Li2C2 has the highest theoretical capacity (1400 mA?h?g?1) as the electrode material for Li-ion battery, but suffers from low conductivity. Here we found that under external pressure its conductivity was irreversibly enhanced by 109-fold. To explain that, we performed X-ray diffraction, Raman, IR, gas chromatography?mass spectrometry, and theoretical investigations under external pressure. We found that the C2 2? anions approached to each other and polymerized upon compression, which is responsible for the irreversible enhancement of conductivity. The polymer has a ribbon structure and disproportionates into Li3C4 (Li2?0.5C2) ribbon structure, Li6C3 (Li propenide) and Li4C3 (Li allenide) upon decompression, implying that the carbon skeletal is highly electrochemically active. Our work reported polymerized Li2C2 for the first time, demonstrated that applying pressure is an effective method to prepare novel Li?C frameworks, and hence shed light on the search for novel carbon-based electrode materials.