Niobium: Magic Metal for Battery Anodes? - Tech Briefs

The element niobium (Nb), a transition metal, stands ready to improve the performance of one of the lithium-ion (Li-ion) battery’s confusing array of possible electrode chemistries — the LTO (lithium titanium oxide) anode, which after graphite is the second most-produced. During battery charging, lithium ions leave the positive cathode and move through the battery’s electrolyte to take up positions of higher energy in the anode. During discharge, this process reverses and drives electrons through an external circuit to power the load.

Most desired in an anode is large surface area in which a great number of lithium ions can take positions, giving potential for high energy storage. Also desirable is an open structure offering little resistance to the movement of ions, to allow rapid charge/ discharge without excess heating. The warm undersides of laptops remind us that this resistance is real. This is analogous to the loading and unloading of airliners. For maximum capacity we must put seats everywhere, but “resistance” — the difficulty of reaching your seat — is extreme. Seating area is sacrificed to provide one or more aisles to speed passenger movement.

Carbon as a Li-ion battery anode offers tremendous surface area but its volume increases and decreases as lithium ions enter and depart, producing cyclic strain that in time detaches carbon particles that may lead to battery self-discharge. Carbon anodes are potentially subject to ‘lithiation’ — the deposition of metallic lithium, a possible consequence of which is the growth of dendrites (like the growth of ‘whiskers’ sometimes seen protruding from tin plating). Dendrite growth can perforate the membrane separating anode and cathode, allowing self-discharge and runaway heating that can burst containment and ignite the organic electrolyte.

All this quickly becomes too complicated to hold the interest of non-specialists. Yet research in countless corporate and university laboratories produces steady reporting of “breakthroughs” by tech web sites, each maximizing ‘clicks’ by giving the impression that the long-hoped-for Super Battery will hit the market by Friday at the latest. When you puzzle through these announcements, you find most relate to small refinements — a new fire-re-tardant for the electrolyte or means of enhancing electrode conductivity with finely-divided graphite. Incremental improvements are important but seldom revolutionary.

There are also the enduring holy grails of battery technology, offering promise of revolutionary energy density, such as the lithium-air cell, offering 10 times the theoretical energy density of the best on today’s market, and on which IBM expended three years’ research. Or the theoretical potential of a nanosilicon anode. Their promise remains to be realized. There is a romantic urge to believe that someone (possibly resembling the comic character Gyro Gearloose), working in a basement, will stumble upon something that works.

The properties of Li-ion batteries are pictured as wheels, the length of each ‘spoke’ corresponding to one aspect of performance: cost, safety, life, specific energy storage, speed of charge, and discharge. No truly well-rounded electrode chemistry has appeared, so the business pursues several chemistries, each specialized for specific applications. Tesla has focused until recently on cathodes of NCA (nickel cobalt aluminum oxide) for their high specific energy.

Along with VW, Rivian, and Ford, Tesla has shifted to the high-current LFP (lithium iron phosphate) cathode long favored by power tool manufacturers and Chinese battery-and-vehicle maker, BYD. LFP cells offer lower specific energy and are more thermally stable. Another strategy is the hybrid battery, which seeks a “rounder wheel” by combining one battery of high specific energy chemistry with another having high current chemistry, with the latter being recharged by the former.

Anodes such as LTO, with an open-spinel chemical structure, allow high current and rapid charging with good safety and outstanding cycle life (thousands, not hundreds of cycles). But their specific energy storage capability is much inferior to that of batteries favored for camera/laptop/phone or EV applications. These qualities have given it a place in powering electric city buses on short, repetitive routes. High charge/discharge current and very high cycle life allow a six-minute charge at either end of a route to provide adequate power and longevity.

And what of niobium? Toshiba in Japan have marketed their LTO battery since 2008, while research into open-structured anodes has continued in many places. It has been discovered that a niobium titanium oxide (NTO) structure has a theoretical volume capacity (mAh/cm3) three times that of LTO, making NTO a potential competitor in the EV market. It is claimed that the resulting battery maintains 90 percent of its initial capacity after 5,000 charge/discharge cycles, retaining LTO’s ability to be rapidly recharged in temperature as low as 14 °F (-10 °C).

These new anode materials are described as Wadsley-Roth crystallographic shear structures, open enough to offer rapid, low-resistance charge/discharge, containing many lattice vacancies for lithium ions, yet not changing volume during charge/discharge as carbon does. Niobium’s ability to assume multiple oxidative states is basic to the expanded capabilities. The intensive and detailed study that has gone into such development gives hope that increasingly, improved battery electrode capabilities can be actively engineered — perhaps even by computational methods — rather than passively discovered by that old stand-by of research, trying everything.

Rapid charging capability sounds good but restrain enthusiasm until you calculate the amperage required. Can foreseeable charging stations deliver it — even with the liquid-cooled cables now being discussed? Creating a new anode composition such as NTO is only a beginning. A whole range of existing and possible future techniques must be applied to optimize the performance of its chemistry. As noted in one paper, such work “requires deep understanding of crystal and electronic structure.”

A primary supplier of ferroniobium (used in tiny amounts as a grain refining agent in HSLA [high-strength low-alloy] steels) and niobium oxide is CBMM (Companhia Brasiliera Metalugia e Mineracao) in Araxa, Brazil. On September 24, 2021, CBMM signed an agreement with Toshiba and Sojitz Corp., a Japanese trading company, to develop a Li-ion battery with NTO anode having higher specific energy yet retaining the rapid charging and long life of its LTO predecessor.

This article was written by Kevin Cameron, an editor at Cycle World magazine since 1991. Cameron also has authored many books on motorcycle engineering and technology.

This article first appeared in the September, 2022 issue of Battery & Electrification Technology Magazine.

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