PerspectiveMaterials Science

Using all energy in a battery

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Science  09 Jan 2015:
Vol. 347, Issue 6218, pp. 131-132
DOI: 10.1126/science.aaa2870

It is not easy to pull all the energy from a battery. For a battery to discharge, electrons and ions have to reach the same place in the active electrode material at the same moment. To reach the entire volume of the battery and maximize energy use, internal pathways for both electrons and ions must be low-resistance and continuous, connecting all regions of the battery electrode. Traditional batteries consist of a randomly distributed mixture of conductive phases within the active battery material. In these materials, bottlenecks and poor contacts may impede effective access to parts of the battery. On page 149 of this issue, Kirshenbaum et al. (1) explore a different approach, in which silver electronic pathways form on internal surfaces as the battery is discharged. The electronic pathways are well distributed throughout the electrode, improving battery performance.

Silver particles grow on the surface.

Scanning electron micrograph of a silver vanadium phosphate cathode after reduction with Li (15). The composition of the material is similar but not identical to that studied by Kirshenbaum et al.

Commercial battery electrodes (anodes and cathodes) are typically created by casting a porous powder composite (a mixture of the active material, a small amount of polymer binder, and a conductive additive such as carbon) onto a metal foil current collector. Electrons are conducted via chains of particles through the composite to the current collector. In contrast, ions move through the liquid or solid electrolyte that fills the pores of the composite. Optimization of both pathways is critical for battery performance. Although this slurry-cast electrode structure works very well, better control of the three-dimensional (3D) architecture would enhance the energy per unit mass and volume of the electrode. Cobb and Blanco recently reported an important step in this direction by creating a cathode consisting of alternating low- and high-density stripes. The low-density stripes provide higher porosity and better access for ions traveling through the liquid electrolyte into the cathode (2).

Kirshenbaum et al. now report the fabrication and performance of a silver vanadium phosphate cathode with a well-defined 3D architecture. The cathode consists of relatively dense thick pellets without binder or conductive additives. When Li+ ions and electrons move into the silver vanadium phosphate particles, V4+ is reduced to V3+ and Ag+ is reduced to metallic silver; the latter remains at the surface of the active material particles as small silver particles, presumably electrically connected by a thin layer of Ag (see the image for a micrograph following such a reaction). Under the right conditions, the silver forms an effective electronic path throughout the electrode, enhancing the insertion of Li+ into the cathode lattice and hence increasing the amount of accessible energy in the battery.

Such reduction displacement reactions, also known as conversion reactions, occur upon Li+ reaction with a wide range of binary and bimetallic oxide, fluoride, and sulfide compounds (3). These materials have potentially very high energy densities that may yield rechargeable and low-cost battery materials. The biggest challenge for practical use of such reversible conversion electrodes is the voltage penalty, where the voltage of the battery during discharge (conversion) is much less than the voltage needed for recharge of the battery (reconversion). It remains unclear how much of this voltage penalty is intrinsic and how much of it is a result of kinetic limitations that could in principle be minimized (4, 5).

Many studies have shown the advantages of intimate “wiring” of the active battery electrode particles with the electronic conducting component. Robust physical or chemical bonds formed during cosynthesis or annealing of the active particles with conductive fibers (see the second figure, panel B) give superior performance during charge-discharge cycling compared with traditional electrodes (see the second figure, panel A) (68). However, fibrous or templated 3D structures are generally difficult to form as a dense body. Conversion electrodes such as those reported by Kirshenbaum et al. (see the figure, panel C) provide improved density by forming an internal conductive network through electrochemical reaction. Bonding of the conversion particles (as in the second figure, panel C) to a conductive carbon fiber scaffold through high-temperature processing (as in the second figure, panel B) not only prevents capacity loss but also reduces the voltage penalty to recharge the battery (9).

Fabricating Li-ion battery electrodes.

(A) Traditional electrodes rely on a random arrangement of contacts and may result in parts of the battery not being accessible. (B) Electrodes with physical or chemical bonds connecting the active particles to a conductive fiber perform better, but are difficult to make into a dense structure. (C) In Kirshenbaum et al.'s study, the electrode starts as a porous monolith. Upon reaction via lithium addition, a conductive metal such as silver moves to the particle surface as a thin conductive coating and small nodules. The metal coating and nodules provide a conductive pathway for electrons along all interior surfaces. If well formed, it should penetrate the entire volume of the electrode for full access of the stored energy. (D) In a conceptual, completely optimized battery electrode, every active particle is perfectly shaped, sized, and wired to the current collector and to the solid or liquid electrolyte (not shown).

IMAGES: ANDREW SPROLES

Kirshenbaum et al. analyze the distribution of discharge products for their model cathode materials in an extraordinary level of detail. They use energy dispersive x-ray diffraction (EDXRD) to probe the intact battery. Although not noted by the authors, the results indicate a slight accumulation of metallic Ag at both faces of the electrode. This distribution is similar to that in at least one other study of a thick electrode, where the distribution was attributed to the relative transport rates of reactants from opposing directions (10).

Kirshenbaum et al. focus instead on the equally intriguing result that the silver metal distribution and discharge performance is very sensitive to the discharge rate: When the current drawn early in the discharge is reduced by a factor of 3, the metallic silver is distributed more uniformly and capacity utilization is higher. The authors attribute this sensitivity to the tendency for the active particles to crack upon rapid lithium addition, which creates more internal surface and nucleation sites for the silver clusters. Support for this idea comes from Woodford et al. (11), who have predicted a critical discharge rate based on particle size and the diffusion-induced stress above which brittle battery particles are likely to fracture.

Other advanced in situ methods are also helping researchers to visualize the complex chemical and structural changes during charge-discharge reactions. Transmission neutron diffraction analysis has revealed strong changes in the active electrode material located close to the edges of a large battery electrode sheet after substantial cycling (12). This method cannot resolve any gradients across the thickness of the battery electrodes, but can map lateral inhomogeneities along the battery area from edge to edge. Synchrotron radiation x-ray tomographic microscopy provides dramatic maps of changes in both electrode structure and chemical content with cycling. Ebner et al. recently used this method to study Li addition into SnO particles in a carbon matrix, revealing swelling and crack formation in 20-µm particles (13). Yang et al. used transmission x-ray microscopy combined with x-ray absorption near-edge structure to reveal much more subtle changes in a cycled intercalation electrode, reporting not only a distortion in the particle shape, but also a redistribution of Mn, Ni, and Co transition metals and formation of a new phase (14).

Kirshenbaum et al.'s study is an exciting step toward understanding how optimized battery electrode architectures can maximize the energy per unit volume and weight (see the second figure, panel D). Silver compounds may be too expensive for applications other than medical ones, but bimetallic polyanionic materials (crystalline or amorphous) containing Cu or Fe have promise as active electrode materials with widespread application. To further improve access to full capacity, future, thicker electrodes could also include gradients in morphology spanning the thickness of the electrode and the distance from the electrode terminal. Using the battery chemistry itself to drive the formation of the electrode structure, as shown by Kirshenbaum et al., is an elegant approach toward such an optimized structure.

References and Notes

  1. Acknowledgments: Work by N.J.D. is supported by the Center for Mesoscale Transport Properties, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). Work by J.L. is supported by the U.S. DOE, Office of Science, BES, Materials Sciences and Engineering Division. We thank A. Sproles (ORNL) for help with graphics.

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