Solid-state batteries are seen as a critical enabler for compact, energy-dense battery-powered devices. Solid-state batteries are considered the natural successor to liquid electrolyte batteries such as lithium-ion because they have the potential to exhibit 2.5 times the energy density of current liquid electrolyte batteries with reduced weight and lesser degradation in battery performance from rapid charging. Though currently restricted to practical applications in devices such as wearables and pacemakers, solid-state batteries are potential enablers for the total electrification of the on-road light-duty transportation industry. They are also expected to play a significant role in the electrification of the heavy-duty fleet.

To increase the market penetration of solid-state batteries, the best material combination for the anode, cathode, and solid electrolyte must be determined. Because of their high specific energy densities, lithium and silicon are preferred candidates for the anode material. Both, however, have shortcomings (lithium dendrite development and silicon ‘swelling’) that must be solved before they can be implemented as a production-ready solution. Sulphide, oxide, and polymer-based chemicals are particularly appealing as solid electrolytes, but again, all have shortcomings that lead to limited market penetration.

Hydrides are hydrogen-containing chemical compounds that are essential for novel battery applications. They have long been employed as negative poles in Nickel-Metal Hydride alkaline batteries which are used in some electrified vehicles. However, they have found favour as an anode material in new fields of study. Furthermore, complex hydrides have good ionic conductivity and can be used as a solid electrolyte, replacing the highly flammable liquid-electrolyte in modern lithium-ion batteries. The use of solid hydrides as electrolytes and electrodes in batteries might make them safer and allow them to store more power. New hydride materials are being discovered and created, demonstrating how hydrogen may be employed with other battery energy storage methods.

Benefits and Shortfalls of Solid-State Batteries

In solid-state batteries, the solid electrolyte is smaller in size than the liquid electrolyte in lithium-ion batteries. This enables solid-state batteries to be more energy-dense than lithium-ion batteries with small solid-state batteries providing the same power output as larger lithium-ion batteries. This allows for the use of additional batteries in the same limited area.

Since there is no flammable liquid electrolyte in solid-state batteries, they are safer than lithium-ion batteries.

Solid-state batteries are also more rechargeable than lithium-ion batteries. The liquid electrolyte in lithium-ion batteries reduces battery life by gently corroding the electrodes over time. However, this does not happen in solid-state batteries. A solid electrolyte is estimated to provide a battery life length of approximately 5 times that of a liquid electrolyte lithium-ion battery.

It is difficult to manufacture solid-state batteries on a large scale. This is mostly owing to the newness of the technology and the challenge of transitioning from laboratory expertise to mass manufacturing. The optimum material for a solid electrolyte with perfect ionic conductivity is yet to be discovered, making their implementation somewhat difficult.

Latest Developments in Hydrides for Solid-State Batteries

Scientists from Tohoku University and the High Energy Accelerator Research Organization, Tokyo, Japan, have recently developed a new complex hydride lithium superionic conductor that could result in all-solid-state batteries with the highest energy density to date. It is expected that the battery will have an energy density of 2,500Wh/kg – approximately 10 times that of the latest liquid-electrolyte lithium-ion batteries. According to the researchers, the novel material, created by constructing structures of hydrogen clusters (complex anions), has extremely high stability against lithium metal, making it the perfect anode. This new solid electrolyte exhibits high ionic conductivity and high stability against lithium metal and is thought to be a real breakthrough for all-solid-state batteries that use a lithium metal anode.

Researchers from France’s Saft and Université Paris Est employed a nanocomposite metal hydride as the anode in a full solid-state battery with a sulphur cathode and LiBH4 electrolyte for the first time. With discharge plateaus at 1.8V and 1.4V, the cell has a high reversible capacity of 910mAhg-1. Over the first 25 charge/discharge cycles, it was found that the capacity stays at 85% of its initial value. Metal hydrides have been presented as potentially game-changing negative electrode materials. These hydrides work well in all-solid-state cells using LiBH4 as the solid electrolyte and metallic Lithium as the anode. However, their use as negative active materials is yet to be realised.

A team of experts from the Lawrence Livermore National Laboratory, USA and Sandia National Laboratories, USA, suggested using aluminium hydride (AlH3) to store hydrogen in late 2021. This solid-state metal hydride, sometimes known as “alane,” is widely utilised in rocket fuels, explosives, alkali batteries, and as a hydrogen source in low-temperature fuel cells. According to the researchers, this material can overcome the difficulty of hydride thermodynamic limitations in hydrogen storage. Many high-capacity metal hydrides have poor thermodynamics of hydrogen absorption after initial release, necessitating high hydrogen pressures to renew. This disadvantage is frequently associated with their metastable character and impedes their real-world uses. To couple them, appropriate lithium-based positive materials with comparable theoretical capacities and chemical compatibility with the electrolyte must be found. The S/Li2S redox pair is a good option in this scenario. Indeed, it has been demonstrated to have great compatibility and performance in all-solid-state batteries using LiBH4 as the solid electrolyte (800mAhg-1 after 50 cycles).

What Next?

In 2020 the global solid-state battery market was valued at US$590.9 million. It is expected to grow at a Compound Annual Growth Rate (CAGR) of 36% between 2021 and 2028. The introduction of hydrides as solid electrolytes and electrodes in solid-state batteries brings new opportunities for practical applications. As solid electrolytes, borohydrides i.e., BH4 and BnHn containing compounds can rapidly conduct ions due to the effective paddle-wheel mechanism. After being improved in ionic conductivities and electrode compatibilities, these compounds can enable stable cycling of various solid-state batteries, showing competitive performances as compared to other solid electrolytes. As electrodes, binary hydrides and alanates can react with Lithium or Sodium through conversion reactions, delivering ultrahigh theoretical specific capacities. After being improved by changes in composition and structure, these hydrides can exhibit high specific reversible capacities as well as excellent rate capabilities and cycling stabilities. Thus, further exploration of hydrides as key components for solid-state batteries will be highly desirable in the future.