Comparison between Thin-Film and Bulk Solid-State Battery Technologies for the Mobility Sector
Lithium-ion batteries are used in everything from laptop computers to electric vehicles to electric aircrafts. These types of batteries have thrived in the last decade due to their relative ease of manufacture, relatively high energy density (compared to legacy nickel-metal hydride and common alkaline batteries) and existing in-depth technical understanding of their battery chemistry. One of the main limiting factors of these types of batteries is their use of a liquid electrolyte which results in high weight for a given energy. Liquid electrolytes also have issues with safety due to thermal runaway events that can occur if battery cell or pack integrity is compromised.
Solid-state batteries utilise solid electrolytes instead of the liquid electrolytes used in contemporary lithium-ion batteries. These solid electrolytes are fabricated from materials such as glass, ceramics, or other lightweight solid materials. Solid-state batteries are similar in structure to traditional lithium-ion batteries. However, by eliminating the liquid electrolyte, the batteries can be made more compact.
Solid-state batteries of the future are expected to be over twice as energy-dense as the leading liquid electrolyte lithium-ion batteries of today. Unfortunately, the ion conductivity through solid materials is lower as compared to liquid electrolytes which is the reason for their limited use in devices larger than wearables or small personal items at present.
Solid-state batteries can be broadly classified into two categories: 1.) thin-film micro-batteries made using thin-film deposition processes such as Radio Frequency (RF) sputtering, spin-coating, and Pulsed Laser Deposition (PLD) and 2.) bulk solid-state batteries manufactured by pressing electrodes and solid electrolyte powder together under extreme environmental conditions. Thin-film solid-state batteries have thicknesses of hundreds of nanometres to several microns, whereas bulk solid-state batteries exhibit a typical thickness of hundreds of micrometres or more.
Thin-film solid-state batteries are generally used in small-scale applications such as pacemakers and wearables as they are of low energy storage capacity – although it is hoped that recent innovations might find favour in the electric micro-mobility sector and in other electric vehicles. Lithium-phosphorus-oxy-nitride (LiPON) is the most commonly used electrolyte for thin-film solid-state batteries and has a lithium-ion conductivity of 10-6S/cm at room temperatures. In addition to the benefits of employing a solid electrolyte, thin-film fabrication can lead to improvements in specific energy, energy density, and power density. It may also result in lower manufacturing costs by allowing the use of low-cost materials due to scalable roll-to-roll processing.
Bulk solid-state batteries find favour in larger applications such as batteries for electric vehicles or even aircrafts due to their higher energy density as compared to thin-film solid-state batteries. The conductivity of the solid electrolyte in bulk solid-state batteries should be in the range of 10-4 to 10-3S/cm to reach reasonable power densities. This is similar to the conductivity performance of liquid electrolytes without the weight penalties and safety concerns.
Innovations in Thin-Film and Bulk Solid-State Batteries for Mobility
Solvay, a materials and chemical company headquartered in Belgium, is using its innovative technologies to develop the next generation of solid-state electrolytes for thin-film and bulk solid-state batteries. These products will cover everything from wearables to electric vehicles and electromobility in general. Investments began with the installation of a dry room laboratory in Solvay’s research facility near Paris, France, in early 2021, followed by a new research and development centre of competence in La Rochelle, France later that year. Solvay is pioneering the research of innovative inorganic materials for solid electrolytes, a crucial component of solid-state batteries, with support from the Île-de-France and Nouvelle Aquitaine regions, and is expediting the scale-up of these materials.
In mid-2020, researchers at Sandia National Laboratories, USA, created a higher-power thin-film lithium-ion battery for electric vehicles and small, portable electronic items that is more robust and energy-dense than legacy thin-film solid-state batteries. This technology offers a solid-state lithium battery with high power and a small footprint by using a lithium-stable, higher-conductivity electrolyte — lanthanum lithium tantalate (Li5La3Ta2O12) — and affordable metal foil substrates. Researchers found that their LiPON thin-films have orders of magnitude higher conductance, higher power density, and a wider operating temperature range than industry standard LiPON thin-films. The novel materials and battery designs overcome restrictions like stress-induced film fracture to provide compact, resilient, and higher-energy-density batteries that have the potential to transform the electric vehicle market by improving driving range, battery life, and lowering the cost per unit of energy. Reel-to-reel manufacturing, flexible electronics, and applications needing low profiles all benefit from a thin and flexible battery substrate.
In late 2021, LionVolt BV, a spin-off from the Netherlands Organisation for Applied Scientific Research (TNO), reported that it had raised €4 million in funding from various technology start-up investors to develop its 3D solid-state thin-film battery technology, which it plans to deploy for use in wearables, micro-mobility, and other electric vehicles. According to LionVolt, the conformal coating of the entire battery stack and the development of scalable manufacturing methods are two important challenges that the product must overcome to ensure it is commercially scalable. According to reports, the construction of micropillar arrays reduces the technological obstacle of conformal coating, and the open and regular structure helps the deposition of successive layers.
The battery development company, Solid Power, USA, partnered with SK Innovation, South Korea, in late 2021, to produce automotive-scale bulk all-solid-state batteries. Using Solid Power’s sulphide-based solid electrolyte, patented cell designs, and production techniques, the partnership’s purpose is to confirm Solid Power’s all-solid-state battery and electrolyte production processes and collaborate on technology development. This collaboration builds on the joint venture established between SK Innovation and Ford Motor Company, USA, for the construction of a battery manufacturing facility in mid-2021. Whilst this facility will manufacture contemporary liquid electrolyte lithium-ion batteries at first, it has been designed to be compatible with the latest manufacturing techniques required for large-scale solid-state battery production.
In early 2022, Dongfeng Motor, China, delivered 50 electric vehicles utilising pseudo-solid-state batteries for demonstration in China – the first of its kind. These batteries, categorised as ‘bulk solid-state’ batteries, adopt a hybrid of conventional liquid electrolyte lithium-ion and solid-state battery technologies, known as semi-solid lithium metal (Li-OH). Although strictly not all-solid-state, there is significantly less liquid electrolyte used and energy density is up to 80% higher as compared to an equivalent conventional liquid electrolyte lithium-ion battery. The pseudo-solid-state batteries here use advanced separation technology utilising flexible diaphragms that allow lithium-ions to flow between the electrodes whilst mitigating lithium dendrites that can cause durability issues.
Solid-state batteries in general are safe and have the potential to be more energy-dense than modern lithium-ion batteries if the correct combination of materials for anodes, cathodes, and electrolytes can be established. However, solid-state battery cost is currently prohibitively expensive with some studies estimating costs of US$400-800 per kWh by 2026. This is far in excess of today’s (2022) cost of approximately US$132 per kWh for mature liquid electrolyte lithium-ion battery technology.
Currently, a variety of solid electrolyte, anode, and cathode materials are being investigated both in the thin-film and bulk solid-state battery fields, each of which necessitates quite distinct working environments, processing conditions, and costs. Costs are influenced by the processing environment and the cell operating parameters. The requirement for high pressure and temperature during production and/or cell operation will inevitably increase the size of the manufacturing facility, its expenses, and the time taken for battery production. Traditional manufacturing processes must be therefore altered in the long run for solid-state batteries to become economically viable.
Finally, with the proliferation of electric vehicles and electrified mobility in general, solid-state battery technology is expected to mature over the next decade. With predictions suggesting that over half of all new vehicles sold in 2032 will be fully electric, the demand from the electric vehicle sector is likely to initiate further significant spending in solid-state battery research and development in the very near future.