Titanium Carbide Nanolayers Investigated for Lithium Ion Batteries.

[NNadir]

The paper I will discuss in this post is this one: First-Principle Study of Li-Ion Storage of Functionalized Ti2C Monolayer with Vacancies (Liu et al, ACS Appl. Mater. Interfaces, 2018, 10 (7), pp 6369–6377)

Recently in another group here, I expressed my hostility toward the inappropriately admired Tesla car, which I regard as nothing other than more consumer junk for rich people, by noting that one of its key components is a "conflict metal," cobalt: Wow. This is different. CNN actually notices there might be an ethical problem with your Tesla.

Actually problems with cobalt and its mining have been known for a long time, and in fact, was a motivator for the development of a superior magnet to the one time most common magnet, the Alnico magnet, an alloy of aluminum nickel and cobalt. The superior magnet is the neodymium iron boride magnet.

Issues with Neodymium and its environmental impact aside - I certainly think it not only technologically superior to the Alnico magnet but also less morally onerous - the development of the neodymium iron boride magnet did address issues with the political and instability problems that arose in the Congo region in the 1970's (which was then known as "Zaire" ).

The problem of "critical metals" will not go away; it will only get worse, particularly as the best ores for many technologically important metals have been mercilessly exploited by our generation, leaving tailings, landfill and low quality ores for all future generations, barring a thermodynamically viable and logistically comprehensive and tightly organized program of recycling.

Thus when I read about research for things that may become technologically important in the future, I always do so with an eye toward sustainable materials.

The above cited paper refers to a new class of materials, called "MAXenes." The MAXenes, which like the MAX phases from which they are made have been largely discovered and developed by Michel Barsoum of the Materials Science Department of Drexel University, and Arab-American Scientist who I personally believe should probably be a candidate for the Nobel Prize, not that I am competent to decide such things. I've been fascinated by the MAX phases - which combine the best properties of metals with the best properties of ceramics - for many years now because of their refractory, chemical resistant and (in some cases) radiation resistant - even for neutrons in some forms - properties.

Barsoum has written a nice book giving an overview of these materials: MAX Phases: Properties of Machinable Ternary Carbides and Nitrides. I recommend it to scientists who might be interested in this class of materials.

A famous MAX Phase is a ternary alloy of titanium, silicon and carbon, all of which are abundant elements not subject to depletion in the next few centuries, if ever. Another is a ternary alloy of titanium, aluminum, and carbon, also earth abundant elements.

These alloys are highly structured and highly layered. If the discrete aluminum layers of the titanium aluminum carbide are dissolved in HF, what results is monolayers of titanium carbide.

Here, from the cited paper, is a graphic representation of the resulting titanium carbide layered "MAXene" phase:



The authors write:

Two-dimensional (2D) materials have attracted great interest due to their unique physical structures and chemical properties.(1-3) Graphene, as the 2D honeycomb-like material with monolayer carbon atoms, exhibits good electrochemical performances in application for energy storage.(4, 5) Other free-standing 2D materials, such as silicene(6) and MoS2(7) monolayers, have also been proved to be promising anode materials for lithium-ion batteries (LIBs). Recently, a new family of 2D transition metal carbides and/or nitrides (MXene) was synthesized by extracting the “A” element from the MAX phases by hydrofluoric acid (HF) solutions.(8) The MAX phases can be described with a general formula Mn+1AXn, where “M” stands for an early transition metal, “A” represents an A-group (mainly IIIA and IVA) element, “X” denotes C and/or N, and “n” can be 1, 2, or 3.(9) Because MXene is usually etched in HF solution, it has a mixture of O–, F–, and OH– terminations,(8) which are definitely crucial for the distinctive properties.(10, 11) Nonterminated MXenes are yet to be synthesized. For the sake of brevity, this is usually denoted as Mn+1XnTx, where Tx stands for the surface terminations. As reported in the reviews,(12) more than 15 different MXene compositions have subsequently been synthesized, like Ti2CTx, V2CTx, Nb2CTx, Mo2CTx, Ti3C2Tx, Ta4C3Tx, and (Ti0.5Nb0.5)2CTx.(13-15) There are many researches about MXenes with specific terminations (O–, F–, and OH− ) and mixed terminations.(16, 17) Via chemical treatment, thermal annealing, and mechanical exfoliation processes, the carrier transport behavior of MXene can be tuned by modifying the surface groups.(16)

Because of the versatile chemistry of MXene, it is regarded as a potential material in a variety of fields, including reinforcement for composites,(18, 19) water purification,(20-23) and energy storage.(12, 24) Many studies have shown that MXenes are promising anode materials for ion batteries due to their fast ion diffusion and good rate capability.(25-27) Ashton et al.(28) predicted that the most lightweight members of MXene family (M = Sc, Ti, V, or Cr) used in LIBs have gravimetric capacity above 400 mAh g–1, higher than that of graphite. Zhou et al.(29) calculated the theoretical capacity of Mn2C sheet as 879 mAh g–1, which is greater than that of most 2D electrode materials of LIBs...

This paper is a computational paper, designed to evaluate the properties of these materials for use in lithium batteries which might eliminate the ethical problems of these batteries, much as neodymium reduced the ethical problem with magnets.

The authors have chosen to evaluate titanium carbide MAXenes because these are the lightest and most sustainable of this class of materials.

They concern themselves with the potential effects of minor impurities and defects, particularly along the edges of layers, stuff like what is pictured here:



The caption:

Figure 1. (a) Top view of the pristine monolayer Ti2C (5 × 5 supercell). (b) The considered adsorption sites on the surface of Ti2C and Ti2CT2 (T = F and OH) monolayers. (c–f) Side views of monolayers Ti2C, Ti2CO2, Ti2CF2, and Ti2C(OH)2, respectively

I don't have a lot of time today, and won't go into the details too much, but I'll simply jump to the authors conclusions:

In summary, using first-principle calculations, we choose functionalized Ti2C monolayer, the lightest material in MXene family, to assess the influence of intrinsic vacancies (carbon vacancy and titanium vacancy) on the Li-ion storage performance of MXene as a promising candidate for LIB electrodes. Our calculations reveal that the carbon vacancies tend to enhance the adsorption of Li in Ti2C monolayer, whereas the titanium vacancies play a similar role in Ti2CT2 when functional groups are present. The presence of vacancies further leads to a change in the diffusion behavior of lithium atoms. On the basis of our calculations of energy barriers for Li on Ti2C and Ti2CT2 monolayers, we propose an idea for mitigating the adverse effects on Li diffusion performance by regulating the surface functional groups. In the presence of VC, the surface of Ti2C monolayer is suggested of being modified with OH– functional groups due to its relatively low diffusion barrier in the range of 0.025–0.037 eV when Li diffuses around VC, whereas in the presence of VTi, the surface of Ti2C monolayer is suggested of removing the functional groups, resulting in a decrease of energy barrier of about 1 eV when Li atom diffuses around VTi. Our study may provide a guideline to improve the rate performance of Ti2C monolayers as electrode materials in LIBs, with the atomic vacancies being taken into account.

There's a long way from a computational paper to an industrially viable approach, but I thought this paper of interest because it offers some hope for solving very difficult issues in such a way as to not spit on all future generations.

It's nice to contemplate what is possible, perhaps not likely, but possible.

As I approach the end of my life, and learn more and more about what is going on, and feeling the pain of what we have done, these things seem very important in assuaging my guilt over living in the times I have lived and the role I played as a participant, albeit a very minor participant, but a participant all the same.

Have a pleasant evening.

[Anon-C]

1. You did not choose the era you were born to, of course.

You should have a sense of pride for having lived your experience as well as you could and you can. Please don't feel guilty, and have a great night.