Doubling the capacity of solid-state lithium batteries

Scientists at Japan’s Tokyo Institute of Technology (Tokyo Tech), Tohoku University, National Institute of Advanced Industrial Science and Technology and Nippon Institute of Technology have demonstrated that a clean electrolyte–electrode interface is key to realising high-capacity solid-state lithium batteries — a breakthrough that could pave the way for improved battery designs with increased capacity, stability and safety for both mobile devices and electric vehicles. Their research has been published in the journal ACS Applied Materials & Interfaces.

Liquid lithium-ion batteries can be found in the majority of everyday mobile devices. While they possess a fair share of advantages, liquid-based batteries carry notable risks as well. This has become clear to the public in recent years after reports of smartphones bursting into flames due to design errors that caused the battery’s liquid electrolyte to leak and catch fire. Other disadvantages — such as fabrication cost, durability and capacity — led scientists to look into a different technology: solid-state lithium batteries (SSLBs).

SSLBs comprise solid electrodes and a solid electrolyte that exchange lithium (Li) ions during charging and discharging. Their higher energy density and safety make SSLBs very powerful sources; however, there are still many technical challenges preventing SSLBs’ commercialisation. For the current study, researchers conducted a series of experiments and gained insight that could take SSLBs’ performance to the next level.

“LiNi_0.5Mn_1.5O₄ (LNMO) is a promising material for the positive electrode of SSLBs because it can generate comparatively higher voltages,” said study leader Professor Taro Hitosugi, from Tokyo Tech. “In this study we showed battery operations at 2.9 and 4.7 V, and simultaneously achieved large capacity, stable cycling and low resistance at the electrolyte/electrode interface.”

Previous studies had hinted that producing a clean electrolyte/electrode interface was essential to achieve low interface resistance and fast charging in LNMO-based SSLBs. Scientists also noted that Li ions spontaneously migrated from a Li₃PO₄ (LPO) electrolyte to the LNMO layer upon fabrication, forming a Li₂Ni_0.5Mn_1.5O₄ (L₂NMO) phase in LNMO with unknown distribution and impact on battery performance.

The team investigated what the L₂NMO phase was like, analysing the changes in crystalline structure between the Li₀N_i0.5Mn_1.5O₄ (L₀NMO) and L₂NMO phases during charging and discharging. They also studied the initial distribution of L₂NMO at clean LPO/LNMO interfaces fabricated in a vacuum, as well as the effect of electrode thickness.

Strikingly, the clean interface facilitated the intercalation and deintercalation of Li during charging and discharging of the SSLBs. As a result, the capacity of SSLBs with a clean interface was twice that of conventional LNMO-based batteries. Moreover, the study marked the first time stable reversible reactions were found between the L₀NMO and L₂NMO phases in SSLBs.

“Our findings indicate that the formation of a contamination-free, clean LPO/LNMO interface is key to increasing the capacity of SSLBs while ensuring low interface resistance for fast charging,” said lead author Assistant Professor Hideyuki Kawasoko, from Tohoku University.

Aside from mobile devices, SSLBs could find a home in electric cars, for which cost and battery durability act as major barriers for widespread commercialisation. The results of the study thus provide important insight for future SSLB designs and pave the way for a transition away from fossil fuels and towards more eco-friendly ways of transportation.

Image credit: ©stock.adobe.com/au/pickup

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