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Quasi-solid-state batteries (QSSBs) are gaining widespread attention as a promising solution to improve battery safety performance. However, the safety improvement and the underlying mechanisms of QSSBs remain elusive. Herein, a novel strategy combining high-safety ethylene carbonate-free liquid electrolyte and in situ polymerization technique is proposed to prepare practical QSSBs. The Ah-level QSSBs with LiNi 0.83 Co 0.11 Mn 0.06 O 2 cathode and graphite-silicon anode demonstrate significantly improved safety features without sacrificing electrochemical performance. As evidenced by accelerating rate calorimetry tests, the QSSBs exhibit increased self-heating temperature and onset temperature (T 2 ), and decreased temperature rise rate during thermal runaway (TR). The T 2 has a maximum increase of 48.4 °C compared to the conventional liquid batteries. Moreover, the QSSBs do not undergo TR until 180 °C (even 200 °C) during the hot-box tests, presenting significant improvement compared to the liquid batteries that run into TR at 130 °C. Systematic investigations show that the in situ formed polymer skeleton effectively mitigates the exothermic reactions between lithium salts and lithiated anode, retards the oxygen release from cathode, and inhibits crosstalk reactions between cathode and anode at elevated temperatures. The findings offer an innovative solution for practical high-safety QSSBs and open up a new sight for building safer high-energy-density batteries., (© 2024 Wiley‐VCH GmbH.)