Elsevier

Journal of Power Sources

Volume 196, Issue 16, 15 August 2011, Pages 6688-6694
Journal of Power Sources

Challenges for rechargeable batteries

https://doi.org/10.1016/j.jpowsour.2010.11.074Get rights and content

Abstract

Strategies for Li-ion batteries that are based on lithium-insertion compounds as cathodes are limited by the capacities of the cathode materials and by the safe charging rates for Li transport across a passivating SEI layer on a carbon-based anode. With these strategies, it is difficult to meet the commercial constraints on Li-ion batteries for plug-in-hybrid and all-electric vehicles as well as those for stationary electrical energy storage (EES) in a grid.

Existing alternative strategies include a gaseous O2 electrode in a Li/air battery and a solid sulfur (S8) cathode in a Li/S battery. We compare the projected energy densities and EES efficiencies of these cells with those of a third alternative, a Li/Fe(III)/Fe(II) cell containing a redox couple in an aqueous solution as the cathode. Preliminary measurements indicate proof of concept, but implementation of this strategy requires identification of a suitable Li+-ion electrolyte.

Introduction

The primary commercial targets for rechargeable batteries are: (1) portable electronics, (2) power tools and electric vehicles (EVs), and (3) stationary electrical energy storage (EES) for a grid supplied by wind, radiant-solar, and nuclear power. The first of these targets has already been realized commercially by the advent of the Li-ion rechargeable battery, and Li-ion batteries for power tools and small EVs are under active commercial development. The principal remaining challenge is to develop safe, rechargeable batteries for larger plug-in hybrid and all-electric vehicles (PHEVs and EVs) of larger driving range, faster charging rates, and lower cost as well as for EES for the grid.

A battery converts chemical energy stored in its two electrodes (the anode is the reductant and the cathode is the oxidant) into a discharge electronic current I = Idis at a voltage V = Vdis for a time Δt = Δtdis, and a rechargeable battery restores the chemical energy by the application of a charging current Ich at a voltage Vch over a time Δtch. The capacity of a battery delivering a current Idis is the total amount of electronic charge Q(I) transported to the cathode over the time Δtdis for a complete discharge of the chemical energy available at Idis. Resistances Rb to transport of the working ion (H+, Li+, or Na+) across the electrode/electrolyte interfaces and within the electrolyte and any insertion-compound electrodes give a voltage loss IRb = η = VOC  V, where the open-circuit voltage VOC = (μA  μC)/e is the difference of separated anode and cathode electrochemical potentials and e is the magnitude of the electronic charge. At I = 0, transport of the working ion inside the cell of a battery from the anode to the cathode is not charge-compensated by an externally delivered electronic charge, so a positive potential is created at the cathode and a negative potential at the anode until the internal electric field prevents further flow of the working ion. The battery parameters of primary interest are, therefore, its

  • maximum specific power output ImVm/wt (W kg−1 or mW g−1);

  • specific capacity Q(I)/wt (Ah kg−1 or mAh g−1) where Q=0ΔtIdt=0Qdq;

  • specific and volumetric energy densities 0QV(q)dq/wt and 0QV(q)dq/vol (W kg−1 or mW g−1 and Wh L−1);

  • cycle life (number of charge/discharge cycles at which Q(I) retains 80% of its initial value) and calendar life;

  • polarization η = VOC  V(q,t);

  • percent efficiency of EES at a given I, (100×0QVdisdq/0QVchdq).

The commercial constraints of safety and cost apply to all applications, but the required energy density versus I, calendar and cycle life, EES efficiency, and response or charging times vary greatly between batteries for electronic devices, EVs, and grid EES. Present-day strategies for the Li-ion battery rely on liquid-carbonate electrolytes and insertion-compound cathodes. We need to address the limitations of these strategies relative to alternatives for batteries that power larger EVs and store electrical energy of the grid.

Section snippets

Electrolytes

The “window” of an electrolyte, Eg = ELUMO  EHOMO or EC  EV is the energy difference between the lowest unoccupied and the highest occupied molecular orbitals (LUMO and HOMO) of a liquid electrolyte; between the bottom of the conduction band, EC, and the top of the valence band, EV, of an inorganic solid electrolyte. An anode with a μA > ELUMO or EC can transfer electrons to the LUMO or conduction band of the electrolyte, thereby reducing it; a cathode with a μC < EHOMO or EV can receive electrons from

Alternative cathode strategies

Two alternative cathode strategies have been suggested: (1) use of gaseous O2 in a Li/air battery [20] and (2) breaking of the S–S bonds of S8 in a Li/S battery [21]. Both of these alternatives may require a Li+-ion solid-electrolyte separator to enable use of a lithium-metal anode.

The Li/air cell depends on a catalyst for both the oxygen-reduction reaction (ORR) on discharge and also for the reverse oxygen-evolution reaction (OER) on charge: 2Li+ + 2e + O2 = Li2O2. With α-MnO2 as the catalyst, the

Conclusions

In order to develop Li-ion batteries of adequate energy density for EVs and grid EES at an acceptable cost, it will be necessary to go beyond present strategies that are based on insertion-compound cathodes. Fig. 10 shows the voltage profiles versus Li+/Li0 of the available insertion-compound cathodes. The step drop of 1 V in the output voltage of the Li2x[Mn2]O4 cathode at x = 0.5 shows that oxide electrodes with a spinel framework have a capacity limited to 1 Li per 2 host cations, but the

Acknowledgments

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under contract no. DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) program. This work was also supported by the Robert A. Welch Foundation, Grant#F-1066.

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    Present address: Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, USA.

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