Low-temperature battery thermal management in electric vehicles via CO2 refrigerant direct heating: Mitigating temperature-glide mismatch and two-phase heating-window shrinkage

CO2 refrigerant direct thermal management provides a compact, low-GWP route for integrated battery cooling and heating in electric vehicles by reducing intermediate heat-transfer losses. However, the application of CO2 refrigerant direct battery heating remains insufficiently understood because the heating process is governed by refrigerant heat rejection rather than evaporation-driven heat absorption. Building on the known non-isothermal heat rejection and pressure-dependent phase behavior of CO2, this study identifies two heating-specific limitations in direct battery heating: temperature-glide-induced spatial heating mismatch and shrinkage of the effective two-phase heating window as battery temperature increases. To address them, a validated cabin–battery coupled transient CO2 thermal management model was developed. The influence of temperature glide on heat transfer driving force and battery uniformity was quantified. A semi-series unequal high-side pressure architecture was proposed to suppress temperature-glide-induced heating mismatch by separating supercritical cabin heating from two-phase battery heating. The effects of supply air temperature and battery heating driving temperature difference were evaluated, and air-side switching was introduced to sustain two-phase heating at elevated battery temperatures. Results show that the proposed architecture reduces the maximum battery temperature difference from 17.3 °C to 2.4 °C, while air-side optimization further reduces it from 4.8 °C to 1.1 °C. These results clarify the heating-specific origins of battery temperature non-uniformity in CO2 refrigerant direct heating and provide a mechanism-guided pathway for uniform and sustained low-temperature battery preheating in electric vehicles.