Batteries have moving parts

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The race is on to make lithium-ion batteries safer, to increase the amount of energy that can be drawn from these devices, and to reduce the time it takes to recharge them.

Transistors and other electronic components depend on the movement of electrons, which are effectively massless and dimensionless to the semiconductor, metal, and dopant atoms surrounding them. A battery, on the other hand, is an electrochemical cell, using the movement of positive ions to store and release an electrical charge. Unlike electrons, ions have mass and take up space. Charge / discharge rates, battery capacity, and most other aspects of battery performance are consequences of the massive nature of ions.

Lithium-ion batteries, in particular, store charge by extracting lithium ions from a positive electrode into an electrolyte, then intercalating them into a negative electrode. When the battery is discharged, the reverse happens. The positive electrode is generally referred to as the cathode and the negative electrode is the anode. (This terminology is only strictly correct when the battery is discharging, but it is used here in accordance with industry practice.)

Anode and cathode materials are discussed in more detail below, but in most cases, electrode materials are not good conductors of electrons or ions. So, rather than depending on slow transport through bulk materials, typical electrode designs combine active particles engineered with a polymer binder and, if necessary, conductive carbon particles. The structure is deliberately porous to allow infiltration by the electrolyte.

The interactions between the electrolyte and the electrodes depend on the specific materials involved. But in general, designers should take into account the chemical reactions between the electrodes, the electrolyte, and potential contaminants such as moisture and metal particles. As the battery charge / discharge cycles move the ions back and forth, the electrode particles expand and contract. Cracks and mechanical failures tend to occur over the life of the battery. Mechanical stress can also disconnect the electrode particles from the carbon conductive particles, increasing the resistance of the cell.

At the system level, the structure of the electrodes contributes to specific capacity (Wh / kg), charge and discharge rates, and battery life. Among the lithium ions present in the cathode material, which fraction can be extracted and transferred to the anode? How much energy supplied by the charging circuit is available to extract the ions and how much is lost due to resistive heating? How fast does ion extraction and transfer take place? And how do these characteristics change over the life of the battery?

Battery management systems are outside the scope of this article, but part of their role is to balance the charge / discharge cycles of individual cells to maximize capacity and life without compromising performance or safety. Beyond the general principle that battery fires are undesirable, specific details depend on the battery application.


Fig. 1: Charge transfer mechanism for lithium-ion batteries to protect against overcharging. Source: Argonne National Laboratory

Anodes, electrolytes and separators
Lithium-ion batteries depend on lithium compounds rather than lithium metal for safety reasons. While early designs used lithium metal, it is prone to dendrite growth, causing shorts and fires. Today most anodes are LiC based6, intercalating lithium in C6 rings. Overloads and other battery management failures, however, can allow dendrites to grow, as intercalation is only slightly more energy-friendly. Since overcharging forces more ions into the material than the anode can hold, it can also cause the battery to swell.

The electrolyte consists of a good ionic conductor, typically LiPF6, dissolved in an organic solvent. LiPF6 degrades on exposure to moisture, forming HF acid. The acid, in turn, can release oxygen from the cathode. For this reason, the solvents are typically non-aqueous organic carbonates. These solvents are however very reactive with carbon. When the electrolyte first comes into contact with the anode, a passivation layer (SEI, or solid electrolyte interface) forms on the anode particles. This layer prevents further reactions, but also increases the internal resistance of the cell. Over time, it reduces the capacity of the battery by capturing dissolved ions from the electrolyte. The characteristics of the SEI layer are an essential factor in the design of the electrolyte.

Since lithium-ion batteries are designed to prevent the formation of free lithium metal, the electrolytic solvent is usually the most flammable part of the battery. It is the component that burns in most lithium-ion battery fires.

An insulating separator – a long sheet coiled inside the battery case – prevents the anode and cathode from coming into contact with each other. It is saturated with electrolyte and porous with lithium ions. It flexes as ions pass through it, and materials on either side expand and contract. Metal particles, whether due to dendrite formation or manufacturing contamination, can abrade the separator, ultimately puncturing it and causing a short circuit. And since the separator is polymer based, it can melt or catch fire if the battery overheats. The design of the separator is a compromise between the ability to easily pass ions through and the ability to resist abrasion and other damage.

Cathode design and manufacture
The last major component, the cathode, is at the center of most battery development research. While most commercial batteries use a LiCoO2 cathode, metallic cobalt is both expensive and toxic. In addition, the layered oxide crystal structure of LiCoO2 collapses if more than about half of the lithium is removed. Such a collapse releases oxygen, potentially igniting the electrolyte.

Many alternative electrode materials have been proposed. The objective is to find a material allowing a more complete extraction of lithium and using less cobalt. Some of the most promising alternatives are compounds rich in nickel such as Li (Ni0.8Co0.1Mn0.1) O2 (abbreviated NCM811). As Jianan Zhang, a former MIT researcher and colleagues explained in work presented at the Materials Research Society’s fall meeting, nickel-rich NCM compounds offer high energy density, but particles polycrystallines that are generally available are prone to cracking and the resulting instability. The MIT group combined metal salts with a solvent in a flame spray synthesis chamber to produce single crystal particles. By varying the heat treatment conditions, they were able to control the particle size and achieve high particle size uniformity.

A complementary project by Dries De Sloovere, former researcher and colleagues at the University of Hasselt in Belgium, fabricated “core-shell” particles, enclosing LiNi0.5Mn1.5oh4 (LNMO) and NMC622 particles in a titanium oxide shell. This coating serves as a conductive agent while also protecting the cathode material from HF dissolution. The deposition process, based on the solution deposition of a titanium-based precursor, is scalable to other shell and core materials.

Regardless of the specific electrode materials used, Chuan Cheng, senior researcher at the University of Warwick, UK, pointed out that charge rates are limited by the ability of lithium ions to propagate through the material of the l ‘electrode. The electrode is typically a homogeneous mixture of active material, conductive particles and binder. Under fast charge conditions, however, ions tend to accumulate at the separator.

Without enough time for them to propagate through the electrode, much of the available active material is not actually used. As a result, the usable capacity of the battery is reduced, while the material near the separator is subject to stress and overheating. As an alternative, this group used layer-by-layer spray deposition to vary the fractions of conductive and active material. The ideal compositional gradient depends on the material, but in general this approach improves electronic and ionic conductivity and slows battery degradation.

Beyond lithium ions
In addition to improvements to lithium-ion batteries, researchers are studying alternative designs. Sodium-ion batteries are likely to have similar characteristics, but depend on sodium, a much more abundant metal. Solid state batteries seek to replace organic solvents with a polymer or a glass-ceramic composite. A future article will examine these developments in more detail.


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