Technical review outlines challenges for both batteries and fuel cells as basis for electric vehicles
In an open-access invited review for the Journal of the Electrochemical Society, Oliver Gröger (earlier post), Volkswagen AG; Dr. Hubert A. Gasteiger, Chair of Technical Electrochemistry, Technische Universität München; and Dr. Jens-Peter Suchsland, SolviCore GmbH, delve into the technological barriers for all-electric vehicles—battery-electric or PEM fuel cell vehicles.
They begin by observing that the EU’s goal of 95 gCO2/km fleet average emissions by 2020 can only be met by means of extended range electric vehicles or all-electric vehicles in combination with the integration of renewable energy (e.g., wind and solar). Based on other studies, they note that without an increasing percentage of renewables in the European electricity generation mix, the only vehicle concept which could meet the 95 gCO2/km target is the pure battery electric vehicles. (Hydrogen produced via electrolysis using the EU mix or by natural gas reforming would exceed the target.)
Theoretically, with renewable electricity, the 95 gCO2/km target could also be met by extended range electric vehicles with 40 miles all-electric range if 50% of driving is powered by the battery, or by fuel cell electric vehicles (FECVs), with hydrogen produced by water electrolysis.
While these propulsion concepts look promising, their contribution to CO2 emission savings in the transportation sector would only be meaningful if their market penetration were substantial. In the absence of government regulations, the latter largely hinges on consumer acceptance, which in turn strongly depends on cost. In addition, in the case of BEVs, recent studies clearly showed that BEV driving range (closely followed by cost) are the predominant variables determining consumer acceptance.
Since vehicle cost and range largely control market penetration, we will first provide a rough estimate of the cost/range projected for BEVs and FCEVs. Next, we will briefly review the current status and the expected future progress in lithium ion battery (LiB) technology, which is currently used to power BEVs. This will be followed by an assessment of the perceived technological barriers and the potential energy density gains for so-called post-LiBs, namely lithium-oxygen and lithium-sulfur batteries. Last, we will discuss the materials development challenges for FCEVs, focusing on approaches to reduce platinum catalyst loadings and to improve fuel cell durability.—Gröger et al.
(Volkswagen’s Gröger, who holds a number of battery patents, was responsible for the Li-sulfur section of the paper.)
As others, including automakers such as Toyota, have done, Gröger and his colleagues suggest that that BEVs will be the preferred option for short-range vehicles, while FCEVs are more suitable for large driving distances. In explaining this, they note that:
Since the societal value of electromobility requires substantial market penetration, it hinges on consumer acceptance of all-electric vehicles. … the main obstacles to BEV consumer acceptance are driving range and cost. The observed paradox with regards to driving range is the difference between actually needed and preferred driving range, which is related to several factors: i) inaccurate understanding of needed driving range; ii) habitually large driving range of conventional vehicles; iii) so-called “range anxiety”, i.e., the fear of getting stranded; and, iv) lack of experience with limited-range vehicles.
… Since driving range is a critical factor determining market penetration, it is useful to briefly examine the options to increase the actual or the perceived range of all-electric vehicles… Quite clearly, increasing the gravimetric and volumetric energy density by means of advanced LiBs and so-called post-LiBs would be the most straightforward way toward long-range BEVs; alternatively, H2-powered FECVs would be another path toward long-range all-electric vehicles. Driving range could also be increased by reducing energy consumption per mile … which can be accomplished by the use of light-weight materials (e.g., carbon composite chassis explored by BMW), by improvements in electric-drive efficiency, and by advanced vehicle climatization concepts (e.g., for the UDDS (urban dynamometer drive schedule) drive cycle, BEV driving range reductions by ≈17 and ≈50% have been reported for cooling and heating, respectively).
In some studies it has been noted that the adequate BEV range perceived by the customer could be lower if the recharging time would be sufficiently short. Therefore, companies have been establishing fast-charging stations (ranging, e.g., from 24 kW by BMW to 120 kW by Tesla). For a 100 mile-range BEV requiring ≈21 kWhnet, complete recharge could be accomplished within ≈60 min. using a 24 kW charging station and within ≈12 min. using a 120 kW charging station (assuming 90% charging efficiency). … On the other hand, FCEVs requiring ≈5 kg H2 for 300 miles range can be refilled within conventional filling times (≈8 min. acc. to the 0.6 kgH2 /min. filling rate reported by DoE, and only 3.5 min. from 20 to 95% fill-level for Toyota’s Mirai vehicle), so that they would have similar range and refilling attributes as conventional vehicles.—Gröger et al.
In either case, however, cost and durability challenges still remain and at least evolutionary technology advances are still required, the authors go on to explain. Some of their main points, backed up by references to the literature, with respect to major technology options include:
Despite the enormous amount of research and development efforts put into Li-ion batteries, specific energies significantly larger than 0.25 kWhname-plate/kgbattery-system are not yet on the horizon. As a result, lithium-ion battery-based BEVs with 200 miles range or more will probably be out of scope for mid-size car market/pricing that would be attractive for mass adoption.
With respect to one of the targeted post-LiB systems, the Li-air or Li-O2 battery, no stable electrode components and electrolytes which would result in a demonstrated/reproducible 100% O2-recovery over a Li-O2 battery charge/discharge cycle have yet been confirmed. Therefore, the authors note, further fundamental research and materials development is required to determine the viability of Li-O2 batteries.
If successful, the expected gains in specific energy of a practical battery-system would likely be not better than ≈1.5–fold compared to advanced lithium-ion batteries (Si/C-composite anodes with HE-NMC, NMC811, or NCA cathodes), they suggest.
On the Li-sulfur side, it is difficult to achieve the expected gravimetric energy density from a lithium sulfur battery-system. Along with that, the requirements of the automotive industry have also changed over the years, with increased focus on volumetric energy density rather than only gravimetric energy density. The achievable volumetric energy densities for lithium—sulfur batteries, independent of the anode, will always be substantially lower than that of lithium ion batteries, the authors observe.
To deliver cell-level gravimetric energy densities competitive with advanced LiBs (Si-anode and HE-NMC, nickel-rich NMC811, or NCA), lithium-sulfur batteries would require relatively large areal capacities (≥4 mAh/cm2) and high cathode sulfur content (≥60 %wt). Such Li-S batteries with silicon anodes could reach 350–400 Wh/kgcell—at best ≈1.3-fold larger than the values projected for advanced LiBs.
450– 500 Wh/kgcell could be obtained using a Li-metal anode—assuming the issues of safety and durability were dealt with successfully. This is ≈1.3-fold larger than the gravimetric energy densities for advanced LiB cathode materials coupled with lithium anodes.
In terms of volumetric energy density, lithium-sulfur battery cells are definitively inferior to LiBs. However, with regards to cost, lithium-sulfur batteries might be superior, if the additional components which might be needed to improve cycle-life and safety (diffusion barriers, etc.) can be realized at low cost, the authors suggest.
Further using silicon anodes instead of metallic lithium might enable higher power densities and longer cycle-life, if SEI-stabilizing electrolytes/additives can be developed which prevent the continuous consumption of electrolyte during cycling. One open issue with silicon anodes in lithium-sulfur batteries is the incorporation of lithium by either industrially feasible pre-lithiation procedures or by the use of LiS- rather than S-cathodes.
For hydrogen fuel cells, they note that much progress has been made over the last 10 years. Concepts for platinum-based cathode catalysts with high mass activity catalysts are putting the targeted Pt loading reduction to the 10 g/FCEV in reach. The new class of Pt-alloy catalysts formed by dealloying also shows improved voltage-cycling stability, reaching the targets set by the DOE.
The challenge is now to combine these catalyst concepts with support materials with higher durability in order to ensure fuel cell performance over FCEV service life. In addition, the origin of the yet unassigned mass transport losses at low Pt loadings must be understood and mitigated, they write.
The main challenge in fuel cell membrane research seems to be to identify materials suited for higher operating temperatures and at low relative humidity in order to simplify system design, improve heat rejection, and reduce energy losses by the air compressor.
An analysis of the system-level energy density of lithium ion batteries (LiBs) suggests that the gravimetric energy density of advanced LiBs is unlikely to exceed 0.25 kWhname-plate/kgbattery-system, which would limit the range of BEVs for the compact car market/pricing to ca. 200 miles, with recharging times substantially larger than that of conventional vehicles. Whether this will suffice for a large market penetration will depend not only on the needed but also on the perceived range requirement by customers. Higher energy densities would only be possible, if one were able to develop durable and safe metallic lithium anodes. While the so-called post-LiBs, viz., lithium-air and lithium-sulfur batteries have been assumed to revolutionize battery energy storage, cell- and system-level gravimetric energy densities are not expected to substantially exceed that of advanced LiBs; volumetric energy densities will most definitely be lower.
In contrast to BEVs, H2-powered FCEVs are capable of large driving ranges (>300 miles) and can be refilled within several minutes. Besides the need for a hydrogen infrastructure based on hydrogen produced from renewable energy, a reduction of the platinum requirement per vehicle (currently ≈20–40 gPt/FCEV) still requires further development. Nevertheless, current data suggest that advanced catalysts (dealloyed Pt-alloys) are able to meet the long-term DOE activity and durability targets, but their integration into MEAs which can operate at high current densities and low Pt loadings still needs to be demonstrated.—Gröger et al.
Oliver Gröger, Hubert A. Gasteiger, and Jens-Peter Suchsland (2015) “Review—Electromobility: Batteries or Fuel Cells?”, J. Electrochem. Soc. volume 162, issue 14, A2605-A2622 doi: 10.1149/2.0211514jes