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Solid state battery tutorial

What is a battery? What is a solid-state battery? A tutorial on the solid-state battery

Batteries: general

A battery is a device that coverts chemical energy within its material constituents into electrical energy. In the process electrons are transferred from one material to another: this process is called a redox reaction because one material is reduced and another oxidized. A battery consists of three major components shown in a typical design in the following illustration.
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(Image source: Christian Julien & Gholam-Abbas Nazri, Solid State Batteries: Materials Design and Optimization, Boston, Dordrecht & London: Kluwer Academic Publishers, 1994, p. 2.)
The anode, or negative electrode, is oxidized, that is to say it releases electrons into the external circuit and ions into the electrolyte. This electrolyte has to be an ionic conductor, most commonly a liquid such as water with dissolved salts, acids or alkalis. The cathode, or positive electrode, is usually reduced, that is to say it accepts electrons from the external circuit. However, when the cathode is an intercalation compound, the reaction is different: ions migrate into the interstitial spaces of the cathode, from where they can again be extracted. Intercalation compounds are now widely used in rechargeable batteries. 


 

Theoretically speaking, the most advantageous electrode materials are light and a combination of anode and cathode material yielding high voltage and capacity. However, the choice of materials is affected also by cost and by complications caused by reactivity with other components and by polarization. For example, the goings-on at the electrode-electrolyte surface is in fact rather complex:
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(Image source: http://www.britannica.com/bcom/eb/article/

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The anode should be a good reducing agent, exhibit good conductivity and stability, and be easy and cheap to produce. Metals are most commonly used as anodes. The lightest metal, lithium, has most often been chosen. Lithium is also very electropositive (so that combined with an electronegative cathode, a large electromotive force will result). In the 1990s, rocking-chair batteries have been the subject of R&D, in which the anode is made of a non-metal compound capable of storing and exchanging a large quantity of lithium ions.

The electrolyte must be an insulator of electrons to prevent the battery cell from self-discharging. It also serves as a physical separator of the two electrodes. At the same time it must be an ionic conductor.

Cathode materials are especially important for the quality of the battery: its available energy is proportional to the cathode size and directly related to many other characteristics.


 

Secondary batteries (reversible or rechargeable batteries)

Secondary batteries are systems that are electrically rechargeable after discharge. The battery is returned to its original state (or at least something very close to it) by passing a current in the opposite direction to that of the discharge. Desirable characteristics of secondary batteries include high power density, high discharge rates, flat discharge curves, and a good low-temperature performance. Solid electrolytes are of particular interest for secondary batteries and for fuel cells.

The classical secondary battery contains two reversible solid-reactant electrodes and a liquid electrolyte: S-/L/S+. The Planté lead-acid cell is a typical example: Pb/H2SO4/PbO2. This type is most commonly employed in car batteries. A 15kg battery will deliver the 300-400A required to ignite an automobile engine. During discharge, the so-called double sulphate reaction occurs: Pb + PbO2 + 2H2SO4 ---> 2PbSO4 +2H2O (both electrodes are converted into lead sulphate). The processes at the two electrodes involve dissolution and precipitation, as opposed to solid-state ion transport or film formation. The cadmium-nickel battery is a second example of the classical secondary battery, used for heavy-duty tasks such as materials-handling trucks, mining vehicles, railway signalling, and emergency (standby) power. In addition, sealed cadmium-nickel batteries are widely used for smaller appliances, portable tools, electronic and photographic equipment, memory back-up etc. The basic electrochemistry of discharge is: 2NiOOH + 2H2O + Cd ---> 2Ni(OH)2 + Cd(OH)2. In this discharge reaction, trivalent nickel hydroxide is reduced to divalent nickel hydroxide through the consumption of water, and metallic cadmium is oxidized into cadmium hydroxide. Obviously, the charging reactions of both the lead-acid and the cadmium-nickel cells go in the opposite direction.

The reactions at cathodes made from intercalation compounds differ from those in the classical secondary batteries, although the case of the cadmium-nickel battery contains some similarities. The cathode reaction of that battery consists of an insertion of a proton into the trivalent nickel hydroxide thereby turning it divalent. The cathode structure remains NiO2 sandwich layered but the hydrogen bonding between the layers is reoriented. Intercalation compounds also allow for such an insertion, but of larger ions than the proton: usually that of lithium. Even when lithium ions, which are after all substantially larger than protons, are inserted into the intercalation compound, the latter experiences no extensive bond breakages or atomic reorganization. The compound is merely slightly expanded. A well-known intercalation reaction is the absorption of water by clay.

 

Intercalation cathode expansion with the introduction of lithium ions into the interstitial spaces.

By permission of McGraw Hill. Source: McGraw-Hill Encyclopedia of Science and Technology, entry on Intercalation.

 

Research on solid-state batteries

In 1967, researchers at Ford Motor Company at Dearborn, Michigan, discovered that so-called b-alumina (polycrystalline ceramics) conduct sodium ions very well at temperatures above 350°C. They developed a sodium/sodium-b-alumina/sulfur battery. At temperatures above 350°C the electrodes are liquid. In the mid-1990s, these batteries were still seen as having the greatest potential for electrical vehicle and for energy storage at central power plants. Unresolved problems included the short lifetime of the ceramic electrolyte and the lack of reproducibility of battery systems. There is a 10% failure in the b-alumina electrolyte, the origin of which remained in dispute.

But many other solid-state electrolytes have been developed, the most surprising of which was polymeric electrolytes, introduced by Michel Armand. This was surprising because at first ions were thought to move only in channels of the conductor, so as not to bump into its nuclei. By contrast, in polymeric and vitreous electrolytes ions diffuse within a disordered host network. Similarly, crystal solid electrolytes are made with pressed powders and not with single crystals, again revealing that solid ion conduction is feasible in a disordered host. Many new solid electrolytes operate at ambient temperatures.

Measurement of materials for battery purposes are primarily chemical, such as conductivity studies as a function of temperature. Analysis of solid-state electrolytes has involved primarily x-ray diffraction that readily yields distances of the crystal layers depicted in the above illustration, also for powders. Other techniques, such as scanning tunneling microscopy (available off-the shelf since c1990) are less well suited to powdered crystal analysis, and are only being applied now, in the year 2000. Computer simulations have been used too since the late 1980s, especially Monte Carlo. Monte Carlo is well suited to simulate the 'hopping' of ions from one interstitial space to the next, 'hopping' being one theoretical model for understanding the conduction of ions in a crystal. The utility of such modelling is primarily heuristic. Battery makers and chemists investigating specific substances for battery use are very loosely guided by theoretical principles, and rather tend to develop a great many substances, the qualities of which are then put to the test. For example, Stanley Whittingham, one of the central figures in the field, has synthesized and investigated many manganese and vanadium oxides (promising intercalation compounds) in the last two years (1999-2000).

Generally speaking, because high currents will not cross solid-solid interfaces easily, solid-state batteries are low-power density. But they make up for this by being high-energy density, easily miniaturized (even as thin films), long-lived and without marked changes in performance at high or low temperatures. And of course, a solid electrolyte cannot leak. For these reasons, solid-state batteries are well suited for electronic devices.

Reversible lithium solid-state batteries mostly have a glass electrolyte, an anode made of metallic lithium and an intercalation compound (e.g. titanium disulfide) as the cathode. One advantage of this type of battery is that the overall resistance does not increase with discharge. The emf is approximately 2V with only a slight and continuous decrease with loss of capacity. This contrasts with conventional batteries, which experience an abrupt loss of voltage without warning upon depletion. Research has been conducted in the 1990s both on the electrode and electrolyte materials. First electrodes: in the early 1990s, two companies announced commercialisation of the above-mentioned rocking-chair battery using lithium ions in D, AA, or coin-size cells. Both electrodes are made of intercalation compounds and the electrolyte is an amorphous polymer, for example: LiCoO2/El/carbon; LiNiO2/El/carbon; and LiNi0.2CoO0.8/El/carbon. The anode contains lithium safely between graphitic coke layers thus facilitating rechargeability. Lithium cobalt oxide has the same structure as and similar behaviour to titanium sulfide (see above illustration). But LiCoO2 is much more oxidizing than TiS2, and this leads to a cell emf of about 3.5V. This is almost exactly three times as much as the nickel-cadmium or the nickel-hydride batteries. With such a high emf, one cell also suffices as supply for a portable computer, rendering battery management much simpler. Secondly electrolytes: polymer electrolyte materials were first suggested by Michel Armand in the late 1970s, and this turned into the most active field in the 1990s, especially electrolytes for reversible batteries. The challenge was to find materials with higher conductivity at ambient temperatures. One might speak of several generations of polymers: 1st: high molecular weight polyethylene oxide hosts with lithium salts; 2nd: polyvinyl ethers as hosts; 3rd: electrolytes formed by trapping a low molecular weight liquid solution of a lithium salt in aprotic (this means not a proton donor) organic solvent, within the polymer matrix of a high molecular weight material.

This page was written by Arne Hessenbruch and last updated 18 December 2000.