Sodium-ion batteries (SIBs) have been studied as one of the most promising alternatives to lithium-ion batteries (LIBs), due to the abundance of sodium in the earth’s crust and its lower cost. Hard carbons (HC) are non-graphitizable carbons and have proven to be the best anode materials for SIBs in terms of insertion chemistry and specific electrochemical capacity (300 mAh.g-1 and beyond). These carbon materials can be synthesized from very diverse precursors and depending on their synthesis route, their mechanical properties differ inducing different behaviors during the electrode formulation and electrochemical cycling. The impact of the mechanical properties (particularly hardness) of these carbon materials on their electrochemical properties is very poorly reported in the literature. This work developed an indentation in-situ in a scanning electron microscope using a micromanipulator and associated force measurement tips to determine the mechanical properties of carbon anode materials for sodium-ion batteries. The results showed that graphitizable carbons are generally softer than hard carbons. Sucrose-derived HC proved to be harder than hard carbon synthesized from pitch while also showing a better specific capacity than the latter. Upon sodiation (i.e. discharge down to 0 V vs. Na+/ Na), graphitizable carbons showed an increased hardness while the hard carbons showed a reduction in their mechanical property with a decrease of around 33% in their hardness. However, a commercially available HC showed a different result when compared to the lab-synthesized HCs, exhibiting a lower hardness while also having an increasing hardness upon sodiation. These anomalous results are not understood yet but they could be attributed to the unknown synthesis route of the commercial HC.
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