As a crucial component of substation energy storage equipment, lithium-ion batteries effectively alleviate a series of issues associated with the integration of new energy sources by means of peak shaving, frequency regulation, voltage support, and standby capacity. However, as the DC voltage level of lithium-ion battery energy storage systems increases, higher demands are placed on the insulation performance of the battery system. This paper employs multi-physics simulation software to establish a three-dimensional model of a square lithium-ion battery. Based on this model, thermal field and electric field simulations are conducted to investigate the thermal and electric field distributions of the square lithium-ion battery. The findings indicate that both the temperature peak and the extent of high-temperature regions within the lithium battery grow and spread as the discharge rate increases, with the highest temperature concentration occurring centrally on the battery casing's surface. Under a DC voltage of 1500 V, the maximum field strength is primarily concentrated at the junction between the outer insulating film and the cold metal plate, especially at the bottom corners of the battery cell. This study lays the foundation for future research into the effects of thermal ageing in lithium-ion batteries and the effects on the insulating elements of battery clusters under multi-physical field coupling.