All-solid-state batteries with LiFePO4 cathodes and polyethylene oxide (PEO) based electrolytes are optimized with organic polymer synthesized from allyl amine and tartaric acid which show better properties including ionic conductivity, electrochemical window, charge-discharge capacity and rate performance. The rate of PEO ( MW ¯ = 4,000,000) and lithium salt (Lithium bis(trifluoromethanesulphonyl)imide, LiTFSI) is 75:20 (w/w) and polymers of different molecular weight are tested at a wide temperature range from 27 °C (ambient temperature) to 120 °C. Electrolytes with larger amount (weighting more than half of PEO) of low-molecular-weight polymer show conductivity of above 10−4 S cm−1, which is similar to typical gel-polymer-electrolyte. Electrolytes made of high-molecular-weight dopant, PEO and LiTFSI through ball milling show better conductivity (5.4 × 10−6 S cm−1) as well as better strength than electrolytes without dopant (1.3 × 10−6 S cm−1) at room temperature. And a sensitive dependence on temperature increasing is also observed constructing to the sample without dopant. Stability below 120 °C also indicates promising use of this composite electrolyte in solid state lithium ion batteries.