Perspective Chapter: Design and Performance of Lithium-Ion Batteries for Achieving Electric Vehicle Takeoff, Flight, and Landing

Abstract

Today, the burgeoning drive towards global urbanization with over half the earth’s population living in cities, has created major challenges with regards to intracity and intercity transit and mobility. This problem is compounded due to the fact that almost always urbanization and increase in standard of living drives individual automobile ownerships. Over 95% of automobiles are presently powered by some form of fossil fuel and as an unintended consequence, urban centers have also been centers for peak greenhouse gas emissions, a major contributor to global climate change. A revolutionary solution to this conundrum is flight capable electric automobiles or electric aerial vehicles that can tackle both urban mobility and climate change challenges. For such advanced electric platforms, energy storage and delivery component is the vital component towards achieving takeoff, flight, cruise, and landing. The requirements and duty cycle demands on the energy storage system is drastically different when compared to the performance metrics required for terrestrial electric vehicles. As the widely deployed lithium ion-based battery systems are often the primary go-to energy storage choice in electric vehicle related applications, it is imperative that performance metrics and specifications for such batteries towards areal electric vehicles need to be established. In this nascent field, there exists ample opportunities for battery material innovations, understanding degradation mechanism, battery design, development and deployment of battery control and management systems. Thus, this chapter comprehensively discusses battery requirements and identifies battery material chemistries suitable for handling aerial electric automobile duty cycles. The chapter also discusses the battery cell-level metrics pertaining to electrochemical, chemical, mechanical, and structural parameters. Furthermore, specific models for battery degradation, state of health (SOH), capacity and models for full cell performance and degradation are also discussed here. Finally, the chapter also discusses battery safety and future directions of batteries that would power these next generation urban electric aircrafts.

1. Introduction

By the year 2050, it is projected that ∼70% of the global population will live in urban centers [1]. With the rapid rise in terrestrial automobiles (fossil fuel based) accompanying this urbanization drive, traffic congestions with high greenhouse gas emissions are unavoidable. Presently, on an average (according to a recent report based on 2019 statistics), American’s waste ∼99 h/year in traffic congestions [2], which is equivalent to a productivity loss worth ∼88-billion USD. The revolutionary concept of urban air mobility (UAM) is increasingly being considered as a potential solution to mitigate this two-pronged conundrum. Recent reports suggest that even a small fraction of traffic diversion to UAM platforms can substantially reduce the vehicle fuel due to traffic congestions [3, 4]. With the recent successes and rapid advancements in electric vehicle technologies, the concept of electric Vehicle Take-off and Landing (eVTOL) has been garnering enormous research and development (R&D) consensus [5, 6, 7]. According to a recent Roland Berger report, there are over 95 reported eVTOL projects currently under various stages of development around the world and it even predicts that commercial passenger UAM routes are potentially possible by 2025 [8]. On the other hand, steady progress and advancements spanning several decades in battery technologies (specifically Lithium-ion batteries (LIBs)) has led to advanced microelectronics to power tools and grid load leveling to electric vehicles (EV). EVs powered by LIBs are already denting the carbon footprint of major cities worldwide. These LIB packs that power modern EVs are a product of continuous R&D efforts into the constituent material and design aspects driven by a positive feedback loop as a result of considerable data obtained on modern EVs. EVTOLs however, are a nascent technology and have unique operating profiles and duty cycles which places drastically different demands on LIB packs (with regards to calendar life, energy and power densities, safety, etc.,) that power them when compared to terrestrial EVs. Typical eVTOL batteries need to operate at both high C-rates (during take-off) and low C-rates during flight with intermittent high C-rate operation during cruise and areal maneuvering (see Figure 1), thus resulting in such batteries to have longer peak-power durations than EV batteries [9]. The operation of eVTOL batteries also vary depending on the distance traveled in a single flying time. It is also essential to fast charge to get sufficient energy in passenger-swapping gaps to ensure uninterrupted eVTOL operation in busy hours specifically in the case of public UAM platforms. In other times during lazy hours, the battery can be charged at lower C-rates or other charged batteries can be used to interchange with the exhausted one. The typical eVTOL design and its battery requirements metrics are displayed in Figure 1 and detailed discussion is given later. It should be noted that such high utilization rates cause a critical challenge to battery cycle life as well as calendar life. Additionally, eVTOL batteries need to be designed in such a way that they should continue functioning and sustain the platform even after a safety incident occurs till a safe landing is achieved. In this chapter we highlight the materials selection for battery safety, fast charging capability, high -specific power, and -specific energy delivery for eVTOL applications. To meet the performance metrics for eVTOLs platforms, metrics such as energy density, power density, safety, cycle, and calendar life of battery systems need to be optimized and R & D focus should also be directed towards incorporating the energy dense and safe all solid-state batteries. Thus, this chapter highlights the opportunity for innovation in material developments, investigation and need to understand degradation mechanisms, development and deployment of battery control and management systems for eVTOL platforms. Specific objectives of this chapter include identification and down-selection strategies for battery chemistry selection for eVTOL applications, necessity, and strategies to implement predictive models for battery degradation and state-of-health (SOH), pathways to optimize advanced Li-ion materials and cell fabrication techniques for high energy and power density along with development of extensive electrochemical, chemical, mechanical, structural testing protocols towards eVTOL systems.

Figure 1.

(a) Designing of high-power density and high-energy density batteries for convenient eVTOL operation and (b) eVTOL design aspects for efficient energy utilization on boarding (multi-copter, tilt rotor, lift and cruise, and ducted vector thrust).

2. Battery metrics relevant for eVTOL applications

2.1 Specific energy/energy density

The energy density or specific energy of a lithium-ion battery depends on the capacity and operational cell voltage of the active materials, used inactive materials content and engineering design. The cell level energy density is defined by the (product of capacity and operational cell voltage)/total cell weight. Therefore, for batteries total system level energy density will be lower than the energy density at the cell level. The system level energy density can be enhanced if the use of inactive materials can be reduced. This can be achieved through the selection of appropriate materials combination. The energy landscape of various electrodes materials is depicted in Figure 2a. It can be inferred from the Figure 2a that selecting a high voltage cathode (positive electrode) and low voltage anode (negative electrode) with a high capacity will be the most favorable strategy to increase specific energy or energy density. Therefore, metallic lithium and silicon are excellent candidates to deploy as anodes for the next generation lithium-ion battery for eVTOL applications. The integration of high voltage cathode materials with the metallic lithium or silicon anode is not a straightforward process. The major barrier is to establish a suitable electrolyte which will be electrochemically stable at higher voltage and do not react with metallic lithium at lower voltage interval. The dendrite formation and shorting of the cell is the critical issue for lithium anode at high current rate either in liquid or solid electrolyte [10, 11]. It was anticipated that all solid-state battery (ASSB) can suppress the dendrite growth and protect the electrical cell shorting due to the high mechanical modulus solid electrolyte. However, the dendrite can grow through the solid electrode and can short circuit the battery cell. Because, ASSB has several interfaces such as anode | solid electrolyte (SE), cathode | SE, catholyte | active material, etc. These interface properties vary depending on the contact condition, energy states, type defects, and chemical/electrochemical stability. ASSB life and performance rely largely on these interfaces since dendrite formation, Li-depleted space-charge layer generation, and spatial variation in interfacial adhesion originate at the interfaces, which leads to battery failure. Stabilizing Li | SE interface is crucial for the development of high-energy density solid-state batteries. Current approaches in Li metal stabilization employ the energy and cost-intensive protocols that have a detrimental impact on the techno-economic feasibility of the ASSBs. Recently our group reported the facile, electrochemical protocol for improving the interfacial impedance and contact at the Li | Li_6.25Al_0.25La₃Zr₂O₁₂ (LALZO) interface [10]. Noting that the implementation of a fraction of second short duration high voltage pulse to a poorly formed interface leads to a sustained improvement in contact impedance and lower overpotentials for electrodeposition and electro-dissolution. The good news is that our group invented the operando electrochemical dendrite healing process by using the current-voltage protocol to revive the battery cell from the failure state [11]. The problem of silicon is different than the metallic lithium. The volume expansion-contraction of lithiated and delithiated silicon is about 400% due to conversion/alloy formation reaction [12]. The silicon particle is disintegrated and electronically isolated during charge-discharge cycle and resulted sever capacity fade. However, the capacity fading issue is being mitigated partially by formation of silicon composite with carbon material. The various microstructural modification and coating design have been reported for silicon electrode which could not attract commercial viability due to difficulty of scalable processing [13].

Figure 2.

(a) Energy landscape of various electrode materials: Capacity as a function of cell voltage against lithium electrode and (b) crystal structure of three main cathode materials, layered, spinel and olivine.

Crystal structure, particle size and morphology, ionic and electrode conductivity of cathode materials play important role for the practical capacity and electrochemical performances of batteries at required current rate. The cathode materials can be classified into three main crystallographic structure (Figure 2b), (i) Layer structure (ii) spinel structure and (ii) olivine structure which are intercalation electrode and volume expansion-contraction is minimum during charge-discharge process. Each of the crystal structure has inherent advantages and disadvantages in terms of capacity, stability, cell voltage and electronic and ionic transport properties. Layer structure cathode exhibits higher practical capacity than spinel and olivine structure cathode. However, spinel and some olivine phases exhibit higher cell voltage than layer structure cathode. Details comparison is given in Table 1. Sulfur is only a high-capacity conversion type cathode material which has several short comes e. g. volume expansion-contraction, electronically insulator and poly sulfide dissolution in liquid electrolyte and shuttling impact [14]. These are the very critical issues which need to be addressed before commercial applications. The power density and energy density of a battery depend on the cell fabrication and battery design aspect as well as intrinsic properties of material. The energy cell or power cell can be fabricated deliberately by designing the coating thickness. The thicker electrode can provide higher energy density and lower power density for the same electrode material. On the other hand, thinner electrode design can produce higher power density but lower energy density. However, a specific electrode material can deliver a certain rang of practical power and energy density which depends on the ionic and electronic transport properties of the material and its inherent capacity and cell voltage. The elaborate discussion about power and energy density is given in session 2.2. The power density can also be tuned by playing around the particle size and morphology of the electrode materials for conventional lithium-ion battery. In all solid-state battery, electrolyte also play an important role for the power density. The smaller electrode particles are supposed to deliver higher power density compared to the bigger particles. Noting that the battery calendar and cycle life might be varied depending on the particle size. The smaller particle has high surface energy compared to the bigger particle which could originate parasitic reaction with other components of battery and induce shortening life of battery.

		Voltage Window (V)	Nominal Capacity (mAh/g)	Current Cyclability	Worthiness for EVTOL Applications		Areas of Improvement
					Power Cell	Energy Cell
Commercial Battery Chemistries	NMC Type (LiNixMnyCozO2, x + y + z = 1)	3–4.4	>200	Excellent (>1000 Cycles)	Compatible	Compatible	Electrode formulation and cell fabrication
	NCA Type (LiNixCoyAlzO2, x + y + z = 1)	3–4.4	>200	Good (500–1000 Cycles)	Compatible	Compatible	Electrode formulation and cell fabrication
	Lithium Cobalt Oxide (LCO)	3–4.3	∼180	Excellent (>1000 Cycles)	Compatible	Not Compatible	Structural stability and plugging it up into solid-state battery
	Lithium Iron Phosphate (LFP)	2.7–4.2	165	Excellent (>1000 Cycles)	Compatible	Not Compatible	Need to focus on higher voltage olivine phases
Battery Chemistries Close to Commercialization	Spinel Type (LNMO)	3–4.95	140	Excellent (>1000 Cycles)	Compatible	Not Compatible	Development of high voltage stable electrolyte
	Li-Silicon	0.05–1	3600	Average (100–500 Cycles)	Not Compatible	Compatible	Formation of good composite and minimization of first cycle capacity fade
	Nickel Rich & Cobalt Free	3–4.4	>200	Average (100–500 Cycles)	Compatible	Compatible	Cycling performance, particle size and morphology optimization and coating
Emerging Battery Chemistries	Li-Sulfur	1.8–2.8	∼1600	Below Average (100–300 Cycles)	Not Compatible	Compatible	Translate to solid-state battery, formation of better composite electrode
	Lithium Rich Cathodes (LMR)	2–4.8	250	Below Average (100–300 Cycles)	Compatible	Compatible	Development of high voltage stable electrolyte and minimization of phase transition
	All-Solid-State Batteries	3–5	3862	Poor (100–300 Cycles)	Compatible	Compatible	Minimization of interface resistance and scalable fabrication
	Li Metal	3–4.4	3862	Average (100–500 Cycles)	Not Compatible	Compatible	Mitigation of dendrite effect
	Disordered rock salt cathodes	2–4	200–435	Average (100–500 Cycles)	Compatible	Compatible	Lower temperature synthesis and sintering.
Battery Chemistries in Lab scale R&D	Li-Air	2–4	>2000	Very Poor (<100 Cycles)	Not Compatible	Compatible	Reduction of cell polarization and improving the cyclability
	Alloying anodes	0.05–1	>2000	Very Poor (<100 Cycles)	Not Compatible	Compatible	Minimization of impact of volume expansion-contraction
	LiMPO4, M = Ni, V etc.	2.5–5	∼200	Average (100–500 Cycles)	Compatible	Compatible	Tuning the transport properties and stabilization of nano particles

Table 1.

Comparison of electrochemical properties of various electrode materials and their compatibility with eVTOL applications and suggested the possible area of improvements.

Specific energy is an important metric as the overall range or drive/flight distance of the eVTOL platform will depend on it. Higher the energy density of the constituent battery, longer the distance traveled. Moreover, high specific energies can also improve the payload of the eVTOL platform in addition to the range. The Breguet range equation provides the relationship between the trip distance and the energy density requirement of the eVTOL battery [5, 15]:

$$R_{\mathit{\text{trip}}}={\mathit{SE}}_{\mathit{\text{trip}}}\frac{L}{D}\frac{{\eta }_c}{g}{\omega }_{\mathit{bat}}$$E1

where R_trip denotes trip distance, SE_trip the specific energy consumed for the trip, and η_c the system efficiency, ω_bat is the battery weight fraction_, and L/D is the lift-to drag ratio. It is evidenced from the above equation that the trip distance R_trip is directly proportional to SE_trip and ω_bat in the asymptotic limit of long range where energy consumption during takeoff and landing becomes negligible. The specific energy and specific power consumption of eVTOL batteries is very complex when compared to that of EV batteries. The traveling distance of an eVTOL platform also depends on the design and flying mechanism (flight apparatus configuration, for example: (a) multirotor, lift and cruise, tilt rotor and ducted thrust vectoring). Yang et al. identified three possible representative model designs of eVTOL aircraft configuration considering the parameter of energy consumption [9]. Contemporary designs have the lowest L/D ratio and represents the worst-case scenario for eVTOL configurations. The best design is expected to have a high L/D-ratio for efficient cruise as well as efficient hover which denotes the best-case scenario. The energy consumption should be considered sequentially for both safe landing and possible emergency diversions. Therefore, eVTOL batteries should not discharge to state of charge (SOC) below 10%, as voltage drops drastically in this region and leads to current spikes [16]. Additionally, eVTOL batteries should have reserve energy for balked landing or diversion to alternative locations in case of emergencies. As the battery ages (calendar/cycle life increase) the same distance cannot be travel with the same battery and the range would shrink further as the battery degrades. If a battery end of life is defined as one with 25% energy lost from its initial value, the nominal travel distance will drop accordingly. Potential eVTOL services can be intercity and intracity and thus aging batteries can be replaced to the intracity service from the intercity travel where short distances and reduced payload options can be considered. The maximum use of battery can be thus secured, and economic viability of eVTOL platforms can be achieved in such a way. EVTOL services are also envisioned as city taxi, airport shuttle, and intercity flights for passenger-UAM [8]. The first two scenarios are intracity hops with short trip distances and aging batteries with near end of life from the intercity vehicle can be installed for intra-city hops with short trip distances. Areal capacity of the battery cell can also be enhanced by fabricating thicker electrode architectures which could provide higher specific energy which can assist long distance travel, but the major challenge would be insufficient specific power for eVTOL operation as this imposes a limitation of fast charge/discharge rates.

2.2 Specific power/power density

The power density of a battery depends on two major factors. The intrinsic ionic and electronic transport properties of electrodes and electrolyte materials and design architecture of battery electrodes. The higher the transport properties of active materials, higher would be the power density. Some materials which have low conductivities may have other desirable properties in terms of eVTOL applications. The low conductivity materials need to be optimized by engineering specific to microstructures, particle morphologies and wiring with conductive additives. For instance, olivine structure (LiFePO₄) electrode materials exhibit lower ionic and electronic conductivity and one directional ionic diffusivity, however, boast stable cycling performances which would be vital for long calendar life. In such cases, nano particle with flake like morphologies or other desired architectures are highly recommended for high power battery development. While this is the case for the olivine structure, micron size particles with spherical morphologies are shown to deliver better performances in the case of layered electrode materials. Sulfur is the higher capacity insulator material which needs to be composited with a conductive carbon or polymer materials. The ionic conductivity and diffusivity of electrolyte materials are also very crucial for achieving higher power density in batteries. The ionic conductivity of various widely used electrolyte materials are shown in Figure 3 as a function of operating temperature. This mode of comparison is valuable as operational temperature is also a critical parameter in battery performance for eVTOL platforms which would experience a wide range of thermal environments. The ionic conductivity of an electrolyte should lie within the red rectangle for convenient operation at a practical power density specific for eVTOL applications.

Figure 3.

Ionic conductivities of various electrolyte materials as function temperature.

The relationship between power density and energy density of lithium-ion batteries is compared in Figure 4. It is evidenced from the Figure 4 that power density is almost inversely proportional to the energy density i.e., higher the specific energy, lower is the specific power. Typically, eVTOL platforms cycle through different duty cycles with varying power demand and energy consumption while under operation. These phases typically include takeoff, hovering, climbing, cruising, descending, and landing. Yang et al., demonstrated a model of power and energy profile for a sample vehicle design for over an 80-km trip [9] which is redrawn in Figure 5. It is seen from Figure 5 that takeoff and landing hovers have the highest power demand that determines the battery’s peak discharge rate, and the cruise power defines the battery’s continuous discharge rate.

Figure 4.

Battery energy density as a function of power density consumption.

Figure 5.

The requirements of power and energy consumption in eVTOL battery for real world applications. Schematic illustration of a typical eVTOL trip (a) [17] representative battery power profile during an eVTOL trip and it depends on the vehicle design and operating pattern (B) [9] required battery specific power in hover versus in cruise for the aircraft configurations being pursued by the industry (C). Trip distance versus consumed specific energy for three representative aircraft configurations discussed in ref. [9] (D) and battery energy breakdown for eVTOL trips with specific design as mentioned in the ref. [9] (E).

$${\mathit{SP}}_{\mathit{\text{hover}}}=\frac{1}{{\omega }_{\mathit{bat}}}\frac{g}{{\eta }_h}\sqrt{\frac{\sigma }{2{\rho }_{\mathit{air}}}}$$E2

$${\mathit{SP}}_{\mathit{Cr}}=\frac{1}{{\omega }_{\mathit{bat}}}\frac{g}{{\eta }_c}\frac{V_{\mathit{Cr}}}{L/D}$$E3

It is seen from the Eqs. (2) and (3) that, the specific power (SP) depends highly on battery weight fraction (ω_bat) and aircraft configuration—disk loading (σ) for hover-power and lift-to-drag (L/D) ratio for cruise power. It should be noted that aircraft design configuration determines the consumption of specific power while hovering and cruising [18]. For example, the wing-based eVTOL design demands less power efficiency during hovering and more power efficiency on cruising, opposite is applicable for multirotor based eVTOL [19, 20]. Thus, present designs demand a trade-off between hover and cruise efficiencies.

It has been previously discussed that eVTOL batteries operate at much higher C-rates than EV batteries [9]. The average C-rate of a typical EV battery is 0.3C in high-way-driving and 0.1C in city driving, whereas a similar eVTOL battery averages around 1C over the 80 km trip and possibly even higher at hovering [9]. High specific power is required for both peak and continuous discharges which poses huge challenges to eVTOL batteries. As batteries face a power-energy trade-off an increased discharge power inevitably reduces the deliverable energy. Based on that the battery pack size should be optimized for a specific vehicle configuration to ensure sufficient energy output at the designed C-rates. On the other hand, both battery energy and power decrease substantially at freezing temperatures. Even for EVs, the cruise range could fall by <40% as the temperature drops from 24 to 7°C [21]. It is thus evident that the eVTOL batteries need to operate at higher C-rates and its operation would be more troublesome in cold weather when compared to conventional EVs.

2.3 eVTOL versus EV battery requirements

The degree of safety is an important platform specific parameter that varies based on the operational conditions of the battery. Battery for microelectronics has much more different degree of safety than batteries for EVs. On the other hand, the eVTOL batteries have more rigorous safety requirements than EV batteries in all aspects. The high cruise power leads to a larger average discharge rate for eVTOL batteries. Thus, the specific energy of eVTOL batteries should be rated at a higher C-rate (e.g., 1C) than EV batteries (C/3, even C/10). The high fast-charging frequency and utilization rates make eVTOL batteries at present, more vulnerable than EV batteries which pose huge challenges to overall battery life. It should be noted that the duty cycle induced aging is more important than calendar aging for eVTOL batteries. The major differences between eVTOL and EV batteries are compared in the Table 2.

Operational conditions	EV battery	eVTOL battery	References
Driving/cruising C-rate	Need C/3 or C/10	Need 1C	[9, 12, 13]
Size of battery	Relatively small	Relatively big
Power requirements	Relatively lower	Need higher
Peak power duration	Typically, 10s	Typically lasting 30–120 s
Battery lifetime	longer	shorter
Charging frequency	less	more

Table 2.

Comparison of EV and eVTOL batteries.

2.4 Need of fast charging

It is discussed in Eq. (1) that the eVTOL traveling range can be extended by elevating ω_bat. This is one way of increasing the travel distance. However, a higher ω_bat favorably reduces the required power density and fast charge rate. Lithium plating is big challenge for fast charging in energy-dense Li-ion cells due to the high cell polarization at an enhanced current. Energy dense cells needs thicker electrode in which fast charging and discharging event might be limited by ionic diffusion of electrolyte solution. Nonetheless, eVTOL needs larger batteries to ensure highly repeated operation during rush hours which have a small SOC consumption per trip. Therefore, the charge rate can be relaxed by recharging only partially in passenger-swapping gaps. However, larger the battery higher will be the costs and thus increasing payload demands. The aerospace batteries could cost 4.13 times more than automotive batteries on a US$/kWh basis as estimated by Uber [22].

Increasingω _bat is constrained by the payload, as:

$$m_{\mathit{pay}}=\mathit{\text{GTOM}}\left(1-{\omega }_{\mathit{bat}}-{\omega }_{\mathit{\text{empty}}}\right)$$E4

where m_pay is the payload, GTOM the gross takeoff mass, and ω_empty the empty weight fraction. It should be noted that the ω_empty is an essential parameter of an aircraft. It is seen that a lower ω_empty is favorable but can only be achieved at a higher GTOM [21]. A constant ω_empty is typically presumed in eVTOL vehicle design and optimization for the sake of simplicities. It is obvious from the Eq. (4) that the raising ω_bat without sacrificing the payload requires increasing GTOM. It is shown in the above comparison in Table 2 that the peak-power duration is typically 30–120 s for eVTOL takeoff or landing. It is estimated that annual effective flexible capacity (EFC) for eVTOL (∼1600 EFCs) is much higher than EV battery (∼45 EFCs) [9]. For fast-charging repetition frequency of eVTOL is much higher than EV battery [16] and within 5-min it should add energy from 50 to 62% SOC at the pack level, which is insufficient for eVTOL takeoff. The second fast-charging option can be adopted with a high-power electrodes design (thinner electrodes), that cannot offer sufficient specific energy for eVTOLs apparently. Furthermore, various steps have been taken to optimize charge algorithms which is simply able to manage the exclusion of Li plating without fundamentally relaxing the underlying electrochemical and transport limitations. The procedure can only incrementally enhance the fast-charging capability. A major technological challenge for eVTOL batteries is charging an adequate amount of energy in a very short time (very fast) without causing huge degradation to the batteries. In principle, Li plating can occur due to physicochemical processes at anode and electrolyte solution at higher C-rate which generate overpotential (cell polarizations). These are ion transport limitation in electrolyte solutions at high current, lithium intercalation reaction at interface between graphite-electrolyte and solid-state lithium-ion diffusion in graphite particles interior. It should be noted that ion transfer in the electrolyte is typically the limiting factor due to the thick and dense electrodes [22, 23] for high-energy LiBs. The possible solution would be enhancing electrolyte conductivity, diffusivity, and transference number [24, 25, 26, 27] by tuning composition and proper formulation of the chemical components. However, it is not straight forward to improve one parameter without sacrificing other metrics due to the sheer complexity of these systems. Some solvents and salts can enhance ionic transport properties while electrolyte stability can be considerably compromised [28, 29]. As a result, battery cycle and calendar life are reduced compared to in normal operations. Another way would be aligning the electrode particles to reduce tortuosity [30, 31, 32]. It can be inferred from the above discussion that high energy density (thicker electrode) and big size batteries are not suitable for eVTOL operation on their own. The bigger batteries will cost on payload and high energy batteries limit fast charging. Therefore, energy density and size of battery needs to be critically assessed for eVTOL operation.

2.5 Charge pattern and cycle life

The cycle life depends on the depth of charge and discharge of battery during under operation and how fast it is charged and discharged. Lithium-ion batteries for eVTOL poses some unique challenges for fast energy storage and delivery on demand requirements. The electrical powertrains and batteries for eVTOL are needed to be coordinated for the energy and power delivery and consumption. The battery needs to supply huge energy instantly while the heavy gear of eVTOL needs to be hauled into the air depending on the payload of the eVTOL. There are some limitations to how much energy a battery can store and keep the weight light enough for the aircraft to achieve easy take off and flight. Developing an energy and power dense battery is one of the key challenges facing the electric aviation industry among other factors. In the case of public transit eVTOLs, it is worth noting that these need to run during rush hours more frequently (6–10 am and 4–8 pm per workday) in order to earn a profitable return on investment. Typically, we can envision a 5–10 min gap between two trips for passenger swapping in the peak hours. Thus, it is possible to charge the battery with enough energy needed for the next trip within this time. Based on the operational pattern it is assumed that eVTOL battery may need to charge several times in a day. For a short distance trip, depth of discharge might not cause a huge issue while for the relatively long-distance trip the depth of discharge expected to be huge to cover whole distance along with landing hovering. Thus, it can be concluded that fast charging is crucial for maximizing vehicle utilization rates in order to achieve eVTOL return on investment. Uber reported detailed analysis of earning revenues and predict eVTOL profit when compared with a typical car [7]. It should be noted that most charging events of eVTOL batteries are fast charging except in slow hour. The frequent charging and discharging will reduce the calendar life of battery and depth of charge and discharge will reduce both cycle and calendar life of battery. An alternative would be interchanging the discharged battery with charged battery in the station during passengers swapping for public eVTOL transit platforms. Furthermore, an interchangeable battery approach requires piles of reserve battery packs stored at each station which raises significant capital costs, safety, and space requirements. Making battery packs interchangeable would require standardization among battery and aircraft manufacturers, and market competition for consumers [33] and thus would be great challenge to overcome to establish a viable business.

2.6 eVTOL performance models and battery dataset generation

Operating conditions of eVTOL battery are different from other battery systems, and thus, performance models need to be developed for the whole calendar life of the battery system. Machine learning is a new modeling technique which provides more reliable performance prediction. However, the development of machine learning based models requires relevant battery performance datasets to train and test the models. There are only limited number of openly available datasets which poses a challenge to bring advanced machine-learning methods to battery performance prediction specific for eVTOL applications [34, 35]. In addition, there are no openly accessible datasets which follow the characteristic eVTOL duty cycle (Take-off, Cruise, Landing, Rest and Charging). Bills et al., [36] generated an experimental battery performance dataset specific to the power requirements of an eVTOL platform in order to fill this gap. Twenty-two cells were run in a variety of operating conditions that an eVTOL might experience during normal operation. The following charge-discharge cycling profiles were applied in the same generic format in all test cases:

1. Take-off: the cell was discharged at a high constant power for a period (around 120 s at least).

2. Cruise: the cell was discharged at a lower constant power for a longer duration,

3. Landing: the cell was discharged at high constant power (same rate as takeoff) for a slightly longer time,

4. Rest: the cell was allowed to rest until it cooled to a temperature of less than 27°C and again

5. Charging: the cell was charged using a constant current-constant voltage (CCCV) charging protocol,

6. Again Rest: The cell was allowed to rest until cell temperature reached 35°C, then allowed to rest 15 minutes further before beginning the next cycle.

After every 50 cycles, using the above eVTOL protocol, the capacity of each cell was tested and discharged at constant current until its voltage was 2.5 V. Also, the cells were cycled at temperatures exceeding 70°C throughout the followed charge-discharge protocols. Noting that during each test, temperature, voltage, current and discharge-charge capacity were recorded. The baseline profile was parameterized to test different conditions which could be encountered on an eVTOL operation mission. These test parameters, however, were not enough to implement good performance modeling. Furthermore, diffusion, transport and kinetics information of the cell components/interfaces were needed and were used in this model which cannot be directly obtained from these measurements. However, physically they correspond to processes which include electrochemical reactions, reaction kinetics, and transport. A subset of the parameters of the performance model was also chosen to capture the evolution of the performance model as the battery ages [15]. It should be mentioned that the parameters of battery models were very difficult to identify using local optimization methods [37]. Details about the model, method and discussion are reported in the Reference [36].

The reported aging model for discharge curves is shown in Figure 6 which is important to model the life of the aircraft. Design decisions are thus made based on changes in the battery performance characteristics over time. It should be noted that the aging parameters must be allowed to evolve over the life of the battery in order to model the changes in performance characteristics over the battery’s life.

Figure 6.

Cellfit voltage and temperature predictions voltage (a-c) and temperature (d-f) pre-dictions along with experimental data generated at a sub-milliecond run time of Cellfit for 3 cells in the dataset [33].

Battery degradation model needs to be developed along with performance model. Bills et al., developed two degradation models [36]:

1. Mechanistic degradation model (MDM),

2. Universal battery degradation model (UBDM),

MDM can include contributions from solid electrolyte interface (SEI) growth, lithium plating, and active material loss [15, 25, 26]. These are three different degradation mechanisms and depend on the specific chemistry used in the battery. Some battery chemistries can exhibit severe impact, and some might be mild under operational conditions. Therefore, battery chemistry-based model should also be developed. UBDM approach can capture more complex effects of charge/discharge, active material loss as well as change in resistance [27, 30]. This was reported to be a differential education-based model, and thus any mechanistic model could be substituted for the currently used mechanistic model in the UBDM, to account for changes in chemistry, operating conditions, etc.

2.7 Safety of eVTOL battery

Safety is paramount for eVTOL batteries. Catastrophic failure of battery can occur due to various reasons. Fast charging is one of them which induces the plating of metallic lithium at the anode under extreme eVTOL duty cycle conditions. The plated Li metal is highly reactive and can trigger thermal runaway even without internal shorting [38, 39]. It should be noted that most eVTOL battery charging events are fast charging in nature and should be expected to be performed in all types of weather conditions. Thus plating-free, fast charging battery technologies are essential for secure eVTOL operation in extreme temperature conditions that an eVTOL platform experiences during its duty cycle. Additionally, high discharge rates of eVTOL batteries create severe challenges to battery thermal management systems (BTMS). Batteries have several resistive components which can cause heat on applying current. Typically, the battery heat generation rate is proportional to the square of the applied current. Therefore, heat generation of an eVTOL battery is higher than EV batteries as the discharge rate of eVTOL battery is generally considered to be higher (∼1C) than a typical EV battery (C/3 for highway-driving and C/10 for city driving). Thus, a powerful BTMS is essential for the safe operation of eVTOL platforms. However, it is equally critical to limit BTMS mass to minimize the loss in pack-level-specific energy. The good news is that the rate of catastrophic failure for commercial LiBs has been reported to be only one in 40 million cells [40]. Statistics from Tesla has shown that for their electric vehicles, rate of fire incidents was only one in 205-million driving miles from 2012 to 2020 [41]. However, for eVTOL applications more rigorous battery design architectures need to be adopted to prevent all possible failures. Some novel features need be installed in the battery such as individual cell temperature monitoring abilities [42], self-healing/sealing electrodes and separators [43], etc. Furthermore, flammable liquid electrolyte in the conventional lithium-ion battery has been the critical component causing fire related hazards and thus alternatives such as all solid-state electrolytes can be considered when the technology matures. All solid-state-batteries (ASSB) are expected to provide more safety guarantees than conventional batteries. However, materials selection, development, and failure mode analysis are very critical for achieving sustainable operation of SSBs in eVTOL applications.

2.8 Confluence of EVTOL battery electrochemistry and mechanics

EVTOL platforms routinely experience a multitude of mechanical stimuli during take-off, cruise, and landing. These mechanical stimuli (vibrations) can be most often translated to the batteries that power these platforms. Such platforms can experience mechanical vibrations in the range of 10 Hz to 40 Hz [44]. Batteries that would power these EVTOL platforms need to be assessed for their airworthiness in terms of their mechanical robustness in addition to their electrochemical performance. Additionally, battery systems are inherently mechano-electrochemically coupled [45, 46, 47] and as a consequence, evaluation of battery material and component mechanical properties are essential for building EVTOL worthy battery packs. A multitude of mechanical properties pertaining to LIB systems for electric automobiles have been widely reported in literature. Commercial LIB cells used in electric vehicles which can be adapted for EVTOL platforms contain liquid electrolytes. These cells store a considerable amount of energy and unwarranted mechanical stimuli can lead to catastrophic short circuit events which can cause thermal runaways leading to structural compromise of the Battey pack which can cause damage to the EVTOL platform. Therefore, the individual structural components of the LIB cell have to be reassessed with regards to mechanical impulses experienced during EVTOL duty cycles. For example: the cell component that is expected to prevent short circuiting in LIB cells by preventing the direct contact between the two electrodes is the separator (usually made of a polymeric material). A good separator should be mechanically robust and should have sufficient puncture resistance without compromising electrochemical performance [48]. Apart from puncture resistance, the separator should also exhibit low thermal shrinkage. The electrodes should also be mechanically robust that there should be good adhesion between the electrodes and the active material coatings. High frequency mechanical vibrations experienced by the EVTOL platform during a duty cycle could compromise the adhesion between the active materials and the current collector. Specifically, during fully charged/discharged states as well as battery materials that have been subjected through hundreds of charge/discharge cycles could be adversely affected by high frequency mechanical vibrations. All solid-state batteries are also increasingly being considered for next generation EVTOL platforms that can operate at extreme temperature conditions. In such solid-state batteries, the inherent rigidity of the components when compared to their liquid-state counterparts, is expected to exacerbate the adverse effect of mechanical impulses experienced during EVTOL duty cycles. Moreover, current solid-state batteries require pressure application to achieve desired electrochemical performances. In such scenarios, a systematic and thorough R&D investigation of this phenomenon of mechano-electrochemical coupling is paramount for the development of mechanically robust batteries for EVTOL platforms.

3. All solid-state-battery (ASSB)

Based on the prior sections, we suggest that an ideal battery for eVTOL platform should be ultra-safe, delivering high energy density and preferably all solid-state (SSB) [49, 50, 51, 52, 53, 54, 55, 56] that employs a high ionically conductive solid electrolyte (SE) which can operate in extreme temperatures (from −80–100°C) and at extreme environmental conditions like intense radiation and vibration (Figure 7). It is envisioned that such a battery can be deployed in applications pertaining to even outer space, upper altitudes, and harsh terrestrial environments. Different R&D efforts across the globe have been rigorously working towards achieving practical ASSBs. However, many challenges need to be overcome prior to successful commercialization. The hurdle is not only materials development but also strategizing scalable approaches.

Figure 7.

Schematic diagram highlighting the design, focused area and application range, and aspects to be investigated for high energy density and high-power density all solid-state battery.

Unlike conventional battery, high-capacity single crystalline cathode materials (LiNi_xMn_yCo_zO₂, NMC and xLi₂MnO₃, LMR) can be integrated with the best reported solid electrolyte (SE) combinations which demonstrate strong interfacial stability with both anode and cathode. Such cell assemblies when optimized, are expected to deliver high energy and power densities. Ideal metrics include areal capacities >3 mAh cm⁻², demonstrating high Coulombic efficiencies over hundreds of cycles under extreme operating conditions. The ASSB has a transformational impact on secondary energy storage under harsh conditions as well as on specific battery deployments for conventional applications. At the cell level, inter and intra material resistances in solid-state batteries can significantly deteriorate the system performance in terms of capacity retention and coulombic efficiency. Identifying an upper bound of performance intrinsic to the material system can help in engineering the SSBs to mitigate the performance deficit in the meso-scale batteries. For this purpose, single crystal battery cathode is a promising approach and can be engineered using layered growth approaches (molecular beam epitaxy, atomic layer deposition, chemical vapor deposition, among others). Composite cathode designs can be tailored by utilizing material as well as engineering approaches. Material approaches can include high voltage cathode materials comprising of standard NMC622 material and Li- and Mn- rich cathode (LMR). LMR cathodes have high voltage and capacity but suffer from chemical degradation with conventional electrolytes which can be overcome with solid-state batteries. In order to mitigate intra-particle grain boundaries and transport limitations, single crystal/particle cathode materials can be a viable option for both NMC and LMR materials. In order to achieve high electrochemical performance, coating layers at the inter-particle interface between cathode active material and other components can be investigated that typically include LiA_xO_y (A = Nb, Ti, etc.), and LiX (X = halides). In addition to the identifying a stable cathode and solid electrolyte, R&D efforts should also focus on binder and electronic additives for battery stability in extreme conditions. Rigorous electrochemical testing (long term stability, capacity retention tests, GITT) should be performed on these batteries prior to deployment in eVTOL systems.

The economic viability is extremely important for the commercial deployment of EVTOL. It is a great challenge to transport people and cargo economically and efficiently in short and long-distance and is generated revenue by moving their luggage and freight. The cost and revenue are important but also very complex for EVTOL. The battery price, calendar and cycle life, and design cannot give a complete picture of cost and revenue. The payload, range and hours flying per day, and design of aircraft are key metrics. For instants, aircraft volume is a big concern because most flights are volume-limited rather than weight-limited. The batteries need to be carried within the wings as is fuel on current airliners, otherwise, the size of the aircraft must be increased to maintain payload volume. There are other direct costs including ownership, fuel, maintenance, and crew, airborne electrical equipment and gas turbines now cost about the same per kilowatt. All of these aspects must be considered when generating the cost rate or how much energy is consumed per passenger mile. The cost per kilowatt-hour is driven by the degradation of the cell which refers to the effect the charging itself does to degrade the use of the battery. Details cost analysis and economic comparison between electric aircraft and conventional airliners is reported in nature recently [57].

4. Future outlook

1. In ideal scenario, battery life is expected to last as long as vehicle life is sustained. Similar to terrestrial EVs, eVTOL platforms are also typically designed for achieving tens of years in service. However, design for available batteries would be based on the near term of 15 years at best. Additionally, the maximum range of an eVTOL vehicle is essentially proportional to the product of battery weight and specific energy as discussed in this chapter. Therefore, in future, R&D efforts have to be directed towards innovative battery designs and chemistry specific for eVTOL platforms. Specifically, enhancements to improve the specific energy of the battery would be ideal to achieve a larger payload while minimizing battery size for both short-range intracity commutes as well as for long-range intercity trips.

2. With the conventional LIB chemistries, increase the areal loadings of active materials is the most practical approach to enhance specific energy. However, this approach creates a critical barrier for achieving fast charging capability. Yang et al., [9] previously reported that the battery cells (∼215 Wh/kg) could be charged with 6C rates, enabling 5 min charging for 80 km trips. This implies that eVTOL batteries can be optimized based on their specific duty cycles. For example, a private eVTOL platform would require a specific type of battery optimized to serve a specific duty cycle while a public transit based eVTOL platform.

3. Based on our discussions, it can be inferred that high areal capacities would reduce the charge rate further thus limiting eVTOL operation at rush hour which may help in achieving a better return on investment. Therefore, it is essential to develop advanced battery technologies and protocols that can safely achieve fast charging of ultrahigh-energy density lithium-ion batteries.

4. The high utilization rates of eVTOL battery requires higher cycle life sustainability. Enhancing the specific energy and improvement of fast-charging capability could assist the cycle life longevity. The battery functionality (payload minimization) and performance are always a tradeoff. Presently, high voltage Ni-rich oxide cathodes with high-capacity silicon or Li-metal anodes are a prospective avenue to increase specific energy, however, such combinations when subjected to fast charging (potential eVTOL duty cycle requirements) can induce significant volume expansion of silicon anode, or formation and propagation of Li dendrites thereby resulting in either catastrophic failure or low cycle life expectancy. Nevertheless, significant breakthrough is needed in fast charging and cycle life advancements for new generation battery technologies to be viable for eVTOL applications.

5. Another way to increase the battery performance is to operate at higher temperature. Elevating the operational temperature can lower the cooling demand by an order of magnitude because of reduced heat generation rates and higher temperature differences between cells and ambient atmosphere [58, 59]. Elevated temperature operation can also improve battery power capability due to fast charging possibilities. Our opinion is that battery components with thermally modulated structures enables outstanding battery performance at low ambient temperatures, while the use of stable materials offers high safety and durability at high temperatures. This synergistic combination can achieve durable, safe, and high-performance, eVTOL operation even at different atmospheric conditions and thermal cycles.

Acknowledgments

This manuscript has been supported by Oak Ridge Nation Laboratory (ORNL) managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE) and the Laboratory Directed Research and Development (LDRD) Program at Oak Ridge National Laboratory.

Notice

This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for the United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).