Batteries Have Moving Parts

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

Transistors and other electronic components depend on the movement of electrons, which are effectively massless and dimensionless relative to the semiconductor, metal, and dopant atoms that surround them. A battery, in contrast, is an electrochemical cell, using the movement of positive ions to store and release electrical charge. Unlike electrons, ions have mass and occupy 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, and then intercalating them into a negative electrode. When the battery is discharged, the reverse occurs. The positive electrode is typically called the cathode, and the negative electrode is the anode. (This terminology is strictly accurate only when the battery is discharging, but is used here in accordance with industry practice.)

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

Interactions between the electrolyte and the electrodes depend on the specific materials involved. But in general, designers need to consider chemical reactions between the electrodes, the electrolyte, and potential contaminants like moisture and metallic particles. As battery charge/discharge cycles move ions back and forth, the electrode particles expand and contract. Cracking and mechanical failures tend to occur over the life of the battery. Mechanical stress also can disconnect the electrode particles from carbon conducting particles, increasing cell resistance.

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

Battery management systems lie outside the scope of this article, but part of their role is to balance charge/discharge cycles for individual cells in order to maximize capacity and lifetime without compromising performance or safety. Beyond the general principle that battery fires are undesirable, the 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, leading to short circuits and fires. Today, most anodes are based on LiC₆, intercalating lithium into C₆ rings. Overcharging and other battery management failures can still allow dendrite growth though, as intercalation is only slightly more energetically favorable. Because overcharging forces more ions into the material than the anode can accommodate, it also can 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 liberate oxygen from the cathode. For this reason, solvents are typically non-aqueous organic carbonates. These solvents are highly reactive with carbon, though. When the electrolyte first comes in contact with the anode, a passivating 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 electrolyte design.

Because lithium-ion batteries are designed to avoid the formation of free lithium metal, the electrolyte solvent is typically the most flammable part of the battery. It is the component actually burning in most lithium-ion battery fires.

An insulating separator — a long sheet coiled up inside the battery case — keeps the anode and cathode from coming in contact with each other. It is saturated with electrolyte and porous to lithium ions. It flexes as ions pass through it and the 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 leading to a short circuit. And because the separator is polymer-based, it can melt or ignite if the battery overheats. Separator design is a tradeoff between the ability to pass ions easily and the ability to resist abrasion and other damage.

Cathode design and manufacturing
The last major component, the cathode, is the focus of most battery development research. While most commercial batteries use a LiCoO₂ cathode, cobalt metal is both expensive and toxic. Moreover, the layered oxide crystal structure of LiCoO₂ collapses if more than about half of the lithium is removed. Such a collapse liberates oxygen, potentially igniting the electrolyte.

Numerous alternative electrode materials have been proposed. The goal is to find a material that allows more complete lithium extraction and uses less cobalt. Some of the most promising alternatives are nickel-rich compounds such as Li(Ni_0.8Co_0.1Mn_0.1)O₂ (abbreviated as NCM811). As Jianan Zhang, a former MIT researcher and his colleagues explained in work presented at this year’s Materials Research Society Fall Meeting, nickel-rich NCM compounds offer high energy density, but the polycrystalline particles that are typically available are prone to cracking and resulting instability. The MIT group combined metal salts with a solvent in a flame spray synthesis chamber to produce monocrystalline particles. By varying heat treatment conditions, they were able to control the particle size and achieve high particle size uniformity.

A complementary project by Dries De Sloovere, a former researcher and colleagues at Belgium’s Hasselt University fabricated “core-shell” particles, encasing LiNi_0.5Mn_1.5O₄ (LNMO) and NMC622 particles in a titanium oxide shell. This coating serves as a conductive agent while also protecting the cathode material from dissolution by HF. The deposition process, based on solution deposition of a titanium-based precursor, is extensible to other shell and core materials.

Regardless of the specific electrode materials being used, Chuan Cheng, senior teaching fellow at the University of Warwick, U.K., pointed out that charging rates are limited by the ability of lithium ions to propagate through the electrode material. The electrode is typically a homogenous blend of active material, conductive particles, and binder. Under fast charging conditions, though, 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 utilized. As a result, the usable capacity of the battery is reduced, while the material near the separator is prone to stress and overheating. As an alternative, this group used layer-by-layer spray deposition to vary the fractions of conducting and active material. The ideal composition gradient depends on the material, but in general this approach improves electron and ion conductivity and slows battery degradation.

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