Editing IPCC:AR6/WGIII/Chapter-10 (section)

=== 10.3.2 Electric Technologies ===

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Widespread electrification of the transport sector is likely crucial for reducing transport emissions and depends on appropriate electrical energy storage systems (EES). However, large-scale diffusion of EES depends on improvements in energy density (energy stored per unit volume), specific energy (energy stored per unit weight), and costs ( [[#Cano--2018|Cano et al. 2018]] ). Recent trends suggest EES-enabled vehicles are on a path to becoming the leading technology for LDVs, but their contribution to heavy-duty freight is more uncertain.

'''Electrochemical storage of light and medium-duty vehicles.''' Electrochemical storage, i.e., batteries, are one of the most promising forms of energy storage for the transport sector and have dramatically improved in their commerciality since AR5. Rechargeable batteries are of primary interest for applications within the transport sector, with a range of mature and emerging chemistries able to support the electrification of vehicles. The most significant change since AR5 and SPR1.5 is the dramatic rise in lithium-ion batteries (LIB), which has enabled electromobility to become a major feature of decarbonisation.

Before the recent growth in market share of LIBs, lead-acid batteries, nickel batteries, high-temperature sodium batteries, and redox flow batteries were of particular interest for the transport sector ( [[#Placke--2017|Placke et al. 2017]] ). Due to their low costs, lead-acid batteries have been used in smaller automotive vehicles, e.g., e-scooters and e-rickshaws ( [[#Dhar--2017|Dhar et al. 2017]] ). However, their application in electric vehicles will be limited due to their low specific energy ( [[#Andwari--2017|Andwari et al. 2017]] ). Nickel-metal hydride (NiMH) batteries have a better energy density than lead-acid batteries and have been well optimised for regenerative braking ( [[#Cano--2018|Cano et al. 2018]] ). As a result, NiMH batteries were the battery of choice for hybrid electric vehicles (HEVs). Ni-Cadmium (NiCd) batteries have energy densities lower than NiMH batteries and cost around ten times more than lead-acid batteries (Table 6.5). For this reason, NiCd batteries do not have major prospects within automotive applications. There are also no examples of high-temperature sodium or redox flow batteries being used within automotive applications.

Commercial application of LIBs in automotive applications started around 2000 when the price of LIBs was more than USD1000 per kWh ( [[#Schmidt--2017|Schmidt et al. 2017]] ). By 2020, the battery manufacturing capacity for automotive applications was around 300 GWh per year ( [[#IEA--2021a|IEA 2021a]] ). Furthermore, by 2020, the average battery pack cost had come down to USD137 per kWh, a reduction of 89% in real terms since 2010 ( [[#Henze--2020|Henze 2020]] ). Further improvements in specific energy, energy density (Nykvist et al. 2015; [[#Placke--2017|Placke et al. 2017]] ) and battery service life ( [[#Liu--2017|Liu et al. 2017]] ) of LIBs are expected through additional design optimisation (Table 6.5). These advances are expected to lead to EVs with even longer driving ranges, further supporting the uptake of LIBs for transport applications ( [[#Cano--2018|Cano et al. 2018]] ). However, the performance of LIBs under freezing and high temperatures is a concern ( [[#Liu--2017|Liu et al. 2017]] ) for reliability. Auto manufacturers have some pre-heating systems for batteries to see that they perform well in very cold conditions ( [[#Wu--2020|Wu et al. 2020]] ).

For EVs sold in 2018, the material demand was about 11 kilotonnes (kt) of optimised lithium, 15 kt of cobalt, 11 kt of manganese, and 34 kt of nickel ( [[#IEA--2019a|IEA 2019a]] ; [[#IEA--2021a|IEA 2021a]] ). IEA projections for 2030 in the EV 30@30 scenario show that the demand for these materials would increase by 30 times for lithium and around 25 times for cobalt. While there are efforts to move away from expensive materials such as cobalt ( [[#IEA--2019a|IEA 2019a]] ; [[#IEA--2021a|IEA 2021a]] ), dependence on lithium will remain, which may be a cause of concern ( [[#Olivetti--2017|Olivetti et al. 2017]] ; [[#You--2018|You and Manthiram 2018]] ). A more detailed discussion on resource constraints for lithium is provided in Box 10.6.

Externalities from resource extraction are another concern, though current volumes of lithium are much smaller than other metals (steel, aluminium). As a result, lithium was not even mentioned in UNEP’s global resource outlook ( [[#IRP--2019|IRP 2019]] ). Nonetheless, it is essential to manage demand and limit externalities since the demand for lithium is going to increase many times in the future. Reuse of LIBs used in EVs for stationary energy applications can help in reducing the demand for LIBs. However, the main challenges are the difficulty in accessing the information on the health of batteries to be recycled and technical problems in remanufacturing the batteries for their second life ( [[#Ahmadi--2017|Ahmadi et al. 2017]] ). Recycling lithium from used batteries could be another possible supply source (Winslow et al. 2018). While further R&amp;D is required for commercialisation (Ling et al., 2018), recent efforts at recycling LIBs are very encouraging ( [[#Ma--2021|Ma et al. 2021]] ). The standardisation of battery modules and packaging within and across vehicle platforms, increased focus on design for recyclability, and supportive regulation are important to enable higher recycling rates for LIBs ( [[#Harper--2019|Harper et al. 2019]] ).

Several next-generation battery chemistries are often referred to as post-LIBs ( [[#Placke--2017|Placke et al. 2017]] ). These chemistries include metal-sulphur, metal-air, metal-ion (besides Li), and all-solid-state batteries. The long development cycles of the automotive industry ( [[#Cano--2018|Cano et al. 2018]] ) and the advantages of LIBs in terms of energy density and cycle life (Table 6.5) mean that it is unlikely that post-LIB technologies will replace LIBs in the next decade. However, lithium-sulphur, lithium-air, and zinc-air have emerged as potential alternatives for LIBs. These emerging chemistries may also be used to supplement LIBs in dual-battery configurations, to extend the driving range at lower costs or with higher energy density ( [[#Cano--2018|Cano et al. 2018]] ). Lithium-sulphur (Li-S) batteries have a lithium metal anode with a higher theoretical capacity than lithium-ion anodes and much lower-cost sulphur cathodes relative to typical Li-ion insertion cathodes ( [[#Manthiram--2014|Manthiram et al. 2014]] ). As a result, Li-S batteries are much cheaper than LIB to manufacture and have a higher energy density (Table 6.5). Conversely, these batteries face challenges from sulphur cathodes, such as low conductivity of the sulphur and lithium sulphide phases, and the relatively high solubility of sulphur species in common lithium battery electrolytes, leading to low cycle life ( [[#Cano--2018|Cano et al. 2018]] ). Lithium-air batteries offer a further improvement in specific energy and energy density above Li–S batteries owing to their use of atmospheric oxygen as a cathode in place of sulphur. However, their demonstrated cycle life is much lower (Table 6.5). Lithium-air batteries also have low specific power. Therefore, lithium-air require an extra battery for practical applications ( [[#Cano--2018|Cano et al. 2018]] ). Finally, zinc–air batteries could more likely be used in future EVs because of their more advanced technology status and higher practically achievable energy density ( [[#Fu--2017|Fu et al. 2017]] ). Like Li-air batteries, their poor specific power and energy efficiency will probably prevent zinc-air batteries from being used as a primary energy source for EVs. Still, they could be promising when used in a dual-battery configuration ( [[#Cano--2018|Cano et al. 2018]] ).

The technological readiness of batteries is a crucial parameter in the advancement of EVs ( [[#Manzetti--2015|Manzetti and Mariasiu 2015]] ). Energy density, power density, cycle life, calendar life, and the cost per kWh are the pertinent parameters for comparing the technological readiness of various battery technologies ( [[#Manzetti--2015|Manzetti and Mariasiu 2015]] ; [[#Andwari--2017|Andwari et al. 2017]] ; [[#Lajunen--2018|Lajunen et al. 2018]] ). Table 6.5 provides a summary of the values of these parameters for alternative battery technologies. LIBs comprehensively dominate the other battery types and are at a readiness level where they can be applied for land transport applications (cars, scooters, electrically-assisted cycles) and at battery pack costs below USD150 per kWh, making EVs cost-competitive with conventional vehicles ( [[#Nykvist--2019|Nykvist et al. 2019]] ). In 2020 the stock of battery electric LDVs had crossed the 10 million mark ( [[#IEA--2021a|IEA 2021a]] ). [[#Schmidt--2017|Schmidt et al. (2017)]] project that the cost of a battery pack for LIBs will reach USD100 per kWh by 2030, but more recent trends show this could happen much earlier. For example, according to IEA, battery pack costs could be as low as USD80 per kWh by 2030 ( [[#IEA--2019a|IEA 2019a]] ). In addition, there are clear trends that now vehicle manufacturers are offering vehicles with bigger batteries, greater driving ranges, higher top speeds, faster acceleration, and all size categories ( [[#Nykvist--2019|Nykvist et al. 2019]] ). In 2020 there were over 600,000 battery electric buses and over 31,000 battery electric trucks operating globally ( [[#IEA--2021a|IEA 2021a]] ).

LIBs are not currently envisaged to be suitable for long-haul transport. However, several battery technologies are under development (Table 6.5), which could further enhance the competitiveness of EVs and expand their applicability to very short-haul aviation and ships, especially smaller vehicles. Li-S, Li-air, and Zn-air hold the highest potential for these segments ( [[#Cano--2018|Cano et al. 2018]] ). All three of these technologies rely on making use of relatively inexpensive elements, which can help bring down battery costs ( [[#Cano--2018|Cano et al. 2018]] ). The main challenge these technologies face is in terms of the cycle life. Out of the three, Li-S has already been used for applications in unmanned aerial vehicles ( [[#Fotouhi--2017|Fotouhi et al., 2017]] ) due to relatively high specific energy (almost double the state of the art LIBs). However, even with low cycle life, Li-air and Zn-air hold good prospects for commercialisation as range extender batteries for long-range road transport and with vehicles that are typically used for city driving ( [[#Cano--2018|Cano et al. 2018]] ).

'''Alternative electricity storage technologies for heavy-duty transport.''' While LIBs described in the previous section are driving the electrification of LDVs, their application to railways, aviation, ships, and large vehicles faces challenges due to the higher power requirements of these applications. The use of a capacitor with a higher power density than LIBs could be suitable for the electrification of such vehicles. It is one of the solutions for regenerating large and instantaneous energy from regenerative brakes. Classical capacitors generally show more attractive characteristics in power density (8000–10,000 watts per kilogram (W/kg)) than batteries. However, the energy density is poor (1–4 watt-hours per kilogram (Wh/kg)) compared to batteries, and there is an issue of self-discharge ( [[#González--2016|González et al. 2016]] ; [[#Poonam--2019|Poonam et al. 2019]] ). To improve the energy density, electrochemical double layer capacitors (EDLCs; supercapacitor) and hybrid capacitors (10–24 Wh/kg, 900–9000 W/kg at the product level) such as Li-ion capacitors have been developed. The highest energy density of the LIC system (100–140 Wh/kg in the research stage) are approaching that of the Li-ion battery systems (80–240 Wh/kg in the product stage) ( [[#Naoi--2012|Naoi et al. 2012]] ; [[#Panja--2020|Panja et al. 2020]] ). Examples of effective use of capacitors include a 12-tonne truck with a capacitor-based kinetic energy recovery system that has been reported to save up to 32% of the fuel use of a standard truck ( [[#Kamdar--2017|Kamdar 2017]] ). Similarly, an EDLC bank applied to electric railway systems has been shown to result in a 10% reduction in power consumption per day ( [[#Takahashi--2017|Takahashi et al. 2017]] ). Finally, systems in which capacitors are mounted on an electric bus for charging at a stop have been put into practical use, for example by a trackless tram ( [[#Newman--2019|Newman et al. 2019]] ). At the bus stop, the capacitor is charged at 600 kW for 10 about 40 seconds, which provides enough power for about 5 to 10 km ( [[#Newman--2019|Newman et al. 2019]] ). In addition, more durable capacitors can achieve a longer life than LIB systems ( [[#ADB--2018|ADB 2018]] ).

Hybrid energy storage (HES) systems, which combine a capacitor and a battery, achieve both high power and high energy, solving problems such as capacity loss of the battery and self-discharge of the capacitor. In these systems, the capacitor absorbs the steeper power, while the LIB handles the steady power, thereby reducing the power loss of the EV to half. Furthermore, since the in-rush current of the battery is suppressed, there is an improvement in the reliability of the LIB (Noumi et al. 2014). In a hybrid diesel train, 8.2% of the regenerative energy is lost due to batteries’ limited charge-discharge performance; however, using an EDLC with batteries can save this energy ( [[#Takahashi--2017|Takahashi et al. 2017]] ; [[#Mayrink--2020|Mayrink et al. 2020]] ).

The development of power storage devices and advanced integrated system approaches, including power electronics circuits such as HES and their control technologies, are important for the electrification of mobility. These technologies are solutions that could promote the electrification of systems, reduce costs, and contribute to the social environment through multiple outcomes in the decarbonisation agenda.

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