The Solid-electrolyte interphase (SEI) is as essential as electrode materials are to rechargeable batteries. Hence, SEI films with controllable properties are highly desired to improve the performance of batteries. Here, we reported the importance of tuning the SEI in controlling the Li and Na-ion storage in hard carbon. First, we elucidated the SEI formation on hard carbon in Li and Na-ion batteries and the Li/Na ion transfer using a combined experimental and theoretical approach. We demonstrated that a stable SEI layer can be designed by pre-cycling the electrode in the desired Li- or Na-based electrolyte to kinetically control the ionic transport. We discovered the selective intercalation of Li-ion with a preformed SEI layer on the electrode’s surface and corroborated with a mixed electrolyte. The Na-based SEI allows easy transport of Li-ions, while the Li-based SEI shuts off the Na-ion transport. The specific capacity of Na ions in hard carbon can be controlled to &lt;25 mAh g−1, ≈1/10 of the normal capacity (250 mAh/g<sup>-1</sup>). Furthermore, we developed an advanced electrolyte to have controlled SEI on hard carbon anodes and hence fast Na-ion transfer. Hard carbon anodes of high rate and good low temperature performance were demonstrated for Na-ion batteries. The specific capacity is ~180 mAh/g at the current density of ~250 mA/g at room temperature. The specific capacity can be ~200 mAh/g at -20<sup>o</sup>C/ discharge at the current density of ~25 mA/g. A new approach for high Coulombic efficiency of hard carbon anode also was developed based on the electrolyte. The first cycle Coulombic efficiency can be &gt;92%.

Tuning the solid electrolyte interphase to control the Li- and Na-ion storage in hard carbon

254th National Meeting (2017)

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