chemistry

Lithium aluminium germanium phosphate

Lithium aluminium germanium phosphate is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Lithium aluminium germanium phosphate rather than just read about it. In short: Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is Li1+xAlxGe2-x(PO4)3. LAGP belongs to the NASICON (Sodium Super Ionic Conductors) family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries.

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Reference excerpt

Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is Li1+xAlxGe2-x(PO4)3. LAGP belongs to the NASICON (Sodium Super Ionic Conductors) family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries. Typical values of ionic conductivity in LAGP at room temperature are in the range of 10–5 - 10–4 S/cm, even if the actual value of conductivity is strongly affected by stoichiometry, microstructure, and synthesis conditions. Compared to lithium aluminium titanium phosphate (LATP), which is another phosphate-based lithium solid conductor, the absence of titanium in LAGP improves its stability towards lithium metal. In addition, phosphate-based solid electrolytes have superior stability against moisture and oxygen compared to sulfide-based electrolytes like Li10GeP2S12 (LGPS) and can be handled safely in air, thus simplifying the manufacture process. Since the best performances are encountered when the stoichiometric value of x is 0.5, the acronym LAGP usually indicates the particular composition of Li1.5Al0.5Ge1.5(PO4)3, which is also the typically used material in battery applications.

Properties

Crystal structure Lithium-containing NASICON-type crystals are described by the general formula LiM2(PO4)3, in which M stands for a metal or a metalloid (Ti, Zr, Hf, Sn, Ge), and display a complex three-dimensional network of corner-sharing MO6 octahedra and phosphate tetrahedra. Lithium ions are hosted in voids in between, which can be subdivided into three kinds of sites:

Li(1) 6-fold coordinated sites at Wyckoff 6b position; Li(2) sites at Wyckoff 18e position; Li(3) sites at Wyckoff 36f position.

In order to promote lithium conductivity at sufficiently high rates, Li(1) sites should be fully occupied and Li(2) sites should be fully empty. Li(3) sites are located between Li(1) and Li(2) sites and are occupied only when large tetravalent cations are present in the structure, such as Zr, Hf, and Sn. If some Ge4+ cations in the LiGe2(PO4)3 (LGP) structure are partially replaced by Al3+ cations, the LAGP material is obtained with the general formula Li1+xAlxGe2-x(PO4)3. The single-phase NASICON structure is stable with x between 0.1 and 0.6; when this limit is exceeded, a solid solution is no more possible and secondary phases tend to be formed. Although Ge4+ and Al3+ cations have very similar ionic radii (0.53 Å for Ge4+ vs. 0.535 Å for Al3+), cationic substitution leads to compositional disorder and promotes the incorporation of a larger amount of lithium ions to achieve electrical neutrality. Additional lithium ions can be incorporated in either Li(2) or Li(3) empty sites. In the available scientific literature, there is not a unique description of the sites available for lithium ions and of their atomic coordination, as well as of the sites directly involved during the conduction mechanism. For example, only two available sites, namely Li(1) and Li(2), are mentioned in some cases, while the Li(3) site is neither occupied nor involved in the conduction process. This results in the lack of unambiguous description of LAGP local crystal structure, especially concerning the arrangement of lithium ions and site occupancy when germanium is partially replaced by aluminium. LAGP displays a rhombohedral unit cell with a space group R3c.

Vibrational properties

Factor group analysis LAGP crystals belong to the space group D63d - R3c. The factor group analysis of NASICON-type materials with general formula MIM2IVPO4 (where MI stands for a monovalent metal ion like Na+, Li+ or K+, and MIV represents a tetravalent cation such as Ti4+, Ge4+, Sn4+, Zr4+ or Hf4+) is usually performed assuming the separation between internal vibrational modes (i.e. modes originating in PO4 units) and external modes (i.e. modes arising from the translations of the MI and MIV cations, from PO4 translations, and from PO4 librations). Focusing on internal modes only, the factor group analysis for R3c space group identifies 14 Raman-active modes for the PO4 units: 6 of these modes correspond to stretching vibrations and 8 to bending vibrations.

On the contrary, the analysis of external modes leads to many available vibrations: since the number of irreducible representations within the rhombohedral R3c space group is restricted, interactions among different modes could be expected and a clear assignment or discrimination becomes unfeasible.

Raman spectra The vibrational properties of LAGP could be directly probed using Raman spectroscopy. LAGP shows the Raman features characteristic of all the NASICON-type materials, most of which caused by the vibrational motions of PO4 units. The main spectral regions in a Raman spectrum of NASICON-type materials are summarized in the following table.

The Raman spectra of LAGP are usually characterized by broad peaks, even when the material is in its crystalline form. Indeed, both the presence of aluminium ions in place of germanium ions and the extra lithium ions introduce structural and compositional disorder in the sublattice, resulting in peak broadening.

Transport properties LAGP is a solid ionic conductor and features the two fundamental properties to be used as a solid-state electrolyte in lithium-ion batteries, namely a sufficiently high ionic conductivity and a negligible electronic conductivity. Indeed, during battery operations, LAGP should guarantee the easy and fast motion of lithium ions between cathode and anode, while preventing the transfer of electrons. As stated in the description of the crystal structure, three kinds of sites are available for hosting lithium ions in the LAGP NASICON structure, i.e. the Li(1) sites, the Li(2) sites and the Li(3) sites. Ionic conduction occurs because of hopping of lithium ions from Li(1) to Li(2) sites or across two Li(3) sites. The bottleneck to ionic motion is represented by a triangular window delimited by three oxygen atoms between Li(1) and Li(2) sites. The ionic conductivity σ {\displaystyle \sigma } in LAGP follows the usual dependency on temperature expressed by an Arrhenius-type equation, which is typical of most of solid-state ionic conductors:

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Illustrations

Worked examples

Example 1 — a first encounter with Lithium aluminium germanium phosphate

Start with the simplest possible case. Write down what Lithium aluminium germanium phosphate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lithium aluminium germanium phosphate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lithium aluminium germanium phosphate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lithium aluminium germanium phosphate

In research: Lithium aluminium germanium phosphate appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lithium aluminium germanium phosphate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lithium aluminium germanium phosphate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrochemistry, Germanium compounds, Lithium-ion batteries, so understanding it makes those chapters shorter.
In everyday life: Look for Lithium aluminium germanium phosphate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

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