Regulating Ceramic-Polymer Interphase in Composite Solid Electrolytes
Authors: Xue Shida , Pan Feng , Yang Luyi 
Published: February 2024

Exhibiting satisfactory safety and high energy density, all-solid-state lithium-ion batteries (ASSLBs) is considered as the next-generation energy storage device for green power. Composite solid electrolytes (CSEs) consisting of polymer matrix and active inorganic fillers have shown great potential for practical applications owing to desirable mechanical properties and improved ionic conductivity. However, mechanisms of how different active fillers enhance the ion transport in CSEs still remain inconclusive and comprehensive properties of CSEs need to be improved to meet the demand for high-performance ASSLBs. Our work focuses on the mechanism of Li ion transportation in CSEs and the interaction among polymer matrix, inorganic active fillers and Li-salts. Further, the specific insights into the improving mechanism are applied to the design of CSEs with ideal overall performance.

First, NASICON-type (LATP) and garnet-type (LLZTO) inorganic solid electrolytes are utilized as active fillers to incorporate with poly (ethylene oxide) PEO matrix. The experimental and computational results shown that the high affinity between LATP and PEO facilitates unhindered interfacial Li+ transfer, so that LATP functions as bulk-active filler to provide additional inorganic ion pathway. Instead, anion-trapping LLZTO mainly improves the ion conductivity by dissociating lithium salt, making it an interface-active filler. Therefore, the as-prepared PEO-LATP CSEs achieve the highest ion conductivity with medium filler content (50wt%) due to the well-built synergistic ion transport tunnels. By contrast, PEO-LLZTO peak at low filler content (10wt%) attributed to the maximized interfacial interactions.

Next, based on the above findings, surface defects are introduced to interface-active LLZTO to not only enhance the anion-trapping effect, but also serve as anchoring points for polymer chains, forming a firmly bonded polymer-ceramic interface. The as-prepared OV-LLZTO/PEO composite exhibits high mechanical strength, reduced interfacial resistance with electrodes as well as improved Li+ conductivity.

Further, Li-salt combination in CESs is optimized to improve the electrode compatibility for higher energy density. With the interaction of interface-active LLZTO and main lithium salt, functional additive has access to more preferable coordination. As a result, robust inorganic-rich electrode/electrolyte interface with high ionic conductivity can be synergistically constructed. More importantly, stable and fast ion transport network is built inside the high-volt LCO cathode, thus high rate performance and high capacity retentions achieved in Li|PEO-LLZTO|LCO cells with charging cut-off of 4.2 V.

To summarize, we have first revealed ion conduction mechanisms in CSEs, then optimization approaches such as surface defects introduction on the active fillers and Li-salt modification are employed to improve the critical properties of CSE (e.g. ionic conductivity, mechanical strength, interfacial contact and interfacial stability between electrode and electrolyte). These works jointly point out a new principle for the design of high-performance CSEs via regulating the ceramic-polymer interphases.

Figure 1