Electrifying Off-Road Vehicles: Is 1000 [Wh/kg] Enough?

Abstract

Electrified transportation is a critical component of our decarbonized future. In the past two decades, we have made strides in decarbonizing light duty vehicles through advances in lithium-ion battery technology. With the battery fundamentals reasonably known for these passenger electric vehicles (EVs), further scientific progress is motivated by the need to electrify other transportation modes shown in Figure 1. Of these, performance metrics of batteries for electric flight (1−5) have been considerably discussed in recent years, and the first federally sponsored program─Propel 1k (6,7)─was launched this February to incentivize corresponding battery research (in comparison, most previous efforts focused using lithium-ion batteries for prototype flight operations, e.g., NASA’s X-57 (8)). Electric flight demands the most ambitious battery performance to-date: battery packs with 1000 [Wh/kg] energy density. Figure 1. Decarbonization opportunities related to different modes of transportation based on 2022 U.S. greenhouse gas emissions (underlying data are from ref (23) and discussed in section S2). Contributions from various transportation modes are shown to highlight the importance of electrifying off-road vehicles. As per Figure 1, another transportation mode with aviation-equivalent decarbonization opportunity is off-road vehicles, e.g., excavators, forest machines, cranes, etc. No systematic study exists in the scientific literature (9−12) or policy debates (13−15) identifying battery performance metrics to chart a path to electrifying off-road vehicles, and is the focus of this Viewpoint. Unlike light duty vehicles (16) and aviation (1−5) modes with well-defined vehicle performance, off-road vehicles cover a wide range of behaviors. For example, a Volvo EC950F Crawler Excavator (17) digs to a depth of about two stories (∼30 [ft]) and lifts a load equivalent to about ten pickup trucks (18) (∼50 000 [lbs]). A Caterpillar 793D Mining Truck (19) can carry twice the weight of total copper and steel in the Statue of Liberty (20) (∼250 [tons]), and a typical person is only tall enough to reach half the height of its tires (∼12 [ft] diameter). Yet another example is a Liebherr L538 Wheel Loader (21) whose bucket carries 10+ bathtubs (22) full of gravel (∼660 [gal]) at a time. To capture such a broad range of behaviors of off-road vehicles, we surveyed leading off-road vehicle manufacturers (24) for quantitative information about the energy and power requirements of these machines. We found that only two of the manufacturers─Caterpillar and Liebherr─reported enough information for our analysis. These two manufacturers represent ∼30% share of the off-road vehicles market (24) and cover a broad portfolio of vehicle performances such that an analysis based on their vehicles is representative of the off-road sector. The raw data are curated as a database and provided as Supporting Information. The key question is if we have the batteries to replace internal combustion engines in these vehicles. Each of these vehicles performs two functions: driving from one location to another; its task, e.g., digging for excavators, lifting for loaders, etc. Figure 2. Behavior of Caterpillar off-road vehicles in terms of operating time with a full fuel tank and the time needed to refuel the fuel tank using heavy duty refueling stations. For a battery powered vehicle, operating time relates to the time it takes to discharge the battery, while refueling time is the battery charging time. Representative battery powered EVs are shown alongside to emphasize that the off-road ICE vehicles represent a fundamentally different behavior than existing EVs. Raw data for these plots along with references are provided in Tables S3–S4 and the database of off-road ICE vehicles. Hand-drawn schematics of different off-road vehicle categories (“trucks” include mining and articulated trucks; “loaders” include wheel, backhoe, skid steer, track, multiterrain, and compact loaders; and “miscellaneous” category is composed of forest machine, soil compactor, wheel dozer, and wheel skidder) are also shown. This is a very stringent requirement for batteries. For comparison, equivalent specifications for the present-day passenger EV batteries (16) are also plotted in Figure 2. These batteries typically charge overnight and provide 6–8 h of run time (Table S4). For the past five years, efforts (28−32) have been underway to decrease the charging time of the passenger EV batteries to a few minutes. However, as Figure 2 reveals, present-day passenger EV batteries or its near future variants are insufficient to satisfy the requirements of off-road EVs. Hence, it is not surprising that prototype off-road EVs (33−35) running on Li-ion batteries are smaller sized and/or offer short runtimes compared to the off-road ICE vehicles. (36−38) Designing batteries that discharge very slowly over hours and days, but charge very quickly in seconds and minutes, can be the next milestone for ongoing lithium-ion battery materials research. (28) Interestingly, some system-level engineering solutions (39) do exist to relax the constraint of beyond extreme fast charging rates identified in Figure 2 and Figure S1: battery swapping, overhead catenary charging, and wireless charging. Given the rugged terrains these off-road vehicles traverse, wireless charging is infeasible. Similarly, catenary charging may only work for some off-road vehicles traversing a (semi)regular path during their operation. Of these solutions, battery swapping offers the most flexibility for off-road vehicles. For passenger EVs, battery swapping faces challenges (40,41) due to economies of scale and standardization of battery packs. However, unlike passenger EVs, there are only a few off-road vehicle manufacturers, standardization of battery packs should be easier, and battery swapping can be the leading solution. (42) These solutions can only be deployed if we have batteries that can deliver enough energy to power these off-road vehicles. Hence, we next estimate the energy density based on the engine power, P, as Figure 3. (a) Behavior of Caterpillar off-road vehicles in terms of energy density and operating time. The same vehicles as Figure 2 are shown here with identical symbols. Panel (b) shows which of these vehicles can be electrified using different energy density batteries. For comparison, representative energy density goals for ongoing battery research are identified. It is evident that about 10% of the off-road vehicles can be electrified with ongoing battery research, but we need new battery materials research to electrify the rest of these vehicles. Combining these findings, we need new kinds of batteries that are denser than 1000 [Wh/kg] and can deliver energy very slowly. Figure 4 expresses such futuristic batteries compared to battery technologies for other transportation modes (refer to Tables S4–S6). It is evident that the batteries for off-road EVs represent a very different operational regime. The present-day lithium-ion batteries cannot be repurposed for off-road EVs since corresponding materials fundamentally lack the requisite high energy densities. Figure 4. A comparison of battery requirements for different transportation modes in terms of the required energy density and operating rate during discharge. While the data for passenger EVs and electric aircrafts are borrowed from the literature (summarized in sections S4 and S5), the performance metrics for off-road EVs are estimated from data reported in Figure 3 (a) and Figure S2 (a) (respectively, right and left error bars herein Figure 4). It is evident that the off-road EVs represent very different and stringent battery requirements as compared to existing batteries or the ones expected from ongoing battery research, and we need to catalyze battery materials research to enable off-road EVs. Based on theoretical energy densities, candidate electrode materials are identified that can potentially electrify the off-road vehicles. As we imagine more energy dense batteries to electrify other transportation modes at scale, our choices are limited to a few elements from the periodic table that are lightweight as well as abundant. These choices lead to battery chemistries that theoretically represent outstanding energy densities but have proven difficult to operate reversibly due to myriad poorly understood limiting mechanisms. As a starting point, we can borrow the list of potential battery chemistries for electric flight (e.g., Table 1 in ref (5)). The choices are further constrained for the off-road EVs due to their even greater energy densities. Based on energy densities, the possibilities (46,47) reduce to the ones shown on the right-hand side of Figure 4. While superficially the slower rates may not seem like a limitation, chemical parasitic reactions, e.g., polysulfide shuttle in sulfur cathodes, (48) and other slow effects, e.g., electrolyte drying in air-breathing oxygen cathodes, (49) play a more dominant role at slower rates. Consequently, even enabling the same battery chemistry for electric flight and off-road EVs requires us to focus on understanding different fundamental behaviors of the underlying materials in order to design corresponding batteries. A well-founded concern is if the battery chemistries (i.e., electrodes) identified on the right-hand side of Figure 4 represent plausible solutions or just a theoretical exercise. With the electrode materials chosen for energy density, the remainder of the problem is identifying an appropriate electrolyte that enables reversible operation at a desired rate. In the past decade, our battery community has witnessed many solutions that were believed to be impossible for the longest time such as dendrite-free lithium metal anodes with liquid electrolytes, (50,51) lithium-ion batteries with aqueous electrolytes, (52,53) and reversible calcium electrodeposition. (54−56) In each of these, the key was to identify an appropriate electrolyte. Our understanding of electrolyte behavior, (57−64) though not complete, has grown considerably in recent years, and we should optimistically dedicate focused efforts in electrolyte research to enable these high energy density batteries. As we embark on the quest to find batteries for off-road EVs, we should be mindful that beyond meeting the energy density and C-rate targets, we will have to solve additional problems like thermal safety as we make progress. It will be curious to see if we can shorten the development time for these batteries as compared to the lithium-ion batteries with our community’s experience in successfully scaling-up different chemistries, focused research efforts, and advanced tools (65,66) at our disposal. To summarize, off-road vehicles emit an appreciable amount of greenhouse gases and are a sizable opportunity for decarbonization. While multiple off-road vehicle manufacturers and end users have pledged to electrify these vehicles, (33−35,67−71) it is not clear if the electrification of off-road vehicles is an engineering question of integrating present-day batteries or a scientific quest requiring materials research for beyond lithium-ion batteries. To answer this question, we analyzed off-road vehicles from leading manufactures and found that these vehicles represent a very unique regime of battery performance: they must be much more energy dense, i.e., ≫1000 [Wh/kg], compared to present-day lithium-ion or any near future variants as well as deliver energy at much slower rates. Given these unique performance requirements, ongoing battery research will not lead to batteries for off-road EVs, and we need dedicated battery materials research. Potentially, these batteries will also electrify medium and heavy vehicles (72−74) and contribute to multi-hour and multi-day stationary storage (75−77) given their high energy density, long operation time, and abundant electrode materials. Only a handful of battery chemistries can theoretically offer the desired energy density, and the critical scientific question for our community is to develop electrolytes that mitigate the underlying poorly understood limiting mechanisms to enable these chemistries. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.4c01276. A checklist (78) summarizing key theoretical details of the study; nomenclature; S1. Potential uncertainties associated with the present analysis; S2. Historic trends of U.S. greenhouse gas emissions; S3. Off-road electric vehicle prototypes; S4. Batteries for light duty (passenger) vehicles; S5. Batteries for electric flight; S6. Performance analysis of Liebherr off-road vehicles; S7. Can existing batteries with more frequent recharging electrify off-road vehicles? (PDF) A database of off-road ICE vehicles manufactured by Caterpillar and Liebherr (XLSX) Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html. This work is supported by the Colorado School of Mines faculty research grant. This article references 78 other publications. This article has not yet been cited by other publications.