The challenges, they note, are many:

• Unresolved fundamental issues remain in the quest to fully understand the behavior of all-solid batteries, especially in the area of electrochemical interfaces.

• Parameters that require robust understanding from a product development standpoint are material cost, cell lifetime and shelf life, cell energy density on a volumetric and gravimetric basis, operable capabilities for given temperature conditions, and safety.

• The advantage of energy density remains to be realized in solid state electrolytes (SSEs) since most studies to date utilize thick SSEs or cathodes with low active loading compared to liquid counterparts.

• The desire to use SSEs in conjunction with Li metal anodes requires understanding and managing the morphology of Li metal plating, which can impact volumetric energy density.

• Although operation at both higher and lower temperature compared to conventional technologies is a significant potential advantage of SSE systems, reports of solid state cells achieving parity with traditional systems at room temperature or any other temperature do not currently exist.

• The decreased flammability of SSE systems is another potential advantage but requires ongoing validation and study.

• The manufacturability and material component costs of SSEs have not been well characterized, and thus the value of these features will need to be weighed accordingly with any added cost.

• The operating lifetime of SSEs capturing intrinsic materials parameters such as voltage stability as well as catastrophic failure modes such as shorting have been briefly investigated, but in the absence of high energy density electrode formulations and application based testing protocols that are comparable to commercial liquid electrolyte cells.

To enable development and maturation of solid state battery technology, the value propositions of SSEs must be substantiated with relevant data in the coming years. Companies with competencies in ceramic or battery processing and with the resources to engage in a broad level of materials development and failure analysis may be well positioned to enable this technology.

—Kermana et al.

In their paper, the team references more than 200 papers, exploring critical aspects of solid-state battery technology including the basic material properties of solid state electrolytes—along with key differences in theory and understanding of material physics—and the fabrication of those materials.

They discuss the solid electrolyte interfaces, including the number of failure modes that can occur due to phenomena at the electrolyte-electrode interface. Failure modes can range from the catastrophic to mere poor performance.

The development of full cells requires understanding of material limitations, processing capabilities and cell form factor.

Noting that all energy storage devices, including batteries, possess inherent risk as energy is being confined in a closed system that can be physically or electrically damaged, they discuss risk mitigation strategies as well.

A basic fishbone diagram highlighting two of the key high level failure modes (impedance growth or electrical shorting) and the variety of potential contributing factors. Possible solutions and the consequent product risks to their implementation, including reduced safety, power, energy density, throughput, or requirement of material innovation are also shown. Understanding the interplay between material level issues and cell level ramifications are highlighted to reiterate holistic approaches. Kermana et al. Click to enlarge.

There has been incredible progress in the field of solid state electrolytes for Li ion batteries. The discussion in this work strongly emphasizes that, in the context of solid state battery technology, a holistic approach to material development, taking cell and product design considerations into account is powerful and necessary in creating resilient and far reaching solid state material technologies in the Li ion space. Highly conductive solid ionic materials have paved the way for the potentially ground breaking technology of high energy density anodes and a true solid state battery.

… Specific research vectors for advancing solid state battery technology include scalable manufacturing of low defect density thin ionic conducting solids, characterization methods to determine defect densities at relevant scales, increasing ionic conductivity of solid state electrolytes further, protective active cathode particle coatings, developing high ionic conductivity materials that are deformable or have low melting temperature, and increasing active cathode particle fraction in solid state electrodes.

—Kermana et al.

Resources

• Kian Kermana, Alan Luntz, Venkatasubramanian Viswanathan, Yet-Ming Chiang, and Zhebo Chen (2017) “Review—Practical Challenges Hindering the Development of Solid State Li Ion Batteries” J. Electrochem. Soc. volume 164, issue 7, A1731-A1744 doi: 10.1149/2.1571707jes

• Yuki Kato, Satoshi Hori, Toshiya Saito, Kota Suzuki, Masaaki Hirayama, Akio Mitsui, Masao Yonemura, Hideki Iba & Ryoji Kanno (2016) “High-power all-solid-state batteries using sulfide superionic conductors” Nature Energy 1, Article number: 16030 doi: 10.1038/nenergy.2016.30