This article aims to explain how to draw the Li2S lewis structure, its shape, hybridization, formal charge, and the most important application of lithium sulfide as a cathode material in rechargeable lithium-sulfur batteries.
Lithium sulfide is an ionic compound that has the chemical formula Li2S.
Lithium is the lightest metal on earth. It belongs to the alkali metal group or group 1 in the periodic table with an electronic configuration of 1s22s1.
Sulfur is a non-metal which belongs to group 16 in the periodic table with an electronic configuration of [Ne] 3s23p4. It is one of the most abundant elements on earth.
Octet Rule
Before drawing the lewis structure of Li2S, we need to know about the octet rule as well as some guidelines for drawing the structure.
In 1916, two scientists, Kössel and Lewis developed a theory to understand the chemical bonding.
According to this theory, atoms tend to attain eight electrons(octet) in their outermost shell, they do so by either sharing valence electrons(covalent bonding) or transferring valence electrons, by losing or gaining(ionic bonding). This is known as the octet rule. There are some exceptions to this rule(hydrogen, helium, lithium, etc.).
In some cases, the valence electrons in the central atom can be beyond eight, too, as found in PF5 and SF6, where P has ten valence electrons, and S has 12 valence electrons. These are exceptions to the octet rule.
In the case of Li2S, sulfur has six valence electrons(3s23p4) it needs two more electrons to complete its outer shell. Since sulfur is a non-metal, it accepts two electrons, one from each lithium atom, to complete its octet.
Lithium(1s22s1) does not follow the octet rule. It is an alkali metal that gains stability by losing an electron from its 2s orbital. Losing an electron from the 2s orbital, its electronic configuration becomes 1s2 the configuration of helium( a noble gas).
li2s lewis structure
It is a diagram that shows how the lone pairs and bond pair electrons are distributed in a molecule.
Lewis’s structure uses the octet rule. Electrons are represented by dots and bonds by a dash.
The lewis structure does not represent the actual shape of the molecule but helps understand the formation of the molecules.
How to draw li2s lewis structure?
Lithium is an alkali metal, whereas sulfur is a non-metal. Li2S is an ionic compound, and there will be a transfer of electrons to complete the valence shell octet of all atoms.
To draw a Lewis structure, we need to follow some rules and guidelines.
First, we have to count the total number of valence electrons of each atom in a molecule. For example, in the case of CH4, the total number of valence electrons is eight( four valence electrons of carbon and four valence electrons contributed by four hydrogens).
Then we identify the central atom of the molecule. This is usually the atom that is either least in number or least electronegative. In CO2 and NF3, the central atoms are carbon and nitrogen, respectively, as they are the least electronegative in the given molecules.
Another way of identifying the central atom is that it might be the only one that can form more than one bond. In CH4, carbon is the central atom for the same reason.
The valence electrons have to be arranged so that every atom shares a pair of electrons and forms a bond.
Then arrange the rest of the electron pairs to form lone pairs or multiple bonds until each atom completes its octet.
In the case of anions, we add electrons according to the negative charge present on them. If an anion has a -2 charge, we will add 2 electrons.
Similarly, for a cation, the positive charge indicates the loss of an electron from the neutral atoms present in the molecule.
Here, the central atom is sulfur with six valence electrons, and lithium has one valence electron(a total of 8 valence electrons as there are two lithium atoms). Since lithium is a metal, it will donate its electron to sulfur which will accept an electron from both lithium atoms to complete its octet.
Both the lithium atoms will get a positive charge due to the loss of an electron, and sulfur will get a -2 charge by gaining two electrons.
In the case of anions, we add electrons according to the negative charge present on them. If an anion has a -2 charge, we will add 2 electrons.
Similarly, for a cation, the positive charge indicates the loss of an electron from the neutral atoms present in the molecule.
Here, the central atom is sulfur with six valence electrons, and lithium has one valence electron(a total of 8 valence electrons as there are two lithium atoms). Since lithium is a metal, it will donate its electron to sulfur which will accept an electron from both lithium atoms to complete its octet.
Both the lithium atoms will get a positive charge due to the loss of an electron, and sulfur will get a -2 charge by gaining two electrons.
The lewis electron dot structure of lithium sulfide is shown below.
Li2S shape
Li2S is a yellow-white inorganic compound that crystallizes in an antifluorite structure.
In an antifluorite structure, the cations and anions are arranged so that their locations are opposite to that of a fluorite structure.
Each lithium cation is surrounded by four sulfur anions, and each sulfur anion coordinates with eight lithium ions( anti of fluorite structure).
It exists in cubic and orthorhombic crystal structures. Orthorhombic and cubic are part of the seven crystal systems.
In cubic structures all three axis are equal in length and are perpendicular to each other (a = b = c and α = β = γ = 90°).
In orthorhombic structures, all the three axes are unequal and are all perpendicular to each other(a ≠ b ≠ c and α = β = γ = 90°).
Uses of lithium sulfide
Lithium sulfide is a key component of lithium-ion batteries that are used in electronics, electric vehicles, mobiles and smartphones, laptops, digital cameras, etc.
Lithium sulfide is used in the cathode material or as a solid electrolyte precursor in high-performance lithium sulfide batteries.
Lithium-sulfur batteries have the potential to offer cheaper, cleaner, and faster-charging energy storage solutions than lithium-ion batteries. They can be recharged hundreds of times.
Lithium-sulfur cells, in theory, are better than lithium-ion cells due to higher charge density, higher temperature tolerance, and better handling of current loads, which makes lithium-sulfur batteries an exciting prospect in battery advancement.
The key strengths of lithium sulfide batteries are that it’s lightweight, safe, cost-effective, maintenance-free, and their pressure tolerance. Lithium sulfide as an electrode material solves many problems that pure sulfur electrodes face.
Formal charge-
Since different atoms have different electronegativities, electrons in a chemical bond are not shared equally.
If we forget about the electronegativities and assume that the electrons in a bond are equally shared, then the charge assigned to an atom in a molecule would be the formal charge.
If we calculate the number of valence electrons in a free atom(not bonded to any other atom or isolated atom) and then calculate the number of valence electrons to that atom in a molecule, the difference will give us the formal charge on that atom.
It is a hypothetical charge and does not represent the actual electron density distribution.
The formal charge helps predict the most stable lewis structure in case of resonance where more than one lewis structure is possible.
Lewis structures with low formal charge are the most stable.
The formal charge is calculated by the formula, V-1/2[B] -N
Here V is the total number of valence electrons on free atoms, B is the total number of shared electrons, and N is the total number of non-bonded unshared electrons.
The formal charge is calculated for covalent bonds. Since lithium sulfide forms an ionic bond, there is no sharing of electrons(which we require to calculate the formal charge); therefore, we cannot calculate the formal charge of Li2S.
Hybridization of Li2S
Hybridization is an important concept in which atomic orbitals combine to form new orbitals called hybrid orbitals, which have different shapes and energy than the atomic orbitals.
Lithium sulfide does not undergo hybridization.
It explains the molecular geometry and atomic properties in bonding.
The concept of hybridization is used only in covalent bonding(the bonding in which valence electrons are shared).
When two or more atomic orbitals overlap(partially), a covalent bond is formed.
In ionic bonds, there is no hybridization (no overlapping of bonds). There is only a transfer of electrons due to the high electronegativity difference between atoms.
Stability of Lithium sulfide batteries
In lithium-sulfide batteries, sulfur undergoes a series of changes in composition and structure during each battery cycle.
This leads to difficulty in maintaining the stability of the electrode, utilization of the active material, and good battery efficiency.
The anode in lithium-sulfur batteries has solid lithium metal as anode. A lithium atom detaches itself from the metal and travels to the cathode via the electrolyte. It carries charge while doing this. This is called charging.
It travels back to the anode via the electrolyte, called discharging.
Lithium likes to bond to the elements present in the electrolyte when it is moving back to the anode. This happens with each battery cycle.
This makes the lithium-sulfide battery unstable, and eventually, it just dies.
To make them stable, researchers are trying to modify the chemical composition of the electrolyte and the lithium anode.
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Hello, I am Sakshi Anand completed my Master’s in Chemistry. My area of specialization is Inorganic Chemistry. I am here to make chemistry easy and enjoyable to read. Complex ideas do not require complex language. I am an avid reader and enjoy researching intensively.