Lithium-ion batteries comprise a family of battery chemistries that employ various combinations of anode and cathode materials. Each combination has distinct advantages and disadvantages in terms of safety, performance, cost and other parameters. The most prominent technologies for automotive applications are NCA (lithium-nickel-cobalt-aluminium), NMC (lithium-nickel-manganese-cobalt), LMO (lithium-manganese spinel), LTO (lithium titanate) and LFP (lithium-iron phosphate). All automotive battery chemistries require elaborate monitoring, balancing and cooling systems to control the chemical release of energy, prevent thermal runaway — a positive-feedback loop whereby chemical reactions triggered in the cell exacerbate heat release, potentially resulting in a fire — and ensure a reasonably long life span for the cells.
There is increasing interest and activity in exploring new technologies that might boost the specific energy and performance of future batteries. But while many universities and research laboratories are working intensely on this, most of these technologies are unlikely to be available for production on a significant scale before 2020.
The recent explosion in innovation is driven by the need to break some fundamental compromises in battery technology. On the technical side, competing lithium-ion technologies can be compared along six dimensions: safety; life span (measured in terms of both number of charge-and-discharge cycles and overall battery age); performance (peak power at low temperatures, state-of-charge measurement, and thermal management); specific energy (how much energy the battery can store per kilogram of weight); specific power (how much power the battery can store per kilogram of mass); and cost, which remains one of the major hurdles. Charge time, which does not vary substantially among existing battery technologies, remains a significant performance challenge.
Currently no single technology wins along all dimensions. A fairly high-performance solution such as NCA presents safety challenges, while LFP is safer at the cell level but provides a low specific energy.
Safety seems to be one of the most important criterion for electric-car batteries. The main concern in this area is avoiding thermal runway, which can be caused by an overcharged battery, too-high discharge rates, or a short circuit. Technologies that are prone to thermal runaway include NCA, NMC and LMO, and they must be used in conjunction with system-level safety measures that either contain the cells or monitor their behaviour. Such measures include a robust battery box, a very efficient cooling system (to prevent the early stages of thermal runaway), and precise state-of-charge monitoring and cell-discharge balancing.
While battery safety is indisputably a valid concern, it is useful to put this concern in context by recalling the significant safety challenges originally associated with the ICE (internal combustion engine) and with gasoline storage, which were largely overcome through improvements in design and engineering.
There are two ways of measuring battery life span: cycle stability and overall age. Cycle stability is the number of times a battery can be fully charged and discharged before being degraded to 80 % of its original capacity at full charge. Overall age is the number of years a battery can be expected to remain useful. Most automotive manufacturers are planning for a 10-year battery life span, including expected degradation. For example, an OEM (original equipment manufacturer) whose EV nominally requires a 12-kilowatt-hour (kWh) battery is likely to specify a 20-kWh battery instead, so that after 10 years and 40 % performance degradation the battery will still have sufficient energy capacity for normal operation. Of course, this approach increases the size, weight and cost of the battery.
Another option may be to install a smaller battery with a shorter life span and plan to replace it every five to seven years. This may allow OEMs to upgrade batteries as the technology continues to advance. Battery-leasing models, such as those proposed by Think, a manufacturer of small city cars, and Better Place, a start-up provider of battery infrastructure, also allow for shorter-lived batteries. These models decouple the battery’s life span from the vehicle’s life span and remove up-front battery costs.
The expectation that the owner of an EV should be able to drive it both at hot summer and sub-zero winter temperatures, poses substantial engineering challenges. It is difficult to engineer batteries that function over a wide range of temperatures without incurring performance degradations. One solution might be for OEMs to rate batteries for particular climates and rely on heating and insulation to make up for climate differences. However, climate-specific batteries would likely hinder mobility across regions and a lower performance may be preferable over the higher costs and other restrictions this would incur.
Specific energy and specific power
The specific energy of batteries — their capacity for storing energy per kilogram of weight — is still only 1 % of the specific energy of gasoline. Unless there is a major breakthrough, batteries will continue to limit the driving range of EVs to some 250 to 300 kilometres between charges. Battery cells today can reach nominal energy densities of 140 to 170 watt-hours per kilogram (Wh/kg), compared with 13 000 Wh/kg for gasoline. The specific energy of the resulting battery pack is typically 30 % to 40 % lower, or 80 Wh/kg to 120 Wh/kg. Even if that energy density were to double over the next 10 years, the range would hardly exceed 300 kilometres.
Specific Power — the amount of power that batteries can deliver per kilogram of mass – is relatively well addressed by current battery technologies. This is particular important in hybrid vehicles, which discharge a small amount of energy quickly. In electric vehicles specific power is less important and equals or exceeds that of ICEs.
Long charging times present another technical challenge and a commercial barrier that must be addressed. It takes almost 10 hours to charge a 15 kWh battery by plugging it into a standard 120 Volt outlet. Charging by means of a 240 Volt outlet with increased power (40 Amps) can take two hours, while charging at a commercial three-phase charging station can take as little as 20 minutes. Battery-swap methods, such as the models contemplated by Better Place, promise to provide a full charge in less than three minutes. But such approaches need OEMs to agree to common standardization requirements.
Without a major breakthrough in battery technologies, fully EVs that can travel up to 500 kilometres are unlikely to be available for the mass market by 2020. Independently of the infrastructure that needs to be built both for charging or swapping batteries, adoption of such vehicles may initially be limited to commercial fleets and commuter cars that are confined to a certain range. Range-extender vehicles, which combine an electric power train with an ICE, can overcome the range and infrastructure limitations, but at an increased cost.
The cost of the cell represents some 65 % of the battery pack. Battery costs will decline steeply as production volumes increase, but some 25 % of those costs — primarily linked to raw materials and parts — are likely to remain stable.
The cost for the charging infrastructure is another major component of EV expansion and operating costs. A large part of this will need to be financed by governments, power companies and private contractors. The infrastructure mix will depend on access to home charging stations, how far and where to people drive, and the use of range extenders.
Increasing electricity demand will also require upgrades to the grid, where the IEC is directly involved in all key Smart Grid initiatives around the world.
The future depends on standards
In the meantime, while engineers work on the possibility of increasing power and storage capabilities of batteries and possibilities for "instant" recharging technologies, it remains vitally important to continue working in a smart manner on standardizing the connectors and sizes of the existing batteries and EVs so that the future of personal transport can continue to progress in a clean and efficient manner.