**6. Perspective and outlook**

Battery research at present is at a stage where SSB technology can either redefine the next generation of batteries or fissile out without making an impact. The key difference between the two scenarios is whether the underlying challenges can be systematically mitigated or not. With more commercial enterprises investing heavily into SSB research, there is momentum slowly building to propel and position SSBs as the batteries of the future. Success of SSBs would depend upon the following factors: (i) improving the interfacial issues, (ii) developing unique processing capabilities while minimizing cost, (iii) addressing cell and pack level design challenges when integrating solid state components, and (iv) demonstrating performances exceeding that of advanced lithium-ion batteries. Some potential challenges and opportunities for SSBs include:

With the electric vehicle revolution driving up the demand for batteries that can deliver high energy density, can fast charge, have long cycle and calendar life while maintaining low manufacturing costs, there is definite potential for SSBs to play a major role in achieving these goals. Moreover, the US Department of Energy's target goal for EV batteries include (i) of reducing battery cost to <\$100/kWh and ultimately to <\$80/kWh, (ii) increasing the range of electric vehicles to 300 miles and (iii) decreasing charge time to 15 minutes or less. A full solid-state battery that can meet these targets that are set for electric vehicles in the next decade is an ambitious endeavor especially when the best anode, cathode, and electrolyte chemistries for such an SSB are not obvious at present.

Urban air mobility is a rapidly pursued avenue that could redefine transportation as we know it. Conventional liquid electrolyte-based batteries are subjected to increasingly stringent safety requirements for use in operation of auxiliary electronics on present day aircraft platforms. Applications with demanding duty cycles such as electric vehicle take off and lift (EVTOL) platforms require batteries that can operate at extreme temperature gradients while being subjected to a multitude of mechanical stresses during operation. SSBs with their safety and energy density advantages are uniquely positioned to be the go-to batteries for such applications. Exploring SSBs for such applications would require coordinated efforts between industrial players and research institutes to initially define the performance requirements for such platforms followed by systematic material and engineering efforts to address the challenges.

i. Extreme environment batteries are a niche yet rapidly growing application due to the increasing public interest into commercial low-earth orbit and interplanetary travel. Certain terrestrial, upper altitude and space applications necessitate secondary energy storage technologies that can function under a very wide modality of extreme environmental conditions including but not limited to temperature, pressure, radiation loads, and mechanical loads. With some present day SSBs operating efficiently at elevated temperatures when compared to the room temperature performance, this presents a unique opportunity for deploying SSBs in these applications. Development of stable, safe, and durable energy storage technologies can have a transformational impact on the application sectors

which can include orbital satellites, outer planetary/deep-space probes, land rovers, polar vehicles/end stations, among others.

ii. As an intermediate step, a less-demanding end use application such as SSBs for portable/wearable electronics can be pursued, however, this would risk diverting the focus of the battery R&D community from the biggest projected market for next generation battery systems – electric vehicles and grid storage.
