 phosphorus crystallographic sites). At 800 °C, the signal centered at -27 ppm related to the phosphate groups of LATP₀.₃ is broader: this phenomenon could indicate Al³⁺ diffusion in LATP₀.₃ by a conversion from small quantity of n in P(OTi)₄-n (OAl)n to a greater quantity. Indeed, the more (OAl) groups compared to (OTi), the more positive is position of the signal.¹⁸,⁴⁴,⁴⁵ Furthermore, the distribution of all these groups broadens the LATP₀.₃ signal. However, the spectrum is composed of several other features: two signals at -3.5 and -5.6 ppm corresponding to Li₄P₂O₇; one at -9.8 ppm indicating the formation of LiTiOPO₄ according to Figure 6d; and another one at -29.7 ppm which remains unidentified. This indicates that the broadening of the LATP signal can also be due to the signals of other unassigned species.

<figure><img src="image_8.png" /><figcaption>Figure 6: ³¹P (a), ⁷Li (b) and ²⁷Al (c) MAS NMR spectra of samples of LATP₀.₃ and LiBr (1:1) quenched from different temperatures. (d, e) MAS NMR spectra of main impurities LiTiOPO₄,</figcaption></figure>

a

b

c

d

e

f
$Li_3PO_4$ and $Li_4P_2O_7$ prepared as pure phases. (f) $^{27}$Al 2DMQMAS of the mixture of $LATP_{0.3}$ and $LiBr$ (1:1) quenched at 800 °C.

Pure samples of $LATP_{0.3}$ and $LiBr$ were analyzed by $^7$Li MAS NMR spectroscopy: two signals are observed at 0.49 ppm and -0.94 ppm for the $LATP_{0.3}$ sample, whereas only one is observed at 1.75 ppm for the $LiBr$. The narrow signal at -0.94 ppm is typical of $LATP$ with a level of substitution of $Al^{3+}$ equal to 0.3,$^{45}$ considering both lithium sites $Li1$ and $Li2$. However, the broad signal at 0.49 ppm remains unidentified and is not present in the other spectra. Only the contributions of $LiBr$ and $LATP_{0.3}$ are visible at 200 °C, while the signal related to $LiBr$ disappears at 400 °C and above. It is accompanied by the appearance of a new signal at 0.08 ppm that could be identified as $Li_3PO_4$ according to the $^{31}$P MAS NMR data. However, the signal does not fit with a sample of pure $Li_3PO_4$ presented in Figure 6e. At 800 °C, this signal disappears and another one appears at 0.21 ppm, corresponding to $Li_4P_2O_7$. The impurity $LiTiOPO_4$ is not visible as its signal is overlapped with that of Li sites from $LATP_{0.3}$. According to the literature, a higher substitution level implies a broader signal corresponding to Li sites from $LATP_{0.3}$ as the substitution of $Ti^{4+}$ by $Al^{3+}$ involves the insertion of additional $Li^+$ to be neutral. The signal is shifted only when the substitution level $x$ exceeds 0.5 in $Li_{1+x}Al_xTi_{2-x}(PO_4)_3$.$^{45}$

Finally, all $^{27}$Al (spin $l = 5/2$) MAS NMR spectra of the samples are presented in Figure 6c. A signal is expected around -15 ppm in the case of pure $LATP$ sample, corresponding to Al in octahedral coordination ($Al^{VI}$), which substitutes for Ti in the $TiO_6$ group.$^{18,45}$ The signal is present across all spectra at -14 ppm, but it is accompanied by other signals around 40 ppm. This range of chemical shift corresponds to Al sites in tetrahedral environments ($Al^{IV}$), which indicates the presence of $AlPO_4$.$^{46,47}$ For the spectrum of pure $LATP_{0.3}$ and that at 25 °C, this signal is split at 41.1 and 39 ppm, probably indicating the presence of two $AlPO_4$ with slightly different structures. From 200 °C, a single signal is present at 40.9 ppm, but it disappears at 800 °C, and a new signal appears at -17.2 ppm. Considering the proximity of this signal to that at -14 ppm, we performed a 2D Multiple Quantum Magic Angle Spinning (2D MQMAS) experiment on the sample thermally treated at 800 °C. The result of this analysis is presented in Figure 6f. Two signals are observed at -14 ppm and -17.2 ppm which originate from two sites that exhibit small quadrupolar coupling constants $C_Q$ with different environments. In the literature, this signal has been
observed when the level of Al³⁺ substitution is greater than x = 0.5 in Li₁+xTi₁-xAlx(PO₄)₃, but the authors explained it as the formation of a new phase containing Al octahedral site.⁴⁴,⁴⁵

Based on these results, the appearance of new signals in ³¹P and ²⁷Al MAS NMR can be attributed either to the formation of a new phases or to the substitution of Ti⁴⁺ by Al³⁺ and Li⁺. In the latter case, it is assumed in literature that a greater level of Al substitution does not imply any shift of LATP signals from ²⁷Al and ³¹P MAS NMR spectra. However, the impact of the Li and Al distributions within the structure has not been studied. Thus, we decided to conduct some DFT calculations in order to understand the impact of these distributions on the ²⁷Al and ³¹P chemical shifts of the signals related to LATP₀.₃.

For this purpose, different models (Figure S9 in SI) were obtained starting from the crystalline structure of Li₁-xAlxTi₂-x(PO₄)₃⁴³ and using 1×1×1 and 2×1×1 supercells in which Al atoms substitute some of the Ti positions with addition of Li atoms (in crystallographic positions of Li2-type) to keep the structure neutral. The corresponding calculated NMR spectra are presented in Figure S10 in SI. These results show that the distribution of lithium and aluminium impacts the ³¹P and ²⁷Al signals associated with LATP. In particular, the more an Al site local environment is enriched in Li, the more its δiso(²⁷Al) is shifted towards positive values and the more its Cq is important. In the sample treated at 800 °C, the new signal appears at more negative values, indicating that the lithium enrichment of one of the aluminium sites is not the cause. All in all, we could not find a model that explains both the new ³¹P and ²⁷Al contributions satisfyingly.

We further explored the possible formation of unknown impurities. In the literature, Waetzig *et al*.²³, presented the possibility of Al substitution on Ti sites in LiTiOPO₄. This substitution must be accompanied by the insertion of Li⁺ in order to keep the neutrality of the structure. However, crystallographic data for this phase has not been reported in the literature to our knowledge. As a first approach, we assumed that a new phase Li₁+xTi₁-xAlxOPO₄ would retain the same *Pnma* symmetry as LiTiOPO₄⁴⁸ and used the Bond Valence Sum (BVS) method⁴⁹ to identify potential sites where an additional lithium atom could be placed in this structure. Again, this model was not successful in reproducing the new contributions in ³¹P and ²⁷Al signals appearing at 800 °C (see Figure S11 in SI).
In view of our results, a change in Al doping level in LATP seems unlikely. For now, these
contributions and the fate of aluminium are most likely explained by the presence of a new
phase which remains unidentified.

**3.8.** $Li_4P_2O_7$ as a sintering aid without formation of impurities

The reactivity mechanism uncovered suggests that using Li₃PO₄, directly as a sintering
aid or formed *in situ* via the decomposition of another salt, systematically results in the
decomposition of LATP and the formation of LiTiOPO₄ and Li₄P₂O₇. It appears therefore
interesting to investigate the reactivity of Li₄P₂O₇ itself as a sintering aid (Figure 7). A
mixture of LATP₀.₃:Li₄P₂O₇ with 1:0.25 ratio was analyzed by TGA-DTA up to 900 °C. It
shows no mass loss over the whole range of temperature (Figure S12 in SI), and two
reversible thermal events (Figure 7a). The first peak at 632 °C is attributed to the phase
transition from the low temperature phase β-Li₄P₂O₇ to the high temperature phase α-
Li₄P₂O₇. The second happens at 768-805 °C. This temperature does not correspond to a
known structure transition of Li₄P₂O₇ and is significantly lower than its expected melting
temperature (876 °C ± 10 °C), at which no thermal event is measured.²⁹,⁴⁸,⁵⁰ In the
literature, only one study has observed these DTA signals around 770 °C, from a system
consisting of LTP and LiNO₃. The authors identified it as the melting/solidification of a
mixture made up of the solid electrolyte and Li₄P₂O₇, formed *in situ* by the reaction of
LiNO₃ with LTP.²¹ The partial solubility of LTP in Li₄P₂O₇ explains the decrease in melting
temperature of Li₄P₂O₇. Although their system is slightly different due to the use of LTP,
the reactivity of the lithium salts is similar to that with LATP at an aluminium level of x = 0.3.
In other words, this thermal event corresponds to the melting of a LATP₀.₃-Li₄P₂O₇ mixture
in our system.
<figure><img src="image_9.png" /><figcaption>Figure 7: (a) DTA curve of a mixture of LATP<sub>0.3</sub> and Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> with a ratio of 1:0.25, compared to the DTA curve of pure LATP<sub>0.3</sub> and Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub>. (b) XRD patterns of the LATP<sub>0.3</sub>:Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> mixture before and after the heat treatment at 900 °C. No decomposition products are observed after the heat treatment and Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> is preserved.</figcaption></figure>

Finally, XRD data obtained after heating a LATP<sub>0.3</sub>:Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> (1:0.25) mixture up to 900 °C reveals only LATP<sub>0.3</sub> and Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub> Bragg peaks (Figure 7b), without the formation of impurity phases. We can therefore conclude that despite the partial solubility of LATP in Li<sub>4</sub>P<sub>2</sub>O<sub>7</sub>, this happens without any decomposition reactions.
4. Discussion

Through a careful investigation of the chemical reactivity of LATP with various Li salts, we
were able to propose a unified reactivity mechanism, schematized in Figure 8. This
mechanism explains the lack of correlation between the melting point of the salt and
densification temperature raised by Waetzig et al.²³ It also clarifies the sequential
formation of impurities observed depending on the thermal treatment. It should be noted
that an excess of Li precursor during the synthesis of LATP would also lead to the
formation of such impurities. It is relatively common in solid-state synthesis protocols that
5-10% molar excess Li₂CO₃ can be used to "compensate for Li volatilization" at high
temperature. In this case, it leads to the formation of unwanted reaction products and
should therefore be controlled carefully.

<figure><img src="image_10.png" /><figcaption>Figure 8: Schematic of reactivity mechanism uncovered for (a) general salts, exemplified by LiBr, which reacts with LATP to form Li₃PO₄ and TiO₂ intermediate products, (b) Li₃PO₄ which reacts with LATP to form LiTiOPO₄ and Li₄P₂O₇, and (c) Li₄P₂O₇ which melts and partially solubilizes LATP without the formation of decomposition products.</figcaption></figure>

For the purpose of understanding the reactivity mechanism, we chose to use a larger
excess of salt (15 wt% of LiBr for a LATP₀.₃:LiBr ratio of 1:1) than in literature studies that
focus on optimization of the conductivity properties (1 to 10 wt% of Li salts). The
mechanism observed remains valid for smaller amounts of Li salt additive, as the same
impurities are observed for 1 wt% of Li₃PO₄ (Figure S13 in SI), despite being more difficult

a

b

c
to detect. It should be noted that using a much larger excess of Li salt can lead (i) to the
complete consumption of LATP and (ii) to the formation of other intermediate
decomposition products (Figure S14 in SI). This can be understood *via* the phase diagram:
Li-rich zones could result in the formation of Li₃PO₄ and Li₂Ti₃O₇, Li₄Ti₅O₁₂ or Li₂TiO₃
instead of TiO₂. One can understand that, locally, zones with a large excess of Li salt exist
which can lead to the formation of these impurities, although longer sintering times would
anyway lead to the formation of thermodynamically stable products.

The homogeneity of the starting LATP:Li salt mixture should therefore be considered with
care, as the formation of different intermediate products would certainly affect the sintering
process and the resulting microstructure. Furthermore, the nature, quantity and repartition
of the decomposition products in the sintered material will necessarily impact the
conductivity properties. All these aspects remain to be understood to fully develop the
potential of oxide-based solid electrolytes.

The whereabouts of aluminium remains elusive at this point. NMR investigations point to
the possible formation of an impurity phase containing aluminium and phosphate anions.
Furthermore, several unindexed Bragg peaks after heat treatments of LATP with LiBr
could be linked to the formation of such an impurity as these peaks are not present when
reacting LTP with LiBr (Figure S15 in SI). Further work is underway to isolate and
characterize this phase which will also play a role in the conductivity properties of densified
LATP. Recent results highlight that new crystalline phases still remain to be discovered in
the Li-Al-P-O phase diagrams.⁵¹

Finally, we have established that the use of lithium salts of different nature to decrease
the densification temperature of LATP can be rationalized by their reactivity, and the
formation of impurities upon decomposition of LATP. To decrease further this temperature
and optimize the conductivity properties of the final densified ceramic, additional studies
are underway to relate the densification behavior and ionic conductivity with the reactivity
mechanism of the salt.
## 5. Conclusion

Li salts can be used as sintering aids to decrease the densification temperature of LATP in order to facilitate its utilization in the assembly of solid-state batteries. Through careful study of the reactivity mechanism *via* thermodiffraction, NMR and Raman spectroscopies and thermal analysis, we have established a unified reactivity mechanism that explains the modification of the sintering process at lower temperature. In particular, the formation of new intermediate impurity products ($TiO_2$, $Li_3PO_4$) from the decomposition of LATP have been identified. $Li_3PO_4$ further reacts with LATP to form the final impurity products $Li_4P_2O_7$ and $LiTiOPO_4$, commonly observed in most literature reports. This work sheds new light on the reactivity of LATP and opens new perspective to process it at lower temperature by a careful selection of non-reactive salts. Such fundamental understanding will help move forwards in the development of all-solid state batteries with oxide-type solid electrolytes.
**Supporting information**

Supporting data to this article can be found online at ...

Fluorescein test results, additional X-ray diffractograms, Raman spectra and thermogravimetric analysis curves of LATP/salt reactions mixtures, high temperature X-ray diffraction data of LTP/LiBr mixture, structures of models used for DFT calculations of $^{27}$Al and $^{31}$P NMR parameters.

**Notes**

The authors declare no competing financial interest.

**Acknowledgements**

This research was funded, in whole or in part, by the French National Research Agency [ANR, Grant ANR-21-CE50-0013-01]. DFT calculations were performed using HPC resources from GENCI-IDRIS (Grant AD010-097535). The French Région Ile de France-SESAME program is also acknowledged for financial support (700 MHz NMR spectrometer). We thank all our collaborators for their kind help and discussion during this project, in particular the “Plateforme de spectroscopies vibrationnelles et optiques (PLASVO, Sorbonne Université)” for access to the HR800 Raman spectrometer and Assia Hessani and Ludovic Bellot-Gurlet (MONARIS laboratory) for their technical assistance in performing the measurements, Isabelle Génois for WDXRF, Mohamed Selmane for thermodiffraction, Sandrine Leclerc and Polytech Sorbonne for access to DSC, Dalila Seghouine for ATG-ATD and Marina Soustre for ATD.

A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions.
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