Challenges for rechargeable batteries
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
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maximum specific power output ImVm/wt (W kg−1 or mW g−1);
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specific capacity Q(I)/wt (Ah kg−1 or mAh g−1) where ;
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specific and volumetric energy densities and (W kg−1 or mW g−1 and Wh L−1);
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cycle life (number of charge/discharge cycles at which Q(I) retains 80% of its initial value) and calendar life;
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polarization η = VOC − V(q,t);
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percent efficiency of EES at a given I, .
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.