Improvement In Low-temperature Performance Of Li-ion Batteries And Their Application In Photovoltaic Energy Storage System

With the development of photovoltaic energy storage system, the technology of energy storage had been concerned widespreadly, specially, the chemical energy storage technologies. Lithium ion battery is considered as the most important battery for the photovoltaic energy system, because of its excellent structural stability, environmental compatibility. However, the main limitation is that both the energy and power output capability of Li-ion batteries are significantly deceased when the operation temperature falls below. That reduced the energy efficiency of photovoltaic energy storage system. So the low-temperature performances of the Li-ion battery are very important for the development of photovoltaic energy system. In this present work, we aimed at the low-temperature performances of the Li-ion battery and photovoltaic energy storage system. The research contents can be divided into the following two parts.1. The electrochemical performances of different electrolytes containing ethylene carbonate (EC), propylene carbonate (PC) and ethylmethyl carbonate (EMC) were investigated in the temperature range of 25℃to -40℃. The conductive behaviors of EC-based electrolytes strongly depend on the compositions and contents of solvent components. EMC and PC as a co-solvent can largely lower down the freezing point and effectively extend the low temperature operation range of the EC-based electrolytes. The ternary system of EC-PC-EMC has a moderate conductivity at room temperature but exhibits excellent conductive behaviors at low temperatures. And the volume fraction of PC in the EC-PC-EMC ternary electrolyte should be less than 0.33. The cycling performance, high current rate and capacity retention of the Li-ion cell can be improved by using an EC-PC-EMC electrolyte. However, the optimal performance is achieved only when a volume fraction of PC in the electrolyte mixture is less than 0.11. EIS analysis shows that the SEI film formed with 1.0 M LiPF6/EC-PC-EMC (1:0.5:3, v/v) electrolyte has higher conductivity at low temperatures. Therefore, the improved low-temperature performance of the Li-ion cells can be ascribed to a less resistive SEI film on the interfaces between the electrolyte and LiFePO4 electrodes.2. The photovoltaic energy storage system and Li-ion batteries was synthesized by design in one year. In Hangzhou, the photovoltaic energy system should not be used in winter and June for decreasing the used time of Li-ion battery. The 190 W photovoltaic energy system can produced 100-300 W energy after charged 5-7 h in one day and 1000-4000 W energy in one month. The photovoltaic energy system increased the used period, and optimized the reliability and the way of charge.